What Does Knifeline Attack (KLA) Mean?
A knifeline attack (KLA) is a type of intergranular corrosion involving a stabilized stainless steel or an alloy. It typically occurs along adjacent or connected weld posts, heated into a temperature range suitable for sensitization. The corrosion is restricted to an extremely narrow line, neighboring the line of fusion.
This corrosion attack appears as sharp as razor, thus the term "knifeline attack." It is highly possible to recognize the attack visually if the lines have already formed next to the weld. In this type of corrosion, the crystalline boundaries of materials are exceptionally prone to attack rather than the interiors.
Several alloys can go through KLA, but the best example is that of stainless steel. This is due to the depletion of chromium at the surrounding area of the boundaries due to chromium carbides’ intergranular precipitation during a thermal cycle.
Corrosionpedia Explains Knifeline Attack (KLA)
KLA is caused when, in stabilized alloys and steels, the carbon comes in contact with stabilizers such as niobium and titanium. In such, there will be no weld corrosion in the zones affected by heat during the welding process. However, when chromium carbide precipitates during subsequent welding or heat treatment, there is a possibility that the narrow band close to the fusion line undergoes a knifeline attack.
A good example of a knifeline attack is one involving steels that are stabilized by niobium, like stainless steel 347. Niobium and titanium, along with their associated carbides, can dissolve the steel at extremely high temperatures. With certain cooling systems, niobium does not undergo precipitation, so the steel acts like unstabilized steel, building chromium carbide as an alternative. This affects a very thin zone in the surrounding weld, making it difficult to notice the speed of corrosion. Materials that are made from steel should undergo heating at around 1950°F as the niobium carbide develops and the chromium carbide disbands. The cooling process after this treatment is not essential, since the carbon which is supposed to turn into chromium carbide is seized as niobium carbide.