Following the preceding article, a theory that considers the primary role of strain-induced vacancies in hydrogen-related failure is presented. Detection of lattice defects the creation and agglomeration of which are enhanced by hydrogen is described. As a support of the theory, correlations between the mount of strain-induced vacancies and the susceptibility to hydrogen-related failure are shown with steels of different microstructures. Interaction of fatigue and delayed fracture is shown as another evidence of the theory. The role of hydrogen concentration in the failure is discussed in regard to a time-dependent alteration of defects during delayed fracture test.
In order to quantitatively evaluate the resistance of a candidate overpack material for geological disposal of high-level nuclear waste to the crevice corrosion, the optimized test method for determining the corrosion-crevice repassivation potential, ER,CREV, of a Ni-Cr-Mo alloy (Alloy 22) was developed based on that of stainless steels (JIS G 0592). It was found that two restrictions shall be satisfied for determining the valid value of ER,CREV for Alloy 22. Restriction (a) was to avoid transpassive dissolution, and (b) was to obtain a penetration depth of 65 μm or more in creviced areas. The recommended procedure in JIS G 0592 at the corrosion-crevice initiation stage, which involved the potentiodynamic anodic polarization at a scan rate of 30 mV min−1, could not satisfy the restriction (a). Consequently, we adopted the potentiostatic holding at the potential below the critical potential for transpassive dissolution. The recommended procedure in JIS G 0592 at the corrosion-crevice propagation stage, which involved the galvanostatic holding at an applied current of 200 μA for 2 hours, could not always satisfy the restriction (b), and the applied current of 1600 μA or more could not satisfy the restriction (a). Therefore, we adopted the galvanostatic holding at a current of 800 μA for 2 hours. The limits of safety usage of Alloy 22 were evaluated by values of ER,CREV which were measured with the optimized procedure in 0.1 to 4 mol dm−3 sodium chloride solutions at 90ºC.
An outdoor exposure and a simulated airborne sea salt corrosion test were carried out to investigate the effect of Al content of Mg-Al-Zn alloys on atmospheric corrosion. In both of the corrosion tests, the alloy containing high Al had the superior corrosion resistance. In the outdoor exposure, Al was concentrated in the surface film during the exposure, which has the beneficial effect for suppressing the following corrosion, and the corrosion form was changed from general corrosion to localized one with increasing Al content in the alloys. In the simulated airborne sea salt corrosion test, mass loss of Mg alloys increased with the amount of the deposited salt and relative humidity. From the relationship between the water film thickness and Cl− ion concentration, it was also clarified that the corrosion rate of the Mg alloys mainly depended on the water film thickness.
The mechanism of crystal adhesion and growth of calcium carbonate on the stainless steel plate surface from supersaturated solution was investigated at 25.0ºC. The amorphous CaCO3 (ACC) was immediately precipitated in solution from the highly supersaturated solution. Crystal growth of calcite in solution started after 2 minutes of induction period. The crystal growth of vaterite started at 3.5 min. All the ACC has transformed to crystalline polymorphs by about 8 min. The metastable crystals, vaterite, transformed into stable form, calcite, by 7 hours. When the stainless steel plate was inserted in solution prior to ACC precipitation, ACC adhered at first on the plate surface, then it disappeared within a couple of minutes. After that, the calcite crystals nucleated on the surface and grew to large size. When the plate was inserted in solution just after the precipitation of ACC, vaterite crystals were simultaneously formed with the calcite. The vaterite crystals lined straight on the surface. Calcite crystals gradually grew, whereas the vaterite crystals dissolved and finally disappeared. When the plate was inserted after the growth of vaterite in the solution, no calcium carbonate was formed on the metal surface. Etching of the metal surface by acid reduced the formation of calcite. Scratch on the metal surface scarcely accelerated the adhesion of crystals.