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Cadmium plating corrosion products

August 27, 2009

I have been looking at some anodized aluminum parts which were believed to be mated with some CRES screws with Cd plating. The aluminum part had both a red and blue residue in a counter bore where the screw was believed to be inserted.

SEM/EDS analysis of the red powdery material reveals Cd, O and Fe.
SEM/EDS analysis of the blue material reveals elemental Cd and O (trace Si)
FTIR spectroscopy is consistent with cadmium acetate or at least a family of inorganic acetates.

I'm guessing that the Cd plating of the screw was scratched, creating a galvanic cell which resulted in the corrosion of the cadium to produce the cadmium acetate.

Does this sound reasonable?

I can't get my hands on any of the CRES screw.

Daniel Feeney
engineer - Tucson, Arizona, United States of America

Cadmium forms a very low voltage battery with aluminum and SS forms a much higher voltage with aluminum. That is why the screw was plated. Unfortunately, cad is soft and it is highly probable that it was scraped off during the process, especially if it was removed and reinserted. Many SS screws are of 400 series for the higher strength than the 300 series screw, so that is why you saw some iron. Some SS will have a trace of Si, but several aluminums have Si as an alloy.

James Watts
- Navarre, Florida
August 28, 2009