Dear Sir /Madam
We generally receive fire accident cases where we find some steels structures /Mechanical components /motor engine parts/ motor body parts
Our challenge is to find out part is which is corroded naturally and which part has been oxidized during fire .
You've mentioned motor parts or parts of some engine, which can be corroded by hot/warm grease/char/combustion product as well. It this case, High temperature corrosion and low temperature corrosion form some sort of continuum. Suppose the exhaust of motorcycle. Hot gases leaving through its exit can have temperature easily above 150 degrees celsius and can contain Carbon monoxide, char, PAH, organic pyrolysis products like lower aldehydes etc. The final end of exhaust, when the motorcycle stopped and cool down, may be hydrated by moisture, or often penetrated by mud/sludge/dirt. Again heated up, the high and low temperature corrosion components would mutually react, and corrosion history of the surface would be overwritten (e.g. hydrated rust drying, then replaced by high boiling point organic compounds, then pyrolyzing......) and of course, the exhaust can contain Carbon monoxide, and of course water vapour by combustion of hydrocarbon fuel, that can react with metallic iron to catalyze some rather complicated reaction (air Nitrogen included as well)
High temperature failure of structural materials in usual fire accident is almost always accompanied with some hint of char. Frequently thermal fatigue and spalling causes more structural failure. Th hot, failed specimen can undergo regrowth of crystal grains as well, due to high temperature. Low temperature corrosion product are hydrous, and biofouling is often observed (e. g. moss growing on rust, mussel growing on ship) which may be charred due to fire incident, or show sign of desiccation. No such sign would be seen for purely high temperature corrosion.
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