Have certainly seen that in lead-free solder process (wave and selective) 
before. Presume you are seeing from assay of your bath contents. We never 
really got to the bottom of rising levels in the wave solder bath, but 
coincident rising levels of Fe content gave a clue to the problem with the 
selective machine which had some minor stainless steel erosion issues. High tin 
content and higher alloy temperature will attack solder pump impellor and 
ducting etc if not treated to impede erosion. I have a presentation on this 
story if you think it might be helpful and I could send a copy by email offline 

Higher than normal levels of Ni can supposedly cause "grittiness" and "grainy 
joints" with SnPb alloy, but of course at least one patented lead-free alloy in 
wide use deliberately adds 0.05% Ni, so it can't be all bad. 

Nigel

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