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Chromate passivation peels off from Zn-Fe (zinc-iron) plating

Q. Sir,

I am running zinc-iron alloy system. I am maintaining Zn at 8 to 10 gpl and caustic at 100-120 gpl. Cathode to anode distance is about 10 inches. The current density is of about 2- 3 amps per dm sq. Continuous filtration and with recommended additions of chemicals. Weekly once zinc dust [on eBay or Amazon] packing are also done in correct manner. The problem is with the passivation peel off and I can see the zinc coating. I am using dm water for rinse after passivation and a caustic dip as predip. What may be the cause for it?

Thanking you,

Jude
Industrialist - Tamil Nadu, India
2004

A. Dear Mr Jude,

Why use caustic as a pre-dip? Why not regular HNO3? You should reduce all alkalinity before coming to the passivation step.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

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