Failure Analysis, Cadmium Brush Plate Qualification and 

Salt Fog Testing of the AH-64 Shock Strut Mount, 

P/N 7-311113409 

by Marc Pepi 

ARL-TR-2870 November 2002 

Approved for public release; distribution is unlimited. 
NOTICES 

Disclaimers 

The findings in this report are not to be construed as an official Department of the Army position unless 

so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the 

use thereof. 

Destroy this report when it is no longer needed. Do not return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2870 November 2002 

Failure Analysis, Cadmium Brush Plate Qualification and 

Salt Fog Testing of the AH-64 Shock Strut Mount, 

P/N 7-311113409 

Marc Pepi 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
ii 

Abstract 

A failure analysis was performed on a shock strut mount from an AH-64 Apache attack 

helicopter. It was concluded that the failure was attributed to stress corrosion cracking and/or 

corrosion fatigue and had initiated at a region where the protective cadmium plating was worn 

away in service. Based upon this conclusion, it was necessary to qualify two facilities (Hunter 

Army Air Field, Savannah, GA and Ft. Lewis, Dupont, WA) for the cadmium brush plating 

rework of these components found to have worn coatings during routine inspections. The U.S. 

Army Research Laboratory was involved in the qualification process for both facilities, and the 

results of each quality audit are included. Finally, the results of salt fog testing of cadmium 

brush plated specimens, plated at the respective facilities, are also included, as well as a 

comparison of the grain size of the material in the failed component vs. the material from the 

struts plated at each facility.
iii

Contents 

List of Figures v

List of Tables ix

1. Introduction 1

2. Failure Analysis of an AH-64 Shock Strut Mount, Part 7-311113409 1

2.1 Background .....................................................................................................................1 

2.2 Manufacturing Criteria....................................................................................................2

2.3 Miscellaneous..................................................................................................................2 

2.4 Applicable Specifications................................................................................................2 

2.5 AH-64 Shock Strut Mount ..............................................................................................2 

2.6 Visual Inspection/Light Optical Microscopy ..................................................................3 

2.7 Metallography .................................................................................................................9 

2.8 Electron Microscopy/EDS.............................................................................................10 

2.9 Chemical Analysis.........................................................................................................21 

2.10 Hardness Testing ...........................................................................................................21 

2.11 Discussion .....................................................................................................................22 

2.11.1 Maraging 300 Grain Size ..................................................................................22 

2.11.2 Effect of Grain Size on Mechanical Properties .................................................23 

2.11.3 Effect of Grain Size on Resistance to SCC .......................................................23 

2.11.4 Vacuum-Deposited Cadmium Coating .............................................................24 

2.11.5 Hydrogen-Assisted Failures ..............................................................................24 

2.11.6 SCC and CF.......................................................................................................24 

2.11.7 Titanium-Carbonitrides .....................................................................................24 

2.12 Failure Scenario.............................................................................................................25 

2.13 Conclusions ...................................................................................................................25 

2.14 Recommendations .........................................................................................................25

3. References 27
iv

Appendix A. Qualification of Hunter Army Air Field (HAAF), Savannah, GA 

for Cadmium Brush Plating Rework of Shock Strut Mounts 29

Appendix B. Qualification of Ft. Lewis, Dupont, WA for Cadmium Brush Plating 

Rework of Shock Strut Mounts 35

Appendix C. Salt Fog Testing of Cadmium Brush Plated Specimens From 

Hunter Army Air Field (HAAF) and Ft. Lewis 41

Appendix D. Grain Size Comparison of Failed Strut Mount, Hunter Army 

Air Field (HAAF) Specimen and Ft. Lewis Specimen 61

Appendix E. U.S. Army Aviation and Missile Command (AMCOM) 

Deficiency Report 65

Appendix F. Change of Manufacturer’s Engineering Drawing 69 

Report Documentation Page 71 
v 

List of Figures 

Figure 1. Schematic of the strut mount attached to the helicopter and the location of failure. ......3 

Figure 2. Fracture surface of the failed part in the as-received condition (mag. ~1H). .................4 

Figure 3. The fracture surface using oblique lighting. Note the shear lip separating the 

primary and secondary cracks, as well as the beach marks (mag. ~1.4H). ................................4 

Figure 4. Intergranular morphology observed within primary crack using light optical 

microscopy (mag. 30H). .............................................................................................................5 

Figure 5. Intergranular morphology observed within secondary crack using light optical 

microscopy (mag. 30H). .............................................................................................................5 

Figure 6. The only evidence of the green epoxy primer was along this small section of the 

shoulder......................................................................................................................................6 

Figure 7. The only evidence of cadmium on the failed part was in the threads, shown 

here as a white, powdery corrosion product energy dispersive spectroscopy (energy 

dispersive spectroscopy [EDS] confirmed the presence of cadmium) (mag. ~1.5H). ...............6 

Figure 8. Corrosion pits adjacent to the primary origin (mag. 10H)...............................................7 

Figure 9. Corrosion pits adjacent to the secondary origin (mag. 10H). ..........................................7 

Figure 10. Group of pits observed along the spindle of the failed part. Pitting was 

prevalent along the spindle region (mag. 25H). .........................................................................8 

Figure 11. Competing crack path adjacent to primary fracture surface. Note the 

intergranular nature of the crack, as well as the region of intergranular attack 

(mag. 15H)..................................................................................................................................8 

Figure 12. Microstructure of the material showing aged, low-carbon martensite in the 

longitudinal direction. Marble’s Reagent (mag. 50H)...............................................................9 

Figure 13. Microstructure similar to that in Figure 12 shown at higher magnification 

and in the transverse direction. Marble’s Reagent (mag. 100H).............................................10 

Figure 14. Microstructure of the material with grain size reticle overlay. The grain 

measured between nos. 2 and 3. Marble’s Reagent (mag. 100H). ..........................................11 

Figure 15. Microstructure of the material with incremented reticle overlay. The grain size 

measured between 0.005 and 0.01 in. Marble’s Reagent (mag. 100H)...................................11 

Figure 16. Schematic of fracture features as determined through SEM. Numbers in boxes 

correspond to figure locations..................................................................................................12 

Figure 17. Morphology of the primary fracture surface. “Rock candy” was prevalent 

within this region (mag. 100H). ...............................................................................................13 

Figure 18. Morphology of the secondary fracture surface. “Rock candy” was prevalent 

within this region (mag. 100H). ...............................................................................................13 

Figure 19. Magnified view of the intergranular morphology showing secondary cracking 

(mag. 200H)..............................................................................................................................14 
vi

Figure 20. Ductile morphology noted within shear lip between the primary and secondary 

fracture (mag. 1500H). .............................................................................................................14 

Figure 21. Interface between the secondary fracture surface and the shear lip (mag. 50H). ........15 

Figure 22. Interface between the primary fracture surface and the transgranular/ overload 

region (mag. 50H). ...................................................................................................................15 

Figure 23. Morphology of the surface on the same plane as the primary fracture surface 

subjected to fast-fracture, consisting of a transgranular/ ductile mode (mag. 1000H).............16 

Figure 24. Morphology of the surface on the same plane as the secondary fracture surface 

subjected to fast-fracture, consisting of a transgranular/ductile mode (mag. 1000H)..............16 

Figure 25. Transgranular/ductile morphology at higher magnification (mag. 1800H)................17 

Figure 26. Representative EDS spectra of the spindle of the failed part. No evidence of 

cadmium was noted along the spindle. ....................................................................................18 

Figure 27. Representative EDS spectra within the threads of the failed part. Cadmium was 

noted within the threads...........................................................................................................18 

Figure 28. SEM micrograph of polished and etched surface showing titanium precipitate 

(white arrow) within a grain boundary that was subjected to EDS (mag. 2000H)...................19 

Figure 29. EDS spectra showing evidence of titanium within the precipitate shown in 

Figure 28. Other elements constitute the alloy. ......................................................................19 

Figure 30. SEM micrograph of intergranular morphology of the primary fracture, 

showing the titanium-carbonitride precipitate that was examined using EDS (Figure 31) 

(mag. 1500H)............................................................................................................................20

Figure 31. EDS spectra of the Ti (C, N) precipitate shown in Figure 30. Note the presence 

of titanium, carbon, and nitrogen, as well as the alloying elements. .......................................20 

Figure 32. SEM micrograph of intergranular morphology of the primary fracture, showing 

another titanium-carbonitride precipitate (mag. 1300H).........................................................21 

Figure C-1. Intact shock strut mount shown in the as-received condition (reduced 75%)...........41 

Figure C-2. Fixtures used for the stress corrosion testing of bent beam specimens 

(reduced 64%)..........................................................................................................................42 

Figure C-3. Grain size of a representative HAAF specimen (mag. 100H). ..................................45 

Figure C-4. Grain size of a representative Ft. Lewis specimen (mag. 100H). ..............................46 

Figure C-5. Montage of a typical fracture surface of a HAAF specimen (specimen no. 1) 

(mag. 12H)................................................................................................................................47 

Figure C-6. Schematic illustrating a mapping of the fractographic features of the surface 

shown in Figure C-5. Boxes show locations of Figures C-7 and C-8.....................................48 

Figure C-7. Intergranular morphology prevalent on the HAAF specimens, indicative of a 

brittle fracture (mag. 200H)......................................................................................................48 

Figure C-8. Ductile morphology noted on the HAAF specimens, indicative of final fast 

fracture (mag. 1000H). .............................................................................................................49 

Figure C-9. Montage of a typical fracture surface of a Ft. Lewis specimen (specimen no. 3) 

(mag. 12H)................................................................................................................................50 
vii

Figure C-10. Schematic illustrating a mapping of the fractographic features of the surface 

shown in Figure C-9. Boxes represent location of figures......................................................51 

Figure C-11. Transgranular fracture morphology prevalent on the Ft. Lewis specimens 

(mag. 500H)..............................................................................................................................51 

Figure C-12. Intergranular attack noted at origin no. 1 of Ft. Lewis specimen no. 3 

(mag. 100H)..............................................................................................................................52 

Figure C-13. Intergranular attack noted at origin no. 2 of Ft. Lewis specimen no. 3 

(mag. 100H)..............................................................................................................................52 

Figure C-14. Intergranular attack noted at origin no. 3 of Ft. Lewis specimen no. 3 

(mag. 200H)..............................................................................................................................53 

Figure C-15. Higher magnification of the intergranular attack noted at origin no. 1 

(Figure C-12) of Ft. Lewis specimen no. 3 (mag. 200H). ........................................................53 

Figure C-16. Higher magnification of the intergranular attack noted at origin no. 2 

(Figure C-13) of Ft. Lewis specimen no. 3 (mag. 200H). ........................................................54 

Figure C-17. Higher magnification of the intergranular attack noted at origin no. 3 

(Figure C-14) of Ft. Lewis specimen no. 3 (mag. 200H). ........................................................54 

Figure C-18. Mixed mode morphology (intergranular and transgranular) noted close to 

origins within zone A Ft. Lewis specimen no. 3 (see Figure C-10) (mag. 300H). ..................55 

Figure C-19. Morphology noted further from origins (closer to final fast fracture region) 

within zone A Ft. Lewis specimen no. 3 (see Figure C-10) (mag. 200H)................................55 

Figure C-20. Ductile morphology observed in the final fast fracture region (zone B) of 

Ft. Lewis specimen no. 3 (see Figure C-10) (mag. 200H). ......................................................56 

Figure C-21. Chart showing the effect of increased cobalt composition on the 

time-to-failure of maraging 250 steel. Plot compares 8% cobalt, 2% cobalt, 

and cobalt-free maraging 250 steel. Tests were conducted in stagnant 3.5% NaCl 

solution for 1000 hr using proof-ring specimens.....................................................................58 

Figure D-1. Grain size of the failed strut mount with reticle overlay (mag. 100H)......................61 

Figure D-2. Grain size from the intergranular fracture surface of the failed strut mount 

(mag. 200H)..............................................................................................................................62 

Figure D-3. Grain size of the HAAF specimen with reticle overlay. Compare to 

Figure D-1 (mag. 100H). ..........................................................................................................63 

Figure D-4. Grain size from the intergranular fracture surface of the HAAF SCC 

specimen. Compare to Figure D-2 (mag. 200H). ....................................................................63 

Figure D-5. Grain size of the Ft. Lewis specimen with reticle overlay. Compare to 

Figure D-1 (mag. 100H). ..........................................................................................................64 

Figure D-6. Grain size from the intergranular fracture surface of the Ft. Lewis SCC 

specimen. Compare to Figure D-2 (mag. 200H). ....................................................................64 
viii

INTENTIONALLY LEFT BLANK. 
ix 

List of Tables 

Table 1. Chemical composition (weight-percent).........................................................................22 

Table 2. Hardness readings on spindle (Rockwell “C” scale). .....................................................22 

Table 3. Hardness readings on metallographic samples (Rockwell “C” scale)............................23 

Table A-1. Calculated thickness and deflection of HAAF specimens..........................................30 

Table A-2. Specimen plating variables (HAAF). .........................................................................32 

Table B-1. Calculated thickness and deflection of Ft. Lewis specimens......................................36 

Table B-2. Specimen plating variables (Ft. Lewis). .....................................................................37 
x 

INTENTIONALLY LEFT BLANK. 
1 

1. Introduction 

The U.S. Army Research Laboratory (ARL) was tasked by the U.S. Army Aviation and Missile 

Command (AMCOM) to perform a failure investigation of an AH-64 Apache shock strut mount. 

This technical report summarizes the results of this analysis, as well as ARL’s involvement in 

qualifying two Army facilities to cadmium brush plate worn strut mounts (Appendices A and B). 

Also included are the results of salt fog testing cadmium brush plated specimens from the two 

facilities (Appendix C), and grain size comparison of various specimens (Appendix D). 

Appendix E includes an AMCOM Deficiency Report based on the results of ARL’s 

investigation, while Appendix F contains the updated Engineering Drawing, which included a 

material change and the addition of shot peening. The failure analysis report and appendices are 

entitled and dated as follows: 

• Failure Analysis of an AH-64 Shock Strut Mount, Part 7-311113409 (Final version, 18 

May 1998); 

• Qualification of Hunter Army Air Field (HAAF), Savannah, GA for Cadmium Brush 

Plating Rework of Shock Strut Mounts (17 June 1997); 

• Qualification of Ft. Lewis, Dupont, WA for Cadmium Brush Plating Rework of Shock 

Strut Mounts (8 April 1998); 

• Salt Fog Testing of Cadmium Brush Plated Specimens from HAAF and Ft. Lewis (6 July 

1998); 

• Grain Size Comparison of Failed Strut Mount, HAAF Specimen and Ft. Lewis Specimen 

(26 October 1998); 

• AMCOM Deficiency Report as a Result of ARL’s Failure Investigation (22 January 1998); 

• Change of Manufacturer’s Engineering Drawing (undated). 

2. Failure Analysis of an AH-64 Shock Strut Mount, Part 7-311113409 

2.1 Background 

AMCOM requested that ARL and the Weapons and Materials Research Directorate (WMRD) 

perform a failure analysis of an AH-64 shock strut mount, which is a component of the main 

landing gear assembly. The right hand (R/H) mount, which secures the R/H landing gear trailing 

arm to the fuselage failed in flight. It was the first of these mounts to fail during flight. Stress 

corrosion cracking (SCC), which initiates at the transition point between the base of the 

underside of the shaft and the mount base, has been attributed to numerous failures of this item in 

the past. This has been the result of cadmium wearing off and corroding away in service. The 
2

part under investigation failed after 1148 flight hours, and the gross weight of the aircraft at the 

time of failure was 12,040 lb. 

2.2 Manufacturing Criteria

Material: 18% Nickel Maraging Steel per Hughes Material Specification (HMS) 6-1081, 

300,000 psi grade. 

Strength: 280,000 psi Ultimate Tensile Strength (UTS) required per specification HMS 

6-1081. 

 275,000 psi UTS required per MDHS drawing 7-311113409. 

Treatment: Annealed at 1500 °F for 1 hr/in, cooled in air, then aged at 

900 °F for 4–6 hr and cooled in air. 

Coating: Vacuum Cadmium Plating (Finish 61A per EPB 4-230; MIL-C-8837, 

Class 2 equivalent [1]), followed by Epoxy Primer per MIL-P-23377, Type I, 

Class 3 [2].

2.3 Miscellaneous

Aircraft Flight Hours: 1148 

Intended Service Life: Unlimited 

2.4 Applicable Specifications

• McDonnell Douglas Engineering Drawing No. 7-311113409 

• HMS 6-1081 

• McDonnell Douglas Bulletin EPB 4-230 

• McDonnell Douglas Process Specification HP 1-1 

2.5 AH-64 Shock Strut Mount 

The shock strut mount was fabricated in accordance with McDonnell Douglas Engineering 

Drawing No. 7-311113409-2. The component is part of the main landing gear and is located on 

the helicopter as shown in Figure 1. This figure also shows the location of the failure. The 

component is forged from Maraging 300 steel; and after heat treatment and final machining, it is 

cadmium coated using a vacuum deposition process. The part is subsequently painted using an 

epoxy primer. The failure of the part occurred at the radius separating the spindle from the base 

of the component. This region is subjected to the loads generated by the aircraft performing 

level and running landings. ARL examined a similar part, in which the failure occurred at the 

same location and was attributed to SCC and/or corrosion fatigue (CF) [3]. 
3

Figure 1. Schematic of the strut mount attached to the helicopter and the location of failure. 

2.6 Visual Inspection/Light Optical Microscopy 

Figure 2 shows the fracture surface of the sectioned component in the as-received condition. The 

part failed at the machined radius of the spindle section. Figure 3 is a close-up of the fracture 

surface using oblique lighting, showing the fracture features, such as shear lips and beach marks. 

Two distinct cracks were noted and labeled “primary” and “secondary,” based upon the severity 

of corrosion on the fracture surface (the crack with the most severe corrosion was labeled 

primary, while the other was labeled secondary). These two crack fronts initially propagated on 

different planes and converged during fast fracture in the location shown in Figure 3. The size of 

the grains within these intergranular regions was so large that the resultant “rock candy” 

morphology was observed using light optical microscopy. Figures 4 and 5 show the 

intergranular morphology of the primary and secondary cracks, respectively. The green-colored 

primer was nonexistent along the length of the spindle, and the only evidence of the primer 

coating on the entire section was along the small portion of the shoulder shown in Figure 6. 

Visual evidence of the cadmium coating, as noted by white corrosion products, was only 

observed along the threads, as shown in Figure 7, and was not present anywhere else on the 

failed section. These coatings most likely wore off in service. The spindle itself showed areas of 

heavy corrosion and pitting. Figure 8 shows the radius adjacent to the primary origin. Note the 

corrosion pit that was only 0.031 in away from the fracture origin. Corrosion pits were also 

adjacent to the secondary origin, as shown in Figure 9. Corrosion pitting was prevalent along the 

spindle, and Figure 10 shows an example of a typical grouping of these pits within a corroded 
4

Figure 2. Fracture surface of the failed part in the as-received condition 

(mag. ~1H). 

Figure 3. The fracture surface using oblique lighting. Note the shear lip separating the primary and secondary 

cracks, as well as the beach marks (mag. ~1.4H). 

crack front convergence 

beach marks 

primary crack 

shear lip secondary crack 
5

Figure 4. Intergranular morphology observed within primary crack using light 

optical microscopy (mag. 30H). 

Figure 5. Intergranular morphology observed within secondary crack using 

light optical microscopy (mag. 30H). 
6

Figure 6. The only evidence of the green epoxy primer was along this small 

section of the shoulder. 

Figure 7. The only evidence of cadmium on the failed part was in the threads, 

shown here as a white, powdery corrosion product energy dispersive 

spectroscopy (energy dispersive spectroscopy [EDS] confirmed the 

presence of cadmium) (mag. ~1.5H). 
7

Figure 8. Corrosion pits adjacent to the primary origin (mag. 10H). 

Figure 9. Corrosion pits adjacent to the secondary origin (mag. 10H). 

region. Also observed was a competing crack that was adjacent to the primary origin. Figure 11 

shows the intergranular nature of this crack. There was also evidence of intergranular attack 

along this crack path. It was difficult to discern the exact location of the fracture origins; 

however, based upon the heavy corrosion in certain areas on both the primary and secondary 

crack surfaces, the general location of the origins was surmised. 

fracture 

f

fracture 

i i

corrosion pits 

fracture surface

corrosion pit

Fracture origin

fracture surface 

fracture origin 

corrosion pits 
8

Figure 10. Group of pits observed along the spindle of the failed part. Pitting 

was prevalent along the spindle region (mag. 25H). 

Figure 11. Competing crack path adjacent to primary fracture surface. Note 

the intergranular nature of the crack, as well as the region of 

intergranular attack (mag. 15H). 

Intergranular attack 
9

2.7 Metallography 

A section of material was taken approximately 3/4 in away from the fracture plane, such that 

metallographic examination could be performed. A transverse and longitudinal sample were 

polished then etched with Marble’s reagent (4-g CuSO4, 20-mL HCl, and 20-mL deionized 

water). The structure was similar in both orientations and is shown in Figures 12 and 13. The 

structure was typical of Maraging 300 steel, consisting of an aged, low-carbon martensite. In 

addition, the material was fairly clean, exhibiting no gross defects or inclusions and containing 

no appreciable decarburization. However, the structure was not consistent with the prior 

treatment (annealed at 1500 °F for 1 hr/in, cooled in air, then aged at 900 °F for 4–6 hr and 

cooled in air). Section 3.4.1.3 of specification HMS 6-1081 requires a grain size of no. 7 or finer 

with an occasional no. 5 unless otherwise specified for material up to 3 inches in diameter or 

square. The grain size was determined using four different methods: (1) American Society for 

Testing and Materials (ASTM) E112 [4] charts overlaid onto photomicrographs at 100H, (2) 

using a metallograph with grain size reticle, (3) by means of an incremented ruler reticle, and (4) 

examining SEM micrographs of the intergranular fracture. The grain size of this material was 

Figure 12. Microstructure of the material showing aged, low-carbon martensite 

in the longitudinal direction. Marble’s Reagent (mag. 50H). 
10

Figure 13. Microstructure similar to that in Figure 12 shown at higher 

magnification and in the transverse direction. Marble’s Reagent 

(mag. 100H). 

between nos. 2 and 3 at 100H magnification using the ASTM E112 charts over Figure 12. To 

verify this, a photomicrograph was taken using a grain size reticle, as shown in Figure 14. 

Again, the size was between nos. 2 and 3. The ASTM E112 charts indicate that the nominal 

diameter of the average grain is between 0.005 and 0.007 in, respectively for a grain size of 

nos. 3 and 2. A photomicrograph was subsequently taken of this structure at 100H, using an 

incremented reticle (Figure 15). As shown, the grain appear to have diameters ranging from 

0.005–0.01 in, which correlated to the previous measurements. Subsequently, the intergranular 

fracture surface shown in Figure 18 (also see section 2.8) was also examined for grain size using 

the ASTM E112 “Fracture Grain Size” method. This technique is typically used for highhardness, brittle steels with a predominantly martensitic structure, such as tool steels, highcarbon steels and martensitic stainless steels, and should be done with the specimen in the asquenched or lightly tempered condition. Even though the Maraging 300 steel is a low-carbon 

alloy, the results obtained compared favorably to those from the other three methods. From 

Figure 18, the grain size ranged from ~100–200 μm, which correlated to a range of 0.004–0.008 

in. 

2.8 Electron Microscopy/EDS 

The fracture surface of the component was subsequently examined using the scanning electron 

microscope (SEM) in order to determine the fracture morphology and fracture mechanism. The 

fracture surface was ~50% intergranular (the primary crack comprised 20%, while the secondary 

crack was 30%). Figure 16 is a schematic showing the fractographic features of the surface. The 
11

Figure 14. Microstructure of the material with grain size reticle overlay. The 

grain measured between nos. 2 and 3. Marble’s Reagent 

(mag. 100H). 

Figure 15. Microstructure of the material with incremented reticle overlay. 

The grain size measured between 0.005 and 0.01 in. Marble’s 

Reagent (mag. 100H). 
12

 crack front convergence 

 secondary crack plane (transgranular/ductile) 

 24 

 primary crack plane 

 (transgranular/ductile) 

 23 

 22 

 

secondary origin 

 primary crack (intergranular) secondary crack (intergranular) 

 17 19 18 

 primary origin shear lip 20 21 

Figure 16. Schematic of fracture features as determined through SEM. Numbers in boxes correspond 

to figure locations. 

morphology of the both the primary crack and the secondary crack was intergranular, as shown 

in Figures 17 and 18. Note the textbook “rock candy” morphology and the secondary cracking 

typical of a brittle fracture. Figure 19 (taken within the secondary crack fracture surface) shows 

this morphology at increased magnification. Note the evidence of micro-pores, which are typical 

of 18% nickel maraging steel hydrogen-assisted failures [5]. These two crack fronts were 

separated by a shear lip, which failed in a ductile manner, as evidenced by the morphology 

shown in Figure 20. Figure 21 shows the transition zone between the secondary crack and the 

adjacent shear lip that separated the two crack fronts. Figure 22 shows the transition zone 

between the primary crack and the transgranular/overload morphology. This was also typical of 

the transition zone between the secondary crack and the transgranular/overload region. 

Figures 23 and 24 show the transgranular/overload morphology that made up ~45% of the 

fracture surface. Figure 25 shows this morphology at higher magnification. As shown, this 

surface morphology represents very low cycle fatigue, in which the loading produces not only 

the transgranular morphology typical of fatigue, but also ductile dimples indicative of tensile 

overload. No fatigue striations were discernible within the transgranular/overload regions. An 

attempt was made to examine the fracture origins; however, the corrosion, as well as the nature 

of brittle failures, made it difficult to pinpoint the exact sites with 100% accuracy. 
13

Figure 17. Morphology of the primary fracture surface. “Rock candy” was 

prevalent within this region (mag. 100H). 

Figure 18. Morphology of the secondary fracture surface. “Rock candy” was 

prevalent within this region (mag. 100H). 
14

Figure 19. Magnified view of the intergranular morphology showing secondary 

cracking (mag. 200H). 

Figure 20. Ductile morphology noted within shear lip between the primary and 

secondary fracture (mag. 1500H). 
15

Figure 21. Interface between the secondary fracture surface and the shear lip 

(mag. 50H). 

Figure 22. Interface between the primary fracture surface and the transgranular/ 

overload region (mag. 50H). 
16

Figure 23. Morphology of the surface on the same plane as the primary fracture 

surface subjected to fast-fracture, consisting of a transgranular/ 

ductile mode (mag. 1000H). 

Figure 24. Morphology of the surface on the same plane as the secondary 

fracture surface subjected to fast-fracture, consisting of a 

transgranular/ductile mode (mag. 1000H). 
17

Figure 25. Transgranular/ductile morphology at higher magnification 

(mag. 1800H). 

EDS was used in an effort to determine the presence of cadmium on the surface of the spindle. 

Exhaustive EDS spectra were generated along the spindle (and adjacent to the fracture origins), 

without revealing the presence of cadmium. Figure 26 is typical of the spectra generated in these 

regions. In contrast, analysis of the threaded region (where white, powdery corrosion products 

were noted) revealed the presence of cadmium as shown in Figure 27. The wear within the 

threads may not have been as severe as along the spindle, such that the cadmium was still 

somewhat intact. This was evidence that the part had been cadmium coated at one time. 

Paragraph 3.3.3d of Process Specification HP 1-1, “Steel, Nickel-Base and Cobalt-Base Alloys, 

Heat Treatment of,” states that maraging steel bars and forgings shall exhibit an aged martensitic 

structure, fine grained, with no evidence of titanium-carbonitride at the grain boundaries. The 

mounted samples were repolished and etched with ferric chloride (5-g FeCl3, 50-mL HCl, and 

50 mL of distilled water) in order to reveal the austenitic grain boundaries. EDS was 

subsequently used to determine the chemical composition of precipitates noted within the grain 

boundaries. Figure 28 shows an example of one of these precipitates (see white arrow). EDS of 

this precipitate (Figure 29) revealed a small amount of titanium, but no carbon or nitrogen. 

EDS was subsequently conducted on the intergranular fracture surface, in an attempt to locate 

Ti (C, N) precipitates along the grain boundaries. Figure 30 shows a precipitate on a grain 

boundary of the “rock candy” surface that was analyzed. The resultant spectrum is displayed in 

Figure 31. Note the high level of titanium as well as the presence of carbon and nitrogen. This 

was evidence of a titanium-carbonitride at the grain boundaries. Figure 32 shows an additional 

precipitate that revealed a similar spectrum. 
18

Figure 26. Representative EDS spectra of the spindle of the failed part. No evidence of cadmium 

was noted along the spindle. 

Figure 27. Representative EDS spectra within the threads of the failed part. Cadmium was noted 

within the threads. 
19

Figure 28. SEM micrograph of polished and etched surface showing titanium precipitate (white arrow) 

within a grain boundary that was subjected to EDS (mag. 2000H). 

Figure 29. EDS spectra showing evidence of titanium within the precipitate shown in Figure 28. 

Other elements constitute the alloy. 

 

 

Gr boundaries 
20

Figure 30. SEM micrograph of intergranular morphology of the primary 

fracture, showing the titanium-carbonitride precipitate that was 

examined using EDS (Figure 31) (mag. 1500H). 

Figure 31. EDS spectra of the Ti (C, N) precipitate shown in Figure 30. Note the presence of 

titanium, carbon, and nitrogen, as well as the alloying elements. 
21

Figure 32. SEM micrograph of intergranular morphology of the primary 

fracture, showing another titanium-carbonitride precipitate 

(mag. 1300H). 

2.9 Chemical Analysis 

A section of the shock strut mount was analyzed in order to determine the chemical composition 

of the alloy. The material was required to conform to Specification HMS 6-1081. The 

composition was determined by the Leco combustion method (carbon and sulfur) and inductively 

coupled argon plasma emission spectroscopy. The material met the requirements of 

HMS 6-1081, as listed in Table 1. 

2.10 Hardness Testing 

Rockwell hardness testing was conducted on the failed component, as well as the mounted 

samples used for metallography. The part is required to exhibit a hardness between HRC 50–54, 

in accordance with HMS 6-1081. The average of six readings taken along the cylindrical spindle 

conformed to this requirement, as shown in Table 2. 

Table 3 contains the results of hardness testing on the mounted metallographic samples, 

representing the longitudinal and transverse orientations of the component. A total of five 

readings were taken on each sample. The results conformed to the governing requirement. 
22

Table 1. Chemical composition (weight-percent). 

Element Failed Component HMS 6-1081 

Carbon 0.012 0.03 max. 

Nickel 18.5 18.0–19.0 

Cobalt 9.12 8.5–9.5 

Molybdenum 4.88 4.6–5.2 

Titanium 0.65 0.50–0.80 

Aluminum 0.12 0.05–0.15 

Copper 0.23 0.25 max. 

Tungsten 0.036 0.25 max. 

Chromium 0.22 0.25 max. 

Manganese 0.042 0.10 max. 

Silicon 0.081 0.10 max. 

Phosphorus 0.001 0.010 max. 

Sulfur 0.003 0.010 max. 

Zirconium 0.012 0.020 max. 

Boron 0.0026 0.003 max. 

Calcium 0.0014 0.050 max. 

Iron remainder remainder 

Table 2. Hardness readings on spindle 

Rockwell “C” scale. 

Reading HRC 

1 52.5 

2 50.9 

3 52.0 

4 49.8 

5 51.3 

6 52.9 

Average 51.6 

HMS 6-1081 50–54 

2.11 Discussion 

2.11.1 Maraging 300 Grain Size 

The grain size of this material was determined to be approximately twice as large as required. 

The nominal grain size of no. 7 or finer as specified in HMS 6-1081 is to be expected from 

annealing this material at the 1500 °F requirement and can be expected after annealing up to a 

temperature of ~1800 °F, while a grain size of no. 4 can be expected after annealing at 2100 °F, 

and as large as no. 0 after annealing at 2300 °F [6]. It has also been stated that a grain size of no. 

1.5 could be expected at 2100 °F, no. 4 at 1880 °F, and no. 7 after three more cycles at 1880 °F 

[7]. Figure 2 of reference [8] shows the course grain (size no. 2) of 18% Nickel maraging steel 

that had been annealed at 2200 °F. Further, the ASM Source Book on Maraging Steels [9] states 

that grain size remains at no. 7 until the annealing temperature between 1700 °F and 1900 °F was 
23

Table 3. Hardness readings on metallographic samples Rockwell “C” Scale. 

Reading HRC 

Transverse 

1 51.4 

2 51.7 

3 51.4 

4 51.6 

5 51.2 

Average 51.5 

Longitudinal 

1 50.5 

2 51.3 

3 51.9 

4 51.6 

5 52.7 

Average 51.6 

HMS 6-1081 50–54 

reached, and that annealing above 1900 °F increased the grain size rapidly to no. 1 at 2100 °F. 

The general trend is that the grain size increases with increasing annealing temperatures. 

2.11.2 Effect of Grain Size on Mechanical Properties 

It is well documented that increased grain size leads to a reduction in the tensile and fatigue 

properties. However, the UTS of Maraging 300 steel drops only slightly (~10 ksi), as a result of 

annealing temperatures from 1600 °F to 1900 °F [10]. As mentioned earlier, these temperatures 

correspond to grain sizes of approximately no. 7 to no. 4. This is consistent with the fact that the 

hardness of the part under investigation still met the governing requirement. However, fatigue 

life decreases progressively as annealing temperatures are raised above 1400°–2200 °F, and 

fatigue life shows little to no correlation with UTS for this alloy [8]. As the solution annealing 

temperature is raised, the fatigue fractures became increasingly intergranular in nature. It has 

been shown that a large austenite grain size is beneficial to toughness when direct rapid quenches 

from the annealing range is employed [8]. However, this condition is detrimental upon air 

cooling (as required for this alloy) or intermediate holding [11]. 

2.11.3 Effect of Grain Size on Resistance to SCC 

Stavros and Paxton [12] is a study of the SCC behavior of Maraging 300 steel. The material was 

subjected to different annealing temperatures ranging from 1400 °F to 1830 °F (grain size nos. 

9–4). It was concluded that cracking in fine-grained material that was produced by lower 

annealing temperatures branched extensively thus increasing the time to failure relative to the 

nonbranching case. It is highly likely that the large grain noted within the part under 

investigation contributed to the SCC failure. 
24

2.11.4 Vacuum-Deposited Cadmium Coating 

The part is required to be cadmium coated using the vacuum deposition method, per Finish 61A 

per EPB 4-230 (MIL-C-8837, Class 2 equivalent). The coating thickness is required to be 

0.00030-in minimum. ARL attempted to determine whether the coating was indeed vacuum 

deposited, rather than electrodeposited. However, there was not sufficient coating remaining on 

the part to perform this task. An improperly applied, or non-post-baked electrodeposited coating 

could have imparted nascent hydrogen into the component, which is potentially detrimental. The 

vacuum process does not contaminate the subject part with hydrogen, and therefore, would not 

be considered contributory to this type of failure if applied in accordance with the governing 

specification. 

2.11.5 Hydrogen-Assisted Failures 

Maraging 300 steel, because of its high strength, can fracture under low stress when exposed to 

hydrogen in a corrosive environment. Nascent (or atomic) hydrogen is formed by the corrosion 

process (evident on the fracture surface of the part under investigation) and can migrate into the 

material causing failure by embrittlement. This hydrogen permeates into the areas of high-stress 

concentration, such as pits, defects, inclusions, voids or crack tips. The hydrogen required to 

cause embrittlement decreases as the strength of the material increases. Hydrogen can induce 

SCC, and hydrogen-assisted fractures in Maraging 300 steel occurs by intergranular decohesion, 

as exhibited in the material under investigation. 

2.11.6 SCC and CF 

SCC occurs in service only if all three of the following conditions are met: (1) the alloy has to 

be susceptible to SCC, (2) there must be a corrosive environment, and (3) a tensile stress is 

present in the form of actual loading or residual stresses. The morphology of SCC fractures are 

usually intergranular (hydrogen-assisted), but can be transgranular due to anodic dissolution. CF 

occurs when a part is subjected to a corrosive environment that reduces the fatigue resistance. 

CF failures are difficult to distinguish from SCC failures because both exhibit intergranular and 

transgranular morphologies [3]. The fracture surface of the strut mount exhibited beach marks 

that were the result of pauses in crack propagation. Beach marks are usually associated with 

cyclic loading (fatigue), but may also be formed during SCC by differences in the rate or degree 

of corrosion and/or crack growth on different planes.

2.11.7 Titanium-Carbonitrides 

The presence of titanium-carbonitrides [Ti (C, N)] within the austenitic grain boundaries of this 

alloy is prohibited per Specification HP 1-1. This is due to the fact that these intermetallic 

precipitates, formed during cooling or intermediate isothermal holding below 2000 °F (usually 

1500°–1800 °F), have been shown to drastically reduce toughness in this alloy [11]. Evidence of 

these precipitates at the grain boundaries suggested that a higher than optimal annealing 

temperature was most likely used by the contractor. 
25

2.12 Failure Scenario 

The failure of the shock strut mount occurred after the protective vacuum-deposited cadmium 

coating was worn off and corroded away at the radius in service. Corrosion pits developed along 

the radii that were prime crack origination sites. The corrosion process allowed nascent (atomic) 

hydrogen to migrate into the high-strength component causing SCC and/or CF. The part had a 

larger than acceptable grain size, and unacceptable titanium-carbonitride precipitates were 

detected along the grain boundaries, which offered decreased resistance to SCC. 

2.13 Conclusions 

• The cadmium coating along the spindle and radius was worn off and corroded in service. 

• Pitting corrosion occurred along the radius and spindle, leading to a hydrogen-assisted 

failure resulting from SCC and/or CF. 

• The grain size was approximately double the required size, most likely caused from a 

higher than nominal annealing temperature. The large grains offered less resistance to 

fatigue and SCC. 

• Titanium-carbonitrides were detected on the grain boundaries of this material. This phase 

is formed at higher annealing temperatures (as is larger grain size) and tends to embrittle 

the alloy. The phase may have contributed to the intergranular SCC attack. 

2.14 Recommendations 

• Cadmium Plating: Numerous shock strut mounts have failed in the same location. A 

common link to each failure is the fact that the cadmium coating along the spindle and 

radius is worn off and corroded away, leading to corrosive attack. The use of a dry film 

lubricant could reduce wear in these regions. 

• SCC and CF: A stress analysis or instrumented field test could be performed on the part to 

determine the magnitude of loading during worst case scenarios (hard landing, etc.). If 

loading is well below the threshold for Maraging 300, a material with less notch sensitivity 

and greater corrosion may be suitable, such as 200–250 Maraging steel. 

• Heat Treatment: The manufacturer should exercise greater care and workmanship during 

the heat treatment of this alloy. Higher than nominal annealing temperatures have been 

shown to produce larger grain in this alloy, as well as detrimental titanium-carbonitride 

precipitates. These features decrease resistance to SCC and CF. 
26 

INTENTIONALLY LEFT BLANK.
27 

3. References 

1. U.S. Department of Defense. Plating, Cadmium, Vacuum Deposited. MIL-C-8837, 

Washington, DC, 2002. 

2. U.S. Department of Defense. Epoxy-Polymide Primer. MIL-P-23377, Washington, DC, 

2002. 

3. Champagne, V. K., and G. Wechsler. “Failure Analysis of the Shock Strut Mount Located 

on the AH-64 Apache Helicopter.” Letter Report, U.S. Army Ballistic Research Laboratory, 

Aberdeen Proving Ground, MD, 10 May 1991. 

4. American Society for Testing and Materials. “Standard Test Methods for Determining 

Average Grain Size.” ASTM E112, West Conshohocken, PA, 1996. 

5. Engel, L., and H. Klingele. An Atlas of Metal Damage. Englewood Cliffs, NJ: PrenticeHall Inc., p. 130, 1981. 

6. Floreen, S., and R. F. Decker. “Heat Treatment of 18% Ni Maraging Steel.” ASM Source 

Book on Maraging Steels, p. 23, 1979. 

7. Saul, G., J. A. Roberson, and A. M. Adair. “The Effects of Thermal Treatment on the 

Austenitic Grain Size and Mechanical Properties of 18% Ni Maraging Steels.” ASM Source 

Book on Maraging Steels, p. 55 (Figure 4), 1979. 

8. Birkle, A. J., D. S. Dabkowski, J. P. Paulina, and L. F. Porter. “A Metallographic 

Investigation of the Factors Affecting the Notch Toughness of Maraging Steels.” ASM 

Source Book on Maraging Steels, p. 127 (Figure 2), and p. 130, 1979. 

9. Tuffnell, G. W., D. L. Pasquine, and J. H. Olson. “An Investigation of the Fatigue Behavior 

of 18% Nickel Maraging Steel.” ASM Source Book on Maraging Steels, p. 77, 1979. 

10. Carter, C. S. “Fracture Toughness and Stress Corrosion Cracking Characteristics of a High 

Strength Maraging Steel.” ASM Source Book on Maraging Steels, p. 181 (Figure 1), 1979. 

11. Kalish, D., and H. J. Rack. “Thermal Embrittlement of 18% Ni Maraging Steel.” ASM 

Source Book on Maraging Steels, p. 37, 1979. 

12. Stavros, A. J., and H. W. Paxton. “Stress-Corrosion Cracking Behavior of an 18 Pct. Ni 

Maraging Steel.” ASM Source Book on Maraging Steels, p. 179, 1979. 
28 

INTENTIONALLY LEFT BLANK. 
29 

Appendix A. Qualification of Hunter Army Air Field (HAAF), Savannah, GA 

for Cadmium Brush Plating Rework of Shock Strut Mounts 

A.1 Background 

The U.S. Army Research Laboratory (ARL) was asked by the U.S. Army Aviation and Missile 

Command (AMCOM) to qualify Hunter Army Air Field (HAAF), Savannah, GA for cadmium 

brush plating rework of shock strut mount components that are removed from service due to 

worn vacuum cadmium coating. The information that follows was included in the trip report 

sent to Dr. Kirit Bhansali of AMCOM. The qualification took place 8–10 June 1997 by Marc 

Pepi (T.O. no. PEP3283TJ80934) and Sharad Pednekar (T.O. no. PED82102J80964). 

Qualification was performed by the use of three-point bend test specimens (a total of four), tested 

in accordance with American Society for Testing and Materials (ASTM) F519.1

 The four 

specimens were machined from the webbing of the failed AH-64 Landing Support Strut and 

cadmium brush plated by HAAF. The testing was performed to determine whether the brush 

plating process would embrittle this material (Maraging Grade 300 steel) under high stress (80% 

and 92% of the ultimate tensile strength). 

A.2 Brush Plating Process and Theory 

Brush plating, also known as selective plating, is a portable method of applying coatings to the 

localized areas of damaged work pieces and components without using an immersion tank.2

 This 

process is advantageous to the part under investigation in that it will minimize repair turnaround 

time. The process is described in general terms as follows: The operator soaks a plating tool in 

the plating solution. The tool has an adsorbent cloth wrapped around its anode, which is used to 

apply the plating solution to the work area of the part being repaired. A power supply produces 

the direct current necessary for the plating process to occur. Two leads coming from the power 

supply connect to the plating tool and the part being plated, respectively. When the tool is 

placed on the part, plating occurs as a result of the completed circuit. The precleaning operation 

is performed in a similar manner with a cleaning solution. Subsequent to cadmium brush 

plating, the part is dipped into a chromic acid solution to provide additional corrosion protection. 

A.3 Test Plan 

The thickness of each of the four specimens was measured and listed as well as the deflection in 

Table A-1.

 1 American Society for Testing and Materials. “Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of 

Plating Processes and Service Environments.” ASTM F519, West Conshohocken, PA, 1997. 

2

 Surface Finishing. “Brush Plating Applies Coatings On-Site or In-House to Repair Equipment.” 

<www.surfacefinishing.com>, 27 October 1999. 
30 

Table A-1. Calculated thickness and deflection of HAAF specimens. 

Specimen Thickness 

(in) 

Deflection at 80% UTS 

(mils) 

Deflection at 92% UTS 

(mils) 

1 0.101 NA 193 

2 0.103 NA 189 

3 0.101 168 NA 

4 0.102 166 NA 

The deflection was calculated from the following formula, listed within ASTM G393 for 

three-point bend testing. Textbook values for strength were used. 

2

6

H

Ety σ = , (1) 

where 

σ = stress, 

E = Young’s modulus, 

t = specimen thickness, 

y = deflection, and 

H = distance between contact points. 

The mechanical properties of Maraging Grade 300 steel are listed as follows: 

UTS = 275 ksi, 

YS = 270 ksi, 

%El = 4%T, 8%L, and 

E = 26.5 H 106 psi. 

A.4 Synopsis of Essential Information 

Aviation and Troop Command representatives (Mr. Randy McFarland and Mr. William Alvarez) 

accompanied the ARL representative (Mr. Marc Pepi and Dr. Sharad Pednekar) to HAAF. ARL 

had fixturing developed that would test the flat specimens (3/4 H 4 H ~0.100 in) in three-point 

bending per ASTM F519. Cadmium brush plating was performed by Engineering Technician 

John Cook of HAAF on four specimens fabricated from an AH-64 Landing Support Strut. Mr. 

Cook was directed to plate these specimens in the exact manner he would have plated an actual 

component fielded from service. Mr. Cook was certified by the LDC Corporation for plating 

processes involving LDC products. The brush plating method followed was LDC-4803, 

“Process Procedure for Preparing and Electroplating Ultra-High Strength Steel with LDC-4803 

Cadmium S (Low Hydrogen Embrittlement).” Mr. Cook used a portable LDC Model 3025120-

1C power supply, serial no. 1996. The cadmium solution used was clear and appeared relatively 

new. It had the following characteristics, as taken from the label: 

 3

 American Society for Testing and Materials. “Standard Practice for Preparation and Use of Bent-Beam Stress-Corrosion 

Test Specimen.” ASTM G39, West Conshohocken, PA, 1999. 
31

Type: LDC-4803, Cadmium (No Bake) 

Batch: 041597 

Recommended Voltage Range: 6–20 V 

Plating Factor: 0.007 

Manufactured: 4/1/97 

Solution Embrittlement Test Date: 4/15/97. 

The chromate conversion coating solution used had the following characteristics: 

Type: LDC-1721, DPM 3689. 

Batch: 051997 

Shelf Life: Unlimited. 

The first task was to unload specimen no. 3 from a test fixture, which was loaded to 92% of its 

UTS on Thursday, 5 June 1997. It was unloaded at ~0900 hr on 9 June 1997 and exhibited no 

sign of plastic deformation. The dial gauge recorded a value of !0.192 in upon removal of the 

load, which was consistent with the loading deflection imparted on the specimen. 

The first specimen brush plated by HAAF was specimen no. 1. The specimen was immersed 

into a bath of 1,1,1 trichloroethane for degreasing purposes. Mr. Cook attached a cotton batting 

to the graphite-tipped anode and dipped it into electrocleaning solution LDC-01. The power 

supply was set to reverse polarity and ~12 V (10–15 V required). The current ranged from 

5–10 A and varied depending upon the amount of force placed on the specimen. The part was 

cleaned with the solution on all sides, for a total of 0.083 A-hr. The specimen was rinsed with 

tap water and examined. If the water did not break, there was no film remaining, and the part 

was ready for plating. Further cleaning would be necessary if the water did break. The part was 

subsequently cadmium brush plated with the LDC-4803 solution. The voltage ranged from 

11.5–13 V and the current ranged from 3.5–5 A. This process was performed using the 

“forward” polarity setting for a total of 0.141 A-hr on all sides of the specimen. The part was 

again rinsed in tap water. Mr. Cook expressed that no guideline existed, and the cadmium 

plating process was complete when it simply “looked” acceptable. The part was subsequently 

dipped (for ~1 to 2 s) into the LDC-1721 chromate solution, water rinsed, and air blasted. 

Again, this was performed until the part looked visually acceptable. The chromate solution was 

used in the undiluted state by Mr. Cook, who later agreed that the LDC specification required a 

dilution. Because the part was already dipped into the full-strength solution, it was determined 

that an 80% specimen would also be dipped into the full-strength solution. The part was placed 

into the test fixture and loaded to 0.193-in deflection. Specimen nos. 2–4 were prepared in a 

similar manner with the variables listed in Table A-2. 
32

Table A-2. Specimen plating variables (HAAF). 

Feature Specimen no. 1 Specimen no. 2 Specimen no. 3 Specimen no. 4 

Cleaning current (A) 5–10 5–10 5–10a 5–10 

Cleaning voltage (V) 13.5 15 15a 15 

Cleaning duration (A-hr) 0.083 0.087 0.090a 0.172 

Plating current (A) 3.5–5 5–10 5–10a 5–10 

Plating voltage (V) 11.5–13 15–18 15–18a 15–18 

Plating duration (A-hr) 0.141 0.181 0.242a 0.134 

Chromate solution undiluted diluted 50%b diluted 50%b undiluted 

% UTS 92% 92% 80% 80% 

Deflection (in) 0.193 0.189 0.168 0.166 

Load start time 0915, 6/9/97 0950, 6/9/97 1008, 6/9/97 1024, 6/9/97 

a

Estimated. 

b

Diluted to 50% with tap water. Note: Item 16 of LDC solution data sheet states to dilute the solution to 4–9:1 with deionized

water. This was overlooked by Mr. Cook while he performed the brush plating process. 

The loaded parts were placed into a padded storage box and returned to ARL. 

The plating thickness of the cadmium deposited on each specimen was estimated using the 

formula listed in the Tool and Manufacturing Handbook, Volume 3:4

 EAH = (f)(a)(t) (2) 

where 

EAH = estimated A-hr, 

f = plating factor (0.007 A-hr/in2

/10-4 in), 

a = surface area plated (in2

), and 

t = thickness of plating H 1000 (i.e., t = 5 correlates to 0.0005-in-thick plating). 

These calculations are estimates at best, especially for specimen no. 3, for which the EAH was 

estimated, rather than recorded. These thickness estimates are averages and are based on the 

premise that the operator evenly plated the entire specimen. The estimated plating thickness for 

specimen no. 1 was calculated as follows: 

EAH = (f)(a)(t) 

0.141 = (0.007)(0.75 H 4 H 2 + 0.75 H 0.101 H 2 + 4 H 0.101 H 2)(t) 

 t = 0.00029 

The estimated thicknesses were calculated in a similar manner for specimen nos. 2–4 and are 

listed below: 

Specimen no. 1 estimated plating thickness: 0.00029 in, 

Specimen no. 2 estimated plating thickness: 0.00037 in, 

Specimen no. 3 estimated plating thickness: 0.00050 in, and 

Specimen no. 4 estimated plating thickness: 0.00028 in. 

 4

 Society of Manufacturing Engineers. Tool and Manufacturing Engineers Handbook. “Materials Finishing and Coating,” 

vol. 3, pp. 20–44, 1985. 
33

A.5 Results of Testing 

The specimens remained under load for 8 weeks at each loading without failure, and there was 

no sign of cracking. It was determined that HAAF was qualified to perform cadmium brush 

plating rework on the AH-64 Apache shock strut mount. 
34 

INTENTIONALLY LEFT BLANK.
35 

Appendix B. Qualification of Ft. Lewis, Dupont, WA for Cadmium Brush 

Plating Rework of Shock Strut Mounts 

B.1 Background 

The U.S. Army Research Laboratory (ARL) was also asked to qualify Ft. Lewis, Dupont, WA 

for cadmium brush plating rework of shock strut mount components that are removed from 

service due to a worn vacuum cadmium coating. This would ensure that there would be a rework 

facility on the West coast as well as the East coast. The information that follows was included in 

the trip report sent to Dr. Kirit Bhansali of the U.S. Army Aviation and Missile Command 

(AMCOM). The qualification took place 17–18 March 1998 by Marc Pepi (T.O. no. 

PEP3283TE84502). Similar to Hunter Army Air Field (HAAF), qualification was performed by 

the use of three-point bend test specimens (a total of four), tested in accordance with American 

Society for Testing and Materials (ASTM) F519.1 The four specimens were machined from the 

webbing of an intact AH-64 Landing Support Strut, sent to ARL from AMCOM, and cadmium 

brush plated by Ft. Lewis. Again, the purpose of this testing was to determine whether the brush 

plating process would embrittle this material (Maraging Grade 300 steel) under high stress (80% 

and 92% of the ultimate tensile strength). 

B.2 Test Plan 

Four specimens were fabricated from AH-64 Main Landing Gear Mount Part No. 1560-01-341-

6213 7-311113409-2 by ARL. The specimens were three-point bend tested using the fixture 

shown in Figure 1b of specification ASTM G39.2 Deflection was measured using a fixture 

similar to that shown in Figure 3 of that specification. The deflection was calculated from the 

following formula, in accordance with ASTM G39 guidelines: 

2

6

H

Ety σ = , (1) 

where 

σ = stress, 

E = Young’s modulus, 

t = specimen thickness, 

y = deflection, and 

H = distance between contact points. 

Table B-1 lists the calculated deflection based on converted Rockwell hardness values and 

specimen thickness. Hardness testing was used rather than textbook values for strength. 

 1 American Society for Testing and Materials. “Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of 

Plating Processes and Service Environments.” ASTM F519, West Conshohocken, PA, 1997. 

2

 American Society for Testing and Materials. “Standard Practice for Preparation and Use of Bent-Beam, Stress Corrosion 

Test Specimens.” ASTM G39, West Conshohocken, PA, pp. 1–7, 1994. 
36

Table B-1. Calculated thickness and deflection of Ft. Lewis specimens. 

Specimen Thickness 

(in) 

Hardness 

(Avg. of 4) HRC Converted UTS 

(ksi) 

Stress Level 

(%) 

Deflection 

(mils) 

1 0.101 52.8 281 80 175 

2 0.103 51.7 270 80 168 

3 0.101 52.3 276 92 202 

4 0.102 51.6 269 92 192 

B.3 Synopsis of Essential Information—Plated Specimens 

Cadmium brush plating was performed by Mr. Brian Norling of Ft. Lewis. Mr. Norling was 

directed to plate these specimens in the exact manner he would have plated an actual component 

fielded from service. Mr. Norling was not certified to perform brush plating, but was trained 

by certified personnel. The brush plating method followed was a SIFCO∗

 document entitled, 

“Plating With Cadmium Code 2023 Solution.” The specimens were first electrocleaned using 

the LDC Corporation Electroclean -01, with a required voltage range of 10–15 V. The cadmium 

solution used the was SIFCO Code 5070, which has recently replaced SIFCO Code 2023 (each 

solution has the same Federal Stock Number). Although Ft. Lewis did not possess the procedure 

sheet for the Code 5070 solution, ARL was assured that the procedure was the same for the Code 

2023 solution. The Code 5070 solution appeared fresh and clear, and had the following 

characteristics: 

Type: SIFCO Cadmium LHE Code SPS 5070 

Batch: 9605014 

Recommended Voltage Range: 6–12 V small, 8–20 V large 

Plating Factor: 0.007 

Manufactured: 5/2/96 

Tested for Embrittlement: Yes (no date or material listed). 

Ft. Lewis had two chromate conversion solutions. The representative characteristics are listed 

below: 

Manufacturer: LDC Corporation 

Type: LDC-1721, DPM-3689 

Batch: 051997 

Shelf Life: Unlimited. 

Manufacturer: SIFCO 

Type: SIFCO Code 5005 

Batch: Not Listed 

Shelf Life: Unlimited. 

Mr. Norling used a portable SIFCO Power Supply, Model CP60-25-115-1, S/N 92024 for brush 

plating. The number of amp-hours necessary to deposit ~0.0003 in of cadmium was calculated 

 ∗

 SIFCO, 5708 Schaaf Road, Cleveland, OH 44131. 
37

prior to plating. The four specimens were then degreased using toluene in accordance with 

TT-T-548E, and a cheesecloth swab. Mr. Norling had prepared two graphite anodes with cotton 

batting overlays one each for the electroclean solution and the cadmium solution. After the 

specimens were degreased, they were electrocleaned using the LDC-01 solution with the power 

supply on reverse polarity. The specimen was swabbed on the faces, and not the edges. The 

edges were coated by the throwing power of the anode. A voltage of ~10–11 V was used for up 

to 0.0911 A-hr. The electrocleaned specimens were rinsed with tap water, and tested for water 

break. If the water did not break, the samples were prewet with the Code 5070 cadmium 

solution and the power supply was set to forward polarity. A plating voltage of 6–9 V was used 

(SIFCO recommends 9 V, maximum, to avoid burning the specimen) for ~0.2000 A-hr. After 

plating, the specimens were rinsed with tap water and tested for water break. If the water did not 

break, the specimens were blown dry using compressed air. The specimens were then buffed 

using a ScotchBrite∗

 pad (Type A, Very Fine), applying no pressure. The LDC Code 1721 

solution was diluted 4:1 with deionized water (4–9:1 requirement) prior to use and placed into a 

shallow container. The specimens were dipped into the solution for ~2 s, or for as much time as 

was needed to produce a dark gold, uniform coating. Blotched areas were touched up by 

swabbing additional solution. Specimen 1 was mistakenly left in the solution for ~10 s. 

Table B-2 lists the variables for each specimen. 

Table B-2. Specimen plating variables (Ft. Lewis). 

Feature Specimen no. 1 Specimen no. 2 Specimen no. 3 Specimen no. 4 

Cleaning voltage (V) 10.4 10.7 11.2 11.2 

Cleaning current (A) 6.6 max. 8.1 max. 8.6 max. 9.7 

Cleaning duration (A-hr) 0.0697 0.0657 0.0504 0.0911 

Plating voltage (V) 6.4 and 7.4 6.5 and 7.4 6.5 and 7.4 and 

8.0 and 8.8 

6.8 and 7.8 and 

8.2 and 8.8 

Plating current (A) 3.0 max. 3.3 max. 4.9 max. 4.9 max. 

Plating duration (A-hr) 0.1307 0.1231 0.1240 0.1260 

Total (A-hr) 0.2004 0.1888 0.1744 0.2171 

Chromate solution 4:1, 10 s 4:1, 4 s 4:1, 2 s (touchup = 2 s) 4:1, 2 s 

% UTS 80% 80% 92% 92% 

Deflection (in) 0.175 0.168 0.202 0.192 

Load Date 3/17/98 3/17/98 3/17/98 3/17/98 

Specimen 4 was cleaned with denatured alcohol prior to the chromate treatment in an effort to 

reduce the blotching noticed on the first three specimens. It was believed that the blotches might 

have been caused by oil that was in the compressed air line. Mr. Norling created a condensed 

procedure sheet for the specimens. The loaded specimens were placed into a padded storage box 

and returned to ARL for monitoring. As with HAAF, the plating thickness of the cadmium 

deposited on each specimen was estimated using the formula listed on pp. 20–44 of the Tool and 

Manufacturing Handbook, Volume 3.3

 ∗

 ScotchBrite is a registered trademark of the 3M Company. 

3

 Society of Manufacturing Engineers. Tool and Manufacturing Engineers Handbook. “Materials Finishing and Coating,” 

vol. 3, pp. 20–44, 1985. 
38

 EAH = (f)(a)(t), (2) 

where 

EAH = estimated amp hours (A-hr), 

f = plating factor (0.007 A-hr/in2

/10-4 in), 

a = surface area plated (in2

), and 

t = thickness of plating H 1000 (i.e., t = 5 correlates to 0.0005-in-thick plating). 

These thickness estimates are averages and are based on the premise that the operator evenly 

plated the entire specimen. The estimated plating thickness for specimen no. 1 was calculated as 

follows: 

 EAH = (f)(a)(t) 

 0.1307 = (0.007)(0.75 H 4 H 2)(t) (3) 

 t = 0.00031 in. 

The estimated thicknesses were calculated in a similar manner for specimen nos. 2–4, and are 

listed as follows: 

Specimen no. 1 estimated plating thickness: 0.00031 in, 

Specimen no. 2 estimated plating thickness: 0.00029 in, 

Specimen no. 3 estimated plating thickness: 0.00030 in, and 

Specimen no. 4 estimated plating thickness: 0.00030 in. 

The estimated thicknesses indicate that the Ft. Lewis brush plating procedure is highly controlled 

and repeatable because a cadmium coating thickness of 0.0003 in was sought. 

B.4 Synopsis of Essential Information—Plated Strut Mount 

Ft. Lewis was subsequently directed to brush plate an actual component that was in inventory. 

The component had been taken out of service due to a large radius crack. The process was 

optimized by trying various methods. Initially, the part was placed into a SIFCO Turning Head 

apparatus, but the weight imbalance of the part and the delicate nature of the grip prohibited the 

required pressure from being applied to the part by the anode. This led to an uncoated radius 

(where cracking initiates in the field), as evidenced by the inability of the chromate solution to 

react at this region. It was determined that the part would be plated by turning the part by hand 

on the tabletop. This would allow the pressure necessary to coat the radius. Mr. Norling grit 

blasted the area of concern (1.75-in radius) with aluminum oxide and wiped the region with 

toluene. The part was prewet with the preclean solution, then precleaned in reverse polarity at 

11.1 V for 0.0192 A-hr. The part was rinsed with tap water (water break test) and plated at 7.0, 

7.5, 8.0, and 9.0 V for a total of 0.02518 A-hr (0.2501 A-hr was the goal for a coating thickness 

of 0.0003 in). The part was subjected to the water break test again, and air dried. The part was 

hand buffed with a very fine ScotchBrite pad and chromated for 2 s by dipping in the LDS 

solution diluted to 4:1. The area around the radius (which did not receive adequate cadmium 

coverage while the part was in the SIFCO Turning Head) was determined to have ample 
39

cadmium using this method. This may have been because the operator could exert more pressure 

with the rectangular graphite anode as opposed to the Turning Head method where the radius 

was perpendicular to the table. It was determined that the SIFCO chromate solution resulted in a 

more uniform and aesthetically pleasing finish than the LDS chromate solution. The part was 

subsequently air dried, and Mr. Norling recorded the condensed procedure onto a Dalic Process 

Procedure form. The estimated plating thickness for this part was consistent with the goal of a 

0.0003-in-thick coating and was calculated as follows: 

 EAH = (f)(a)(t) 

 0.2518 = (0.007)(12.35)*(t) (4) 

 t = 0.00029 in 

* = the area was calculated as not only the cylindrical region, but also the base that comes into 

contact with the anode. The area of the base was calculated to be: 

 a = π/4(D2

-d2), (5) 

where 

D = 2.375 in, 

d = 1.75 in, and 

a = 2.02 in. 

The area of the cylinder was calculated as follows: 

 a = 2πrh, (6) 

where 

r = 0.875 in, 

h = 1.88 in, and 

a = 10.33 in2

. 

After the part was plated, it was magnetic particle inspected to determine whether the plating 

would mask the discontinuity. The part was subjected to 2000 A for a split second and sprayed 

with fluorescent particles. The discontinuity was revealed as a result of this inspection. 

B.5 Test Results 

The specimens remained under load for 8 weeks at the respective loads. It was determined that 

HAAF was qualified to perform cadmium brush plating rework on the AH-64 Apache shock 

strut mount. ARL furnished Ft. Lewis a procedure sheet to be used while plating the actual main 

landing gear mounts, which is included on the following page: 
40 

AMCOM-APPROVED PROCEDURE FOR PREPARING AND CADMIUM BRUSH 

PLATING AH-64 MAIN LANDING GEAR MOUNTS AT FT. LEWIS, WASHINGTON, 

8 APRIL 1998 

1. Preclean the part. Degrease with a non-flammable solvent. Remove any oxides, corrosion, 

scale or any organic coatings by lightly blasting with aluminum oxide media. Be careful not 

to damage threads with grit blast. Clean off grit blasted region with nonflammable solvent. 

2. Prepare 2-in H 3-in graphite anode with cotton batting, and soak in LDC-01 solution. 

Prepare another 2-in H 3-in graphite anode and soak in SIFCO 5070 LHE cadmium solution. 

3. Prewet the area to be plated with LDC-01 solution. 

4. Electroclean with LDC-01 Reverse (Negative) polarity at 10-15 V, for ~0.05 A-hr. 

5. Water rinse thoroughly with clean tap water. Check to make sure no water breaks occur. 

6. Air dry part with filtered compressed air. 

7. Prewet part with SIFCO 5070 LHE cadmium solution. 

8. Brush plate with SIFCO 5070 LHE cadmium Forward (Positive) polarity at 6–9 V for 

~0.2501 A-hr (for a coating thickness of 0.0003 in). Do not exceed 9 V. 

9. Water rinse thoroughly with clean tap water. Check to make sure no water breaks occur. 

10. Air dry part with filtered compressed air. 

11. Hand buff coated region lightly (apply no pressure) with Type A “Very-Fine” ScotchBrite 

pad. 

12. Dip or swab coated region in LDC-1721 or SIFCO 5005 chromate solution. The LDC 

solution must be diluted 4-9:1, while the SIFCO product is used as-is. Chromate solution 

should be kept on part for only about 2 s, or until a uniform dark gold coating results. 

Ensure radius is adequately chromated. Radius and spotty areas may be retouched by 

swabbing chromate solution. 

13. Rinse off chromate solution in water. 

NOTES: 

1. Use a different anode for precleaning and plating process. 

2. If radius does not react to chromate solution, it is most likely not plated. Part must be 

replated (repeat steps 1–13). 
41 

Appendix C. Salt Fog Testing of Cadmium Brush Plated Specimens From 

Hunter Army Air Field (HAAF) and Ft. Lewis 

C.1 Background 

The U.S. Army Aviation and Missile Command (AMCOM) requested the qualification of two 

facilities for the cadmium brush plating of AH-64 landing support struts (shown in Figure C-1). 

The facilities would perform rework in the spindle to webbing radius that has been the origin of 

many failures over the past decade. As part of the qualification process, the U.S. Army Research 

Laboratory (ARL) performed hydrogen embrittlement testing on specimens that were cadmium 

brush plated by the Hunter Army Air Field (HAAF), Savannah, GA, and Ft. Lewis, WA. Each 

facility used a “no-bake,” non-embrittling solution for plating. The specimens were produced in 

accordance with American Society for Testing and Materials (ASTM) G391 from two separate 

AH-64 Landing Support Struts comprised of Maraging 300 Grade steel. Each specimen 

withstood 8 weeks of ambient exposure (75 °F and 50% relative humidity) at 80% and 92% 

ultimate tensile strength (UTS) loading without failure. Based upon the results of ambient 

loading, the two facilities were qualified to perform cadmium brush plating operations. 

Figure C-1. Intact shock strut mount shown in the as-received condition 

(reduced 75%). 

 1

 American Society for Testing and Materials. “Standard Practice for Preparation and Use of Bent-Beam, Stress Corrosion 

Test Specimens.” ASTM G39, West Conshohocken, PA, pp. 1–7, 1994. 
42

After hydrogen embrittlement testing was completed, the specimens were unloaded and 

subjected to salt fog exposure while reloaded to 75% and 90% of the yield strength. This testing 

was performed in order to gain an understanding of how these coatings would behave in a stress 

corrosion cracking (SCC) environment and to provide exposure data for these two stress levels. 

C.2 Salt Fog Testing 

Salt fog testing was conducted in accordance with ASTM B117.2 A sodium chloride level of 5% 

was used for all testing. Table C-1 contains the results of salt fog testing of the specimens that 

were brush plated at HAAF. One of the HAAF specimens was loaded to 90% yield strength 

while another was loaded to 75% yield strength. The specimens were loaded into the fixtures 

shown in Figure C-2. The final specimen was not plated, and loaded to 90% yield strength. As 

anticipated, the unplated specimen was the first to fail, followed by the 90% loaded specimen, 

then finally, the 75% loaded specimen. 

Table C-1. Results of salt fog testing of HAAF brush-plated specimens. 

Specimen Prior History 

ARL Salt Fog 

Testing Deflection 

(in) 

Salt Fog Duration 

to Failure 

(hr)

a

Comments 

1 

Cadmium brush 

plated, loaded to 92% 

UTS at HAAF 

Loaded to 90% YS, 

placed into salt fog 0.182 428–492 

White corrosion products and 

some rusting at the contact points. 

Some base metal exposed. 

2 

Cadmium brush 

plated, loaded to 80% 

UTS at HAAF 

Loaded to 75% YS, 

placed into salt fog 0.154 1495–1510.5 

Rust of base metal on tensile side, 

white corrosion products on 

compressive side 

3 Unplated/unloaded Loaded to 90% YS, 

placed into salt fog 0.208 124.5–143.5 ~95% of entire surface contained 

general corrosion a

 Hours are estimated because continuous monitoring was not feasible (failed overnight or weekend). 

Note: YS = yield strength. 

Figure C-2. Fixtures used for the stress corrosion testing 

of bent beam specimens (reduced 64%). 

 2

 American Society for Testing and Materials. “Standard Practice for Operating Salt Spray (Fog) Apparatus.” ASTM B117, 

West Conshohocken, PA, 1997. 
43

Table C-2 lists the results of salt fog testing of the specimens that were brush plated at Ft. Lewis. 

A total of two Ft. Lewis specimens were loaded to 90% yield strength while the remaining two 

were loaded to 75% yield strength. 

Table C-2. Results of salt fog testing of Ft. Lewis brush-plated specimens. 

Specimen 

Prior 

History 

Salt Fog 

Testing Deflection 

(in) 

Salt Fog 

Duration to 

Failure 

(hr)

a

Comments 

1 

Cadmium 

brush plated, 

loaded to 

80% UTS at 

Ft. Lewis 

Unloaded after 

8 weeks, then 

loaded to 75% 

YS, placed into 

salt fog 

0.158 5671.5–5766 

Evidence of base metal 

corrosion (red rust ~40% of 

entire surface) as well as, 

greenish streaking. 

2 

Cadmium 

brush plated, 

loaded to 

80% UTS at 

Ft. Lewis 

Unloaded after 

8 weeks, then 

loaded to 75% 

YS, placed into 

salt fog 

0.158 7133.5–7248 

Tension side exhibited 

almost 100% base metal 

corrosion (red rust) with 

greenish streaking. 

Compressive side exhibited 

~40% corrosion. 

3 

Cadmium 

brush plated, 

loaded to 

92% UTS at 

Ft. Lewis 

Unloaded after 

8 weeks, then 

loaded to 90% 

YS, placed into 

salt fog 

0.193 2976–3051.5 

Tension side exhibited 

almost 100% base metal 

corrosion (red rust) with 

greenish streaking. 

Compressive side exhibited 

~40% corrosion. 

4 

Cadmium 

brush plated, 

loaded to 

92% UTS at 

Ft. Lewis 

Unloaded after 

8 weeks, then 

loaded to 90% 

YS, placed into 

salt fog 

0.189 4320–4393 

Tension side exhibited 

almost 100% base metal 

corrosion (red rust) with 

greenish streaking. 

Compressive side exhibited 

~40% corrosion. a

 Range is provided since continuous monitoring was not feasible (failed overnight or weekend). 

Because the time-to-failure for the Ft. Lewis specimens drastically surpassed that of the HAAF 

specimens, the chemical composition of the HAAF vs. the Ft. Lewis specimens was investigated, 

as well as the hardness, cadmium-coating thickness, grain size, and the fracture morphology. 

C.3 Chemical Analysis 

A section of each of the shock strut mounts used for specimen fabrication was analyzed in order 

to determine the chemical composition. The composition was determined by the Leco 

combustion method (carbon and sulfur) and inductively coupled argon plasma emission 

spectroscopy (remainder of elements). The HAAF material exceeded the requirements for nickel 

as listed in Boeing Mesa Specification HMS 6-1081 (Table C-3), while the Ft. Lewis material 

exceeded the requirements for nickel and cobalt. These levels were verified with rechecks. 
44

Table C-3. Chemical composition (weight percent). 

Element HAAF Component Ft. Lewis Component HMS 6-1081 

Carbon 0.02 0.03 0.03 max. 

Nickel 19.55, 19.18a 19.32, 19.30a 18.0–19.0 

Cobalt 9.34 9.77, 9.73a 8.5–9.5 

Molybdenum 4.94 4.94 4.6–5.2 

Titanium 0.67 0.66 0.50–0.80 

Aluminum 0.15 0.16 0.05–0.15 

Copper 0.15 0.11 0.25 max. 

Tungsten 0.10 0.08 0.25 max. 

Chromium 0.17 0.14 0.25 max. 

Manganese 0.01 0.01 0.10 max. 

Silicon 0.07 0.06 0.10 max. 

Phosphorus 0.005 0.007 0.010 max. 

Sulfur <0.005 <0.005 0.010 max. 

Zirconium <0.005 <0.005 0.020 max. 

Boron 0.0025 0.0025 0.003 max. 

Calcium <0.005 <0.005 0.050 max. 

Iron remainder remainder remainder a

 Recheck value. 

C.4 Hardness Testing 

Rockwell hardness tests were conducted on a representative HAAF specimen, as well as a 

representative Ft. Lewis specimen. The “C” scale was used, which uses a 150-kg major load. 

The results of this testing are included in Table C-4. No significant difference in hardness was 

noted. 

Table C-4. Results of Hardness Testing Rockwell C Scale, 

150 kg. 

HAAF Component 

(in) 

Ft. Lewis Component 

(in) 

53.3 52.6 

53.5 53.2 

53.6 53.2 

53.3 52.8 

53.4 53.7 

53.3 53.1 

54.1 54.1 

53.4 53.6 

53.6 53.4 

53.9 53.2 

Avg. 53.5 Avg. 53.3 

C.5 Cadmium-Coating Thickness 

The thickness of the brush cadmium coating and subsequent chromate treatment was analyzed to 

determine whether this had a role in the difference in time-to-failures. The coating thickness on 

each of the HAAF specimens and the four Ft. Lewis specimens were theoretically calculated 
45

prior to salt fog testing (see qualification reports and Table C-5). As listed, the coating 

thicknesses of the HAAF specimens were similar to those of the Ft. Lewis specimens, with the 

exception of the HAAF specimen that measured 0.00050 in. 

Table C-5. Theoretical cadmium plating thickness of HAAF and Ft. Lewis specimens. 

HAAF Components 

(in) 

Ft. Lewis Components 

(in) 

0.00029 0.00031 

0.00037 0.00029 

0.00050 0.00030 

0.00028 0.00030 

C.6 Metallography 

Representative SCC specimens coated by HAAF and Ft. Lewis were sectioned in order that a 

metallographic examination could be performed. Transverse and longitudinal samples were 

polished then etched with Marble’s Reagent (4-g CuSO4, 20-mL HCl, and 20-mL deionized 

water). The structure was similar in both orientations, consisting of an aged, low-carbon 

martensite. In addition, the material was fairly clean, exhibiting no gross defects or inclusions. 

Paragraph 3.4.1.3 of specification HMS 6-1081 requires a grain size of no. 7 or finer with an 

occasional no. 5 unless otherwise specified for material up to 3-inches in diameter or square. 

The grain size was determined using a metallograph with grain size reticle conforming to ASTM 

E112.3

 It was determined that the grain size was between ASTM nos. 6 and 7 at 100H

magnification for both the HAAF and the Ft. Lewis specimens (Figures C-3 and C-4), and no 

appreciable difference existed between the two. 

Figure C-3. Grain size of a representative HAAF specimen (mag. 100H). 

 3

 American Society for Testing and Materials. “Standard Test Methods for Determining Average Grain Size.” ASTM E112, 

West Conshohocken, PA, 1996. 
46

Figure C-4. Grain size of a representative Ft. Lewis specimen (mag. 100H). 

C.7 Scanning Electron Microscopy (SEM) 

The broken specimens were subjected to SEM to characterize the fracture morphology. The 

corrosion on the fracture surfaces was cleaned ultrasonically in a solution of (Brainerd Chemical) 

Rodine 213 (inhibited HCl). This product has been shown not to attack steel, as concentrated 

HCl would. The montage in Figure C-5 shows a typical fracture surface of a HAAF failed 

specimen. As the schematic map of the HAAF specimen illustrates (Figure C-6), the fracture 

surface was ~75% intergranular (Figure C-7), while the remaining surface consisted mainly of 

ductile dimples (Figure C-8). The fracture surface consisted of one plane and had the appearance 

of a brittle fracture. It was very difficult to pinpoint the exact location of the origin(s) on the 

fracture surface of this specimen. 

Figure C-9 contains a montage of a typical Ft. Lewis failure, while Figure C-10 contains the 

accompanying schematic map. The fracture surfaces of the broken Ft. Lewis specimens 

consisted primarily of a transgranular, quasi-cleavage morphology (Figure C-11). The fracture 

of one of the specimens consisted of two planes with three distinct origins on the tensile side of 

the specimen and had the appearance of a ductile fracture. The surface was predominantly 

transgranular with evidence of intergranular attack and secondary cracking only at each of the 

three origins. Ductile dimples were noted on the compressive side of the specimen, which was 

the last to fail. Figures C-12 through C-14 represent the three origins at 100H magnification, 

while Figures C-15 through C-17 show the origins at higher magnification (200H). Figure C-18 

represents the morphology noted within the center of the specimen (mixed mode; ductile and 

transgranular) as illustrated in Figure C-10, while Figure C-19 represents the morphology of near 

the compressive side of the specimen. Figure C-20 is a high-magnification scanning electron 

micrograph representing the ductile region (final fast fracture) on the fracture surface.
47

Figure C-5. Montage of a typical fracture surface of a HAAF specimen (specimen no. 1) (mag. 12H). 
48

 primarily ductile 

 8 

 7 

 intergranular with secondary cracking 

Figure C-6. Schematic illustrating a mapping of the fractographic features of the surface shown in 

Figure C-5. Boxes show locations of Figures C-7 and C-8. 

Figure C-7. Intergranular morphology prevalent on the HAAF specimens, 

indicative of a brittle fracture (mag. 200H). 
49

Figure C-8. Ductile morphology noted on the HAAF specimens, indicative of 

final fast fracture (mag. 1000H). 
50

Figure C-9. Montage of a typical fracture surface of a Ft. Lewis specimen (specimen no. 3) (mag. 12H). 
51

 Zone B 

 20 

 19 

 

 Zone A Zone A 

 18 

 origin no. 1 origin no. 2 origin no. 3 

 

 12 15 13 16 14 17 

Figure C-10. Schematic illustrating a mapping of the fractographic features of the surface shown in 

Figure C-9. Boxes represent location of figures. 

Figure C-11. Transgranular fracture morphology prevalent on the Ft. Lewis 

specimens (mag. 500H). 
52

Figure C-12. Intergranular attack noted at origin no. 1 of Ft. Lewis specimen 

no. 3 (mag. 100H). 

Figure C-13. Intergranular attack noted at origin no. 2 of Ft. Lewis specimen 

no. 3 (mag. 100H). 
53

Figure C-14. Intergranular attack noted at origin no. 3 of Ft. Lewis specimen 

no. 3 (mag. 200H). 

Figure C-15. Higher magnification of the intergranular attack noted at origin 

no. 1 (Figure C-12) of Ft. Lewis specimen no. 3 (mag. 200H). 
54

Figure C-16. Higher magnification of the intergranular attack noted at origin 

no. 2 (Figure C-13) of Ft. Lewis specimen no. 3 (mag. 200H). 

Figure C-17. Higher magnification of the intergranular attack noted at origin 

no. 3 (Figure C-14) of Ft. Lewis specimen no. 3 (mag. 200H). 
55

Figure C-18. Mixed mode morphology (intergranular and transgranular) noted 

close to origins within zone A Ft. Lewis specimen no. 3 (see 

Figure C-10) (mag. 300H). 

Figure C-19. Morphology noted further from origins (closer to final fast 

fracture region) within zone A Ft. Lewis specimen no. 3 (see 

Figure C-10) (mag. 200H). 
56

Figure C-20. Ductile morphology observed in the final fast fracture region 

(zone B) of Ft. Lewis specimen no. 3 (see Figure C-10) 

(mag. 200H). 

C.8 Discussion 

C.8.1 Fracture Path 

The fracture paths of the HAAF specimens were all intergranular, while those of the Ft. 

Lewis specimens were each mostly transgranular. This was puzzling because the 

specimens were fabricated from the same location on actual components used in service 

that were required to conform to the same specifications, machined to the same 

dimensions at the same facility (ARL), and coated using similar chemicals to similar 

thicknesses. In addition, each specimen passed the hydrogen embrittlement test of eight 

weeks (suggesting the hydrogen content within the steel as a result of the coating process 

was of no concern), and the salt fog testing and fixturing was the same for each specimen. 

A survey of open literature revealed that it has been demonstrated that the fracture path of 

a 300 grade maraging steel can travel either in an intergranular or transgranular mode. 

Most references4, 5, 6

 indicate that the fracture path is almost always intergranular, while 

 4 Stavros, A. J., and H. W. Paxton. “Stress-Corrosion Cracking Behavior of an 18% Ni Maraging Steel.” 

Metallurgical Transactions, vol. 1, no. 11, pp. 1, 5, 7, 9, 10, 12, 13, 17, April 1970. 

5 Dean, W., and H. R. Copson. “Stress Corrosion Behavior of Maraging Nickel Steels in Natural Environments.” 

Proceedings of 20th Annual Conference of the National Association of Corrosion Engineers, pp. 95, 101–103, 

9–13 March, 1964. 

6 Dautovich, D. P., and S. Floreen. “The Stress Corrosion and Hydrogen Embrittlement Behavior of Maraging 

Steels.” Proceedings of the Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys Conference, 

pp. 798–799, 801–804, 810, 812, 12–16 June 1973. 
57

transgranular cracking occurs much less often. Intergranular cracking is characterized by 

a path that follows the prior austenitic grain boundaries, while transgranular cracking 

occurs around the martensitic platelet boundaries. Many references contain explanations 

as to what factors may influence the mode of cracking. These include composition, 

microstructure, environment, annealing, aging effects, and geometry, each of which are 

discussed in the following pages.

C.8.2 Effect of Composition on SCC Fracture Path 

When comparing the results of chemical analysis of both materials, it is evident that only 

the cobalt content was significantly different. As listed in one source, some hardening 

results from a short-range ordering reaction in the matrix involving cobalt, but it appears 

that the main contribution of cobalt is to lower the solubility of molybdenum in the 

martensite matrix and thus increase the amount of Ni3Mo precipitate.7 Cobalt is added to 

maraging steels to lower the Ms temperature and thereby produce a finer microstructure 

according to the ASM Specialty Handbook.8 No difference in hardness or grain size was 

noted between the Ft. Lewis and the HAAF specimens. Other sources9, 10 indicate that 

the primary strengthening effect of this alloy comes from the combination of cobalt and 

molybdenum. Most importantly, however, according to the ASM Specialty Handbook 

cobalt additions increase the time-to-failure for maraging steels in stagnant 3.5% NaCl.8

 

This is illustrated graphically in Figure 21 of the ASM Specialty Handbook).8

 Although 

these data were generated for the 250 Grade (no such data was found for the 300 Grade), 

the specimens were proof-ring type (not bent-beam type), and the specimens were 

immersed in salt water (not within a salt fog environment), it does show the effect cobalt 

has on the time-to-failure of maraging steel. The differences shown graphically in 

Figure C-21 are for 8% nominal cobalt (top curve), 2% nominal cobalt (middle curve), 

and cobalt-free (bottom curve). The tests were conducted in stagnant 3.5% NaCl solution 

for 1000 hr using proof-ring speciments.7

 The specimens in the ARL study only differed 

by 0.43 weight-percent (9.77 minus 9.34 weight-percent). However, with the lack of any 

other contributory factor, it is believed that the difference in cobalt composition may have 

attributed to the extended duration times Ft. Lewis specimens, as compared to the HAAF 

specimens. 

 7 ASM. ASM Metals Handbook, Ninth Edition, Volume 1, Properties and Selection: Irons and Steels. “Maraging 

Steels,” p. 446, 1978. 

8

 ASM. ASM Specialty Handbook, Carbon and Alloy Steels. “Corrosion Behavior,” pp. 465–466, 1996. 

9 Vishnevsky, C. “Literature Survey on the Influence of Alloying Elements on the Fracture Toughness of High 

Alloy Steels.” AMMRC CR 69-18/1, Interim Report, Contract DAAG 46-69-C-0060, Army Materials and Mechanics 

Research Center, Watertown, MA, pp. 31, 32, February 1970. 

10 Psioda, J. A., and J. R. Low, Jr. “The Effect of Microstructure and Strength on the Fracture Toughness of an 

18 Ni, 300 Grade Maraging Steel.” NASA Technical Report No. 7, Carnegie Mellon University, Department of 

Metallurgy and Materials Science, Pittsburgh, PA, p. 13, August 1975. 
58

Figure C-21. Chart showing the effect of increased cobalt composition on 

the time-to-failure of maraging 250 steel. Plot compares 

8% cobalt, 2% cobalt, and cobalt-free maraging 250 steel. 

Tests were conducted in stagnant 3.5% NaCl solution for 

1000 hr using proof-ring specimens. 

C.8.3 Effect of Microstructure on SCC Fracture Path 

It has previously been shown that crack propagation rates within high-strength steels are 

generally faster at higher strength levels and are greatly dependent on microstructure.11 

Generally, maraging steel with a coarse grain size does not offer the time-to-failure 

resistance of finer grained maraging steel. The structures and grain size of specimens 

coated at each facility were similar; therefore, it was determined that the microstructure 

had no effect. 

C.8.4 Effect of Environment on SCC Fracture Path 

Because the pH, temperature, and the sodium chloride concentration for all testing were 

kept at specified levels within the salt fog chamber, it is highly unlikely that the 

environment had a role in the different fracture paths. 

 11 Wanhill, R. J. H. “Microstructural Influences on Fatigue and Fracture Resistance in High Strength Structural 

Materials.” Engineering Fracture Mechanics, vol. 10, p. 351, 1978. 
59

C.8.5 Effect of Annealing on SCC Fracture Path 

It has been shown that coarse grain size is detrimental to SCC resistance for 300 grade 

maraging steels according to Dautovich and Floreen.6

 Subsequent failure occurs along 

the prior austenitic grain boundaries, and results in shorter time-to-failure values. The 

landing support struts are required to be annealed at 1500° ± 25 °F. Higher annealing 

temperatures (in excess of 1500 °F) produce larger grains within the structure of 

maraging steels. Because the grain size of both the HAAF and Ft. Lewis specimens was 

similar, and within specification, it was determined that prior annealing temperatures had 

no effect on the fracture path. 

C.8.6 Effect of Aging on SCC Fracture Path 

Open literature contains the effect of aging treatments and times on the hardness of 

maraging steels.10 The range of aging treatments listed in this reference is from 600° 

to 1000 °F. The required aging range for the shock strut mount is 900 °F ± 25 °F. In 

general, the high strength of maraging alloys is developed by aging,12 and it has also been 

shown that crack rates in a 350 grade maraging steel could be varied over 2 orders of 

magnitude by changes in the aging temperature.11 Therefore, different aging treatments 

could produce material with different hardness levels. However, the fact that the 

hardness measurements were similar for both the HAAF and Ft. Lewis specimens 

indicates that prior aging was not an issue. 

C.8.7 Effect of Geometry on SCC Fracture Path 

Crack path observations in this material are somewhat difficult to interpret because SCC 

attack generally produces intergranular fractures, while transgranular morphologies may 

occur as a result of geometry.13 This does not explain the phenomenon noted by ARL 

because the geometry was similar for each specimen (bent-beam specimens 

4 in H 0.75 in H 0.10 in).

 12 Battelle. Aerospace Structural Metals Handbook. “18 Ni Maraging (300),” Metals and Ceramics Information 

Center, Columbus Division, Columbus, OH, p. 1, 1987. 

13 Jeanfils, C., C. A. Pampillo, and H. W. Paxton. “The Effect of Reverted Austenite on the Stress Corrosion 

Cracking Resistance of Two Maraging Steels.” Office of Naval Research Technical Report N00014-67-A-0314-0004, 

p. 3, November 1971. 
60

C.9 Conclusions 

Although the Ft. Lewis specimens outperformed the HAAF specimens with respect to 

time-to-failure within the salt fog chamber, the specimens were practically similar in 

every way. The specimens were sectioned from two different shock strut mounts, but in 

the same locations. The specimens were loaded into similar fixtures and loaded to 

similar levels. The hardness of both sets of specimens was similar, as was the brush 

cadmium thickness, the grain size, and the microstructure. 

One difference was noted: the Ft. Lewis specimens contained more cobalt than the 

HAAF specimens. Increased cobalt has been shown to increase the time-to-failure of 

maraging steels with respect to SCC. It may be possible that the difference in cobalt 

composition was enough to prolong the time-to-failure of the Ft. Lewis SCC specimens 

in the salt fog environment. 
61

Appendix D. Grain Size Comparison of Failed Strut Mount, Hunter 

Army Air Field (HAAF) Specimen and Ft. Lewis Specimen 

D.1 Background 

The grain size of failed Shock Strut Mount P/N 7-311113409, determined to be 

approximately double the required size (refer to section 2), was compared to the grain 

size of two scrap strut mounts. These strut mounts included one which had been 

sectioned into specimens that were cadmium brush plated at HAAF, Savannah, GA, and 

one that was sectioned for brush plating at Ft. Lewis, WA. The governing material 

specification, HMS 6-1081, 18% Nickel Maraging Steel - Bars and Forgings, 300,000 psi 

(2069 Mpa) Yield Strength Grade, requires a grain size of no. 7 or finer with an 

occasional no. 5. It was determined that the grain size of the failed strut mount was 

between American Society for Testing and Materials (ASTM) nos. 2 and 3. Figure D-1 

shows the grain size of the failed part, with a grain size reticle overlay at 100H

magnification. Figure D-2 shows a scanning electron micrograph of the actual grain size 

observed within the intergranular fracture surface at 200H magnification. 

Figure D-1. Grain size of the failed strut mount with reticle overlay (mag. 100H).
62

Figure D-2. Grains from the intergranular fracture surface of the failed strut 

mount (mag. 200H). 

D.2 HAAF Specimen Grain Size 

For comparative purposes, the grain sizes of the scrap strut mounts were documented in a 

similar manner. Specimens were sectioned from these struts, cadmium brush plated at the 

two facilities, and loaded into a three-point bend fixture in accordance with ASTM G39.1

 The 

test fixtures were subsequently placed into a salt fog chamber and tested in accordance with 

ASTM B1172 until failure. A representative failed stress corrosion test specimen from both 

HAAF and Ft. Lewis was sectioned and metallographically prepared. Figure D-3 shows the 

grain size of the HAAF strut mount with the reticle overlay. These grains measured between 

nos. 6 and 7. Compare to the grains shown in Figure D-1. Figure D-4 shows the fracture 

surface of the HAAF specimen that had failed in the salt fog chamber. Compare these grains 

to those of the failed strut mount (Figure D-2). 

D.3 Ft. Lewis Specimen Grain Size 

Similarly, the Ft. Lewis specimen grain size was documented, as shown in Figure D-5 with 

the reticle overlay. The size of these grains were similar to those observed within the HAAF 

specimen. Also, the fracture surface of the stress corrosion cracking (SCC) specimen is 

shown in Figure D-6 at 200H magnification. Although the mode of failure was mixed mode 

(transgranular, occurring along the martensitic platelets with evidence secondary cracking), 

the cracking occurred at the austenitic grain boundaries, allowing for grain size comparison. 

Again, the grain size was similar to the HAAF specimen. 

 1 American Society for Testing and Materials. “Standard Practice for Preparation and Use of Bent-Beam, Stress 

Corrosion Test Specimens.” ASTM G39, West Conshohocken, PA, pp. 1–7, 1994. 

2

 American Society for Testing and Materials. “Standard Practice for Operating Salt Spray (Fog) Apparatus.” ASTM 

B117, West Conshohocken, PA, 1997. 
63

Figure D-3. Grain size of the HAAF specimen with reticle overlay. 

Compare to Figure D-1 (mag. 100H). 

Figure D-4. Grains from the intergranular fracture surface of the HAAF SCC 

specimen. Compare to Figure D-2 (mag. 200H). 
64

Figure D-5. Grain size of the Ft. Lewis specimen with reticle overlay. 

Compare to Figure D-1 (mag. 100H). 

Figure D-6. Grains from the intergranular fracture surface of the Ft. Lewis SCC 

specimen. Compare to Figure D-2 (mag. 200H). 

D-4. Conclusion 

This study was undertaken in order to compare the grain size previously noted within the 

failed strut mount to that of two additional strut mounts. This comparison is further evidence 

that the failed strut mount contained a grain size that was larger than specified.
65

Appendix E. U.S. Army Aviation and Missile Command (AMCOM) 

Deficiency Report 

As a result of the failure analysis performed by the U.S. Army Research Laboratory, a 

Category I/Category II Deficiency Report was issued by AMCOM. This type of report is 

submitted when there is a critical failure, and in this case, it details a description of the 

problem, and the short- and long-term recommendations concerning the shock strut 

mount. The AMCOM deficiency report is included as an attachment in this appendix.
66
67
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INTENTIONALLY LEFT BLANK.
69

Appendix F. Change of Manufacturer’s Engineering Drawing 

Because this part has had a history of failures, the manufacturer performed an 

engineering study and concluded that a change in the alloy used to make this component 

was necessary. Corrosion resistant precipitation hardenable PH 13-8Mo steel was chosen 

to be the replacement for the high-strength maraging steel. Another change, was that the 

part would now be required to be shot peened (which was not a requirement for the 

maraging steel component). The change to the manufacturer’s engineering drawing is 

shown as an attachment in this appendix. 
70
failure analysis, maraging steel, stress corrosion cracking 77

A failure analysis was performed on a shock strut mount from an AH-64 Apache attack helicopter. It was concluded

that the failure was attributed to stress corrosion cracking and/or corrosion fatigue and had initiated at a region where the

protective cadmium plating was worn away in service. Based upon this conclusion, it was necessary to qualify two

facilities (Hunter Army Air Field, Savannah, GA and Ft. Lewis, Dupont, WA) for the cadmium brush plating rework of

these components found to have worn coatings during routine inspections. The U.S. Army Research Laboratory was

involved in the qualification process for both facilities, and the results of each quality audit are included. Finally, the

results of salt fog testing of cadmium brush plated specimens, plated at the respective facilities, are also included, as well

as a comparison of the grain size of the material in the failed component vs. the material from the struts plated at each

facility.

ARL-TR-2870

U.S. Army Research Laboratory

ATTN: AMSRL-WM-MD 

Aberdeen Proving Ground, MD 21005-5069

Marc Pepi

Failure Analysis, Cadmium Brush Plate Qualification and Salt Fog Testing of the AH80

AH-64 Shock Strut Mount, P/N 7-311113409 

November 2002 Final, 1997–January 2002

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