November 28, 2017

Composition of Alloy and Process Limitation

Composition of aluminum alloy make the process condition of anodizing is conditioning in order can get good performance of anodized alloy. Surface finish, prior processing, temper or heat treatment are influence to the quality of anodic coatings. The limitations imposed by each of these variables on various condition of aluminum alloy are as follows:

Chromic acid process should not use for anodize aluminum casting alloys containing more than 5% of Cu or more than 7.5% of total alloying elements, because this alloy can resulting of excessive pitting, and may burning. Sulfuric acid bath can be used for commercial alloys, while hard anodizing process is usually apply in limited alloys containing less than 5% Cu and 7% Si. Choice of alloys is important when the product should have maximum corrosion or abrassive resistance required. Alloys contain of copper and copper magnesium is able to produce hard, corrosion resistance coating.

Two kind of alloy can be anodized together in the same anodizing bath if the voltage requirement are identical, even simultaneous anodizing of two different alloy is not normal applied. This process can be more difficult for sulfuric acid process than chromic acid process.

Finished anodizing will form an anodic film, and often time formed surface irregularities that are emphasized more by chromic acid bath than by sulfuric acid bath. For this reason, suggest that sulfuric acid bath is use instead of chromic acid bath where optimum corrosion or abrasion resistant surfaces are required. Clad sheet should be handled with careful to prevent mechanical abrasion or exposure of the core material. Scratch on surface of aluminum make this magnifies exposed, with different color of anodized finished.

Anodizing grade must be specified for extruded products so that mill operations are controlled to minimize longitudinal die marks and other surface blemishes. Surface irregularities must be removed from forgings, and surface of forgings must be cleaned by a process that removes trapped and burned-in die lubricants.

Castings can be anodized provided their composition is within the process limits described under alloy composition. Uniform will appearance, however anodizing usually is undesirable for castings.

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