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Blistering with corrosion of Silver/Copper Plating on Aluminum
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Silver plated aluminum (probably 6061)parts with a copper underplate exhibit blistering after 4-5 years in hermetically sealed environment. Parts submitted to me for failure analysis. These are commercial off the shelf parts purchased by my company.
I have 21 years experience with metal finishing and associated problems. Visually, this looks like a classic cleaning/rinsing problem. The external surface of the silver appears fine. I have performed an analysis of the corrosion residues using Energy Dispersive X-ray. The analysis shows pretty much what I expected. Potassium, copper, carbon, oxygen, silver, magnesium, zinc, aluminum, and a trace of chlorine (probably as chloride). Question is why do I not see nitrogen from the cyanide, assuming a cyanide based silver or copper bath was used?
Jay Otts- Garland, Texas
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You did not mention where the blistering occurs: from the Al or from the Cu. If the blisters separate from Al, cleaning and preparation should be reviewed, however, even with good preparation, if the copper thickness is insufficient to prevent near complete diffusion into the silver, blistering will occur. My experience tells me that a minimum of 0.003" of copper is required if any heating process such as soldering etc. is used. If the blisters are from the silver separating from copper, the problem is likely in the silver strike. (too much silver, low cyanide, low amps. contaminated solutions or rinses) chemical replacement deposits of silver on cu have little adhesion. Parts should go into the copper with current on (contact prior to entering the tank.)
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Don Baudrand Don Baudrand, Consultant  Poulsbo, Washington |
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