Detection and evaluation of material degradation of thermally aged duplex stainless steels: Electrochemical polarization test and AFM surface analysis

Thermal aging embrittlement of cast duplex stainless steels was evaluated by Charpy impact test and microhardness measurement. Also, for the purpose of the non-destructive evaluation of thermal aging embrittlement in cast duplex stainless steels, electrochemical polarization measurements for service exposed and laboratory aged materials were carried out in 0.1 M nitric acid solution and the depth of dissolution during the polarization measurement was measured with an atomic force microscope (AFM). In the polarization curves, the peak current densities of all specimens tested were appeared in the negative range of current and aged materials showed larger peak values than unaged ones. In the dissolved depth of austenite phase, no trend can be seen with aging time but those of ferrite phase were increased according to aging time, which has the same tendency as the polarization results. A good correlation between the dissolved depth measured by the AFM and the changes in mechanical properties of materials and, based on these results, an evaluation method for thermal aging embrittlement of duplex stainless steels was suggested.