The Inhibitive Effect of Carbon Containing Corrosion Products Inside Corrosion Pits on the Repassivation of Carbon Steels

摘要

The effect of carbon containing corrosion products within corrosion pits on the repassivation of pitshas been studied on 20# and 45# carbon steels in sodium chloride and sodium hydroxide / nitrate itrite solutions. Anodic polarisation tests have been carried out in an attempt to cause pittingcorrosion and then measure the repassivation potential. The presence of corrosion products within pitshas been demonstrated via EDX. Pits in 0.1 M NaNO2 solutions with different concentrations of NaCl(20 mM, 40 mM, 0.1 M and 0.2 M) have been found to repassivate at different potentials. Therepassivation potential of 45# carbon steel is lower than that of 20# when carbon containing corrosionproducts are detected within corrosion pits, but the difference is negligible when the products are notdetected. The presence and absence of carbon containing corrosion products is found to depend on theconcentration of chloride. The difference on the repassivation potentials has been ascribed to theinhibitive effect of corrosion products on metal ion diffusion, which may maintain aggressive solutionwithin pits and inhibit repassivation at a low dissolution rate. It has also been found that pits cannotrepassivate in NaOH and NaCl solutions, while pitting corrosion cannot take place in NaNO3 and NaClsolutions.