Non-destructive evaluation of thermal aging embrittlement of duplex stainless steels

The non-destructive evaluation procedure for detecting thermal aging embrittlement of cast duplex stainless steels has been investigated. As a novel measurement technique for the thermal aging embrittlement, an electrochemical method was used and anodic polarization behaviors were measured on new, service exposed, and laboratory aged materials and then were compared with the results of the mechanical tests and microstructural changes. During the polarization experiments performed in potassium hydroxide solution (KOH), M₂₃C₆ carbides on phase boundary were preferentially dissolved, which was confirmed by the SEM after polarization measurements. The preferential dissolution of M₂₃C₆ carbides appeared as the large anodic current density on the polarization curves. Through the electrochemical potentiokinetic reactivation (EPR) experiments in sulphuric acid solution, the relation between reactivation ratio and microstructural change was discussed. In EPR experiments, the reactivation peaks corresponding to Cr depleted zone formed on ferrite/austenite boundaries by the precipitation of M₂₃C₆ carbides were obtained. Also, the non-destructive measurement and evaluation method of spinodal decomposition, which has been known as the primary mechanism of embrittlement in ferrite phase, was reviewed. When the materials, where spinodal decomposition occurred, were polarized in an acetic acid solution (CH₃COOH), larger critical anodic current densities were observed than those observed on new materials, and these results were consistent with the result of the microhardness measurement. Concerning these polarization results, a critical electric charge, which was required for stable passive films in passive metals, was defined and the relationship between the microstructural changes and this charge amount was reviewed under various polarization conditions in order to verify the polarization mechanism of the spinodally decomposed ferrite phase.