Tribocorrosão em meio salino do aço super duplex UNS S32750 nitretado por plasma em baixas temperaturas

Abstract

Low-temperature plasma nitriding of duplex stainless steels has been employed to enhance surface hardness and wear performance without detrimental effects on corrosion resistance. However, few works evaluated the performance on simultaneous action of corrosion and wear, even less employing in situ electrochemical techniques. Open potential circuit test (OCP) and, more recently, linear polarization resistance test (LPR) have been used as tools to study the tribocorrosion behavior of stainless steels. The main goal of this work was to investigate the tribocorrosion performance in 0.5 M NaCl solution of plasma nitrided UNS S32750 super duplex stainless steel, treated at 300, 350 and 400 °C for 4 hours. To achieve that goal, ball-on-flat reciprocating tests in NaCl solution integrated to LPR, OCP and friction monitoring were employed. Two rubbing conditions were employed: first an intermittent one, where repassivation is likely to occur between sliding cycles, and second a continuous one, where the metal is continuously exposed to the harsh environment. Nitrided layers characterization by means of optical and scanning electron microscopy, grazing incidence X-ray diffraction (GIXRD) tests, microhardness tests, topography evaluation by optical interferometry, and corrosion by cyclic polarization tests (CP) were done to support discussions about the nitriding temperature effect on tribocorrosion performance. The results show that it was possible to obtain expanded layers with thickness up to 10 µm. GIXRD analysis and for the CP’s curves indicated that there wasn’t corrosion resistance loss by CrN precipitation for those conditions treated up to 350 ºC. Tribocorrosion results revealed significant volume loss reduction for nitrided samples, regardless the rubbing condition. The tribocorrosion performance was better for higher temperature nitrided conditions.