On September 3, Stephane Menard posted a question about a problem with pure
tin as a protective coating, namely whiskers. There is another problem with
tin plating that deserves comment: so-called "tin pest". Tin pest is the
conversion from a metallic crystal structure of tin (white) to a
nonmetallic, powdery form (gray).  According to Klein Wassink (p.148, 2d
edition), "the transformation may occur after a long incubation period at
temperatures below 13 C, the rate of transformation being highest at
approximately -30 C."  I don't know what the term "long incubation period"
means.  If someone were interested for a particular tin plating process, it
obviously wouldn't cost very much to find out, although it would take a
while, and of course, the most that anyone could say would be, "It didn't
happen during my investigation." They wouldn't be able to say, "It never
occurs" (and this applies as well to studies of whisker formation).  If it
did occur, it seems certain that that would spell the end of any protection
which the tin would provide. Parts are unlikely to be stored under
conditions that would result in phase transformation, and the condition
would of course be discovered during manufacturing if it were to occur. But
if the finish is being counted on for corrosion prevention during storage
and service of the delivered product, then tin pest may be a concern.


Gordon Davy
Northrop Grumman ESSS
Baltimore, MD
[log in to unmask]
410-993-7399

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