Chromate conversion coatings (brand name Alodine, or generic Iridite) are chemical conversion treatments that create a corrosion-resistant layer on aluminum without changing dimensions. Used as paint primer, corrosion protection, or standalone finish on aluminum parts. This guide covers types, applications, and RoHS considerations.
The aluminum part is immersed in a chromium-based solution. A chemical reaction creates a thin (0.5-1 μm) layer of mixed chromium compounds on the surface. This layer:
Chromate conversion is the standard prep before painting or powder-coating aluminum, particularly for aerospace and military applications.
Two chromate chemistries exist:
Hexavalent chromium (Cr⁶⁺): traditional, best performance, but toxic. Regulated under RoHS (restricted), REACH, and WHO guidelines. Phased out of most consumer and industrial applications since 2006. Still allowed for certain aerospace and military uses under specific exemptions.
Trivalent chromium (Cr³⁺): modern replacement. RoHS-compliant, REACH-compliant, dramatically less toxic. Performance approaches but doesn't quite match hexavalent — ~80% corrosion resistance, comparable paint adhesion. The default for civilian applications now.
Unless you have a specific aerospace or military callout for hex-chrome, specify trivalent. Chrome-free alternatives (zirconium-based, cerium-based) are emerging for the most environmentally-sensitive applications.
MIL-DTL-5541 defines two classes and color conventions:
Colors:
Standard prep for all aluminum that will be painted. Paint adhesion to bare aluminum is poor; chromate conversion creates the needed bond.
Chassis grounding, EMI shielding, busbars. Class 3 coating conducts while providing corrosion protection that anodize cannot.
Class 1A for outdoor aluminum parts where appearance isn't critical. Cost-effective alternative to anodize for industrial parts.
Standard prep per MIL-DTL-5541. Either hex-chrome for severe service (with exemption documentation) or trivalent for commercial aerospace.
Galvanic corrosion prevention. Chromate conversion on aluminum mated to steel reduces galvanic current flow, extending assembly life.
Chromate conversion and anodize have overlapping but different sweet spots:
| Criterion | Chromate conversion | Type II anodize |
|---|---|---|
| Thickness | < 1 μm | 5-25 μm |
| Electrical conductivity | Conducts (Class 3) | Insulating |
| Wear resistance | Poor | Moderate |
| Dye/color options | Limited (gold, clear) | Full range |
| Dimensional impact | Negligible | Half grows outward, half consumed |
| Cost (typical) | +5-10% of part cost | +15-25% |
| Paint adhesion | Excellent (primary purpose) | Excellent with correct sanding |
| Corrosion resistance | Good | Very good to excellent |
For parts that will be painted anyway, chromate conversion is usually the better choice — cheaper, faster, preserves dimensions. For parts where color or wear resistance matters as the final finish, anodize is better.
Email [email protected]. Specify trivalent (RoHS-compliant default) or hexavalent (aerospace/military with exemption). We can also suggest anodize alternatives when that's a better fit.
Start a quote →