Karimi, Gilbert
(2019)
Corrosion of hardfacings for nuclear (PWR) applications.
PhD thesis, University of Nottingham.
Abstract
There are a number of moving parts such as valves and bearings in a pressurised water reactor primary circuit where resistance to both wear and corrosion is required. The hardfacing cobalt based alloys Stellite 6® and Stellite 3® have proven track record in these situations. However due to neutron activation and gamma radiation associated with cobalt-containing corrosion and wear debris, candidate iron-based hardfacings like, Tristelle 5183 are being considered as replacements for the cobalt-based alloys. Moreover, there is a need to consider the design basis for the design of new iron-based alloys with enhanced performance, and as part of that, the corrosion behaviour of alternative hard phases (ceramics) for such hardfacings is considered.
The corrosion behaviour of three hardfacings materials namely, Stellite 3, Stellite 6, Tristelle 5183 in both cast and HIPed versions, along with pure carbides (Cr3C2, Cr7C3, Cr23C6, Mo2C, NbC and TiC) was evaluated in conditions replicating the operating regime of the primary circuit of a pressurised water reactor (PWR) reactor. The extent and mechanisms of corrosion were evaluated with the aid of a number of characterisation tools: optical microscopy, SEM with EDX, TEM with EDX, STEM and XRD.
Both of the Stellite 6 analogues exhibited low rates of overall corrosion. The cast Stellite 6 analogue alloy exhibited localised corrosion at the boundary between the matrix and the M7C3 carbides, with this being attributed to chromium depletion in the matrix associated with the carbide growth. In contrast, the HIPed Stellite 6 analogue alloy exhibited no localised corrosion. As well as differences associated with the method of manufacture, it was noted that the carbon content of the cast Stellite 6 analogue was right at the top end of the allowable range (with the chromium content being towards the lower end of the allowable range), and whilst this results in a high fraction of carbides, it also results in a general depletion of the chromium content of the matrix.
The corrosion behaviour of the wear resistant cast and HIPed Stellite 3 analogue following exposure for 30 days in lithiated water at 300°C was also investigated. Corrosion of the surface of the cast Stellite 3 analogue was again observed to be limited, with this resulting in the formation of a chromium and silicon-rich oxide layer, only ~100 nm thick. Preferential corrosion of the matrix at its interface with just one of the carbide types (M7C3) was observed to a depth of ~1 µm; for the first time, this was shown to be not due to any inhomogeneity in the matrix with it being argued that it was instead an electrochemical effect. For the HIPed Stellite 3 analogue, surface corrosion and preferential corrosion of the matrix at the interface with the Cr-rich carbides was also observed. Moreover, there was interfacial corrosion of the Co-rich matrix at the interface with the W-rich carbide; this oxide surrounded the W-rich carbide and it was rich in chromium, iron, and silicon. The boundary between Co-rich matrix and the Co-rich matrix was also corroded in certain places. The preferential corrosion in the HIPed Co-based alloy was due to a low chromium content in the Co-rich matrix. The difference in chemical composition made HIPed S-3 more susceptible to corrosion than cast S-3.
Among the carbide ceramics, Cr3C2 and TiC were the most resistant to corrosion. Cr23C6 had a high corrosion resistance Cr7C3 experienced corrosion with a brown corrosion product and small crystals formed. Most likely, this was a chromium oxide-passivating layer that prevented further corrosion. Mo2C experienced corrosion by dissolution whilst NbC was heavily corroded with the formation of niobium oxide (Nb2O5). Iron based hardfacing Tristelle 5183 (both cast and HIPed versions) were attacked with the formation of magnetite (Fe3O4) that covered the majority of the sample surface.
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