Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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bake has been found to both relieve hydrogen embrittlement and enhance cor- rosion properties.2 In any case, alkaline zinc-nickel appears to be the stronger can- didate for this application, due to the reduction in required maintenance of the bath and the aforementioned current interest in the properties of this coating. ION VAPOR DEPOSITED ALUMINUM AND ALLOYS Ion vapor deposited (IVD) aluminum is a physical vapor deposition (PVD) process in which a part is placed in a vacuum chamber and glow discharge cleaned. Pure aluminum is then melted in heated ceramic boats until it evaporates and condenses on the part to form a coating. Concurrently, ions from the dis- charge bombard the forming coating to enhance its density. IVD aluminum is a mature process that has been used successfully to deposit a variety of coatings for many years, and has traditionally been one of the most promising technologies for cadmium replacement. It is non-embrittling and galvanically compatible with aluminum substrates. In addition, it has excellent high temperature properties and can be conversion coated. Corrosion resis- tance has been reported to be comparable to, or better than, cadmium in some environments.2,21 even better corrosion protection; IVD aluminum-magnesium alloys with 10% mag- nesium have demonstrated significant pitting corrosion protection.17 Alloying the IVD aluminum coating is reported to provide Past NDCEE work found that aluminum-tungsten and aluminum-molybdenum also demonstrated improved passivation over pure aluminum.6 As mentioned previously, Boeing has qualified IVD aluminum to replace cadmium on component parts made of low strength steel (less than 200 ksi), stain- less steel, aluminum, and copper alloys. In a past TARDEC study, IVD alu- minum demonstrated the best overall performance on aluminum connectors. Specifically, on MIL-C-38999 circular connectors, IVD aluminum performed sim- ilar to or better than cadmium, with lower shell-to-shell resistance, but slightly less corrosion resistance. It was noted that, on MIL-PRF-24308 D-subminiature connectors, cadmium demonstrated the best overall performance, with IVD aluminum being the best performing alternative. It was also noted that on MIL- C-83513 microminiature D-subminiature connectors, IVD aluminum was report- ed to have a significant drawback for use on these connectors. During the IVD process, aluminum coated the entire connector surface (including the phenolic material), causing the pins to be electrically continuous with each other and the connector shell, resulting in shorts and eventual connector failure. As seen above, there are numerous drawbacks to using IVD aluminum for elec- trical connector shells. These include the aforementioned overcoating issues, as well as high start-up and operations costs because the equipment that is used to apply this finish is expensive. Also, while IVD aluminum is not completely lim- ited to line-of-sight coverage, the conventional process cannot "throw" into deep recesses on some parts—particularly holes.1, 5 There are some coating per- formance concerns as well. IVD aluminum coatings display a columnar structure with a high degree of porosity. As a result, the coatings must usually be glass-bead peened to densify the coating and alleviate porosity and corrosion concerns. The NDCEE found that IVD aluminum coatings, even with CCC, provide only mar- ginal cyclic corrosion results19 (see Table 1), underscoring the importance of a dense aluminum coating. Also, like many pure aluminum coatings, IVD alu- minum has also been reported to have poor wear resistance, and has demonstrated 316

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