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The following work divides into two parts: a: a study of the effect of stress on the inter-granular stress assisted corrosion attack of sensitised 20% Cr, 25% Ni, 0.7% Nb in HNO₃ environments. This problem was suggested by the C.E.G.B. and relates to the potential corrosion problems of AGR fuel cladding during storage after use. The aim of this work was therefore to determine how metallurgical condition, test potential and mechanical test variables affect corrosion behaviour. Low frequency cyclic loading offers a way to investigate the stress corrosion of systems at realistic stress levels and strain rates found in practice. b: an investigation into the effect of a low frequency cyclic stress on the process of stress assisted corrosion. The aim of this work was to gain information on the effect of stress cycling on the process of stress assisted corrosion attack. Tensile specimens were subjected to static loads both alone and with superimposed low frequency (10^-⁴ to 10^-² Hz) saw-tooth stress cycles. Cycling was carried out potentiostatically in HNO₃ environments, at below yield stress levels and ambient temperatures. Different frequencies, cyclic amplitudes and levels of background tensile stress were used. Irrespective of loading conditions the optimum potential for accelerated stress assisted corrosion attack was found to be -200mV (SCE). The results of tests showed that test potential, cycle frequency, cycle amplitude and level of background stress strongly affect rates of attack. Grain boundary penetration rates were found to increase as frequency decreased and as peak stress and stress amplitude increased. Different kinetics of penetration were seen for cyclic and static loading. Increase of penetration depth with time for cyclic loading experiments was found to vary with (time)^0.5 whilst that for static loading experiments increases linearly with time. A number of reasons are discussed to explain the difference in observations between cyclic and static penetration rates. Such reasons included the difficulty of ion transport down narrow paths, blunting of the penetration front, the possibility of local strain induced martensite transformation leading to hydrogen embrittlement and plastic strain enhanced dissolution resulting during cyclic loading. The anomalous effects observed during cyclic loading (such as "strain softening") were examined for tensile specimens cycled under a range of mechanical conditions. It was found that the extent of plastic strain increased for higher stress and larger cycle amplitudes. The process of thermal sensitisation of 20 wt% Cr, 25 wt% Ni, 0.7 wt% Nb stainless steel in three different material starting conditions (bar, "reworked bar" and tube) was investigated. Both Cr depletion and impurity segregation are discussed as mechanisms of sensitisaton. An attempt was made to correlate response in chemical and electrochemical tests with both microanalytical (STEM/EDX) observations on the shape of Cr depletion profiles and with analytical modelling. The collector plate model was found to describe AEM measured Cr depletion profiles well.

WhisperGenTM heater head components are currently machined from the austenitic stainless steel Sandvik 253MA. The inner surface of the heater heads is subjected to the working gas of the engine, which is a high-pressure, high-temperature nitrogen-based environment with carbonaceous impurities. As a result of this exposure, a scale forms during operation and eventually spalls. This spalled scale causes abrasive damage to piston seals and guides, which leads to pressure loss and eventual failure of the engine. The aim of the present work was to compare the performance of the austenitic alloy 253MA with two alternative alloys, Incoloy 800H and AISI 310, thereby enabling a material recommendation. A literature review provided information about many general aspects of high temperature corrosion in similar alloys. However, little was found about the application of these alloys in environments similar to those experienced by a WhisperGenTM heater head. Therefore, laboratory experiments were conducted to indicate the relative performance characteristics of the three potential alloys (253MA, Incoloy 800H and AISI 310). To overcome the difficulties with testing at high temperatures and pressures, Thermo-Calc™ was used to calculate gas mixtures at 1 bar that approximated the chemical potentials of carbon and nitrogen in the working gas at 24 bar. Comparisons of the different materials were made via weight loss/gain measurements and metallographic analysis, which included optical microscopy, scanning electron microscopy, X-ray mapping and electron back scatter diffraction (EBSD). The laboratory test sample results were also compared with results from heater heads of the same materials that were run in an actual WhisperGenTM engine. The experimental results taken in total indicate that 253MA is the least suitable alloy for the heater head application because it exhibited poor spalling performance, internal oxidation and formation of a large amount of Cr23C6. AISI 310 was shown in all cases to develop the detrimental sigma phase, although this alloy was the least susceptible to internal oxidation. Incoloy 800H was the most resistant alloy to all forms of degradation and is thus recommended for the heater head application.

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