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| Chromic acid anodizing is performed on aluminum to enhance the corrosion resistance of the part. Chromic acid anodizing offers several advantages over sulfuric acid anodize. The advantages must be balanced with environmental concerns and the softer thinner film formed by the process. Chromic acid anodize is formed by placing a part into a solution of chromic acid which is controlled for concentration, temperature and contamination levels. An electrical current is applied to the parts with the part being the anode and the tank being the cathode. The power is applied to the parts at a voltage of 22 or 40 volts depending on the alloy to be anodized. The process is held at power for 60 minutes or more. The anodic coating formed by the chromic acid process is thin, ductile and non-conductive. A typical thickness for chromic acid anodize is .00005-.0001 inches. Approximately ½ of this thickness is buildup. Except for parts with exceptionally tight tolerance no dimensional change is noticed in the before and after anodize part. The thin coating can be dyed with some success. Darker colors work well but do not generate the vibrant colors that sulfuric acid anodize can due to the fact that the chromic acid anodize coating if light to dark gray in color compared to the relatively clear color of sulfuric acid anodize. The big advantage of chromic acid anodize over other anodizing processes is that it does not lower the fatigue strength of the part as sulfuric acid anodize will. The chromic acid electrolyte will also not damage the base material if it is entrapped in lap joints or porosity as the sulfuric acid electrolyte can. |