Perovskite-type lanthanum chromium composite oxide, LaCr(III)O3, was prepared from La(III)-Cr(VI) mixed solution, and the pyrolytic process was investigated. It was found that the pyrolytic reaction of La(III)-Cr(VI) precursor was affected by partial pressure of oxygen, pO2, in the atmosphere. For pO2≥0.1atm, the reaction clearly proceeds in two steps via the formation of monazite-type LaCr(V)O4 as an intermediate. The structure and the stability of LaCrO4, the electronic configuration of CrO43- cluster, and the kinetics of the conversion of LaCrO4 to LaCrO3 in different atmospheres were studied. In nitrogen, the rate determing step was three-dimensional phase-boundary reaction, and in oxygen, it was three-dimensional diffusion of oxygen through the LaCrO3 layer. LaCrO3-base oxide films were formed on the several substrates successfully by ultrasonic spray pyrolysis from La(III)-Cr(VI)-base solutions. The oxide films improved oxidation resistance of SUS 304 stainless steels at 1000°C in 20% H2O-air.
Present authors have investigated the corrosion characteristics of plasma-spray ceramic coated stainless steel by various electrochemical corrosion testings methods. It was indicated that microcracks and micropores in ZrO2 top coated layer play important role as the path through which aqueous solution comes into inner layers. And, intense corrosion was recognized on the interface regions between ZrO2 top coat and NiCrAlY undercoated layer. In some cases, these corrosion brought about peeling off the top coated layer. Therefore in this paper, to improve corrosion resistance of plasma-sprayed ceramic coating in an aqueous solution environment, sealing treatment and microstructural grading were conducted employing NiCrAlY and ZrO2 system. Then, several investigations concerning corrosion characteristics of these plasma-sprayed ceramic coating system, were conducted from electrochemical view points. As a result, it was recognized that microstructurally graded coating shows little improvements in the corrosion properties. On the contrary, sealing treatment shows much improvements in corrosion resistance especially in the case when heat treatment was employed at 300°C for the period of 2 hours. Then, some evaluations of defect ratio in plasma-spray ceramic coatings were conducted.
Attempts have been made to clarify the cause of blowouts of score on aluminum alloy 5182 EOE in a carbonated soft-drink solution at 38±2°C. Polarization measurements and metallurgical observation indicated that the blowouts is caused by pitting of groove of the score. It is pointed out that the exposure of intermetallic compounds containing copper is detrimental to the pitting of 5182 in a carbonated soft-drink solution concentrated by vaporizing.
In order to assess the thermal stability of Al2O3 coatings of 0.2, 0.4 and 0.8μm thickness formed on SUS 304 stainless steel by ion plating, the specimens were annealed at 800-1000K for 1ks in a vacuum. The assessment was carried out by SEM observation and polarisation curves obtained in an aqueous solution of 0.5kmol m-3 H2SO4. The annealing at 800K does not cause mechanical defects in the coatings, while that at 1000K results in small cracks in the coatings and accordingly increases the current density nearly to the value for the uncoated specimen.
The dielectric and semiconductive properties of the passive film on titanium were investigated under potentiostatic polarization in neutral phosphate and borate solutions by using the impedance technique. Further topographic change of the passivated surface with potential was observed by AFM. From comparison between the film thickness and the capacitance of the space charge layer, the dielectric constant of the passive film was estimated at about 40 in the potential region lower than 3V and about 85 in the potential region higher than 3V. From the Mott-Schottky plot the donor density of the passive film was calculated, which is in the order of 1026m-3, and decreased with an increase of the film formation potential. The decrease may correspond to a change of film structure from amorphous to crystalline. At potential higher than 7.5V, breakdown of the passive film was observed, probably due to influence of very high electric field in the film. The breakdown brings about the increase of donor density, and a roughening surface due to micro-crystallization.
To evaluate the corrosion resistance of passive film on titanium in reductive enviroment, change in electronic property of the anodic oxide film was pursuited with passage of time by using an impedance technique in neutral borate solution. When the passive film was formed at the potential noble than 6V, the surface states was induced to the film which was eliminated at the potential less noble than 0.5V. If the electrode potential was set at potentials less noble than -0.5V, hydrogen penetration would occur to transform the film composition from TiO2 to TiOOH with reducing electric resistivity of the film. If the potential was held between 0.0V and -0.5V, the gradual reduction of space charge layer thickness was observed. Although TiO2 is considered to be stable from the view point of thermodynamics, the result suggests that the change such as hydration of the film may occur.