Hydrochloric Acid in Zinc Phosphate Pretreat
We are thinking of replacing the HCl acid bath that we are currently using on our zinc phosphate line, with the idea being that we would like to avoid using an acid scrubber. Would phosphoric acid get the job done?
Q. We are thinking of replacing the HCl acid bath that we are currently using on our zinc phosphate line, with the idea being that we would like to avoid using an acid scrubber. Would phosphoric acid get the job done? How about liquid/solid acid salts as an HCl replacement? –K.R.
A. It is not clear to me why you would be using hydrochloric acid (HCl) in a zinc phosphate line or at what stage you would be using it. I have never heard of this being done. If you are in need of some scale or rust removal prior to the zinc phosphating, phosphoric acid would be a much better alternative and more compatible with the remaining chemistries in you zinc phosphate line. That should allow you to remove your acid scrubber.
Additionally, acid cleaning of any type can lead to a modified zinc phosphate crystal structure compared to one with no acid exposure. In general, it is a larger, more porous crystal structure and is not as desirable as a smaller, less porous crystal structure. The use of the HCl may make this situation even worse since it is a stronger acid than the phosphoric.
In this paper, a review of several process solutions, examining coolants, solvent cleaning, alkaline clean/etch and deoxidizing/desmutting, listing intended and unintended chemical reactions along with possible mechanisms that would favor corrosion formation.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
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