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 Eb = A + B log10[Cl−] relationship, where A = 0.7 VSCE 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.
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1 May 2021
Research Article|
February 16 2021
Intergranular and Pitting Corrosion in Sensitized and Unsensitized 20Cr-25Ni-Nb Austenitic Stainless Steel
Ronald N. Clark;
Ronald N. Clark
‡
*National Nuclear Laboratory, Unit 102B, Sperry Way, National Nuclear Laboratory, Stonehouse GL10 3UT, United Kingdom.
‡Corresponding author. E-mail: [email protected].
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Choen May Chan;
Choen May Chan
**Jacobs, Walton House, Faraday street, Birchwood Park, Warrington WA3 6GA, United Kingdom.
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W. Steve Walters;
W. Steve Walters
***National Nuclear Laboratory, Building D5 (First Floor), Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, United Kingdom.
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Dirk Engelberg;
Dirk Engelberg
****Materials and Performance Centre & Corrosion and Protection Centre, Department of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom.
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Geraint Williams
Geraint Williams
*****Swansea University, Materials Research Centre, Bay Campus, Fabian Way, Crymlyn Burrows, Swansea SA1 8EN, Wales, United Kingdom.
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CORROSION (2021) 77 (5): 552–563.
Citation
Ronald N. Clark, Choen May Chan, W. Steve Walters, Dirk Engelberg, Geraint Williams; Intergranular and Pitting Corrosion in Sensitized and Unsensitized 20Cr-25Ni-Nb Austenitic Stainless Steel. CORROSION 1 May 2021; 77 (5): 552–563. doi: https://doi.org/10.5006/3725
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