Knife corrosion.

Due to elevated welding temperatures or unreasonably high heat treatment temperatures, chromium carbides concentrate at the grain boundaries of stainless steel. This leads to oxidation of the chromium on the grain boundaries, creation of a galvanic pair in relation to the base material and corrosion of the weld in aggressive environments. Special alloys, low carbon or with additions of titanium and niobium (in alloys, 321 and 347) can eliminate this effect, but with high temperature treatment after welding to prevent knife corrosion. The name itself indicates that the corrosion zone is limited. Often it is only a few microns across. Such narrow gaps make it difficult for oxygen to enter, without which a protective oxide film cannot form, resulting in accelerated corrosion. The shear corrosion zone is usually close to the weld and is almost invisible.

Modern metallurgy has largely avoided these problems by reducing the percentage of carbon in stainless steels below 0.3%. Such steels are called L-grades, such as 316L. Most modern stainless steels are melted with low carbon content.

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