Evaluation of the localized corrosion resistance of 316 L austenitic and 430Ti ferritic stainless steel in aqueous chloride/sulphate media for application in petrochemical crude distillation units

Abstract

Corrosion problems are frequently encountered in petrochemical crude distillation units due to increased levels of chlorides and sulfur anions from the constituents of crude oil. Potentiodynamic polarization test, potentiostatic measurement, open circuit potential measurement and optical microscopy characterization was performed on 316 L austenitic and 430Ti ferritic stainless steel in 3.5 M H2SO4 solution at 0%–6% NaCl concentration to study and compared their general and localized corrosion resistance for application in industrial operating conditions simulating petrochemical crude distillation unit. Results obtained showed 316 L is significantly more corrosion resistant than 430Ti with comparable corrosion rates of 0.222 mm y−1 and 3.174 mm y−1 (316 L), and 24.218 mm y−1 and 12.238 mm y−1 at 0% and 6% NaCl concentration. Corrosion potentials of 316 L showed dominant cathodic reaction mechanisms, shifting from −0.211 VAg/AgCl to −0.290 VAg/AgCl due to higher resistance to localized corrosion reactions. Values obtained for 430Ti showed active-passive behavior resulting from general surface deterioration coupled with localized corrosion reactions. Polarization plots of 316 L displayed extensive passivation behavior during potential scanning from 0%–5% NaCl concentration. Passivation was completely absent on 430Ti plots due to accelerated destruction of the protective oxide. The morphology of 430Ti severely deteriorated with visible corrosion pits due to depassivation of specific sites on the steel surface by SO42− and Cl- anions compared to the morphology of 316 L, which starkly contrasts due to higher resistance to surface oxidation. The extent of degradation on 316 L was superficial. The open circuit potential plots of 316 L were significantly electropositive compared to 430Ti which was electronegative.