Stainless_steels

Stainless_steels - Stainless Steels Surface Oxidation Iron:...

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Stainless Steels
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Surface Oxidation – Iron: Take a piece of iron and convert the surface layer of iron to iron oxide (Fe 2 O 3 or “rust”) The oxide is less dense than the metal, so there is a volume expansion If the next layer of iron is oxidized, the expansion will cause the brittle layer of existing oxide to spall off, and corrosion becomes progressive – hence the term “rusting through”
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Surface Oxidation – Tungsten: Now take a piece of tungsten and convert the surface layer to tungsten oxide. The oxide is more dense – so there is a surface contraction. The brittle oxide cracks, and the surface looks like the dried mud on a river bank after the water has receded Oxidation progresses down the cracks.
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Surface Oxidation – Aluminum: Now take aluminum and convert the surface to aluminum oxide (Al 2 O 3 ) In this case, the densities of the metal and oxide match, so there is no expansion or contraction The aluminum effectively coats itself with a highly stable, unreactive (i.e. corrosion resistant) ceramic
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Surface Oxidation – Stainless Steels By adding at least 12% chromium to iron, oxidation now forms an iron- chromium oxide with matching density, and steel now behaves like aluminum It is important to note that the corrosion resistance of stainless steel is NOT due to a low chemical activity, but is the result of an adherent oxide (stable ceramic) coating.
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Stainless Steel: Stainless steel is an iron alloy with high
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Stainless_steels - Stainless Steels Surface Oxidation Iron:...

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