Surface Layer Formation in the Earliest Stages of Corrosion of Steel in CO2-Saturated Brine at 80°C: Studied by In Situ Synchrotron X-ray Methods

Abstract

Grazing-incidence small-angle X-ray scattering (GISAXS) from polycrystalline steel shows features associated with the underlying microstructure, and, in the initial stages of corrosion, with the development of very small-scale surface roughness, on the nanometer height scale. A 1Cr0.25Mo pipeline steel in hot, CO2-saturated brine develops the very small-scale surface roughness significantly faster than a simple carbon steel, although the overall dissolution current density for the two steels is almost the same. We speculate that it is due to the presence of a layer comprising ‘blobs’ of amorphous FeCO3, which grow to spread over the surface and eventually cover it, because it is significantly larger in height scale than the roughness developed during the initial stages of anodic dissolution in acidified NaCl solution, where no surface film is expected. The greater roughness on the 1Cr0.25Mo steel can be interpreted as due to small pre-crystalline nuclei, that form at much lower supersaturation, and grow faster, than on mild steel. Grazing-incidence X-ray diffraction studies show at later stages the apparent preferential dissolution of smaller crystallites of iron, with spatial size scale 0.1–1 μm. This develops significantly more slowly on the 1Cr0.25Mo than on the simple carbon steel.