Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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knows that surface modification of a substrate, such as etching, can cause a dras- tic decrease in corrosion resistance. A study of one particular additive indicates an improvement rate for corrosion resistance of 50% on etched substrates and 38% on non-etched substrates when using an enhanced bath (see Table 1). The study included 163 different processes that utilized several different cleaners, deoxidizers, and etchants at various temperatures, concentrations, and cycle times. All panels treated with an enhanced performance additive (EPA) were tested against a standard, non-enhanced (NAVAIR) trivalent chromium pre- treatment bath. Test panels were sent to an outside party (a NADCAP-certified laboratory) for neutral salt spray testing (ASTM-B117) as well as undergoing in- house inspection. Panels were examined at intervals of 96, 168, 212, 267, 336, and 407 hours and beyond. For the sake of brevity, only one parameter will be report- ed in this article: etched vs. non-etched. The total number of test panels was in the hundreds. The data in Figure 1 indicates that, statistically, out of 100 different process variations, 81 of them will pass MIL-DTL-5541 on etched substrates using an enhanced version of a trivalent conversion coating, whereas only 31 process vari- ations will pass using a standard, non-enhanced trivalent chromium chemistry. Although the improvement on non-etched substrates is not quite as dramat- ic as it is for severely modified substrates, using an additive in a tri-chrome bath is still beneficial. The data in Figure 2 indicate that, statistically, out of 100 different process variations, 90 of them will pass using an enhanced version and only 53 will pass using a standard, non-enhanced bath. Baths that include an EPA perform much better than standard trivalent chro- mate baths (NAVAIR). The addition of an EPA in a standard trivalent chro- mate pretreatment bath can vary from 15–30% by concentration. We used a 25% EPA concentration in a standard trivalent chromate bath for our test eval- uation. The objective here was to study the influence of an EPA in a standard (NAVAIR) trivalent chromate pretreatment bath and to evaluate the difference in performance with the addition of the EPA. In order to evaluate the effects, we set up a test plan and selected the appro- priate chemistry. We chose the 2024 aluminum alloy as our baseline standard. The aluminum alloy 2024 contains up to 5% copper and a major fraction of the intermetallic inclusions are composed of Al2 MgCu (S-phase), which has cathod- ic potential relative to the aluminum alloy matrix. The intermetallic precipitates make the alloy very susceptible to localized corrosion. We assumed that if we could achieve superior results on this alloy, then the other alloys such as 5052, 6061, and 7075 would perform as well or better than 2024 alloy panels. We aimed to study whether the EPA helps enhance corrosion resistance performance of trivalent chromium pretreatment on aluminum. Salt spray resistance: Figure 3 illustrates enhancement in salt spray resistance as the concentration of EPA increases. Coating weight: Data in Table 2 list the coating weight of 2024 T-3 alloy. Coating weight of 11.4 mg/ft2 was achieved with only two minutes of immersion time, and this exceeds MIL-DTL-81706. The detergents for this trial were selected based on some major customers' sug- gestions as well as our history of various testing with trivalent chromium pre- treatment applications. The temperature was one of the important factors that 305

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