Metal finishing Q&As since 1989

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Spotting out in Rack plated Acid Zinc components.

Hi and thanks for looking in,

Our Chloride Zinc plated steel pressings are sometimes spotting out with grey dots appearing like a rash after two or three months in store.

Whilst porosity in the steel sheet cannot be ruled out, our experience tells me that spotting does not generally occur in steel pressed components in sheet metal parts not more than 2.00 mm thick. There is something we are doing wrong in our bath maintenance which is causing this. Our normal conc. is like this: Metal 25 -30 gms/lit, Boric likewise, Potassium Chloride 120 -135 gms/lit. Is the brightener system to blame or could any other criteria be the cause of this ?

Many thanks in advance and all the best to readers for the new milennium.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2000

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Is this in barrel plating? Do the spots look like they could be lining up with the barrel perforations. This could be perhaps a very mild case of the well known measles of iron contamination.

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

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First I would check the iron as Ted recommended. The iron that is able to cause this kind of problem is iron plus 2. Take a small sample (like 100 ml) and add a few drops of hydrogen peroxide. If you get a brown precipitate it means that you have a lot of iron plus 2 contamination. The way to get rid of it is adding hydrogen peroxide or potassium permanganate [on eBay or Amazon] to turn the iron plus 2 into plus 3 and then to filter out the sludge.

In case you find no iron, raise the concentration of your make up addition agent.

Sara Michaeli
Tel-Aviv-Yafo, Israel
2000

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Duh. I should have read the title before I asked whether this was barrel plating, huh?

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

Hey K Vahanwala,

The bath chemistry does not know the difference between the thick and thin metals. The difference is in the steels (pores and / or inclusions). If the steel is spotting out after a few weeks, it's salts in the pores of the steel that is the problem. Surface roughness needs to be limited (spec it out)use a surface profiler to measure the surface of the two steels and use your results to find a pass/fail number for the steels. Other controls may be: better rinsing (alternate hot and cold water rinses can pump out the pores), more thickness in the pores (better throwing power).

Regards,

Fred Mueller, CEF
- Royersford, Pennsylvania
2000

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Thank you all, Sometimes my common sense goes holidaying. It was on vacation when I posted the question!

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2000

Dear Sir/Madam.

We are in the business of Electro-Galvanizing of Mild Steel Wires using the Zinc Sulphate solution. We are having a problem of the finished goods getting rusted too soon. We also want to know what we should use as brightener for good finishing and texture.

We really appreciate your kind cooperation in this regard. Thank You.

Mohammad Noor Ali
- Karachi, Pakistan
2001