It took a bit of finding but here is a couple of shots, taken some 8 years ago, of a single skin 316 SS flue which had been allowed to cool the combustion gases below their dew point. the acidic conditions had created an oxygen free environment that had allowed the chromium oxide (a thin layer which protects the iron content from rusting in normal use) to be stripped and the tube then perforated. I suspect there was something other than clean wood being burnt because the stain underneath looks