A sodium chloride aqueous solution saturated with calcium hydroxide and its gel solution were used as simulated concretes for investigating electrochemical behavior of corroded steel in concrete. Results obtained are as follows: (1) From cathodic polarization measurements of platinum in the gel solution, the gel solution was considered to act as an uniform diffusion layer for dissolved oxygen. The effect of concrete cover is evaluated experimentally by changing the thickness of the gel solution and theoretically by applying the finite element method. (2) The polarization resistance of steel increased with decreasing cathodic potential until it reached its maximum at a specific potential, showing that the anodic resistance increases and the effect of cathodic protection can be estimated by this value. (3) The changes of pH and chloride-ion concentration on the surface of steel were investigated after the polarization current had been applied on the steel. In the gel solution pH decreased with anodic polarization and increased with cathodic polarization, but in the aqueous solution pH was independent of polarization. In both the aqueous and gel solutions chloride-ion concentration was independent of polarization under the experimental conditions investigated. This phenomenon was explained by considering mass balance of chemical species in the solution close to the surface of steel. Above results show that the gel solution is more adequate as an simulated concrete than the conventional simulated solutions.
The corrosion behavior and corrosion mechanism of air-preoxidized carbon steel have been investigated in concentrated LiBr solution at elevated temperature. The structure and composition of the surface films before and after corrosion were analyzed by XRD, AES, SEM and XPS. The air-preoxidized treatments at above 673K are effective in reducing corrosion weight loss about one order of magnitude in LiBr solution with Li2MoO4 inhibitor. Mass loss of the carbon steel air-preoxidized at 673K does not depend on the presence of Li2MoO4 in the LiBr solution, because the films have no defects. Mass loss of carbon steel air-preoxidized at 573K varies within a range in LiBr solution with Li2MoO4 which is due to defects of the preoxidized film. However, the maxium variation is not larger than that of no-preoxidized carbon steels. The oxide films both before and after corrosion are composed of Fe3O4 and α-Fe2O3. After corrosion, in the case of preoxidation at 573K, concentrated Mo layer which was observed at the oxide films/solution interface above 673K, is not observed in the oxide films. Li2MoO4 in the LiBr solution can repair the defects in the air-preoxidized film. The relationships between the corrosion behavior and surface film structures are considered using an interfacial ion-selective diffusion layer.
In order to improve the ability of ceramics to withstand cavitation attack, two schemes were tried. One was to strengthen the bulk of ceramics through preparing powder of very fine aluminum oxide particles with uniform diameter, which was casted tightly by applying centrifugal force on it before sintering. The alumina ceramics manufactured by this method had a fracture toughness twice that of current alumina ceramics, resulting in a cavitation erosion rate as low as one sixth of that of current alumina ceramics. The other was the dynamic ion mixing method where a very hard and adherent layer of titanium nitride was built on the surface of a base metal of austenitic stainless steel (SUS 304) by implanting nitrogen ion in it and depositing titanium on it simultaneously. The titanium nitride layer of 10μm thickness on the metal had a Vickers hardness of 3500, and prolonged the incubation period of the composite material by fifty times that of base metal. It was discussed that manufacturing cost, characteristics of the materials as well as the level of size precision should be taken into account when these newly developed materials are put in the material selection for hydraulic machine parts.
In order to prevent dew point corrosion by sulfuric acid, the roles of chromium and molybdenum in enhancing the corrosion resistance in 60 and 80% H2SO4 at 120°C were investigated. Particular attention was paid to the addition of active carbon to H2SO4. The chromium contents of alloys were 16 and 20% and the molybdenum contents were 17, 20, 24 and 25%. In carbon-free H2SO4 increasing molybdenum content gives rise to a decrease in corrosion rate and Ni-20Cr-24Mo alloy shows high corrosion resistance by spontaneous passivation forming a remarkably molybdenum-enriched passive film. In carbon-containing H2SO4 increasing chromium content is effective in decreasing the corrosion rate. Three Ni-20Cr-Mo alloys are spontaneously passive in 80% H2SO4 with carbon. Because the carbon addition increases the open circuit potential exceeding the stability limit of tetravalent molybdenum as a result of enhancement of the catholic activity, the spontaneously passivated film consists of remarkably concentrated chromium ions along with some molybdenum ions.
On the approach of corrosion protection of metal by TiO2 coating under illumination, the effects of iron oxides on the photoelectrochemical response of TiO2/carbon steel were studied. The iron oxides, magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3), were prepared on the carbon steel by using both the chemical conversion method (blackening treatment) and the heat treatments. From the examinations for the structures and the photoelectrochemical behavior of sol-gel derived TiO2 coating, it is found that the interfacial α-Fe2O3 demonstrates a significant photoelectrochemical response while the interfacial Fe3O4 and γ-Fe2O3 show poor performance. Although a vivid photoeffect for the TiO2/α-Fe2O3/steel system was observed, the Fe was still found to diffuse into TiO2 during the heat treatment of TiO2. Consequently, by changing the TiO2 coating procedure, particularly the heat treatment of TiO2 to suppress the diffusion of Fe ions, the photoeffect of TiO2/α-Fe2O3/steel could be improved. For TiO2/α-Fe2O3/steel under illumination, the stability of iron oxides was also studied. It was found that α-Fe2O3 was stable in aerated solution, while in a thoroughly deaerated solution α-Fe2O3 was reduced slightly by the photo-excited electrons.
With the aim of visualizing the fast cleavage fracture suggested by the AE source simulation of bulk waves, both the microscopic video image and Lamb wave monitoring were attempted for a loaded type 304 foil in a polythionic acid solution. A laser ultrasonic system revealed that the Mode-I fracture in the foil emitted the Lamb wave with the largest amplitude in the direction of crack opening vector. PZT transducers used for SCC test of the thin foil SUS 304 monitored only the zero-th order anti-symmetric Lamb mode (Ao-Lamb) whose amplitude and velocity are dispersive depending on the fracture kinetics. Polythionic intergranular SCC of a sensitized Type 304 foil emitted a number of high frequency Ao-Lamb waves. Direct observation by a CCD video demonstrates the fast crack opening at the source location of the Lamb waves, The developed system, applicable to a thin foil, possesses a potential capability for visualizing the microscopic fracture mechanism of SCC, when both the time and frequency resolution of the system are improved.
IS (Iodine-Sulfur) thermochemical hydrogen production process has corrosive environments, such as a boiling sulfuric acid and hydrogen iodine. An Fe-Si alloy is known as a corrosion resistant material in H2SO4 environment because of SiO2 film formation. Although the corrosion resistance increases with Si concentration, it becomes brittle. Hence, a compositionally graded Fe-Si alloy (CG Fe-Si alloy), 14wt% Si at free surface and 3wt% at substrate, was tentatively made by using CVD (Chemical Vapor Deposition) technique. The corrosion resistance of the CG Fe-Si alloy in the boiling 17.7kmol/m3 sulfuric acid was examined through weight loss, SEM microscopic observation and an electron probe X-ray microanalyzer. The CG Fe-Si alloy shows equivalent corrosion resistance to the Fe-Si alloy including more than 12wt% Si homogeneously. However, after 300 hours immersion the SiO2 film formed on the surface became likely to break away around surfaces where microcracks caused by cooling-down process during the CVD treatment were present.