Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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was controlled within a range of 120–125°F. As much as cleaning of a part sur- face is important, it is also important to clean the surface without drying out dur- ing the transfer stage. The concentration of these detergents was based on the low- er end of chemical manufacturers' suggested operating range. The results of NSS testing did not indicate any reason for a detergent to be responsible for failure. Therefore, we concluded that the process parameters for the various detergents were acceptable as tested for evaluation with the EPA in a standard trivalent chromium pretreatment bath. The test panels were etched using a selected etchant as suggested by major cus- tomers. The study examined the difference in performance with and without the etchant for EPA performance. The results were favorable for the use of this etchant in a trivalent chromium pretreatment bath with the EPA additive. The process parameters were kept at the same level for etched and non-etched test pan- els for comparison. Out of the 164 total test panels, 119 (73%) of the etched panels passed. There were fewer failures for the etched panels (22%) with the EPA than with- out it in a standard trivalent chromate pretreatment bath. On the other hand, there were fewer failures of non-etched panels (8%) when used with the EPA than when used in a standard trivalent chromium pretreatment bath. It was con- cluded from this study that the EPA increases corrosion resistance by 51% on etched panels. We also determined that EPA increases corrosion resistance by 39% on non-etched panels. These data show that this extended protection additive helps to enhance cor- rosion resistance performance with or without the etchant when compared with a standard (NAVAIR) trivalent chromium pretreatment application. The results of NSS testing conducted by NADCAP-certified laboratory showed that standard trivalent chromium pretreatment (NAVAIR) performed from 96–768 hours with the etched panels. The non-etched panels in the same bath per- formed better in NSS, with a range of 168–2,786 hours. The same process for evaluation of panels tested for EPA effectiveness showed that etched panels performed in NSS from 174–912 hours, whereas non- etched panels performed from 72 to more than 3,120 hours. To activate the part surface, an acid in combination with a deoxidizer was used. There were different mix ratios of acid and deoxidizer, as well as the process cycle time, for testing purposes. Results were based on various combinations of cycle time and con- centration ratios. NSS testing results indicated that lower concentrations of acid in a deoxidizer performed better than higher concentrations of acid/deoxidizer for the standard trivalent chromium pretreatment (NAVAIR) bath. However, acid concentration did not substantially influence the trivalent chromium pretreatment with EPA- treated substrates. Test panels processed with a low concentration of acid/deox- idizer in a trivalent chromium pretreatment (NAVAIR) bath failed at 168 hours, whereas test panels with an EPA passed 267 hours. SUMMARY The trivalent chromium pretreatment formulated and invented by NAVAIR performs very well for replacing hexavalent chromium on aluminum. Its per- formance varies depending on the type of detergent and its concentration, tem- perature, and cycle time. Also important is the surface activation and the type of acid and/or deoxidizer that is used for the application. However, the most 306

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