Editing Corrosion (section)

=== Weld decay and knifeline attack ===
{{Main|Intergranular corrosion}}
[[File:Unsensitised structure of type 304 stainless steel.jpg|thumb|Normal microstructure of Type 304 stainless steel surface]]
[[File:Sensitized structure of 304 stainless steel.jpg|thumb|Sensitized metallic microstructure, showing wider intergranular boundaries]]
Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a major alloying component ([[chromium]], at least 11.5%). Because of the elevated temperatures of [[welding]] and heat treatment, [[chromium carbide]]s can form in the [[crystallite|grain boundaries]] of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a [[galvanic corrosion|galvanic couple]] with the well-protected alloy nearby, which leads to "weld decay" (corrosion of the grain boundaries in the heat affected zones) in highly corrosive environments. This process can seriously reduce the mechanical strength of welded joints over time.

A stainless steel is said to be "sensitized" if chromium carbides are formed in the microstructure. A typical microstructure of a normalized [[SAE 304 stainless steel|type 304 stainless steel]] shows no signs of sensitization, while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries.

Special alloys, either with low carbon content or with added carbon "[[getter]]s" 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, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometers across, making it even less noticeable.