Statistical and stochastic aspects of localized corrosion such as pitting corrosion and stress corrosion cracking have been reviewed and discussed. Stochastic theory was introduced to rationalize time dependent pitting process and distribution of pitting potential. A birth and death stochastic process was found properly to describe the generation and repassivation process of pitting. The stochastic analysis could provide rational estimates of the pit generation rate and repassivation rate, which, in turn, decide pitting potential. Statistical analysis of spatial distribution of pits for evaluation of atmospheric exposure test and recent advances of extreme value analysis were discussed.
The chemical properties of metallic biomaterials used for artificial joints, bone plates, and dental implants in human body were discussed on the basis of the empirical data. In particular, maturation of the surface oxide film on titanium and destruction and regeneration of the film in bioliquid were considered. The release of metallic ion and the behavior of the ion in living body were also discussed. In addition, the surface modification of the materials to improve their corrosion resistance, wear resistance, and bone conductivity, were explained. Surface modification methods were classified according to their purpose and environment in which the modifications are performed. Calcium ion implantation into titanium for the improvement of the bone conduction was explained as an example.
Electrochemical behavior and environment-induced degradation in strength for a carbon steel, SM400B, have been investigated in 0.025M Na2CO3+0.075M NaHCO3 solutions containing various concentrations of NaCl at 363K. A slow strain rate test (SSRT) was employed to evaluate the degradation in mechanical properties. It was found from polarization curves that there generated pitting corrosion beyond 0.01M NaCl. The SSRT was carried out applying a potential below the pitting potential to the specimen in the test solutions containing 0.5 and 0.01M NaCl. Under both solution conditions, an increase in the applied potential resulted in a reduction in maximum stress and fracture strain, as well as an increase in current density during SSRT. From observation of the surface of the fracture specimens, however, there was no crack on it. Based on assumption of homogeneous deformation and general dissolution until maximum load, true stress at the maximum load was calculated. It was revealed that the true stress was almost independent of the applied potential, although the maximum load decreased with increase in the applied potential. It means that the homogeneous dissolution is accelerated by applying the stress and/or the dynamic strain. It is concluded from the facts that the degradation in the mechanical properties results from a rapid reduction in the cross section due to homogeneous dissolution, instead of localized corrosion.
High temperature corrosion behavior of a SCH 13 steel in a waste incineration furnace, where used plastics and tires were burned in the day-after-day operation, was investigated by means of optical microscope and scanning electron microscope, electron probe micro-analysis, and X-ray diffraction analysis. The SCH 13 steel was put at the exit of the furnace, and the temperature of exhaust gases maintained above 1273K (maximum is 1423K) for 7ks and then at 1150K for 17ks, followed by furnace cooling. Corrosion products in the surface were mainly oxides as Cr2O3, Fe3O4, NiMn2O4, and FeCr2O4. Internal corrosion products with a network structure were composed of Cr-Mn oxide, and at the internal corrosion front SiO2 and CrS were formed in link with Cr-Fe carbides and σ-phase, which were precipitated in a network structure during the repeated furnace operation. Laboratory tests on sulfidation or oxidation of both as-received and heat-treated SCH 13 steels showed that the Cr-Fe carbides and σ-phase were preferentially corroded, forming a network structure. It could be concluded that the internal corrosion of the SCH 13 steel suffered from the waste incinerator atmosphere is due to selective sulfidation of Cr-Fe carbides and σ-phase in the internal corrosion front, followed by oxidation to form SiO2 and Cr-Mn oxide.
Environmentally Assisted Cracking (EAC) is characterized by crack growth rate of 40-100 times faster than that in air for reactor pressure vessel (RPV) steels in some specific light water reactor (LWR) environments. It is known that the water chemistry at a crack tip produced by dissolution of MnS inclusions in RPV steel is dominant factor for crack growth and electrochemical potential at crack tip affects the dissolution behavior of a bare metal at a crack tip. In order to elucidate environmentally assisted cracking behavior of materials, it is very important to evaluate a crack tip water chemistry. In this study electrochemical potential and water chemistry at the advancing crack tip of semi elliptical surface cracked compact tension specimens tip of reactor pressure vessel steel, SA 533B in oxygenated high temperature water were directly measured. It was observed that the measurement of crack environment and the modification of specimen had no effect on crack growth behavior of steels. The internal potential inside of crack is about 420-550mV lower than external potential of crack. The change of loading frequency had no effect on electrochemical potential inside of the crack. The crack tip water chemistry was microsampled under cyclic and constant loading conditions and analyzed by ion chromatography. Sulfate ion concentration measured during the test had a range between 0.1 and 2.5ppm depending upon the conditions and it is clear that the change of sulfate ion concentration was affected strongly by electrochemical potential and loading mode. At higher crack growth rates and at higher potentials in oxygenated environments, the sulfate concentration was found to be higher than that of constant load and lower potential at nitrogen charged environment.