It is very important from a practical viewpoint to study hydrogen behavior in metals because hydrogen embrittlement has been related with concentration of hydrogen at the crack initiation sites. In order to reveal the mechanism of the embrittlement, experimental methods to characterize hydrogen concentration to localized area should be established. Hydrogen microprint technique (HMT) is a simple and powerful method to visualize hydrogen behavior in high resolution. When polished surface of a specimen is covered with nuclear emulsion containing numerous submicron particles of silver bromide, hydrogen in the specimen reduce the silver bromide. Thereby silver particles left on the surface after fixing represent the sites of hydrogen emission. In this article, details in experimental procedure of HMT are described with emphasis on functions of nuclear emulsion and fixing solution, and the results on hydrogen behavior in metals using HMT are reviewed. The effects of plastic deformation and stress gradient on hydrogen transport in metals are also presented in relation to the mechanisms of hydrogen embrittlement that have been proposed so far.
Secondary ion mass spectrometry (SIMS) enables us to visualize hydrogen trapping sites in steels. Information about the hydrogen trapping sites in high-strength steels by SIMS is very important to discuss environmental embrittlement mechanism for developing steels with a high resistance to the environmental embrittlement Secondary ion image analysis by SIMS has made possible to visualize the hydrogen and deuterium trapping sites in the steels. Hydrogen in tempered martensite steels containing Ca tends to accumulate on inclusions, at grain boundaries, and in segregation bands. Visualization of hydrogen desorption process by secondary ion image analysis confirms that the bonding between the inclusions and the hydrogen is strong. Cold-drawn pearlite steels trap hydrogen along cold-drawing direction. Pearlite phase absorbs the hydrogen more than ferrite phase does. This article introduces the principle of SIMS, its feature, analysis method, and results of hydrogen visualization in steels.
Recently, an imaging plate has been applied to examine the hydrogen distribution in metals, and the tritium radioluminography has been developed. In this review, the principle and the experimental procedure of this newly developed method is described. Some results such as observation of hydrogen distribution, determination of local hydrogen concentration or the measurement of the change of local hydrogen concentration are also presented.
The lattice locations of hydrogen dissolved in V, Nb, Ta and Nb-Mo alloys are investigated by the channelling method utilizing a nuclear reaction 1H(11B, α) αα with a 11B beam. The H atoms are located at tetrahedral (T) sites in V, Nb and Ta. In V, under the  compressive stress of 7 kg/mm2, the H atoms are displaced from T sites by about 0.44Å towards octahedral (O) sites, and after the release of the stress they return to the T sites. In Nb-3 at%Mo alloys the H atoms are located at the sites displaced from the T sites by about 0.6Å towards the nearest neighbour lattice points at room temperature and become located at the T sites at 373K, indicating the trapping of H by Mo atoms at room temperature. With increasing Mo concentration, the fraction of the H atoms located at the trapped sites decreases, and most of them are located at the T sites and some portion of them are at the O sites.
Carbon steel has been utilized well as a structural material in a commercial adsorption refrigerating system with help of corrosion inhibitors : most typically, hydroxide and molybdate ions. In this study, X-ray photoeletron spectroscopy (XPS) analysis has been carried out on the films formed on carbon steel exposed in the 17.3 mol kg-1 LiBr+0.1 mol kg-1 LiOH+0.01 mol kg-1 Na2MoO4 solution at 393 and 433K. Carbon steel passivates well in the solution at 433K, forming a doubly layered film. The inner layer is mainly composed of Fe3O4 and the outer one of MoOx, where the value of x decreases from 3 to 2 with immersion time. The film thickness increases in proportion to the cubic root of immersion time suggesting that movement of ions through inner oxide layer is governing the film formation process. Carbon steel exposed in the solution at 393K suffers from pitting occasionally. The film formed on a specimen without pits shows incorporation of Mo species within it whereas that with pits shows no incorporation. Molybdate ion would inhibit corrosion through 1) competitive adsorption with Br-, 2) acceleration in formation of the protective inner layer and 3) restriction for anions to permeate trough the outer layer. It can be concluded that higher temperature is advantageous for molybdate ion to work better because the chemical process for 2) proceeds more efficiently.
The present study is concerned with the construction of aqueous chemical potential diagrams of multiple metallic elements systems and the interpretation of the equilibrium diagrams to the effect of alloying elements on the stabilization of rust of weathering steels. Among chemical potential diagrams, our attention is focused on E-pH diagrams of bimetallic systems. In the corrosion process of weathering steels it is recognized that stabilization of rust proceeds accompanying the coprecipitation or sorption of minor elements to ferric hydroxide. Since the data are scarce for stability of aqueous reaction the tendency for the coprecipitation and sorption were estimated on the analogy of complex oxide formation based on the high temperature thermodynamical data. New E-pH diagrams of bimetallic systems led to a conclusion that elements beneficial for weathering steels are those forming insoluble complex oxide with iron at relatively low pH. A clear correlation exists between tendencies of complex oxide formation and of coprecipitation and sorption with rust.
It has been known that an ion-beam-assisted deposition (IBAD) process can produce thin films with different physical properties by changing conditions of assist-ion-beam. The aim of this study is to produce Fe thin films by the IBAD and an ion-beam-sputter deposition (IBSD) process which uses no assist-ion-beam and then to examine relationship between the microstructure and the corrosion resistance of the films. Five types of the films were prepared using the IBSD and the IBAD with various conditions of assist-ion-beam. The corrosion resistance of the films prepared was examined in 0.1 kmol·m-3 Na2SO4. The micro-structure of the films was observed by transmission electron microscope (TEM) and the surface topography of them was measured by atomic force microscope (AFM). All the films were consisted of α-Fe microcrystals. The grain size of the IBAD films was half of that of the IBSD films. The corrosion tests in 0.1 kmol·m-3 Na2S04 exhibited that corrosion pits on the IBAD films initiated immediately but those on the IBSD films occurred at about 5 ks after immersion. The number of the pits on both the films increased for a short time and became constant a fter a given time. The increase rate of pit radius on the IBAD films was larger than that on the IBSD films. According to the observation by AFM, the surface of the IBAD films was roughened by the irradiation of assist-ion-beam. The roughness of the films with low corrosion resistance was larger than that with high corrosion resistance. The corrosion resistance of the IBAD films was influenced by Ar flow rate in the hollow cathode compared with assist-beam-voltage.
A mathematical model to explain quantitatively the existence of the threshold stress intensity factor (KISCC) and the growth rate(da/dt) of stress corrosion cracking (SCC) of 18-8 austenitic stainless steel in a chloride environment at room temperature was investigated using electric circuit theory applied to the closed corrosion path with a metal-electrode (1 anode-2 cathode) -solution system of SCC crack geometry. Several conceptual assumptions and simplifications of the equations were attempted to derive the circuit current and the concentration of the chemical components of solution in SCC crack. It is predicted from the derived equation that the accumulation of solution in SCC crack may arise in the condition of f-CCl-×K×g2>0 holds where f and g2 are parameters due to the circuit resistances etc, CCl- is the free chloride ion concentration inside the crack, and K is the equilibrium constant or stability constant of the crack solution. A relation between da/dt and applied external force was simulated using an anode dissolution parameter relating to stress intensity factor Kl. The SCC growth rate of about 10-8 m/s obtained is reasonable compared with the data in the literatures. A parameter ε(KISCC) corresponding to KISCC was derived and it may predict quanitatively the concentration dependence of KISCC regarding metallic ions and hydrogen ions.
An atomically flat surface is very difficult to make on a less novel bulk material, for the surface of the material is ordinarily covered with rough oxide film in air. In the preparation of a test specimen we deposited 304 stainless steel on the (111) surface of silicon substrate by ion-assisted sputtering. Oxide film, which was formed on the deposited 304 SS after sputtering process were removed in a 0.1 M sulfuric acid solution by cathode reduction method, and then the surface potential was quickly shifted to passive state and was held on at the state for 15 minutes in the solution. After picked up from the solution and dried up, the treated surface of the specimen placed in air was observed with scanning tunneling microscope. The dimension of particle-like structure increased from 30 nm to 130 nm in diameter in several hours. The atoms on the structure were arranged at the steps of which step-terrace structure was constructed in the duration. The observed atoms were determined as oxygen according to an angle-resolved X-ray photoelectron spectrometry (XPS) analysis. The atomically flat surface of the step-terrace structure lasted for about 100 hours in air. Such a flat surface is expected to be of practical use for the suppression of corrosion occurrence.