Pitting corrosion behavior of lean duplex stainless steels in chloride and thiosulfate containing environments

Lean duplex stainless steels;Pitting corrosion;Thiosulfate

He, Liang ; Singh, Preet Materials Science and Engineering Gokhale, Arun Alamgir, Faisal Garmestani, Hamid McDowell, Matthew ; Singh, Preet

University:Georgia Institute of Technology

Department:Materials Science and Engineering

关键词: Lean duplex stainless steels;    Pitting corrosion;    Thiosulfate;   

Others  :  https://smartech.gatech.edu/bitstream/1853/62650/1/HE-DISSERTATION-2019.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

A number of chemical process industry streams contain chlorides along with thiosulfate or sulfate ions. Focus of this study is white water environment in paper machines, which is a neutral to acidic environment that mainly contains chloride, thiosulfate and sulfate anions. Because of the occurrence of problems like pitting corrosion, crevice corrosion and stress corrosion cracking, pulp and paper industry tends to select austenitic stainless steels and duplex stainless steels (DSSs) as construction materials for paper machine. DSSs are dual-phase Cr-Mo-Ni-Fe steels systems that possess both high mechanical strength and high corrosion resistance. Compared with standard DSSs like DSS 2205, lean DSSs (LDSSs) such as UNS S32101 and UNS S32003 with lower Cr and Ni content, are more cost-effective in less aggressive environments such as paper machine white water. However, as newly developed grades, their corrosion behavior has not been thoroughly investigated. In this work, pitting corrosion behavior of four LDSSs, including UNS S32304, S32101, S32003 and S82441 in near-neutral environments containing chloride and thiosulfate anions was systematically studied. UNS S32003 and S82441 with higher molybdenum content are found to be resistant to pitting corrosion in the studied environments. The addition of thiosulfate to chloride solution promotes pitting corrosion of UNS S32101 and S32304 mainly through preventing the repassivation of pits. A preferential dissolution of the ferrite phase was found during pitting corrosion of S32101 and S32304 samples. The comparison between different grades of LDSSs in simulated white water provides insights into materials invention and selection for paper machines in a cost-effective way. Scratch tests on the single ferrite phase and single austenite phase showed a higher pitting corrosion resistance of the austenite phase. Galvanic corrosion tests on the single ferrite and austenite phase in hydrochloric acid with the addition of thiosulfate showed that the ferrite phase was anodic to the austenite phase in the simulated pit solution. Further, effects of heat-treatment on the microstructural evolution and pitting corrosion behavior of LDSSs were studied. Effect of annealing temperature on the microstructure of relatively newly developed LDSS UNS S32003 and its effect on the pitting corrosion behavior was discussed and it was found that the maximum pitting corrosion resistance was reached when the pitting resistance of the two constituent phases was similar. Precipitation of secondary phases in LDSSs is slower compared with standard and super DSSs, but pitting corrosion behavior is sensitive to the microstructural change even when the fraction of the precipitates is below 1%.