In Situ Electrochemical Study On Residual Stress-assisted Localized Corrosion Of Stainless Steel

With strong corrosion resistance and high mechanical properties, stainless steel has been widely applied in many areas such as aviation, aerospace, energy, chemical industry, transportation and daily life. Residual stress which generated during the producing, processing and manufacturing process of metals and alloys may cause brittle failure of the materials. Based on the mechano-chemical interaction, the metal corrosion under the synergistic effect of residual stress and aggressive media will lead to the degradation of material performances and consequent reduction of service life-time. In serious cases, the corrosion damages may give rise to a major accident, leading to direct and indirect economic losses. It is of great importance in the aspect of theory and application, therefore, to deeply study microscopic mechanism of localized corrosion of stainless steel under the action of stress, and to clarify the mechanism that residual stress accelerates the local corrosion.In this dissertation, by combining traditional electrochemical methods (e.g. polarization curves and cyclic voltammetry, etc.) with scanning electrochemical microscopy (SECM) technique (e.g. area scan and approach curve methods, etc.), the correlation between the SECM images showing local electrochemical activities and the localized electrochemical reactions occurring at the metal/solution interface has been established on the basis of the relationship between the Faraday current on the tip and corresponding stress profiles on the steel specimen. The motivation to carry out the above research is to investigate the effects of a variety of factors such as residual stress, concentration of aggressive ions and substrate polarization on the localized corrosion behaviors of stainless steel, and to deeply understand the initiation and development process and its reaction mechanism of localized corrosion of stainless steel under the mechano-chemical interaction.The main work is as follows:1. The SECM measurements are conducted with current feedback (CF) and substrate generation/tip collection (SG/TC) modes on the steel specimens in the absence or presence of residual stress, respectively. On the basis of the relationship between the Faraday current on the tip and corresponding stress profiles on the steel specimen, moreover, the SECM images showing local electrochemical activities have been correlated with the localized electrochemical reactions occurring at the metal/solution interface, in which the concentration and distribution of electroactive species originating from the corrosion reactions is characterized.2. Upon scanning in the vicinity of the substrate surface, the current of redox reactions on the tip has the feedback feature, which may be associated with the solution compositions, the distance between probe and substrate surface features, etc. Comparing the correlation between the stress gradient pattern and the corresponding Faraday current on the tip, the mechano-chemical effects and related laws which consequently lead to localized corrosion of metal have been analyzed and discussed further. The objective of this study is to finally establish the correlation between material surface heterogeneity induced by residual stress and the susceptibility to localized corrosion of metals.3. The effects of varying substrate polarizations on the corrosion behaviors of type316stainless steel in Cl-ions-containing solution has been investigated. The results show that, if different polarization potentials are applied on the steel substrate, the morphology and the micro-electrochemical activity of the steel substrate under the synergistic effect of residual stress and electrochemical polarizations could be monitored directly by means of the SECM image (3D) which is certainly related to the redox reactions on the tip.