Stainless steel

In metallurgy, stainless steel (inox) is defined{{ref|AISI}} as a ferrous alloy with a minimum of 10.5% chromium content. The name originates from the fact that stainless steel stains, or rusts, less easily than ordinary steel. Stainless steel has higher resistance to oxidation (rust) and corrosion in several environments.

Corrosion in stainless steel

Even a high-quality alloy can corrode under certain conditions. Because these modes of corrosion are more exotic and their immediate results are less visible than rust, they often escape notice and cause problems among those who are not familiar with them.

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Pitting corrosion

Passivation relies upon the tough layer of oxide described above. When deprived of oxygen (or when another species such as chloride competes as an anion), stainless steel lacks the ability to re-form a passivating film. In the worst case, almost all of the surface will be protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen. In extreme cases, the sharp tips of extremely long and narrow pits can cause stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in stainless alloys, but it can be prevented by ensuring that the material is exposed to oxygen (for example, by eliminating crevices) and protected from chloride wherever possible.

Related Topics:
Chloride - Anion - Stress concentration

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Weld decay and knifeline attack

Due to the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless steel. This chemical reaction robs the alloy of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries near welds) in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name implies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable{{ref|Jones}}.

Related Topics:
Welding - Carbide - Grain boundaries - Galvanic couple - Titanium - Niobium

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Rouging

Stainless steel can actually rust quite rapidly if it fails to form its protective oxide layer. This tends to happen when the stainless has had carbon steel forced into its surface, as by being dragged over carbon steel during installation, brushing with carbon steel, grinding with a contaminated wheel, or temporary welds to carbon steel.

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See Corrosion Doctors on Rouging.

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