The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding

Abstract

The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding. Corrosion Science, [108365].

Pitting corrosion of 17-4PH stainless steel manufactured by laser beam melting

“…Such an acidic electrolyte led to the preferential dissolution of both the grain boundaries and the matrix surrounding the NbCs. Therefore, these observations could suggest that NbCs might act as preferential pit initiation sites, as demonstrated by Clark and its co-workers [43]. However, considering the very small size of the NbCs, it was not trivial to establish a clear relationship between pit initiation and NbCs.…”

“…To better evaluate the susceptibility to pitting corrosion of the 17-4 PH MSS depending on the forming process, stable pits were first characterised: a current density of 1 mA cm − ² was applied on 1-cm² samples in order to compare both the density and morphology of the stable pits, for a similar charge passed for the different microstructures. Indeed, recent work of Clark and its co-workers demonstrated the nobility of NbC precipitates in austenitic stainless steels [43]. The authors suggested that NbC precipitates were cathodic with respect to the matrix and could create a micro galvanic couple promoting the initiation of localised corrosion.…”

The effect of sodium hydroxide on niobium carbide precipitates in thermally sensitised 20Cr-25Ni-Nb austenitic stainless steel

“…The NaOH pH-moderation of these ponds is used to inhibit corrosion of the stainless steel cladding. The 20Cr-25Ni-Nb alloy is stabilised by ~0.6% niobium (Nb) which precipitates as niobium carbide (NbC) during the manufacturing process [5]. Despite Nb stabilisation, during reactor operation a small proportion of fuel cladding can become sensitised through an alternative mechanism, known as radiation induced segregation (RIS), which leads to a redistribution of alloying elements at the microstructural level (notably, Cr diffusion away from grain boundaries), rendering the alloy sensitised [5] [6] [7].…”

“…The 20Cr-25Ni-Nb alloy is stabilised by ~0.6% niobium (Nb) which precipitates as niobium carbide (NbC) during the manufacturing process [5]. Despite Nb stabilisation, during reactor operation a small proportion of fuel cladding can become sensitised through an alternative mechanism, known as radiation induced segregation (RIS), which leads to a redistribution of alloying elements at the microstructural level (notably, Cr diffusion away from grain boundaries), rendering the alloy sensitised [5] [6] [7]. The alloy can also be sensitised through the use of a two-stage heat treatment which causes Cr depletion at grain boundaries by the formation of Cr carbide precipitates [5] [8] [9] [10].…”

“…Whilst NbC precipitates play an important part in stabilising the free carbon content within this alloy; investigators have also sought to improve the fundamental mechanistic understanding by both identifying and characterising the sites where corrosion initiates in relation to these precipitates [5] [11] [12] [13] [14]. Phuah et al [14], during post corrosion imaging, using scanning electron microscopy (SEM), noted the presence of pits and crevices adjacent to NbC precipitates in thermally sensitised 20Cr-25Ni-Nb when polarised in a Clenvironment.…”