Effect of sulfate reducing Citrobacter sp. strain on the corrosion behavior of API X70 microalloyed pipeline steel

Abstract

In this study, the effect of sulfate reducing Citrobacter sp. strain on the corrosion behavior of API X70 micro-alloyed pipeline steel has been evaluated in a simulated soil solution at different immersion times (varying from 7 days to 40 days) under both abiotic and biotic conditions using open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). The corrosion products and biofilm morphology developed on the metal surface at various immersion times were evaluated by field emission scanning electron microscopy (FESEM). Formation of a protective layer of corrosion products including ferric oxide, sulfide and carbon-based compounds accumulated on the underlying steel leads to an increase of the corrosion resistance for the first 21 days under abiotic condition. The results of electrochemical impedance spectroscopy in the solution containing bacteria show a significant reduction in charge transfer resistance from 400 Ω cm2 after day 7–55 Ω cm2 after day 21 due to the biofilm formation. This reduction in corrosion resistance is consistent with the high concentrations of sulfide produced by the metabolism of bacteria and the formation of a relatively compact and thick biofilm that is heterogeneously dispersed over the surface. In addition, a higher susceptibility to pitting corrosion is observed for samples immersed in the bacteria containing medium.