Corrosion and Passive Film Formation Studies on Modified 9Cr–1Mo Steel in Different Sodium Hydroxide Concentrations at Room Temperature and in Boiling Condition

Studies were carried out on modified 9Cr–1Mo steel to understand its corrosion as well as passive film characteristics in caustic environment. Potentiodynamic anodic polarization studies were carried out in 1–8 M sodium hydroxide solutions at room temperature (RT) and in boiling conditions. The specimens were passivated at 0.0 V(SCE) for 1 h in 3 and 8 M sodium hydroxide solutions at RT as well as in boiling condition. Laser Raman spectroscopic (LRS) analysis was carried out to examine the nature of oxides/hydroxides formed on the surface of the specimens. Corrosion rates increased by one order of magnitude whereas passive current density increased by almost two orders of magnitude in boiling solution compared to the RT values. Appearance of only maghemite (γ-Fe2O3) peaks in passivated steel in 3 M sodium hydroxide solution at RT compared to that in the 8 M sodium hydroxide solution, which showed Fe(OH)2, maghemite, magnetite, goethite (α-FeOOH) and CrO(OH), Cr2O3 peaks, indicated corrosion attack on the outer layer of the passive film. The passivated steel specimens in 3 and 8 M boiling solutions showed maghemite, magnetite, goethite, hematite (α-Fe2O3) (only in 8 M) and extremely weak peaks of Cr(OH)3 and Cr2O5. These observations indicated dissolution of the outermost part of the passive film with superficial attack on the inner part of the passive film exposing Cr oxide/hydroxides in boiling 3 M solution. However, passivated steel in 8 M solution showed molybdenum oxides, apart from the other iron and chromium oxides/oxyhydroxides. The scanning electron microscopic (SEM) studies on the morphology of the corrosion products along with LRS analysis/characterization confirmed these observations. These results showed increased corrosion attack during passivation with increase in concentration of alkali and temperature.