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Black Stain on Aluminum Extruded Electroless Nickel Part

Hello,

We manufacture an aluminum extruded component that has an Electroless Nickel plating.
Many of the parts in the latest batch display a 1/8"~ diameter black stain in the same general area. This stain is not in the racking area, and the parts do not touch during the plating process. Sometimes, the mark is not black, but only slightly discolored. If the parts are stripped, and re-finished, the mark goes away.
Any thoughts as to the probable cause?

Thanks in advance.

Matt Travers
Designer - Randolph, Massachusetts
August 1, 2008

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August 12, 2008

1-Check if the stain will go if left for longer time in plating.
2-Check if ,some how ,that spot was stressed by previous work.

Khair Shishani
aircraft maintenance - Al Ain, UAE