Non-Destructive Evaluation of Material Degradation for Austenitic Stainless Steel by Means of Electro-Chemical Method

Austenitic stainless steels are extensively used for high temperature components, such as heater tubes and boiler tubes in chemical and electric power generation plants. It is well known that, for these materials, the metallurgical degradation occurs at an elevated temperature which is caused by the nucleation and growth of carbides and intermetallic phases. In this study, such metallurgical characteristics of the degraded SUS321 steel and SUS316 steel for a long term in service were investigated, and the non-destructive evaluation of the material deterioration was performed by means of an electro-chemical method. The results obtained are summarized as follows: (1) The microstructural change after a long term exposure in a high temperature environment for SUS321 steel is the nucleation and growth or the decrease of TiC, M₂₃C₆ type carbide and σ phase. These precipitates except TiC cause the material degradation. (2) These precipitates have active dissolution peaks at a particular potential independent each other in the anodic polarization measurement in 1N·KOH solution. (3) The metallurgical damage by the precipitation can be estimated in terms of the active dissolution current density (I_P value), since the amount of the precipitate has a good relation with I_P value at the specific potential obtained by the anodic polarization measurement. (4) The material degradation occuring on SUS316 stainless steel in a high temperature exposure can also be evaluated.