Cavitation erosion tests were carried out on five metallic materials which are the standard materials of the International Cavitation Erosion Test Program which is co-ordinated by Dr. J. Steller, Institute of Fluid-Flow Machinery, the Polish Academy of Science, Gdansk, Poland. Three kinds of testing apparatus were used: a vibratory unit, a vibratory unit with a stationary specimen, and a water tunnel. In the two vibratory tests, the ranking order of the materials according to their durability to cavitaition attack showed a complete coincidence with each other. However, the ranking based on the water tunnel test was different from those of the vibratory units. This was because the damage rate of some materials was suddenly accelerated in the middle of water tunnel test. It was revealed that the sudden increase in erosion rate is caused by the electrochemical corrosion which is enhanced by the activation of the damaged surface in atomic level. The corrosion is further promoted when a macrocell of corrosion is formed between the cavitation-damaged area (anode) and the undamaged area (cathode) of the specimen surface.
Cavitation erosion tests were carried out on five metallic materials which are the standard materials of the International Cavitation Erosion Test Program which is co-ordinated by Dr. J. Steller. Three kinds of testing apparatus were used: a vibratory unit, a vibratory unit with a stationary specimen, and a water tunnel. The damage depth was chosen as the index of damage. This increased linearly against testing time after an incubation period during which no damage appeared. Damage depth vs. testing time lines under different intensity of cavitation attack were extrapolated back to the beginning of test to obtain a single characteristic point on the O-axis at which all these lines crossed the axis. By using Tabor's “the relation of one to one correspondence” between hardness and strain, the point was correlated to the amount of plastic strain which was accumulated within the incubation period. The slope of the line (damage rate) was related to the sum of occurrence frequency of cavitation impulsive pressure of which the intensity was larger than the tensile strength of the material.
Electrochemical impedance spectroscopy has been applied to study atmospheric corrosion. A two electrode cell type atmospheric corrosion probe has been constructed. Quantitative analysis on IR drop and current distribution on the electrode, which produce serious errors in electrochemical measurements of corrosion rate of metal with a very thin moisture layer, have been made on the basis of transmission line model. The analysis reveals the optimum cell sizes and the limitation of atmospheric corrosion monitoring. The instantaneous corrosion rates of copper in the atmosphere of 30% to 95% Relative Humidity (RH) have been monitored by the continuous measurements of the impedances at 10mHz and 10kHz. Below approximately 60%RH, the corrosion rate shows extremely low values. When the atmosphere is switched from 60%RH to 70%RH, the corrosion rate abruptly increases, and then reaches a maximum value at 90%RH. The further increase of relative humidity slightly decreases the corrosion rate. These phenomena can be explained by the dependence of atmospheric corrosion rate on the thickness of electrolyte layer.
Investigation has been made on the corrosion resistance of stainless steels which were polished with coated abrasive papers wetted by various kinds of solutions. Atmospheric exposure tests, salt spray tests, and pitting potential measurements were carried out, and it was discovered that the rust resistances of SUS 430 and SUS 430 LX polished together with Na2S solution were much degraded. It was assumed that the deterioration of the rust resistance was due to sulfides on the polished surface which were detected by XPS and were inferred to be formed by tribochemical reaction during the process of polishing. SUS 304 was deteriorated much less than SUS 430 or SUS 430 LX. Na2S2O3, NaSCN and Na2SO3 deteriorated the rust resistance to less degree than Na2S, while Na2SO4, NaCl and NaF did not. Sulfide formation reactions were discussed based on measured potentials of the specimens which were being polished, and pH values of the polishing solutions.
On using the surface treated materials, properties such as corrosion resistance and mechanical properties depend on chemical composition of the surface layer, which are effected strongly by treatment condition and kinds of base metal. The resistance of pitting and Strees Corrosion Cracking (SCC) of SUS 304 in chloride solution was improved remarkably by using Chromize. Application of Cu/Ni composite plating on SUS 304 improved crevice corrosion and SCC. And Ni-Fe-Cr-Mo layer which was produced by diffuision heat treatment after Ni plating on high purity ferritic stainless steel prevented SCC. And resistance of local corrosion was improved by PVD, laser, metalspraying method.