Influence of anodic polarization products of NiCu low alloy steel in compacted bentonite on its corrosion behavior

The corrosion products of steel overpacks for high-level radioactive waste disposal significantly affect their cathodic depolarization behavior during corrosion and may even alter the depolarization mechanism. In this study, the anodic polarization products of NiCu low alloy steel were prepared by potentiostatic polarization in compacted bentonite with 20% and 40% water contents. Combined with the chemical composition, structure of corrosion products, and corrosion kinetic parameters, the influence mechanism of different anodic polarization products on the corrosion behavior of steel at open circuit potential was revealed. The results showed that the products formed under weak anodic polarization potentials were mainly Fe6(OH)12CO3 and Fe6(OH)12SO4. With rising polarization potential, Fe3O4, α-FeOOH, and Fe2O3·H2O emerged with increasing relative contents. Meanwhile, the residual Fe3C also accumulated in products. At open circuit potential, α-FeOOH, Fe2O3·H2O and Fe3C from strong polarization showed stronger cathodic depolarization and micro-galvanic effects than weak polarization-derived green rust, inducing a sharp drop in the cathodic and anodic polarization resistances of steel. Additionally, products formed in the 20% water content system at weak polarization resulted in lower polarization resistances than the 40% system, whereas the opposite trend occurred under strong polarization. The former was due to more abundant and easily diffused oxygen in unsaturated bentonite’s capillary pores, which oxidized green rust to α-FeOOH/Fe2O3·H2O and enhanced cathodic depolarization capacity. The latter was attributed to the loose and porous corrosion products formed under strong polarization potential when the water was sufficient, which decreased the inhibition on steel anodic dissolution.