A Ni aluminide layer was formed on Ni substrate by electrodepositing Al in molten salt. The electrodeposition of Al was conducted using potentiostatic polarization method at constant potentials in an equimolar NaCl-KCl melt containing 3.5 mol% AlF3 at 1023 K. The deposited layer consisted of Ni2Al3. Nickel covered by the electrodeposited layer was more resistant than bare nickel to high temperature oxidation. Furthermore, the formation of a Ni aluminide layer containing Zr on a Ni substrate was attempted by the Zr deposition, followed by the Al deposition for the purpose of improvement of cyclic oxidation resistance. A layer consisting of Ni2Al3 and a Ni aluminide layer containing Zr on the Ni2Al3 layer were formed. Furthermore, when the Zr electrodeposition conditions were changed, the concentration of Zr in the Ni aluminide layer containing Zr, which was formed in the surface region, changed. The cyclic oxidation test showed that for the Ni sample with the Al deposition, the mass reduction due to scale exfoliation took place, whereas for the sample treated with a small deposit of Zr, followed by Al deposition, no mass reduction was observed. Furthermore, a synchronous electrodeposition of Al and Zr was attempted in order to decrease the number of electrodeposition. As a result, it was found that the synchronous electrodeposition of Al and Zr could be achieved by decreasing of ZrF4 content in the molten salt. The Ni aluminide layer formed by using the molten salt containing 3.5 mol% AlF3 and 0.05 mol% ZrF4 contained a small amount of ZrAl3 particles at the surface region and showed a high cyclic-oxidation resistance.
The corrosion of iron plates in flowing water has been more inhibited by applying a magnetic flux density of 7.0 T with a superconducting magnet than in the case of non-magnetic treatment (NMT). X-ray diffraction (XRD) quantitative analysis has supported that magnetic treatment (MT) of flowing water is effective for increasing amount of amorphous or nanocrystalline rusts which may be helpful for inhibition of rusting. Experimental results with a monitor of inside of pipes in which water flows in both NMT and MT cases have shown that sticking of iron rust to inside of pipe is more suppressed in the MT case.
Cathodic protection has been widely applied to protect the holidays of buried steel pipes from corrosion. Since it is difficult to measure the density of cathode current flowing directly into the holidays, buried steel pipes are controled by the density of cathode current flowing into the probes attached on the pipes. If we can estimate the density of cathode current flowing into the holidays, it is possible to repair the holidays and to control cathodic protection precisely. We have constructed a theoretical equation with potential gradient on the earth in order to estimate the density of cathode current flowing into the holiday. Experiments using simulated pipes with holidays have proven the validity of our estimation method.
A modified middle scaled CBB test method with a notched specimen is developed and its stress conditions are evaluated, in order to investigate stress corrosion cracking behavior around fusion boundary between nickel base alloy (Alloy 182) and low alloy steel (LAS) under high stress condition. Stress conditions of crack tips are evaluated by a newly developed ‘equivalent stress intensity factor, KJ’, which is defined by J integral considered plastic deformation behavior around a crack tip. Stress analyses revel that KJ value of the notched specimen is greater than that of a smooth specimen with multi cracks. By adequately changing a configuration of notch, including its depth and radius of curvature, a CBB test could be carried out under a stress condition at arbitrary KJ value. In addition, multi notched specimens with various notch configuration are useful for investigation of crack propagation under different stress conditions at the same time.
Some researchers recently point out that Ni based alloys used in nuclear power plants have the ordering tendency, which is a potential to decrease mechanical properties within the expected lifetime of the plants. In the present study, authors evaluated the effect of 8 alloying elements on the ordering tendency in Alloy690 based upon thermodynamic calculation by Thermo-Calc. It is clarified that the additive amount of Fe, Cr, Ti and Si, particularly Fe and Cr, was influential for the stability of Ni2M, while that of Mn, Cu, B and C had almost no effect for that. Authors therefore designed the Ni2M stabilized alloy by no addition of Fe in Alloy690. Ni2M is estimated to be stable even at 773 K in the Ni2M stabilized alloy. The influence by long range ordering or precipitating of Ni2M in Alloy690 for mechanical properties or SCC susceptibility is expected to be clarified by the sample obtained in the present study.
Copper plates were exposed in sulfur containing atmosphere made by sodium sulfide solutions for up to 180 days. The cathodic reduction method was used to execute the qualitative analysis of the film formed on copper plate. The thickness and constituents of the film have been investigated by cross-sectional observation and EPMA analysis. Cathodic reduction curves revealed that the film was mainly composed of CuO, Cu2O and Cu2S. The thickness of the film formed in sulfur containing atmosphere made by 1 wt%Na2S solution has increased with an increase in an exposure period and reached about 40μm for 180 days. Multilayered film composed of sulfide and oxide was identified by the EPMA analysis.
We have developed the apparatus for monitoring changes in humidity in the test environment and mass of the rusted steel in order to detect the moisture absorbed into and desorbed out of the steel rust. The rusted steel was set in the container in which the air was controlled to be dry or humid. Change in mass of the rusted steel due to absorption or desorption of moisture was continuously monitored by an electric analytical microbalance, and humidity in the container was spontaneously determined by a digital hygrometer. In the air of a relative humidity (RH) more than 90%, mass of the rusted steel increased rapidly and parabolically due to absorption of moisture into the rust, and then increased gradually and linearly due to adsorption of moisture on the surface. As the test lasted, the mass oscillated ; sudden decrease in mass corresponded to falling a water droplet with some pieces of the rust, and linear increase in mass thereafter did to re-adsorbing moisture. In the case of RH less than 90%, mass change was similar to that of RH more than 90%, but there was no falling of a water droplet from the rusted steel. It is revealed from the long-term test in the air of RH 88% that mass of moisture absorbed into the rust was independent of test time and that mass of new rust increased linearly with test time. In addition, it is suggested that the mass increase rate, that is corrosion rate, corresponds to the cathodic reaction rate limited by diffusion of dissolved oxygen in the thick water film.