Zinc Plating with Blue Chromate is turning color over time

A. Hi Elaine!

Photos of the problem could help us understand "how much" is the issue and how can we help. So, having said that, I would recommend:

1) Check abnormalities in chromating baths (pH, concentration, zinc or iron contamination as you mentioned).
2) Check abnormalities in the top sealer (it's inorganic? organic? a lacquer? This could be the source of your problem, too).
3) Check organic contamination of your zinc plating bath (could be, this is a long shot but happened once here).

So, if you kept digging into the problem and have some more information, send it so we can lend you a hand.

Have a nice week!

A. Hi Elaine,

Seems a low thickness problem to me. The greyish parts may be the parts with very low to no zinc, and the other ones with a little more zinc. But I could look around in your plating process instead of the chromating.

Hope.you can solve this. If you can share more data we will try yo guide you to interpret it and take some course of action.

Best of luck!

A. Hi Elaine
Iron contamination is a very common problem in acid zinc plating, and may well be your problem especially since it is supported by lab tests. The first thing to consider is that parts must not be allowed to fall off of the racks into the zinc tank and slowly dissolve in the acid; make sure you have good measures in place against this. The second issue is how to deal with the iron contamination. I am not familiar with the ion exchange method you speak of, but hydrogen peroxide treatment is a commonplace -- have you tried it? Thread 4169, "Iron contamination / buildup problem in acid zinc plating" is a very thorough discussion of that topic.
Luck & Regards,


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

A. Khadar,
Could you please provide us with some more information? For example is this a hex or tri clear? What types of parts are these? What conditions are the parts subjected to during these 10-12 weeks? Is the corrosion performance still up to par after the yellowing? What are the chemical properties of the bath? Etc.

The more information that you can give us the better.

Good Luck,

Dave

A. Dear Arun,

When there are yellowish tinges on the edges, mainly is a iron content problem of the zinc bath, or the blue chromate concentration is too high.

We see that problem in barrel pieces and solve it with diluted (5%) hydrogen peroxide. Do you test the zinc bath with Hull Cell ⇦ ? ? If you do, you could test it in Hull cell (2 A, 5 minutes, with agitation) and chromate the plates with and without peroxide added. Then you could make a conclusion!

Hope you could solve your issue. Best regards,

A. Hi Shazer. You are fortunate to have a rather consistent issue rather than an off and on problem. I think Step 1 is to do Hull Cell testing of the four baths. Because of the consistent nature of the problem this testing ought to quickly reveal whether the problem is in the chemistry of the bath vs. elsewhere. Good luck and please get back to us.

Regards,

A. When I used to have this problem it was mostly due to iron contamination of the bath. But maybe there could be another reasons for that problem.

A. The yellow chromate of a blue chromate formation coming out of a chloride solution is most likely high iron contamination.

A. Hi James. I think the first step is to leave a part unwrapped, wrap a part in paper, & wrap a part in different plastic and see whether the problem is related to the wrapping material/method or whether the yellowing always appears irrespective of the wrapping. Good luck.

Regards,

A. Hello Swamy,

Most likely iron content in your blue trivalent PASSIVATION tank, not in your plating bath. If it were in your tank, you would see black staining in high current zones, but yellow staining may be because of high iron content in your chromate :)

Best of luck!

A. Good day.

We had that kind of problem a few years ago.

The main reason is the dissolved iron in passivate, activation or sealer.

Please try to analyze iron by sulfosalisilic acid (Fe+3) or phenantroline + sodium disulfite (Fe+2).

Once a week take out the passivation to a relay tank and clean the walls and remove the pieces from the bottom.

In my experience the concentration over 150 ppm of iron (Fe+3) generates the problem of yellow pieces

A. This defect is related to copper contamination in your plating bath. Use titanium anode hooks instead of copper anode hooks.

A. Hello there,

We need more information about your zinc tank. Is it alkaline non-cyanide, or is it Acid zinc ? I assume is acid zinc since you have co-deposited impurities in the LCD area. Because of the geometry of your part plus being a barrel plating, your LCD area have very low current density, if its lower than 0,3 A/dm2 you have co-deposition of Iron. (Imagine doing dummy plating but with your parts instead of panels).
If you want correct passivate film on your zinc parts, the thickness should be minimum 5 µm. Also keep your Iron level low in both the tank and the passivation tank.
To check your bath for Iron, you simply can add a drop of H2O2 in a jar with sample from your bath: if it becomes cloudy brown, then your iron is high and you should treat it with H2O2. For barrel plating you can treat directly in the tank but don't add more than 0,1 ml/L at once. The iron will precipitate (considering your pH is higher than ~5.3-5.5) and will drop on the bottom of the tank. Repeat these steps until there is no precipitation occurring.

A. Mr. GANESAN,

If parts are yellowish after blue chromate, check blue chromate chemistry. Check the concentration, iron contamination.