Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 423 of 707

dissolved iron in noncyanide zinc plating solutions can create chromating prob- lems. Furthermore, the activity of zinc deposits from cyanide and noncyanide solutions can differ sufficiently to produce variations in the chromate film char- acter. Variations in the composition of zinc die casting alloys and hot-dipped galvanized surfaces can also affect chromate film formation; however, in the latter case, the result is usually difficult to predict, due to the wide variations encountered in spelter composition, cooling rates, etc. Large differences in the chromate coating from spangle to spangle on a galvanized surface are not uncommon. This is especially evi- dent in the heavier films. Copper Alloys. Since chromate treatments for copper and its alloys can be used to polish chemically as well as to form protective films, the grain structure of the part becomes important, in addition to its alloying content. Whereas fine-grained, homo- geneous material responds well to chromate polishing, alloys such as phosphor bronze and heavily leaded brass usually will acquire a pleasing but matte finish. In addition, treatment of copper alloys, which contain lead in appreciable amounts, may result in the formation of a surface layer of powdery, yellow lead chromae. One of the more important factors in controlling the formation of the chromate film is the pH of the treatment solution. For any given metal/chromate solution system, there will exist a pH at which the rate of coating formation is at a maximum. As the pH is lowered from this point, the reaction products increasingly become more soluble, tending to remain in solution rather than deposit as a coating on the metal surface. Even though the rate of metal dissolution increases, the coating thick- ness will remain low. Chemical-polishing chromates for zinc, cadmium, and cop- per are purposely operated in this low pH range to take advantage of the increased rate of metal removal. The chromate films produced in these cases can be so thin that they are nearly invisible. Beyond this point, further lowering of the pH is suf- ficient to convert most chromate treatments into simple acid etchants. Increasing the pH beyond the maximum noted above will gradually lower the rate Effects of pH of metal dissolution and coating formation to the point at which the reaction, for all practical purposes, ceases. Hexavalent Chromium Concentration Although the presence of hexavalent chromium is essential, its concentration in many treatment solutions can vary widely with limited effect, compared with that of pH. For example, the chromium concentration in a typical aluminum treatment solution can vary as much as 100% without substantially affecting the film-formation rate, as long as the pH is held constant. In chromating solutions for zinc or cadmium, the hexavalent chromium can vary fairly widely from its opti- mum concentration if the activator component is in the proper ratio and the pH is constant. Activators Chromate films normally will not form without the presence of certain anions in regulated amounts. They are commonly referred to as "activators' and include acetate, formate, sulfate, chloride, fluoride, nitrate, phosphate, and sulfamate ions. The character, rate of formation, and properties of the chromate film vary with the particular activator and its concentration. Consequently, many proprietary for- mulations have been developed for specific applications and they are the subject of numerous patents. Usually, these proprietary processes contain the optimum con- 422

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