Corrosion and dissolution behaviors of Al2O3 ceramics prepared by the pressureless sintering method in demineralized water at 275°C were studied using the small once-through type experimental loop. Experiments were carried out for two specimens with different Al2O3 contents of 92% and 99.5%, respectively. The weight loss of exposed specimen with 92% Al2O3 content was large and reached to 3.48mg/cm2 for 100h exposure. The surface of specimen was corroded deeply, of which corrosion depth was 150μm and changed to porous one. The release levels of the minor constituents of Al2O3 ceramics, Si, Ca, Na and K, were relatively high compared with that of Al, and the level of Al was about one forth of Si. On the other hand, the weight loss of exposed specimen with 99.5% Al2O3 content was very small and was 0.145mg/cm2 including particles released from the corroded layer by ultrasonic cleaning. In addition, γ-AlOOH crystals were formed on corroded surface. These results suggested that the corrosion resistance of Al2O3 ceramics in high temperature water was dependent strongly on Al2O3 content, and then that the corrosion resistance of AO2O3 grain was very high, and on the hand, grain boundary itself was eluted selectively into the solution.
To apply catholic protection technology for steel structures in an arctic sea, performance characteristics of four kinds of aluminum galvanic anodes in the artificial sea water at 273K were investigated by galvanostatic test. It was clarified that in these anodes, Al-Zn-Hg-Si alloy had an excellent anode performance, followed by Al-Zn-In alloy, and on the other hand, anode performance of both Al-Zn-Mg-In-Ca and Al-Zn-Sn-B alloy was strikingly deteriorated due to potential noblement at 273K. Hg containing anodes have an excellent anode performance, but it is difficult to apply these anodes for real fields because of a poisonous effect of Hg to the living things. In order to improve the poor anode performance at 273K of Al-Zn-Mg-In-Ca alloy, which has an excellent anode performance without Hg content at ambient temperatures, galvanostatic tests were carried out to evaluate the effect of the increase of In and Mg content on anode performance. These alloys with increase of In and Mg demonstrated excellent anode performance at a low temperature due to inhibited potential noblement in the low temperature region.
The influence of cyclic frequency and sea water temperature on surface fatigue crack growth retardation of mild steel was investigated by varying the testing frequency from 0.1Hz to 15Hz and the temperature from 278K to 308K. A particular frequency was found at which the environmental acceleration factor, (dl/dN)CF/(dl/dN)air, showed maximum. The particular frequency increased with increasing the temperature, while the maximum acceleration factor decreased with increasing the temperature. The particular frequencies at the temperature of 288, 298, and 308K were found to be 0.7, 1.0 and 10Hz, and the maximum values of the acceleration factor were 2.2, 2.0 and 1.6 respectively at a ΔK of 5MN/m3/2. The acceleration factor decreased with increasing the testing frequencies above the particular frequency and decreased with decreasing the testing frequencies below it. When testing at lower frequencies, there was a critical frequency at which or below which the rate of crack growth in sea water slowed down and finally stopped. It increased with increasing the temperature. The critical frequencies at the temperature of 298, 303 and 308K were found to be 0.15, 0.3 and 0.6Hz respectively. These phenomena can be explained by crack tip blunting by dissolution in sea water.
Electrochemical potentiokinetic reactivation (EPR) tests were carried out for the Fe-28Cr-5Ni ferritic stainless steel aged at 450°C for up to 5000h. The unaged steel showed a reactivation peak and this is attributed to the Cr-depleted zone in the vicinity of carbides. The ageing at 450°C promotes the diffusion of Cr into the Cr-depleted zone and the Cr-depletion recovers. However the ageing produces new reactivation peaks and severe pitting attacks in the grains were observed for the steel aged for longer than 1000h. This reactivation is considered as a result of spinodal decomposition of the steel into Cr-rich α′ and Cr-depleted Fe-rich α phases, which has been confirmed by atom-probe and Mössbauer effect. The reactivation peaks in the aged steels may be identified with Fe-rich α-phase with Cr concentration below 12 at%, and 14-20 at%.
Al-Si alloys have been used as a brazing material for constructing the car radiator, and are however corroded greatly in aqueous solution containing chloride anion even at a low concentration. A new surface treatment against the pitting corrosion of the alloys has been developed. The treatment consists of two steps, where a specimen of Al-Si alloy is immersed in neutral aqueous solution of Na4SiO4 at 2.5gdm-3 concentration at 95°C for 30min and then immersed in neutral aqueous mixture of 0.05 moldm-3 MgSO4 and 0.005 moldm-3 NaHCO3 at 95°C for 30min. The Al-Si alloy after the treatment is covered with a compact hydrated oxide film about 2μm thick and its resistance against pitting corrosion is much improved.
Magnetic bacteria synthesize magnetic particles which are aligned in chains and enveloped by a membrane. Magnetic particles isolated from magnetic bacteria could be useful because they are small in size, covered with lipid bilayers and do not aggregate. We have studied the mass culture of magnetic bacteria and their characterization and the application of bacterial magnetic particles. This review describes some of the recent advances towards bio-technological and medical applications of bacterial magnetic particles. Enzymes and antibodies may be immobilized on bacterial magnetic particles. These enzyme-magnetic particle and antibodymagnetic particle conjugates may then be used to produce biosensors for glucose and immunoglobulins, respectively. Bacterial magnetic particles have also been introduced into red blood cells by cell fusion, and into leucocytes by phagocytosis. Moreover, gene cloning system of magnetic bacteria was also developed.