Metal Finishing Guide Book

2012-2013

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other hand, is the conversion of the surface layer of a metal to an oxide. The metal most commonly anodized is aluminum, but other metals, such as magnesium and titanium, can also be successfully anodized. Aluminum will naturally form an oxide layer when exposed to oxygen, but this is a relatively thin layer. Anodizing provides a much thicker coating. Anodized finishes exhibit a number of desirable properties. They are capable of being processed further to modify the appearance of the aluminum. For example, colored finishes are easily obtained by such techniques as dyeing or color anodizing. Anodizing also improves the wearability of aluminum. An anodized finish is much more resistant to abrasion than the base metal. Anodizing is also extensively used in environments where corrosion is a problem. A number of anodizing processes are employed for aluminum. The most common is the sulfuric acid anodizing process. This provides a coating typically 0.1 to 1.0 mil. thick and lends itself to further color processing. Other conventional aluminum anodizing processes are those utilizing chromic acid (found in marine and aircraft applications) and phosphoric acid (used as a surface preparation for adhesive bonding and as a base for electroplating). These conventional anodizing processes require a DC power supply similar in nature to those found in electroplating, except that the voltages typically used in conventional anodizing (18-50 V) are higher than those commonly found in plating (6-18 V). Otherwise, the design of the rectifiers for DC electroplating and DC anodizing is basically the same. Hard-coat anodizing is often employed in applications where a more abrasive or corrosion-resistant oxide layer than that obtained with conventional anodizing is desired. Hard-coat anodizing processes typically demand voltages between 50 and 150 V, and in many cases, pulse power supplies are utilized to obtain specific results. As in electroplating, the pulse rectifiers are very similar in design, options, and usage. Color Anodizing Many architectural aluminum anodizing applications require that color be applied to the finished product. Colored finishes are obtained through the use of dyeing, integral, or electrolytic color processes. Dyeing is a simple process. A dye bath is composed of water and dyeing material, and the anodized aluminum is placed in the dye bath for some minutes. After removal from the dye bath, the aluminum is then rinsed and sealed in a normal manner. Integral color is a process by which the color is produced during the conventional anodizing process. Organic acids are added to the anodizing bath, and these acids produce a color, ranging from amber through black, in the aluminum oxide. Standard DC rectifiers are used, though at a voltage approximately three times that found in sulfuric acid anodizing. The electrolytic or two-step process begins by conventional sulfuric acid anodizing using DC power. The parts are then placed into a coloring solution consisting of salts of various metals such as tin, nickel, and cobalt, and AC power is applied. The AC current causes the deposition of metallic particles in the pores of the anodic coating. By varying the relative amplitudes and times of the positive and negative half cycles of the AC output, numerous colors and finish characteristics can be obtained. The electrolytic coloring processes have become popular as they require less energy than competing methods. An ideal power supply for the two-step process will provide the opportunity 774

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