Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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heaters where chlorides are absent. Containers and tank linings can be made from plastics such as polyvinyl chloride (PVC), polyvinylidine chloride (PVDC), poly- ethylene, and polypropylene. Acid-resistant brick or chemical stoneware is satis- factory for some applications, but is subject to attacks by fluorides. Parts-handling equipment is made of stainless steel, plastisol-coated mild steel, or plastic. Mild steel can be used for leaching tanks because the solutions are generally alka- line, whereas tanks for dyeing solutions, which are slightly acid, should be of acid-resistant material. Usually, ventilation is not necessary because most chromate solutions are oper- ated at room temperature and are nonfuming. Where chromating processes are heated, they should be ventilated. FILM FORMATION Mechanism The films in most common use are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of other components, or "activators," in an acid solution. The hexavalent chromium is partially reduced to trivalent chromium during the reaction, with a concurrent rise in pH, forming a complex mix- ture consisting largely of hydrated basic chromium chromate and hydrous oxides of both chromium and the basis metal. The composition of the film is rather indefinite, because it contains varying quantities of the reactants, reaction products, and water of hydration, as well as the associated ions of the particular systems. There are a number of factors that affect both the quality and the rate of for- mation of chromate coatings. Of the following items, some are peculiar to chro- mating; many derive simply from good shop practice. A working understanding of these factors will be helpful in obtaining high-quality, consistent results. Different formulations are required to produce satisfactory chromate films on various met- als and alloys. Similarly, the characteristics of the chromate film produced by any given solution can vary with minor changes in the metal or alloy surface. Commonly encountered examples of this follow. Effect of Basis Metals Aluminum Alloys. The ease with which coatings on aluminum can be produced, and the degree of protection afforded by them, can vary significantly with the alloying con- stituents and/or the heat treatment of the part being processed. In general, low alloying constituent metals that are not heat treated are easiest to chromate and pro- vide the maximum resistance to corrosion. Conversely, wrought aluminum, which is high in alloying elements (especially silicon, copper, or zinc) or which has under- gone severe heat treatment, is more difficult to coat uniformly and is more suscep- tible to corrosive attack. High silicon casting alloys present similar problems. The effect of these metal differences, however, can be minimized by proper attention to the clean- ing and pretreatment steps. Most proprietary treatment instructions contain detailed information regarding cleaning, desmutting, etc., of the various alloys. Magnesium Alloys. As in the case of aluminum, the alloying element content and the type of heat treatment affect the chromating of magnesium. With the excep- tion of the dichromate treatments listed as Type III in Military Specification MIL-M-3171, all of the treatments available can be used on all the magnesium alloys. Zinc Alloys. Chromate conversion coatings on zinc electroplate are affected by impurities codeposited with the zinc. For example, dissolved cadmium, copper, and lead in zinc plating solutions can ultimately cause dark chromated films. Similarly, 421

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