The effect of surface roughness on re-passivation and pitting corrosion of super duplex stainless steel UNS S 32760

Abstract

Stainless steel is used as a battery case material because of its excellent strength and corrosion resistance. Among the stainless steels used in fields requiring superior corrosion resistance, duplex stainless steels, composed of austenite and ferrite, exhibit improved strength and corrosion resistance. In particular, super duplex stainless steel (SDSS), with a pitting resistance equivalent number (PREN) between 40 and 50, offers excellent strength and corrosion resistance, making it suitable for structural components in various applications. However, the actual corrosion resistance of a material is often lower than its theoretical value, which has led to extensive research efforts to enhance practical corrosion resistance. Recent studies reported atomic-scale measurements of corrosion resistance and identified the influence of atomic layers on corrosion. Corrosion properties were studied by Trinh et al. (2020) and Nilsson (1992) [26]; however, they did not examine the re-passivation of the SDSS. SDSS requires re-passivation because of real corrosion. Although structural stainless steels are processed to achieve surface roughness in the micrometer range and undergo passivation, research on this aspect is still lacking. To enhance the performance of SDSS, research on re-passivation before and after pitting is required. Therefore, this study analyzed the effect of surface roughness variation from 100 to 1000 nm on the corrosion resistance and re-passivation with or without pitting of SDSS. The surface conditions corresponding to different roughness levels were analyzed using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM), while corrosion resistance was evaluated through open circuit potential (OCP) measurements, potentiodynamic polarization tests, and cyclic polarization tests at 0.6 V (before broken passivation layer) and 1.3 V (after broken passivation layer), as well as critical pitting temperature (CPT) tests. Surface roughness reduces the corrosion potential (Ecorr) and increases the corrosion current density (Icorr) for activation polarization. With passivation, as the surface roughness increased, uniform corrosion decreased, resulting in a stronger passive layer. The surface roughness lowered the OCP and increased the corrosion rate, thereby reducing the corrosion resistance. Therefore, when SDSS is used in an environment prone to corrosion, a surface roughness of less than 100 nm exhibits superior corrosion resistance.