General corrosion behaviour of modified 9Cr–1Mo steel weldments in acidic environments

Abstract

The general corrosion behaviour of modified 9Cr–1Mo (T91) steel weldments has been studied in acid media (sulphuric and phosphoric acids containing 1% nitric acid).The work has involved weight loss measurements and potentiodynamic polarisation and impedance measurements for different regions of the weldment at different temperatures. The corroded surface and the nature of the corrosion products have been examined and characterised by scanning electron microscopy, electron probe microanalysis (EPMA), and X -ray diffraction. In sulphuric acid the material exhibits more corrosion than in phosphoric acid. The lower rate of corrosion in phosphoric acid is attributed to a possible phosphate film, as confirmed by impedance and other surface examination studies. With an increase in temperature it is found that the overall corrosion rate increases in all zones of the weldment. In general the weld zone and heat affected zone have been found to show greater corrosion than the parent metal. Impedance studies have clearly demonstrated the formation of the passive film in phosphoric acid by the presence of a Warburg's impedance in the impedance diagram. The possible species present in the corrosion products are Fe₂O₃, Fe₃O₄, FesO₇(OH).4H₂O, Fe₂(SO₄)₃. 9H₂O, and FeMO₄. The EPMA results have revealed chromium depletion of the surface regions after corrosion in the acid media.