Influence of Solidification Structure on Pitting Corrosion Resistance of 317L Steel in Simulated wet-process Phosphoric Acid Environment

ZOU Dening; LI Miaomiao; LI Yunong; HUI Pengbo; HE Chan; HUANG Dongsheng; DU Chuanxiang

doi:10.16490/j.cnki.issn.1001-3660.2025.18.007

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Surface Technology ›› 2025, Vol. 54 ›› Issue (18) : 65-76. DOI: 10.16490/j.cnki.issn.1001-3660.2025.18.007

Influence of Solidification Structure on Pitting Corrosion Resistance of 317L Steel in Simulated wet-process Phosphoric Acid Environment

ZOU Dening^1,2, LI Miaomiao^2,*, LI Yunong², HUI Pengbo², HE Chan², HUANG Dongsheng², DU Chuanxiang¹

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Abstract

The corrosive environment (especially in the filtration section) within a wet-process phosphoric acid (WPA) system is particularly complex, where elevated high-chloride concentrations induce localized pitting corrosion of the filter plates, posing a significant challenge to the integrity of 317L austenitic stainless steel. However, it is important to emphasize that the enhanced alloying content present in 317L steel contributes to a complex and heterogeneous solidification microstructure. This intricate microstructure is instrumental in dictating the material's overall corrosion resistance. This study aims to investigate the corrosion mechanism of 317L steel under varying cooling rates (6, 100, and 1 000 ℃/min) in a simulated WPA environment, with emphasis on microstructure-corrosion property relationships. The corrosion behavior of the test steel is significantly influenced by the grain size, phase distribution and secondary phase, which is critical to optimizing the performance of 317L stainless steel in demanding applications. A high-temperature confocal laser scanning microscopy (HT-CLSM) is employed to meticulously control the solidification dynamic within the mushy zone, generating specimens with distinct solidification morphologies. This approach facilitates the preparation of samples exhibiting distinct solidification microstructures. Comprehensive microstructural characterization of the 317L steel is conducted through advanced techniques such as scanning electron microscopy (SEM) coupled with backscattered electron diffraction (BSD) analysis. Furthermore, the pitting corrosion resistance and passive film composition within the simulated WPA environment are systematically evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The findings reveal that 317L steel solidified with the presence of δ-ferrite and austenite phases, and accelerated cooling rates lead to gradually transformation from dendritic to cluster-like organization, accompanied by significantly refined solidified-organization and diminished dendritic spacing of the 317L steel progressively. Concurrently, a downward trend is exhibited in the self-corrosion current density (J_corr) value, while the upward trajectories are demonstrated both the self-corrosion potential (E_corr) and pitting potential (E_P) values. And the expanded radius of the impedance spectrum indicates an enhancement in pitting resistance. The sum of R_f and R_ct obtained using the equivalent circuit fitting reaches a maximum at 1 000 ℃/min, which indicates that the charge transfer at the interface between the passive film and the solution as well as in the pitting holes is subject to the greatest resistance. At a cooling rate of 1 000 ℃/min, the characteristic peaks of various elements are most pronounced in the XPS fine spectrum of 317L steel, revealing that the passive film is predominantly composed of Cr₂O₃, CrO₃, Fe₃O₄, Fe(OH)O, MoO₂, and MoO₃. Furthermore, the pitting corrosion resistance of 317L stainless steel in the WPA simulated environment is mainly related to the degree of segregation of various alloying elements (especially Cr and Mo) within its solidification microstructure. Increasing the cooling rate during the solidification process can effectively refine the solidification microstructure of 317L stainless steel, thereby reducing the segregation of elements that are prone to strongly segregation. This microstructural refinement process is crucial as it eliminates the formation of distinct chromium-depleted and molybdenum-depleted zones within the steel. Consequently, the enhanced elemental homogeneity contributes to a compact passive film on the 317L stainless steel surface, which is essential for improving its overall corrosion resistance and longevity in aggressive environments.

Key words

austenitic stainless steel / solidification structure / wet-process phosphoric acid / pitting corrosion / passive film

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ZOU Dening, LI Miaomiao, LI Yunong, HUI Pengbo, HE Chan, HUANG Dongsheng, DU Chuanxiang. Influence of Solidification Structure on Pitting Corrosion Resistance of 317L Steel in Simulated wet-process Phosphoric Acid Environment[J]. Surface Technology. 2025, 54(18): 65-76 https://doi.org/10.16490/j.cnki.issn.1001-3660.2025.18.007

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Funding

Key Research and Development Program of Shanxi Province (202202050201019); National Natural Science Foundation of China (52271067); Key Scientific Research Program of the Shaanxi Provincial Education Department (24JR138)