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  for Metal Finishing since 1989
  

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Surface Pitting

Can you give any advise on the problem of surface pitting of aluminium 6082 plated electroless nickel?

Rob Dummer
- Mansfield, Notts, UK
2003

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Rob,

Somewhere to begin: First you have to find out if pits come from the substrate or are only contained in the plated layer. For this, you can slowly sand or lap a coupon until the first trace of substrate metal shows. If the pores are gone then it's you plating process or chemistry. If they are still there, it can be a poor base metal or the pretreatment prior to plating that is attacking it.

Guillermo Marrufo
Monterrey, NL, Mexico
2003

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Pitting in electroless nickel deposits on aluminum is most often caused by the basis material. To minimize pitting, do not caustic etch, Use mild acids after cleaning (e.g., 5% phosphoric acid) double zincate using either nitric acid 50-60% by vol, or 5% sulfuric acid with hydrogen peroxide or sodium persulphate. Over etching of aluminum alloys creates pits by dissolving either the alloying constituents or causing the aluminum to dissolve around the alloy materials by electrochemical corrosion.

Also, organic contamination of the electroless nickel may cause pitting, along with particle inclusions, bacterial mold, etc. Generally though, EN solutions do not pit unless badly contaminated.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
           and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])
2003