INTERGRANULAR CORROSION AND PHOSPHOR SEGREGATION AT GRAIN BOUNDARY OF NON-SENSITIZED 00Cr25Ni22Mo2N SUPERPURE AUSTENITIC STAINLESS STEEL
超纯奥氏体不锈钢00Cr25Ni22Mo_2N非敏化态晶间腐蚀与磷(硅)的晶界偏聚

The corrosion behaviour and mechanism of superpure austenitic stain-less steel 00Cr25Ni22Mo2N in urea processing environment was studied. The resultsshow that the gas extraction tube made of non-sensitized 00Cr25Ni22Mo2N steelsuffered intergranular corrosion. Corrosive media penetrated not only into the tubewall through grain boundaries but also expanded from the boundary towards theinterior of the grain. Therefore, funnel shape penetration paths were formed, whichwere wider but less deep than common intergranular corrosion of sensitized stain-less steel. Neither depletion of Cr nor precipitates were found at the grain bound-ary. However, P (and Si) was segregated at grain boundaries to a great extent.Semiquantitative calculation indicates that P content at the grain boundary isabout 25 wt-%, three oders of magnitude higher than the content within the grain.Suggestion is made that the potential difference between the grain and its bound-ary due to the segregation resulted in the observed intergranular corrosion.