Chromate Conversion Coating for Aluminum

Published by Advanced Surface Technologies on

Chromate conversion coatings are the most widely used coating for corrosion protection of Aluminum

Coatings and Corrosion Protection

Chromate conversion coatings are used to passivate metals using an immersion bath process. The chromate conversion coating is a thin film that is applied by immersing the metal in a solution containing hexavalent chromium ions, which react with aluminum oxide on the surface of the metal to form a protective layer. This reaction forms a very stable complex with the chromium ion. The coating can be applied as either a single-stage or two-stage process. In the first stage, the metal is immersed in a dilute acid solution containing hexavalent chrome (Cr(VI)) ions, which reacts with the aluminum oxide to produce CrO3−2Al2O3.

During this step, some of the hexavalent chromium is reduced to trivalent chromium (Cr(III)). After rinsing, the coated metal is then dipped into a second solution containing hexavalent and/or trivalent chromium ions, usually at higher concentrations than those used in the first stage. This causes further reduction of any remaining Cr(VI) to Cr(III), forming additional CrO3−2 Al2O3. The resulting coating has a high concentration of chromium oxide, but it also contains residual amounts of hexavalent chromium.

Corrosion Control is a key benefit of Chromate Conversion Coating . Chromate conversion coating is one of the best methods available today for protecting aluminum from corrosion.

The following are the benefits of Chromate Conversion Coatings:

• Excellent Resistance Against General & Pitting Corrosion
• Excellent Resistance Against Crevice Corrosion
• Good Resistance Against Stress Corrosion
• High Durability
• Easy Application
• Low Cost
• Long Service Life

Chromate Conversion Coating Types

There are three types of chromate conversion coatings for aluminum:

1. Single Stage Chromate Conversion Coatings – These coatings contain only hexavalent chromium and do not require a second dipping step.

2. Two Stage Chromate Conversion CoatINGS – These coatings contain both hexavalent and trivalent chromium. The first dip is done in a solution containing only hexavalent chromate. A subsequent dip may be required if there is still too much hexavalent chromium left after the first dip.

3. Dual Stage Chromate Conversion CoatINGS – These coatings contain hexavalent and tri valent chromium. The initial dip is done in a mixture of solutions containing both hexavalent and tetravalent chromium. A subsequent dip may need to be performed if there is still too little hexavalent chromium present after the first dip.

Aluminum Chromate Conversion Coatings are used in many applications including:

• Automotive Industry
• Aerospace Industry
• Building Industry
• Construction Industry
• Electrical Industry
• Food Industry
• Industrial Equipment
• Marine Industry
• Medical Industry
• Military Industry
• Oil Refinery Industry
• Power Generation Industry
• Rail Transportation Industry
• Shipbuilding Industry
• Steel Industry
• Telecommunications Industry
• Water Treatment Industry
• Warehousing Industry
• Wind Energy Industry

 Advanced Surface Technologies works with our customers to create just the right finish or solution for your product or needs. Give us a call today at 303-432-8500.

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