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Definition Needed

 

I need to explain what clear chromate and yellow dichromate coatings are to an off shore vendor. Is there a standard definition available? If so, where would I find it?

Patti Becker
- Racine, Wisconsin

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For a good technical article explaining chromate conversion coatings, try to get a copy of the Metal Finishing Guidebook, available through Metal Finishing Magazine.

A chromate conversion coating is a very thin gel that passivates whatever it is on top of. Generally, these coatings are used as a post-treatment after a zinc or zinc-alloy plating. "Passivation" is the process of making something react or rust less readily. Chromate conversion coatings on top of a zinc plating greatly slows the onset of white corrosion and the following red rust.

Tim Neveau
Rochester Hills, Michigan

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Sir,

Clear or blue or transparent Chromate conversion coating on zinc appears chrome-like with a light bluish shade whereas yellow chromate exhibits an iridescence,which means appearance of more than one color. The clear passivation contains less hex.chromium as compared to the heavier yellow passivation. The passivation colors are interference colors due to the incident light being interfered with in the uv or visible spectrum with increasing thickness of the film. The clear passivated zinc is less susceptible to fingerprint corrosion whereas the heavier yellow chromate has better general corrosion resistance (5% salt spray to white corrosion usually 96 hrs) but more susceptible to fingerprint corrosion. I hope the difference is explained.

S.RAMAJAYAM
- Bangalore, Karnataka, India