# FINAL REPORT

Corrosion Finishing/Coating Systems For
DoD Metallic Substrates Based on Non-Chromate Inhibitors and UV Curable, Zero VOC Materials

SERDP Project WP-1519

AUGUST 2010

Prof. Matt O’Keefe
Missouri S&T

This document has been approved for public release.

### Table of Contents

Page

List of Acronyms iii List of Figures and Tables iv Acknowledgments x

1. Abstract 1

2. Objective 2

3. Background 2 3.1 Non-Chromate Cerium Based Conversion Coatings 3 3.2 Zero VOC Coatings 4 3.3 UV Curable Coatings 4
4. Materials and Methods 6 4.1 Pre-Treatment Conversion Coatings 6
4.2 Substrate Materials 6 4.3 MUV Curable Coatings 6 4.4 Integrated Coating Systems 7
4.5 Characterization 9
5. Results and Discussion 10 5.1 Health and Safety Assessment 10 5.2 Cerium-based Conversion Coatings 10 5.3 Multifunctional UV (MUV) Curable Coatings 36 6. Conclusions and Implications for Future Research/Implementation 6 4
7. Literature Cited 6 6 8. Appendices 6 7
8.1 List of Scientific/Techni cal Publications 6 7
A. Articles in Peer-Reviewed Journals 6 7

B. Technical Reports 6 7
C. Conference or Symposium Scien tifically Recognized 6 8

D. Conference or Symposium Abstracts 6 8

ii

### List of Acronyms

AES – Auger Electron Spectroscopy ASTM – American Society for Testing and Materials CeCC – Cerium Conversion Coatings CrCC – Chromate Conversion Coatings DoD – Department of Defense EDS – Energy Dispersive Spectroscopy EIS – Electrochemical Impedance Spectroscopy EPA – Environmental Protection Agency
FIB – Focused Ion Beam HALS – Hindered Amine Light Stabilizers
HAPs – Hazardous Air Pollutants
HMIS – Hazardous Material s Identification System IBOA – Isobornyl Acrylate LCC – Light Curable Coatings
MUV – Multifunctional UV NCLT – Non-Chromated, Low Temperature OSHA – Occupational Safety and Health Administration PEA – Phenoxyethyl Acrylate PEL – Permissible Exposure Limit SEM – Scanning Electron Microscopy SERDP – Strategic Environmental Re search and Development Program
TCP – Trivalent Chromium Passivation TEM – Transmission Electron Microscopy TRI – Toxic Release Inventory
UV – Ultraviolet
UVAs – Ultraviolet Absorbers VOCs – Volatile Organic Compounds XPS – X-ray photoelectron spectroscopy XRD – X-ray Diffraction

iii

### List of Figures and Tables

Figures Page

Figure 1. Left: Existing three layer, hexavalent chromium based corrosion 2 coating system. Right: Two layer non-chromate, multifunctional UV (MUV) curable corrosion coating system under development.

Figure 2. Auger Electron Spectroscopy depth profiles of spray coated samples 12 cleaned at (a) 45oC, (b) 55oC, and (c) 65oC showing that the cerium oxide thickness increased with cleaning temperature.

Figure 3. Optical images of alkaline cleaned Al 2024-T3 substrates after 6, 13 12, and 25 spray cycles.

Figure 4. Optical images of an acid cleaned Al 2024-T3 substrates after 1 13 spray cycle.

Figure 5. Auger depth profiles of Al 2024-T3 panels after (a) alkaline and 14 (b) acid cleaning.

Figure 6. Nyquist plots for aluminum 7075- T6 panels prepared using 15
different methods.

Figure 7. Nyquist plots for aluminum 7075-T6 panels with CeCCs 16 deposited on panels prep ared using each method.

Figure 8. Optical images of prepared, unc oated aluminum 7075-T6 panels 17 after exposure to salt fog for 18 hours A) chemical immersion, B) chemical wipe, C) ultrasonic processi ng, and D) abrasive blasting.

Figure 9. Optical images of coated alumin um 7075-T6 panels after exposure 18 to salt fog for 18 hours A) chemical immers ion, B) chemical wipe, C) ultrasonic processing, and D) abrasive blasting.

Figure 10. SEM micrographs of CeCC deposited using pH 2.5 and pH 1.0 19 spray solutions.

Figure 11. Plot of wt.% Ce de tected by EDS vs. wt.% of Ce in the spray solution. 20

Figure 12. Plot of wt.% Ce de tected by EDS vs. ml of H 2 O 2 in the spray solution. 20

Figure 13. Nyquist plots of CeCC Al 2024-T3 panels in prohesion solution 21 after 3 hours of testing for panels depo sited using 1, 2, 3, and 4 wt.% Ce in the deposition solution.

ivFigure 14. Nyquist plots of CeCC Al 2024-T3 panels in prohesion solution 22 after 3 hours of testing for panels deposited using 0, 5, 10, and 20 ml of

H 2 O 2 in the deposition solution.

Figure 15. Images of CeCC Al 2024-T3 samples deposited using 0, 5, 10, 22
and 20 ml of H 2 O 2 in the spray solution followi ng 5 hours of EIS testing in prohesion solution.

Figure 16. a) Scanning electron microgra ph of a CeCC deposited on an 24 Al 2024-T3 panel with no gelatin in th e spray solution; b) optical image of panel after 2 days of salt spray testing.

Figure 17. Auger depth profile of a 5 sp ray cycle CeCC deposited on an 24 Al 2024-T3 panel with no gelatin in the spray solution.

Figure 18. a) Scanning electron microgra ph of a CeCC deposited on an 25 Al 2024-T3 panel with 3200 ppm gelatin in the spray solution; b) optical image of panel after 7 days of salt spray testing.

Figure 19. Auger depth profile of a 5 sp ray cycle CeCC deposited on an 25 Al 2024-T3 panel with 3200 ppm gelatin in the spray solution.

Figure 20. Electrochemical impedance of CeCC deposited on an Al 2024-T3 26 panel with 3200 ppm and no gelati n (0 ppm) in the spray solution.

Figure 21. a) Scanning electron micrograph of an as deposited (unsealed) 27 CeCC on an Al 2024-T3 panel; b) optical image of panel after 14 days of salt spray testing.

Figure 22. a) Scanning electron micrograph of a CeCC post treated in a 27

NH 4 H 2 PO 4 solution at 85oC for 5 minutes on an Al 2024-T3 panel; b) optical image of panel after 14 days of salt spray testing.

Figure 23. Electrochemical impedance of CeCCs on Al 2024-T3 panels post 28 treated in different phosphate solutions.

Figure 24. X-ray diffraction patterns of Ce CCs post treated in different 28 phosphate solutions, and one not post tr eated (unsealed), on Al 2024-T3 panels.

Figure 25. Coating thickness as a function of the number of brush or spray 30 deposition cycles.

Figure 26. XRD spectra for coatings deposit ed using three, five, and seven 30 brush cycles, and three and five spray cycles.

vFigure 27. Schematic drawing of reactions that increase the pH near the 31
surface of Al substrates that causes cerium precipitation from solution.

Figure 28. Schematic drawing of reacti ons that occur during CeCC 32 precipitation from solution.

Figure 29. Electron micrograph image of a CeCC on an Al 2024-T3 panel 33 after FIB sectioning.

Figure 30. Cross sectional micrograph image of an Al 2024-T3 panel after 33 cleaning (no sub- surface crevices).

Figure 31. Cross sectional micrograph image of an Al 2024-T3 panel after 34 CeCC deposition (contains s ub-surface crevices).

Figure 32. Cross sectional micrograph image of an Al 2024-T3 panel after 35 spraying a H 2 O 2 + NaCl solution on the surface.

Figure 33. Cross-sectional TEM images of cerium based conversion coatings: 36 a) as-deposited, b) after 14 days of salt spray testing.

Figure 34. Auger electron sp ectroscopy depth profiles of cerium based 36 conversion coatings: a) as-deposited and b) after 14 days of salt spray testing

Figure 35. Optical images of Stage 1 Al 2024-T3 coated panels after 3000 38 hours of neutral salt spray testing.

Figure 36. Optical images of Stage 1 Al 2024-T3 coated panels after 3000 39 hours of neutral salt spray testing an d removal of the corrosion coatings.

Figure 37. Auger maps for Cr, Al, and Zn n ear the scribe for two different 41 Hybricor 204 MUV coated panels; one not exposed to salt spray and the other after passing 3000 hours salt spray.

Figure 38. Auger maps for Ce, Al, and Zn n ear the scribe for two different 42 Hybricor 204 MUV coated panels; one not exposed to salt spray and the other after passing 3000 hours salt spray.

Figure 39. Optical images of Round 2 Al 2024-T3 coated panels after 2000 43 hours of neutral salt spray testing.

Figure 40. Optical image of a Round 2 Hybr icor 204 MUV coated Al 2024-T3 44 panel with a TCP conversion coa ting after 2000 hours salt spray

Figure 41. Optical image of a Round 2 Hybr icor 204 MUV coated Al 2024-T3 44 panel that did not have a conversion co ating (bare) after 2000 hours salt spray.

viFigure 42. Images of integrated coatings on CrCC Al clad Al 2024-T3 after 45 1000 hours of filiform corrosion testing

Figure 43. Optical images of MUV coated Al 2024-T3 panels with different 46 charge levels of Hybricor 204 corrosi on inhibitor after 3000 hours of salt spray testing.

Figure 44. Optical images of MUV coated Al 2024-T3 panels on CeCC and 48 CrCC pretreatments.

Figure 45. Optical images of wet scribe test results. 49

Figure 46. Optical images of Al 2024-T3 control panels afte r 3000 hours of 50 salt spray corrosion testing.

Figure 47. Optical images of MUV coated Al 2024-T3 panels with CrCC and 51 CeCC pretreatments after 3000 hours of salt spray corrosion testing.

Figure 48. Optical images of MUV coated Al 2024-T3 panels with TCP 52 pretreatments after 3000 hours of sa lt spray corrosion testing.

Figure 49. Optical images of Al 7075-T6 control panels after 3000 hours of 53 salt spray corrosion testing.

Figure 50. Optical images of MUV coated Al 7075-T6 panels with CrCC and 54 CeCC pretreatments after 3000 hours of salt spray corrosion testing.

Figure 51. Al 2024-T3 chromate contro l panels after 3000 hours ASTM 5 9 B117 testing.

Figure 52. Al 2024-T3 panels with MUV on CrCC after 3000 hours ASTM 5 9 B117 testing.

Figure 53. Optical images of Al 2024-T3 panels after 2000 hours of ASTM 60 B117 salt spray testing: a) chromate standard and b) MUV on CrCC.

Figure 54. Optical images of Al 2024-T3 panels after 2000 hours of ASTM 6 1 B117 salt spray testing: a) non-chromate standard, b) MUV on TCP, and c) MUV on CeCC.

Figure 55. Optical images of Al 2024-T0 panels after low temperature 6 2 flexibility tests.

Figure 56. Optical images of Al 2024-T0 panels after room temperature 6 2 flexibility tests: a) chromate controls and b) MUV on CrCC.

viiviii Tables Page

Table I Health and Safety Assessm ent of Coating Materials 10

Table II Summary of CeCC Phos phate Sealing Study 29

Table III Summary of MUV Testing 37

Table IV Coating Systems Evaluated (Round 3) 47

Table V Test Matrix for MUV/CeCC Panels Done at LCC Oct 2008 55

Table VI Pencil Hardness, Adhesion, Glo ss, MEK Rub and Skydrol Results 56
for MUV/CeCC Panels Done at LCC Oct 2008

Table VII Salt Spray Ratings for MUV/ CeCC Panels Done at LCC Oct 2008 5 8

Table VIII Summary of MUV Ev aluation Results 6 3

### Acknowledgments

Thanks are due to the many people that contribute d to this effort. The project was co-managed by Bill Fahrenholtz (Missouri S&T), Ben Curatolo (Light Curable Coatings), and John DeAntoni (Boeing). Jim Stoffer’s contributions and dedication to the project while in retirement is both admirable and appreciated. The worked done by graduate students Will Pinc, Simon Joshi, Daimon Heller, William Gammill, Phillip Jones, and Andrew Thomas along with post-doctoral fellows Dr. Ajay Padwal, Dr. Shujiang Geng, Dr . Becky Treu, Dr. Surender Maddela, and Dr. Beth Kulp at Missouri S&T has been of the hi ghest quality. The support of the staff of the
Graduate Center for Materials Research Center at Missouri S&T is gratefully acknowledged.
Thanks are extended to Donna Ballard at AFRL/ML in the first few years of the project and Gary Wright of AFRL/ML in the final year for their support and encouragement. The contributions of
Dr. Paul Yu and Prof. Tom O’Keefe from the st art of the project until the spring of 2008 are acknowledged in memoriam.

The financial support of the St rategic Environmental Research and Development Program and guidance of program managers Chuck Pellerin an d Bruce Sartwell are gratefully acknowledged.

x

### 1. Abstract

Corrosion resistant coatings containing non-chro mate inhibitors and no volatile organic compounds were developed and evaluated for DoD applications. The tech nical effort involved cerium based conversion coatings (CeCCs) and ultraviolet (UV) light curable coatings containing corrosion inhibitors. This approach would replace the current three layer, environmentally unfriendly coating system with a benign, two layer multifunctional UV (MUV) coating system. Replacement of corrosion coatings on high strength aluminum alloys used for military aerospace applications was the main focus for the technology developed. Work on the impact of processing parameters on CeCC corrosion performance, including th e use of gelatin in the deposition solution and post deposition treatment in phospha te solution, resulted in significant improvement in CeCC protection of high strength aluminum from corrosion. The
identification of sub-surface crev ices in the Al substrates that form during CeCC deposition helped explain results previously not understood. Application of MUV co atings with Hybricor 204 corrosion inhibitor repeated ly demonstrated the ability to protect scribed, chromate conversion coated Al substrates during ASTM B117 salt spray testing. Drawbar applied MUV coatings on CeCC passed salt spray testing whil e spray applied MUV on CeCC formed blisters during ASTM B117 testing. MUV coating properties su ch as flexibility and fluid resistance were improved throughout the project and demonstrated the potential to meet aerospace requirements. Results indicate that the technology holds prom ise for replacing existing environmentally hazardous corrosion coatings for military applications.

1

### 2. Objective

The primary objective of this project was to resear ch, develop, characterize, evaluate, integrate, and demonstrate an environmentally friendl y, chromate-free, zero TRI/VOC/HAPs coating system using rare-earth oxide based pretr eatments and corrosion inhibiting, multifunctional UV (MUV) curable self-priming coatings for metallic substrates that meet or exceed current military requirements. The integrated two layer non-chro mate coating system replaces the existing three layer system (Figure 1), and utilization of a UV curing system will greatly reduce the time required to paint DoD equipment. Aluminum alloy substrates commonly used in military aerospace applications were the main focus of the work. The integration, compatibility, and effectiveness of the novel low and high gloss ul traviolet light curabl e, corrosion preventing,
MUVs with rare-earth based conve rsion coatings has the potential to meet or exceed all of the objectives contained in th e SERDP statement of need PPSON-06-04

.

Chromate Co n v e r sion Coating Organic Top Coat

Strontium Chromate Primer

Metallic Substrate MUV Coating

Non-Chromate Conversion Coating

Metallic Substrate

Figure 1. Left: Existing three layer, hexavalent chromium based corrosion coating system. Right: Two layer non-chromate, multifunctional UV (MUV) curable corrosion coating system under development.



### 3. Background

Military corrosion protection coating systems typically consist of a metal pretreatment conversion coating followed by an epoxy primer and a polyurethane topc oat. Although very effective at preventing corrosion of the underlying metallic subs trate, these coatings contain volatile organic compounds (VOCs), hazardous air pollutants (HAPs) and chemicals listed on the toxic release inventory (TRI). The chromate co rrosion inhibitors used in the conversion coating in direct contact with the metallic substrate, as well as in the epoxy pr imer, present a serious health risk to workers that paint and de-paint military assets. In addi tion, the epoxy primer is formulated at 340 grams per liter VOC while polyur ethane topcoats are ty pically formulated at 420 grams per liter VOC and are crosslinked with hazardous isocyanates that are on the TRI.

The annual cost of corrosion to the U.S was estimated at $276 bill ion dollars. According to the “Cost of Corrosion” study published by CC Tec hnologies and NACE International, corrosion and metal wastage arising from oxidation as cau sed by exposure to the elements and reactivity between dissimilar materials costs the U.S. military about $20 billion annually [1]. Aluminum alloys important in both military and civilian airc raft are prone to such corrosion. The result is an annual maintenance cost for Navy aircraft of about $800 million per ye ar. According to USAF

2sources, aircraft painting/strippi ng/repainting processes and handling associated hazardous waste is one of the highest co st maintenance activities

. Chromates are an essential part of the present coating systems used on many DoD substrates.

The EPA regulates chromium and its compounds under the Clean Air Act, Clean Water Act, Comprehensive Environmental Response, Co mpensation and Liability Act, Resource Conservation and Recovery Act, Superfund Amen dments and Reauthorization Act, and Toxic Substances Control Act. Federal, state, and local agencies have issued re gulations that limit or prohibit the use of chromate- containing material s. OSHA regulates the amount of hexavalent chromium to which workers can be exposed an d has reduced the Permissible Exposure Limit (PEL) for hexavalent chromium and for all Cr(VI) compounds in construction, shipyards, and general industry from the previous 52 μg/m 3 to 5 μg/m 3 as an 8 hour time-weighted average [2]. This also includes provisions for employee protec tion such as preferred methods for controlling exposure, respiratory protection, protective work clothing and equipment, hygiene areas and practices, medical surveillance, hazard communi cation, and record keepin g. These regulations will severely impact the use of hexavalent chromium throughout the DoD and the defense industry. Use of hexavalent chromium compounds is also tightly controlled by EPA regulations issued under the Toxic Substances Control Act, Safe Drinking Water Act, Clean Water Act, Clean Air Act, Resource Conservation Recovery Act, Comprehensive Environmental Response, Compensation and Liability Act; and by OSHA under the Occupational Safety and Health Act. Chromium has been found in 1,036 of 1,591 National Priority List (Superf und) sites identified by the EPA. Development of a chrome free altern ative corrosion protection coating system would not be subject to these restrict ions while maintaining the corr osion protection needed for DoD platforms.

There has been extensive research on the develo pment of alternative treatments of metallic surfaces for corrosion protection. Among the major cl asses of chromate alte rnatives are cerium, cobalt, molybdenum, and manganese-based coati ngs, titanium and zirconium fluorocomplexes, hydrotalcites, boehmites and silanes, and inherently conducting polymers. Recent work includes some of the current efforts investigating the use of magnesium rich primers and conducting polymers to protect aluminum alloys, contro lled release of inhib itors from polyaniline conductive polymers, and the use of cerium modifi ed bentonite clay for corrosion inhibition and sensing.

3.1 Non-Chromate, Cerium Based Conversion Coatings

The non-chromate coatings group at Missouri S&T has identified cerium compounds as potentially viable corrosion inhi bitors and replacements for chromate conversion coatings on aluminum alloys. Success in de veloping the technology ha s led to several patent applications being filed and issued [3,4]. Work at Missour i S&T on conversion coatings over the past few years has involved the deposition of cerium oxi de-based conversion coatings from aqueous solutions. Feasibility studies on other rare-earth compounds as well as el ectrochemically active
elements have been conducted.

3Initial attempts to deposit ceriu m-based conversion coatings on aluminum alloys consisted of immersion in a cerium chloride solution for a pe riod of several days, which resulted in the deposition of a thin, cerium (III) oxide layer [5]. Some of the methods used to reduce coating deposition time have included galvanostatically po larizing the aluminum surface in an aqueous
CeCl 3 bath or boiling the panel in an aqueous cerium salt solution in combination with an anodic
treatment in Na 2 MnO 4 . The addition of hydrogen peroxide to a cerium chloride bath can reduce the coating time to approximately ten minutes [6 ]. This method can be further enhanced by the
use of a deoxidizer to activat e the surface before immersion into the coating bath [7].

Cerium-based conversion coatings have been de posited using electrolytic (current-driven) and spontaneous (immersion) deposition methods. Pr ocesses that deposit cerium-based conversion coatings that meet the DoD standard by provi ding corrosion protection for 336 hours (2 weeks) in ASTM B117 salt spray testing have been developed for both aluminum alloy 7075-T6 and
2024-T3. Mechanistic studies revealed that coating depos ition occurred due to an electrochemically-driven increase in the pH near the alloy panel surface that led to the precipitation of insoluble hydrated Ce 4+ compounds that attached to the panel surface [8]. Work done on this project focused on developing and optimizing a spray proc ess for deposition of cerium-based conversion coatings on aluminum alloy 2024-T3. Based on work done at Missouri S&T, surface preparation, deposition solution com position, and post treatment steps significantly impact the corrosion resistance of cerium oxide conversion coatings.

3.2 Zero VOC Coatings

Zero VOC corrosion coatings can be formulated by substituting VOC exempt solvents that are not photo-chemically reactive. N on-HAP VOC exempt solvents, such as acetone, can be used in coatings formulations. In some cases use of these solvents increases the flammability of the coating and can have a negative impact on a ppearance and application parameters. More desirable, zero VOC corrosion coatings formulat ed with water have been introduced to the marketplace, but it is not known if these coatings will meet all of the stringent performance requirements of military specifications, necessitating a different approach.

3.3 UV Curable Coatings

Ultraviolet (UV) light curable co atings are a class of coatings that can be formulated to be solvent free, and therefore zero VOC. UV curi ng is a photochemical process by which a liquid coating changes to a cross-linked solid polymer through exposure to UV radiation without the need for isocyanate cross-linking agents. Benefits of UV technology include zero VOC, HAPSfree coatings that fully cure in seconds, re sulting in faster thr oughput and labor savings, consistent with the needs outlined in the statement of needs for this project. UV technology is recognized as a Best Available Control Technology and a Lowest Achievable Emission Rate technology by the EPA. An ultraviolet light curable coating qualifie s as a “Superclean” technology defined by the Southern California South Coast Air Qu ality Management District. This allows for immediate installation of UV processes without the need for a permit.

Ultraviolet curing is one of th e radiation curing techniques that are used in coatings, inks, adhesives and sealants. UV curable coatings can be formulated to be 100% solids, and can be instantaneously polymerized and cross-linked to give films that are decorative and functional [9].

4The original driving forces be hind the commercialization of UV-curing technology were energy saving and freedom from solvents [10]. Improved economics, and efficiency result from instantaneous curing characteris tic in the case of UV polymerization. Since all of the formulation reacts or remains effectively within the final film, UV curable coatings are near zero VOC. As the pressure to reduce VOC continue s to mount, UV cured coatings are becoming
more attractive. There are two classes of UV-cu rable coatings, free radical (mostly acrylate) and cationic (epoxy) depending upon the nature of photoinitiators and resin chemistry.

Improvement in the coating/substr ate interaction is one of the mo st critical areas of performance in the practical application of UV curing. Du e to adsorption, the substrate/coating interface
receives the lowest dose of UV radiation and the interacti on between the coating and the substrate can inhibit the cure [11]. Pigment sele ction also plays an impor tant role in the overall curing characteristics of pigmented systems. In ge neral, the larger the pigment particle size, the smaller the surface area and, therefore, the better the curing. UV cured coatings that are used for exterior application usually requir e the use of light stabilizers to prevent degradation of the resin system due to damaging UV radiation. The two li ght stabilizers typically used are ultraviolet absorbers (UVAs) and hindered amine light stab ilizers (HALS). HALS have, in most cases, little or no impact on the cure properties of the photopolymers. UVAs, however, can compete with photoinitiators for the availa ble light and therefore have an impact on the cure speed of most formulations [12]. Incorporation of i norganic corrosion inhibi ting compounds into a UV
curable matrix results in a multifunctional UV (MUV) coating that provides the weathering protection of a top coat and the corrosion pr otection of a primer into a single layer.

5

### 4. Materials and Methods

The overlap of the technical appr oach necessitated that all members contributed to each task and significant interaction occurred within each ta sk. Responsibility for the project tasks was distributed by expertise and capabilities in each area, cognizant that each team member contributed to every task. Missouri S&T was th e lead organization for non-chromate conversion coatings, LCC headed up the formulation and ap plication of MUV curabl e coatings, and Boeing was responsible for integrati on, evaluation and implementation.

4.1 Substrate Materials

Aluminum alloys 2024-T3 and 7075-T6 were used throughout the study. Panels were sheared from large sheets into either 1” x 3” or 3” by 6” size for studies. The bulk chemical composition of Al 2024-T3 is 90.7-94.7 wt% Al, 3.8-4.9 wt% Cu, 1.2-1.8wt% Mg, 0.3-0.9 wt% Mn, maximum of 0.5 wt% Fe, maximum of 0.1wt% Cr , maximum of 0.5wt% Si, maximum of 0.15 wt% Ti, maximum of 0.25 wt% Zn, balance ot her trace elements. The bulk chemical composition of Al 7075-T6 is 87.1-91.4 wt% Al, 5.1-6.1wt% Zn, 1.2-2.0wt% Cu, 21.-2.9wt% Mg, maximum of 0.3wt% Mn, maximum of 0.5 wt% Fe, 0.18-28wt% Cr, maximum of 0.4wt% Si, maximum of 0.20wt% Ti, ba lance other trace elements.

4.2 Pre-Treatment Conversion Coatings

Spray deposition of cerium oxide based convers ion coatings onto Al 2024-T3 and Al 7075-T6 panel substrates was the focus of the pre-treatmen t technical efforts. Cleaning of the substrates prior to coating was done by an isopropyl alcohol rinse; an acid, alkaline, or a combination of acid and alkaline solutions; and water rinses. The substrates were kept wet throughout the process. Unless otherwise noted, the depositi on solution was 250 ml in volume with a nominal composition of 4 wt.% CeCl 3 ·xH 2 O, 20 ml of 30 vol.% H 2 O 2, 0.8 g of organic gelatin with the balance water. Spray deposition was accomplished using a commercially available detail air gun. The panels were placed upright at an angle of ~60° to the horizontal and sprayed evenly for a few seconds, followed by a 35 second delay; this was considered one spray cycle. Repetition of spray cycles was used to build up coati ng thickness and improve coating uniformity. The hydrated cerium oxide that was de posited by this method was then pos t treated, or “sealed”, in an 85°C phosphoric acid based solution at pH 4.5 to convert the coating, either partially or completely, into hydrated cerium phosphate. All panels were sealed in a phosphate solution
unless otherwise noted. Chromate conversion coated panels used for controls during evaluation were produced at Boeing or obt ained from commercial vendors.

4.3 MUV Curable Coatings

Several formulations of MUV curable coatings were developed and ev aluated throughout the project. Corrosion inhibitors evaluated included Hybricor 204, HT-13, and HT-20 from Wayne Pigments and praseodymium oxide based material s from Deft. The initial studies used 36% inhibitor loading; subsequent la dder studies investigated load ings of 0%, 7%, 15%, 22%, 29% and 36%. Isobornyl acrylate (IBOA) and phenoxyet hyl acrylate (PEA) were the two monomers evaluated. Dispersants, fillers, photoinitiators, stabilizers, and other additives were also

6evaluated. Five sets (rounds) of MUV coatings were evaluated. The first two rounds were deposited by drawbar with a 36% inhibitor loading. The final three rounds used Hybricor 204 inhibitor at a loading of 29% and were deposit ed using a spray gun. The MUV formulation was heated to ~105°F prior to spray application in or der to produce consistent flow characteristics. Panels were mounted horizontally and the MUV wa s sprayed using an overlapping pattern until a final film thickness of ~0.002” (2 mils) was ach ieved. Curing was done sequentially using 10” long, 600W/in V and H ultraviolet lamps from Fu sion, Inc. The panels were passed under the bulbs at a conveyor speed of 1”/sec and at a distance of 2.5” from the lamps. This setup resulted in an initial exposure of the pa nels to a V bulb for about 0.5 second and then to an H bulb for an additional 0.5 second, which fully cured the coatings.

4.4 Integrated Coating Systems

Evaluation of the integrated corrosion coating systems under development against standard test procedures was done throughout th e year. Cerium conversion co ated panels were made at Missouri S&T and sent to LCC for MUV application in most cases, with one set of panels made at LCC by S&T students in October of 2008. Commercial chromate and non-chromate comparison systems were used as control sample s during testing. Evaluation of the coatings were done at Boeing-St. Louis and included ne utral salt spray testi ng, wet adhesion tests, Skydrol fluid resistance, low and room temperat ure flexibility, filiform corrosion testing, and Xe arc weathering. Boeing’s testing pr ocedures are listed below. Th e ratings criteria for evaluating scribed panels was developed at Boeing to provi de a relative performa nce evaluation; panels with 1,A ratings are considered the best.

Boeing Test Procedures

Surface Treatment : Coordinate test panel chromate surface treatments with the chem ical process area. Cerium conversion coating will be performed by Missouri S&T.

Primer and Topcoat Application : Apply primer and topcoat per the manufacturer’s instructions. Apply primer to dry film
thickness between 0.7 and 1.3 mils. Apply topcoat to a dry film thickness between 1.7 and 2.3 mils. Let topcoat air dry two weeks prior to test . Measure and record a ll coating thicknesses. UV coating will be applied by Light Curable Coatings.

3000 hour 5% salt spray : Panels shall be scribed with an “X” across the test face using a NewHermes engraving machine. The scribe lines should penetrate to the substrate. Wrap the edges and back of the test panels with tape. Expose the test panels to 5% neutral salt fog for 3000 hours per ASTM B 117.

Check the panels for corrosion at 1000, 2000 and 3000 hours.

For each evaluation period, provide the following: Rank primers from best to worst. Rank the panels alpha/numerically using the ra nking method established by the laboratory.

7Ratings Criteria for Panels after ASTM B117 Testing

Code Description

1 Scribe line beginning to darken or shiny scribe.

2 Scribe lines > 50% darkened.

3 Scribe line dark.

4 Several localized sites of white salt in scribe lines.

5 Many localized sites of white salt in scribe lines.

6 White salt filling scribe lines.

7 Dark corrosion sites in scribe lines.

8 Few blisters under primer along scribe line. (<12)

9 Many blisters under primer along scribe line.

10 Slight lift along scribe lines.

11 Coating curling up along scribe.

12 Pin point sites/pits of corrosion on orga nic coating surface (1/16” to 1/8” dia.).

13 One or more blisters on surface away from scribe.

14 Many blisters under primer away from scribe.

15 Starting to blister over surface.

Code Scribe line ratings - corro sion creepage beyond scribe

A. No creepage

B. 0 to 1/64

C. 1/64 to 1/32

D. 1/32 to 1/16

E. 1/16 to 1/8

F. 1/8 to 3/16

G. 3/16 to 1/4

H. 1/4 to 3/8

Provide a checklist of all the types of corrosion / defects at each interval: After the 3000-hour exposure period get photographs taken of the test panels. Strip one of the test panels with Turco 5469 or equivalent paint stripper and get a photograph of the stripped panels.

Scribed Wet Tape Adhesion Tests : Perform a scribed wet tape adhesi on test on the test panels as fo llows: Scribe two parallel lines with an X-acto knife along the 6" panel directio n, approximately 3/4" ap art and 2 inches long. Scribe an "X" between the two parallel lines. Apply 3M #250 tape para llel to the first two parallel scribe lines. Roll a 4 lb., 60 durometer rubber roller acr oss the tape 4 times back and forth. Immediately remove the tape at a 45-de gree angle with one ab rupt motion while holding the panel steady with the opposite hand. Examine the test panel for removal and uplifting of the coating. Evaluate the adhesion of the panels per Method A of ASTM D 3359 - 97 and provide the rating of 5A, 4A, 3A, 2A, 1A, or 0A. Also provide the percentage of the coating removed.

8Low Temperature Flexibility : Expose panels in a cold box maintained at -60 ° F+/-5 ° F for 5 hours. After exposure the panels
shall be bent over a 1⁄2 inch mandrel which has been conditioned at the same temperature. Examine for cracks with unaided eye.

GE Room Temperature Reverse Impact Flexibility : Using the GE impact tester the flexibility sha ll be 20% minimum. Test method shall be ASTM D2794. Do not allow the impacter to boun ce on the panel. Report results under 10x magnification and with the unaided eye.

1000 Hour Filiform Corrosion : The test panels shall be scribed with an “X” across the test face using a New Hermes engraving machine. The scribe lines should penetrate to the substrate. Wrap the edges and back of the test panels with tape. Place the panels vertically in a dessicator containing 12N hydrochloric acid for
one hour. Within 5 minutes of removal from th e desiccator, place the panels in a humidity
cabinet maintained at 104 +/- 4 o F and 80 +/- 5% RH for 1000 hours. After removal from the humidity cabinet, examine the test panels fo r filiform corrosion, as described in ASTM D2803. The applied coating shall exhibit no filiform corr osion extending beyond 1⁄4 in. from the scribe. A majority of the filaments shall be less than 1/8 i n. in length. Rank coatings from best to worst. After the 1000-hour exposure period, photograph the test panels. Strip one of the test panels with Turco 5469 or equivalent paint stri pper and photograph th e stripped panels.

Xenon Arc Weather Resistance : The test panels shall be exposed for 1000 hours in a xenon-arc weatherometer that is cycling between 102 minutes of light onl y, and 18 minutes of light and wa ter spray. Check panels for initial color (E value), 85 a nd 60-degree gloss and at 500 and 1000 hours for Delta E and gloss.

30-day Room Temperature Skydrol Resistance Determine the initial pencil har dness of the film per ASTM D3363. Scribe the panels and wet with Skydrol LD-4 once daily for 30 days at ambient temperature. Remeasure the pencil hardness after this period. Afte r immersion there shall be no so ftening below a hardness of H, any blistering, any wrink ling, or loss of adhesion.

4.5 Characterization

Electrochemical and materials characterization techniques were used throughout the project. Surface analysis was done using Auger electr on spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS), imaging a nd chemical analysis was done using scanning electron microscopes (SEMs) with energy dispersive sp ectroscopy (EDS) capability, and cross sections were made in a focused ion beam (FIB) micr oscope. Electrochemical evaluations included potentiodynamic polarization scans and electrochemical impedance spectroscopy (EIS).

Specific details are provided in the publications li sted at the end of the re port, with articles by S. Geng, et al, “Screening Study of Spray Soluti on Parameters for Depositing Cerium Based Conversion Coatings on Al Alloy 2024-T3”, J. Applied Electrochemistry, 40, 551, (2010) and W. Pinc, et al, “Effects of acid and alkaline based surface prepar ations on spray deposited cerium

9based conversion coatings on Al 2024-T3”, Applied Surface Science, 255, 4061 (2009) representative of the types of characterization that were done.



### 5. Results and Discussion

5.1 Health and Safety Assessment

Boeing conducted an environmental, health, and safety assessment of the coating materials and systems being used on the project per the Hazardous Materials Identific ation System (HMIS), including the chromate materials used as controls . Shown in Table I belo w are the results of the assessment. The top two rows are for the two-layer CeCC and MUV coating system and the
three lower rows are for the current chromate coa ting system. The assessment indicated that the materials used in the project are much more benign than the standard chromate system.

Table I - Health and Safety Assessment of Coating Materials

Health (HMIS Rating) Reactivity (HMIS Rating) Flammability (HMIS Rating) Carcinogenic (Cal. Prop. 65) Teratogenic (Cal. Prop. 65) SARA

313 VOC

Cerium
Oxide 1 1 0 No No No None

MUV 2 2 1 No No Yes None

Chromate

conv. coating 3 1 0 Yes No Yes None

Epoxy
chromate primer 3 1 3 Yes No Yes 334 g/l

Polyurethane
topcoat 4 1 3 No Yes Yes 420 g/l

5.2 Cerium-based Conversion Coatings

The corrosion performance of CeCCs on high strength aluminum alloys was found to be dependent on many factors, especially the prep aration of the surface pr ior to deposition, the solution chemistry of the deposition soluti on, and post-deposition tr eatment in phosphate solutions. Development of non-spray deposition methods were investigated for possible field application. Characterization of the coatings wa s done throughout the project to provide insight into coating formation that led to pro posal of a deposition mechanism for CeCCs.

10Surface Preparation

Alkaline Cleaning Temperature

Studies during the initial phase of the project investigated the influence of alkaline cleaning temperature on coating deposition and corrosion pe rformance. The alkaline cleaner used, Turco 4215-NLCT from Henkel Corp., has a recommended operating range of 40-60°C. Prior work on
Al 7075-T6 substrates had identified 55°C as the optimum cleaning temperature for immersion (dip) CeCCs. Since the focus of this project wa s on using a spray deposition process, a series of experiments was conducted to determine if a 55°C was the prefe rred alkaline cleaning
temperature.

During spray deposition on Al 7075-T6 it was obse rved that the deposi tion rate was lower on panels cleaned at 45oC compared to panels that were cleaned at 55oC. After 20 sprays, the panels had changed color slightly, but only to a light ora nge, and the coatings were not as uniform as for a standard panel cleaned at 55oC. Therefore, mo re than 20 spray cycles were needed for panels cleaned at 45oC before the color of the panels ch anged from gray to a dark orange color. The panels cleaned at 55oC only re quired 20 spray cycles and they coated evenly and changed color as expected. The panels cleaned at 65oC change d color the fastest, but spraying continued for the standard number of 20 spray cycles over 15 minutes to ensure a thick coati ng. There was little to
no measurable difference in the microstructures of the coatings examined using optical and electron microscopes. However, Auger depth pr ofiling analysis indicated that the thickness of the cerium oxide layer increased from 100 nm for panels cleaned at 45oC, to 143 nm for panels cleaned at 55oC, and to 296 nm for panels cleaned at 65oC (Figure 2). Ther efore, the color of the CeCC was a reasonable indication of coating thickness and could be used as process control parameter by operators in the field. Chemical surface analysis of the substrates after cleaning, but before CeCC deposition, did not reveal any significant differen ces in composition after using the different cleaning temperatures.

11100

O Al

Ce

Figure 2. Auger Electron Spectro scopy depth profiles of spray coated samples on Al 7075-T6 cleaned at (a) 45oC, (b) 55oC, and (c) 65oC showing that the ce rium oxide thickness increased with cleaning temperature. (c) (b) (a)

Al

O

Ce 296 nm Al

Ce

O 143

Acid Activation

Standard cleaning procedures for 20 24-T3 Al alloys initially used just an alkaline solution, Turco 4215 NCLT, prior to spray deposition. This resulte d in a surface that was able to be evenly
coated with ~300 nm of hydrated cerium oxide (CeO 2 •xH 2 O) after approximately 30 spray cycles. It is more desi rable from a manufacturi ng standpoint to have fewer spray cycles so work
was done to determine methods to decrease the nu mber of spray cycles to obtain ~300-400 nm thick films with fewer spray cycles. It was determined that a 1 wt% sulfuric acid activation after an alkaline clean, but prior to deposition, on Al 2024-T3 reduced the number of spray cycles needed for a 400 nm thick film to five, with one spray cycle on acid activated panels resulting in
a film ~275 nm in thickness. Shown in Figure 3 are a series of images from alkaline (only) cleaned Al 2024-T3 panels afte r a number of spray cycles wh ile Figure 4 depicts an acid activated panel after just one spray cycle. It is clear from the images that very little coating

12forms on the alkaline cleaned ( only) panel up to ~10 spray cycl es while a single spray cycle on
acid activated substrates resulted in a fairly uniform coating.

6 sprays 12 sprays 25 sprays Figure 3. Optical images of alkaline cleaned Al 2024-T3 substrates after 6, 12, and 25 spray cycles.

1 spray

igure 4. Optical images of an acid cleaned Al 2024-T3 substrates after 1 spray cycle. F

In order to determine why the two cleaning processes produced such a dramatic difference in deposition rate, Auger electron spectroscopy (AES) was used to characteriz e the substrates after cleaning but prior to deposition. Shown in Fi gure 5 are AES depth profiles of Al 2024-T3
substrates after alkaline cleani ng in Turco alkaline (Figure 5a) and activation in 1 wt% sulfuric acid (Figure 5b) solutions. The profiles show th at the surface of the alkaline cleaned substrate had a significant amount of Mg and ~25 nm thick oxide film, along with lit tle Cu. In contrast, the acid activated substrate surface had very lit tle Mg, only a ~5 nm thick oxide, but ~10 at.% Cu after acid cleaning, which was an indication thatthe acid was more effective at removing the MgO/Al 2 O 3 layer and exposing the underlying metal su rface. The copper on the surface, most likely from intermetallic compounds in the alloy, act s as cathodic sites that facilitate precipitation of cerium oxide during deposition and increases the deposition rate . Concentrations of Cu and Mg in Figure 5 are different from the bulk values due to surface enrichment and depletion that occurs during heat treatment.

13(a)

(b) Figure 5. Auger depth profiles of Al 2024-T3 pane ls after (a) alkaline and (b) acid cleaning.

Alternative Surface Preparation Methods

The alkaline and acid cleaning described above wa s done by immersion of the panels in a beaker
of solution. This method would not be feasible fo r field repair and touch ups on existing aircraft. During the final year of the program alternativ e methods to prepare surfaces for coating were studied, including hand application (wiping) of the cleaning solutio ns, ultrasonic agitation in water, and abrasive media (“sand”) blasting. Th ese methods were compared to the standard immersion processes. After the alternative surfa ce preparation techniques the aluminum surfaces were optically different from the unprepared surfaces and from each other. Immersion prepared and chemically wiped panels were both dark in color, but the immersion cleaned panels tended to be darker than the chemical wiped ones. Ultrasonically processed panels did not appear different than unprepared panels. Abrasively blasted panels had visibly rough surfaces that tended to be a brighter color than unprepared panels.

14Electrochemically, treated Al 7075-T6 surfaces were more active than unprepared panels. Impedance scans (Figure 6) showed the blasted pane ls had the lowest charge transfer resistance (3.4 k Ω -cm 2 ) and that the ultrasonically cleaned panels had the highest charge transfer resistance (5 k Ω -cm 2 ). The tests found that all of the prepared panels had higher corrosion currents and lower charge transfer resistances than as-recei ved panels and, therefore, had higher corrosion rates. Based on these results, the protective oxi de layer was affected by the alternative surface preparation techniques.

-3000

-2500

-2000

-1500

-1000

-500

0

0 1000 2000 3000 4000 5000 Immersion ( )

Chemical wipe ( ) Ultrasonic ( )

Abrasive blast ( )

Z’ ( Ω -cm^2)Z” (Ω-cm^2) -3000

-2500

-2000

-1500

-1000

-500

0

0 1000 2000 3000 4000 5000 Immersion ( )

Chemical wipe ( ) Ultrasonic ( )

Abrasive blast ( )

Z’ ( Ω -cm^2)Z” (Ω-cm^2)

Figure 6. Nyquist plots for aluminum 7075-T6 panels prepared using different methods.

Electrochemical tests performed on alternativel y prepared panels coated with CeCC found an
increase in the measured corrosion resistance af ter coating. Results varied with the surface preparation used. Electrochemical impedance sp ectroscopy (EIS) tests found variation in charge transfer resistance values for pa nels with CeCCs, from about 5 k Ω -cm 2 for the abrasively prepared coating to about 44 k Ω -cm 2 for the immersion prepared coating (Figure 7). The charge transfer resistance values for panels with CeCCs were higher than the unco ated surfaces, but the immersion prepared and chemically wiped pane ls improved the most, consistent with the potentiodynamic scans. All of the coatings offe red some protection compared to the prepared panels.

15

### -20000

-15000

-10000

-5000

0

5000

10000

0 10000 20000 30000 40000 Immersion ( )

Chemical wipe ( )

Ultrasonic ( )

Abrasive blast ( )

Z’ ( Ω -cm^2)Z” (Ω-cm^2) 

### -20000

-15000

-10000

-5000

0

5000

10000

0 10000 20000 30000 40000 Immersion ( )

Chemical wipe ( )

Ultrasonic ( )

Abrasive blast ( )

Z’ ( Ω -cm^2)Z” (Ω-cm^2)

Figure 7. Nyquist plots for aluminum 7075-T6 pane ls with CeCCs deposite d on panels prepared using each method.

Salt spray testing was also performed to compar e corrosion performance of panels with CeCCs. Figure 8 contains images of the uncoated, prepar ed panels after 18 hours of exposure. The salt spray tests on the uncoated panels show the sa me trend as the electrochemical tests on the uncoated panels. The abrasively blasted panel corrode d the most; it had a continuous layer of corrosion product on the surface. The immersion pr epared panel had many la rge pits with tails, while the chemically wiped panel had fewer pits a nd smaller tails. The ultrasonically prepared panel had the fewest pits and th e least amount of taili ng of the prepared panels. Panels with CeCCs were less corroded than the uncoated pane ls (Figure 9). The salt spray results were
consistent with the electrochemical tests on the coated panels. The most corroded panel was the blasted panel, again with a continuous corro sion product layer. The CeCC on the immersion prepared and chemically wiped panels performed the best in salt spray, with few pits and little tailing. The immersion pretreatme nt had slightly fewer pits. Th e ultrasonically prepared panel was the least corroded of the uncoated panels but the coating on the ultras onically prepared panel did not protect as well as th e coatings on the immersion prep ared and the chemically wiped panels.

16

### A B

C D

Figure 8. Optical images of prepared, uncoated aluminum 7075-T6 panels after exposure to salt fog for 18 hours A) chemical i mmersion, B) chemical wipe, C) ultrasonic processing, and D) abrasive blasting.

17

### A B

C D

Figure 9. Optical images of coated aluminum 707 5-T6 panels after exposure to salt fog for 18 hours A) chemical immersion, B) chemical wipe , C) ultrasonic processing, and D) abrasive blasting.

18Deposition Conditions

pH, Ce and H 2 O 2 Concentration Effects

Efforts to develop and optimize cerium oxide based conversion coatings on high strength aluminum alloy substrates focused on how the pH, cerium, and H 2 O 2 concentration in the spray solution influenced coating formation and corrosi on performance. The influence of pH (1.0-2.5), Ce concentration (1 to 4 wt.%), and H 2 O 2 concentration (0 to 20 ml per 250 ml) on coating morphology, thickness, and corrosion performance afte r 5 spray cycles were investigated. It was
found that the amount of Ce detect ed in the film by EDS in the SEM was ~30 wt% for all of the pH values evaluated but there were fewer cracks observed in the coating from pH 2.0 or 2.5 as compared to pH of 1.0 or 1.5 (Figure 10). As th e concentration of Ce in the spray solution was increased from 1 to 4 wt.%, however, the amount of Ce detected on the surface by EDS
increased from ~8 wt.% Ce in th e film from the 1 wt.% Ce solution to ~30 wt.% Ce in the film for the 4 wt.% Ce solution (Figure 11), an indire ct but reproducible met hod of evaluating coating thickness. As the coating thickness increase d the corrosion resistance also improved up to 4 wt.% Ce in solution but then degraded beyond that point. Similarly, as the hydrogen peroxide concentration increased from 0 to 20 ml, the am ount of Ce detected by EDS increased (Figure 12) with peroxide concentrations above 20 ml resulting in poorer corrosion performance.

pH 2.5 

## 2.5

20 μ m

pH 1.0 

## 1.0

20 μ m

Figure 10. SEM micrographs of CeCC deposite d using pH 1.0 and pH 2.5 spray solutions.

19Figure 11. Plot of wt.% Ce detected by EDS vs. wt.% of Ce in the spray solution. 

# 0 10 20 30 40

012345

CeCl 3 

# ·7H 2 

# O concentration of solution (wt.%)Ce wt.% in coatings

·

Figure 12. Plot of wt.% Ce de tected by EDS vs. ml of H 2 O 2 in the spray solution.

20In a similar fashion, EIS measurements were ma de on CeCC panels in prohesion solution for up to 5 hours and the data indicated that the higher th e impedance, or graphically the larger the loop and intercept value on a Nyquist plot, the better the perfor mance during salt spray testing. Nyquist plots for Al 2024-T3 panels with CeCC af ter 3 hours in prohesion solution for coatings deposited with different concentr ations of Ce (Figure 13) and H 2 O 2 (Figure 14) indicated that as
both the Ce and H 2 O 2 concentration in the solution increa sed the impedance of the CeCC panel (which had been sealed in a phosphate solution) also increased. Note that no coating was deposited in the case of 0 ml of H 2 O 2 since no oxidizer was pres ent in the solution. The difference in pitting corrosion after five hours of EI S testing was also confirmed visually in that the higher the impedance, the better the corrosion resistance. Shown in Figure 15 are images of the peroxide test samples after 5 hours of EIS testing in prohesion solution.

Figure 13. Nyquist plots of CeCC Al 2024-T3 panels in prohes ion solution afte r 3 hours of testing for panels deposited using 1, 2, 3, a nd 4 wt.% Ce in the deposition solution. 

# 3.0-h

0.0 5.0x10 4 1.0x10 5 1.5x10 5 2.0x10 5 0.0 5.0x10 4 1.0x10 5 1.5x10 5

4 wt.%
3 wt.%
2 wt.%
1 wt.%- ZIm(Ωcm2)

Z
Re ( Ω cm 2 ) 0 1x10 4 2x10 4 3x10 4 4x10 4 0 1x10 4 2x10 4 3x10 4- ZIm(Ωcm2)

Z
Re ( Ω cm 2 )

21Figure 14. Nyquist plots of CeCC Al 2024-T3 panels in prohes ion solution afte r 3 hours of testing for panels deposited using 0, 5, 10, and 20 ml of H 2 O 2 in the deposition solution.

Figure 15. Images of CeCC Al 2024-T3 samples deposited using 0, 5, 10, and 20 ml of H 2 O 2 in the spray solution following 5 hours of EIS testing in prohesion solution. 

## 0 ml 5 ml

10 ml 20 ml 

# 3.0-h

0.0 5.0x10 4 1.0x10 5 1.5x10 5 2.0x10 5 0.0 5.0x10 4 1.0x10 5 1.5x10 5

20ml
10ml

5ml

0ml- ZIm(Ωcm2)

Z
Re ( Ω cm 2 ) 0 1x10 4 2x10 4 3x10 4 4x10 4 0 1x10 4 2x10 4 3x10 4- ZIm(Ωcm2)

Z
Re ( Ω cm 2 )

22Gelatin Addition

The addition of a water soluble gelatin to the cerium-based conversion coating solution greatly improved the salt spray corrosion performa nce of the CeCC on high strength aluminum substrates that have been pos t-treated (those without post-trea tment do poorly in salt spray regardless). Video recording of the deposition process had dem onstrated that the presence of gelatin in the solution modifies the formation and stability of bubbles that form during the precipitation of cerium oxide that results in coa ting formation on the substrate surface. When gelatin is not present in the deposition solution the bubbles are unstable, readily forming and breaking; when gelatin is present the bubbles are very stable. If no gelatin is added to the deposition solution a CeCC with a lot of crack s and areas where the coating was no longer present, exposing the underlying su bstrate, was observed (Figure 16a). Coatings made without gelatin in the spray solution do very poorly during salt spray tes ting, with many pits and tails after just a few days (Figure 16b) regardless of any other changes in the overall process. Analysis of the films made without gelatin af ter 5 spray cycles using Auger depth profiling
indicated that the coat ings were ~850 nm thick (Figure 17) . Many ceramic th in films exhibit cracking and loss of adhesion at thicknesses appr oaching 1000 nm. In contrast, CeCCs deposited with gelatin in the spray solution had very fe w to no cracks or uncoated areas (Figure 18a) and performed well during salt spray testing (Figure 18b) . The thickness of film s made with gelatin after 5 spray cycles was typically in the 350-400 nm range (Figure 19), meaning that the deposition rate is significantly lower when gelatin is present in the spray solution.

23a) b)

Figure 16. a) Scanning electron micrograph of a CeCC deposited on an Al 2024-T3 panel with no gelatin in the spray solution; b) optical image of panel afte r 2 days of salt spray testing.

0 10 20 30 40 50 60 70 80

0 200 400 600 800 1000 1200
Depth (nm)Atomic Percent 

## Al

Figure 17. Auger depth profile of a 5 spray cy cle CeCC deposited on an Al 2024-T3 panel with no gelatin in the spray solution. 

## Ce

O 850 nm

24a) b)

Figure 18. a) Scanning electron micrograph of a CeCC deposited on an Al 2024-T3 panel with 3200 ppm gelatin in the spray solu tion; b) optical image of pane l after 7 days of salt spray testing.

0 10 20 30 40 50 60 70 80

0 100 200 300 400 500 600 700 800 900
Depth (nm)Atomic Percent 

## Al

Ce

O 375 nm

Figure 19. Auger depth profile of a 5 spray cy cle CeCC deposited on an Al 2024-T3 panel with 3200 ppm gelatin in the spray solution.

25Electrochemical impedance charact erization of films made wit hout (0 ppm) and with 3200 ppm of gelatin in solution indicated that the impeda nce of the CeCC was significantly higher for the films made with gelatin (>100 k Ω -cm 2 ) than those without gelatin (~50 k Ω -cm 2 , Figure 20). This correlated very well with the corrosion te sting, as films with hi gher impedance performed better during salt spray exposure.

0 5 0 00 1 0 000 1 5 000 20

000 25000 30000 35000 40

000 45

0 0 0

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 Z' ( Ω -cm 2 )Z" (Ω-cm2) 

## 3200 ppm

0 ppm

Figure 20. Electrochemical impedance of CeCC deposited on an Al 2024-T3 panel with 3200 ppm and no gelatin (0 ppm) in the spray solution.

Post-Treatment

The formation of cerium phosphate during post treatment greatly improves the corrosion protection of CeCCs on Al substrate. Investigati ons to determine the influence of post treatment parameters on corrosion performance were c onducted to understand the improvement. The initial study focused on the use of different phos phate sources for the post treatment solution. Included in the study were orthophosphates (PO 4 ), pyrophosphates (P 2 O 7 ), and polyphosphates (P 3 O 10 ), along with as deposited (unsealed) CeCCs for comparison. SEM and optical images of specimens that were as deposited and post treated in a PO 4 solution (Figure 21), and post treated
in a P 2 O 7 solution (Figure 22) indicate the differences possible using different phosphate sources.
It was determined that the PO 4 based solutions had the fewest cracks in the films, performed the best in salt spray, and had the highest electr ochemical impedance valu es (Figure 23). The presence of a hydrated CePO 4 phase after post treatment (Figur e 24) correlated strongly with good corrosion performance during salt spray testing.

26a) b)

Figure 21. a) Scanning electron micrograph of an as deposited (unsealed) CeCC on an Al 2024- T3 panel; b) optical image of panel after 14 days of salt spray testing.

a) b)

Figure 22. a) Scanning electron microgr aph of a CeCC post treated in a NH 4 H 2 PO 4 solution at 85oC for 5 minutes on an Al 2024-T3 panel; b) optic al image of panel after 14 days of salt spray testing.

27Figure 23. Electrochemical impedance of CeCCs on Al 2024-T3 panels post treated in different phosphate solutions.

Figure 24. X-ray diffraction patterns of CeCCs post treated in diffe rent phosphate solutions, and one not post treated (unsealed), on Al 2024-T3 panels.

28A summary of the phosphate sealing studies is presented in Table II. Corrosion performance was qualitatively evaluated by assigning a ranking to each condition. In Table II 5 dots corresponds to passing 2 weeks of ASTM B117 salt spray te sting, 3 dots corresponds to passing 1 week of ASTM B117, and 1 dot is failure after 1 day of salt spray testing; 2 and 4 dots correspond to
conditions that fall between those values. The el ectrochemical testing of the panels correlated very well with the other characterization methods. In general, higher charge transfer resistance,

R ct , lower corrosion current density, i
corr , and more positive p itting potentials, E pit , were associated with panels that we re found to have formed CePO 4 , had fine cracks, and performed best during salt spray testing.

Table II - Summary of CeCC Phosphate Sealing Study

R ct (k Ω -cm 2 ) i
corr (μA/cm 2 ) E pit (mV) CePO 4 Formed Morphology (Cracks) Corrosion

Performance

8 1.50 -435 No Large 

### ••

NH 4 H 2 PO 4 (2.5 wt%) 60 0.60 -261 Yes Fine 

### •••••

Na 3 PO 4 (2.5 wt%) 51 0.45 -280 Yes Fine 

### •••••

K 4 P 2 O 7 (2.5 wt%) 25 0.74 -275 No Fine 

### ••••

Na 2 H 2 P 2 O 7 (2.5 wt%) 15 1.17 -322 No Defects 

### •••

Na 5 P 3 O 10 (2.5 wt%) 4 2.14 -343 No Defects 

### •

10 sec (85 °C) 17 1.41 -401 No Large 

### ••

30 sec (85 °C) 25 1.98 -340 No Fine 

### •••

2 min (85 °C) 52 0.84 -329 Yes Fine 

### ••••

10 min (85 °C) 62 1.00 -245 Yes Fine 

### •••••

55 °C (5 min) 49 0.83 -384 No Large 

### ••

70 °C (5 min) 63 0.66 -320 Yes Fine 

### ••••

85 °C (5 min) 91 0.65 -258 Yes Fine 

### •••••SourceTimeTemp. Conditions

Unsealed

Deposition Methods

Brush Application

During the last year of the proj ect, work was done to determin e the feasibility of depositing a CeCC onto Al 2024-T3 substrates using a standard paint brush application. This process could be used for field repairs and ot her locations where a spray boot h, immersion tank, or electrolytic
bath were not available. The same solution was used to deposit CeCCs by spray and brush methods in order to compare the application methods. Usi ng similar substrate cleaning processes, it was found that a uniform CeCC c ould be deposited using ei ther a spray or brush application method. The brush samples were do ne by immersing the paint brush into the CeCC sample and swabbing it across the surface for a few seconds, waiting ~30 seconds, and reapplying CeCC solution with the brush up to 7 times . In all cases it was possible to obtain a CeCC using a brush application method. A plot of spray and brush applied CeCC thicknesses as a function of deposition cycles is presented in Figure 25. The spray application resulted in a thicker coating per deposition cycl e than the brush application but the thicknesses were in the range of ~250 to 400 nm that had previously be en demonstrated to pr ovide corrosion protection during salt spray testing.

290 50 100 150 200 250 300 350 400 450 500

012345678

CyclesCoating Thickness (nm)

Brush

Spray

Figure 25. Coating thickness as a function of the number of brush or spray deposition cycles.

X-ray diffraction analysis of brush and spray applied CeCCs coatings after post treatment indicated that the 3, 5, and 7 depos ition cycle brush coatings did not have peaks associated with hydrated CePO 4 as compared to the spray 3 and 5 cycle samples that did have CePO 4 peaks (Figure 26). Corrosion performance of brush pane ls during salt spray test ing was similar to, but not quite as good as, the spray panels. It was not determined why the brush deposited CeCCs did
not transform into CePO 4 during post treatment but does re inforce the importance of deposition conditions on the structure and performa nce of cerium-based corrosion coatings.

3 Brush

5 Brush

7 Brush

3 Spray

5 Spray

CePO 4 hydrate

CeO 2

Aluminum

10 20 30 40 50 60 70 80 90
2 Θ angle (degrees)Intensity 3 Brush

5 Brush

7 Brush

3 Spray

5 Spray

CePO 4 hydrate

CeO 2

Aluminum

10 20 30 40 50 60 70 80 90
2 Θ angle (degrees)Intensity

Figure 26. XRD spectra for coatings deposited us ing three, five, and seven brush cycles, and three and five spray cycles.

30Deposition Mechanism

Based on all of the data available and experime ntal observations, a mechanism describing what is thought to occur during deposit ion (precipitation) of CeCCs on high strength Al alloys was developed. Although it can’t be shown interactiv ely in this document as it is a step-wise mechanism, the two major components of the mechanism are shown in Figures 27 and 28. Figure 27 shows an increase in pH associated with electrochemical reactions involving the alloy
substrate that contains cat hodic (intermetallic compounds) a nd anodic (Al matrix) sites. Reactions that increase the pH of the soluti on near the Al/solution interface drive the precipitation reactions, including peroxide decomposition, generati on of water, and consumption
of H + . The actual precipitati on reactions in the region of higher pH (> 4.5) can involve cerium hydroxide or peroxide species de pending on the local conditions (Fig ure 28). In either case these can decompose into hydrated cerium oxide or cerium hydroxide phases. The corrosion performance of the films is much better after phosphate post treatment (“sealing”), so conversion into phosphates is usually done. Whether or not the cerium oxide/peroxide species present after deposition can affect the phosphating process is not fully understood and requires more study.

Figure 27. Schematic drawing of reactions that incr ease the pH near the su rface of Al substrates that causes cerium preci pitation from solution.

31Figure 28. Schematic drawing of reactions that occur during CeCC precipitation from solution.

Characterization

Sub-Surface Crevices

During calendar year 2008 Missouri S&T insta lled and began operation of a FEI Nanolab 600 focused ion beam (FIB) scanning electron microsc ope. The FIB is able to selectively remove material from specific sites on a sample, enablin g cross sectional views at and just below the surface of the specimen. Shown in Figure 29 is a low magnification image of a sample after removing ~20 μ m long, ~10 μ m deep section from a CeCC on an Al 2024-T3 substrate (essentially looking down into a ho le dug out from the surface). Shown in Figure 30 is a cross section image of an Al 2024-T3 panel after acid activation cleaning, with no CeCC present. The figure shows that the Al 2024 substrate is contin uous and without any sub-surface damage prior to CeCC deposition.

32

## CeCC

Al 2024

Figure 29. Electron micrograph image of a CeCC on an Al 2024-T3 panel after FIB sectioning.



## Pt Protection Layer

Surface

Al 2024-T3 Substrate

Figure 30. Cross sectional micrograph image of an Al 2024-T3 panel after acid activation cleaning (no sub-surface crevices).

Presented in Figure 31 is a FIB cr oss sectional image of samples after spray deposition of CeCC
onto cleaned Al 2024-T3 substrates . The areas examined were near the large cracks in the CeCC, ~10% of the total surface area on a sample (90% have fine or small cracks). It was
observed that the Al 2024-T3 substrate unde r the CeCC with large cracks had sub-surface crevices, an indication that the Al substrate had been preferentia lly etched and material removed during deposition. The crevices can be near the surface only or extend down into the substrate

33several micrometers. It should be noted that the areas with fine crack s (90% of total surface area) do not have the sub-surface crevices and look similar to Figure 30.



## CeCC

Sub-surface crevices

Figure 31. Cross sectional micrograph image of an Al 2024-T3 panel after CeCC deposition (contains sub-surface crevices).

To investigate the origin of the sub-surface crevices a solution consisting of H 2 O 2 and NaCl was
made to simulate the H 2 O 2 plus CeCl 3 solution used to deposit CeCCs . Without the presence of
Ce in the solution there is no film de posited but the active etching species, Cl - and H 2 O 2 , are still present. As seen in Figure 32 below, the sub- surface crevices were observed after spraying the salt plus peroxide solution on the surface of a cleaned Al 2024-T3 panel. Therefore, the
concentration of the chloride a nd hydrogen peroxide in the CeCC spray solution is sufficient to preferentially etch the Al 2024-T3 substrate. Studies were conducted that demonstrated the reducing the amount of Cl - and H 2 O 2 in the deposition solution reduced, or eliminated, the formation of sub-surface crevices. However, samples without sub-surface crevices did not perform well during corrosion testing.

34Figure 32. Cross sectional micrograph image of an Al 2024-T3 panel after spraying a H 2 O 2 + NaCl solution on the surface.

Post Salt Spray Surface Analysis

Characterization of CeCC panels before and af ter salt spray testing wa s done throughout the project. Panels that passed the 14 day visual inspection criteria after salt spray testing were analyzed to see if differences in material co mposition or structure coul d be measured. Cross sectional transmission electron microscopy (TEM) of the phosphate sealed CeCC coating before salt spray exposure indicated that the film was a dense, con tinuous coating that was a few hundred nanometers in thickness (F igure 33a). After 14 days in the salt spray cabinet the CeCC was still present but an additional layer was present on the surface of the CeCC (Figure 33b). Additional analysis of the panels using depth profiling revealed that the CeCC before salt spray testing was a mixture of Ce, O, and P (Figure 34 a) but after salt spray exposure Al and O were present on top of the CeCC (Figure 34b). The pr esence of an oxide layer on top of the CeCC after salt spray testing was detected on multiple samples throughout the project. Collectively, the
TEM and AES results show that an aluminum ox ide layer was present on the surface of the CeCC after 14 days of salt spray exposure on panels that passed 336 hours of testing.

a)

35b) Figure 33. Cross-sectional TEM images of cerium ba sed conversion coatings: a) as-deposited, b) after 14 days of salt spray testing.

0 100 200 300 400 0 25 50 75 100ATOMIC%

SPUTTER DEPTH (nm) O Al

Ce

P

0 200 400 600 800 0 25 50 75 100

Ce O AlATOMIC%

SPUTTER DEPTH (nm) P

a) b)

Figure 34. Auger electron spectrosc opy depth profiles of cerium ba sed conversion coatings: a) as-deposited and b) after 14 da ys of salt spray testing.

5.3 Multifunctional UV (MUV) Curable Coatings

Development and evaluation of MUV coatings wa s done throughout the project. A total of 5 rounds of experimental testing we re conducted. Within the first fe w months of the project it was decided that each round of testing would use co mmercially available chromate and non-chromate coatings as controls. This was done in order to compare results from military approved coatings to the ones being developed and te sted in this project. A brief overview of the 5 rounds (rds) of testing is presented in Table III. The detailed results from the testing are available from the project Principal Investigator.

36Table III - Summary of MUV Testing

• Completed 5 Rounds of MUV Test and Evaluation on A1 2024 and
A1 7075 Substrates

3⁄4 Chromate & Non-Chromate Controls As Reference (CrCC/TCP, Epoxy Primer, Polyurethane Top Coat)
3⁄4 MUV on CrCC, CeCC, TCP and Bare A1 2024-T3
3⁄4 B117 Salt Spray, SO 2 Exposure, Filiform, Cross Hatch Adhesion, Skydrol Resistance, GE Impact, Low Temp Mandrel, Accelerated UV Weathering

• Rd 1 (FY06) – Drawbar MUV, 2 Corrosion Inhibitors Evaluated

• Rd 2 (FY07 – Drawbar MUV, 3 Co rrosion Inhibitors Evaluated

• Rd 3 (FY08) – Spray MUV, Wayne Pigment Inhibitor

• Rd 4 (FY08/09) – Spray MUV, Wayne, Different Formulations

• Also Evaluated Gloss White

• Rd 5 (FY09) – Spray MUV, Wayne, Flexibility Adjustments

Round 1 MUV Evaluations

Round 1 testing was limited to salt spray testing on ly of MUV coatings with Hybricor 204 or
Deft corrosion inhibitor on chromate and ceriu m coated Al 2024-T3 or Al 7075-T6 panels. Commercial chromate and non-chromate coating were used as controls. Th e controls were spray applied at Boeing and the MUV coatings were draw bar applied at LCC. The MUV coatings with Hybricor 204 inhibitor performed as well as or better than the controls in terms of preventing salting in the scribe after 3000 hour s of testing. Shown in Figure 35 are some of the panels after 3000 hours testing, with the Hybricor 204 MUV coated panels on CrCC and CeCC having no visible salting in the scribe. This result was very significant as it demonstrated that a UV curable
formulation loaded with corrosion inhibitors has the capability of protecting against corrosion of scribed high strength aluminum panels on ch romate and non-chromate conversion coatings.

37Chromate Control Non-Chromate Control

Deft MUV on CrCC Deft MUV on CeCC

Hybricor 204 MUV on CrCC Hybricor 204 MUV on CeCC

Figure 35. Optical images of Stage 1 Al 2024-T3 coated panels after 3000 hours of neutral salt spray testing.

38The MUV coatings were stripped off some of the panels following 3000 hours of salt spray testing to determine if corrosi on under the coating had occurre d. As shown in Figure 36 the
surface and scribe of the Al had no visible corr osion or staining for the Hybricor 204 panels while the Deft MUV and control panels each ha d corrosion by-product in and around the scribe.

Chromate Control Non-Chromate Control

Deft MUV on CrCC Deft MUV on CeCC

Hybricor 204 MUV on CrCC Hybricor 204 MUV on CeCC

Figure 36. Optical images of Stage 1 Al 2024-T3 coated panels after 3000 hours of neutral salt spray testing and removal of the corrosion coatings.

39Characterization of the MUV st ripped Hybricor 204 panels us ing mapping Auger spectroscopy was done to determine what, if any, differences be tween the panels could be detected. Panels which were coated, scribed, and th en stripped, but not subjected to salt spray testing, were used as control panels for comparison of “before” and “after” salt spray testi ng. Shown in Figure 37 is Auger maps with the percentage of Al, Cr, and Zn measured near the scribe for Hybricor panels on a CrCC before and after salt spray testing. Zinc is one of the components of the Hybricor 204 compound. The scribe runs vertica lly down the middle in all cases; it is clearly demarked for the Cr and Ce signal maps with no salt spray exposure as the blue strip. The data indicate that corrosion protection of the Al substrate and scribe is related to the corrosion inhibitor from the MUV, in this case indicated by the presence of Zn on the panel. There was no Cr found in the scribe and Zn was not found in the scribe or on the surface before testing. After 3000 hours of salt spray testing, Zn was detected in the scribe and on the surface of the Al substrate, a strong indication th at the corrosion inhibitor in M UV migrated and protected the panel. Similar results were found for Hybric or 204 panels that pa ssed 3000 hours of ASTM B117 testing on CeCC (Figure 38), additional sup port that the MUV was actively protecting the Al against corrosion.

40Cr Signal: No salt spray Cr Signal: After Passing 3000 hours 20% 21% 0.5%
19% 20% 0.1%

Al Signal: No salt spray Al Signal: After Passing 3000 hours 4% 2% 28%

6% 3% 40%

Zn Signal: No salt spray Zn Signal: After Passing 3000 hours <1% <1% <1% 3% 5% 3% 2% 2%

Figure 37. Auger maps for Cr, Al, and Zn near th e scribe for two different Hybricor 204 MUV coated panels; one not exposed to salt spra y and the other after passing 3000 hours salt spray.

41Ce Signal: No salt spray Ce Signal: After Passing 3000 hours 34% 37% 3% 45% 35% 30% 2% 22%

Al Signal: No salt spray Al Signal: After Passing 3000 hours 8% 36% 4% 3% 2% 29%

Zn Signal: No salt spray Zn Signal: After Passing 3000 hours <1% <1% <1% 2% 1% 3% 2% 4%

Figure 38. Auger maps for Ce, Al, and Zn near the scribe for two diffe rent Hybricor 204 MUV coated panels; one not exposed to salt spra y and the other after passing 3000 hours salt spray.

42Round 2 MUV Evaluations

Based on the promising results of Round 1 testing, a more complete set of tests was conducted on MUV coatings during Round 2, including filiform, adhesion, fuel resistance, SO 2 spray, and Xe arc weathering in addition to the neutral salt spray testing. Multifunctional UV coatings were applied to CrCC, CeCC, tri-valent chromium (T CP), and bare Al 2024-T3 panels, and chromate and non-chromate panels were again used as controls. Hybricor 204, Deft, HT-13, and HT-20 corrosion inhibitors, as well as different monomers, were evalua ted. The experimental matrix was exploratory in nature such that the test results were expected to vary widely. This approach was taken to help identify and optimize the ke y performance variables in the MUV coating system. In general, the MUV coatings w ith Hybricor 204 corrosi on inhibitor and IBOA monomer on CrCC performed better than other MUV inhibitor, monomer, and conversion coating systems, but not as well as the chroma te controls. In most cases the MUV coating systems with Hybricor 204 inhibitor on CrCC we re close to passing a ll of the performance requirements. The results were considered enco uraging but not definiti ve. Shown in Figure 39
are the chromate and non-chromate Al 2024-T3 cont rol panels after salt spray testing along with Hybricor 204 loaded MUV coatings on CrCC and CeCC Al 2024-T3 panels.

Chromate Control Non-Chromate Control

Hybricor 204 MUV on CrCC Hybricor 204 MUV on CeCC

Figure 39. Optical images of Round 2 Al 2024-T3 co ated panels after 2000 hours of neutral salt spray testing.

43In response to a SERDP action item, results fr om salt spray testing of MUV coatings with Hybricor 204 on TCP and bare Al 2024-T3 ar e presented in Figures 40 and 41. The TCP conversion coatings with a Hybricor 204 loaded MUV (Figure 40) looked similar to the CeCC results with a little salting in th e scribes but the MUV in direct co ntact with the Al substrate had numerous blisters and significant sa lting in the scribe (Figure 41).

Figure 40. Optical image of a Round 2 Hybricor 2 04 MUV coated Al 2024-T3 panel with a TCP conversion coating after 2000 hours salt spray.

Figure 41. Optical image of a Round 2 Hybricor 2 04 MUV coated Al 2024-T3 panel that did not

ave a conversion coating (bar e) after 2000 hours salt spray. h

44Filiform corrosion testing was conducted on Alclad 2024- T3 substrates (i.e. Al roll bonded to Al 2024-T3) out to 1000 hours of testin g. Figure 42 depicts the contro l, HT-13 MUV, and Hybricor 204 MUV panels after filiform testing. Chrome and chrome-free controls passed filiform testing as did the Hybricor 204 on CrCC as all corrosion was less than 1/ 4” from the scribe and most were less than 1/8” away from the scribe. Th e corrosion away from th e scribe for the HT-13 MUV panels after filiform testing was just outside the test specifications, as was also the case for the CeCC panels with MUV coatings. These results were considered promising for a first
attempt.

Chromate control Non-chromate control

MUV with HT-13 Inhibitor MUV with Hybricor 204

Figure 42. Images of integrated coatings on CrCC Al clad Al 2024-T3 after 1000 hours of filiform corrosion testing.

45L adder Studies between Round 2 and Round 3

y

e

r Al

f

charge level was chosen for bsequent work to develop a MUV spray deposition process. Multifunctional UV curable coatings applied by drawbar (draw-dow ns) resulted in Hybricor 204 corrosion inhibitor from Wayne Pigments demons trating the best perfor mance during salt spra testing of scribed Al 2024-T3 pa nels with chromate conversion coatings (CrCC). The initial work used a ~36 wt% charge level of the Hybric or 204, and ladder studies indicated that charg levels near 30% were also effective at prevent salting of scribe lines during salt spray testing.
Additional ladder studies were then conducted at charge levels of 36.4%, 34.6%, 32.8%, 30.9%, 29.1%, and 27.3% to determine the optimum value, as lowering the charge level improves othe properties such as flexibility. All of the MUV coatings in th e study were applied to CrCC 2024 substrates using a drawbar. Other than sa mples that had adhesion failures related to substrate preparation, each of th e charge levels had a panel wi th 1,A ratings after 3000 hours o ASTM B117 testing. Images of the panels af ter 3000 hours of salt spray testing are shown in Figure 43. Taking into account a number of fact ors, the 29.1%

su

36.4% Charge Level 34.6% Char ge Level 32.8% Charge Level

30.9% Charge Level 29.1% Charge Level 27.3% Charge Level

rent charge levels of ybricor 204 corrosion inhibitor af ter 3000 hours of salt spray testing. Figure 43. Optical images of MUV coated Al 2024-T3 panels with diffe

H

46R ound 3 MUV Evaluations

onding

e

nd an

uble rubs, and good appearance with 20 ° gloss = 0.5, 60 ° gloss = 4.3, and

5 ° gloss = 7.6.

UV

repare d. A listing of the panels tested and coatings sed in the study are presented in Table IV.

Table IV - Coating Systems Evaluated (Round 3) After determining the amount of Hybricor 204 corrosion inhibitor to use in the MUV, efforts were then focused on developing a MUV fo rmulation that could be spray applied. It was found that the gloss values for sprayed pane ls were higher than those for corresp draw-downs, so formulation adjustments we re made to lower gloss and to improve sprayability, flow, and appearance. A study of additional dispersing aids for the Hybricor
204 corrosion inhibitor indicated that lower visc osity and better coating sprayability could be obtained with Solsperse hyperdispersants. Th e best results were obtained with Solspers 39000, which was incorporated into the formulat ion. Adjustments were also made to the photoinitiator blend, monomer blend, oligomer, and level of filler use d. A different black pigment was incorporated along with an improved package for stable w eatherability a additive for improved flow. The modifi ed MUV coating on CrCC aluminum panels exhibited 5B adhesion (100%), reverse impact st rength flexibility of 14 inch-lbs, resistance

to >200 MEK do

8

After adjustments to the MUV formulation had b een made, coatings were applied to Al 2024-T3 and Al 7075-T6 substrates with CrCC, CeCC, an d TCP using the modified low gloss gray M coating sprayed onto the panels. Chromate a nd non-chromate coated control panels using commercially available materials were also p

u

Panel ID Pretreatment Coating system

A
MIL-C-5541/PS13209 and Thin

P
dichrom ibility panels only) MUV (Hybricor 204) Chromate conversion coating,

film sulfuric acid anodized per

S 13201 TY II with 2 minute ate seal (flex

B CeCC MUV (Hybricor 204)

C TCP PPG CA8211/F36375/CA8200B top Deft 65GN015 primer,

coat(chrome free control)

D
MIL-C-5541/PS13209 and Thin

P
dichrom ibility panels only) CA8211/F36375/CA8200B (chromated control) Chromate conversion coating,

film sulfuric acid anodized per

S 13201 TY II with 2 minute ate seal (flex Deft 44GN072 primter, PPG

47Chromate conversion coated panels had good a ppearance but the appearance of the coa
on cerium conversion coa ting ted panels was not as good, with some wrinkl ing at higher coating icknesses (Figure 44). App earance was better on Alclad panels which had a lighter
erium conversion coat.

in

also

0

l loss of less than 5 at 60° and a delta E of less than 1. The MUV coatings had gloss values of ~2, meeting the spec ification, but the delta E values were close to 5, well above the requirement. th

c

Cerium conversion coating Cerium conversion coating Chromate conversion coating

Figure 44. Optical images of MUV coated Al 20 24-T3 panels on CeCC and CrCC pretreatments.

After the panels were prepared, evaluation was conducted at Boeing. The wet tape adhesion tests indicated that the MUV and chromate primer on Cr CC passed with images of the panels shown Figure 45. Exposure of the panels to Skydrol fluid for thirty days and measurement of the pencil hardness resulted in only the chromate control pane ls passing the test. Al l of the other coating combinations failed, with complete loss of adhe sion occurring for MUV spray coatings on CeCC. Exposure of panels to xenon-arc radiation for 1000 hours as an accelerated w eathering test was conducted. Measurement of the change s in optical appearance and co lor were record ed at the 50
and 1000 hour mark. The results i ndicated that the chromate a nd non-chromate control panels passed the test, with a spectra

48MUV on CrCC MUV on CeCC

Non-chrome primer on TCP Chromate primer on CrCC

Figure 45. Optical images of wet scribe test results.

Filiform corrosion for 1000 hours on Alclad 20 24-T3 and Alclad 7075-T6 substrates was performed. The chromate control panels fo r both 2024 and 7075 passed the test while the nonchromate controls passed on 2024 but failed on 7075, with the MUV coatings on CrCC and CeCC failing on both substrates.

Neutral salt spray testing of control and M UV coated CrCC, CeCC, a nd TCP pretreatments on Al 2024-T3 and Al 7075-T6 panels was done out to 3000 hours with ratings assessed every 1000
hours. The chromate and non-chromate controls performed the best, with 1,A or 2,A ratings, with the MUV on CrCC (2,4,A) ratings next best . The MUV coatings on TCP and CeCC had the lowest rankings (2,4,9,H typical). Pictures of the panels after 3000 hour s of ASTM B117 testing are presented in Figures 46-48.

49Chromate Primer on CrCC

Non-chromate Primer on TCP

Figure 46. Optical images of Al 2024-T3 contro l panels after 3000 hours of salt spray corrosion testing.

50MUV on CrCC

MUV on CeCC

Figure 47. Optical images of MUV coated Al 2024-T3 panels with CrCC and CeCC pretreatments after 3000 hours of sa lt spray corrosion testing.

51MUV on TCP

Figure 48. Optical images of MUV coated Al 20 24-T3 panels with TCP pretreatments after 3000 hours of salt spray corrosion testing.

52Overall the performance of th e coatings was not as good on Al 7075-T6 panels as the Al 2024- T3 panels, with no coating systems rated 1,A after 3000 hours. The chromate and non-chromate control ratings were ~2,4,A as were the M UV on CrCC panels. The MUV on CeCC had a rating of ~3,5,9,H. The MUV on CrCC was given the #1 rank after 3000 hours. Figures 49 and 50 are images of the panels after salt spray testing.

Chromate Primer on CrCC

Non-chromate Primer on TCP Figure 49. Optical images of Al 7075-T6 contro l panels after 3000 hours of salt spray corrosion testing.

53MUV on CrCC

MUV on CeCC

Figure 50. Optical images of MUV coated Al 7075-T6 panels with CrCC and CeCC pretreatments after 3000 hours of sa lt spray corrosion testing.

54MUV Coated CeCC Studies

The results from the Round 3 testing clearly indicated that improvement in the salt spray performance of the MUV on CeCC was needed. A test matrix of CeCCs deposited under different conditions that were then sprayed with the same formulation of MUV was carried out. Salt spray testing was done out to 2000 hours. The results were encouraging as a few panels had 1,A ratings while most of the panels had 1 rating s in combination with blisters (rating 8). The
heat treated and fluoroboric acid cleaned Al 2024-T3 samples did we ll as did the fluoroboric acid cleaned Al 7075-T6 substrates. Blistering unde r the MUV coating was a common flaw for all
conditions.

In October 2008 all of the team members met at the Light Curable Coatings (LCC) facility in Berea, OH to process and coat panels (Table V) . This was done in order to investigate how depositing the CeCC and then MUV coating within minutes of each operation affected coating performance. In addition to the CeCC samples made at LCC, CeCC panels were also made in Rolla the day before the trip so that a comparis on could be made. All samples were MUV coated
and cured at LCC. The CeCCs made at LCC were blow dried with air for 5 minutes before the MUV coating was spray deposited. Among the CeCC variables investigated were the number of spray cycles, post treatment sealing temperature, and cleaning/surface preparation process. In some cases the MUV was sprayed directly onto the cleaned panel withou t a conversion coating.

Table V - Test Matrix for MUV/ CeCC Panels Done at LCC Oct 2008

Condition Allo y Surface Pre p . Coatin g Post Treatment Notes Panel Numbers

1 2024 standard 3 s p ra y s 55 C 1- ( 1-3 )

1R 2024 standard 3 s p ra y s 55 C s p ra y ed at MST 1R- ( 1-6 )

2 2024 standard 3 s p ra y s 85 C 2- ( 1-6 )

2R 2024 standard 3 s p ra y s 85 C 2R- ( 1-6 )

2A 2024 standard 3 s p ra y s 85 C p H 2 A

- ( 1-3 )

2B 2024 standard 3 s p ra y s 85 C dr y overni g ht 2B- ( 1-6 )

3 2024 standard 1 s p ra y 85 C 3- ( 1-6 )

3A 2024 standard 1 s p ra y 85 C p H 3 A

- ( 1-3 )

3B 2024 standard 1 s p ra y 85 C dr y overni g ht 3B- ( 1-6 )

4 2024 standard 5 s p ra y s 85 C 4- ( 1-6 )

4A 2024 standard 5 s p ra y s 85 C p H 4 A

- ( 1-3 )

4B 2024 standard 5 s p ra y s 85 C dr y overni g ht 4B- ( 1-3 )

5 2024 turco onl y no Ce coatin g N/A 5- ( 1-6 )

5R 2024 turco onl y no Ce coatin g N/A s p ra y ed at MST 5R- ( 1-6 )

6 2024 turco and aci d no Ce coatin g N/A 6- ( 1-6 )

6R 2024 turco and aci d no Ce coatin g N/A s p ra y ed at MST 6R- ( 1-6 )

7 2024 standard b rush ( 1 stroke ) 85 C 7- ( 1-6 )

7R 2024 standard b rush ( 1 stroke ) 85 C b rushed at MST 7R- ( 1-6 )

8 2024 standard wi p e ( 1 stroke ) 85 C 8- ( 1-6 )

8R 2024 standard wi p e ( 1 stroke ) 85 C b rushed at MST 8R- ( 1-6 )

9 2024 standard 5 s p ra y s 85 C UV treatment before MUV a pp lication 9- ( 1-6 )

9A 2024 standard 5 s p ra y s not sealed UV treatment before MUV a pp lication 9A- ( 1-6 )

10 2024 Cu seedin g 3 s p ra y s 85 C 10- ( 1-6 )

10R 2024 Cu seedin g 3 s p ra y s 85 C s p ra y ed at MST 10R- ( 1-6 )

11 7075 Cu seedin g 3 s p ra y s 85 C 11- ( 1-6 )

11R 7075 Cu seedin g 3 s p ra y s 85 C s p ra y ed at MST 11R- ( 1-6 )

12 2024 Luna no Ce coatin g 85 C 12- ( 1-5 )

12R 2024 Luna no Ce coatin g 85 C p re p ared at MST 1x3 12R- ( 1-6 )

13 7075 standard 3 s p ra y s 85 C 13- ( 1-6 )

13 R 7075 standard 3 s p ra y s 85 C s p ra y ed at MST 13R- ( 1-6 )

55Presented in Table VI are some of the results from evaluation of the MUV/CeCC panels done at
LCC. Most of the test conditions on Al 2024-T3 substrates resulted in adequate pencil hardness values (>H), reasonable adhesion ra tings (>4B), 60° gloss values <5, greater than 200 MEK rubs without coating removal, and maintaining integr ity after 24 hour Skydrol fluid exposure. The panels that did not do as well we re related to using a lower te mperature (55°C) post treatment step, a thicker CeCC layer (5 spray cycles), and brush or wipe applicati on of the CeCC. Data from MUV on bare Al 2024-T3 substrates (no conversion coating) were comparable to the results from the panels with a CeCC. The re sults on Al 7075-T6 panels (condition 13) were poor
in almost all cases.

Table VI - Pencil Hardness, Adhesion , Gloss, MEK Rub and Skydrol Results
for MUV/CeCC Panels Done at LCC Oct 2008

PENCIL GLOSS 24 HR

CONDITION/PANEL HARDNESS ADHESION 20 0 60 0

85 0 MEK RUBS SKYDROL

CONDITION 1-3 F 0B 0.6 5.0 9.6 60 RUBS FAILED

CONDITION 1R-5 4H 3B 0.5 3.6 7.9 >200 RUBS PASSED

CONDITION 2-6 4H 3B 0.5 3.8 7.8 >200 RUBS PASSED

CONDITION 2R-1 3H 4B 0.7 5.4 11.3 >200 RUBS PASSED

CONDITION 2A-1 2H 3B 0.5 3.7 7.9 >200 RUBS PASSED

CONDITION 2B-4 F 3B 0.6 4.6 9.5 >200 RUBS PASSED

CONDITION 3-1 4H 5B 0.6 5.1 12.7 >200 RUBS PASSED

CONDITION 3A-3 4H 4B 0.5 3.8 8.7 >200 RUBS PASSED

CONDITION 3B-2 4H 4B 0.5 4.5 8.8 >200 RUBS PASSED

CONDITION 4-3 F 1B 0.5 3.6 8.9 >200 RUBS PASSED

CONDITION 4A-3 H 2B 0.5 3.6 7.6 >200 RUBS PASSED

CONDITION 4B-3 2H 2B 0.5 4.5 9.7 >200 RUBS PASSED

CONDITION 5-1 3H 3B 0.6 4.6 9.4 >200 RUBS PASSED

CONDITION 5R-1 4H 4B 0.5 4.3 8.0 >200 RUBS PASSED

CONDITION 6-1 4H 5B 0.5 4.1 8.2 >200 RUBS PASSED

CONDITION 6R-1 4H 5B 0.5 4.4 9.5 >200 RUBS PASSED

CONDITION 7-1 4H 2B 0.5 4.4 11.3 195 RUBS PASSED

CONDITION 7R-1 4H 5B 0.5 4.1 9.2 >200 RUBS PASSED

CONDITION 8-1 4H 2B 0.5 4.4 8.4 >200 RUBS PASSED

CONDITION 8R-1 3H 1B 0.7 5.6 11.6 190 RUBS PASSED

CONDITION 9-3 4H 5B 0.6 4.4 8.2 >200 RUBS PASSED

CONDITION 9A-2 2H 4B 0.4 3.6 9.3 >200 RUBS PASSED

CONDITION 10-1 4H 5B 0.6 4.5 8.2 >200 RUBS PASSED

CONDITION 10R-1 4H 5B 0.6 5.2 10.8 >200 RUBS PASSED

CONDITION 11-3 4H 3B 0.6 4.6 11.3 >200 RUBS PASSED

CONDITION 11R 4H 5B 0.6 5.0 10.3 >200 RUBS PASSED

CONDITION 12-3 4H 4B 0.4 3.3 7.6 >200 RUBS PASSED

CONDITION 12R-1 F 4B 0.6 4.8 10.1 145 RUBS FAILED

CONDITION 13-4 2H 0B 0.6 4.6 11.8 175 RUBS FAILED

CONDITION 13R-5 2H 0B 0.8 6.3 12.7 98 RUBS FAILED

Pencil Hardness Scale: 9B<8B<7B<6B<5B<4 B<3B<2B<B<HB<F<H<2H<3H<4H<5H<6H<7H<8H<9H Cross-hatch adhesion percentages: 0B (0% to 35%), 1B (35% to 65%), 2B (65% to 85%), 3B (85% to 95%), 4B (over 95%), 5B (100%)

56Salt spray testing of the MUV/CeCC panels made at LCC was done along with control panels of MUV on CrCC (labeled QCCC) and MUV dir ectly on bare Al-2024 T3 substrates. The ratings for the panels after 500 and 1000 hours ar e listed in Table VII. Although the scribe lines were still shiny for many of the test pane ls after 1000 hours of salt spray testing, most of the cerium conversion coated panels contained blisters, and the only conditions that had any 1A ratings after 1000 hours of testing were conditions with no CeCC and one that had UV treatment prior to MUV deposition, and the control MUV on QCCC panels.

Table VII - Salt Spray Ratings for MUV/CeCC Panels Done at LCC Oct 2008

57Effect Of CeCC Variables On Corrosion Resistance Of Sprayed MUV Films For MS&T 10/2008 Test Matrix

CONDITION/PANEL Left Top Right Bottom 500 Hrs 1000 Hrs

CONDITION 1-1 1.90 2.00 1.90 2.00 1,14,B 3,6,8,14,H

CONDITION 1-2 2.10 1.70 2.00 2.20 1,4,8,14,G 3,6,8,14,H
CONDITION 1R-1 1.15 1.50 1.45 1.50 1,A 1,4,8,E

CONDITION 1R-2 2.20 2. 00 2.30 2.20 1,1,8,B 1,4,8,E

CONDITION 2-4 2.60 2.20 2.10 3.00 1,8,13,B 1,8,13,H

CONDITION 2-5 2.60 2.40 2.20 2.60 1,8,13,B 1,8,13,E
CONDITION 2R-5 1.70 2.10 1.15 2.20 1,8,B 1,8,G
CONDITION 2R-6 1.80 2.30 2.00 1.90 1,A 1,8,B
CONDITION 2A-2 2.10 2.20 2.10 1.80 1,8,13,B 1,8,13,D
CONDITION 2A-3 2.30 2.60 1.90 1.95 1,8,14,B 1,4,8,14,H
CONDITION 2B-1 2.40 1.55 2.40 1.85 1,4,8,13,B 1,4,8,13,H

CONDITION 2B-3 1.95 2. 00 2.40 1.85 1,8,13,B 1,8,13,G

CONDITION 3-2 1.70 1.65 1.95 1.70 1,8,B 1,8,F

CONDITION 3-3 1.75 2.10 2.10 2.30 1,8,B 1,8,E
CONDITION 3A-1 2.10 1.70 1.40 2.00 1,8,B 2,5,8,B

CONDITION 3A-2 1.60 1. 70 1.75 1.70 1,4,8,C 2,4,8,H

CONDITION 3B-1 2.60 2. 50 2.15 2.20 1,4,8,C 1,4,8,13,D

CONDITION 3B-3 1.70 1.85 2.30 1.90 1,8,B 1,4,8,G

CONDITION 4-1 1.75 2.20 1.95 2.30 1,A 2,4,8,D

CONDITION 4-4 1.75 1.75 1.65 1.75 1,13,B 1,13,B
CONDITION 4A-1 1.50 1.80 1.35 1.45 2,4,8,14,B 3,6,8,14,E
CONDITION 4A-2 1.45 1.80 1.60 1.35 2,4,8,13,B 2,4,8,14,B

CONDITION 4B-1 2.40 2. 60 2.50 3.00 1,4,8,14,C 1,4,8,14,H
CONDITION 4B-2 2.20 2.20 1.75 1.95 1,4,8,14,B 2,4,8,14,G

CONDITION 5-3 2.60 2.50 2.10 2.60 1,8,B 1,8,G

CONDITION 5-4 2.30 2.10 2.40 2.80 1,4,8,10,B 1,4,8,10,E
CONDITION 5R-5 1.80 2.40 1.75 1.60 1,8,B 1,8,C
CONDITION 5R-6 1.80 2.10 1.55 1.65 1,8,B 1,8,10,C

CONDITION 6-3 1.80 2.20 2.10 1.85 1,A 1,A

CONDITION 6-4 2.40 2.90 3.00 2.50 1,A 1,8,B
CONDITION 6R-2 1.85 2.10 2.20 2.20 1,A 1,A
CONDITION 6R-4 2.20 2.30 2.10 2.40 1,8,B 1,8,C

CONDITION 7-3 1.60 1.60 1.90 1.70 1,8,B 2,4,8,G

CONDITION 7-4 1.50 1.70 2.10 2.20 1,8,B 2,4,8,H
CONDITION 7R-3 1.65 1.55 1.60 1.75 1,8,C 1,8,C

CONDITION 7R-4 1.80 2.00 2.10 1.85 1,8,C 1,8,G

CONDITION 8-3 1.85 1.90 2.50 2.30 1,8,B 1,8,G

CONDITION 8-4 1.55 1.50 1.30 1.35 1,8,B 1,8,G

CONDITION 8R-2 1.90 1. 70 1.80 1.60 1,4,8,D 1,4,8,F
CONDITION 8R-3 1.80 2.20 1.60 1.85 1,8,B 1,8,C

CONDITION 9-1 1.65 2.30 2.10 1.85 1,8,B 1,13,B

CONDITION 9-2 1.95 2.10 2.30 2.10 1,8,13,B 1,8,13,F
CONDITION 9A-1 1.75 1.70 1.45 1.20 1,A 1,A
CONDITION 9A-3 2.30 2.30 2.10 2.20 1,8,13,C 1,8,13,F
CONDITION 10-3 2.10 1.75 2.20 1.95 1,8,B 1,8,C
CONDITION 10-4 2.30 2.10 2.20 3.00 1,13,B 1,8,14,H
CONDITION 10R-3 1.70 2.10 2.10 2.40 1,8,B 1,8,C

CONDITION 10R-4 1.65 1.85 1.55 1.30 1,8,B 1,8,C

CONDITION 11-1 2.60 2.80 2.20 2.50 1,4,8,G 2,4,8,H

CONDITION 11-2 2.60 2.90 2.60 3.00 1,4,8,H 2,4,8,10,13,H

CONDITION 11R-2 1.85 2.00 1.60 1.70 1,4,A 2,4,8,C

CONDITION 11R-3 2.30 1.55 1.60 1.65 1,4,A 2,4,8,B

CONDITION 12-4 1.75 1.40 1.40 2.20 1,4,8,C 2,4,8,G

CONDITION 12-5 1.45 1.95 1.65 2.20 1,4,8,10,C 1,4,8,10,F

CONDITION 13-5 2.10 1.70 1.60 1.55 2,4,8,13,C 2,4,8,14,D

CONDITION 13-6 1.80 1.90 1.50 2.00 2,4,8,13,C 2,4,8,14,C

CONDITION 13R-2 2.20 1.95 1.90 2.10 2,4,8,B 2,4,8,C

CONDITION 13R-3 2.40 2.30 1.85 2.40 2,4,8,C 2,4,8,D

CONTROL QCCC-1 1.95 2.00 1.65 2.20 1,A 1,A
CONTROL QCCC-2 1.85 2.20 1.90 2.10 1,A 1,A

CONTROL 2024 BARE-1 2.20 2.00 2.10 2.20 1,8,B 1,8,E

CONTROL 2024 BARE-2 1.75 2.20 1.75 1.90 1,8,B 1,8,E Initial Film Build (mils) Corrosion Inspection & Scribe Line Ratings

Round 4 MUV Evaluations

For the panels made and evaluated during Round 4 the corrosion inhibitor level was the same as Round 3 with changes to the formulation made to adjust the mechanical and physical properties of the MUV coating. Results indicated that AS TM B117 neutral salt sp ray test ratings were similar for MUV coatings on CrCC, chromate c ontrols, and non-chromate controls on Al 2024 and Al 7075 substrates (Figures 51 and 52). Ratings for MUV on TCP on 2024 were ~ 2,A while MUV on cerium conversion coatings (CeCC) were ~3,E with many blisters.

58Figure 51. Al 2024-T3 chromate control pa nels after 3000 hours ASTM B117 testing.

Figure 52. Al 2024-T3 panels with MUV on CrCC after 3000 hours ASTM B117 testing.

The MUVs were able to pass 200 MEK rub testing but coatings were unable to retain their integrity after a 24 hour Skydrol so ak. The chromate and non-chromate controls passed wet tape adhesion and room temperature flexibility te sts but the MUV coatings on CrCC and CeCC did not. Only the chromate controls passed low temperature flexibility test s. Xenon arc weathering of the chromate and non-chromate controls were within specifications but the change in the MUV coatings optical appearance were outside of the acceptable limits.

Chromate control and MUV coa tings on CrCC passed filiform corrosion testing on Al 2024 and Al 7075 substrates. MUV coatings on CeCC pa ssed on Al 7075 while non-chromate controls

59passed on Al 2024. The corrosion performance of MUV coatings was excellent but adjustments to the MUV formulations did not improve th e non-corrosion propertie s of the coatings.

Round 5 MUV Evaluations

Changes in MUV formulation were made to try to improve the physical properties of the MUV
films from Round 4 while still maintaining corrosion inhibition. After 2000 hours of ASTM B117 neutral salt spray testing, the MUV coatin gs on chromate conversion coated (CrCC) Al 2024 and Al 7075 substrates had comparable ratings (1, 4, A) with the chromate controls in testing done at Boeing and Light Cu rable Coatings (Figure 53). Small blisters away from the scribe were present on the MUV coated panels . The non-chromate controls and MUV on TCP also had salting in the scribe after 2000 hours of ASTM B117 testing while the MUV panels on cerium conversion coated (CeCC) s ubstrates had wide spread bliste r formation and salting in the scribe (Figure 54).

a) b)

Figure 53. Optical images of Al 2024-T3 pane ls after 2000 hours of ASTM B117 salt spray testing: a) chromate standard and b) MUV on CrCC.

60a) b) c)

Figure 54. Optical images of Al 2024-T3 pane ls after 2000 hours of ASTM B117 salt spray testing: a) non-chromate standard, b) MUV on TCP, and c) MUV on CeCC.

MUV coatings with CrCC on Alclad 2024 and Al clad 7075 substrates passed filiform corrosion testing. MUV coatings with CeCC on Alclad 2024 passed filiform corrosion while MUV with CeCC on Alclad 7075 failed filiform testing. The chromate controls passed on Alclad 2024 and
7075 but the non-chromate controls failed on Al clad 2024 and 7075 substrates. Accelerated Xenon-arc weathering after 500 hour s indicated that the 60 degr ee gloss reading was < 2 on the MUV coatings on CeCC and CrCC and 3 to 4 on the top coated chromate and non-chromate controls (maximum allowed value of 5). The de lta E values for the controls was <1, within specifications, while the MUV coatings had a delta E of ~5 due to colo r darkening associated with the carbon pigment.

Initial pencil hardness of the control and MUV pa nels was 2H. The chromate and non-chromate control samples passed Skydrol resistance testing (3H after 30 days) while the MUV coatings on CrCC and CeCC failed (less than 6B after 30 days). All 3 of the chromate control and 2 of the 3 MUV/CrCC samples passed the low temperature flex ibility tests (Figure 55). Room temperature flexibility test results indicated that the chromate and non-chromate controls had 20% elongation before cracking was observed at 10X magni fication, 2 of the 3 MUV on CrCC had 10% elongation before cracking, and the MUV on CeCC ha d cracks visible at only a few % elongation (Figure 56). Investigatio ns into the source and nature of the blisters that were observed after salt spray testing of MUV coatings on CeCC were conduc ted. Results indicate that blisters did not form after exposure to 40°C ambient or 85% relative humidity at 40°C for up to two weeks. Blisters only formed during submersion of the panels into DI water, but the blisters decreased in size, or were not observable, afte r the panels were left out in ai r for a day. Panels with fewer blisters performed better in salt spray testing than panels with many blisters, and the adhesion strength of the MUV was higher on pane ls with fewer blisters as well.

61Chromate controls

MUV on CrCC

Figure 55. Optical images of Al 2024-T0 panels after low temperature flexibility tests.

a) b)

Figure 56. Optical images of Al 2024-T0 panels af ter room temperature flexibility tests: a) chromate controls and b) MUV on CrCC.

62Summary of MUV Evaluations

Table VIII presents an overall summary of the MUV results from the project. The tests that were consistently passed by the MUV coa tings during the project are highli ghted in blue (Note: the Al 7075 substrates were not used for many of the te sts because the specification calls for Al 2024.) Adjustments to MUV formulation throughout the proj ect resulted in the ab ility of the MUV to pass many of the other tests, including wet tape adhesion and low temperature flexibility. Room temperature flexibility and acceler ated weathering values were cl ose to required specifications and did pass requirements in certain instances. The main objective of developing and demonstrating a UV curable coating that can provide corrosion protection to high strength aluminum alloy substrates was achieved.

Table VIII - Summary of MUV Evaluation Results

63

### 6. Conclusions and Implications fo r Future Research/Implementation

The project was successful in achieving many of the goals and objectives in the original proposal. Cerium conversion coatings were able to pass 336 hours of ASTM B117 salt spray testing on Al 2024-T3 and Al 7075-T6 with prop er surface preparation, deposition conditions, and post-treatment. It was determined that subs trate preparation significantly altered the surface chemical and electrochemical prope rties of high strength aluminum alloys. Activation of panels in strong acid or base solutions removed the na tive oxides and exposed intermetallic compounds in the alloys. Acid activation results in faster CeCC deposition rates, reducing the number of spray cycles required to completely cover a substrat e from 40 for panels that were degreased to 1 for panels that were degreased and activated. Alternative surface prepar ation methods such as ultrasonic cleaning, use of abrasive media, and hand application of cleaning solutions, were shown to be suitable for and compatible with subsequent CeCC deposition processes. The addition of 0.8 g gelatin to deposition solutions was found to decrease the effective deposition rate of CeCCs by slowing the nucleation pro cess, improving coating quality and corrosion resistance. Investigations on the effect of the pH, Ce concentration, and amount of H 2 O 2 in the deposition solution found that a pH of 2.5, concentration of 4 wt.% Ce using CeCl 3 salt sources,
and 20 ml of H 2 O 2 resulted in coatings that performed well in salt spray and electrochemical testing and optimized phosphate post-treatment. However, the use of chloride and peroxide containing deposition solutions was found to produ ce sub-surface crevices in the Al substrates. Phosphate solution post-treatment of CeCCs could significantly im prove the corrosion protection of the films. Several phosphate sources and pr ocessing conditions were evaluated and it was determined that using NH 4 H 2 PO 4 at pH 4.5 and 85°C for 5 minutes was able to cons istently pass ASTM B117 criteria. Correlation of salt spray results and electrochemical polarization and impedance spectroscopy was excellent after modifi cation of a prohesion-b ased electrolyte. The majority of CeCC depositions were done using a spray method but brush application, immersion, and electrolytic processes were also demonstrated to be effective. In almost all cases, it was
found that a ~400nm thick CeCC was the most effec tive corrosion coating. All of the knowledge gained during the project was used to deve lop and publish a deposition mechanism for CeCCs that has gained recognition and acceptan ce within the scientific community.

Development and demonstration th at UV curable coatings with inorganic corrosion inhibitors could be fully cured in just a few seconds, have good flexibility, adhesion, and fluid resistance, and pass corrosion testing was a major breakthro ugh. There was skepticism at the start of the project that highly loaded polymer coatings would be able to pass the requirements for existing epoxy based coatings; therefore, chromate a nd non-chromate epoxy coatings were used as reference standards throughout th e project in order to directly compare results. Evaluation of MUV coatings on TCP conversion coated panels was also done during the project. The multifunctional UV curable system repeatedly passed 3000 hours of ASTM B117 salt spray testing on chromate conversion coated high stre ngth Al alloys with ~30 wt. % Hybricor 204 corrosion inhibitor from Wayne Pigments in the MUV. An acrylate-based polymer system was used throughout the project along with a two bul b curing system from Fusion, Inc. Initially,
corrosion inhibitors from Deft and Wayne Pigments were evalua ted but it was evident from the early stages of the project that Hybricor 204 was the best inhibitor. Appl ication of the MUV was done using a drawdown method during the first tw o rounds of testing in order to conserve material. A complete non-chrome coating syst em of Hybricor 204 inhibitor on CeCC did pass

64ASTM B117 in the first round of testing. A la dder study was done after th e first two rounds in order to determine the optimum amount of Hybr icor 204 and, while a range of values between 27
and 36 wt% were all effective at inhibiting corrosion, a 29 wt% load ing was chosen to develop a MUV spray formation by considering all of the desired properties. The first spray deposited MUV formulation was done during round three and again demonstrated excellent filiform and salt spray corrosion performance, with ~10% el ongation, good adhesion, and low gloss values on chromate conversion coated panels. Panels with MUV on CeCC developed blisters during corrosion testing that led to failure. In an attemp t to improve the flexibility of the MUV, lower molecular weight oligomers were used duri ng round 4 and, although corrosion performance was still very good, there was degrad ation in the non-corrosion relate d performance of the MUV. The final round of testing was done with a formulation that was a modified version of the formulation used in round 3. Darkening of the MUV during accelerated weathering tests and poor performance on CeCC, as was the case in round 3, were observed in round 5. Positive results from round 5 on chromate conversion coated pa nels included excellent corrosion protection, good adhesion, samples that passed low temperature flexibility testing, and ~10% reverse impact flexibility at room temperature. Collectively, th e results indicated that the MUV coatings were capable of passing most of the required aeros pace requirements and that adjustments in
formulation were able to alter properties as needed.

The results of this project indicate that a two la yer coating system consisting of a non-chromate CeCC and UV curable polymer with inorganic corrosion inhibitors is capable of meeting aerospace coating requirements, as demonstrated in the first round of testing. Additional research is needed understand and optimize the CeCC process and MUV formulation such that reproducible results are obtained. The major conc lusions that point to the need for future
research are summarized below.

• When deposited on high strength aluminum alloys with CrCCs, MUV coatings demonstrated excellent performance. Th is combination appears to be ready for demonstration on non-critical components on military aircraft, and implementation of the technology onto specific areas of weapons platforms would not require extensive
investment.

• The CeCC and MUV consistently pass independen t, individual tests but when combined into a system do not meet specifications. In particular, the elimination of blisters on MUV/CeCC panels during salt spray testi ng would likely result in passing not only ASTM B117 testing but also significantly improve adhe sion and flexibility. Some
additional research focused on elimination of peroxide species from CeCCs or integrating
the MUV with other non-chromate surface treat ments is needed to produce a viable chromium-free system.

• Although the MUV coating passes the most cr itical performance sp ecifications (i.e., corrosion performance, adhesion, etc.), a co lor change was observed in accelerated weathering testing. Changing the type of pi gment used in the MUV could address the changes in color during accelerated weathering.

• Coating processes were devel oped to be compatible with field application. However, demonstration of the UV curing system would initially need to be done on a small scale for proof of concept followed by large scal e operations based on existing processes in
commercial use.

65The results of this project have shown that th e MUV can meet the important performance criteria for coatings used on military aircraft. The path forward to implement the technology is straightforward and appears to be re ady for demonstration/validation.



### 7. Literature Cited

1. “Corrosion Cost and Preventive Strategies in the United States”, Report No. FHWA-RD01-156, CC Technologies and NACE International, September 30, 2001.

2. “OSHA Proposes Revised Rule on Hexavale nt Chromium”, OSHA News Release, October 1, 2004.

3. “Electrodeposition of Cerium Based Coati ngs for Corrosion Protection of Aluminum Alloys”, J. O. Stoffer, T.J. O’Keefe, S. P. Sitaram, P. Yu, X. Lin, E. Morris, U.S. Patent 5,932,083, Aug. 3, 1999.
4. “Cerium Based Spontaneous Coating Process For Corrosion Protection of Al Alloys”, James O. Stoffer, Thomas J. O’Keefe, Matthew O’Keefe, Eric L. Morris, Scott Hayes, Alex Williams, Berny Rivera, Xu an Lin, Patent # 7,048,807, May 23, 2006.

5. B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Materials Forum 9 (3), 162-73 (1986).

6. B. Rivera, B. Johnson, M. O’Keefe, and W. Fahrenholtz, Surface and Coatings Technology, 176 (3), 349 (2003).

7. S. Geng, W. Pinc, P. Yu, P. Jones, M. O’ Keefe, W. Fahrenholtz, and T. O’Keefe, J. of Surface Finishing , 2(4) 276-282 (2007).

8. Pu Yu, Scott A. Hayes, Thomas J. O’Keefe, Matthew J. O’Keefe, and James O. Stoffer, J. of the Electrochem. Soc.

, 153 (1), C74-79 (2006).

9. “Radiation Curing in Coatings”, J. V. Ko leske, ASTM International, US, pp.3 (2002).

10. V.Shukla, M.Bajpai, D.K. Singh, M.Singh, R.Shukla, Pigment & Resin Technology
33 (5),272 (2004).

11. “New Perspectives in product development a nd pollution control thr ough the use of ultra violet polymerizatio n”, A. De Raeve, http://www.unitex.be/Downloads/niet-leden/to2cderaeve.pdf

. 12. “UV curing technical principle and m echanism”, Ciba Specialty Chemicals, http://www.cibasc.com/pf/docMDMS.asp?t argetlibrary=CHBS_CE_MADS&docnumber

=1628

.

66

### 8. Appendices

8.1 List of Scientific/Technical Publications

A. Articles in Peer Reviewed Journals

1. S. Geng, P. Yu, M O’Keefe, W. Fahrenho ltz, and T. O’Keefe, “Screening Study of Spray Solution Parameters for Depositing Ce rium Based Conversion Coatings on Al Alloy 2024-T3”, J. Applied Electrochemistry, 40, 551, (2010).

2. D. Heller, W. Fahrenholtz, and M. O’Keefe, “The Effect of Post-treatment Time and Temperature on Cerium Based Conversi on Coatings on Al 2024-T3”, Corrosion Science, 52, 360 (2010).

3. W.R. Pinc, P. Yu, M. O'Keefe, and W. Fa hrenholtz, "Effect of Gelatin Additions on
the Corrosion Resistance of Cerium Base d Conversion Coatings Spray Deposited on Al 2024-T3," Surface and Coatings Technology, 203(23) 3533 (2009).

4. D. Heller, W. Fahrenholtz, and M. O’Keef e, “Effect of Phosphate Source on the
Post-treatment and Corrosion Performance of Cerium Based Conversion Coatings on Al 2024-T3”, J. Elec. Soc. 156(11), C400 (2009).

5. D.K. Heller, W.G. Fahrenholtz and M.J. O' Keefe, "The Effect of Phosphate Source
on the Post-Treatment of Cerium-Based Conversion Coatings on Al 2024-T3 and Its
Correlation to Corrosion Performa nce," ECS Trans. 16, 47 (2009).

6. W. Pinc, S. Geng, M. O’Keefe, W. Fahre nholtz, and T. O’Keefe, “Effects of acid and alkaline based surface pr eparations on spray deposited cerium based conversion coatings on Al 2024-T3”, Applied Surface Science, 255, 4061 (2009).

7. S. Geng, W. Pinc, P. Yu, P. Jones, M. O’ Keefe, W. Fahrenholtz, and T. O’Keefe, “Influence of Cleaning on the Depositi on Rate of Cerium Based Conversion Coatings on Aluminum Alloy 2024”, J. of Surface Finishing, 2(4) 276-282 (2007).

8. P. S. Jones, P. Yu, W.R. Pinc. M.J. O'K eefe, W.G. Fahrenholtz, and T.J. O'Keefe, "Spray Deposition of Cerium Oxide-Base d Conversion Coatings on Al 2024-T3," International Journal of Applied Ceramic Technology, 5(1) 63-73 (2008).

9. Shaoxin You, Philip Jones, Ajay Padwal, Paul Yu, Matthew O’Keefe, William Fahrenholtz, and Thomas O’Keefe, “Res ponse of Nano-crystalline Cerium Based Conversion Coatings on Al 2024-T3 To Chlo ride Environments”, Materials Letters, 61 (17), 3378 (2007).

B. Technical Reports

1. Philip Jones, May 2007, MS Thesis, Impact of Processing Parameters on Spray Deposition of a Cerium-Based C onversion Coating on Al 2024-T3

2. William Gammill, December 2009, MS Thesis, Evaluation of Alternative Methods for Surface Preparation and Deposition of Cerium-based Conversion Coatings on Aluminum Alloys

3. “Multifunctional UV (MUV) Curable Corrosion Coatings for Aerospace Applications”, Matthew J. O’Keefe, Willia m G. Fahrenholtz, and Ben S. Curatolo, Metal Finishing, pp. 28-31, February 2010.

674. “Cerium-Based Conversion Coatings as Alte rnatives to Hex Chrome”, Matthew J. O’Keefe, Shujiang Geng, and Simon Jo shi, Metal Finishing, pp. 25-28, May 2007.

C. Conference or Symposium Scientifically Recognized

1. A. Thomas, D. Heller, W. Gammill* W. Fa hrenholtz, M. O’Keefe, J. DeAntoni, and B. Curatolo, “Performance of Multifunctional UV (MUV) Curable Corrosion Coating Systems to Aerospace Military Test Criteria”, DoD Corrosion Conference, Washington DC, Aug 2009.

2. S. Geng, W. Pinc, S. Joshi, W. Fahrenho ltz, M. O’Keefe, T. O’Keefe and P. Yu, “Influence of Processing Parameters on Cerium Based Conversion Coatings”, TriServices Corrosion Conference, Denver, CO, Dec 2007.

3. J. DeAntoni, B. Curatolo, M. O’Keef e and W. Fahrenholtz, “Development and Evaluation of Multifunctional Ultraviolet (MUV) Light Curable Coatings for High Strength Aluminum Alloys”, Tri-Services Corrosion Conference, Denver, CO, Dec

2007.
4. Philip Jones, Ajay Padwal, Paul Yu, Ma tthew J. O’Keefe, William Fahrenholtz, Thomas O’Keefe, “Characterization of Al kaline Cleaned and Cerium Oxide Coated Al 2024-T3”, Materials Science and Technology Conference, Product Manufacturing Symposium, pp 591-597, Cincinnati, OH, Oct 2006.

D. Conference of Symposium Abstracts

1. W. Pinc , D. Heller, W. Fahrenholtz, and M. O’Keefe, “The Electrochemical and Structural Changes of Cerium Based Conve rsion Coatings During Exposure to a Salt Spray Environment”, Electrochemical Society Meeting, Oct 5-8, 2009, Vienna,
Austria.

2. D.K. Heller, W.G. Fahrenholtz , and M.J. O'Keefe, "Electrochemical Analysis of Phosphate Treated Cerium Based Conve rsion Coatings on Al 2024-T3," PRIME 2008: Pacific Rim Meeting on Electrochemi cal and Solid State Science, October 12- 17, 2008, Honolulu, HI.

3. Heller , W. Fahrenholtz, and M. O’Keefe, “The Effect of Phosphate Post-treatment
Process Variables on Cerium Based Conve rsion Coating of Al 2024-T3”, MS&T Conference, Pittsburgh, PA, Oct 2008.

4. W. Gammill, W. Fahrenholtz, and M. O’Keefe, “Non-Immersion Surface Preparation for Cerium-based Conversion Coatings On Aluminum Substrates”, MS&T Conference, Pittsburgh, PA, Oct 2008.

5. S. Joshi, W. Fahrenholtz, and M. O’Keefe, “T he Effect of Phosphate Additions on
the Corrosion Resistance of Cerium C onversion Coatings on Aluminum Alloy 7075 – T6”, MS&T Conference, Pittsburgh, PA, Oct 2008.

6. W. Pinc, W. Fahrenholtz, and M. O’Keefe, “Effect of Gelatin Additions on the
Corrosion Resistance of Cerium Based C onversion Coatings”, MS&T Conference, Pittsburgh, PA, Oct 2008.

7. S. Geng , P. Yu, M. O’Keefe, W. Fahrenholtz, and T. O’Keefe, “Corrosion Behavior of Ce-Based Conversion Coatings on Al Alloy 2024-T3”, MS&T Conference, Detroit, MI, Sep 2007.

6869 8. S. Joshi , W. Pinc, S. Geng, M. O’Keefe, W. Fahrenholtz, and T. O’Keefe,
“Characterization of Cerium Conversion Co ated Al 7075-T6 Prepared with Alkaline and Acid Cleaning Solutions”, MS&T Conference, Detroit, MI, Sep 2007.

9. Materials Science and Technology (MS&T) Conference, “Corrosi on Performance of Cerium-Based Conversion Coatings on Aluminum Alloys”, Bill Fahrenholtz and Matt O’Keefe, Oct 28, 2009.

10. SERDP 2008 Symposium, “Development and Evaluation of Multifunctional UV (MUV) Curable, Non-Chromate Corrosion Coating Systems”, Matt O’Keefe , Bill Fahrenholtz, John DeAntoni, and Ben Curatolo, Wash DC, Dec 3, 2008.

11. University of Cincinnati, Materials Science and Engineering Seminar, “NonChromate Coatings for Corro sion Protection”, Matt O’Keefe and Bill Fahrenholtz , 8 May 2008.
12. Surface Finishing and Repair Issues for Su staining New Military Aircraft Workshop,
“Multifunctional UV Coa tings and Ce-Based Mate rials”, Matt O’Keefe , Bill Fahrenholtz, John DeAntoni, and Ben Curatolo, Tempe, AZ, Feb 28 2008.

13. DoD Metal Finishing Workshop, “Ce Base d Conversion Coatings”, Matt O’Keefe

, Hill AFB, UT, May 16, 2007.

14. GM Tech Center Symposium, “Developmen t of Environmentally Friendly Corrosion Coating Systems”, Matt O’Keefe , Warren, MI, Apr 16, 2007.
15. W. Pinc , S. Geng, S. Joshi, M. O’Keefe, W. Fahrenholtz, and T. O’Keefe, “Effects
of Acidic and Alkaline Based Surface Preparations of Al 2024-T3 on Spray Deposited Cerium-Oxide Conversion Coati ngs”, MS&T Conference, Detroit, MI, Sep 2007.