The stress corrosion of a sensitised stainless steel : a study of the effect of low frequency cyclic loading on the process of stress assisted corrosion in 'sensitised' 20%Cr, 25%Ni, 0.7%Nb stainless steel, whilst in HNO3 solution
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.