As a footnote: some of the (all tin) tincans used in Antarctic expeditions at the
beginning of this century are reportedly still sound and I guess that is as long an
ageing test as can be practically done.
Mike Fenner
BSP, OX15 4JQ, England
T: +44 1295 722 992
M: +44 789 999 7715
F: +44 1295 720 937
-----Original Message-----
From: Kay Nimmo <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 06 September 1999 13:17
Subject: Re: [LF] Problems with pure tin
>Dear Gordon,
>
>The occurrence of tin pest is very unlikely. ITRI has investigated a number
>of occurrences over the years which have been attributed to tin pest but
>these have all been found to be due to other factors (mainly various forms
>of corrosion).
>
>As you say, crystal transformation is theoretically possible below 130C but
>in practice any occurrence is preceded by an incubation time of several
>years during which no sign of transformation can be seen. We now believe the
>fastest transformation rate to be at -400C, however, time to transformation
>depends on a number of factors:
>
>In particular, high purity tin is more prone to tin pest than standard
>purity tin or any tin alloys. Even so very high purity tin has been stored
>at -400C for over three years with no signs of tin pest. When alloyed, the
>likelihood of transformation is greatly reduced and e.g. commercial purity
>tin-lead has been stored for over 10 years at -400C with no signs of
>transformation.
>
>It has also been found in laboratory studies that innoculation of white
>(standard) tin with grey tin is necessary to promote the crystal change.
>Even so, low temperature for several years is still required. A number of
>studies on tin pest had to be abandoned because no grey tin was available
>for this innoculation. Many (unsuccessful!) efforts were made to form grey
>tin for this purpose.
>
>Exposure to low temperature must also be continuous for the number of years
>stated. Intermittent exposure to -400C or other low temperatures does not
>'add up' to the same effect.
>
>Gordon is right that the experiments are fairly easy to carry out but most
>people give up/retire before any observations are made! It is also correct
>in assuming that few parts will be stored under the necessary conditions for
>tin pest to occur .... if some have then they are likely to have problems
>due to other factors than tin pest.
>
>Kay Nimmo
>
>+++++ Visit our lead-free.org website +++++
>ITRI Ltd, Kingston Lane, Uxbridge, Middx, UK, UB8 3PJ
>tel: +44 (0)1895 272406 fax: +44 (0)1895 251841
>email: [log in to unmask] www.itri.co.uk and www.lead-free.org
>
>-----Original Message-----
>From: Leadfree [mailto:[log in to unmask]] On Behalf Of Davy, Gordon
>Sent: 03 September 1999 20:44
>To: [log in to unmask]
>Subject: [LF] Problems with pure tin
>
>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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