Intergranular and Pitting Corrosion in Sensitized and Unsensitized 20Cr-25Ni-Nb Austenitic Stainless Steel

Advanced gas-cooled reactor (AGR) oxide fuels used in the United Kingdom are clad in bespoke grade 20%Cr-25%Ni-Nb austenitic stainless steel. Electrochemistry was first applied to correlate the breakdown potential with chloride ion concentration, temperature, and pH for this alloy. At near-neutral pH the unsensitized material exhibited a linear E_b = A + B log₁₀[Cl⁻] relationship, where A = 0.7 V_SCE and B = –0.098 V/decade. Scanning Kelvin probe force microscopy revealed that grain boundary regions in the heat-treated material were up to 65 mV less noble to the matrix, whereas undissolved niobium carbide (NbC) precipitates were up to 55 mV more noble to the matrix. In situ time-lapse microscopy and postcorrosion observations confirmed that sensitized grain boundaries were susceptible to pitting corrosion, further developing along intergranular corrosion pathways. It has, however, been shown that microgalvanic coupling between the Nb precipitates and matrix and/or sensitized grain boundary regions is not a factor in corrosion initiation as all experiments were performed under external potential control. Postcorrosion observations showed the presence of pits at NbC precipitates promoting grain boundary corrosion. It is postulated that corrosion initiates at NbC precipitates as a pit, and when in close vicinity to Cr-depleted grain boundaries, then propagates along grain boundaries as intergranular corrosion.