Long-Term Performance of Cadmium Alternatives

120-Cycle galvanic response: AA 5083 to E/M 6386 gray

Cosmetic Response: Cd at 120 cycles

120-Cycle galvanic response: AA 5083 to Zn/Ni (11%)

120-Cycle galvanic response: AA 5083 to E/M 6386 black

Cosmetic Response: Tin/zinc 120 cycles

120-Cycle galvanic response: AA 5083 to Sn/Zn

120-Cycle galvanic response: AA 5083 to Dorrltech over Zn plate

120-Cycle galvanic response: AA 5083 to Cd (yellow)

Cosmetic Response: Dorrltech 120 cycles over zinc-rich

Cosmetic Response: Zinc/nickel 120 cycles

120-Cycle galvanic response: AA 5083 to Dorrltech over Zn rich

Cosmetic Response: Dorrltech 120 cycles over zinc plate

Cosmetic Response: EM 6386 120 cycles

120-Cycle galvanic response: AAS 5083 to Cd (olive drab).

120-Cycle galvanic response: AA 5083 to Zn

Cosmetic Response: Zinc 120 cycles

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This report provides fastener specific data on seven commercially available cadmium substitutes and four lubricants. Priority was given to obtaining a statistically acceptable clamp load and second to obtaining both cosmetic and galvanic corrosion resistance when coupled to aluminum under accelerated service conditions. The corrosion resisting capability and comparative ranking of the various finish systems changed dramatically with test duration. An organic alternative was found to have performance comparable to cadmium in all respects except for electrical conductivity.

Background. Industry has developed numerous coating systems in an attempt to find a performer equivalent to cadmium. Unfortunately, complete engineering data to support their use as direct replacements in all military applications is not available. The Army has a huge variety of tactical systems in its inventory (many of which have predominately cadmium-plated hardware) and a logistics support system that is attempting to minimize seemingly insignificant product variability (the finish on "standard hardware"). To compound the problem, once a yellow chromate finish is applied to an inorganic fastener finish, all finishes look identical when new. Alternate finishes can only be used if they provide the same torque-tension relationship with the standard military torque charts. These same torque charts are used by many commercial businesses as well.

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Loose and missing fasteners have been reported on a wide range of products. Bradley Fighting Vehicles have had problems retaining their bolt-on armor¹; Boeing 737 inspections have detected missing screws² and 1997 Ford Expeditions have had recalls for loose lug nuts³. A primary cause for a loose fastener (other than obvious installer error with an incorrect torque) is insufficient clamp load. Reliance on generic torque charts may be the problem. This situation is more prevalent than one might imagine since both cadmium- and zinc-plated fasteners in common installations will have clamp-load problems if certain conditions are not maintained. One way to establish if your applied torque is statistically adequate to achieve the necessary clamp load is to determine the statistical data spread at plus/minus three sigma. Finishes that do not provide clamp load through the statistical confidence range have the potential for working loose if the applied load exceeds the clamp load and a prevailing torque (locking) feature is not provided.

There are a large number of variables that will directly affect the applied clamp load. These variables all impact the amount of friction created in making the connection. The more energy that goes into overcoming friction, the less that is available for obtaining clamp load. Consequently, a statistical spread rather than single values are more appropriate for defining this outcome. The obvious variables include type and class of thread, size and grade of fastener, finish material properties, finish thickness and uniformity, type and concentration of lubricant applied and whether a prevailing or free-running nut is used. Less obvious variables include the condition of the roll dies for forming the threads, surface condition of the bearing surface of the substrate and type of prevailing torque feature used. The number and complexity of these variables makes reliance on any single material value such as coefficient of friction (nut factor, K) an unreliable indicator of clamp-load capability. Although it could be argued that a relatively small K range at low K values (0.11 to 0.13) of the complete fastener finish system could be used to maintain compliance with the military torque charts, it would not preclude the user from breaking the fastener upon installation. Furthermore, it would eliminate perfectly acceptable finish systems with higher K values that have lower torque-tension variability.

The variability associated with this method is obvious when one attempts to conduct a literature search for recommended torque values. A zinc- or cadmium-plated, Grade 8, ½-13 bolt with or without lubricant has a recommended installation torque range of 58 to 127 ft-lb. (Military torque charts recommend 80-90 ft-lb installation torque). Furthermore, few seem to make a distinction between a free-running and a prevailing torque application. For some finishes there is a significant difference in clamp load between these applications. In fact the finishes with the greatest difference in clamp load for the same applied torque are zinc and cadmium.

The accepted practice for reducing torque-tension variability and lowering the nut factor (K) is to add a dry lubricant to the nut. A review of product data sheets for lubricants would lead one to believe that these products are significantly different from one another. Specifications that invoke their use, however, address them in a generic category. There appears to be no industry standard that mandates their use in a prevailing torque application. The military specification that defines prevailing torque nut requirements, MIL-N-45913B (Self-Locking, Hexagon, Prevailing Torque Nuts), requires the use of a dry lubricant on cadmium-plated nuts. This requirement kept the military torque charts for cadmium-plated fasteners on track. When the military specification was "improved" in 1998 as a detail specification, the lubricant requirement was removed. The impact on the torque charts was not considered.

A commercial concern that does not have to rely on a military torque chart would have a different approach for establishing the optimum finish. A finish system should have minimal torque-tension variability throughout the applied torque range for the type and grade of fasteners used. For these finishes torque values can be easily defined that provide a high confidence that both the clamp load had been obtained and the fastener will not break on installation. The major drawback to this pursuit is that data collection and analysis requires sophisticated equipment/software. It is also a relatively expensive endeavor.

Corrosion resistance has been historically benchmarked with a neutral salt spray (NSS) test per ASTM B117 at an arbitrary number of exposure hours. This is a good quality control tool but has minimal merit when trying to establish or qualify performance in a real time correlation. There is also the question of test reproducibility with the NSS. TACOM has conducted comparison tests at different test facilities with identical product and obtained different test results based upon the test chamber. ASTM has recognized this problem and has instituted a recommended practice to standardize the corrosivity of the test. Unfortunately, it is only a recommendation and not a requirement. Both General Motors and Ford have established accelerated corrosion tests that attempt to correlate a specific number of test cycles to years of cosmetic corrosion protection. The GM test appears to be more widely publicized. It has one of the highest statistical correlations of all accelerated corrosion tests to four-year fielded autos in Montreal, Quebec⁴. It also claims approximate correlation of up to 10 years of cosmetic corrosion (80 cycles), and controls the corrosivity of the test conditions with standard coupons that must have a specified mass loss at cycles 8,16, 40 and 80.

The overall corrosion resistance of a bolted connection, however, is not limited to cosmetic corrosion. Other conditions such as stress (preload), galvanic coupling, operational damage, automotive fluid contact and heat can have significant impacts. Although there is no published correlation of the GM test to accelerate these conditions, unofficial, preliminary findings suggest that the GM 9540P (Accelerated Corrosion Test) protocol has a correlation of ~1.0 to 1.5 for galvanic corrosion and a 0.7 to 1.0 correlation for stress-accelerated corrosion.

Zinc alloys have received most of the attention as cadmium alternatives. Much of their corrosion resistance capability was established on pristine surfaces in NSS with an unspecified plating thickness. The high nickel (10-14%), zinc-nickel alloys and tin-zinc were reported to give superior performance.

Even pure zinc has been touted as a cadmium substitute and has actually been converted as such in some military programs as a "cost-savings, environmentally compliant" initiative. A major reservation in the Army community with using the cadmium alternatives was the unknown impact on the torque charts. Organic alternatives to cadmium are being used in most commercial automotive designs. Many military engineers are uncomfortable with an organic replacing an inorganic system. Organic systems are highly dependent on process control to maintain substrate adhesion, adequate thickness and durability. They tend to be applied at a higher thickness (approximately 1.0 mil) which would tend to affect the torque-tension relationship. As externally threaded fasteners are typically rolled to class 2 dimensions, only a ~ 0.3 mil nominal plating build is available to meet class 3 thread tolerances. Previous negative experiences with "brittle" thermoset coatings created a high level of uncertainty if an organic would remain intact with normal handling abuse, installation torque and the highly abrasive, off-road military environment. Many military designs are highly dependent on aluminum for weight savings and corrosion resistance. There is no data available to assess the impact of these replacements in a galvanic couple. One published article⁵ reported the electrochemical potential of the high nickel, zinc-nickel alloy at _0.80 V which would indicate a minimal galvanic interaction with our aluminum armor that is typically AA5083 at a potential of _0.87 V.

The $100K research grants are a rare commodity in DoD these days. There were many questions, a directive to replace cadmium wherever practical (but no significant funding to support the change) and enough concerned parties with the talent, facilities and dedication to get the answers. This report summarizes the cooperative efforts of eight organizations to accomplish this end.

Test Format. The direction for this effort was to obtain engineering information relative to actual product obtained with normal manufacturing processes and not laboratory controlled samples. These samples would be procured, handled, assembled and tested in a manner that would duplicate as close as possible the actual service conditions. Rather than controlling the variables that affect the outcome of the numerous tests involved, factual data would be gathered and reported to define the conditions under test. The GM 9540P accelerated test protocol was selected as the test mechanism not only for the amount of published data relative to the test but also because it was already widely used by the automotive supplier base. The test applies a 1.25% salt solution as a mist in four consecutive intervals over a 4.5 hr period followed by eight hours in high humidity @ 49 ± 2C and an eight-hour dryoff at 60 ±2C. A 24-hr test period constitutes one test cycle. Test samples would be evaluated at 80 and 120 cycles of accelerated testing. The nominal life of an Army system is 25 years including at least two overhauls/upgrades. A 10-year (80-cycle) prediction would be totally inadequate in this context. It is highly unlikely that the corrosion rates would be linear when extending the test duration past 80 cycles, but the 120-cycle duration appeared more appropriate for military applications.

The ½ -13, Grade 8 bolt with a flange nut was selected as the test candidate. It is one of the most widely used bolts in the Army inventory for high-strength connections. The torque chart recommends an installation torque of 90 ft-lb for all applications. The equivalent metric size would be an M12-1.75, Grade 10.9 with an installation torque of 80 ft-lb.

Both free-running and prevailing torque applications were evaluated. The free-running hex and hex flange nuts are Grade 8. The prevailing torque hex nuts (all metal type) are Grade C; the prevailing torque flange nuts (all metal type) are Grade G. The flange nut was selected over the conventional hex for four reasons. 1) It is extremely difficult to barrel process the organic finishes on flat surfaces such as flat washers. They tend to stick together during the curing cycle. 2) A hardened washer or flange nut is required in most fastener connections. 3) The flange nut creates a large bearing surface that would make the galvanic reaction on the aluminum surface easier to evaluate. And 4) it was believed that the galvanic interaction would be restricted to the bearing surface of the flange nut and the top periphery of the flange. This would leave the flats and the top of the flange nut unaffected by the galvanic reaction and intact for the cosmetic corrosion evaluation.

The aluminum substrate was aluminum armor, AA 5083. There were 12 aluminum fixtures (18 × 2.5 × 1.38 inch) that would accommodate 12 through-bolted connections each. The bolt hole diameter was 0.525 ± .002 inch. The test fixtures received a chromate conversion coating per MIL-C-5541 prior to fastener installation. The twelve test fixtures were the maximum allowable that could be tested in the ACT chamber at one time. The test fixtures were rotated 180 degrees and progressively moved from the center to the sides of the test chamber every 20 cycles starting with cycle 40 to equalize the conditions. This action was necessary as differential corrosion rates were noted at the cycle-40 inspection interval. The test fixtures and their fastener systems were prepared in duplicate so that a separate evaluation could be made at both 80 cycles and 120 cycles. There would be six replicates of each system in both a prevailing torque and free-running application (except for the zinc and zinc-nickel finishes where three each were tested). The controls were cadmium with a yellow chromate finish and also cadmium with an olive drab chromate finish. The olive drab finish is more corrosion resistant due to the thicker chromate and is preferred with the standard Army camouflage topcoat system. Should the chemical agent resistant (CARC) topcoat chip-off, the olive drab finish is more visually compatible than the reflective yellow chromate finish.

Both free-running and prevailing torque applications were tested for two reasons. 1) We needed to determine what impact the prevailing torque feature had on the integrity of the fastener finish relative to corrosion resistance. 2) A break-loose torque test would be performed at each test interval to get a quantitative assessment of the impact of the corrosion products on fastener removal. We were uncertain if the prevailing torque fasteners could be broken loose without fracture after 120 cycles. All bolts in the test fixtures were torqued to 90 ft-lb with a calibrated, dial torque wrench. Actual preload or preload relaxation was not determined. The break-loose test was performed with the same tool.

The cadmium-plated hardware was purchased to Federal specification, QQ-P-416, Class 2, Type 2. The zinc alloy finishes were purchased to GM specification, GM6280M (Corrosion Protective Coatings-Zinc Alloy Plating). The zinc plating was purchased to ASTM B633, Class Fe/Zn 8, Type 2. These specifications dictate a minimum plating thickness of eight microns and a supplementary chromate finish. The intention was to assure that the finishes were procured to the maximum level of performance by applying the maximum allowable plating thickness. The actual plating thickness was verified on flat significant surfaces and documented. The organic finishes are not held to a minimum thickness but to performance. The organic finish film thickness was taken from cross-sectioned, metallographically polished and etched samples. Prevailing torque nuts were specified with a dry lubricant of the plater's choice. Based upon previous data, the Sn/Zn finish was not lubricated for the PT (locknut) application. The only organic finish that came with a factory applied lubricant was the gray E/M 6386, PT nut. The lubricants involved in this study were Adrem 615 (AD 615), Everlube 8000, Jon Cote 615 (JC 615) and MACuGUARD.

Cosmetic corrosion resistance was evaluated qualitatively from two perspectives. The ability of the finish system to protect the substrate and its resilience to installation damage (resistance to red corrosion products) was the first standard. The overall resistance of the finish to degrade (white corrosion products) combined with the red corrosion products define the overall appearance of the fastener over time and was the "cumulative" standard for ranking corrosion resistance.

The extent of the galvanic corrosion was established quantitatively. As there are no established standards for acceptance, the guidance provided in ASTM G46, Examination and Evaluation of Pitting Corrosion, was used to prepare and evaluate the aluminum surfaces. The microscopic method was used at 50X to determine both the maximum pit depth as well as the total pitted volume under and adjacent to the contact area with the flange nut. The total average pitted volume was calculated by multiplying the deepest pit depth (average) by the pitted area (average).

Preload/clamp load data and statistical spread were based upon 12 replicates. Bolts and nuts had the finish as specified, but in all cases the square test washer was zinc plated. Test format was per SAE J174 for the free-running applications and IFI 100/107 for the prevailing torque applications. The tension range was established for the same applied torque. A GSE-Model FTS 860, Fastener Test System was used with the data analysis by the Keithley "Asyst" Scientific Program. From a practical point of view, the maximum clamp load must be below the minimum required tensile strength, which is 21,285 lb for this Grade 8 bolt. Clamp load in excess of this value has the potential for fastener fracture on installation. The minus three sigma value could be conservatively established at 12,750 lb, which is nominally determined at 75% of the yield strength. A major manufacturer has had no field problems with a -3 sigma of 12,000 lb and this value was used as the approximate low limit for this study. The coefficient of friction (a.k.a. nut factor K) was also reported for information at the noted torque.

The final test was electrical conductivity after each test interval. This is an important aspect if electromagnetic interference (EMI) or grounding is a concern.

A cost factor was also included for all nine finishes based upon major sources of supply in the Detroit area. It was not our intention to promote "gold plated" commercial hardware. Performance must be prorated against product cost.

Test results and data. The scope of the project increased as a result of two unforeseen variables: 1) The plating thickness of both the Sn/Zn and Zi/Ni alloy finishes was found to be excessively high (close to 0.001 inch) on the original hardware. A free-running nut could not be advanced more than one revolution by hand. The plating specifications do not limit the maximum thickness. Although new hardware was plated, we tested the impact of the high plating build on the Sn/Zn for a comparison to the "nominal" build. This could be a common situation with a fastener purchase noting the lack of control in the specifications. 2) We noticed a significant difference in the torque-tension relationship with the various lubricants. We tested systems with and without a lubricant and also compared different lubricants over similar plating thicknesses.

The data is presented in the attached spreadsheets and tables. Corrosion rankings are provided at both the 80 and 120 cycle ACT interval. Only the best and worst finishes maintained their same ranking with respect to resistance to cosmetic corrosion and galvanic corrosion at both intervals. With regard to cosmetic corrosion resistance, the cadmium finishes were always the best; the zinc with yellow chromate was the worst. Dorrltech over zinc rich had the best resistance to galvanic corrosion, and zinc with yellow chromate was the worst. The other finishes shifted in ranking based upon test duration. As the rankings are comparative, photographs are provided of each system at the 120-cycle interval. The photographs document the worst case of each sample group. A finish may have finished third with respect to cosmetic corrosion but be totally acceptable for general use.

Findings and conclusions. The test data indicate that there is no direct cadmium replacement for all engineering applications. There was no significant difference in cosmetic or galvanic corrosion resistance between the two controls. The criteria for visual acceptability (cosmetic) are quite subjective. Most acceptance standards only evaluate the flat (significant) surfaces of the fastener. The cosmetic, cadmium benchmark was corrosion free on all surfaces. The only finishes that could not be recommended for long-term general use are zinc and both of the E/M finishes. The severe corrosion on the zinc-plated hardware is obvious. The exposed bolt threads on the E/M finished bolts in the PT applications were corroded, and the gray finish was showing signs of disbond at 120 cycles. The galvanic test data (pitted volume) indicate a dramatic increase in corrosion rates — up to two orders of magnitude from the 80 to 120 cycle interval. The only exceptions were the cadmium and Dorrltech over a zinc-rich finish where the increased corrosion of the aluminum substrate was negligible. The primary site for the galvanic attack was adjacent to the flange nut outside diameter and run-off area. The only finish that showed a different attack site was the zinc-nickel finish where both the inside and outside diameter areas directly under the flange nut bearing area were equally attacked. This is a significant finding, as a material loss at the bearing surface will result in loss of preload over time. The galvanic test data for the zinc-nickel finish does not agree with the electrochemical data. The data suggested that rather than acting as an alloy, the zinc is preferentially removed (dezincification). To confirm this suspicion a semi-quantitative, x-ray analysis was performed on the SEM on the 120 cycle, flange nuts. Corrosion attack sites had a nickel content as high as 29% (as compared to the original nickel content of 11% by atomic weight percent). The solution potential of the corroding zinc-nickel has changed over time, and this finish has become extremely cathodic with respect to the aluminum.

Many military programs have EMI or grounding requirements. The organic systems that tested relatively well for corrosion resistance lack this capability. The tin-zinc finish, although able to meet this criterion, had relatively poor results in a galvanic couple with aluminum; this finish does have merit if used in ferrous systems. If electrical conductivity is not an issue, then the Dorrltech finish over zinc-rich base is actually superior to cadmium when the cost factor is introduced. This organic finish has the additional benefit of being applied with a process that does not induce hydrogen embrittlement in the substrate (provided the steel is not pickled prior to the zinc phosphate pretreatment).

The break-loose torque data was more complex than originally anticipated. The best and worst finishes with respect to cosmetic corrosion resistance (cadmium and zinc, respectively) provided the two extremes in change from the 80 to 120 cycle test interval. The Dorrltech finishes and the Gray E/M 6386 in the free-running application had relatively low break-loose torque values at the 80-cycle interval. These low values suggest a possible loss of preload or it may be a by-product of the coating chemistry.

What was totally unexpected was a drop of up to 23% in break-loose torque over time with three of the finishes (Dorrrltech over zinc-rich coated bolt/ Dorrltech over zinc-plated nut, Gray E/M 6386 and zinc-nickel). The Dorrltech system had negligible metal loss at the bearing surfaces. Metal loss without a substantial increase in corrosion products would reduce the preload of the connection and explain the drop. Although the zinc-nickel finish had relatively high metal loss by cycle 120, only the free-running application showed a drop in torque. The prevailing torque application displayed the anticipated reaction of a torque increase of 17%. This increase is probably attributed to the volumetric increase in corrosion products at the thread interface. This same anomaly between applications was also noted in the dual Dorrltech system. Only the gray E/M 6386 showed a slight decrease in torque between the two test intervals for both applications (8-11%).

Many automotive designs are only looking for 10-year performance. The visual appearance of all the finishes, with the exception of zinc plate, is acceptable. The black E/M finish had no red corrosion on the exposed bolt threads. Both the silver/gray E/M 6386 and the Dorrltech/zinc-rich finishes were beginning to develop red rust at the thread run-off area. From the galvanic perspective, all the finishes, with the exception of zinc with a yellow chromate, should provide adequate performance coupled to aluminum. The same electrical conductivity issues still apply if grounding is a concern. The user is only left with cadmium and tin-zinc alloy finishes.

The torque-tension data provided four major conclusions. 1) Separate torque charts are warranted for prevailing torque and free-running applications. Only one (zinc) of the nine systems tested in a free-running application had a statistically acceptable preload using the military torque charts. If the torque charts are to remain unchanged, the appropriate lubricant must be identified for each individual finish. The Army has been using insufficient torque for the free-running cadmium-plated hardware from day one. This explains the loosening phenomena on the applique armor that was fastened with free-running cadmium-plated bolts. 2) There is no such thing as a "generic" lubricant. There are major differences in these products. The optimum lubricant will provide an acceptable preload in both prevailing torque and free-running applications. The fastener user should become totally familiar with the performance aspects of the various lubricants and specify only those lubricants that fit his torque requirements. It may be possible to employ a more lubricious lubricant to rectify a heavy plate condition. 3) A procurement specification must control maximum material condition dimensions with a thread assembly or functional size gage to maintain the validity of the torque chart. 4) The relatively thick, organic finishes do not have a negative impact on torque-tension values. Testing should be conducted on fine threaded fasteners to verify that this statement holds true for all fastener systems.