Rotary bending fatigue tests of prestressing wires were conducted potentiostatically in sat. Ca(OH)₂ solution containing 2wt% CaCl₂. Under the stress amplitude of 29.3kg/mm², which is lower than the fatigue limit in air, failures of smooth specimens occurred only at potentials more noble than a critical potential, V_cf≅-0.47V(SCE). At -0.35V, more noble than V_cf, under σ_a=29.3kg/mm² the grown cracks showed a pit-like form up to 26μ in depth. The crack depth of 26μ coincides with a threshold crack depth, l_th, which is determined from stress amplitude of 29.3kg/mm² and the threshold stress intensity range, ΔK_th, of 12kg·mm^-3/2. The fact that a crack initiates at the bottom of a pit of l_th in detph was also observed under other stress amplitudes lower than the fatigue limit. Under stress amplitude of 29.3kg/mm², potentials of specimens which had been potentiostated at -0.35V to grow the cracks were changed to less noble potentials than V_cf in order to observe the successive growth of the cracks. The growth of cracks of which depth is smaller than l_th was found to be ceased by cathodic protection at potentials less noble than about -0.8V. Cracks with a depth larger than l_th, however, continue to propagate independently of electrochemical procedures. So the cracks of which depth is smaller than l_th are ‘pits’ from the standpoint of not only shape but also dynamical behavior, and the pitting process up to the depth of l_th is the necessary one for the initiation of propagating corrosion fatigue cracks. Use of galvanized prestressing wires is effective in preventing the growth of cracks, of which depth is smaller than l_th, by cathodic protection of the steel.

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The hydrogen content in mild steels at the reversible potential of hydrogen electrode reaction was successfully measured by using an electrochemical method reported previously (Part 1). It was confirmed that the hydrogen content thus measured was not affected by pH and the kind of ion in electrolyte and was equal to the solubility of hydrogen in hydrogen atmosphere at 1atm. The additive elements in mild steels was found to have the secondary effect on hydrogen occlusion. The solubilities in 20 steels containing different additives except for Ti were 2.1-0.2×10^-8gr atom/cm³. It was also found that the temperature at hot rolling excerted no affect on the solubility of hydrogen in mild steels.

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An improved intensity-following method of ellipsometry which allows continuous measurements of ellipsometric parameters to be made of changing surfaces is described and applied to measurements of the passive film on iron. The merits of this method are as follows, (1) it can be used not only for the case of small incidence angle but also for the case of large incidence angle in which the change of P and A due to the film formation is usually large, (2) it is not affected by the initial light intensity which often differs with different measurements, and (3) only two successive measurements instead of three runs with conventional method are required to estimate ellipsometric parameters P and A. The method was used to measure the passive film formed on iron in a sodium borate solution at pH 8.42 and its catholic reduction at pH 6.48 at which the film can be reduced layer by layer without changing its composition. For the steady state film there are three different potential regions in the ΔP-ΔA curve with respect to its slope; the first region corresponds to the films formed in the potential region between -0.3V and +0.245V, the second region to those formed between +0.245V and +1.54V and the third region to those formed above +1.54V. From cathodic reduction measurements, it is found that the passive film consists of two layers, an inner barrier layer and an outer deposit layer. The optical constants of the barrier layer, calculated from the slope of ΔP-ΔA curve obtained during cathodic reduction, differ with different film formation potential regions described above. The optical constants of the deposit layer, however, scarcely change with the film formation potential except for a potential region below +0.245V. From comparison of the optical constants of the passive film with those of known iron oxides and hydroxides, the deposit layer is estimated to be an amorphous hydroxide and the barrier layer to be an oxide close to γ-Fe₂O₃.

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The stress corrosion cracking behaviors of 18-8 stainless steel have been investigated in H₂SO₄/NaCl solutions using potentiostatically controlled and dynamically loaded specimens at room temperature. Transgranular stress corrosion cracking occurred in 5N-H₂SO₄/(0.2-0.8)N-NaCl solutions. The crack depth increased with increasing anodic potential, except that a general corrosion rate increased with anodic potential. The critical anodic potential which was observed on the stress corrosion cracking shifted to noble potential with increasing the concentration of NaCl in the solution. In this acid solution, the corrosion rate and anodic current at the steady state were decreased with increasing chloride ions. It is suggested that the stress corrosion cracking occurrs in the condition in which chloride ions acts as a corrosion inhibitior.

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Scale plugging trouble of stainless steel superheater tubes has been experienced by a considerable number of power stations. This report describes the phenomenon, the cause and the preventive measures of scale plugging trouble with brief review of steam oxidation of stainless steels. Scales of double layer structure developed on the internal surface of stainless steel superheater tubes. Outer layer of the scale exfoliates, accumulates at a certain part of superheater and causes the plugging trouble. Preventive measures proposed from the standpoint of materials are to use (1) higher chromium austenitic steels such as 310 or Incology 800, (2) 300 series stainless steels of fine grains such as fine grain 347H, (3) 300 series stainless steels of cold worked surface such as shot blasted or ground, (4) surface coating such as chromium plating, and so on.

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Mass transfer studies of corrosion of iron and steel are reviewed. It is demonstrated through corrosion experiments under well defined hydrodynamic conditions that corrosion of iron and mild steel in acidic and neutral solutions of pH>1.0 takes place under diffusion control with dissolved oxygen as the diffusion-rate determining species and that the rates of corrosion calculated with mass transfer equations and correlations fitted for electrochemical systems are in good agreement with observed rates of corrosion of iron and mild steel with no rust formation (pH 1-3) for the following cases: forced laminar and turbulent flow in pipes, rotating rods in forced turbulent flow, and rotating disk electrodes in forced laminar flow. For diffusion of oxygen through porous rust layers on iron, a treatment based on diffusion through heterogenous media is suggested.

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