Also known as “Iridite” “Alodine” or “Chemical Film”

Type 1 A/3 MIL-DTL-5541

Type III Class 1 A MIL-DTL-81706

*Noncompliant (non-RoHS)

Compatible with Aluminum, Steel, and Zinc

– Circle Finishing only does Chromate finish process on Aluminum.

Used as base coating for paint or powder coat, or as a final finish.

ASTM-B921Class 1
ASTM-B921-02Class 3

Chromate is a “conversion coating” which is achieved through an immersion process. The process creates a chemical reaction which requires no electricity. A very thin “micro-coating” is formed which inhibits corrosion and passivates the top layer of a metal. Chromate is commonly used for aluminum, steel, and zinc. It serves as an excellent base coating for paint or powder coat, although it is used as a final finish in some applications.

Chromate chemistries can be hexavalent, trivalent, and non-chrome versions. Trivalent and non-chrome chromates meet the restrictions of RoHS. In yellow chromate (hexavalent), Class 3 is a lighter coating when electrical conductivity is important (30 second to 1 minute immersion) and class 1A is a heavier coating (1-2 minutes) when maximum corrosion protection is important.

Although chromate coatings are initially gelatinous and hold onto water molecules, they become harder and more hydrophobic as they age. The chromate process for aluminum is generally:

  • Soak Clean (remove oils, surface oils)
  • Etch (remove top layer of aluminum/aluminum oxide, open pores, clean metal)
  • De-oxide/De-smut (remove smut formed by etching process)
  • Chromate Immersion
  • Final rinse
  • Dry