超纯奥氏体不锈钢00Cr25Ni22Mo2N非敏化态晶间腐蚀与磷(硅)的晶界偏聚

运用金相、电子显微镜观察及表面分析技术,研究了超纯奥氏体不锈钢00Cr25Ni22Mo2N制造的汽提管在尿素生产环境下的腐蚀形态和机理。结果表明:该钢种在生产条件下产生非敏化态晶间腐蚀。其特征是腐蚀介质不仅沿晶界向钢的内部渗入,而且同时向晶界的两侧扩展,因而在管断面上观察到的晶间腐蚀路径较宽,但深度较浅并呈现漏斗形貌。电子显微镜观察、二次离子质谱、俄歇能谱分析及计算表明,这种材料晶界不贫铬、无第二相沉淀,但有磷(硅)的高度偏聚。晶界区磷的含量(约25％)比晶内高约三个数量级。作者认为,磷的晶界偏聚造成的与晶内在腐蚀电解质溶液中的电位差,是这种超纯奥氏体不锈钢非敏化态晶间腐蚀的原因。

Intergranular Corrosion And Phosphor Segregation at Grain Boundary of Non-sensitized _(00)Cr_(25)Ni_(22)Mo_(2)N 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.