The passive current noise of sensitized SUS 304 was analyzed by the FFT method in 0.05M H2SO4 to investigate correlation between the degree of the sensitization and electrochemical noise pattern, comparing with results obtained by the electrochemical potentiokinetic reactivation method (EPR method). It was found that the sensitization effect appeared markedly in an early stage after polarization start, on the power spectral density (PSD) of lower frequency region (∼0.2Hz), when the specimen was polarized at a less noble potential, +0.2V vs. Ag/AgCl. Also, in the early stage (∼5h), the burst type noise with small amplitude was very frequently observed on the passive current, which corresponding to local dissolution of sensitized SUS 304. The micronized and local dissolution at grain boundaries and its neighborhood was concluded to bring mainly the higher level of PSD in the early stage for sensitized SUS 304. Further, the comparison of the PSD level in the early stage gave a possibility to estimate the degree of the sensitization.
In order to clarify the effect of dissolution of MnS inclusions on environmentally assisted cracking of low alloy steel in high temperature water, dissolution behavior of MnS inclusions was investigated. The effects of temperature, dissolved oxygen concentration, specimen orientation and the composition of the test environments are discussed. Corrosion tests in high temperature water were carried out by using coupon specimens with crevice and without crevice to simulate the conditions of inside and outside of the cracks. Marked MnS inclusions were observed before and after 72 hours corrosion by a scanning electron microscope. The MnS inclusions dissolved at temperatures higher than 473K in crevice. The dissolution of MnS was accelerated by crevice and temperature up. Dissolved oxygen accelerated the dissolution of matrix around MnS inclusions but took little effect on dissolution of MnS inclusions. Specimen orientations and a simulated PWR environment showed little effect. The dissolution of MnS inclusions is considered to accelerate fatigue crack growth at temperatures higher than 473K.
In the NaCl solution, 4 points bending tests were attemped to investigate the corrosion fatigue behavior of SiC/7075 Al metal matrix composite (MMC) comparing with ordinary matrix Al alloy (Al alloy). Time to failure of both MMC and Al alloy decrease with an increase of loaded stress or applying at more anodic potential, and under the same condition, corrosion fatigue life of MMC was longer than that of Al alloy. The crack initiation and growth in corrosion fatigue process explained by passive region, repassivation process, and dissolution of newly created surface can be monitored by measuring the potential at the opencircuit and the current at the potentiostatic condition. As the results of this technique, it was shown that corrosion fatigue process, fracture morphology, and the site of the crack initiation for MMC was different from them for Al alloy, and the crack occurred from the bottom of pits on the MMC. Current amplitude corresponded to the defect of corrosion fatigue. According to the measurement of current amplitude, it was shown that once corrosion fatigue started, MMC got marked damage, however for almost all the period of corrosion fatigue test, it was resistive.
We studied the initial stage of electrodeposition of copper on gold electrode in 0.1M CuSO4+0.6% H2SO4 aqueous solution by using a scanning tunneling microscope (STM). We have found that the enhanced electrodeposition occurred under STM tip when the tunnel current was increased. The electrodeposition rate was strongly dependent on the tunnel current but not on the potential of gold electrode, and became 7600 times faster than that estimated from catholic polarization curve of gold as the tunnel current was increased. The STM observation after the test showed that the electrodeposition of copper occurred only under the STM tip. It may be considered from these results that some part of the tunnel current was consumed to the electrochemical reaction.
It is well known that the corrosion resistance of stainless steels depends on Cr depleted layer formed near the surface of matrix and the change in passive film compositions due to various surface treatments. There has, however, been little only a few quantitative studies on this phenomenon. In this study, the Cr depth profiles near the surface of SUS 304 polished mechanically and passivated in respective HNO3 solution (10, 30%), were measured by electrochemical method and AES. Then the atmospheric corrosion resistance, anodic polarization behavior in NaCl solution (Cl-: 10-2g·dm-3) and repassivation potential for crevice corrosion of SUS 304 with various Cr depth profiles were evaluated respectively. These results clarified the quantitative relation between surface Cr concentration and corrosion resistance, suggesting that the initiation process of corrosion, such as the first current increase in anodic polarization curves depends on the outermost surface Cr concentration of a passive film and the propagation process, such as rusting and the growth of pitting or crevice corrosion depends on the Cr concentration near the surface of matrix.
Ellipsometry is one of optical methods for characterization of solid surfaces by using polarized-light reflection. It can determine the optical properties of surfaces and surface layers. Particularly it has a high sensitivity and reliability of the thickness measurement of the surface layers or films. This article describes on the ellipsometry the principle, apparatus and the application to corrosion study.