The work covered in this thesis concerns the performance of a commercially available AI-Zn-In alloy, particularly in above ambient temperature conditions where the anode is completely or partially buried. Initial work involved the construction and commissioning of a heat transfer rig for use in long term performance studies. The anode under test was subjected to a series of long term laboratory performance tests and a series of short term electrochemical tests, at both ambient temperatures and at varying levels of heat transfer. The maximum test temperature used corresponds to a maximum internal temperature in the anode of ~1 05 °C. The surrounding electrolyte was maintained at a constant temperature of 4 °C. The internal and interfacial temperatures of the anode were monitored closely throughout all experiments. The electrolyte used for both tests was a mixture of bentonite clay and/or BS artificial seawater. This electrolyte represents the worst case scenario likely when this anode is in industrial usage. The long term testing of the anodes involved monitoring the potential and current flow over 1000 hours. The anodes were tested over a range of applied current densities, 500, 1000 and 2000 mA/m-2 ; in addition, the anodes were tested as a simple galvanic couple with steel. It was found that under all test temperatures and conditions, the steel was adequately protected. With increasing temperature, the potential of the anode became slightly less negative and the potential of the steel decreased dramatically. As shown by previous workers it was found that the highest value of efficiency at each test temperature was displayed by the anode subjected to the highest impressed current density. The efficiency of the test anodes was found to decrease steadily with increasing temperature, reaching a minimum of approximately 20% at80°C. It was also found that the effect of heat transfer was more detrimental to the current capacity than the same temperature applied isothermally. A series of short term electrochemical tests were undertaken, ac impedance, repassivation kinetics and potentiodynamic sweeps. The following features were identified; the application of ac impedance has shown that at high temperatures, the anode is not activating fully although corrosion processes are occurring, confirming that self corrosion is the major reaction under these conditions. The minium chloride concentration necessary to activate the alloy has been shown to be between 0.5 and 1.0% w/w chloride. Chloride adsorption onto the metal surface is thought to be an essential stage in the activation process. Iron at the surface of the anode' is also thought important in the activation process. There are three main complimentary factors which are causing the reduction in current capacity or efficiency. Firstly, the level of self corrosion increases as the test temperature is increased. Secondly, there is an increase in the formation of corrosion products around the anode, particularly aluminium hydroxide, due to the increased'level of self corrosion. Finally, the level of dissolved oxygen adjacent to the anode is highly dependant on both the temperature and the nature of the test environment. In the clay/seawater mixture at high temperature, the concentration and rate of diffusion of oxygen will be greatly reduced.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:586713 |
| Date | January 1995 |
| Creators | Erricker, S. L. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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