Rei Takei

Evaluation of Initial Corrosion Phenomenon of Magnesium Alloys by SKPFM

Magnesium (Mg) has the lowest standard electrode potential in structural materials, and then it easily corroded for that property contacting with other metals. The corrosion resistance of Mg alloys was generally investigated by conducting saltwater immersion test, salt spray test, and electrochemical corrosion test. However, these tests give only macroscopic information but don’t give microscopic (i.e. galvanic corrosion at the interface). So, the origin of galvanic corrosion at the interface between dispersoids and a-Mg in Mg alloys was investigated for fundamental clarification. In this paper, the surface potential difference (VSPD) at the interface between dispersoids and a-Mg using AZ91D Mg alloy is measured by using Scanning Kelvin Probe Force Microscope (SKPFM). Surface potential of pure metals measured by SKPFM had good correlation with standard electrode potential (SEP), and then VSPD also positively correlated with difference of SEP. Salt water immersion test was conducted using AZ91D in order to compare VSPD with corrosion phenomenon. Topographic map around the interface between dispersoids and a-Mg was measured by using AFM after corrosion test. Corroded phase corresponded to the anodic phase indicated by surface potential.

Variation of Critical Heat Flux by Flow Oscillation in a Small Vertical Channel

The effect of sinusoidal oscillation of inlet mass flux on the critical heat flux (CHF) in forced convective boiling was investigated in experiment and numerical calculation. In the experiment, the test section was a small stainless steel round tube of 5 mm in inside diameter, filtrated and deionized tap water was used as a test fluid, and the flow direction was set to vertical upward. The heated length was 1,600 mm. Electric power supplied to a circulation pump was varied periodically to oscillate the inlet mass flux sinusoidally. Direct current was passed through the test section tube to heat it ohmically. The occurrence of critical heat flux condition was detected using the signal from the thermocouples that were spot-welded on the outer wall of the test section tube. In the present experimental conditions, it was expected that the critical heat flux condition was triggered by the dryout of liquid film in annular two-phase flow regime. The main experimental parameters were the time-averaged inlet mass flux and the amplitude and period of flow oscillation. The system pressure was also used as an important experimental parameter since a boiling water reactor is operated under high pressure condition. If the oscillation period is long enough, it is expected that the critical heat flux under the flow oscillation condition is close to that for the steady state when the flow rate is equal to the minimum flow rate in the oscillatory condition. On the other hand, the decrease of the critical heat flux would be mitigated if the oscillation period is shortened, since interaction would take place between the thin and thick film regions within a boiling channel. In accordance with this expectation, the critical heat flux measured under the flow oscillation condition was reduced with an increase in the oscillation period. It was demonstrated that the reduction of critical heat flux under flow oscillation condition can be correlated fairly well using the concept of dimensionless heated length. Numerical calculations using a one-dimensional three-fluid model were also carried. The calculated critical heat fluxes for flow oscillation conditions increased with increased value of dimensionless heated length, as in the present experiment.Copyright

Corrosion Phenomenon Evaluation of Mg Alloys using Surface Potential Difference Measured by SKPFM

Magnesium is easy to be corroded because it has small standard electrode potential (SEP). In magnesium alloys, the corrosion resistance of dispersoids hasn’t yet evaluated quantitatively as the SEP of dispersoids wasn’t almost investigated. This study surveyed the surface potential difference (SPD) between α-Mg in AZ91D magnesium alloy and the dispersoids in that alloy by using Scanning Kelvin Probe Force Microscope (SKPFM). The corrosion resistance was evaluated and correlated with SPD, where this measured SPD represents the difference of work function between α-Mg and the dispersoid. At this experiment, it was obtained that the difference of SEP generally increases as the SPD increases. The good correlation between difference of SEP and SPD was obtained. The SPD between α-Mg and Mg17Al12 or Al6Mn in AZ91D Mg alloy is measured. In addition, in order to obtain the reference of SPD, the area of α-Mg only and the interface area between Fe and α-Mg were respectively scanned. The result shows that Mg17Al12 has small SPD to α-Mg and that the SPD between Mg17Al12 and α-Mg is mostly the same as that in α-Mg. Therefore, Mg17Al12 is considered of no effect on corrosion. On the other hand, Al6Mn degrades corrosion resistance because the SPD between Al6Mn and α-Mg is as large as that between Fe and α-Mg.