Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Randy Taylor, Advanced Tooling Corporation
November, 2007

White Corrosion with Black Spots on Zinc Coatings

Q. This is regarding to the acid zinc/ alkaline zinc/ caustic pottash plating process. We need to get trivalent yellow passivation appearance.

The material is steel, its a sheet metal component, we tend to get bluish appearance. But we are getting defects like black spots, white patches.

Could you provide me some details about the process and causes of the defects?

A. There are a number of causes for "white corrosion" and "white corrosion with black dots" on zinc coatings. http://www.pavco.com/: supplier of zinc solutions and technical assistance.

Here are some additional comments from an independent zinc plating expert;

Porous materials are prone to corrosion. However you are plating sheet steel and porosity in the base metal may not be the issue.

Let’s assume a robust degreasing step and soak and/or electro-cleaning step are part of your wet process. The next important step is an acid pickle with suitable rinsing. A two step acid (HCL), one at 50% and the second at 10-15% by volume is a proven way to break down surface corrosion products.

Steel parts however tend to rust almost immediately in the atmosphere after pickling, compounded by other air born and liquefied corrosive products including “iron oxide”. This can set up pre-plate rust condition on the part surface which may not be readily detected by the naked eye and that a simple water rinse will not remove.

For caustic/ potash zinc baths, an additive of modest amounts of caustic soda to the final rinse can help to dissolve away some of the atomic particles as well as coating parts with a thin film which holds them temporarily until they an be immersed into the plating tank.

No information on acid zinc baths at this time - RT