Somewhat extinguished member
- 8 Nov 2014
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- West Yorkshire
Thanks MM. That tallies with this (old, but very interesting) paper I found: https://nvlpubs.nist.gov/nistpubs/jres/ ... 63_A1b.pdfMusicMan":17ezf8er said:Yes, colour indicates the thickness of the oxide, and nothing else. The well-known charts are compiled for a standard 'blacksmith' rate of heating, hence they are practically useful.
If you leave the metal at a fixed temperature for some time, the oxide will thicken and hence the colour will change even though the temperature has not.
If you want the maths, there is no such thing as the temperature that oxide starts to form, or rather, it is absolute zero. The rate of growth increases exponentially with temperature, and logarithmically with time at that temperature, assuming a coherent oxide that does not crack off. If it does, then the growth rate is more or less linear with time. Iron oxide is coherent when thin and incoherent when thick.
EDIT: I find a lot of people are also incoherent when thick. Ahem.