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White Spots in Zinc Phosphate type 1 Coating as per TT-C- 490E

A friend of mine uses metal grit shots to mechanically clean heavy heat treating oxide from a medium carbon steel substrate. Upon impinging on the surface of the medium carbon steel substrate, the steel grit left residues of its break away fine particles embedded into the substrate surfaces. Acid etching is not allowed to etch the surface of the substrate except alkaline cleaning. After condition, zinc phosphating (TT-C-490 [from DLA] ) and conversion coating exercise was completed, white spots were evidenced on the roughened surfaces of the mechanically cleaned substrate. EDAX surface analysis of the substrate also show evidence of high carbon concentration (above other elements present) on the Zn- phosphate coated films. What could be suggested to the possible cause (s) of the white spots seen on the finished coated irregular surfaces of the part? Chlorine were also found. Would the inclusions of chlorine and the embedded steel grits (smuts) inclusions be detrimental to the finished zinc coated part?.

Bassey J. Udofot
Musgegon, Michigan
2007

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The first thing that I would do it to reduce the impingement velocity, either air or mechanical. This will take a lot longer to clean, but it will significantly reduce the broken bits in the surface of the part.
I would consider switching to aluminum oxide grit of a largest grit size that will leave a desired surface. A hot soak in caustic will remove any grit that it stuck in the surface, as well as it will be significantly smaller sized pieces stuck in the part..

James Watts
- Navarre, Florida
2007