Inhibition of passive film breakdown on iron in a borate buffer solution containing Cl- by various inhibitors is reviewed on the basis of the hard and soft acids and bases principle. Since Fe3+ is equivalent to a hard acid according to the principle, inhibitors classified as hard bases suppress the pit nucleation by repairing defects in the passive film with deposits of their complexes with Fe3+. The pit growth is inhibited with inhibitors acting as soft bases by adsorption on the surface of substrate iron, a soft acid within the pit. Both pit nucleation and growth processes are suppressed by some mixtures of the hard and soft base inhibitors. Oxidizing inhibitors, some of which can change the substrate surface to an oxidized one, inhibit both processes by hard acid-hard base interaction.
The thermal shock resistance of ceramics is generally estimated by the critical quenching temperature difference that is defined as the lowest applied temperature difference above which a loss in flexure strength occurrs. However, the critical quenching temperature difference can not always be clarified owing to the dispersion of fracture strength. Therefore, the probabilitical estimation of the thermal shock resistance is essential. In this paper, the important factors which affect the thermal shock resistance are investigated by experiments using various kinds of ceramics, specimen diameters and quenching media. It is also mentioned that the probability of crack occurrence after thermal shock can be estimated using Weibull statistics theory. Furthermore, the probabilitical evaluation method of critical quenching temperature difference is proposed.
The resistance of nonchromate treated tinplate for powdered milk cans to yellow staining and smudging with the passage of time was studied. Anodic treatment in a sodium phosphate solution was found to be effective as a nonchromate treatment, and the resistance to smudging was found to be improved by adding catholic treatment as a pretreatment. The O/Sn ratio of the tinplate with anodic treatment determined by using AES was higher than that of tinplate with catholic treatment and the Fe/O ratio of the tinplate with anodic treatment scarcely increased with the passage of time. In addition, the refractive index and thickness of the tin oxide film with anodic treatment measured by ellipsometry scarcely changed after the passage of time, and the binding energy of Sn3d XPS spectra of the tin oxide film with anodic treatment also hardly changed.
In situ IR-RAS (Infrared Reflection Absorption Spectroscopy) is a powerful technique for investigation of the initiation of atmospheric corrosion, but the interpretation of the spectra is not always easy, because the band positions and shapes of the IR-RAS spectra possibly change from those of the transmission spectra due to an abnormal dispersion of the refractive indexes of surface layers. To estimate the band shifts, IR-RAS spectra of corrosion products and related compounds were theoretically calculated by using their optical constants which were derived from the transmission spectra and the Kramers-Kronig analysis. In this procedure a three phase model (air/surface layer/copper) and a four phase model (air/water layer/corrosion product layer/copper) were employed and the results were compared. The band of IR-RAS spectra shifted to higher wavenumber region by 10, 64, 20, and 18cm-1 for CuSO4⋅5H2O, CuSO4, Na2SO3, and Chevreul's salt (Cu2SO3⋅CuSO3⋅2H2O), respectively. A spectrum obtained on copper in air containing 10 ppm SO2 and 80% relative humidity of water was analyzed based on the calculated IR-RAS spectra.
Effect of residual chlorine (R-Cl) on the corrosion behaviors of type 304 and 316 stainless steel tube weldments in hot water was investigated in this paper. The spontaneous potential, ESP, measurements for the weldments and long-term loop tests were conducted in hot waters containing several levels of R-Cl at 80°C. R-Cl concentration had a strong effect on ESP values. The noblest ESP values, ESP, max, were no more than 100mV (Ag/AgCl) in DO (Dissolved Oxygen)-saturated hot water without R-Cl, however, ESP, max values were ennobled up to 310-440mV(Ag/AgCl) by addition of 1.0mg/LR-Cl. After an immersion in a long-term looping test, stainless steel tubes were covered with reddish-brown corrosion products film on the inner surface. The corrosion products film brought ESP values approx-imately 110-160mV (Ag/AgCl) nobler in hot water containing 1.0mg/LR-Cl. These facts indicate that not only R-Cl but also corrosion products film make ESP of welded stainless steel nobler in hot water.
Type 430 stainless steels with 0.09 and 0.9 mass% Mn were heated to 1273 K with heating rates of 5, 33, and 50 Ks-1, and then oxidized in 1.67×104 Pa O2-balanced N2 for up to 300s, in which the total pressure was 1.013×105 Pa and the flow rate of the gas was 200cm3min-1. The effect of the heating-up rate on the oxide thickness, morphology, and crystal structure has been studied. When the temperature reached to 1273 K with 50Ks-1, an iron rich corundum type oxide with a thickness of 40 nm was formed for the steels with 0.09 and 0.9 mass% Mn. Contrary, the oxides with 5 and 33Ks-1 were composed of corundum type one rich in chromium and the thicknesses were two times thicker than that with 50Ks-1. Spinel type oxide was also formed for the steel with 0.9 mass% Mn with 5 and 33Ks-1. As the oxidation proceeds at 1273 K, the thicknesses of the oxides, surface composition, and crystalline structures became similar independent of the heating rates. The surface morphology and the amount of the spinel type oxide were, however, affected by the heating rate at relatively longer oxidation time.
A scanning electrochemical microscope (SECM) was used to evaluate heterogeneity of passive films formed on iron in deaerated pH 8.4 borate solution. The probe current image and specimen current image for the specimen electrode, which consists of two iron plates embedded into epoxy resin, were measured with a tip-generation/substrate-collection (TG/SC) mode in deaerated pH 8.4 borate solution containing [Fe(CN)6]4- as a mediator. The distinctions between epoxy resin and each iron plate were more clear in the probe current image than in the specimen current image. Moreover, the difference in thickness of passive film between two iron plates was distinguishable in the probe current image. The lateral resolution of the probe current image was about 40μm which was two times as much as the diameter (20μm) of the probe electrode.
On the basis of the field research on AC corrosion by induced AC voltage on a cathodically protected pipeline in soil paralleling electric power transmission line, laboratory study was performed with a carbon steel specimen simulating a coating defect to understand the effect of superimposed AC current at 50 Hz of the power-line frequency on corrosion. Results have demonstrated that the occurrence of severe corrosion as high as 6.5mm/y was obtained under applied AC current density of 19 mA/cm2 without DC current imposition. On the other hand, corrosion rates decreased with an increase of DC current densities suggesting effectiveness of catholic protection to suppress AC corrosion. For aiming at the final goal to establish cathodic protection criteria with induced AC voltage, the quantitative relationship among corrosion rate, AC current density, and DC current density has been examined.