Corrosion of low alloy steel in stagnant artificial or stirred natural seawater: The role of Al and Cr

Abstract

AbstractCarbon steel and low alloy (containing Al and Cr) steel coupons were immersed for 6–7 months in stagnant artificial seawater or in natural marine site. The corrosion processes were studied via the combination of electrochemical techniques and the analysis of corrosion product layers by µ‐Raman spectroscopy and X‐ray diffraction. In natural seawater, the low alloy steel showed better resistance to corrosion and the best results were obtained when the mill scale was removed from the steel surface. This shows that improved corrosion resistance is due to a protective layer that forms spontaneously in the environment. In stagnant artificial seawater, the behavior of low alloy steel coupons (without mill scale) was compared with that of carbon steel coupons. The polarization resistance of the carbon steel coupons remained approximately constant over time. The corrosion product layer proved to be mainly composed of magnetite, an electronic conductor that does not hinder oxygen reduction. In contrast, the polarization resistance of the low alloy steel coupons increased with time, as the growing corrosion product layer, enriched with insulating FeOOH compounds, hindered oxygen diffusion.