Influence of copper addition on the passivity and on the pitting corrosion resistance of super duplex stainless steel

摘要

Super duplex stainless steels (SDSS) are important engineering materials, due to their generally high corrosion resistance combined with high strength and moderate alloy cost (lower nickel and molybdenum content) compared to austenitic grades with comparable corrosion resistance properties (6Mo austenitic SS). They are used in various industrial sectors (oil & gas, desalination, geothermal, pollution control and pulp and paper industries). In this paper, the influence of the addition of Cu in SDSS on their corrosion resistance is investigated. Two alloys are studied: UNS S32750 (0.088 %wt. Cu) and UNS S32520 (1.602 %wt. Cu). To our knowledge, the effect of the addition of Cu in SDSS on their corrosion resistance has never been studied using local electrochemical techniques and surface analysis methods. The thickness and chemical composition of the passive films was first investigated after surface preparation (XPS and AES). The pitting corrosion resistance was then investigated in NaCl-based media using the critical pitting temperature (CPT) testing method combined with the electrochemical microcell technique. Cu is present in the passive film formed on UNS S32520. The ratio Cr/Fe and Fe~(3+)/Fe~(2+), the thickness and the nitrogen content were then affected . A mechanistic model was proposed to explain the role of Cu in the passivity of SDSS. Results obtained from CPT measurements were analysed considering changes observed in the passive film properties.