Raman spectroscopy has been applied to the studies of the oxidation of stainless steel and similar alloys and has showed that the sensitivity and specificity of the technique are sufficient to detect the formation of iron, chromium and mixed spinel oxides in surface layers that are too thin for analysis by X-ray diffractometry. This information also has been obtained both during and after exposure to high temperature environments. Therefore the oxide formation is simultaneously monitored using Raman spectroscopy technique. In this report, the characterization and in-situ measurements of oxide layers by Raman spectroscopy are introduced. In addition, it is demonstrated that Raman spectroscopy and photo-luminescence spectroscopy offer methods for determining the stress distribution and semi-quantitative stress values in thin oxide layers.
The method of infrared reflection absorption spectroscopy (IR-RAS) for identification of corrosion products and for determination of their formation rates, which were initially formed on metal in gaseous corrosion environments, was explained by using copper, as an example, in humid air containing sulfur dioxide. Overlapping bands of a series of time-resolved IR spectra were deconvoluted on the basis of the two-dimensional correlation analysis (2D-IR), and the corrosion products were identified. By simultaneous measurements using IR-RAS and QCM, calibration curves for each component of the corrosion products and physically adsorbed water were made, and the quantitative determination of the corrosion products was achieved from only IR-RAS measurements.
This review gives an overview of the principles and experimental methods of UV-visible reflectance spectroscopy including potential-modulated reflectance spectroscopy (PMRS) and real-time spectroscopic ellipsometry (RTSE). The capabilities of in-situ, non-invasive, surface-sensitive analysis are demonstrated for the composition determination of passive films using PMRS and for the real-time monitoring of thin film growth and dissolution processes using RTSE.
Modification of passive film by photo irradiation is described as a photo quantum process in the semiconductor thin film. Ultraviolet light irradiation accelerates Cr enrichment in the passive film of Fe-Cr and Type 304 stainless steel, resulting in improved corrosion resistance. The process observed in neutral and acid solution is described in terms of structure change in the passive film induced by photo excitation. Some quantum effects caused by photo excitation are discussed, and possible mechanisms for such photo protection are proposed.
In view of scarcity of data as to whether or not acid rain that falls in unpolluted areas accelerates atmospheric corrosion of carbon steel, a series of atmospheric exposure test was conducted in the unaffected atmosphere of Maebashi, where specimens were tested with and without periodic exposures to artificial acid or neutral rain. Exposures to acid (pH 3.5) and neutral 5% NaCl solutions 3 times a week as artificial rains accelerated corrosion by 25 times regardless of the pH of the rains. Twice a week exposures to artificial sea water diluted to 1/6 and 1/30 and acidified to pH 2.5 resulted corrosion about 3 times larger than similar exposure to corresponding neutral artificial rains, demonstrating accelerating effect of acid rain. The cause of this acceleration was attributed to the detrimental effect of acid rain on the protective nature of the rust films to atmospheric corrosion. This is caused either by the destructive action of acid rain during its contact or the formation of anion nests by the constituents of acid rain during subsequent atmospheric exposure, but the mechanism has not been specified by this study. From the present data and scanty literature, it is deduced that atmospheric corrosion of carbon steel is accelerated several times when the sea salt deposition is less than moderate and the adverse effect of acid rain is considerable.
The critical chloride concentration above which corrosion of carbon and stainless steel (type 304) reinforcements in concrete occurs has been examined as a function of pH that reflects neutralization of concrete. A saturated Ca(OH)2 solution, pH 12.4-12.5, was used to simulate the fully alkaline concrete and mixed solutions of Ca(OH)2 and CaCO3 saturated solutions in varying ratios to simulate neutralized concrete of lower pH values. The test solution for stainless steel contained active carbon powder and was thoroughly aerated by bubbling through air. The chloride concentration was changed by adding NaCl to the solutions. To simulate crevice between reinforcement and concrete, semi-spherical deposits of an epoxy resin adhesive were provided to the carbon steel specimen, and a multiple crevice washer to the stainless steel specimen. The critical NaCl concentrations below which corrosion of carbon steel was slight in solutions of pH 12 and 11.5 were 0.20, 0.10 and 0.05g/L, respectively. Below pH 11.0 corrosion occurred generally regardless of the chloride concentration. Type 304 showed no corrosion with 150g/L NaCl at pH 12.5 but the critical concentration was in the range of 100-150g/L NaCl at pH 11.5. The critical chloride concentration decreases for both carbon and stainless steels as the pH falls, but that for stainless steel is much higher than the reported maximum chloride concentration near the surface of concrete structures exposed to a high deposition of chlorides from the ocean, indicating a high corrosion resistance of the steel in the hostile environment.
Cobalt based overlaying materials have been widely used as valve seat materials for high temperature and high pressure services, because of the high corrosion and wearing resistances. In nuclear power plants, however, the cobalt-based alloys would be the potential source of 60Co in the reactor water, thus alternate materials (Co-free materials) have been required and studied, especially for use as the large size valve seats on the feed water and condensate lines. From screening test and actual model test, Ni based alloys were selected as Co-free seat materials. The performance test of large valves (nominal size 300 A and 600 A) with Ni based alloys (A-1 and A-3) seats were done under the condition of high temperature and high pressure 100 for the 300 A valve and 50 cycles for the 600 A valve and the results were confirmed to be good.