The corrosion behavior of Zr and Zr-based binary alloys (Zr-Ti, Zr-Nb, Zr-Ta, Zr-Pd, Zr-Pt, Zr-Cu, and Zr-Au) with various concentrations in simulated body fluids were investigated. Pure Zr showed pitting corrosion during the potentiodynamic anodic polarization measurement in Hanks' solution, however, the pitting potential of Zr was much higher than that of type 316L stainless steel. The pitting potential was widely changed by the composition of the alloy. In particular, Zr alloyed with larger amount of Ti, Nb, and Au showed higher pitting potential than pure Zr. Thus, these alloying elements can be considered as beneficial for improving corrosion resistance in physiological environment. On the other hand, Zr-Cu alloys showed low pitting potentials. Therefore, Zr alloys should be carefully designed when they will be used in chloride-containing environments.
Lotus type porous metals and alloys have various properties including advantages to be applied bio-medical use. Ingot with randomly distributed directionally elongated pores is able to be produced by unidirectional solidification in a pressurized gas atmosphere such as hydrogen and nitrogen. This material exhibits apparently low density without loss of mechanical properties. In this review, corrosion behavior and bio-compatibility of lotus-type porous austenitic stainless steels are described based on the authors' experimental results.
Magnesium alloys attract attention as a biodegradable/bioabsorbable metallic material because of the higher strength and toughness than those of biodegradable polymers and ceramics. However, practical application of Mg alloys makes little progress due to the too rapid corrosion in physiological environments. New alloys and surface coatings are developed to improve the corrosion resistance and biocompatibility. On the other hand, in vivo corrosion rate of Mg alloys was often considerably different from that obtained with in vitro corrosion test designed for conventional metallic biomaterials such as Ti alloys. It is thus a critical issue for Mg alloys to develop appropriate corrosion evaluation methods, for that it is necessary to elucidate corrosion characteristics of Mg alloys. We examined some characteristic corrosion factors for Mg alloys using a rotating electrode, and also developed calcium phosphate coatings to control Mg alloy corrosion. In this paper, influence of various physiological corrosion factors is summarized, and corrosion behaviour of developed calcium phosphate-coated magnesium alloys is also briefly described.
Evaluation techniques for fretting corrosion properties of metallic biomaterials in air and in pseudo-body fluid and the representative results are reviewed. Two-stage fatigue test, consisting of the initial fretting fatigue test and the subsequent pure fatigue test, was adopted to investigate whether crack initiation or propagation dominates for fatigue fracture. The influence of fretting on fatigue property is discussed by separating the influence into the friction stress added to axial cyclic stress and the other additional effects, and the fretting fatigue property of some representative metallic biomaterials are compared.
A flange joint with Zinc anode was used. It was immersed in filtered natural seawater. After 84 days no sign of crevice corrosion was found on the SUS304 flange with Zinc anode,while crevice corrosion ,maximum depth of 0.26mm, occurred on the SUS 304 flange without Zinc anode near the edge of the gasket sheet.
The effects of sulfate and sulfite ion on pitting corrosion for Al alloy in chloride solution were investigated by potentiodynamic polarization measurements. If the concentration of sulfate and sulfite ion was equal to each other, the oxygen diffusion －limited current density in the sulfate ion solution was higher than that in the sulfite ion solution and hydrogen ion diffusion －limited current density in the sulfite ion solution was higher than that in the sulfate ion solution. The pitting potential became more noble with an increase in sulfate and sulfite ion concentration. The open circuit potential was independent of sulfate ion concentration and increased with sulfite ion concentration. These results indicated that the difference between the pitting and the open circuit potential became larger with sulfate ion concentration and smaller with sulfite ion concentration. Upon the above results, it was concluded that sulfate acted as an inhibitor and sulfite acted as an accelerator.
Long-term stability operation is requested in the boiler for thermal power generation. Recently, thinning in the tube of heat exchanger due to the grooving corrosion has been actualized in the pulverized coal combustion boiler. Corrosion damage with the grooving corrosion was detected in the burner zone and nearby area with high thermal load as a consequence of cyclic thermal stress (localized tube surface temperature fluctuation on account of slag detachment and tube surface temperature fluctuation induced by wall blower operation and other causes) on the furnace side of the tube wall in an atmosphere containing H2, H2S, and O2, which leads to cracking of the corrosion product layer. Thermal spray coatings applied to the surface of boiler components such as heat exchanger tubes are effective as the countermeasure. In this paper, the Cr3C2-NiCr spray coating was performed by the high-velocity oxygen fuel spraying (HVOF) and the atmospheric plasma spraying (APS). The characteristics of the spray coatings were evaluated by adhesive strength, high temperature hardness, and the abrasive wheel wear test, the high temperature erosion test, and the high temperature corrosion test, etc. The evaluation results proposed the method of evaluating the spray coatings to apply to the coal fired boiler in the laboratory.