For successful application of low-voltage, ultra-high resolution FE-SEM in corrosion studies, the use of test specimens with clean, smooth and deformation-free surfaces is of critical importance. Otherwise, subtle surface evolutions associated with early stages of corrosion, proceeding both generally over the matrix surface and locally at and around fine inclusions of various sizes and compositions, will never be disclosed clearly. Here, a new and novel approach for the preparation of sample surfaces of required quality is presented. A key feature is the use of radio-frequency powered Glow Discharge (rf-GD) sputtering for final follow-up treatment of mechanically polished sample surfaces. This utilizes its unique sputtering characteristics where both conductive and non-conductive surfaces are sputtered very stably with Ar+ ions of very low energies, less than 50 eV, and very high current density of ∼100 mA cm−2 ; the very low energies of Ar+ ions ensure that sputtering proceeds without significant formation of altered surface layers, while high current density allows sputtering to proceed at very high rates, typically 1∼10 μm s−1, making sample preparation time extremely short, normally less than 10 s.
The galvanic behavior of an illuminated rutile-type TiO2 and iron couple and the corrosion mass loss of the iron under the formation of this galvanic cell were investigated as a function of the hydrogen ion concentration in the aqueous solution. The rutile-type TiO2 with the form of film was formed by a high-temperature oxidation and then was reduced by a high-temperature hydrogen gas. The rest potential of the TiO2 fell due to the light irradiation. This drop increased with an increase in temperature of hydrogen reduction. The drop of the rest potential with light irradiation for the TiO2 reduced at 1173 K increased with an increase in solution pH in the aqueous solutions of pH higher than 10. In these solutions, as a result, the rest potential of the TiO2 was lower than that of the iron. When the illuminated TiO2 was in contact with the iron in the aqueous solutions of 10∼12 pH, the cathodic current passed for the iron. This current increased with an increase in the solution pH. It was found that this reason was due to that the photo-anodic current for the TiO2 increased with an increase in the solution pH. The corrosion mass loss of the iron under the formation of galvanic cell with the illuminated TiO2 was lower than that of the iron under the formation of galvanic cell with no illuminated TiO2 and that of the no galvanized iron. Further, the corrosion mass loss of the iron under the formation of galvanic cell with the illuminated TiO2 decreased with an increase in the ratio of surface area of the TiO2 to the iron.
Corrosion acceleration mechanisms of carbon steel pipes exposed in boiler water circumstances of power generation and chemical plants are not necessarily clarified, because of complicated degradations with chemical and mechanical actions caused under flow conditions. In order to clear basic corrosion phenomena of carbon steel in the high temperature water, corrosion batch tests using a small container for 24 hours and longer test duration were conducted up to a temperature of 463 K under stagnant and flow (agitated) conditions. Carbon steel specimens were weighed before and after a corrosion test, and after an electric removal treatment of iron oxide formed on test surfaces. The iron mass in oxide film and the dissolved iron mass in the solution were separated from the total corrosion loss of carbon steel. As results, it is found that the dissolution of iron from a carbon steel surface was predominant accompanying with the maximum mass loss of the specimens at temperatures of 373 to 393 K under stagnant and flow conditions. The total corrosion loss was increasing with testing time in spite of a formation of magnetite films on carbon steel surfaces. The flow condition in the case of this study was found to accelerate the dissolution of iron. An initial corrosion mechanism of carbon steel in the high temperature water was proposed according to the test results.