Corrosion of the weld
Due to the increased welding temperature or the unreasonably high heat treatment temperature, chromium carbides concentrate on the boundaries of the stainless steel grains. This leads to the oxidation of chromium at the grain boundaries, the creation of a galvanic pair in relation to the base material and the corrosion of the weld in aggressive media. Special alloys with a low carbon content or with additives of titanium and niobium (in alloys 321 and 347) can eliminate this effect, but under the condition of high temperature treatment after welding to prevent knife corrosion. The name itself indicates that the corrosion zone is limited. Often — it’s just a few microns across. Such narrow slots make it difficult to access oxygen, without which it is impossible to form a protective oxide film, which leads to faster corrosion. The zone of knife corrosion is usually located near the weld and is almost imperceptible.
Modern metallurgy largely avoids these problems by reducing the percentage of carbon in stainless steels below 0.3%. Such varieties are called L-varieties, for example, 316L. Most of the modern stainless steels are smelted with low carbon content.
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