Summary for chromate passivation peel off Discussion
Chromate passivation peel off is the issue in this zinc-iron plating thread. Jude says he is running a zinc-iron alloy bath with Zn at 8–10 gpl, caustic at 100–120 gpl, 2–3 A/dm², continuous filtration, weekly zinc dust packing, DM water rinses after passivation, and a caustic pre-dip, but the passivation is peeling and exposing the zinc coating. The reply from Bengaluru, Karnataka, India questions the use of caustic as a pre-dip and suggests using regular HNO3 instead. The key advice is to reduce all alkalinity before the parts enter the passivation step, since residual alkalinity may be contributing to the poor adhesion or peeling of the chromate film.
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
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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
