Discoloration of Zinc Plating with Yellow Chromate Treatment

A. Rejectable by the doctrine of common sense :-)

Seriously, how can anyone tell if you have complete coverage, or predict the salt spray resistance or useful life of something that's already such a mess. I am curious what they did to it, although the second photo seems to show water stains. Your vendor needs to send you good parts whether or not it's his plater's fault.

Regards,

A. It is unusable for virtually everyone. It is a problem for the vendor and the plater to sort out.

A. Sir,

Your photos remind me of testing that I did 30 years ago. In that hex chrome galvanized zinc product we found that chromic acid (chromium trioxide) gave very bad staining (like your photos), whereas when we used sodium dichromate and acetic acid [on eBay or Amazon] the chromate quench was not visible but did pass the 40 hour salt spray test chamber.

In later years I learned to keep the pH of such a solution between 3.5 and 4.5 which gave a much longer lifetime of the chromate quench tank.

Regards,

A. Dear Sirs:

The case presented is similar to what we have on our products that is loose, yellowish, and powdery deposits that can be scrubbed off by bare hands. Its true however that the question on acceptability lies on the agreement between the plater (or galvanizer) and the end-user.

Nevertheless, granting quality complaints, I think improvements have to be made based on the following:

a) Acidifying the sodium dichromate solution to activate its chromium trioxide (CrO3) needed to further oxidize the zinc oxide layer on the galvanized item in a direct quenching process. My experiment revealed a varnish like coating that still existed after six months of trial without signs of increase in coating thickness compared to a bare zinc oxide coating that eventually became dull grey in color with slight increase in thickness. It is assumed that quenching zinc oxide first in water will create zinc hydroxide that could interfere with the chromium trioxide's action on the zinc coating. It would be very much appreciated if Dr. Cook can comment on this.

b) The above protective coating has to be water rinsed to reveal the protective zinc chromate coating and prevent dehydration of sodium chromate on the surface typical of the pictures presented. It would be more useful if Mr. Crowley, Sir, or anybody could share their experience on chromate rinse water specially on its disposal if it has to be, or just plain addition with water over time.

c) Has anybody have current total chromium analysis from the smoke (latent heat) generated by the galvanized article quenched directly in a chromate solution in whatever form (.e. acidified sodium dichromate, chromic acid, etc.)? I have read that chrome badges are not reliable but there is this colorimetric detector tube that can measure 0.2 to 1 mg/m3 of chromic acid from zinc chromate gases and vapors. Wonder if anyone who's using it can share data for benchmarking.

Regards.

A. Hi Paul. I am perhaps misunderstanding, but this component with bare chromate is put into a 400 °F oven for 7 minutes; then later put into a 375 °F oven for 75 minutes and people want to know why the chromate isn't reliably holding up?

I don't think anyone reasonably expects a bare chromate surface to survive a 375 °F bake. But perhaps the cured paint could survive activating and chromating if you'd like to reverse the chromating and painting sequence?

Regards,