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Reuseability of chromate rinsewater in alkaline zinc plating?

I am working with a client who does rack alkaline zinc plating and trying to solve its excessive chemical use of its tri-chrome chromate dip. Due to increased production, their chemical usage for the chromate dip has increased significantly due to increased dragout and pH of the last alkaline zinc rinse. Because they do not want to increase rinsewater flow rate to wastewater treatment system, we are considering the counterflow of the chromate rinses back to the alkaline zinc rinse tank just before the chromate tank; the concept here is to have the acid in the rinse neutralize the alkali.

Are there any drawbacks to this concept? If so, are there easy corrections that could be made? Has anyone ever seen this kind of reuse of chromate rinsewater?

Steve Schulte
- Cincinnati, Ohio
2000

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While this can probably be done without adverse impact on plating quality, have you asked the supplier whether a dilute nitric dip is recommended with this zinc & chromate combination? If so, that would achieve your goal while improving quality, rather than risking it.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2000

Dear Steve,

Its an interesting idea and theoretically it may work. I worry however about the adhesion of the passivation to the Zinc plating and the effects on corrosion protection . Ill try this on my shop floor too and give you a reply in a week or two. However since I only Chloride Zinc plate, I do not have the same problem as you do but I am keen on conserving water and reducing discharge.

Regards,(watch this space again)

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2000

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Dear Steve, Tried it. Doesn't work , as it interferes with the quality of passivation. didn't get as far as the salt spray test since getting the passivation right didn't work for us. Maybe it can yet work for you. Regards,

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2000

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2000

Dear Mr. Schulte:

I have heard of attempts to do this, but none successful that I can remember. One problem was that precipitants were generated that settled onto the plating. They interfered with the formation of the chromate film and acted adversely in corrosion tests. Someday, perhaps, someone will come up with a technique that will render this idea practical. Sincerely,

Ed Budman [dec]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018