Variation in Synergy of Corrosion Inhibition, on Mild Steel in NaCl Solution, Due to Rare Earth: Ce, Y, La, Nd in Rare Earth Tartrates

The inhibition efficacy of synthesized rare earth tartrate (RETar) compounds (RE—Ce, Y, La, Nd) on mild steel in NaCl solution has been examined. The experimental results demonstrate that CeTar has the maximum inhibition, and YTar has the least inhibition when added to the optimum concentration. This is because the cerium is present on the metal surface as ceria (CeO₂), which is more stable than the other metal oxides. Weight loss data demonstrate that all RETar compounds reduce corrosion for up to 6 months. Further, when 600 ppm CeTar inhibitor is added to the NaCl solution, the maximum inhibition efficiency obtained is 88% after 6 months of immersion test examined by the weight loss measurement. The electrochemical polarization data suggest that RETar inhibitors inhibit corrosion to some extent after one hour of immersion by inhibiting anodic reactions. Then, inhibition efficiency improves after 24 h because of the simultaneous imposition of inhibiting anodic and cathodic reactions and acting as a mixed inhibitor. SEM images demonstrate these inhibitors form a compact layer with only fine cracks. According to the EDX, FTIR, and Raman results, the film comprises metal oxide/hydroxide and the tartrate group on the metal surface. The inhibition mechanism hypothesized that the protective film forms a bimetallic complex in the initial stage. Then, after the increase in immersion time, rare earth metal ions transform to the metal oxide/hydroxide, which, along with the bimetallic complex (Fe–tartrate–RE), protects the metal for an extended duration of immersion.