Oxygen Depletion in Corroded Steel Vessels

Although steel treated with MgCl2 has shown high corrosion rates when in a warm humid atmosphere, the rapid and extensive oxygen depletion observed in a MgCl2 evaporator was much greater than expected. A plant test of a MgCl2 evaporator showed the oxygen content decreased from normal (20.95%) to approximately 1% in 24 hr. The reason for this unexpected O2 depletion was investigated in the laboratory by enclosing test samples in a simulation chamber and monitoring O2 and N2 with a mass spectrometer interfaced to the chamber with a silicone hollow fiber membrane probe. The tests show that the surface scale (magnetite) increases O2 depletion by more than ten fold over that of clean metal. The tests also show that the formation of Fe2O3 is not a factor in the first 24 hr but that the formation of an iron-magnesium compound, Iowaite [Mg4Fe(OH)8OCl·XH2O], which contains a high percentage of oxygen, was observed in the exposed samples. The study has shown that the corrosion process during equipment downtime can ...