Intergranular Corrosion And Phosphor Segregation at Grain Boundary of Non-sensitized 00Cr25Ni22Mo2N Superpure Austenitic Stainless Steel

The corrosion behavior and mechanism of 00Cr25Ni22Mo2N superpure austenitic stainless steel in urea processing environment was studied using metallography, SEM, TEM, SIMS and AES techniques. The results show that the gas extraction tube made of non-sensitized 00Cr25Ni-22Mo2N steel suffered intergranular corrosion. Corrosive media penetrated not only into the tube wall through grain boundaries but also expan ded from the boundary towards the inside of the grain. Therefore, funnel shape penetration paths were formed, which were wider but less deep than common intergranular corrosion of sensitized stainless steel. Neither depletion of chromium nor precipitates were found at the grain-boundary. However, phosphor (and silicon) was segregated at grain boundaries to a great extent. Semiquantitative calculation indicates that phosphor content at the grain boundary is about 25wt%, three orders of magnitude higher than the content whithin the grain. The authors suggest that the potential difference between the grain and its boundary due to the segregation resulted in the observed intergranular corrosion.