(Digital Presentation) the Role of Steel Microstructures on Sweet Corrosion Scaling by in Situ Confocal Raman

Confocal Raman imaging and spectroscopy has been used in a custom-made flow-cell to map in-situ spatial and temporal growth of siderite and chukanovite on pipeline steel in a hot brine environment, under conditions typical of commercial interest (pH 6.8, temp 80oC, oxygen conc < 10 ppb, 0.5 M NaCl, open circuit potential (OCP)). Ferrite and pearlite domains are found to exhibit an interesting contrast (colour) inversion under sweet environmental conditions compared to their Nital etched contrast – confirmed by identifying the shapes and surrounding of the domains before and after exposure to two different conditions. Ferrite and pearlite phases are found to exhibit differing evolution of the corrosion scale mineralogy, and phase transformation. The process starts by revealing the microstructural phase constituents, significantly the interphase boundaries are identified as the primary sites for scale formation. The ferritic phase forms pure siderite scale while the pearlitic phase shows both siderite and chukanovite scales, with chukanovite transforming into siderite over time. LPR profiles reveal that for carbon steel with a higher pearlite fraction the initial corrosion rate is higher, followed by a faster drop when compared to steel with lower pearlite fraction! Figure 1