Chemical Film

Aluminum Conversion Coating

All aluminum parts should be conversion coated for corrosion protection and as surface preparation prior to powder coating or primer and painting.

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The traditional conversion coat for aluminum is Alodine**. In a typical sequence for finishing aluminum, the parts are soak cleaned in a detergent or solvent. The soak cleaner employed in the pretreatment of the aluminum surface removes finishing oils, grease and difficult-to-remove compounds left on the surface of the aluminum from handling and secondary processes.

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Following the soak clean, the aluminum is immersed in a mild caustic or alkaline etch solution. The mild alkaline etch removes the surface layer and slightly roughens the surface. The final step in preparation is Chromate conversion (Alodine**) coating. This is one of the most common types of conversion coating and is used to passivate aluminum, it is primarily used as a corrosion inhibitor, primer, decorative finish, and to retain electrical conductivity. The process is named after the chromate found in the chromic acid used in the bath, more commonly known as hexavalent chromium. This type of bath is the most widely used treatment for Aluminum.

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Alodine* comes in either a liquid or powder form and can be mixed to the strength determined by the part and time desired to obtain a proper surface finish. Alodine* can also be sprayed, brushed, or sponged onto large surfaces however this is not a preferred method as the surface must remain wet for several minutes or until a light to dark golden brown develops. The preferred method is to immerse the part into a bath of Alodine*. Following the conversion process, the part needs to be thoroughly rinsed with water. It is recommended that the parts be powder coated or primed and painted immediately following the conversion coating in order to eliminate the chance of surface contamination.

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Steel, like aluminum, should have a conversion coat for maximum corrosion protection. 

Before you can put a conversion coat on steel, it must be etched like aluminum. The same steps are followed as in aluminum.As the steel is etched some blacking will often develop on the surface. This is the acid converting rust to iron phosphate and is normal.

*Chromate conversion coatings on an aluminum substrate are known by the following terms: chemical film, yellow iridite and the brand names Iridite and Alodine. The most commonly referenced standard for chromate conversion coating aluminum is MIL-DLT-5541.

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