Effect of the quality of the surface of the channel in tubes on their resistance to intercrystalline corrosion

The result of tests conducted to evaluate resistance to intercrystalline corrosion is an important characteristic of the quality of tubes made of corrosion-resistant steel. Intercrystalline corrosion (ICC) is due to the selective dissolution of grain boundaries under the influence of corrosion-active media. The metal becomes brittle as a result of fracture of the boundaries, and a tube made of such metal can fail under small static and - especially - dynamic loads. The mechanism of ICC is connected with the formation of new phases and segregation at the grain boundaries during processing operations (heat treatment, plastic deformation, welding) or during service of the tubes at high temperatures. In corrosion-resistant steels, these phases are often carbides. The development of ICC in austenitic chromium-nickel steels with and without stablization by titanium depends on the carbon content of the solid solution, the composition of the carbides, and the rate of diffusion of chromium during heating in the high-temperature range associated with rapid carbide precipitation. Steels with a low carbon content have the highest resistance to intercrystalline corrosion. The course of ICC also depends on the grain size. The rate of corrosion resulting from the mechanical erosion of grains from the surface is higher in metal with a fine-grained structure. However, the depth of ICC is shallower, the finer the grains. In this article, we show the dependence of resistance to intercrystalline corrosion on the quality of the surface of the channel of tubes