title = "Chromate and Trivalent Conversion Coatings"

content = """
Chromate conversion coatings are chemical conversion treatments applied primarily to aluminum, zinc (galvanized), cadmium, and magnesium alloys to improve corrosion resistance and paint adhesion. At Ridgecrest Metalworks, chromate conversion is most commonly specified on aluminum components destined for aerospace-adjacent or precision industrial applications.

Hexavalent chromate (Cr⁶⁺) conversion, historically the most effective, produces coatings per MIL-DTL-5541 (aluminum) and MIL-DTL-81706. Class 1A coatings are heavier and provide maximum corrosion protection as a standalone finish; Class 3 coatings are thinner and used where electrical conductivity must be retained across the coated surface. Hexavalent chromate has excellent self-healing properties: Cr⁶⁺ ions in the coating are mobile and migrate to repair mechanical damage. However, hexavalent chromium is a carcinogen regulated under REACH (EU) and restricted by RoHS and many defense specifications. Its use is being phased out where alternatives qualify.

Trivalent chromate (Cr³⁺) conversion coatings have been developed as a direct replacement. Trivalent chromate (TCP) coatings such as SurTec 650 and Alodine T 5900 produce films that meet corrosion resistance requirements of MIL-DTL-5541 Class 1A and Class 3 in salt spray testing. Trivalent coatings do not have the self-healing mechanism but achieve equivalent adhesion performance. TCP processing uses similar equipment and sequence to hexavalent but with different bath chemistry and operating parameters.

Processing sequence for aluminum chromate conversion:
1. Alkaline deox/clean
2. Rinse
3. Deoxidizer (nitric/chromic or non-chrome deox to remove natural oxide and smut)
4. Rinse
5. Chromate conversion bath (immersion or spray)
6. Rinse
7. Dry (air or low-temperature forced air, below 60 °C to avoid coating degradation)

Coating quality is verified by visual inspection (gold-to-brown iridescent for hexavalent; clear-to-gold for trivalent) and salt spray corrosion testing per ASTM B117. Electrical conductivity is verified by contact resistance measurement for Class 3 applications. Coating weight and thickness are measured periodically. Bath parameters — pH, concentration, temperature, and immersion time — are controlled and recorded per process sheets.
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