Browsing by Author "Lyon, Stuart B."
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Item The Influence of Composition and Plastic Strain on the Electrochemistry of Stainless Steels Doped with Platinum Group Metals(Materials Performance Centre, University of Manchester, 2009-03) Govender, Kuveshni; Necib, O.; Ahmed, Ismaila Idowu; Lyon, Stuart B.; Sherry, Andrew H.Corrosion-related degradation of stainless steel components of Naval Nuclear Power Plant (NNPP) can have significant impact on the cost of nuclear ownership through inspection requirements, the reliability and, in some cases, the availability of plant. This project aims to establish an improved physical understanding of the mechanisms of Stress Corrosion Cracking (SCC) of 304 stainless steel of the type used for a range of NNPP components. In order to accomplish this aim, the research is studying the influence of platinum group metal (PGM) alloying additions (i.e. < 1 wt.%, additions palladium and/or ruthenium) on the SCC of 304 stainless steel as these have predictable effects on the electrochemical processes of corrosion. The approach includes mechanical and electrochemical testing together with pre- and post-test metallurgical examination (over an appropriate range of length scales), and finally predictive model development. This Report presents experimental data relating to the influence of PGM additions and plastic-strain (cold work) on the electrochemistry of 304L stainless steel in a range of environments including simulated PWR water. The electrochemical data reveal ruthenium to be more effective than palladium in enhancing the corrosion resistance of 304 stainless steel in PWR water chemistry and this was traced to removal of palladium from the alloy matrix by precipitation of a PdMn Intermetallic 2nd phase precipitate (reported previously). For all alloy variants, increasing temperature shifted the Open Circuit Potential (OCP) to more noble values and increased the anodic current density; observations consistent with enhanced to both metal dissolution and oxygen diffusion rates at higher temperatures. Increasing the oxygen concentration increased the OCP but decreased the passive current density; observations rationalised in terms of the efficiency of the cathodic reaction in which PGM additions enhance the exchange current density for oxygen reduction leading to faster passivation kinetics. In hydrogenated water, the PGM modified alloys displayed a preferred lower OCP and a higher anodic current density than the standard alloy and this is because PGM additions increase the exchange current density for hydrogen oxidation. In deaerated pressurised water at 260oC, the ruthenium-doped alloy displayed an improved corrosion resistance compared with standard 304 stainless steel whilst the palladium-doped alloy displayed a higher dissolution current density. Studies in acidified aerated potassium tetrathionate revealed an abrupt transition of the OCP to more positive potentials the effect being more rapid for the ruthenium-doped alloy. Analysis of the anodic polarisation curves suggests that PGM additions enhance the corrosion resistance, the effect being most pronounced for ruthenium. Regarding the effect of plastic strain (cold work), the influence of strain path on the evolution of internal strain in cold-worked austenitic stainless steel was investigated using neutron diffraction and mechanical testing. As expected, hardness and yield strength increase with the level of cold work, however the magnitude of the increase is dependent upon the strain path due to the mixed isotropic/kinematic hardening response. Neutron diffraction demonstrated that residual strain in cold-worked material increases with the level of cold work. The elastic anisotropy and the lattice strain difference increased consistently with the applied stress during in-situ re-straining tests. In considering all the cold work material reloaded in-situ to 5 % in tension along the 3 principal directions, the largest strain was found to occur in the material with the pre-strain level of 20 % reduction. Electrochemical data indicate that the sample orientation (and, consequently the alloy texture) has a greater effect than the absolute level of cold work. Thus, the L-T plane displayed more noble electrochemical behaviour than the L-S plane, and the T-S plane was found to be least noble. Generally, material cold rolled to 20% reduction showed the least noble electrochemical behaviour of all the cold rolled materials on all three orthogonal planes.Item Stress Corrosion Cracking of Austenitic Stainless Steel in Chloride Environment(Faculty of Engineering, Ahmadu Bello University Zaria, 2015) Ahmed, Ismaila Idowu; Alabi, Abdul Ganiyu Funsho; Odusote, Jamiu Kolawole; Aremu, Ishaq Na'Allah; Adebisi, Jeleel Adekunle; Yahaya, Taiwo; Talabi, Segun Isaac; Yahya, Raheem Abolore; Lyon, Stuart B.This study was carried out to assess the effect of applied potentials on stress corrosion cracking of Austenitic Stainless Steels (ASS), Type 304L in a potentiostatically controlled chloride environment at ambient temperature. The stress corrosion cracking tests were carried out on annealed ASS using a Slow Strain Rate Test (SSRT) technique in sodium chloride solution acidified with hydrochloric acid at ambient temperature. Post-mortem analyses of failed specimens were carried using optical microscopy. The study showed that plastic elongation, ultimate tensile strength and time to failure decrease as the applied potential increases during the slow strain rate test. The study showed that immunity of ASS to chloride SCC was improved when the electrochemical potential was maintained in the primary passive potential range. Post-mortem analysis of failed samples showed evidences of SCC and ductile failure on the fracture surface.Item Stress Corrosion Cracking of Austenitic Stainless Steels in Potentiostatically Controlled Chloride Environments at Ambient Temperature(Universitatea Politehnica Timisoara (UPT), Romania, 2015-11-04) Ahmed, Ismaila Idowu; Yahaya, Taiwo; Adebisi, Jeleel Adekunle; Aremu, Ishaq Na'Allah; Alabi, Abdul Ganiyu Funsho; Lyon, Stuart B.In the earlier study carried out to assess the effect of applied potentials on stress corrosion cracking of Austenitic Stainless Steels (ASS), Type 304L in a potentiostatically controlled chloride environment at ambient temperature, the assessment of cracks in failed specimen was limited to optical microscope. In this present study, Scanning Electron Microscope (SEM) was used in addition to optical microscope to gain a better understanding of the Stress Corrosion Cracking (SCC) mechanism in the failed specimen. Stress corrosion cracking tests were carried out on annealed ASS using a Slow Strain Rate Test (SSRT) technique in sodium chloride solution acidified with hydrochloric acid at ambient temperature. Post-mortem assessments of failed specimens were carried out using both optical and SEM. The study showed that plastic elongation, ultimate tensile strength and time to failure decrease as the applied potential increases during the slow strain rate test. The study showed that immunity of ASS to chloride SCC was improved when the electrochemical potential was maintained in the primary passive potential range. Post-mortem assessment of failed specimens showed evidences of SCC and ductile failure on the fracture surface. The cross sectional analysis of the failed samples showed cracks that were predominantly transgranular stress corrosion cracks.