ELECTROCHEMICAL STUDY OF CORROSION PROCESSES IN HIGH SUBCRITICAL AND SUPERCRITICAL AQUEOUS SYSTEMS
Open Access
- Author:
- Guan, Xueyong
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 13, 2006
- Committee Members:
- Digby D Macdonald, Committee Chair/Co-Chair
Long Qing Chen, Committee Member
Paul Raymond Howell, Committee Member
Fan Bill B Cheung, Committee Member - Keywords:
- supercritical water oxidation
high subcritical and supercritical aqueous systems
electrochemical noise analysis
electrochemical corrosion mechanism
chemical corrosion mechanism
pressure
volume of activation
corrosion rates
energy of activation of corrosion processes - Abstract:
- Severe corrosion damage occurs in high temperature aqueous systems and few materials can withstand these harsh working conditions. In this work, electrochemical emission spectroscopy (EES) is used to identify corrosion mechanisms and to analyze the effect of pressure on corrosion reactions in high subcritical and supercritical aqueous systems. Two corrosion mechanisms, “chemical oxidation” (CO) and “electrochemical oxidation” (EO), have been proposed to describe the corrosion processes in high temperature aqueous systems, depending upon the density and dielectric constant of the systems. EO corrosion processes are featured by partial charge transfer reactions such as the metal dissolution and the reduction of oxygen. On the other hand, CO processes are dominated by direct molecular processes such as the direct reaction of the metal with aggressive species on one site. EES method is used to differentiate two corrosion mechanisms by postulating that only the electrochemical mechanism gives rise to spontaneous fluctuations in current and potential. Experiments show that the electrochemical mechanism is the dominant corrosion mechanism when the temperatures are below 350 oC and that it becomes of progressively lower importance as the temperature increases above the critical temperature (Tc = 374.15 oC). The energy of activation of corrosion of Type 304 SS and titanium in 0.01 M HCl at the temperature range of 50 – 250 oC has been estimated. The result shows that titanium is more corrosion resistant than Type 304 SS in HCl solution due to the formation of the strong protective passive film on the surface of titanium. A model is developed to study the effect of pressure on corrosion rates of metals in high temperature aqueous systems, with emphasis on contributions from the activation, the degree of dissociation of aggressive species, and the system isothermal compressibility. EES was applied to estimate the pressure dependencies of the electrochemical corrosion rates of Type 304 SS and nickel in high temperature HCl solutions. The study shows that the volume of activation of corrosion processes is pressure dependent in high temperature aqueous systems. And the magnitude of the volume of activation decreases with increasing pressure in both high subcritical and supercritical conditions. The contribution from the volume of activation plays a dominant role on the relative dissociation is found to be more important to the relative corrosion rate at supercritical conditions.