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Chromic acid staining on Manganese Phosphated parts.

I work for a company that has a Manganese Phosphating line. We use a chromic acid rinse to seal/prepare the parts for painting. We have noticed some orange/brown staining on some of the parts after the chromic bath. We we wondering what, if anything, we can do to prevent this from occurring.

David Chesnes
plating shop employee - Willoughby, Ohio, USA
August 31, 2011

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September 1, 2011

Mn phosphate is a bit unusual as a pre paint as it tends to be very course as well as the most expensive of the 3 normal phosphate coating. It is typically used as a break in material.
Zn phosphate is an excellent prepaint. Calcium modified Zn phosphate is a relatively smooth phosphate. Our readers in India talk about tri cationic phosphate, but I am not familiar with the process or the benefits.
Many companies use iron phosphate and some do a poor job of controlling it, but most "get by" with it as it is the easiest and the cheapest.

As far as the stains, you will loose a bit of the goodness of the chrome rinse, but adding another warm rinse would probably flush out some of the chrome that is trapped in the phosphate and held there by surface tension.

James Watts
- Navarre, Florida