Papers by Author: Kazuhiko Majima

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: 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.

Abstract: Two kinds of pure titanium specimens, i.e., the first one is prepared by the ingot metallurgy (I/M) process and the second one is prepared by the metal injection molding (MIM) process were used in this experiment, and their corrosion behavior under stress has been investigated in several aqueous solutions by Slow Strain Rate Tensile (SSRT) test. Ti MIM specimen showed the larger maximum stress and the lower maximum stain than I/M Ti specimen in the deionized water. Both Ti I/M and MIM specimens indicated good corrosion resistance in the aqueous solution of 2.5 kmol/m3 H2SO4 + 0.2 kmol/m3 NaCl in which SUS304 stainless steel showed SCC, and showed the decrease of the maximum stress and the maximum strain in the aqueous solutions of CH3OH + 0.1 kmol/m3 HCl, where the brittle fracture appearance was more remarkable with an decrease of H2O in the solution.

Abstract: Three cemented carbide specimens, i.e., WC-Co (Sample A), WC-Cr3C2-Co (Sample B) and WC-βt-Co (Sample C) in which βt indicates (Ti,W,Ta,Nb)C, are prepared by liquid phase sintering, and their corrosion behavior has been investigated by the electrochemical methods. The similar activation, passive state and over passive state regions as SUS304 stainless steel during the polarization test, and the preferential dissolution of Co were observed for all of the three specimens mentioned above. Both Cr3C2 and βt improved the corrosion resistance of WC-Co alloy, in which the effect of the former was better than that of the latter. Sample B also showed the best corrosion resistance in all the samples for the electrochemical impedance spectroscopy (EIS) test.

Abstract: The extruded AZ31B Mg alloy specimens using powders fabricated by roll compaction processing (RCP) was prepared, and their corrosion behavior has been investigated through the polarization test, electrochemical impedance spectroscopy test, immersion test and SEM observation in comparison to that of the conventional AZ31B Mg alloy, hereafter shortened as I/M specimen. The extruded AZ31B Mg alloys using RCP powder showed little change in Ecorr irrespective of number of pass cycles. Both anodic and cathodic current density suppression of the RCP specimens became larger with an increase in number of pass cycles. It was also confirmed that the corrosion characteristics of the RCP specimens depended strongly on their structural morphology and that the corrosion resistance of the RCP specimens subjected to 50 pass cycles was nearly same as that of the I/M specimen.

Abstract: Metal injection molding (MIM) process has the advantage of better formability of three dimensional complex shape products with high density and high performance properties compared to the ingot metallurgy (I/M) process. Two kinds of pure titanium specimens, i.e., the first one is prepared by the I/M process and the second one is prepared by MIM process are used in this experiment, and their corrosion behavior under stress has been investigated in several aqueous solutions by Slow Strain Rate Tensile (SSRT) test. Both I/M and MIM specimens showed good corrosion resistance in the aqueous solution composed of 2.5 kmol/m3 H2SO4 and 0.2 kmol/m3 NaCl as well as saline solution. This aqueous solution is noted one in which Type 304 stainless steel showed SCC. In the aqueous solution composed of CH3OH and 0.1 kmol/m3 HCl containing 8.6kg/m3 H2O, the elongation of MIM specimen was slightly higher than that of I/M specimen though both specimens indicated the remarkable decrease in elongation.

Abstract: The pitting corrosion behavior of sintered SUS 444 stainless steels (P/M specimens) with different kinds of porosity ratios of 13.3, 9.11 and 6.88 % was studied in an aqueous solution containing MgCl2 compared with a conventionally processed SUS 444 stainless steel(I/M specimen). The pitting corrosion potential decreased almost linearly as porosity ratio increased, and the I/M specimen showed the best resistance against the initiation of pitting corrosion. The pitting corrosion morphology by SEM observation showed that more small pits were observed on the specimen with larger porosity ratio while less number but larger diameter pits were observed on the specimen with smaller porosity ratio, and it was also confirmed that the I/M specimen had the least number but the largest and deepest pits.

Abstract: Three kinds of Type 304 stainless steels: the first one was made by the conventional wrought process (I/M), the second one by conventional powder metallurgy process (P/M) and the third one by Metal Injection Molding process (MIM), were used. The corrosion behavior of these specimens was investigated through the potentiodynamic polarization curve measurement and electrochemical impedance spectroscopy method. In addition, SCC characteristics were also studied. Both the potentiodynamic polarization curve and the Nyquist plot curve of MIM specimen were similar to those of I/M specimen, but different from those of P/M specimens. Therefore it might be suggested that MIM process is quite suitable to improve the corrosion resistance. On the other hand, time constants of P/M and MIM specimens are similar, but they differed remarkably from that of I/M specimen, which is considered to be due to the remained pores in P/M and MIM specimens. Further, it was also confirmed that the SCC was more liable to take place on I/M specimen than MIM specimen.