Synergistic effect of cold work and hydrogen charging on the pitting susceptibility of 2205 duplex stainless steel
Introduction
Duplex stainless steel (DSS) consists of ferrite (α) and austenite (γ) and has the combined advantages of ferritic and austenitic stainless steels. DSSs are widely used in petrochemistry, construction, and nuclear, automotive, and marine industries due to their high strength, good corrosion resistance and easy processing [[1], [2], [3]]. In particular, high strength is usually required for stainless steels used in automotive, marine, and other fields [4], resulting in the widespread use of cold-worked stainless steel. Meanwhile, DSSs can be easily attacked by the unavoidable presence of hydrogen due to the specific service environment (like seawater) and pre-treatment processes such as welding, acid pickling and cathodic protection [[5], [6], [7]]. Both deformation [8,9] and hydrogen [[10], [11], [12], [13]] can affect the mechanical and electrochemical properties of DSSs.
Many researchers have investigated the effect of pre-strain applied by stretching or rolling on the anodic dissolution and pitting resistance of DSSs. Yang et al. [14] found that the corrosion resistance of 2205 DSS in 3.5 wt% NaCl solution decreased significantly as the pre-strained level increased. However, pre-strain had no effect on the general corrosion and pitting corrosion of 2507 DSS samples. Lv et al. [15] found that the corrosion resistance of the passive film on the pre-deformed 2205 samples in a borate buffer solution containing chloride ions decreased with increasing strain. However, Guo et al. [9] reported that the pitting potential of 2002 DSS in 3.5 wt% NaCl solution was not influenced by tensile deformation, but the passive current density of specimens increased as the degree of deformation increased. Breda et al. [16] suggested that cold deformation mainly affected the lean DSS grades such as 2101 DSS, whereas the high-alloyed DSSs like 2205 and 2507 steels were stable even after heavy deformation. A consistent mechanism explaining the effect of pre-strain on the pitting resistance of DSSs has not yet been clearly established.
For the effect of hydrogen on the pitting susceptibility of DSS, many studies have focused on the composition and semiconductor properties of the passive films before and after hydrogen charging. Guo et al. [13,17] claimed that hydrogen significantly increased the conductivity of the passive films on 2507 DSS samples, especially on austenite. The presence of hydrogen changed the conductivity type of the passive film from p-type to n-type. Yao et al. [18] also observed the presence of pure n-type semiconductors in the passive films of hydrogen-charged 2205 samples. Both groups reported a declining corrosion potential, increased passive current density, and an unaltered pitting potential after hydrogen charging, which was different from the reduced pitting potential observed after hydrogen charging in our previous research on 445 ferritic stainless steels [19]. In addition, Luo et al. [20] suggested that the γ and α phases in 2205 DSS showed different susceptibilities to the hydrogen-induced change of microstructure, which was similar to the results reported by Örnek [21]. These different susceptibilities may also result in different effects of hydrogen on the corrosion performance of the γ and α phases.
In addition to the respective effects of deformation and hydrogen on the pitting of stainless steels, there may also be synergistic effects of deformation and hydrogen. Synergistic effects of hydrogen and applied stress on passive current density and critical chloride concentration have been reported for several stainless steels [[22], [23], [24]]. These samples were hydrogen-charged under stress conditions below the yield stress. Wang et al. [25] reported a synergistic effect of hydrogen and stress on passive current density, especially for the 304 samples with an intermediate degree of hydrogen-charging, but no synergistic effect of hydrogen and stress was found on pitting potential. Bockris et al. [22,23] found that tensile stress increased the solubility and permeation of hydrogen in the lattice of AISI 4340 steel and iron, while the diffusion coefficient of hydrogen was not affected by the applied elastic stress. Yang et al. [24] claimed that the breakdown potential was unaffected by the applied tensile stress for the uncharged 304 stainless steel unless the concentration of chloride ions was high, but the critical chloride concentration was significantly lowered after hydrogen charging within the applied tensile stress range. The effect of hydrogen on the corrosion susceptibility of pre-deformed DSS samples has rarely been studied. Thus, considering the widespread use of deformed stainless steels in hydrogen-containing environments, it is necessary to investigate the synergistic effect of hydrogen and deformation on the pitting susceptibility of DSSs.
In this research, solution-treated 2205 DSS was cold-rolled to different degrees and then charged with hydrogen. The effects of deformation on the dislocation density, hydrogen content and corrosion performance of 2205 DSS samples were studied. The synergistic influence of pre-deformation and hydrogen-charging on the pitting susceptibility of 2205 DSS samples in neutral chloride solutions was investigated and discussed.
Section snippets
Sample preparation and hydrogen charging
The materials used in this study were 2205 DSS samples provided by Baosteel Inc. with a chemical composition shown in Table 1. The as-received samples were solution-treated at 1050 °C for 0.5 h and water quenched. The solution-treated samples were then subjected to unidirectional cold rolling to gradually reduce the thickness by compression. Six 2205 DSS samples with thickness reductions of 0%, 10%, 20%, 40%, 60%, and 80% were investigated. Prior to the electrochemical tests, all samples were
Crystal structure and dislocation density of the cold-rolled 2205 DSS samples
The crystal structure and dislocation density of the cold-rolled 2205 DSS samples were studied before hydrogen charging. Fig. 1 shows the XRD patterns of the 2205 DSS samples with different degrees of cold work. The diffraction peaks of austenite are γ(111), γ(200), γ(220), and γ(311) according to JCPDS card file 00-052-0512, and the diffraction peaks of ferrite are α(110), α(200), and α(211) according to JCPDS card file 03-065-4899. The peak locations are similar to the values presented in the
Conclusions
In this work, the respective and synergetic effects of hydrogen and deformation on the pitting susceptibility of 2205 duplex stainless steel in 1 M NaCl solution were investigated and discussed, and the following conclusions can be drawn:
- (1)
The dislocation densities of austenite and ferrite inside the 2205 DSS samples both increased as the degree of cold work increased from 0% to 80%.
- (2)
The hydrogen content in 2205 DSS samples increased from 13.52 ppm to 41.49 ppm as the degree of cold work increased
Acknowledgments
This work was supported by the National Key Research and Development Program of China (Grant No. 2018YFB0704400) and the National Natural Science Foundation of China (Grant No. 51671059, 51871061, and 51801028).
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