![]() ![]() ![]() Consult the latest version of Mil-A-8625 for a discussion of type IC 'non-chromate' coatings and type IIB 'thin sulfuric' coatings. Phosphoric acid anodizing is one process that has been substituted for chromic acid, and Boeing has been promoting a proprietary boric acid & sulfuric acid electrolyte which is also capable of producing good quality thin anodized coatings. But the world is very concerned with the environmental impact of chromic acid (hexavalent chrome), so there have been great efforts for several decades towards reducing the use of chromic acid anodizing. ![]() It offers good corrosion resistance while being quite thin (usually less than.0001'), which is extremely important in limiting its impact on reducing the fatigue strength of components.Īlso, the chromic acid is not corrosive like sulfuric acid is, making it safer to use on critical components where entrapped acid in a seam or spot weld could be dangerously corrosive. Type I - Chromic acid anodizing (actually there are also types IB (low voltage process) and 1C (chrome-free process) in recent issues of the specification) Type II - Conventional 'room temperature' sulfuric acid anodizing (plus a newer type IIB 'thin sulfuric') Type III - Hardcoat anodizing, done in sulfuric acid at temperatures close to the freezing point of water.Ĭhromic Acid Anodizing, i.e., Type 1 Chromic acid anodizing is often done on aerospace components. ![]() Because of this attack on the anodized coating by the acid, varying the strength and concentration of acid in the electrolyte, and its operating temperature, will result in anodized coatings with very different properties. Rather, the simultaneous formation and dissolution of the anodized film creates a coating that has porosity in the form of microscopic 'drill holes' extending from the surface almost to the raw aluminum that can be very important to the utility of the anodized coating. While the acid helps to conduct the electricity, it also attacks the anodized film that is being formed and tends to dissolve it - but this is not a bad thing. ![]()
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