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Effect of Cu Addition on the Microstructure and Passivation Behavior of Sn Alloyed Ferritic Stainless Steel in NaCl Solution Yang Li, Conglin Yao, Huabing Li, Lu Bai, Pengfei Du, Changyong Chen, and Zhouhua Jiang Submitted: 2 July 2020 / Revised: 8 October 2020 / Accepted: 18 October 2020 / Published online: 23 November 2020 In this work, the effect of Cu addition on the microstructure and corrosion passivation behavior of Sn alloyed ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C was investigated by optical microscope (OM), scanning electron microscope (SEM), energy-dispersion spectrum (EDS), potentiodynamic polarization curve and x-ray photoelectron spectroscopy (XPS). The results indicate that Cu addition has certain effect on grain refinement of ferritic stainless steel. Meanwhile, Cu addition has little influence on the cathodic corrosion process of ferritic stainless steel in 3.5 wt.% NaCl solution but shows beneficial effect on enhancing both the corrosion resistance of steel substrate and its passivation behavior. It has been found that the deposition of Cu particles at the bottom of corrosion pits is responsible for the better corrosion resistance and passivation behavior of ferritic stainless steel. Moreover, there is synergistic effect between Sn and Cu on enhancing the corrosion resistance of ferric stainless steel matrix and improving its passivation behavior in NaCl solution. Keywords

corrosion, ferritic stainless steel, microstructure, passivation, Sn and Cu alloying

1. Introduction Ferritic stainless steel has gradually become an excellent alternative material in many application fields due to its excellent performance in toughness, ductility, weldability, stress corrosion cracking resistance and lower cost compared with austenitic stainless steel (Ref 1-4). To further expand the application ranges of ferritic stainless steel with improved mechanical properties and corrosion resistance, the addition of small amounts of alloying elements has become one of the hotspots for current research (Ref 2, 5, 6). Tin (Sn), as one of the cost-effective elements, has been used for a long time in the steel industry and possesses abundant reserves in China (Ref 7). It is generally believed that Sn is a harmful element for mechanical property of steel; however, 0.1 wt.% Sn addition to stainless steel or oil tanker cargo tank steel can increase their corrosion resistance in Cl-containing acidic solutions (Ref 8, 9). It has been reported (Ref 10) that the uniform dispersion of Sn in steel can reduce the Fermi energy of steel, weaken the electrochemical activity and improve the corrosion resistance of steel in seawater environment. Kamimura et al. (Ref 11) have investigated the atmospheric corrosion behavior of Sncontaining steels in environment with high Cl, and the results suggest that Sn slows down the corrosion rate of steel by inhibiting the formation of adsorptive intermediates (FeOHad Yang Li, Conglin Yao, Huabing Li, Lu Bai, Pengfei Du, Changyong Chen, and Zho