Twenty-six year Corrosion Study Comparing Decorative Hexavalent and Trivalent Chromium

摘要

This long-term corrosion program had two main objectives: To determine if SC 4 and maybe even SC 3 nickel/chromium plating systems within ASTM B 456 could be used to produce exterior decorative automotive parts that do not develop any substrate corrosion for at least 10 years of service and To determine if there is a performance difference between decorative chromium deposits produced from the hexavalent and trivalent chromium processes used in these plating systems. The advantage of this type of study is that all of the panels are very uniformly plated to the required thickness at the same time using the same processes. The physical properties of the deposits were well controlled within the requirements of ASTM B 456. The panels were also randomly positioned, tested and rated at the same time. This made it possible to make accurate comparisons between the plating systems. Production parts might not perform identically under different corrosion conditions. Many times they have lower quality substrates and less uniform deposit thickness without all the physical properties that improve the performance of the total plating system. Both SC 4 and SC 3 (ASTM_B 456) plating systems remained essentially free of substrate corrosion throughout the 15 years of marine, 12 years of industrial, 7 years of mobile, 96 hr of CASS and 132 hr of Corrodkote testing if the plating system included micro-porous chromium (MP and TC). Micro-cracked chromium systems (MC) had slightly lower corrosion and appearance ratings. This supports the North American automotive companies' exclusion of micro-cracked plating systems. Standard hexavalent chromium systems (HC) all had substrate corrosion early into all of the different corrosion tests. These systems, which are not micro-discontinuous, are not permitted for exterior decorative automotive applications. They have also been removed as acceptable options for SC 3 and SC 4 in the 2003 edition of ASTM B 456. Trivalent chromium deposits (micro-porous as plated, TC) and hexavalent chromium deposits (with micro-porosity generated through an additional step, MP) performed almost identically in substrate protection and after corrosion appearance throughout all the tests. Except for the faintly darker color and slightly more surface pitting of the trivalent chromium deposits, both micro-porous systems were equivalent. The difference in color would be less today because of the changes made in the trivalent chromium electroplating technology over the 26 years of this study. Also, recent service experience has shown that this slight increase in surface pitting would most likely have been almost eliminated if a thin, noble nickel deposit was deposited between the bright nickel and the chromium, as required in North American automotive specifications today. The value of Duplex nickel (semi-bright and bright nickels) versus single nickel (bright nickel) plating systems, both with a total nickel of 25 mu m (0.001 in.), was dramatically demonstrated in this study. The single nickel systems developed substrate corrosion during the first year of exposure to all of the outdoor sites. They developed an extensive amount of corrosion after five years. There was a slight improvement when micro-discontinuous chromium, trivalent (TC) and hexavalent (MP and MC), was used. Some of the Duplex nickel systems developed one or two small substrate corrosion sites early in the testing but all micro-discontinuous systems, trivalent and hexavalent, were about equal in performance even at the end of the long-term outdoor tests.