Outdoor Exposure Results for Pretreated and Topcoated 

Aluminum Armor Alloys 2519, 5083, and 7039 

by John V. Kelley and Brian E. Placzankis

ARL-TR-3283 September 2004 

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Army Research Laboratory 

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ARL-TR-3283 September 2004 

Outdoor Exposure Results for Pretreated and Topcoated 

Aluminum Armor Alloys 2519, 5083, and 7039 

John V. Kelley and Brian E. Placzankis 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
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Outdoor Exposure Results for Pretreated and Topcoated Aluminum Armor Alloys 

2519, 5083, and 7039 

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622618.H80 

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John V. Kelley and Brian E. Placzankis 

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14. Abstract 

Over the last few decades, it has been determined that chemical treatments containing hexavalent chromium (Cr+6) are risks to 

both human health and the environment. One of the mainstays in corrosion prevention of aluminum alloys has been the use of 

chromate conversion coatings to promote adhesion and inhibit corrosion. Because of the push to reduce the use of Cr+6, many 

nonchromate treatments have been evaluated over the years. This report compares the relative performance of six nonchromate 

pretreatments with grit-blasted and chromated control samples in outdoor exposure at Cape Canaveral, FL. Recently obtained 

outdoor exposure data have been analyzed to determine the effectiveness of the pretreatments, indicating a different failure 

mode than typically seen in accelerated corrosion chambers. Corrosion attack at the edges is prevalent in outdoor exposure, 

while attack at scribe is uncommon. The substrate of most concern, aluminum 2519, seemed to be less of a factor than 

originally anticipated when no pretreatment was used. All samples have been monitored throughout the test using 

electrochemical impedance spectroscopy. Adhesion tests were performed upon completion of outdoor exposure. The data is 

compared with earlier accelerated corrosion data performed on matching samples. 

15. SUBJECT TERMS

outdoor exposure, marine exposure, nonchromate, chromate, aluminum armor, impedance, salt fog, cyclic salt spray 

16. SECURITY CLASSIFICATION OF: 

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John V. Kelley 

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iii

Contents 

List of Figures iv

List of Tables iv

Acknowledgments vi

1. Introduction 1

2. Experimental Procedure 2

3. Results 7

3.1 Visual Inspection.............................................................................................................7 

3.2 Electrochemical Impedance Spectroscopy (EIS) ..........................................................10 

3.3 Coating Adhesion..........................................................................................................13 

4. Discussion 13

5. Conclusions 19

6. References 20

Appendix A: Sample Designations, Pretreatments, and Alloy Combinations 21

Appendix B: Postexposure Pull-off Adhesion Results With Statistics 23

Appendix C: Failure Mode Recording for Each Pull-off Test 25

Distribution List 27 
iv

List of Figures 

Figure 1. Exposure racks at Cape Canaveral. .................................................................................3 

Figure 2. Teflon hold-down brackets..............................................................................................4 

Figure 3. Schematic of EIS cell used for field measurements. .......................................................6 

Figure 4. Portable EIS setup at outdoor exposure test site at Cape Canaveral. ..............................6 

Figure 5. Schematic of the pull-off adhesion test performed on all exposed samples....................7 

Figure 6. Alloy 2519 pretreated with Turcoat. ...............................................................................8 

Figure 7. Alloy 7039 pretreated with Patclin 2...............................................................................8

Figure 8. Grit-blasted 2519 after 48 months of exposure. ..............................................................9 

Figure 9. Grit-blasted 7039 after 48 months of exposure. ..............................................................9 

Figure 10. Average area failed of both sets of panels after 48 months (208 weeks) of outdoor 

exposure. ..................................................................................................................................10 

Figure 11. ASTM D 1654-79A procedure A ratings for panels in outdoor exposure. .................11 

Figure 12. ASTM D 1654-79A procedure B ratings for panels in outdoor exposure. .................11 

Figure 13. Al alloy 2519 in outdoor exposure. .............................................................................12 

Figure 14. Al alloy 5083 in outdoor exposure. .............................................................................12 

Figure 15. Al alloy 7039 in outdoor exposure. .............................................................................13 

Figure 16. Grit-blasted panels in (a) lab immersion and (b) outdoor exposure............................14 

Figure 17. Chromated panels in (a) lab immersion and (b) outdoor exposure. ............................15 

Figure 18. Averaged postexposure pull-off coating adhesion strength data.................................16 

List of Tables 

Table 1. Description for notations used for coatings and pretreatments.........................................2 

Table 2. Rating criteria of procedure A for evaluating creepage from scribe of painted panels 

subjected to corrosive environments (8)....................................................................................4 

Table 3. Rating criteria of procedure ASTM 1654-79A procedure B for evaluating 

unscribed areas of painted panels subjected to corrosive environments (8)..............................5 

Table 4. Laboratory conditions for pull-off adhesion (9). ..............................................................7 
v

Table A-1. Energy dispersive spectroscopy verification of aluminum alloy substrates...............22 

Table B-1. ASTM D 45411

 coating adhesion lb/in2 (psi). ............................................................23 

Table C-1. Pull-off adhesion test failure modes. ..........................................................................26 
vi

Acknowledgments 

The authors would like to thank several people for their technical contribution. Most notable is 

Mr. Russell Kilbane, who methodically performed many of the laboratory impedance 

measurements and pull-off adhesion tests and compiled and presented data in graphical format; 

Mr. Cliff Hubbard, who painstakingly performed the energy dispersive spectroscopy alloy 

verifications; and consultations with Dr. John Beatty and Christopher Miller, who aided in the 

understanding of coating technology. Finally, we would like to thank the Strategic 

Environmental Research and Development Plan program for the original funding of this 

program.
1

1. Introduction 

The automotive and defense industry have been relying on accelerated corrosion data to 

determine relative corrosion performance of alloys and coating systems for decades. These 

methods are particularly valuable in determining the ability of coatings to protect steel from 

corrosion. However, the use of salt fog testing to determine the corrosion behavior of coated 

aluminum alloys is not as accurate (1). For this reason, outdoor exposure is a useful tool to 

validate accelerated corrosion data and is the most realistic method of evaluating corrosion 

performance. 

The U.S. Army is continually seeking to lighten the force in an effort to increase its mobility and 

deployability. The use of aluminum armor alloys in the construction of armored vehicles is one 

of the most efficient ways to achieve this. However, aluminum armor can introduce new 

challenges in corrosion protection. For example, the mainstay in corrosion prevention of 

aluminum alloys, and the most effective to date, has been the use of chromate conversion 

coatings. Aluminum alloys are often pretreated with chromate conversion coatings to stabilize 

the native oxide on the surface of the metal to promote adhesion and inhibit corrosion. However, 

over the last few decades, it has been determined that chemical treatments containing hexavalent 

chromium (Cr+6) are risks to both human health and the environment. Therefore, the Army and 

private industry have been challenged with developing environmentally benign pretreatments for 

aluminum armor alloys. 

Al 5083-H131 and 7039-T64 are used for the bulk of lightweight armor applications. Currently 

Al-Mg 5083 is the most widely used aluminum armor alloy because of its high ductility and 

resistance to stress corrosion cracking (SCC) (2). It has been used in armored systems such as 

the amphibious assault vehicle (AAV) and the Bradley fighting vehicle. Alloy 7039 has high 

strength and provides better ballistic protection but is more susceptible to stress corrosion 

cracking and less suitable for structural applications in marine/tropical environments. The 

continuous need to lighten the force while improving armor protection has led to the 

investigation of other aluminum alloys. 

In recent years, Al 2519-T87 has been looked at as a possible replacement for 5083 in some 

applications. Its superior ballistic performance and high strength-to-weight ratio would help 

lighten the overall weight of the vehicle while providing better ballistic protection. Alloy 2519 

was recently selected for the advanced amphibious assault vehicle (AAAV). The AAAV is one 

of the weapon systems presently under development by the Marine Corps to eventually replace 

the AAV. Alloy 2519 must be used with care in marine applications because of its susceptibility 

to pitting corrosion (3). Chemical agent resistant coating (CARC) coated 2519 samples have 

performed poorly in accelerated corrosion tests, regardless of the pretreatments used. 
2

Avoiding corrosion of the metallic components in armored vehicles has always been a challenge. 

Since the Environmental Protection Agency has targeted hexavalent chrome for elimination and 

2519 is becoming more common in armored systems, the task has become even more difficult. 

There are many nonchromate alternatives on the market today but, as yet, none have proven to be 

as effective as chromate in various accelerated corrosion tests. 

The Weapons and Materials Research Directorate of the U.S. Army Research Laboratory (ARL) 

developed and executed a research program under the auspices of the Department of Defense 

Strategic Environmental Research and Development Program (SERDP) to examine 

commercially available nonchromate alternatives for the protection of aluminum alloys. 

The goal of this effort was to assess the effectiveness of several nonchromate alternatives on 

coated test panels of three aluminum armor alloys. Included in the program were samples that 

were subjected to outdoor marine exposure. A study conducted by Dante et al. (1) suggests that 

to get meaningful results from outdoor exposure tests, more than 2 years are needed. The results 

in this report are of samples that completed 4 years of outdoor exposure and will be used to 

attempt to validate the earlier accelerated corrosion data and gage the relative corrosion 

performance of three aluminum alloys. Electrochemical impedance spectroscopy (EIS) and 

adhesion tests were also used to evaluate the condition of the topcoats. 

2. Experimental Procedure 

The selection of nonchromate conversion coatings to be tested was done in conjunction with 

study partners from the National Center for Manufacturing Sciences (NCMS), University of 

Michigan (4). Thirteen vendors of nonchromate coatings were asked to coat test panels. Of 

these, four vendors (for a total of six nonchromate coatings) agreed to have their coatings 

evaluated. For control purposes, a chromate conversion coating and a grit-blasted condition 

were also included (5). The notations used to identify all of the pretreatments and coatings in 

this study are listed in table 1. 

Table 1. Description for notations used for coatings and pretreatments. 

Notation Coatings/Pretreatment

Grit blasted Grit-blasted panel with no conversion coating 

Chromate Alodine 1200 

Chemcote Brent Chemcoat L497260A 

Turcoat Turco Alcoat 6787 

Patclin 1 Patclin 1910A 

Patclin 2 Patclin 1910B 

Patclin 3 Patclin 1910C 

Bulk Chemical Bulk Chemical E-CLPS 923 

CARC/topcoat CARC (6) 

Primer Epoxy primer (7) 
3

At the beginning of the program, ~300 aluminum panels nominally 4 × 6 × 0.375 in, (10 × 15 

× 1 cm) of alloys 2519-T87, 7039-T64, and 5083-H131 were machined from rolled armor plate 

stock and sent to vendors and Army depots for coating application. Twelve panels for each 

conversion coating/alloy combination were prepared. From each set of 12 panels, 5 were painted 

with an epoxy primer (7) and CARC (6) topcoat, 3 panels were coated with the epoxy primer 

only, and 4 panels were left in the conversion coated state. 

From the five sets painted with the CARC system, two sets (a total of 48 panels) were selected 

for outdoor exposure and are the topic of this study. All specimens and their designations are 

tabulated in Appendix A. Each of these panels was “X” scored using a hardened steel scribe 

with a static load of 5 kg and placed on a rack on the beach at Cape Canaveral Air Force Base, 

FL (figure 1). A hole was drilled in each panel near the edge and was used to attach the 

“working electrode” lead with a screw for impedance measurements. The panels were propped 

up at about 30° horizontally, with screw holes on top so water would drain away from hole. The 

racks were positioned approximately 100 yards away from and facing the ocean. Each panel was 

held in place 1.25 in above the surface of the rack using Teflon*

 brackets. Teflon was used 

because of its inert qualities, durability, and electrical insulating properties (figure 2). The other 

seven sets of panels were tested in an earlier study using accelerated test methods and EIS (5). 

Figure 1. Exposure racks at Cape Canaveral. 

 *

 Teflon is a registered trademark of DuPont. 
4

Figure 2. Teflon hold-down brackets. 

Periodic visual inspections were performed at 6, 9, 19, and 48 months outdoor exposure, and the 

condition of the surface of the panel was estimated and recorded in terms of percent area failed. 

At the end of the 48-month exposure, the surface of the panel was evaluated according to 

ASTM D 1654-79A (8). Procedure B was used to rank the areas away from the scribe, and 

procedure A was used to obtain ratings for creepage outward from the scribe. The rating criteria 

for ASTM D 1654-79A are outlined in tables 2 and 3. 

Table 2. Rating criteria of procedure A for evaluating creepage from scribe of painted panels 

subjected to corrosive environments (8). 

Millimeters Inches 

(Approximate) 

Rating No. 

Over 0 0 10 

Over 0 to 0.5 0 to 1/64 9 

Over 0.5 to 1.0 1/64 to 1/32 8 

Over 1.0 to 2.0 1/32 to 1/16 7 

Over 2.0 to 3.0 1/16 to 1/8 6 

Over 3.0 to 5.0 1/8 to 3/16 5 

Over 5.0 to 7.0 3/16 to 1/4 4 

Over 7.0 to 10.0 1/4 to 3/8 3 

Over 10.0 to 13.0 3/8 to 1/2 2 

Over 13.0 to 16.0 1/2 to 5/8 1 

Over 16.0 to more 5/8 to more 0 
5

Table 3. Rating criteria of procedure ASTM 1654-79A procedure 

B for evaluating unscribed areas of painted panels subjected 

to corrosive environments (8). 

Area Failed 

(%) 

Rating No. 

No failure 10 

0 to 1 9 

2 to 3 8 

4 to 6 7 

7 to 10 6 

11 to 20 5 

21 to 30 4 

31 to 40 3 

41 to 55 2 

56 to 75 1 

Over 75 0 

EIS was employed to monitor coating condition over time. A portable EIS apparatus was 

designed to allow measurements to be taken at remote sites in the field (figure 3). It consisted of 

a Schlumberger 1280 combination frequency response analyzer (FRA)/potentiostat, a 12-V DC 

to 120-V AC power inverter with 12-V battery, and a Toshiba laptop computer. Periodic 

measurements were taken at 6, 9, 19, and 48 months of exposure using the cell configuration 

illustrated in figure 4. The cell was filled with 25 mL of 0.5 N NaCl solution and allowed to 

equilibrate for at least 2 hr prior to taking measurements. The software package Zplot was used 

for data acquisition and Z view for data analysis. Measurements were taken at the corrosion 

potential of the sample over the frequency range of 100 kHz–0.01 Hz. The single sine technique 

was utilized, with a sinusoidal potential impressed upon the open circuit corrosion potential of 

the substrate (–0.930 V vs. saturated calomel electrode [SCE]) and an applied 80-mV amplitude. 

An attempt was made to attach the cell and take measurements on a different spot on the panel 

for each inspection period. That wasn’t always possible because, as the samples degraded over 

time, the amount of defect-free area to perform EIS became limited on some samples. 

The EIS data was plotted and evaluated in Bode and Nyquist formats. The Bode formats 

displayed the magnitude (log | Z |) and phase angle (θ) of the impedance as a function of applied 

frequency (log f). The total impedance of the specimen, defined as the log | Z | value at 10 mHz 

in the Bode magnitude plot, was also plotted as a function of exposure time for comparison of 

the effectiveness of each coating system. 

Postexposure paint adhesion tests were performed using a HATE Mark VII coating adhesion 

tester (figure 5) in accordance with ASTM D 4541 (9). A loading fixture commonly referred to 

as a “dolly” was secured normal to the coating surface using a cyanoacrylate adhesive. After 

allowing the adhesive to cure for 24 hr in laboratory conditions (table 4), the attached dolly was 
6

Figure 3. Schematic of EIS cell used for field measurements. 

Figure 4. Portable EIS setup at outdoor exposure test site at Cape 

Canaveral. 

inserted into the test apparatus. The load applied by the apparatus was gradually increased and 

monitored on the gauge until a plug of coating was detached. The failure value in lb/in2

 (psi) 

was recorded and the failure mode characterized. For this test to be accurate, the panels must be 

sufficiently thick to ensure that the coaxial load applied during the removal stage does not distort 

the substrate material and cause a bulging or a “trampolining effect.” On a thin specimen, the 

resultant bulge caused the coating to radially peel away outwards from the center instead of 

uniformly pulling away in pure tension, thus resulting in significantly lower readings. All of the 

panels used in this study were made from 0.375-in-thick aluminum armor and were of adequate 

thickness for the pull-off test procedure. 
7

Figure 5. Schematic of the pull-off adhesion test performed on 

all exposed samples. 

Table 4. Laboratory conditions for pull-off adhesion (9). 

Adhesive type Cyanoacrylate 

Cure time (hr) 24 

Temperature (°C) 25 

Relative humidity (%) 31 

Substrate materials Al 2519/5083/7039 

Coating type CARC 53022/53039A 

Coating thickness (mil) ~4.0 

3. Results 

3.1 Visual Inspection 

At each of the inspection intervals, every panel was visually examined, and any defects were 

noted. Some key photos are presented in figures 6–9. Note: regardless of the orientation of the 

photos, the bottom edge of the panel refers to the edge opposite the screw hole. One of the first 

observations made of all panels was of the noticeable fading of the paint at the 9-month 

inspection. At that time, it was also apparent that corrosion had begun on the edges of panels B, 

J, O, and P in set A, and panels A, I, L, O, Q, and R in set B (see appendix A). At 37 months, 

two of the sample sets were removed from the test because of excessive corrosion damage. They 

were P (7039/Patclin 2) and L (2519/Turcoat). All other samples completed the 48 months of 

exposure. When the test was terminated after 48 months, all samples were given a final thorough 

visual inspection. First, a general overall condition of the panels was estimated, and the average 

of each alloy-pretreatment combination was taken. These results are illustrated in figure 10. 
8

(a) (b) 

Figure 6. Alloy 2519 pretreated with Turcoat. 

(a) (b) 

Figure 7. Alloy 7039 pretreated with Patclin 2. 
9

(a) (b) 

Figure 8. Grit-blasted 2519 after 48 months of exposure. 

(a) (b) 

Figure 9. Grit-blasted 7039 after 48 months of exposure. 
10

Figure 10. Average area failed of both sets of panels after 48 months (208 weeks) of 

outdoor exposure. 

Unlike the subsequent evaluations done according to ASTM D 1654-79A, these data represented 

the overall condition of the top surface of the panel and took into account the corrosion near the 

edges and along the scribes. 

Measurements of corrosion creepage outward from the scribes and the condition of the unscribed 

areas of the panels were done according to ASTM D 1654-79A procedures A and B, 

respectively. These results are shown in figures 11 and 12. For the evaluations of areas away 

from the scribe, an effort was made to discount any corrosion within 0.5 in from the edges and 

any corrosion initiating from the scribes. Despite these efforts, in several cases, the corrosion 

undercutting from the edges went beyond the 0.5-in boundary and therefore was included. 

3.2 Electrochemical Impedance Spectroscopy (EIS) 

EIS was used to measure the dielectric response of the coating as it degraded over the 48 months 

of outdoor exposure and compared with previously recorded laboratory measurements. The 

procedure for the laboratory measurements taken on immersion samples is described elsewhere 

(5). For the outdoor experiment, four measurements were recorded for each sample over the 

48 months. These results are given in figures 13–17. The initial measurement was made in the 

lab on a similarly prepared sample. Measurements made at 6, 9, and 19 months were made on 

site at Cape Canaveral Air Force Base. The final impedance measurement was taken at 48 

months for all samples and was done in the laboratory when the exposure test was terminated. 
11

Figure 11. ASTM D 1654-79A procedure A ratings for panels in outdoor exposure. 

Figure 12. ASTM D 1654-79A procedure B ratings for panels in outdoor exposure. 
12

2519 with Primer and Topcoat in Outdoor Exposure at Cape Canaveral

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0 25 50 75 100 125 150 175 200 225

Time (weeks)

Log Z

Grit Blasted

Chromate

Chemcote(L497260A)

Turcoat

Patclin 1

Patclin 2

Patclin 3

Bulk Chem

Figure 13. Al alloy 2519 in outdoor exposure. 

5083 with Primer and Topcoat in Outdoor Exposure at Cape Canaveral

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0 25 50 75 100 125 150 175 200 225

Time (weeks)

Log Z

Grit Blasted

Chromate

Chemcote(L497260A)

Turcoat

Patclin 1

Patclin 2

Patclin 3

Bulk Chem

Figure 14. Al alloy 5083 in outdoor exposure. 
13

Figure 15. Al alloy 7039 in outdoor exposure. 

3.3 Coating Adhesion 

Postexposure coating adhesion tests were performed on all samples exposed. Two “pull-off” 

tests per panel were done, and the results for each alloy/pretreatment combination were averaged 

and illustrated in figure 18. All of the pull-off results, along with the statistics, are tabulated in 

appendix B. The mode of failure for each pulloff was noted and is given in appendix C. 

4. Discussion 

The objective of this effort was to evaluate commercially available nonchromate conversion 

coatings against a baseline of hexavalent chromate (Alodine 1200) and to characterize the 

corrosion mechanisms and adhesion performance of the coatings systems. This discussion will 

focus on the following two areas: (1) the performance of the pretreatments on each alloy in 

outdoor exposure and (2) a comparison of the corrosion observed and the relative performance of 

these coatings in outdoor exposure and accelerated corrosion tests. 

7039 with Primer and Topcoat in Outdoor Exposure at Cape Canaveral

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0 25 50 75 100 125 150 175 200 225

Time (weeks)

Log Z

Grit Blasted

Chromate

Chemcote(L497260A)

Turcoat

Patclin 1

Patclin 2

Patclin 3

Bulk Chem
14

Gritblasted Panels with Primer and Topcoat in Laboratory Immersion

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

Time (weeks)

Log Z

Gritblasted 2519

Gritblasted 5083

Gritblasted 7039

(a) In lab immersion

Gritblasted Panels with Primer and Topcoat in Outdoor Exposure

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0 50 100 150 200 250

Time (weeks)

Log Z

Grit Blasted 2519

Grit Blasted 5083

Grit Blasted 7039

(b) In outdoor exposure 

Figure 16. Grit-blasted panels in (a) lab immersion and (b) outdoor exposure. 
15

(a) In lab immersion 

(b) In outdoor exposure 

Figure 17. Chromated panels in (a) lab immersion and (b) outdoor exposure. 

Chromate Panels with Primer and Topcoat in Outdoor Exposure

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0 50 100 150 200 250

Time (weeks)

Log Z

Chromate 2519

Chromate 5083

Chromate 7039

Chromated Panels with Primer and Topcoat in Laboratory Immersion

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

Time (weeks)

Log Z

Chromated 2519

Chromated 5083

Chromated 7039
16

Figure 18. Averaged postexposure pull-off coating adhesion strength data. 

The three alloys used in the study (Al alloys 2519, 7039, and 5083) have very different general 

corrosion characteristics. Alloy 2519 is generally very susceptible to pitting corrosion and 

considered difficult to pretreat due to the Cu containing precipitates and constituent particles 

found in the alloy microstructure. Alloy 5083, the most commonly used aluminum armor alloy, 

is very resistant to pitting corrosion, even if the alloy is tested “bare.” Alloy 7039 is somewhat 

susceptible to pitting and can also suffer from stress corrosion cracking. Thus, it is anticipated 

that the conversion coating/organic coating systems will behave differently on each alloy. 

Despite the general corrosion resistance of alloy 5083, some of the pretreatments performed 

poorly on this alloy. As shown in figure 11, most of the alloy/coating combinations showed no 

appreciable creepage from the scribes. However, creepback was observed for both the Turcoat 

and Bulk Chemical pretreatments on alloy 5083. ASTM ratings of the unscribed areas are given 

in figure 12, and, again, two pretreatments (Patclin 3 and grit blasting) showed significant 

performance drops on this alloy. The impedance plots for the coatings on 5083 are shown in 

figure 14, while the postexposure adhesion results are shown in figure 18. Neither the total 

impedance measurements nor the postexposure adhesion results provided a simple explanation 

for this behavior. Overall, several of the nonchromate conversion coatings performed well on 

this alloy, as did the Alodine 1200 control. 

Alloy 7039 displayed the greatest resistance to creepage from the scribes but still had significant 

attack away from the scribe for some pretreatments (figures 11 and 12). The Patclin 2 and gritblasted panels had ratings of 2 and 4, respectively, for unscribed regions. Again, results from 
17

impedance as a function of exposure (figure 15) and postexposure adhesion (figure 18) did not 

provide much insight to this behavior. The impedance values of both these pretreatments were 

within the scatter band. The postexposure adhesion strength of the grit-blasted panels was 

somewhat lower (1515 psi) than the average value of 2367 psi attained by all the alloy 7039 

specimens, but the Patclin 2 adhesion was 2505 psi, which was the third highest. Again, as with 

5083, several of the nonchromate pretreatments performed well, as did the chromate control 

samples. 

As previously mentioned, the high copper contents found in 2000 series aluminum alloys make 

them difficult to pretreat, and alloy 2519 is no exception. Despite this, several of the 

nonchromate pretreatments and the Alodine 1200 control performed quite well when tested in 

conjunction with the organic coating system in outdoor exposure. Five pretreatments showed no 

creepage from the scribe (figure 11), and five pretreatments had ASTM ratings of 9 or 10 for 

unscribed areas. Most surprising was the fact that the grit-blasted panels with no pretreatment 

fell into both these groups and compared extremely well with the chromate controls. Figure 8 

shows scanned images of the grit-blasted 2519 panels after 48 months exposure, showing no 

creepback of the coating whatsoever. The only defect visible was a small blister beside the hole 

used to attach the working electrode contacts for impedance measurements, earning it a rating 

of 9. This result was so surprising that the alloy content of each 2519 panel was reverified via 

energy dispersive spectroscopy in a scanning electron microscope. 

The second area of this discussion will compare the performance results of accelerated corrosion 

tests with the results of the same alloy pretreatment combinations in outdoor exposure. 

Observations of the predominant failure modes is also discussed. The initial evaluation of these 

nonchromate conversion coatings was based on ASTM B117 (10) salt fog and GM 9540P (11) 

data. The outdoor exposure results will attempt to validate the initial findings. 

After 100 cycles of GM 9540P, 7039 had only two cases of blistering near the scribe, fewest of 

the three alloys. Because all of the pretreatments on 7039 performed relatively well compared to 

chromate, no nonchromate conversion coatings emerged as the best for 7039. In outdoor 

exposure, 7039 samples had no measurable corrosion attack initiating at the scribes—again, the 

fewest cases of all three alloys. Corrosion initiating at the edges of samples caused the majority 

of the damage on this alloy in outdoor exposure. Chemcote was marginally better at resisting 

corrosion at the edges than the other nonchromate pretreatments for 7039 in outdoor exposure, 

but none were as good as chromate at resisting undercutting at the edges. 

Accelerated corrosion testing proved to be disastrous for nonchromate conversion coatings on 

2519. Total failure was seen on 2519 with all but three surface treatments—grit blasted, 

chromate, and Chemcote, with chromate performing best. However, as mentioned earlier, the 

2519 samples performed much more admirably in outdoor exposure, with the biggest apparent 

surprise being the grit-blasted 2519 samples with no pretreatment. Although unexpected, the 

impressive performance of the grit-blasted 2519 panels was not in conflict with the accelerated 
18

corrosion results. In the earlier work, the grit-blasted 2519 was one of three that did not 

experience “total failure” after 100 cycles of GM 9540P. Also in agreement was the good 

corrosion performance of the Chemcote on 2519. Turcoat (figure 6) and all the Patclin 

treatments on 2519 did not fair as well, which is also consistent with the previous work. 

The exceptional resistance to general corrosion of 5083 was evident in the results of the 

accelerated testing data. In general, 5083 was clearly the best performer of all three alloys as 

long as it was pretreated. However, when 5083 was grit blasted and without a pretreatment, 

significant blistering occurred. This correlated well with the outdoor exposure data where six 

out of the seven pretreatments on 5083 rated a 9 or above (figure 12) and grit blasted only scored 

a 6 rating. Alodine 1200 performed best in both accelerated corrosion and outdoor exposure 

tests on 5083. However, if not for the corrosion along the edges of the panel, an argument could 

be made for Chemcote and Patclin 2 as the best in outdoor exposure. 

In outdoor exposure, corrosion attack at the scribe was relatively uncommon, contrasting what is 

typically seen in accelerated corrosion tests. Only 5 out of the 24 pretreatment/alloy 

combinations (about 21%) experienced any measurable creepage from the scribe. Thus, most of 

the corrosion that was observed initiated at the edges of the samples. In outdoor exposure, the 

edges and bottom of the panels stayed wetter longer than the sun-exposed top surface. The usual 

“edge effects” (residual stress and thinning of the paint) coupled with ultraviolet embrittling of 

the coating contributed to the edge attack not typically seen in accelerated corrosion chamber 

tests. 

The total impedance of a representative set of samples tested in outdoor exposure are given in 

figures 13–15. There was no real correlation between total impedance and corrosion 

performance of the pretreatment. Several samples had good corrosion performance and low 

impedance, while others had high impedance values with poor corrosion performance. That said, 

samples with high impedance and blistering may indicate that the pretreatment relies on barrier 

properties for protection and offers little resistance to undercutting. If the opposite is true, as in 

the case of chromate, the pretreatment may have inhibiting “self healing” qualities and provides 

better resistance to undercutting, but offers less barrier protection. 

Figures 16 and 17 offer a comparison of the impedance values of the two controls in outdoor 

exposure with the laboratory impedance measurements of similarly prepared control samples in 

laboratory immersion. There appeared to be a similar trend with both sets of data, especially 

with the grit-blasted samples, which eventually leveled off at about 105

 ohms. There was clearly 

more noise in the chromate data for both outdoor exposure and laboratory, perhaps as a product 

of the inhibiting qualities of chromate. Nevertheless, this comparison is offered as a verification 

of the degradation trends of the like-prepared control samples in outdoor exposure. 
19

5. Conclusions 

• The mode of failure in outdoor exposure was distinctly different from that typically seen in 

accelerated corrosion chambers. Corrosion attack at the edges was prevalent in outdoor 

exposure, while attack at scribe was uncommon. 

• All samples maintained good adhesion (above 1500 psi), regardless of corrosion 

performance. A slight correlation between postexposure adhesion and corrosion 

performance was observed within samples of a given surface treatment. 

• Postexposure adhesion was a function of surface treatment and not alloy dependant. 

Samples with a chemical pretreatment tended to maintain adhesion better than those 

without a chemical pretreatment (the exception being grit-blasted 2519 samples where 

postexposure adhesion was comparable to all others, including chromate). 

• Alloy 2519 grit blasted with no pretreatment did surprisingly well, performing best of all 

the samples without a pretreatment, and nearly as well as chromate. 

• Because of its superior ability to retard undercutting and inhibit corrosion, chromate was 

best at protecting the edges of samples as well as guarding against attack at the scribe. No 

other pretreatment was more consistent at protecting all three alloys than chromate. 

• Regardless of the mode of failure observed in outdoor exposure, there was agreement with 

the performance results from accelerated corrosion tests and outdoor exposure. In both 

tests, Brent Chemcote performed best of the nonchromate pretreatments on all three alloys. 
20

6. References 

1. Dante, J. F.; Petry, L.; Peeler, D. T. Comparison of Laboratory Corrosion Test Methods and 

Outdoor Corrosion Test for Air Force Coating Systems. 1999 Tri-Service Corrosion 

Conference Proceedings, Myrtle Beach, SC, November 1999. 

2. Nowak, D. Weldability Study of 2519-T87 Aluminum Armor; U.S. Army Materials 

Technology Laboratory: Watertown, MA, May 1990. 

3. Pickens, J. Aluminum Armor 2519: A Review of Literature Relevant to the Advanced 

Amphibious Assault Vehicle; technical report no. 99-95; Concurrent Technologies Corp.: 

Johnstown, PA, 13 January 2000. 

4. National Center for Manufacturing Sciences. Alternatives to Chromium for Metal Finishing; 

final report NCMS no. 0273RE95; Ann Arbor, MI, October 1995. 

5. Placzankis, B. E.; Beatty, J. H.; Kelley, J. V.; Krebs, L. Evaluation of Nonchromate 

Conversion Coatings on Aluminum Armor Alloys; paper no. 532; National Association of 

Corrosion Engineers (NACE) NACE International: Houston, TX, 1997. 

6. MIL-C-53039A. Coating, Aliphatic Polyurethane, Single Component, Chemical Agent 

Resistant 1993. 

7. MIL-P-53022. Primer, Epoxy Coating, Corrosion Inhibiting, Lead and Chromate Free 

1992. 

8. ATSM D 1654-79A. Standard Method for Evaluation of Painted or Coated Specimens 

Subjected to Corrosive Environments. Annu. Book ASTM Stand. 1984. 

9. ATSM D 4541. Standard Test Method for Pull-off Strength of Coated Specimens Subjected 

to Corrosive Environments. Annu. Book ASTM Stand. 1995. 

10. ASTM B117. Standard Method of Salt Spray (Fog) Testing. Annu. Book ASTM Stand. 

1990. 

11. GM 9540P. Accelerated Corrosion Test. General Motors Engineering Standards 1997.
21

Appendix A: Sample Designations, Pretreatments, and Alloy Combinations 
22

Table A-1. Energy dispersive spectroscopy verification of aluminum alloy substrates. 

Panel Identification Set No. 

Alloy 

Element 

Alloy 

Designation Pretreatment Type 

a Cu 2519 Grit blasted/no conversion coating 

A 

b Cu 2519 Grit blasted/no conversion coating 

a Mg 5083 Grit blasted/no conversion coating B 

b Mg 5083 Grit blasted/no conversion coating 

a MgZn 7039 Grit blasted/no conversion coating C b MgZn 7039 Grit blasted/no conversion coating 

a Cu 2519 Chromate - Alodine 1200 

D b Cu 2519 Chromate - Alodine 1200 

a Mg 5083 Chromate - Alodine 1200 

E b Mg 5083 Chromate - Alodine 1200 

a MgZn 7039 Chromate - Alodine 1200 

F b MgZn 7039 Chromate - Alodine 1200 

a MgZn 7039 Nonchromate - Brent Chemcoat L497260A G b MgZn 7039 Nonchromate - Brent Chemcoat L497260A 

a Mg 5083 Nonchromate - Brent Chemcoat L497260A 

H b Mg 5083 Nonchromate - Brent Chemcoat L497260A 

a Cu 2519 Nonchromate - Brent Chemcoat L497260A 

I b Cu 2519 Nonchromate - Brent Chemcoat L497260A 

a Mg 5083 Nonchromate - Turcoat Alcoat 6787 

J b Mg 5083 Nonchromate - Turcoat Alcoat 6787 

a MgZn 7039 Nonchromate - Turcoat Alcoat 6787 

K b MgZn 7039 Nonchromate - Turcoat Alcoat 6787 

a Cu 2519 Nonchromate - Turcoat Alcoat 6787 

L b Cu 2519 Nonchromate - Turcoat Alcoat 6787 

a Mg 5083 Nonchromate - Patclin 1910A 

M b Mg 5083 Nonchromate - Patclin 1910A 

a MgZn 7039 Nonchromate - Patclin 1910A 

N b Cu 2519 Nonchromate - Patclin 1910A 

a Cu 2519 Nonchromate - Patclin 1910A 

O b Cu 2519 Nonchromate - Patclin 1910A 

a MgZn 7039 Nonchromate - Patclin 1910B 

P b MgZn 7039 Nonchromate - Patclin 1910B 

a Mg 5083 Nonchromate - Patclin 1910B Q b Cu 2519 Nonchromate - Patclin 1910B 

a Cu 2519 Nonchromate - Patclin 1910B 

R b Cu 2519 Nonchromate - Patclin 1910B 

a Mg 5083 Nonchromate - Patclin 1910C S b Mg 5083 Nonchromate - Patclin 1910C 

a MgZn 7039 Nonchromate - Patclin 1910C 

T b MgZn 7039 Nonchromate - Patclin 1910C 

a Cu 2519 Nonchromate - Patclin 1910C 

U b Cu 2519 Nonchromate - Patclin 1910C 

a Cu 2519 Nonchromate - Bulk Chemical E-CLPS 923 

V b Cu 2519 Nonchromate - Bulk Chemical E-CLPS 923 

a MgZn 7039 Nonchromate - Bulk Chemical E-CLPS 923 

W b MgZn 7039 Nonchromate - Bulk Chemical E-CLPS 923 

a Mg 5083 Nonchromate - Bulk Chemical E-CLPS 923 

X b Mg 5083 Nonchromate - Bulk Chemical E-CLPS 923 
23

Appendix B: Postexposure Pull-off Adhesion Results With Statistics 

Table B-1. ASTM D 454112 coating adhesion lb/in2 (psi).

Test Sample Reading No. Statistics 

Alloy Pretreatment 1 2 3 4 5 6 Average Std. Dev. 

2519 Grit blast 1710 1750 2800 2400 — — 2165 528 

2519 Chromate 2500 2650 2400 1680 — — 2308 431 

2519 Chemcote 2320 2700 2790 2200 — — 2503 287 

2519 Turcoat 2010 2100 1350 2100 — — 1890 362 

2519 Patclin 1 1920 1810 2280 2450 2200 2050 2118 237 

2519 Patclin 2 3200 2800 2560 2850 2700 2510 2770 249 

2519 Patclin 3 2820 2600 2980 3200 — — 2900 254 

2519 Bulk Chemical 2200 2640 2500 2710 — — 2513 226 

5083 Grit blast 1800 1490 1800 2300 — — 1693 478 

5083 Chromate 2600 2950 2700 2710 — — 2740 149 

5083 Chemcote 2410 1700 2390 800 — — 1825 759 

5083 Turcoat 2150 2220 2350 2800 — — 2380 292 

5083 Patclin 1 1890 2000 2580 2450 — — 2230 336 

5083 Patclin 2 2190 1900 — — — — 2045 205 

5083 Patclin 3 1620 2200 2910 2530 — — 2315 547 

5083 Bulk Chemical 2590 2450 2500 2400 — — 2485 81 

7039 Grit blast 1780 1400 1480 1400 — — 1515 181 

7039 Chromate 2310 2400 2690 2090 — — 2373 249 

7039 Chemcote 2950 2000 2700 2275 — — 2481 425 

7039 Turcoat 2500 2025 2300 2400 — — 2306 205 

7039 Patclin 1 2690 2300 — — — — 2495 276 

7039 Patclin 2 2480 2600 2490 2450 — — 2505 66 

7039 Patclin 3 2310 2450 2890 2710 — — 2590 260 

7039 Bulk Chemical 2840 2900 2620 2600 — — 2740 152 

 1 ATSM D 4541. Standard Test Method for Pull-off Strength of Coated Specimens Subjected to Corrosive Environments. 

Annu. Book ASTM Stand. 1995. 
24

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25

Appendix C: Failure Mode Recording for Each Pull-off Test 
26

Table C-1. Pull-off adhesion test failure modes. 

Panel Identification Set No. First Reading Second Reading 

a Substrate Substrate 

A 

b Substrate Adhesive/substrate 

a Substrate Substrate B b Substrate/adhesive Substrate/adhesive 

a Substrate Substrate C b Substrate Substrate 

a Adhesive/intralayer Adhesive/substrate D b Adhesive Adhesive 

a Adhesive/substrate Intralayer E b Intralayer Interlayer 

a Substrate/adhesive Adhesive F b Adhesive/substrate Substrate 

a Substrate Substrate G b Adhesive/substrate Adhesive/substrate 

a Intralayer/substrate Intralayer/substrate H b Substrate Substrate 

a Substrate Intralayer I b Adhesive Adhesive 

a Adhesive/substrate Adhesive/substrate J b Substrate Adhesive/substrate 

a Adhesive/substrate Substrate/adhesive K b Substrate Adhesive/substrate 

a Substrate Substrate L b Substrate/adhesive Substrate/adhesive 

a Adhesive/substrate Adhesive/substrate M b Intralayer Adhesive/substrate 

a Adhesive Adhesive N b Substrate Substrate 

a Substrate/adhesive Adhesive/intralayer O b Substrate Substrate 

a Intralayer Intralayer P b Adhesive/substrate Adhesive/substrate 

a Substrate Substrate Q b Adhesive Adhesive 

a Adhesive/substrate Adhesive/substrate R b Adhesive Intralayer 

a Substrate/adhesive Substrate S b Adhesive/substrate Adhesive 

a Adhesive Adhesive T b Adhesive Adhesive 

a Adhesive Adhesive U b Adhesive Adhesive/substrate 

a Adhesive Adhesive/substrate V b Intralayer Intralayer 

a Adhesive/intralayer Adhesive/substrate W b Substrate Adhesive/substrate 

a Adhesive Substrate X b Intralayer Adhesive 
NO. OF NO. OF 

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