Unveiling the pitting corrosion mechanism of borated stainless steel in the wet storage environment of spent nuclear fuels

Borated stainless steels (BSSs) with high neutron absorbability are promising alloys for the wet storage of spent nuclear fuel. However, consensus on the underlying mechanisms for the effects of B content in BSS and H ₃BO ₃ in wet storage environment on the pitting corrosion resistance of BSSs is still lacking. To determine the mechanisms, we designed three hypoeutectic BSSs containing 0.6, 1.2 and 1.8 wt.% B and systematically investigated their electrochemical behavior in solutions with various Cl ⁻/H ₃BO ₃ ratios through experimental and computational studies. Results revealed that the pitting corrosion resistance of BSS was dependent on the synergistic effect of the pit initiation induced by localized Cr depletion and the pit stable growth caused by microgalvanic effect, causing the pitting corrosion preferentially occurred at the (Cr, Fe) ₂B/matrix interface. A higher B content increased the density and width while reduced the Cr contents of the Cr-depleted regions, enhancing the susceptibility to pitting initiation. Meanwhile, the strengthened micro-galvanic effect between the (Cr, Fe) ₂B phase and γ matrix further facilitated the stable pit growth. For the H ₃BO ₃ effect, the first-principles calculations indicated that the competitive adsorption of B(OH) ₄ ⁻ to Cl ⁻ inhibited pitting initiation in solutions with lower Cl ⁻/H ₃BO ₃ ratios. However, as the Cl ⁻/H ₃BO ₃ increased, the inhibition effect of the H ₃BO ₃ was weakened, and the acidizing effect became the dominant factor deteriorating the corrosion property. Ultimately, based on the Cr depletion-microgalvanic synergetic mechanism, the corrosion resistance of the BSS was significantly improved by tailoring the size and distribution of the (Cr, Fe) ₂B phase.