When stainless steels are oxidized in air, surface oxide scales with outer chromium enriched layer and with inner chromium depleted layer are formed due to the delay of chromium diffusion from matrix to surface, deteriorating wet corrosion resistance. In this work, the influence of chromium concentration near the surface on crevice corrosion of stainless steel was studied. SUS 304L specimens were oxidized in air-opened siliconit furnace at 1, 273K for 600-2, 400s, and then removing oxide scale by mechanical polishing to measure the chromium depth-profiles by electrochemical method and repassivation potential, ER for crevice corrosion. ER were measured in 3.5% NaCl (303K, deaerated by argon), and the shapes of crevice corrosion were observed by SEM in plane and cross-section. The concentration of chromium at the surface and the grain boundary were decreased due to the selective oxidation and the quick diffusion of chromium through grain boundary. These changes near the surface induced crevice corrosion with shallow and intergranular dissolution mode, and made ER less nobel from -0.13V to -0.23V (Ag/AgCl, sat. KCl) due to the formation of sub crevice, in which repassivation was restrained.
A new electrochemical evaluation method was presented to estimate the rating number of rust staining of stainless steels in a marine environment. In this accelerated test, rust staining resistance of stainless steels was evaluated from the time to pitting under the thin electrolyte layer that was generated by the capillary phenomenon through a cotton cloth. The incubation time to passive film breakdown under this thin electrolyte layer corresponds to initial atmospheric corrosion resistance. From the results of ten years exposure test, most of stainless steels experienced severe rust staining within the first two years exposure. All the types of stainless steel have the same tendency wherein rating number changes with the square of the inverse time. The rating number of stainless steels in the marine environment can be evaluated from this time dependence and the degree of initial rust staining. The latter can be estimated by newly developed test method.
Investigation has been made on the corrosion resistance of stainless steels which were polished by abrasive belt-grinding using various kinds of polishing oils. Stainless steel sheets of SUS 430, SUS 430 LX, SUS 444, SUS 304 and SUS 316 were polished using a belt grinding machine which was equipped by abrasive endless belts smeared with ten kinds of mineral oils. Corrosion resistance of the polished stainless steels was examined by salt spray test, atmospheric exposure test and pitting potential measurement. The results are summarized as follows: Sulfur or its compounds which are added to the polishing oils in order to improve the abrasive belt grinding performance have harmful effects for the corrosion resistance of the stainless steels. On the other hand, other additives, i. e., phosphorus or chlorine compounds and oleic acid are almost harmless. Surface analysis of the polished stainless steels by XPS (X-ray photoelectron spectroscopy) showed that the surface oxide layer formed by the belt grinding, using the sulfurized mineral oil, contained much sulfides which caused the deterioration of the corrosion resistance of stainless steels.
In order to verify the mechanism of the formation process of hemimorphite (Zn4Si2O7(OH)2·H2O) in galvanized steel pipe for water service, the synthesis of hemimorphite was performed through the application of a co-precipitation reaction, which was named the co-precipitation method. Sodium metasilicate (Na2SiO3·9H2O) solution was mixed with Zn(OH)2 colloidal solution. The precipitate obtained from the mixture was aged in aqueous solution to crystallize it into hemimorphite. The samples which were aged for the prescribed periods were analyzed using X-ray diffraction, infrared spectroscopy and thermal differential analysis. It became clear from the analysis that at the initial stage of hemimorphite formation, dissolved silicate was adsorbed onto the colloidal particles of Zn(OH)2 and precipitated with Zn(OH)2 particles; at the next stage, the adsorbed silicate broke the structure of Zn(OH)2 to form an amorphous compound as a precursor of hemimorphite; and at the final stage, rearrangement of atoms proceeded over a long period to form the framework of the hemimorphite. Furthermore, it was suggested that the OH bond of the Zn-OH-Zn bridge in the hemimorphite crystal was hard to form compared with the other bonds in the crystal.
Recently, various X-ray diffraction methods have been established for surface analysis. For example, the parallel beam method enables us to make qualitative analysis of several nanometers depth of the surface. Brief introduction about some X-ray diffraction methods for surfacelayers will be described.
Small fatigue cracks may propagate at higher rates than expected values based on long crack data and linear-elastic fracture mechanics, and even at lower stress intensity factor ranges than the thresholds of long cracks. Since it was reported that crack size effects were more remarkable in corrosion fatigue (CF) than mechanical fatigue, the small-crack growth mechanism has been one of the most attractive subject for CF researchers. Small CF cracks are classified into some categories according to their size scales and growth stages as well as mechanical fatigue cracks, and further into chemically small cracks of which the range is wider than the others. A number of mechanism models of small-CF-crack growth have been proposed and are classified into several groups in this paper as follows; hydrogen embrittlement models, anodic dissolution models, film rupture/slip dissolution models, chemisorption/softening models, crack closure restraint models, and distribution/coalescence models. The researches on small CF cracks, which are still in progress, are expected to contribute to the clarification of fracture mechanism of metallic materials, the development of sophisticated methods of life prediction, and so on.