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I Forge Iron

attention all metalurgy ...


SLAG

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Oxidation kinetics is diffusion driven, and samples oxidized at various temperatures will plot as a straight line on a log-log plot of time and weight gain (all samples oxidized at a given temperature will be at approximately the same point). Extrapolation along the line is extremely accurate in predicting oxidation kinetics at any temperature between the maximum and minimum. 

Any kink in the plot between straight line segments of different slopes indicates a temperature at which the diffusion rate changes, whether due to a phase change, oxide spallation, or some other mechanism. What wasn't clearly defined in the article was whether the grain boundaries were of a separate phase than the grains themselves (i.e., grain boundary carbides or similar), or whether the mismatch between the crystal lattices at the boundaries (or just a concentration gradient causing lattice deformation)  is responsible for the significant increase in diffusion rate. Interesting either way. 

This is easily investigated with a TGA (thermo gravimetric analysis), in case anyone is in college and has interest in this or related topics. 

(A note for posterity: the above is evidence of a misspent youth if there ever was such.) 

 

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Can't post - trying to edit this. 

Sorry. The more understandable version is that for a specific material composition in a particular phase (i. e., steel in Austenite vs. ferrite vs. Martensite phases) the oxidation rate (as measured by the total weight increase of the sample)  will increase at a predictable rate for that given phase as temperature increases.

Sorry

Some things that cause exceptions to the predictably of the oxidation rate for a material of a specific chemical composition include:

-passing through a phase transition (ex. steel changing to the Austenite phase as it reaches the critical temperature - this is where it becomes non magnetic), 

-things like dislocations (something shifted relative to where it should be) , vacancies (holes in the crystal structure) , inclusions (something that doesn't generally belong where it is) , strain (stretching, compression , or other change in the relative distance or orientation between the atoms in the crystal structure), or other things which can impact diffusion rates. 

This can be measured with a sensitive scale in a controlled atmosphere furnace called a TGA.

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Well, posting little bits at a time seems to work, but one bit got eaten...

-another exception is forming dense, adherent (and protective) oxide scales like chrome oxide on stainless steel vs. rust (which spalls - falls off) on steel or forming different oxides (Fe3O4 vs. Fe2O3, for example) . 

Ugh,  good enough for now I hope . 

Please let me know if I didn't explain everything well, but my posts are being eaten and I cannot even report them without errors.

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Iron oxide (= rust). does not form a protective skin that would shield the underlying deeper iron from oxidation. In other words more iron rusts under the surface coat of rust. The rusting will continue until the iron is completely oxidized away.

Aluminum is a different story. Surface aluminum oxide, (aluminum "rust")  forms a very stable chemical, The oxide surface layer stops further aluminum oxidation under its coating. On average, a crushed iron can, discarded in temperate wet woods, will turn into rusty pieces and powder in about five years. On the other hand, a crushed aluminum can takes about five thousand years to similarly end up as powder.

Chrome oxide, also, forms a resistant surface layer. That property makes for "stainless" steel. ( most of those steels will oxidize given enough time or the right conditions).

I hope this note will add to the expertize given in earlier posts

SLAG.

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Thank you SLAG for expanding on the topic.

I was getting frustrated with the forum software and trying to post small bits quickly enough for my posts to merge, and I can see looking back that I did not adequately discuss dense and adherent vs. spalliing (and/or non-dense) oxides.

Your real world example, in particular, brings this seemingly obscure topic to life. 

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I follow you, I have a handle on oxidization already, you're expanding that handle nicely especially terminology. Yes, some metals and chemicals have extremely stable, almost inert oxides chrome more so than aluminum but gold has them all beat. Now I'm wondering about a gold steel alloy. We just saw an article about a gold titanium alloy as being much harder and stronger than the CP ti normally used in joint replacements and bone reconstruction appliances.

Modern science is sooooo cool.

Frosty The Lucky.

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