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FAQ/TCP-HF Question 3

Why is the CHEMEON TCP-HF trivalent chromium conversion coating not self healing but the hexavalent is?

The trivalent chromium coating layer does not have any entrapped water, but the main reason why it is not self healing is because of the valences. Hex chromate can be reduced to Cr3+ (hydroxide and oxide) on the bare substrate, but trivalent chromium coating is not capable of being reduced. Also, hex chromium is mobile in an aqueous environment. On the other hand, trivalent chromium is in the form of precipitate and immobile. That means it cannot diffuse to the scratched area to heal the defect. Finally, the hexavalent coating loses it's self healing properties and forms micro fissures which promote corrosion when heated over about 140º F. The trivalent does not.