The effect of Cr–Cu modification on the microbiologically influenced corrosion resistance of X70 pipeline steel

Abstract

Microbiologically influenced corrosion (MIC) caused by sulfate-reducing bacteria (SRB) is a critical threat to the integrity and service life of oil and gas pipelines. This study developed a Cr–Cu-modified X70 pipeline steel by microalloying and evaluated its resistance to SRB-induced MIC. The MIC behavior of the modified steel and conventional X70 steel was examined in Desulfovibrio vulgaris cultures by biofilm quantification, weight-loss measurements, surface characterization, and electrochemical tests. Scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and electrochemical impedance spectroscopy (EIS) were used to assess biofilm formation, corrosion-product morphology, and interfacial electrochemical behavior. Biofilm quantification and cell counting showed reduced SRB attachment on Cr–Cu steel. Weight-loss measurements gave a corrosion rate of 0.186 mm per year, corresponding to an 8.8% decrease relative to X70 steel. SEM and CLSM revealed that Cr–Cu steel formed a denser corrosion-product layer. Cr enrichment promoted a compact Cr₂O₃-containing passive film, which limited extracellular electron transfer and suppressed localized corrosion. EIS further confirmed higher charge-transfer resistance and improved film stability during immersion. The enhanced MIC resistance is attributed to the combined effects of Cr-induced passivation and weakened sulfide-assisted corrosion due to the reduced Cu content.