A brief overview was done on the fundamental characteristics of supercritical water and some chemical application examples. Supercritical water has both gas property and liquid one, and could mix with many gases and dissolve organic compounds. Hence it was excellent solvent for chemical destruction of organic wastes.
High temperature corrosion kinetics of Ni-Cr alloys in a 1%HCl-50% O2-N2 atmosphere was investigated by thermogravimetric technique. The mass change curves of the specimens showed a slight gain in the initial period and a subsequent linear loss. Since such corrosion behavior was due to the formation of NiCl2 and its vaporization besides the oxide formation, the equation of mass change curve included the terms of mass gain and evaporation curve. However, the conventional kinetics consisting of the terms of a parabolic mass gain curve and a linear evaporation curve did not completely describe the experimental results in the 1% HCl atmosphere. Therefore, in this study, the kinetics equation was introduced on the assumption that the evaporation curve was divided into two curves; one was the curve which represented the rate of chlorination of Ni and the other was the curve which represented the chlorination of NiO. The chlorination of NiO occurred owing to the solving of NiCl2 into the NiO scale. In addition, it was presumed that some incubation periods existed until the vaporization of NiCl2 became significant. The kinetics equation introduced on these assumptions well described the experimental mass change of Ni-Cr alloys very well in the 1% HCl atmosphere.
Pitting corrosion properties of a tin-coated copper tube in hot water were investigated in this paper. The long-term looping corrosion test had been conducted in hot water containing 1.5-3.0mg/L residual chlorine at 60°C for 745 days. The pitting corrosion life of a tin-coated copper tube could be estimated 2.5 times as long as that of a copper tube by applying the extreme-value statistical analysis to the measured maximum pit depth data. The excellent pitting corrosion resistance for a tin-coated copper tube was attributable to the fact that the corrosion potential, ECORR, values for the pitting corrosion growing period were approximately 30mV less noble than these of a copper tube. It is found that the difference of these ECORR values was caused by the reason of followings: (1) Whenever pitting occurred on the copper tube, the unattacked area on the inner surface was covered with corrosion products film consisting of cupric oxide and silicate precipitated into the film in order to ennoble ECORR up to the pitting potential. (2) The tin-coated layers prevented the formation of cupric oxide and the precipitation of silicate.
Recently, dissolved oxygen (DO) in water has been removed by deaerators equipped with polymeric hollow fiber membrane. By this membrane deaeration method, it is possible to examine effects of relatively low concentration of DO and of flow velocity on corrosion of carbon steel in tap water, systematically. In this study, concentrations of DO in Matsuyama-city water were adjusted to 0.5, 2.0, 4.0mg/dm3 by mixing deaerated water by the membrane deaeration system with non-deaerated water. Flow velocities were controlled to 0.5, 1.0, 1.5 and 2.0m/s by successively reducing the cross sectional area on a water line, and there was turbulent flow in all cases. The experimental system was designed so that it flowed smoothly, considering hydrodynamic conditions. In the case of DO=0.5mg/dm3, the corrosion rate of carbon steel increased with flow velocity. To the contrary, the corrosion rate in non-deaerated water decreased with velocity, although the corrosion rate was larger with all flow velocities. Within the range of velocity of this experiment, the lowest corrosion rate and the minimum depth of pitting were observed in the case of tap water of DO=0.5mg/dm3. This condition is acceptable to prevent corrosion of water piping. It is also recognized that the behaviour of the corrosion rate and/or pit depth is closely related to the total amount of oxygen supplies, i.e., multiplication of DO value by water velocity.
Nitridation behaviour of Ti-50Al, Ti-50Al-3Si, and Ti-47Al-3Si alloys was investigated at 1173K for up to 518.4ks in a N2 gas with 99.9995 purity (less than 0.5ppm O2) and the N2 gases purified with Ti or Ti+AlN getter, i.e., N2(Ti) and N2(Ti+AlN) gases. Structural sequences of products on a Ti-50Al alloy are nitrides (Ti2AlN+TiN)-TiAl2 in the N2(Ti+AlN) gas, nitrides (Ti2AlN+TiN+AlN)-TiAl3-TiAl2 in the N2(Ti) gas, and oxides (TiO2+ Al2O3)-nitrides (Ti2AlN+TiN) in an as-received N2 gas. In cases of TiAl alloys containing Si, oxides (TiO2+Al2O3)-nitrides(Ti2AlN+TiN+AlN)-TiAl3-TiAl2 were formed in an as-received N2 gas, and nitrides (Ti2AlN+TiN+AlN)-TiAl3-TiAl2 in both the N2(Ti) and N2(Ti+AlN) gases. Selective nitridation of Ti yields Al-enriched layers of TiAl2 and TiAl3 on the alloy surface and they could be explained from thermodynamical considerations. A periodic formation of Ti2AlN+TiN and AlN layers, looks like a Liesegang phenomenon, is due to a competitive process of a rapid N2 gas ingress and a so-called back-diffusion of Al toward the alloy substrate. A part of a Ti-Al-N ternary phase diagram at 1173K was given, showing compositional paths for these nitridation reactions. Nitridation rate was very low for a Ti-50Al alloy in the N2(Ti+AlN) gas, while it tended to increase with time in an as-received gas and the N2(Ti) gas. TiAl alloys containing Si showed large nitridation rates, accompanied by a catastrophy in nature in each atmosphere. It was suggested that Si addition and impurity oxygen in N2 gas gave deteriorative effects on nitridation property of Ti-Al alloys at high temperatures.
Recently, ERW (Electric Resistance Welded) steel pipes have been adopted in gas and oil fields, because these are economically more attractive. However, the corrosivity of ERW steel pipes in soils has not yet been examined. The objective of this paper is to ascertain the resistance to grooving corrosion at welds in four kinds of ERW steel pipes which are available on the Japanese market as of 1996. The burial testing of ERW in soils was performed, thereby the rates and form of corrosion were examined. The effects of metallurgical factors on resistance to grooving corrosion were also evaluated with regard to alloying elements and macro- and microstructure. The results indicated that all ERW steel pipes have resistance to grooving corrosion. There was no significant difference among the testing ERW steel pipes.