The atmospheric corrosion resistance of SUS 304·BA, SUS 430·BA, 430M·BA (modified SUS 430) and directly Cr-plated Stainless Steels were studied in volacanic Kagoshima district during three months, showing that SUS 304·BA and SUS 430·BA were strongly inferior in corrosion resistance to directly Cr-plated Stainless Steels. Juding from the potentiodynamic anodic polarization curves and the shapes of corroded surfaces measured by SEM, SUS 304· BA and SUS 430·BA had scarcely corrosion resistance against volcanic ashe attack, because the crevice corrosion was occurred under the ache and the corrosion reaction was promoted by extracted corrosive anion (especially F-) from the ashe. On the other hand, corrosion resistance of directly Cr-plated Stainless Steels were little influenced by the corrosive anion extracted from the ashe and quiet superior.
Localized corrosion on buried ductile cast iron pipes was investigated placing the main emphasis on the morphology of corrosion attack and the action of microorganisms. The form of corrosion was classed as selective leaching attack commonly referred to as graphitic corrosion. It was found that each nodular graphite was supported by amorphous silica and iron carbonate, thereby constituting a porous graphite mass that is characteristic of graphitic corrosion. Microbial enumeration and electrochemical observations strongly suggested a possibility that a symbiotic proliferation of iron bacteria and iron oxidizing bacteria was primarily responsible for accelerated localized selective leaching as well as tubercle formation. Results of EPMA analysis indicated that amorphous silica was formed as a result of metabolism of such bacteria to form both porous graphite mass and tubercles.
It was found that potassium dichromate (PDC), a corrosion inhibitor of passivating type, retards the development of cavitation erosion on commercially pure iron. The process was postulated as follows: PDC dissolved in an environmental liquid might adsorb on the metal surface and exert tension on the surface. This tension might retard cavitation erosion. The surface tension of the environmental liquid and the contact angle of the liquid wetting the surface were measured, and it was confirmed that tension certainly works on the surface in the liquid. It was also confirmed that a tensile stress applied mechanically on the metal retards the development of cavitation erosion in the early stage. The process postulated above was accordingly deemed correct.
The reason why a tensile stress applied parallel with a metal surface retards the development of cavitation erosion on the surface was elucidated. By the finite element method, stress distribution in an elastic solid under the attack of cavitation impulsive pressure was obtained. On this stress, another was superimposed by applying a tensile load in the direction parallel to the solid surface. As a result, the compressive stress caused by the cavitation impulsive pressure was canceled through the superimposed tensile stress. Observation of a metal surface exposed to cavitation attack revealed that the plastic flow of the metal surface caused by cavitation impulsive pressure was retarded by the tensile stress applied. Consequently it was concluded that the tensile stress applied parallel with a metal surface retards the development of cavitation erosion because it cancels the compressive stress caused by cavitation impulsive pressure and this reduces the plastic flow of the metal surface.
The embrittlement during the electrolytic hydrogen charging at 25-55°C and its recovery during hydrogen outgassing at 65-110°C after the charging were examined on Monel metal in some details with regard to tensile properties, fracture surface, and X-ray diffraction. A thin layer of hydride formed on the surface during the charging. It was unstable and disappeared outgassing, leaving local lattice strain in the underlying metal. Brittle fracture always occurred at grain boundaries in tensile test. Tensile strength and strain at fracture decreased with the increases in charging time and temperature. There was a relation, h∝ t1/2, between the depth of brittle layer (h) and charging time (t). The activation energy for the diffusion of hydrogen atoms obtained from the temperature dependence of h vs. t1/2 was estimated to be 10.7kcal/mol. On the other hand, the activation energy for recovery process, calculated from Arrhenius plot of the reciprocat time of 50% recovery of the strain at fracture, was evaluated to be 10.5kcal/mol. A linear relation was held between the mean depth of brittle layer and tensile strength and between the depth and strain at fracture, regrdless of charging and outgassing. These results show that the processes of both embrittlement and its recovery are rate-determined by the diffusion of hydrogen atoms in the metal.
Shape memory alloys have successfully entered the repertoire of serving themselves as different types of prostheses owing to their unique characteristics including the shape memory effect, superelasticity as well as high damping capacity since they were introduced in medical and dental fields more than 10 years ago. For being evaluated as a safe, reliable and biofunctional biomaterial (foreign material) against a living tissue, cell or bone (host environment), they are needed to meet strict requirements including the biocompatibility. This paper was prepared based on a literature survey on the medical applications of shape memory alloys, the mechanical, chemical and biological requirements, evaluation methods, various surface treatments and modifications, and advanced techniques for manufacturing and forming processes from which unexpected applications will be developed in the near future.