46065

Acceptable pH range for rinse bath between pickling and flux?  

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Dear Sir,
I would like to know what pH range is acceptable for the water rinse bath that is located after the pickling hydrochloric acid bath and before the flux bath. I have check it several time for the past weeks and I always get 2.0 despite the fact that the regenerating rinse water station is set on 4.0. Therefore I am wandering if something is going wrong with the rinse water station. Thanks a lot in advance for your advice !

Jean Harray
Dvlpt engineer - STACO SA - Manderfeld, Belgium

The rinse tank will continually have acid being added to it as it rises the work coming from the pickle tanks. The more work (or more surface area, or retained acid due to poor drainage) the more acid into the rinse tank.
Of course the best would be to run a nearly neutral rinse tank, but this requires some acid removal (or neutralisation). There are many systems of raising the pH, including replenishment with fresh water (but that usually means disposal problems), chemical neutralisation (but what is the product of that chemical reaction, and what effect on the flux?), or external treatment. (There are proprietry brands from advertisers on this site)
In my opinion, you should keep the pH above 3-4.
Its increasingly common to find double rinse tanks. Rinse the worst of the acid in 1st rinse, then a "polishing" rinse to complete. This second rinse tank doesn't get so much acid, so stays higher pH.
Whatever you do - there's huge money to be saved in getting this right. Acid carry forward to the flux means Fe with it, and that eventually means dross production. Its well worth per$ervering with!

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland

 

Sir:
Any pH below about 4 is good for the rinse between the acids and flux solution in hot dip galvanizing. Some people are mistaken to think the pH should be higher. I have extensive experience in lab and practical experience in many galvanizing plants.
Regards,

Dr. Thomas H. Cook, Galvanizing Consultant Hot Springs, South Dakota, USA