The corrosion behavior of pure Ti, Ti-6Al-4V, Ti-6Al-7Nb alloys and new Ti-(0.1-2.0)wt%Pt (or Pd) alloys in a fluoride containing environment were examined by anodic polarization and corrosion potential measurements. SEM observation was performed before and after immersion in the test solutions. The artificial saliva containing 0.1% and 0.2% NaF (corresponding to 453 and 905 ppmF) and artificial saliva with a low level of dissolved oxygen were used as the test solutions. In an acidic environment (pH 4.0) including fluoride, surfaces were roughened markedly by corrosion for pure Ti and Ti-6Al-4V and Ti-6Al-7Nb alloys, although the surface of Ti-Pt (or Pd) alloys was not affected for the most part. With a low level of dissolved oxygen, surfaces were damaged microscopically by corrosion for pure Ti, Ti-6Al-4V and Ti-6Al-7Nb alloys even with a fluoride concentration included in commercial dentifrices, although the surface of Ti-Pt (or Pd) alloys did not change. The Ti-0.5wt%Pt (or Pd) alloys were expected to be useful as a new Ti alloy with high corrosion resistance in dental use.
Some studies have been performed on the application of the supercritical fluid environment to the extracting separation, the decomposition of the pollutant and the synthesis of the organic compound. One of the problems to apply the supercritical fluid is the selection of the reliable construction material for this environment. Some examinations have been done regarding to the localized corrosion and the stress corrosion cracking, but the electrochemical behavior of metallic materials has not been studied in this environment. The results of the electrochemical examination in the carbon dioxide supercritical fluid mixed with nickel-plating solution, which is acidic and containing chloride, are introduced in this paper. The electrochemical characteristics of the materials in this environment become obvious from this study. It is also estimated from this examination that 329J4L stainless steel and C276 alloy can be use for the construction materials in this supercritical fluid environment.
To protect steels from water-line attack in seawater, we examined to coat steel with cement containing metals powder of different grain sizes. Steel specimens coated with these cements were immersed in 3 mass% sodium chloride solution. The condition to prevent steels from corrosion was determined by measuring potentials and observing the surfaces of specimens. The potentials of specimens coated with the cement containing aluminum or magnesium powder were about -400mV vs. SHE. The corrosion was observed. The cement containing more than 50 mass% zinc powder kept the potentials less than -700mV vs. SHE when steels were covered fully with the cement. In these conditions, no corrosion appeared, though the potential did not always stay at the immunity domain. The protection ability became less with the increasing grain size. When the one side of specimen was coated with the cement containing more than 67 mass% zinc powder and the other side and edges were left bare, the potential of specimen stayed at less noble than -750mV vs. SHE for 60 days. In these conditions the uncoated areas of a specimen were protected. We concluded that the application of cement coating containing more than 67 mass% zinc powder would be effective for the prevention of water-line attack in a marine environment.
Corrosion characteristics of the materials of rotors and blades for low-pressure turbines in a steam power-generating plant were evaluated by using condensed-water evaluation equipment, which is capable of reproducing the environment of the vapor condensation process for testing, constant-strain testing equipment in the dry-and-wet-alternating condition and low-cycle-fatigue testing equipment. The condensed-water evaluation revealed that impurity ions in steam such as sodium, sulfate and chloride were separated in the initial condensation process, i. e. at the initial stage of condensation, from vapor into the initial condensate. An empirical equation obtained from the measured concentrations of impurities in water vapor and condensates suggested that the concentration of impurities in early condensate can be as high as 50ppb in the environment of an industrial plant. Low-cycle-fatigue life decreased with increasing the concentration of impurities. Meanwhile, the kinds of impurity ions and their mixing exhibited no superposition effect on their concentration. Impurity ions were found to shorten the fatigue life of the material far more than dissolved oxygen. Constant-strain testing of roter material using double U-bend specimen showed that dry-and-wet-alternate and wet-and-air alternate conditions increased crack susceptibility. This could be related to their infiltration and accumulation of impurity ions in interstices and the influence of chloride ion was the strongest.
The geothermal energy is desirable as one of the energy resources for the global environmental protection, since it scarcely emits carbon dioxides (CO2) as compared with the conventional fossil fuels. The geothermal fluid contains a large quantity of corrosive chemicals which occasionally cause the corrosion-related problems such as stress corrosion cracking (SCC). Therefore, the evaluation of the corrosion resistance of the materials applied to the steam turbines is indispensable for the economical and effective utilization of the geothermal energy. In this work, SCC growth behavior of the blade materials (13%Cr and 16%Cr-4%Ni stainless steels) and the rotor material (1%Cr-Mo-Ni-V steel) for geothermal steam turbines were investigated by means of WOL specimens immersed in the corrosive water of the simulated geothermal environment including chloride and sulfate with the carbon dioxide and hydrogen sulfide. As the results, SCC growth rate da/dt and critical stress intensity factor KI SCC of the materials tested were calculated. The blade material 13%Cr steel showed crack growth with relatively low stress intensity factors under the tested condition, while the blade material 16%Cr-4%Ni steel with the improved heat treatment showed no crack growth. It was also clarified that the 1%Cr-Mo-Ni-V steel used as a rotor material had an excellent SCC resistance.