Corrosion behavior of carbon steel in presence of sulfate-reducing bacteria in seawater environment

Abstract

• Carbon steel was exposed to sulfidogenic bacteria seeded seawater in a flow loop. • Localized surface attacks followed inoculations with sulfidogenic bacteria. • Bacteria activity does not necessarily lead to significantly elevated corrosion. • Complex corrosion deposits structure. • Antimicrobial treatment temporarily suppressed bacterial activity. The influence of sulfate-reducing bacteria (SRB) on the corrosion behavior of carbon steel was studied in a laboratory test-loop, continuously fed with nutrient supplemented North Sea seawater. The main parts of the test-loop, represented by two separated flow cells, were fitted with steel specimens. The test-loop was operating anoxically for 2200 h and each flow cell was three times inoculated with Desulfovibrio alaskensis or Desulfovibrio desulfuricans species. Additionally, each flow cell was two times perturbed with antimicrobial treatments. Steel specimens exposed in flow cells exhibited comparable appearance and systems responding similarly to inoculations and antimicrobial treatments. The effect of the inoculations in both flow cells on the steel coupons electrochemical behavior was materialized as lower resistance to corrosion and higher surface activity or occurrence of localized pitting events. The localized surface attacks recognized in both flow cells after inoculations continued to progress with the time, although bacterial activity was temporarily suppressed by antimicrobial treatment. Post-exposure sample evaluations might suggest that, some particular steel surface areas have been subjected to a dramatic change in the corrosion mechanism from initial localized attack to general corrosion. The long-term exposure of the carbon steel specimens resulted in identifiable formation of biofilms and corrosion products. Corrosion deposits were characterized by a specific structure built of iron sulfides (FeS), sulfated green rust (GR(SO 4 2− )), magnetite (Fe 3 O 4 ), Fe(III) oxyhydroxides (FeOOH), chukanovite (Fe 2 (OH) 2 CO 3 ), carbonated green rust (GR(CO 3 2− )) and some calcareous deposits. Presented factual evidence reinforced the idea that sulfidogenic species in natural seawater environment may cause localized damage with a specific surface pattern; however, this does not necessarily lead towards significantly elevated corrosion rates.