Simulation of Atmospheric Field Conditions on Weathering Steels at Laboratory

Abstract

E-pH diagram of iron in Na₂SO₄ shows some passive regions. Raman spectroscopy of 42 months exposed WS panels at field exposure sites revealed a few common oxide and hydroxide phases as seen in E-pH diagram. It was thought that long field exposures of WS in some sites leads to the formation of these oxides/hydroxides phases due to slow interaction with environments. It was envisaged to simulate the slow oxidation process in laboratory by subjecting WS panels to very slow potentiodynamic scanning rate. Experiments were designed to investigate the passivation behaviour by slow polarization and to see whether such oxides/hydroxides phases at all formed or not. Na₂SO₄ solution was taken as test electrolyte because SO₂ was created in situ and this is present in industrial environment at Jamshedpur. Passivated oxide layers were found only on WS but not on MS. Raman spectroscopy of passivated WS panels showed the presence of γ FeOOH along with stable Fe₃O₄ and α Fe₂O₃, which are very similar to the rust of WS exposed at Jamshedpur after 42 months. Even the morphology of simulated rust compared very well with the field exposure sample when studied by SEM.