Laboratory corrosion tests were performed on bare and galvanized steel rods exposed to 14 soil conditions, involving sand, clay, humus, their mixtures, two-layer structures, and soils containing NaCl. The corrosion potentials of the rods were measured during the tests. After 1, 2, 4 and 6 years of exposure, weight loss was measured for corroded rods digged up from soils, and corrosion products were identified by using X-ray diffraction analysis (XRD). This paper reports the results of those corrosion measurements. The magnitude and the progress of corrosion of rods are discussed in relation to the structure and properties of the soils, the corrosion products together with the change of corrosion potential of the rods.
A new portable controlling-atmosphere spraying system was developed to deposit metallic coatings with an excellent performance against wet corrosion. Characteristics and corrosion performance of the coatings deposited by the arc or flame spraying of high Cr-Ni wire in the controlled argon gas were examined and compared with that of the coatings deposited by the conventional spraying in air. Among the coatings, the coating deposited by arc spraying in argon gas was found to be dense without oxides and pores, and possess the alloy composition same that of the spraying wire. Polarization measurement of the coatings in sodium chloride and hydrochloric acid revealed that the anti-corrosion resistance increased in the order of (1) flame spraying in air, (2) arc spraying in air (3) flame spraying in argon (4) arc spraying in argon. Corrosion resistance of the coating deposited by arc spraying of wire in argon gas was improved to a great extent. In hydrochloric acid solution, even the spraying wire suffers slight active dissolution. Therefore, the coating deposited by the arc spraying in argon gas suffers pitting corrosion associated with small pores and crevice corrosion along the paths in the coating. Corrodant slowly penetrates into the coating.
Effect of sulfate and phosphate on the pitting behavior of type 304 stainless steel has been studied in chloride solutions at 423 and 523K. The inhibitive work was evaluated in terms of concentration (or activity) of inhibiter required to inhibit pitting as a function of chloride concentration. The relation was expressed conventionally as log [Cl-]=αlog [inh]+β, where α and β, based on the molarity at 298K, were 0.64 and -1.07, respectively, at 423K and 0.67 and -0.95, respectively, at 523K. The values for phosphate were 1.03 and 0.01, respectively, at 423K. The relation for sulfate at 423K is restricted to 0.5M of sulfate, above which pitting is rather promoted possibly because of the increase of activity coefficient of chloride ion and of the deviation from the adsorption isotherm. Increase in temperature requires more concentration of inhibiter to inhibit pitting; β decreases with temperature while α is not very sensitive to it.
Aluminum iso-propoxide was used for alumina coating on Type 316 stainless steel. The optimum thickness of sol-coating was obtained by proper adjustment of the viscosity of the sol and by proper selection of the specimen removal speed from the sol. After gelation in air, baking the gel-coated specimen in vacuum provided the formation of adhesive alumina coating, while baking in air resulted in serious oxidation of the substrate steel in addition to exfoliation of flaky alumina. X-ray diffraction revealed that the alumina coating was mostly amorphous containing small amounts of microcrystalline γ-Al2O3 when baked up to 900°C in vacuum but baking temperature increase up to 1000°C resulted in transformation into α-Al2O3 in addition to growth of γ-Al2O3 crystal. The protective coating can be obtained by repetition of coating including baking at the highest temperature where the coating is mostly amorphous. For instance, when coating including baking at 900°C for 1h in vacuum was repeated three times, the alumina coating was protective against corrosion of the steel even after immersion for 2 months in 0.5M H2SO4. However, baking at 500 and 700°C was not effective in preventing corrosion of the steel. Growth of γ-Al2O3 and transformation into α-Al2O3 resulted in the formation of island-like coating and hence were detrimental for protective coating.
By employing an interference mode, an acoustic microscope has been developed, which can raise the depth resolution about fivefold from the level of conventional models. This enables the individual layers of multilayer structures such as semiconductor devices to be discriminated. Acoustic microscopes are used in various areas including industrial materials and biological studies. This paper deals with their typical applications for observing of microstructural anistropy in plastically deformed zones of materials subjected to tensile stresses, measuring residual thermal stresses generated in joined interfaces between ceramics and metals, measuring the thickness of machining damaged layer, observing rust and dynamically observing of corrosion.