Papers by Author: Satoshi Sunada

Abstract: Sintered magnesium alloys, which were fabricated by Spark Plasma Sintering (SPS) method, were examined to study corrosion characteristics by electrochemical method, XRD and EPMA. The binary mixtures alloys of a low-melting-point metal powder (Sn, Bi, Sb) of1.0 vol.% and the pure magnesium powder were prepared. In the Mg-1.0vol.%Bi and Mg-1.0vol.%Sn, Mg₃Bi₂ and Mg₂Sn precipitates was recognized by XRD, respectively. In addition, formation of oxide along powder particle boundaries was observed by EPMA elemental mapping in all specimens. In the case of Mg-1.0vol.%Zn, precipitation of metallic compounds was not recognized by these experiments. According to the results of polarization curve measurements, the Mg-1.0vol.%Bi shows highest corrosion potential. However, corrosion rate which was estimated by Tafel method is relatively larger than other alloys due to Mg₃Bi₂ precipitation. This result suggests that Mg₃Bi₂ acts as cathode site. The Mg-1.0vol.%Sn shows superior corrosion rale in these alloys.

Abstract: The copper alloys, which were added Nickel show high corrosion resistance. Therefore, they are generally used in environments of poor quality fuel. However, it is expected that we reduce the amount of Ni in copper alloy, due to increase of Nickel consumption in recent years. We used five different types of sintered material samples that are different in the content of Nickel in the present study, to investigate the influence of Ni content to corrosion resistance quantitatively. These samples were used for the experiment of open circuit potentials, anodic and cathodic polarization curves measurements. To simulate environment in poor quality fuel we employed solution of 0.5 M Na₂S + 0.1 M NaOH (pH = 13.05). The experiment of open circuit potentials shows that samples that the content of Nickel is less than 42 mass% were formed passivation film on surface of sample during aeration. The experiment of polarization curves measurements show that the each sample was implied behavior of forming passivation film corresponding to potential-pH diagram of Cu in anodic region basically. In addition, the 36.4 mass% Nickel was remarkably improved corrosion resistance.

Abstract: Since Fe-Cu-C sintered steels are easily rusted, they are coated with rust preventive oils. High viscosity of those rust preventive oils decrease workability, and low viscosity deteriorates rust preventive performance. Therefore, it is necessary to develop new rust preventive oils with contradictory properties of low viscosity and superior rust prevention. However, precise methodology to evaluate rust prevention ability has not been established. In this study, we developed new technique to quantitatively evaluate rust prevention ability by measuring the open circuit potential through thin corrosive solution on Fe-Cu-C sintered steels coated with a rust preventive oils. As a result, the ability for rust prevention can be measured quantitatively, and it decreases slowly over time, with repeating destruction and restoration. Furthermore, it was found that the deteriorating processes of rust prevention ability for rust prevention oils are composed of three characteristics steps respectively. That is, in the first step the great open circuit potential changes from 0V to-0.3V with repetition were observed where the excellent rust prevention ability was kept, in the second step it decreases slowly from-0.1V to-0.4V with oscillation of the small potential changes where the gradual decrease of rust prevention ability was recognized and in the third step it decreases monotonously in the lower potential than-0.4V where the rust was observed because of the remarkable deteriorating of the rust prevention ability.

Abstract: In order to understand the reactivity of Cr₂O₃ surface towards H₂O molecule, the optimized structure, electronic structure, and the behavior of adsorbates were examined using a first-principles calculation based on density-functional theory (DFT). H₂O coverages varying from a quarter to two monolayers (MLs) were considered. At a low coverage, the oxygen atom of H₂O adsorbs on the Cr atom of the outermost Cr₂O₃ surface layer, the entire H₂O molecule is slanted at the direction of a hollow site, and a molecular plane is nearly parallel to the surface. The hydrogen bond is formed between the surface oxygen atom and the hydrogen atom of H₂O molecule. From the optimized structure, the H₂O dissociation mechanism which passes through a transition state is guessed. For 0.5ML coverage the obtained absorption energy is-82.5 kJ/mol. Our results are in good agreement with other reported theoretical and experimental results.

Abstract: In this experiment two kinds of 410L stainless steel, i.e., the first one is prepared by the I/M process and the second one is prepared by MIM process were used, and their corrosion behavior under stress in deionized water and the aqueous solution of 0.01kmol·m^-3HCl+1.72mol·m^-3MgCl₂ (pH=2.33) has been investigated by Electrochemical Impedance Spectroscopy (hereafter shortened as EIS) under Slow Strain Rate Tensile (hereafter shortened as SSRT) test. The charge transfer resistance (R_ct) of the I/M specimen is larger than that of the MIM specimen irrespective of under stress or non-stress, which means that the I/M specimen has the better corrosion resistance than the MIM specimen in the 0.01kmol·m^-3HCl+1.72mol·m^-3MgCl₂ (pH=2.33) solution. It was also confirmed from the fracture surface observation that hydrogen embrittlement occurred on the MIM specimen in the aqueous solution of 0.01kmol·m^-3HCl+1.72mol·m^-3MgCl₂ (pH=2.33). This result would be confirmed to be due to the existing impurities and defects in the MIM specimen.

Abstract: We present the results from first principle calculations of H₂O adsorption on oxygen-covered Fe (100) surface. The calculations are based on a density-functional theory, surface modeling which uses supercell slab models. As a surface oxygen coverage increases, the surface is not activated, which makes the adsorption of water molecules on Fe surface unfavorable. It has been found that the surface covered oxygen exerts an influence on the adsorption of H₂O molecule on Fe surface.

Abstract: The corrosion behavior of Mg-6mass%Al alloy with different microstructure conditions was studied by electrochemical method in Na₂SO₄ and NaCl solutions. A measurement of polarization curves was carried out in order to investigate the fundamental electrochemical characteristics. Electrochemical impedance spectroscopy was carried out to discuss the corrosion characteristics that were obtained from polarization curves. Electrochemical measurements were carried out with as-cast, as solution-treated and two kinds of aged specimens, respectively. For measurement of polarization curves, the apparent difference was exhibited in behavior showing the pitting corrosion by difference of solutions. In all specimens, the corrosion current density which occurred in four kinds of specimens was higher in the NaCl solution than in Na₂SO₄ solution.

Abstract: Recently, due to increasing popularity of the magnesium alloys, many studies on corrosion resistance of the magnesium alloys have been reported to improve their corrosion resistance. However, it is not clarified yet whether an electrochemical polarization curve or a corrosion rate is a good measure for the corrosion fatigue resistance or not. In the present study, corrosion fatigue tests, the weight loss test, and measurement of the electrochemical polarization curve were performed in 3% sodium chloride solution using both the extruded and the rolled Mg alloys. It was clarified that there are no differences in the corrosion fatigue lives between the extruded and the rolled Mg alloys, though they have different corrosion resistance. So, it was concluded that the corrosion fatigue characteristic does not correspond well with the trend in corrosion resistance.

Abstract: In this study, the electrochemical measurements such as the potentiodynamic polarization and the electrochemical impedance spectroscopy (EIS) tests were carried out in order to clarify the corrosion behavior of the aluminum alloys fabricated by powder extruded (P/E) process compared with those by ingot metallurgy (I/M) process, using 6000 system aluminum alloys. Two kinds of aluminum specimens; one is fabricated by the conventional I/M specimen and the other is fabricated by the P/E process, were used for the electrochemical experiment in the sulfuric acid solution with 0.5 kmol/m3 concentration. Both of I/M and P/E specimens showed the linear relationship between the electrochemical potential (E) and the common logarithm of current density (icorr) in the cathodic region where Tafel law was recognized irrespective of stirring of the test solution though the icorr was slightly increased by stirring for both specimens. On the other hand, in the anodic region, both of the two specimens indicated the almost the same icorr irrespective of stirring. These experimental results interpret that the corrosion is controlled by the chemical reaction. The EIS test indicated that the maximum charge transfer resistance (Rct) was observed at -0.55 V which is 0.11 V higher potential than the corrosion potential (Ecorr) for both of the two specimens.

Abstract: The sintered stainless steel produced by the powder metallurgy process (P/M) has attracted a growing interest because it has the advantage of better formability to fabricate complex shape products without machining and welding. The four sintered stainless steel samples; i.e., the mono-phase SUS304L SS P/M sample (hereafter denoted as 304L), the mono-phase SUS316L SS P/M sample (hereafter denoted as 316L), the duplex-phase SUS316L and SUS434L SS P/M sample (hereafter denoted as 316L+434L), and the duplex-phase SUS316L and SUS434L SS P/M sample with copper (hereafter denoted as 316L+434L+Cu) were used in this experiment, and their corrosion behavior was investigated through the electrochemical procedure. It was confirmed from the potentiodynamic polarization test that their corrosion behavior was clearly classified into two groups. The one is for the mono-phase stainless steel group and the other is for the duplex-phase stainless steel group. Both corrosion current density (Icorr) and passivation current density (Ip) for the latter group were smaller than those for the former group, and especially the duplex-phase 316L + 434L SS sample with copper (316L+434L+Cu) showed the lowest value. This implies that the duplex-phase 316L + 434L SS sample with copper (316L+434L+Cu) has the highest corrosion resistance.