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Tin plated part turns black patches in few days 3 - 5 days

We are doing Bright acid tin plating on Brass washers with an undercoat of copper 3 microns and an upper coat of tin 10 microns.

The parts look very bright and finish is good visually after 3 to 4 days certain area of the part turn greyish black.

Can any body help me how to avoid this problem in Acid tin bath?

Concentration of the bath ;
Tin Metal : 18 to 22g/l
H2So4 : 100 ml/litre.

Shreesha Acharya P
Manager - Karnataka, India
2007

First of two simultaneous responses --

Use a slightly alkaline solution as a post dip. It will help with the tarnish which mainly comes from break down products of the brightener.

Sara Michaeli
Tel-Aviv-Yafo, Israel
2007

Second of two simultaneous responses --

Dear Mr Acharya,

You need a proper post plating rinse additive that will remove the brightener film that is causing the darkening.

Get in touch with your vendor for this.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2007

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