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Dark streaks develop on tin plated-aluminum

I have a tin plated aluminum part that uses a copper flash intermediate. Initially, the part looks consistently tin-gray. Dark streaks have developed over time. Surface analysis shows that copper is more apparent in the dark streak areas. Does anyone have any experience with the source of the dark streaking problem? Could the tin plating have not properly bonded in this region, and over time oxidation of the copper flashing in the low-tin region starts to darken the surface appearance.

A Yerges
elect. manufacturer - Milwaukee, Wisconsin
2004

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2004

Dear A Yergis,

Is your Aluminium a casting or an extrusion?

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

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Yerges.

I suppose you use a acid tin. Maybe you have a lot of contamination of copper in the tin solution. I believe your supplier has some additives to eliminate this problem.

Regards,

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
2004