Cavitation erosion process is a very complex phenomenon depending not only on the type and unsteadiness of cavitation, but also the response of the propeller and rudder materials to the cavitation energy imparted upon. It is highly destructive in nature and can cause severe loss in the performance of the ships that may eventually lead to frequent dry dockings, inspections and preventive maintenance or replacement of the damaged parts, resulting in a rather expensive maintenance. The aim of this PhD project is to look at different aspects in characterising the materials generally used for manufacturing the ship propellers and rudders based on their resistance to cavitation erosion. It aims to understand the cavitation phenomena simulated by the ultrasonic vibratory probe device on various ship propeller material samples. Several ultrasonic vibratory cavitation tests were conducted for cavitation erosion, pure corrosion and combined cavitation erosion-corrosion on the two most common metallic propeller materials, Duplex Stainless Steel (DSS) and Nickel Aluminium Bronze (NAB), especially comprising of well-formed oxide films in different conditions. The investigation of the synergistic effect existing between the cavitation erosion and corrosion was carried out with the help of steady mass loss over a period of time and in-situ electrochemical measurements of corrosion. Alicona was employed for surface analyses, and comparisons between gravimetric and volumetric/optical loss measurements were obtained using precision weighing machine and Alicona respectively. Ag/AgCl reference electrode was used for in-situ electrochemical experiments done on both the sample materials kept at open circuit potential, and electrochemical impedance spectroscopy to study the corrosion behaviour during the experimental tests. In order to understand the combined synergistic effects of cavitation erosion and corrosion in seawater five different methods were employed to measure and quantify synergism. The experiments were conducted using an ultrasonic vibratory horn functioning at 19.5 kHz frequency and 80 μm ± 0.2 μm peak-to-peak amplitude. The test methods used to obtain the synergy included gravimetric mass loss technique, volumetric mass loss and mean depth of penetration rate methods, and polarization technique. Scanning electron microscopy was used to analyse microstructural characteristics of the cavitated sample surfaces, as well as the transverse-sections of the surface features. The materials were subjected to pure erosion, pure corrosion and the combined effect of erosion-corrosion in order to understand and measure the individual contributions of each aspect. The extensive experimental results obtained and the conclusions drawn have attempted to address the aim of the research, and meet all the objectives to the best effect. Synergism was found to have measurable impact on the cavitation erosion-corrosion of both NAB and DSS. NAB underwent selective phase attack, resulting in increased material removal especially in the presence of corrosive environment. Whereas, DSS was observed to undergo ductile failure in the cavitated zone in the form of extrusion of the austenite at the cavity boundary along with microcracks and cleavage facets that could be attributed to the austenite to martensite transformation by either high strain or high temperature generated during cavitation. The implications of immersing as-cast NAB in 3.5% NaCl solution for few months seemed to drastically change the synergy behaviour of the material. Whereas, DSS, having higher mechanical and corrosion property as compared to NAB seemed to give best outcome for cavitation erosioncorrosion.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:720218 |
| Date | January 2017 |
| Creators | Basumatary, Jahnabi |
| Contributors | Wood, Robert |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/412707/ |
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