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How to eliminate red color from pre-black zinc phosphating tank

Q. We are doing Black Zinc Phosphate and have a problem with machined surfaces of free machining (higher sulfur content) steels turning red/orange during the pre-black step of the process. The problem sometimes even appears on heat treated parts. The pre-black bath uses antimony trichloride to obtain the black color.

Dave Tokle
plating shop - Owatonna, Minnesota, USA
2004

Q. Why is the tank bath colour black, whereas it should be milky in colour during zing phosphating process? Due to that, minor pin-holes are coming during powder coating process.

Pradeep Singh
- Delhi, India
May 2, 2012

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