Letter 34029

How to avoid white rust from a metal part that has been plated with zinc chromate?  

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We have a problem where a white rust grows on our finished part after plating. It happens very fast where the white rust can grow less than one day after the zinc chromate plating was done. Our plating supplier claimed that the white rust was due to the material porosity. Is it true? How to solve this problem?

Norseha Ngah Hamid
manufacturer - Bandar Baru Bangi, Selangor, Malaysia

My first guess would be that the parts weren't fully dried immediately after plating.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

MR.TED MOONEY IS RIGHT, PROBLEM WILL GET SOLVED IF POST PLATING TREATMENT IS DONE PROPERLY.

1.ZINC PLATING.
2.WATER RINSE.
3.WATER RINSE.
4.NITRIC ACID DIP 1/2%
5.WATER RINSE.
6.PASSIVATION BLUE.
7.WATER RINSE.
8.WATER RINSE.
9.HOT WATER RINSE PREFERABLY D.I., BELOW 60 DEG.CENT.
10. DRY.

Ajay Raina Punjab, India

Hi,

The material has a lot of porosity , as improper drying may not cause white rust with in a day, what is the present room temp of ur location???

Regards

Praveen Kumar     plating process supplier Mumbai, India

It sounds like what you have is a casting. Castings are notorious for this type of problem.

Platers (I'm not one - all I ever plated was Hull Cell panels) have tricks for dealing with parts like this. One I recall from conversations with them is: use a milder pretreatment cycle to avoid rupturing the "skin" that forms on the surface of the part during the casting process.

Platers?

Dave Wichern - The Bronx, New York

We are also facing this type of problem but in NSS test.

The material is heat treated ( carbonitrided ) before plating. We are facing the salt spray problem in the unmachined surface only.

The above said procedure has been followed.

But the material is travelling somewhere around 2500 Kilometres.

Rengarajan
- Punjab

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