The effect of surface polishing methods and heating atmosphere on the structure of oxide films formed on 316L stainless steel has been investigated. On electropolished surfaces, the oxide films which mainly consist of chromium will form at temperatures above 650°C in vacuum environment of 1Pa. In contrast, the threshold temperature of the formation of such oxide films on mechanically polished surfaces was as low as 450°C. This is attributed to the change of morphology in the surface region caused by polishing. After mechanically polishing, the grain size in the surface region became smaller. This leads to a high diffusion rate of chromium to the surface due to the grain boundary diffusion. As a result, chromium-rich oxide film started to form on mechanically polished surfaces at a relatively low temperature compared to the electropolished surfaces. The thickness of oxide film is increasing with abrasive grit size. The corrosion resistance of the oxide films to chlorine gas has also been examined. The high chromium-content oxide films which were dense and thick showed superior corrosion resistance to chlorine gas. The chromium-rich oxide films will not corrode in chlorine gas at 250°C if the films are thicker than 300Å.
The pitting potential of modified aluminum alloys have been evaluated by potentiodynamic and potentiostatic techniques. It has been observed that pitting potential obtained by potentiodynamic technique does not allow accurate predictions to be made and the results do not agree with the more reliable long term studies. On the other hand the values of Ep obtained by potentiostatic induction time technique provide a fairly accurate measure of pitting resistance of modified aluminum alloys. The results are also in agreement with the long term exposure studies.
In order to study a mechanism of hydrogen induced blistering cracking (HIBC), a new broaband (<10MHz) acoustic emission (AE) monitoring and signal processing for AE source inversion were utilized. The monitoring system developed was applied to measure the outplane displacement of elastic waves produced by a fast dipole phenomenon and to compare with the displacement numerically computed for a point of dilatation and cracking. The AE signals, indicating the point of dilatation with an extremely short rise time of source signal were reproducibly measured during the hydrogen charge to the steel plate with segregation band and low carbon steel rod. The AE source location estimated by the multichannel monitoring system agreed fairly well with the that of characteristic HIBC occurrence in several experiments. Possible mechanisms for hydrogen gas explosion due to a hydride or hydrogen cluster assisted by internal potential fluctuations were discussed.
Kinetics of pit growth under the water droplet containing 0.87 to 5.79mol·kg-1 MgCl2 were studied to elucidate the atmospheric corrosion behavior of stainless steels. Pit growth rate in this test was much higher than that obtained in an actual environment. It can be assumed that pit generation and/or repassivation process was predominant factor affecting the atmospheric corrosion resistance of stainless steels. Pit volume decreased with chloride ion concentration, with chromium, nickel, and molybdenum content, and it was independent of the size of droplet. This indicates that anodic dissolution was rate-limiting, while oxygen reduction chiefly affects initiation of pits. Current density in a pit was calculated based on the variation with time of pit volume and area of the opening. Metal dissolution rate in a pit dropped considerably with time, and the current density after 400ks was around 100μA·cm-2. This was much lower than typical value required to sustain pitting (1mA·cm-2) in aqueous solution such as 0.5mol·dm-3 NaCl. Consequently, the pitting in highly concentrated solution like airborne chloride should differ from that in dilute solution.
One is a material recycling technology for thermosetting plastics composites such as printed wired boards (PWBs) and molding resins for IC packages, which are the main industrial plastics wastes for electronic components and used to be difficult to recycle. The other is a novel technology for the treatment of fluorine waste water in order to reduce sludge volume. This technology utilizes the dependence of the solubility and fluorine adsorption characteristics of Al(OH)3 gel on pH, and Al(OH)3 is used repeatedly as fluorine adsorber.