Electrochemical behavior of Type 304 stainless steel and carbon steel in high temperature pure water has been studied by using an electrochemical impedance spectroscopy. Under high dissolved oxygen concentration, impedance diagram for Type 304 stainless steel and carbon steel are founded to be composed of two capacitive semicircles. Impedance response at high frequency is that of H2O/O2 redox reaction and at low frequency is that of corrosion reaction correlated to oxide film growth. Impedance frequency dispersion of Type 304 stainless steel and carbon steel are divided mainly into those of redox reactions, diffusion of an oxygen bearing ions through oxide films, and dissolution of iron using an equivalent circuit obtained by oxygen diffusion model for film growth. Variation in corrosion rates obtained by weight loss measurement are compared with variation in reciprocal of the Warburg impedance coefficient obtained by numerical analysis. Under low dissolved oxygen concentration, only impedance of H2O/H2 redox reaction was observed, because that redox reaction is independent of corrosion reaction and has high reversibility. The pure charge transfer resistance of the corrosion process of Type 304 stainless steel and carbon steel can not be obtained in high temperature water.
The electrochemical polarization characteristics of coated all aluminum cans in a 1% NaCl solution were studied to establish a method for the determination of the area of coating defects. When specimens were cathodically polarized at a potential of -2.0V vs SCE, which is below the hydrogen evolution potential, the current immediately attained a stationary value. Additionally and importantly, a linear relationship was observed between the current and the area of coating defects, implying that the area of coating defects can be determined quantitatively from the catholic current at the potential of -2.0V. The present method is considerably more reliable than the conventional enamel rater value method where the specimens are anodically polarized in a 1% NaCl solution which causes the development of pits at the defect sites and, hence, overestimation of the defect areas. Application of the present method for the evaluation of coating performance has also been discussed with specific examples.
The filiform corrosion resistance of the sanded surface of aluminum alloy was studied in accelerated corrosion test. It was found that the rougher the sanding paper, the poorer the filiform corrosion resistance, and the stronger the sanding intensity, the poorer the filiform corrosion resistance. The fact that phosphated specimens performed much better than the unphosphated one shows that the phosphate coating is a important factor to obtain high filiform corrosion resistance. Phosphate coating was nonuniform on the sanded surface with few phosphate crystals deposited on the sanding scratch. Filiform corrosion tended to follow the sanding scratch and chloride was detected along it. The nonuniformity of phosphate coating was considered as a result of the nonuniformity of oxide.
The rust layers formed on weathering low-alloy steels, which resist atmospheric corrosion, exposed for various periods have been examined. It is elucidated that the inner stable and protective rust layer which covers the surface of a weathering low-alloy steel exposed for 26 years mainly consists of small particles of α-FeOOH and α-(Fe1-X, CrX) OOH (X=0.03) containing a considerable amount of Cr. The variation in the amount of rust constituents against the exposure period supports well a newly proposed schematic progress of long-term growth of stable and protective rust layer. Namely, the initial rust layer of γ-FeOOH is changed to a final stable rust layer of α-oxyhydroxides consisting of α-FeOOH and α-(Fe1-X, CrX) OOH, probably via amorphous oxyhydroxide, during long-term exposurein an industrial atmosphere. The effect of Cr and Cu on the formation of the stable and protective rust layer is discussed on the basis of transmission electron microscopic observation of synthetic α-FeOOH particles with and without addition of Cr and Cu ions. It becomes evident that densely packed fine-particles of the α-oxyhydroxides are formed as a result of coexisting with Cr and Cu.
Principle and characteristic of reflection high-energy electron diffraction (RHEED) are described paying to the relations between RHEED patterns and surface morphologies. Some examples indicating the relations are shown. Micro-probe RHEED is also described, in which focused electron beams are used for obtaining crystalline information from surface micro-areas. An in situ observation result of Si thin film growth is shown as an example.
A new concept “non-equilibrium fluctuation” has been introduced to describe electrochemical reactions with intense non-uniformity along the electrode surface, such as pitting corrosion and nucleation. This concept comes from the fact that the reactions locally proceed through the breakdown of electrostatic equilibrium formed by electrical double layer owing to thermal motion of solution particles. We call this kind of breakdown non-equilibrium fluctuation because it occurs at the electrode potential not equal to the equilibrium potential. Therefore, the reaction rate is controlled by the fluctuation, i. e., the development of the electrolytic current with time is equivalent to the unstable growth of the fluctuation. After determining the intrinsic spectrum of the fluctuation in experiments, we can calculate the current density, overpotentials, surface morphology, and so on. Consequently, microscopic surface morphology of nickel pitting corrosion has been theoretically related to macroscopic total pitting current we can measure.