Electrochemical Discussion of Various Materials Modified By Friction Reforming Suitable for Marine Environment

In recent years, since the ocean resources development has been emphasized, the importance of machines used in the marine environment seem to be growing. The sliding parts of equipment directly contact with seawater, so friction material with superior both corrosion resistance and wear resistance in marine environment is necessary. Thus, in order to develop friction materials adapted to marine environment, surface modification was conducted to add the properties of tribology and corrosion resistance to several materials. The method of surface modification is grouped into two major processing. One is the friction reforming process with powder and the other is zinc pin coating processing. Reforming layers are formed on the base material by this method. In past study, a lot of modified materials have been developed. To develop much more modified material, it is required that we efficiently find a better combination of base metal and powder to make the base material having the superior properties for wear and corrosion resistance. As the result of past study, we found that Ti+Cr2N and Ti+Al2O3 by friction reforming process had excellent wear resistance properties superior to these of base materials under seawater. On the other hand, it is difficult to clarify the corrosion resistance characteristics of the modifying material under seawater just by observation and the elemental analysis of the surface. In this paper, at first, the electrochemical measurement system was built to evaluate corrosion characteristics of the modified materials under seawater. Corrosion characteristics of modified materials can be obtained from measuring corrosion potential and the polarization curve. The corrosion potential and the polarization curve of austenitic stainless steel (JIS SUS304) and 0.45% carbon steel (JIS S45C) were measured to confirm reliability of the electrochemical measurement system. Then, since this system is a help for the understanding of the electrochemical phenomena during fretting test (reciprocating friction and wear test which amplitude is very small), the corrosion potential and the polarization curve of Ti-6Al-4V as a base material of Ti+Cr2N and Ti+Al2O3 by friction reforming process are measured under the 3.5%NaCl solution in static and dynamic conditions by using this system. The dynamic (fretting test) conditions is a normal load of 9.8N, a slip amplitude of 400μm, a frequency of 7Hz and total sliding distance of 16m (20000 cycles). In the polarization test, potential was scanned from -800mV to 600mV with a scan rate of 1mV/s versus the open circuit potential. Finally, the corrosion potential and the polarization curve of Ti+Cr2N and Ti+Al2O3 by friction reforming process are measured under the 3.5%NaCl solution in static and dynamic conditions and compared with these of Ti–6Al–4V. We discuss corrosion and wear characteristics of Ti+Cr2N and Ti+Al2O3 based on these results.