Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Immersion Time (min) 2 5 8 Coating Weight (mg/ft2 ) 11.4 25.5 43.3 Table 2: Coating Weight of 2024 T-3 Alloy In most cases, corrosion-resis- tance performance of this trivalent chromium pretreatment is equal to or better than conventional trivalent chromates or hexavalent chromates. Results of 168 to 500 hours can be achieved in neutral salt spray testing (ASTM-B117), depending on the aluminum alloy tested. In short, this newly devel- oped trivalent chromium chem- istry offers an overall superior per- formance without any sealer or a topcoat. This novel trivalent chromium treatment has also been used to replace high- and mid-tempera- ture anodizing seals. It has proven to be an environmentally friendly and efficient anodizing seal with- out any hazardous chemicals. Figure 3: EPA = enhanced performance additive. RESULTS AND DISCUSSION Figure 4 Most aluminum alloys were test- ed and able to achieve up to 500 hours of neutral salt spray (NSS) corrosion resistance. The con- centration of trivalent chromium pretreatment material ranged from 15–25% by volume with var- ious time cycles from 3–5 min- utes at ambient temperatures. This new chemistry has been pur- sued due to its tremendous suc- cess in initial testing. It is a drop- in replacement for hexavalent chromate. As mentioned earlier, due to the low concentration and chemistry of non-hazardous sub- stances, it can be operated in a manufacturing environment without any exhaust system requirements. The pretreatment of aluminum is important to achieve desired specifica- tions. Alloys such as 6061 and 7075 are relatively easy to process and can work with most available detergent and activation agents, acids, or deoxidizers. On the other hand, 2024 alloy (with up to 5% copper content) is very susceptible to local- ized corrosion due to segregation of intermetallic particles along the grain boundaries, and it requires specific pretreatment. The nature of this alloy makes it sensitive to some etching cleaners and may require certain acids to activate the part surface. This patented trivalent chemistry by NAVAIR is simple to operate provided that 303

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