The depth-profile of Cr concentration of Cr depleted zone formed under oxide film of stainless steels during oxidation process plays an important role in the corrosion behavior and oxidation resistance. In order to produce stainless steels with high quality surface, speedy and exact measurement of the depth-profile of Cr is required, through which production process should be properly controlled. In this paper, a new method is presented for measurement of the depth-profile of Cr. This method depends on the relation between the Cr concentration of stainless steel and the current density in transpassive region in H3PO4 solution at 323K. SUS 304 steel which was oxidized in air at 1, 273 K, and then removed oxide film by mechanical polishing, was polarized at +1.40 V (SCE) (transpassive region) in 10% H3PO4 (air open, 323K) in order to measure the it, p (10-3A·cm-2)-Q (C·cm-2) curve. From the it, p-Q curve, the depth-profile of Cr was obtained by converting it, p into Cr concentration and Q into depth. The usefulness of this method was evaluated through the comparison between the depth-profile of Cr measured by this method and the one calculated and measured by evaporation model, AES, EPMA, chemical analysis respectively.
The intergranular corrosion of nonsensitized SUS 304 and SUS 316L steels was investigated in a 1Kmol·m-3 oxalic acid solution by means of Inductively Coupled Plasma Emission Spectrometry (ICP) and Scanning Electron Micrography (SEM). It was found that on the both steels, the ditch structure was observed in the transpassive potential below 1.3V (SCE), and the step structure was observed above 1.3V (SCE). The result in the compositions of dissolved metal ions was that the preferential dissolution of P and Si occurred in the transpassive potential below 1.3V (SCE). Furthermore, it was found that above 1.3V (SCE), the amount of oxygen evolution increased. Deduced from the obtained results, dissolution of P and Si which were segregated at intergranular was prior to all other elements, above the break-through potential (about 0.9V). As a result of that, intergranular corrosion occurred. Between 1.0V and 1.1V (SCE), oxygen evolution started, which mainly took place at segregated grain boundary. Amount of oxygen evolution was increased by anodic polarization. That caused susceptibility of intergranular corrosion gradually small. And above 1.3V (SCE), oxygen evolution took place in grain and grain boundary, so that nonsensitized austenitic stainless steels were not susceptible to intergranular corrosion.
The susceptibility to hydrogen embrittlement in martensitic steels containing Cr, Ni and Mo under stress and hydrogen diffusion have been investigated by electrochemical methods. As the content of alloying elements was increased, the apparent diffusion coefficient of hydrogen (D) and the threshold hydrogen permeation rate (Jth) for hydrogen embrittlement were decreased. The threshold hydrogen content (Cth), which was calculated by the formula; Cth=Jth×L/D (L; thickness), was almost constant. This means that the steel with smaller D has the higher susceptibility to hydrogen embrittlement because the hydrogen content in steel (C) becomes higher. On the other hand, the C in NACE solution (0.5% CH3COOH+5% NaCl, saturated with 0.1MPa H2S, 25°C) was increased due to the decrease in D with the increase in the content of Cr although the hydrogen permeability (J×L) was decreased, and the susceptibility to Sulfide Stress Cracking (SSC) was increased as the result. However, Mo showed the remarkable effect of decreasing the J×L and the consequent improvement in the resistance to SSC according to its content.
The resistance to oxidation of Ti-Al Intermetallic compounds (Ti-34% Al and Ti-37% Al) at high temperature under atmospheric oxygen was investigated by continuous (heating for 108 or 180ks) and cyclic oxidation (heating and cooling were repeated) tests. Prior to the oxidation tests, the structures of matrices were controlled by heat treatment and/or isothermal forging. Since the delamination between oxide film and matrix developed on cooling, the oxidation rates of Ti-34Al and Ti-37Al under the cyclic oxidation were faster than those of the continuous oxidation. Under the cyclic oxidation, the oxidation resistance of the Ti-37Al was superior to that of Ti-34Al. The grain size of single phase Intermetallic compounds (Ti-37Al) becomes smaller by forging, but it resulted in the degradation of oxidation resistance. In the case of Ti-37Al, Ti3Al phase was produced at the interface of matrix and oxide scales. The outer layer of oxide scale on matrices consisted of TiO2, but the inner layer the mixtures of TiO2 and Al2O3.
The electron microscope is classified by its principle and structure into two types: the transmission electron microscope (TEM) and scanning electron microscope (SEM). The principle of the TEM is to obtain a transmitted electron image or an electron diffraction pattern from a specimen, by magnifying an image or a pattern formed by electrons transmitted through the specimen. The principle of the SEM is to obtain a secondary electron image generated from the specimen surface by 2-dimensional electron beam scanning. The TEM and the SEM are capable of not only magnifying an image of fine structure but also making elemental analysis of small areas on the specimen. Therefore, both instruments are widely used as indispensable analytical tools in the field of material research such as metals, semiconductors, inorganic materials and polymers. In this article, the methods of image observation and elemental analysis, and application data obtained with both TEM and SEM are introduced.
Infrastructures such as bridges, highways, pipelines and harbor piers sometimes suffer from corrosion damage during their service life. To diagnose deterioration of infrastructures and secure effective maintenance, in-situ corrosion monitoring methods based on electrochemical impedance techniques have been developed. This paper reports four examples of new corrosion monitoring sensors: 1) a sensor for measuring corrosion rate of marine steel structures in sea water, 2) a sensor for measuring corrosion rate and detecting corrosion spots of reinforcing bars in concrete, 3) a sensor for monitoring the protectivity of painted steel structures in atmosphere, and 4) a sensor for monitoring corrosion protectivity of the rust forms of weathering steel structures. The effectiveness of these sensors has been verified by experiments both in the laboratory and in the field.