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topic 610

Some questions about chromate coating.


I am from Taiwan. I am a zinc plating worker.This is my first time using the internet to write a E-mail, so if any impoliteness please excuse me. I have some problems about chromate coating.

1. How to decide a chromate solution's dead cycle time and how to manage the solution using the dead cycle time data?
2. I have a spare part needing zinc plating and chromate coating, then we use a small punching machine to connect this spare part to another one. This process always hurts the chromate film and the parts don't pass the salt fog 96-hr. test.
So how can I:
(1). increase the chromate film characteristics to avoid the punching harm?
(2). have any method to recover the film when its broken (about 10% area)?
Sorry to take up your time. Thank you and hope to accept your assistance.

Have a nice day.

Niann Tsyrr Wen
zinc plating shop - Taipei, Taiwan


I'm glad to accept your help, and to have my e-mail in the net that I can meet other friends.

Sorry, about latest letter; first question my real aim is how to manage the chromate solution. My chromate solution is supplied by a vendor. Because the spare part share limit, it can't dyed complete in 10~30 sec, so I change the solution pH , use dilute NaOH solution, to equal 2., reaction time become 70~120 sec. The part have full good yellow dye color. But when I use some day , the part color become green by the same process, then that part must be processed again. This always puzzles me.
So about the above mention, I have other questions:
(1). Using the NaOH to change the chromate solution's pH, is this right?
Does it have any influence to the chromate film and solution?
(2).Why the spare part color go green when the solution was used some days ago? I think the Cr(III) increase does it . But the color sometimes is verdure, sometime is olive drab, so maybe something is wrong.

About the chromate film being broken by punching question. The assembly includes aluminum and considering acid residual problem, we can't treat after assembly.
Now for my observation:
(1).the film after punch, one time it make easy to remove by hand, and sometimes it look like small teeth of a saw near the broken area. But the film test by art gum, 30 stokes, 5 cm long, is OK.
(2).The assembly uses machine oil to lubricate the spare parts. Does that influence the film corrosion resistance? By the salt fog test, before assembly the treatment spare part can pass 96 hr time; after assembly the spare part no hurt part appear small black point.
Why is that?

Now I contract the chromate time, and make the film thinner. That can increase the film adhesion. And film color looks like some rose red. So now I manage it by color. I feel this is not good method.

Have a nice day.

Niann Tsyrr Wen
zinc plating shop - Taiwan

Hi, Niann. Thank you for your detailed letters, and I apologize that I do not have enough hands-on experience with this to supply the complete answers.

But, first, your supplier should be working with you to optimize the chromating. Normally a plating shop will control color and saturation by varying the immersion time, but will not make adjustments to the chromating solution excerpt according to the technical data sheet supplied with it.

If you are actually "punching" the component after plating, then you are exposing a new area of raw steel with no zinc plating. That is always going to be unsatisfactory. But if you mean you are riveting or staking the parts, such that the chromate film is abraded but all of the steel remains covered by zinc plating, it is possible to re-chromate the damaged area with a chromating "pen". Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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