Effect of W on phase transformation kinetics and its correlation with localized corrosion resistance for UNS S39274

Abstract

Previous research in 25Cr Super Duplex Stainless Steels (SDSS) has shown that an optimal W concentration-as an element in solid solution-exists, in which W improves localized corrosion resistance. Outside this range, W is either ineffective or even detrimental. However, debate still exists as to whether W accelerates or retards the precipitation of deleterious phases such as χ- and σ-phase. The objective of this investigation is to elucidate the influence of W on phase transformation kinetics and its corresponding effect on corrosion resistance. Herein, a SDSS containing 2.1 wt. % W (UNS S39274) has been investigated and compared to a W-free SDSS (UNS S32750) studied previously. Different isothermal heat treatments were performed to precipitate tertiary phases in the microstructure. The alloy microstructure was analyzed and quantified by Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Electron Backscatter Diffraction (EBSD) and the type and volume fraction of deleterious phases quantified. After characterization, critical pitting temperature (CPT) tests were performed in which samples were freely exposed (OCP) in 6 wt%. FeCl3 solution starting at 40ºC. Then the temperature was increased 5ºC every 24 hours until corrosion was initiated. The temperature at which corrosion was initiated was taken as the CPT.