A nitric acid solution containing Cr2O72-ion is used for a corrosion test which simulates the condition in a nuclear fuel reprocessing plant. As the effect of Cr content on the corrosion resistance of stainless steel is not yet fully understood in that solution, the effects of Cr content in an alloy and that of Cr2O72-ion in nitric acid on the corrosion rate were studied by using electro-chemical measurements, The increase of Cr content reduced the general corrosion rate in nitric acid solution without Cr2O72-ion, however it increased that in nitric acid solution containing Cr2O72-ion, It is considered that the reversal of the Cr content dependency is due to a rise of the corrosion potential caused by addition of Cr2O72-ion to the potential region where a high Cr alloy shows higher anodic dissolution rate than a low Cr content alloy.
The effects of heat treatment of A 3003 on the pitting corrosion preventive film prepared by the chemical conversion with aqueous solution containing MgSO4 and NaHCO3 were investigated. Resistance of the chemical conversion film to pitting corrosion was evaluated by polarization mesurements in 0.01M NaCl and the mesurements of the maximum depth of pits formed in aerated NaCl solution containing cupric ions. When A3003 was quenched from 600°C, the SEM photographs showed the intermetallic compounds such as (Fe, Mn) Al6 on the alloy surface was refined. As the result, porous film about 10μm thick deposited on the heat treated alloy surface by the chemical conversion treatment. It raised the pitting potential and depressed reduction of oxygen dissolved in solution, resulting in the improvement of pitting corrosion resistance.
Effects of ferrous ions on the growth and activity of sulfate-reducing bacteria (SRB) have been investigated using biological and analytical techniques for a better understanding of its metabolism in corrosive media with high contents of metal ions. In-situ gravimetry by the quartz crystal microbalance (QCM) technique was also applied to examine the sulfide-producing process in the medium. The growth of SRB increased with increasing ferrous ion concentration up to 0.01M and retarded above 0.01M. In the low concentration region (0.00036M-0.01M) ferrous ions enhanced the growth of SRB, whereas in the high concetration region (0.03M-0.7M) ferrous ions gave the opposite effect due to high osmotic pressure. The FeS producing ability was closely related to the growth of bacteria, therefore it seems that SRB accelerates the corrosion of metal in the medium including 0.01M ferrous ions. QCM technique was able to detect sensitively the amount of FeS attached on the electrode surface, which was proportional to the total amount of FeS in the medium. Thus it was found that QCM technique is powerful for monitoring the FeS producing process.
Field survey on the corrosion life of exhaust systems was performed to determine, the ranking between external and internal corrosion, and also the effect of the engine type and the catalytic converter type to internal corrosion. Concurrently, analysis of condensates and corrosion products were carried out to make clear the difference in corrosion environment which results from the difference in type of engine and type of catalytic converter. It was found that corrosion life is limited mainly by inner corrosion, and the corrosion of exhaust systems equipped with three-way catalytic converter was most aggressive relative to conventional types of engine and catalytic converter. In the corrosion environment of exhaust systems equipped with three-way catalytic converter, it was found that SO42-, SO32-, HCO32-, and HCHO exist usually, and very high concentration of Cl-exists in some cases, while organics such as CH3COO-and HCOO-do not exist. From the analysis of corrosion products, it was found that SO42-and Cl-are enriched to the corrosion interface.
Anodic metal dissolution produces hydrated metal salt concentrated at the anode interface and modifies the ion transport in the interfacial diffusion layer to be anion-selective or cation-selective. The anion-selective diffusion layer formed with monovalent chloride or hydroxide contributes to the formation of a chloride film, giving rise to either the chloride-film-induced passivation if the chloride is insoluble (e.g. Ag/AgCl) or the transition from the active state to the polishing state dissolution if it is soluble (e.g. Fe/FeCl2). The cation-selective diffusion layer formed with multivalent phosphate or sulfate gives rise to the formation of an oxide film, and thus to the oxide-film-induced passivation of metal anode (e.g. Ni/NiO). Furthermore, the transformation from a salt film to an oxide film on metal anodes occurs only if the salt film is cation-selective, and does not if it is anion-selective.
Plasma discharge sintering process is a novel consolidation method that combines a very short time at high temperature with pressure application in a plasma environment. The application of instantaneous pulsed electric power activates and purifies the particle surfaces. This effect contributes to enhanced particle sinterability. The powder particles are then rapidly resistance heated with pressure application. The effect of this combination of mechanical pressure, electrical field, and plasma activation in this process is full densification of difficult-to-sinter materials with minimum microstructural changes. In this paper, principle of plasma discharge sintering process and the latest investigations of development of novel materials, such as nano-structured or non-equilibrium materials, intermetallics and composites, are reviewed.