The Influence of Composition and Plastic Strain on the Electrochemistry of Stainless Steels Doped with Platinum Group Metals
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2009-03
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Materials Performance Centre, University of Manchester
Abstract
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.
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Govender, K., Necib, O., Ahmed I. I., Scenini, F., Lyon, S., & Sherry, A. H. (2009). The Influence of Composition and Plastic Strain on the Electrochemistry of Stainless Steels Doped with Platinum Group Metals. Manchester: Materials Performance Centre, University of Manchester.