STRESS CORROSION CRACKING OF SELECTED AUSTENITIC AND DUPLEX STAINLESS STEELS IN POTASH BRINE ENVIRONMENT

Abstract

Potash processing plants generally use austenitic stainless steels in their piping system due to their relatively good mechanical and corrosion properties. However, when exposed to hot chloride environments, these steels are susceptible to stress corrosion cracking (SCC) attack, which may compromise the equipment's integrity and lead to a costly downtime and product wastage. This research investigated the performance of the UNS S31603 austenitic stainless steel (ASS) and three duplex stainless steels (DSS) (UNS S32205, UNS S32760, and UNS S32550) in relation to SCC damage in a saturated potash brine solution. The electrochemical corrosion behavior of these alloys in a chloride environment and their strain rate sensitivities were also investigated. SCC experiments were performed using the slow strain rate test technique (SSRT). The effect of strain rate (10-6 to 5 x 10-7 s-1) and temperature (25 to 80 °C) on the corrosion performance of the selected alloys were studied. Additionally, potentiodynamic polarization (PP), cyclic polarization (CP) and critical pitting temperature tests (CPT) were performed. Strain rate sensitivity (SRS), strain hardening rate (SHR) and strain hardening exponent (SHE) were also investigated in air at ambient temperature and strain rates ranging between 10-2 and 10-7 s-1. The samples were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), electron-backscattered diffraction (EBSD) and Raman spectroscopy. The results of the SRS experiment revealed that the total strain of the alloys remained almost constant as the strain rate varied from 10-7 to 10-3 s-1, decreasing only at 10-2 s-1. Intense serration was observed in the stress-strain curves when the alloys were tested at strain rates ≥10-6 s-1. Also, SHE and SHR of the alloys increased with decreasing strain rate. The results of the SCC tests showed that UNS S31603 showed high susceptibility to SCC, while DSS exhibited considerable resistance to SCC under the conditions tested. Microstructural evaluation of specimens that underwent CPT and SCC testing showed that the ASS alloy corroded appreciably at low temperatures. SEM and Raman analyses indicated a significant amount of corrosion products in the exposed area of the ASS. PP and CP tests results showed that the DSS performed better than UNS S31603 at all temperatures. SEM evaluation of tested PP and CP specimens revealed stable pits formed on UNS S31603 surface after exposure at 25 °C. On the other hand, pitting in DSS only occurred at 80 °C.