This article reviewed the recent results of nano-indentation tests in solution for in-situ evaluation of the mechanical properties of passive iron and titanium surfaces electrochemically controlled in pH 8.4 borate solution. The hardness of passive metal surfaces in the solution could be determined from the load-depth curves measured with nano-indentation tests. In the absence of chromate treatment, the hardness of the passive iron (110) surface was larger by 10% than that of the passive iron (100) surface which was ascribed to the difference in surface atomic density of the substrate. The chromate treatment increased the hardness of the passive iron (100) surface. The effect of chromate treatment on hardness was explained in terms of high repassivation rate at the ruptured sites of passive film during nano-indentation in solution. The hardness of the titanium surface obtained with nano-indentation under the electrochemical control at 5V (RHE) after anodic oxidation at 5V for 1h in the solution was 3-4 times as large as that of the titanium surface obtained with nano-indentation in air after anodic oxidation at the same condition. This large difference in hardness was attributed to high repassivation rate at the ruptured sites of anodic oxide film under the electrochemical control at 5V in the solution as well as the effect of the chromate treatment on hardness for the passive iron surface.
Aluminum specimen covered with thin barrier type anodic oxide films was scratched with Si tip of AFM probe in a CuSO4 solution, and then cathodically polarized in the solution, using the Si tip as a counter electrode. During AFM probe scratching, oscillation of current was observed and decreased with scratching time. Analysis of the current oscillation enabled to examine the proceeding of the removal of anodic oxide films during AFM probe scratching. Silicon tip wore easily by scratching the oxide film-covered aluminum with a force. After AFM probe scratching, cathdic polarization of aluminum specimen caused the Cu deposition only at the film removed area, using a nitrocellulose film-coated Si tip as a counter electrode.
Mechanical characterization in sub-micron scale by nanoindentation techniques is introduced. Some major apparatus are presented and a method of data analysis to calculate hardness and Young's modulus is described. Examples of application to some materials such as thin films and high-strength steels with fine and complicated microstructure are also shown.
Corrosion behavior of an explosion-bonded dissimilar metal joint Zr/Ta/R-SUS 304 ULC in boiling HNO3 solutions containing Cr6+ ions has been examined by an immersion corrosion test, polarization curve and galvanic current measurement, and a bent-beam stress-corrosion test. Transpassive dissolution followed by intergranular attacks occurred in the R-SUS 304 ULC part of the joint exposed to boiling 3 kmol·m-3 HNO3+1 kg·m-3 Cr6+ and 8 kmol·m-3 HNO3+2 kg·m-3 Cr6+ solutions, while no local attack was observed at the Zr/Ta and Ta/R-SUS 304 ULC interfaces. The corrosion rate of the stainless steel part was independent of the surface area of the Zr part. From the measurement of polarization curves for Zr and R-SUS 304 ULC stainless steel and galvanic current densities for a galvanic couple consisting of these two materials, it was confirmed that the Zr part acts as an anodic site and thus no galvanic corrosion occurs in the stainless steel part. Stress-induced corrosion including failure due to cracking was not observed on the dissimilar metal joint exposed to boiling 3 kmol·m-3 HNO3+1 kg·m-3 Cr6+. In this solution, stress-corrosion cracking (SCC) occurred on Zr polarized at potentials higher than 1.5 V (vs. Ag/AgCl/3.33 kmol·m-3 KCl). The corrosion potential of the joint never exceeded the critical potential for SCC of Zr, because it was primarily determined by electrochemical properties of R-SUS 304 ULC stainless steel and retained on the low potential side of transpassive region of the steel.
The effects of a heat-transfer on the corrosion of zirconium was examined in boiling nitric acid solutions with various concentrations. Corrosion mass losses and electrochemical polarization curves were measured on the heat-transfer and isothermal surfaces in the solutions. It was found that the corrosion rate of zirconium was higher on the heat-transfer surface than that on the isothermal surface. The rate increased with increasing nitric acid concentration and solution temperature. The increased oxidization potential on the heat-transfer surface is attributed to the reduction of nitrous acid concentration by the thermal decomposition on the surface and the removal of the decomposition product from solution by boiling bubbles. The redox potential of 12 mol/dm3 nitric acid on a boiling heat-transfer surface was very close to the breakdown potential of primary passivity of zirconium. This suggests the initiation of SCC on a boiling heat-transfer surface in a nuclear fuel reprocessing.
In order to understand corrosion of metals in nitric acid solutions, it is necessary to know the generation mechanism of high equilibrium potential in the solutions, especially under boiling conditions. Existing nitrogen oxides in nitric acid solutions were first analyzed by Raman spectroscopy and then existing amount of nitrogen oxides were examined by thermodynamic calculation using the SOLGASMIX software. The Raman spectroscopic analysis showed that the existing amount of un-dissociated HNO3 increased with increasing nitric acid concentration and solution temperature. The existing amount of NO2 also increased by thermal decomposition. The thermodynamic calculation showed that the important nitrogen oxides in nitric acid solutions are HNO3, NO3-, HNO2, NO2, and NO. The equilibrium potential of nitric acid solutions is, however, mainly decided by the HNO3/HNO2 equilibrium. The thermodynamic calculation also suggested that the increased oxidization potential on the heat-transfer surface is attributed to the reduction of nitrous acid concentration by the thermal decomposition of nitrous acid on the surface and the continuous removal of decomposition product from the solutions by boiling bubbles.
It frequently reported that corrosion damage progresses at an extraordinary high rate (≥1 mm/y) on the inside wall of heat conduction tubes in power plants and in the heat recovery boilers of chemical plants. In previous studies, the influence of dissolved oxygen (DO) and the pH of boiler feed water on the corrosion of carbon steel was investigated at elevated temperatures under high pressure. As a result, it was found that an increase in DO or pH was not useful to prevent the corrosion. In this paper, the application of low alloyed steel, a material that is currently used to resist corrosion through material improvement, was examined. The maximum corrosion depth rate of low alloyed steel reached levels as high as 1mm/y at pH 9.0. This level decreased slightly with increasing pH and then showed a sudden and drastic decrease in the pH range from 9.25 to 9.75 depending the direction of flow of water over the specimen surface. Concerning the influence of temperature, the low alloyed steel was superior to carbon steel at temperatures over 180°C due to the oxide film which is uniformly deposited over the entire surface. Ditch corrosion, however, occurred on the surface of low alloyed steel at temperatures below 160°C, and developed to a greater extent than on a carbon steel surface. It was also found that vortexes generated in the fluid flow are intimately connected with the formation of ditch corrosion. In conclusion, low alloyed steel cannot be recommended for use in boiler tubes since it might suffer ditch corrosion badly under some conditions although it is excellent under conditions where a single and uniform oxide film is formed on the metal surface.