Gerhard Haubner has asked about the thickness of oxides on tin. The place to look for info on this, as for so many other topics, is _Soldering in Electronics_, by R.J. Klein Wassink, now retired from Philips in Eindhoven. The second edition, dated 1989 (Ayr, Scotland, Electrochemical Publications, ISBN 0 901150 24 X) states on page 221 (I have omitted the references):
Two oxides of tin are known, SnO and SnO2. In the oxidation of tin in air below 200°C both play a part, although SnO is the main one. The tin-oxide layers are transparent to light, thus producing only a slight discoloration. Above 200°C, the thickness increases and interference colours become visible... Fresh layers [of tin oxide] have a thickness of about 1,5 nm and grow approximately logarithmically with time: 2 nm after a week, 3 nm after a year, and only 6 nm after 20 years.
At about 200°C, the growth rate is approximately twice the rate at 100°C. At 200°C after 24 hours, a layer of 30 nm is formed on the tin coating on copper wires. Both water and water vapor enhance the growth of the layer considerably, as is shown [in a table not reproduced in this posting] for tin in boiling water.
The effect of lead (in solid solution in the tin) on the oxidation rate of tin can be neglected. Antimony and bismuth assist oxidation, whilst phosphorus, zinc and indium retard growth... The tin [in tin-lead alloys] oxidizes preferentially, so that for solder alloys the main oxide is SnO, just as for pure tin... [Elsewhere he documents that the dross on wave solder pots containing tin-lead solder is mostly solder encapsulated with tin oxide.]
Dave Hillman and Morgan Tench of Rockwell have studied the oxides of tin using SERA (sequential electrochemical reduction analysis) technique, and since Dave subscribes to this forum, I'm sure he will have info to add.

Gordon Davy 
Baltimore, MD 
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410-993-7399 

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