Knife line corrosion

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The knife line corrosion (ger .: knife-line attack ) occurs only when welding high-alloy , stabilized steels on. These are normally corrosion-resistant, as they form a protective chromium oxide layer from a chromium content of 12%.

In the case of materials stabilized by means of titanium in particular, the high thermal loads, for example from welding, can lead to thermal dissolution of the titanium carbides in a very narrow area next to the melting line. The carbon previously bound to the stabilizing element titanium is now dissolved in the lattice. However, the chromium present in the material reacts in this very narrow area of ​​the heat-affected zone with the free carbon to form chromium carbides. This precipitates preferentially at the grain boundaries. In this area, the steel may no longer be resistant to intergranular corrosion because the chromium bound in the carbide is no longer available for oxide formation. If the chromium content falls below 12% locally, no protective layer of chromium oxide forms, which protects the material from further corrosion. This is where the so-called knife line corrosion occurs.

The name knife line corrosion is derived from the damage pattern. The corrosion occurs in an area fine as a knife line at the edge of the weld seam.

Knife line corrosion is a special form of intergranular corrosion and related to this in terms of the damage mechanism. It can occur when welding austenitic as well as ferritic steels, the latter being more vulnerable due to their more permeable lattice structure.

In the case of corrosion-resistant steels, the binding of the carbon by niobium or titanium to niobium or titanium carbide (stabilized steels) or the lowering of the carbon content (LC = low carbon ) counteract knife line corrosion: These measures prevent the harmful depletion of the chromium content along the grain boundaries.

literature

  • Elsbeth Wendler-Kalsch, Hubert Graefen: Corrosion damage theory . Springer Verlag, Berlin / Heidelberg 2012, ISBN 978-3-642-30431-6 .
  • Günter Schulze: The metallurgy of welding. Ferrous materials - non-ferrous metallic materials. 4th edition. Springer Verlag, Berlin / Heidelberg 2010, ISBN 978-3-642-03182-3 .

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