Nb-19at%Al alloy was heated in oxygen up to 1000°C at a rate of 2°C/min. Variation during the heating was observed by a thermomicroscope and by a thermobalance, respectively. Process of the oxidation was discussed based on the data obtained by SEM, X-ray diffractometry and EPMA techniques. Obtained pieces of information were: 1) While weight change was observed only above 800°C, surface appeared to be obviously changing already at 350°C under the thermomicroscope. 2) A lamellar structure, being discerned faintly already at room temperature, was increasingly better defined with increase in temperature and was preferentially oxidized above 800°C. 3) Scale over the lamellar structure was thick and composed of a unique textile fabric pattern across its cross-section. 4) The lamellar structure was considered to appear because that part of the alloy in as-cast condition comprised a cell structure of A15-phase enmeshed in a network of A2-phase, or because that part of the alloy transformed to such a cell structure by dissolution of oxygen into the alloy, followed by internal oxidation of aluminium which resulted in supersaturation of niobium and, finally, precipitation of a network of A2-phase from A15-phase. The unique structure of the scale was considered to be caused by oxidation of a cell-structure containing internally oxidized alumina, together with gradual expansion of the scale due to a series of stepwise oxidation of various niobium oxides. 5) Scale was composed of alumina and niobium oxides.
In relation to the crevice corrosion mode of type 304 stainless steel in high temperature chloride solutions, which changed from general dissolution to pitting at around 423K, breakdown behavior of passive film has been examined in deaerated acidic chloride solutions up to 423K. Only general dissolution was observed at 373K in solutions of any chloride concentration with pH below 1.8. At 398K, pitting was found in the 2mol dm-3 Cl- solution at pH 2.0, in addition to the usual depassivation in the solution of lower chloride concentrations at pH 1.8. At 423K, pitting became dominant mode of breakdown of passivity; the higher the chloride concentration, the higher the pH below which pits start. Since the corrosion potential of stainless steel was shown to follow the hydrogen electrode potential, the critical condition for pitting was derived by equating the hydrogen electrode potential to critical pitting potential determined separately in neutral solutions. Good coincidence of derived function with experimental data insured that pitting occurred with hydrogen evolution as a catholic reaction. The critical pH for pitting is called pitting pH (pHpit). Thus, pHpit is shown to become higher than depassivation pH (pHD) at 423K in relatively higher chloride solutions, which well explains the change in crevice corrosion mode at 423K.
Humid-environment corrosion of nickel and water vapor adsorption on its corrosion products were investigated at temperatures from 40° to 80°C, relative humilities from 50 to 95% and SO2 concentrations from 0 to 20ppm. Quartz crystal microbalance (QCM) was used to determine the corrosion rate, and X-ray photoelectron spectroscopy (XPS) was used to determine the composition of corrosion products. Corrosion kinetics for nickel without SO2 pollutant was described by cubic rate law. On the other hand, corrosion kinetics containing SO2 more than 1ppm was linear. Dependence on relative humidity of corrosion rate was same as adsorption isotherm by Frenkel-Halsay-Hill theory on its corrosion product. Dependence on ralative humidity of corrosion rate was discussed in connection with adsorption behavior of water.
Development of sensor by which corrosion behavior of long duration can be measure with high degree of sensitivity is useful to evaluate corrosion reliability of device and to study corrosion mechanism of material. Vibration type corrosion sensor is based on a principle that change of resonant frequency of elastic plate corresponds to change of thickness of it. This sensor can be used for corrosion monitoring in both air and liquid environment. By using vibration type corrosion sensor, copper dissolution process in dilute HNO3 aqueous solution at room temperature was measured with a resolution of -0.48μm/Hz.
Present status of corrosion research on the back end of nuclear fuel cycle is described. Spent nuclear fuel reprocessing technology requires anti-corrosive materials under highly oxidizing nitric acid. It is important to make interaction between metals and reprocessing solutions clear. On the other hand, materials must be kept their life time for thousands of years in the field of high-level waste disposal technology. Corrosion research energies are put into elucidation of phenomena under geological environment.