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Stain on EN-plated surface

What is the cause of light stain on EN-plated stainless steel metal surface ? Some people say it is caused by reaction between aqueous solvent (eg. water) and EN-plated surface. Is it true ? If yes, why & how ? But why only a small portion of such parts that went through the DI washing line have such stain ?

Anthony Tan
- Singapore

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Try plating some sample pieces twice as thick and some half as thick and see if this delineates the problem, Anthony. My own experience is that this phenomena is often simply rusting of the substrate under a too thin and too porous plating. Good luck.

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

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Anthony:

1) Some baths inherently stain more than others. Lower phosphorous baths tend to stain more than high phos baths.

2) Deposits containing more sulfur tend to stain more.

3) Deposits from old baths stain more, since the deposit is more porous.

4) Dull deposits tend to show stains more.

5) More active deposits stain more. Reducing the temperature and pH in the plating bath may help.

6) Post plate rinsing is critical. DI is water recommended. Avoid using DI water over 65 degrees C on deposits containing <9%P, since it can tarnish the deposit. If hot DI water (over 65 deg. C) is used, avoid using air agitation, since it increases the chance of tarnishing.

Mike Barnstead
- Waterbury, Connecticut

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Hi,

This time I don't agree with Ted. It is very unlikely to have rust from the substrate even for very thin EN since the substrate is said to be SS. I would suggest after the plating an initial neutralization rinse in a mild alkaline water prior to the DI final rinse. Some EN deposits become stained by the bath's acid residues.

Guillermo Marrufo
Monterrey, NL, Mexico

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Ted, I think that you are "outnumbered" this time. I have to agree with all the other replies stating that the stains are caused by the lack of a post treatment. EN deposits tend to tarnish and a chromate post dip with good rinsing is essential.

Sara Michaeli
Tel-Aviv-Yafo, Israel

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I concede!

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