Study on Effect of Phosphate Species on Green Rust Transformation in Steel in Marine Environment

Green Rusts (GRs) are intermediate unstable Fe (II) - Fe (III) hydroxyl salts between metallic iron and final corrosion products, i.e., iron oxyhydroxides and oxides, such as goethite(α-FeOOH), lepidocrocite(γ-FeOOH), and magnetite(Fe₃O₄). GRs oxidize in the presence of oxygen. Therefore GR I, which is Fe(II)₃Fe(III)(OH)₈Cl.nH₂O in marine environment, may govern the mechanisms and kinetics of corrosion and passivation of steel. Furthermore, foreign ions, e.g., Cr, Cu, Mn, P etc. present in steel influence corrosion process and hence oxidation of GRs and formation of final products. In this work, the effect of phosphorus on final products has been studied by adding Na₂HPO₄ to GR suspensions according to Fe/P molar ratio of 10.5, 55 and 137, equivalent to (5 % P, 94.5 % Fe, rest impurities), (1 % P, 98.5 % Fe, rest impurities) and (0.4 % P, 98.5 % Fe, rest impurities), respectively, all are in wt.%, and without containing any phosphate. The samples syringed out during oxidation at different intervals were analyzed using XRD after preparing in absence of oxygen. The XRD patterns for the freeze dried final products were also obtained and particle size was analyzed using TEM. Based on these results, the beneficial effect of phosphorus in steel in marine environment has been discussed.

Key Words: Green Rust I(Cl^-), Phosphate, Lepidocrocite, Steel, Corrosion, Marine environment