There is often a lot of confusion around these terms and for good reason because by themselves they do not alone define a coating or process. Decades ago they were synonymous with yellow, hexavalent chromate conversion coating of aluminum as MacDermid’s Iridite® 14-2 and Henkel’s Alodine® 1200 products were the leading products used to apply the coating.
These yellow chromate conversion coatings, also known as chemical film, provide corrosion protection and act as an adhesion promoter for paint or adhesive bonding. Unlike anodize, the coating over aluminum is conductive (typically no more than 5 milliohm resistance) and only 10-40 millionths of an inch thick.
As the practice of applying corrosion-resistant chromates grew to cover non-aluminum products, each firm’s brand name family expanded to cover everything from clear chromate products for aluminum to post-plate supplementary treatments for zinc and cadmium. While the legacy product Iridite® 14-2 was used for yellow, hex-chrome on aluminum, Iridite® 17 was for copper, Iridite® 18 for silver and Iridite® 80 for zinc.
These days with regulations favoring the use of non-hexavalent chrome conversion coatings, Iridite® NCP and Alodine® 5200 are aluminum conversion coating products that are free from hex chrome.
Anoplate’s AnoChem TCP process is a non-hexavalent chrome conversion process for aluminum that uses a product qualified by the Naval Air Systems Command and thereby listed in QPL-81706. In addition to Iridite® and Alodine® products, there are dozens and dozens of other qualified manufacturers and products.
With the increased use of aluminum in the auto industry to reduce weight and boost fuel efficiency, one finds such hex-free products used for brake calipers, electrical component housings, battery shell and cover used for electric vehicles, and underride guides. Similarly, in the aerospace industry, the chemical conversion coating of aluminum is used for paint and primer application.
Specification wise, the chemical conversion coating of aluminum is covered by the following specifications:
- MIL-DTL-5541 (formerly MIL-C-5541)
- ASTM B449 (Hexavalent Chromate)
- ASTM B921 (Non-Hexavalent)
- AMS 2473 (General Purpose Applications)
- AMS 2474 (Low Electrical Resistance Applications)