On the Effect of Pre-formed Scales in Mitigating Corrosion of Steels in CO2 Environments

Abstract

Chromium (Cr) containing steels were tested to analyse corrosion behaviour in carbon dioxide saturated water of varying salinities with extended exposure time. Both potentiodynamic and mass loss data were collected to gain a better understanding of the corrosion mechanisms. It was found that both the high Cr steels displayed degradation in the form of pitting with increasing salinities. However, the low alloy steel reference material showed uniform iron carbonate (FeCO3) precipitation. The use of high salinity precipitated layers to aid corrosion protection in lower salinity seawater environments was then established as an interesting area for greater examination. Subsequently, samples of the low alloy steel previously corroded in solutions of 7, 14 and 28% sodium chloride (NaCl) concentration were then tested in seawater salinities of 3.5% NaCl. It was found that both the 7 and 14% NaCl pre-corroded samples resulted in a significant reduction in the corrosion rate when compared with non-pre-corroded samples. The 7% NaCl pre-corroded sample showed the greatest reduction in corrosion rate, and through SEM analysis of the layer both on the surface and cross-section it was found to display an iron carbonate layer more densely packed and defect free. This indicated the potential benefits of high salinity pre-corrosion techniques to aid protection in seawater environments.