Erosion - corrosion mechanisms of stainless steel UNS S31603

Solid particle erosion-corrosion is the wear caused by the combined action of the mechanical process of solid particle erosion and the electrochemical process of corrosion. This joint action leads to a synergistic interaction that enhances the wear rate of the material, causing severe problems to engineering components exposed to these aggressive conditions. This poses a problem to designers and engineers, as there are currently no robust models available to predict erosion-corrosion rates due to the incomplete understanding of the physical erosion-corrosion mechanisms and synergy. The aim of this project is to develop a systematic understanding towards modelling erosion-corrosion by investigating the erosion-corrosion mechanisms of stainless steel UNS S31603. An integrated approach is used in this study consisting of three main thrusts from an environmental, electrochemical and materials perspective. The first part of the thesis, examines the robustness of the semi-empirical model based on an active area principle, which was developed recently at the University of Southampton on a passive metal UNS S31603. Gravimetric experiments were performed using a slurry pot erosion tester. The slurry pot erosion tester was also modified to perform in-situ electrochemical investigations. Results from this novel modification, showed that the erosion-corrosion rates and synergy levels increased with increasing velocity, temperature and sand concentration. Electrochemical current noise measurements for multiple particle impact experiments showed that this was partly due to the continuous rupture of the oxide film leading to an erosion enhanced corrosion synergistic effect. The erosion-corrosion rates were found to be a function of the kinetic energy of the particles, the number and the size of the particles impacting the surface. The amount of charge consumed and the repassivation kinetics were derived from the single particle impact experiments. Lips also appear to crack on the surface believed to be caused by corrosive action accelerating material removal. The results were analysed statistically and for the first time, interaction contour plots have been used to decouple the interactions between the test parameters. These studies showed that the largest interaction occurred between velocity and sand concentration and empirical models were also derived from these analyses. Although the model provided reasonable prediction of the synergy values, the unanswered question of whether the right mechanisms were being modelled formed an important basis for this work. For the first time, in-depth investigation was performed on the evolution of wear on the surface and subsurface of UNS S31603 using SEM, FIB, STEM and TEM. Investigations revealed that a three layer grain structure consisting of nano-grains, micro-grains and deformed bulk grains was seen to evolve with time. An explanation is proposed on reasons why the mass loss rates vary at different stages of erosion-corrosion, by correlating the surface and subsurface wear with the trend of mass loss rate versus time. TEM investigations also revealed the formation of numerous fatigue cracks and folding of lips on the surface believed to be due to strain imposed during repeated particle impact. Other unique features observed are embedment of erodent fragments and chromium oxide layer as well as strain induced phase transformation. It is believed that a thin composite structure consisting of these elements are formed and enhanced by the formation of lips over this structure. All these factors combined with grain refinement and work hardening enhances the fatigue crack formation process. This process is then accelerated by corrosion as confirmed by the higher density of cracks observed in the erosion-corrosion sample, compared to the sample subjected to pure erosion. This is proposed as one of the main corrosion enhanced erosion synergistic mechanism present during erosion-corrosion. Physical models have been developed based on these micro and nano-scale wear observations to integrate the surface and subsurface erosion-corrosion mechanisms. This work has generated an enhanced physical model to explain the erosion-corrosion mechanisms at the subsurface of UNS S31603. The findings of this work would greatly assist engineers and designers in the development of future erosion-corrosion models and in the understanding of synergy between erosion and corrosio