- Seawater contains approximately 840 ppm of sulfates.
- Much corrosion of metals in deep water is caused by anaerobic bacteria that reduce sulfate to sulfide:
SO42- + 5H2O + 8e– → HS– + 9OH–
- This reduction half-equation is accompanied by the oxidation half-equation of the metal involved, such as iron:
Fe → Fe2+ + 2e–
- Combining the above two equations:
4Fe + SO42– + 5H2O → 4Fe2+ + HS– + 9OH–
- The products react to form the insoluble compounds of iron sulfide and iron hydroxide:
4Fe2+ + HS– + 7OH– → FeS(s) +3Fe(OH)2(s) + H2O(l)
- The main sulfate-reducing bacterium is Sporovibrio desulfuricans, which obtains energy from the reduction process.