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"Chrome Plating Pitting and Metals Contamination"





February 18, 2010

I am a chemical engineer, just started working in a chrome plating shop. We are having some pitting problems, and I am slightly confused about what I have read about chrome pitting and metals contamination. Some people say metals contamination will only cause rough chrome, bad conductivity, but not pitting. Others say metals can cause pitting. (when I say metals, copper and iron seem to be the ones that matter)

Can anyone help set this straight?
If metals do cause pitting, what levels of contamination do I need to stay under?

Mark Robison
Engineer - Ogden, Utah, US
^


First of two simultaneous responses -- February 18, 2010

Are you plating decorative or hard chrome? What is your thickness? Do you use a mist suppressant?

Jon Barrows
Jon Barrows, MSF, EHSSC
Independence, Missouri
^


Second of two simultaneous responses -- February 19, 2010

Mark,

Pitting can come in many forms and have many causes. High levels of metal contamination can cause pitting under the right conditions, but I would look at two other possibilities first.

Firstly check that your pre-cleaning prior to plating is doing the job. If the surface is contaminated in any way after cleaning then pitting is very likely.

Secondly if the balance between the chromium (VI) oxide and sulphuric acid is out this causes pitting (usually a characteristic "teardrop" shape).

If neither of these things prove to be the problem then you will need to dummy out the solution. This is plating a large surface area with a low current density. This will help with the copper contamination. I don't think iron will be that much of a problem, unless it is particularly high.

Finally, get yourself one or two textbooks on the subject. My favorite is the Canning Handbook, but the Electroplaters Engineering Handbook is also an excellent reference book. Also check out the How to section on this website, very useful.

Brian Terry
Aerospace - Yeovil, Somerset, UK
^


February 22, 2010

We plate hard chrome, anywhere from flash chrome to 30 mils build, and we have no mist suppressant.

Mark Robison
- Ogden, Utah, US
^


February 24, 2010

Our iron and copper levels are all under 2 gm/L, mostly under 1 gm/L. From what I have read, these levels should be too low to cause problems, do you agree?

Mark Robison
- Ogden, Utah, US
^


February 25, 2010

Mark,

I certainly wouldn't expect the iron at that level to be a concern. I would be rather more worried about a copper level measurable in g/l, I would normally expect that in the ppm range.

I would consider a dummy plate to reduce the copper levels if I was you.

Saying that, your problem still sounds like a surface cleaning problem or a sulphate:chromic acid imbalance.

Brian Terry
Aerospace - Yeovil, Somerset, UK
^

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