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"Tin plating for very cold environments; alloying to avoid tin pest"

2003

I am wanting some parts tin plated. The environment is part of an installation on mountain tops, with temperatures real cold, (possibly below -30 °C and worst case for tin pest). My understanding is that the plating should contain traces of Antimony, Bismuth and Lead up to about 1% total, in order to avoid tin pest. The plating companies I have contacted advise that they plate pure tin, and there are never any troubles. I have a sample assay which shows 99.9% pure, with traces of the above at a level of about 0.02% each.

The parts are spacers, about 75 mm diameter, and 5 mm thick. The question is, do I have a problem. Will standard plating fail, or is the advise I have received indeed correct and I am worrying for nothing.

Cheers,

Tony Instone
Hardware Engineer - Wellington, Wellington, New Zealand
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2003

You are quite correct that pure tin would be troublesome at ù30 C; see letters #740 and #18849.

Refer your platers to Appendix X6.2.2 of ASTM B545 [affil. link to spec at Techstreet] : "Where electroplated tin coatings are subject to long-term storage or use at very low temperatures, it may be advisable, when specified by the purchaser, to codeposit small amounts (<1%) of bismuth, antimony, or lead with the tin. These alloying additions, particularly the first, have been shown to inhibit the transformation." According to ASM Handbook vol. 5 Surface Engineering, Federal spec. QQ-S-571 recommends 0.27% antimony although it seems that tin-bismuth plating solutions using methane sulfonic acid (MSA) are more readily available.

Ken Vlach [dec]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
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