In cold water supply systems corrosion of steel substrate after disappearance of the galvanized layer leads to ‘red water’ problem and loss of the carrying capacity due to tuberculation. In this respect corrosion rate measurements for steel pipes were made in cold soft-water by using a one-through testing apparatus. As a result, the corrosion rate of steel pipe increased with an increase in pH value in the pH range of 6.5-8.5. This can be elucidated by the occurrence of localized corrosion due to low buffer capacity in the pH above 7. No appreciable acceleration was observed with the addition of chloride and sulfate up to 100mg/l Cl^-(SO₄^2-). In the study of the effect of water velocity (v), the corrosion rate of galvanized layer was found to be proportional to ³√v, while non-galvanized steel pipes showed the maximum corrosion rate at a flow velocity of 0.67m/s.
Although dosages of hexametaphosphate into water at 2 to 5mg/l (P₂O⁵) levels increased to some extent the corrosion of steel pipe, good protection was attained at levels of 12mg/l (P₂O⁵). Dosages of sodium metasilicates up to 12mg/l (SiO₂) showed no obvious inhibition for steel pipe.
It should be considered that both phosphate and silicate treatments will provide it sequestering action rather than corrosion inhibition at lower dosage levels.

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Surface area dependence of pitting potential has been examined for SUS 304L stainless steel in 3.5% NaCl solution at 35°C, using a multichannel pitting corrosion testing apparatus. The experimental data was proved to obey the relation which was formulated based on a stochastic theory. The predicted relation between pitting potential and surface area is as follows: E_m=(V/Skα)^1/2+E_crit, where E_m is the most probable value of pitting potentials measured by the potential sweep velocity of V for the surface area of S, k and α are constants, and E_crit is a critical pitting potential under which no probabliity of pit generation is expected. Thus the theory and experimental data demonstrate that the pitting potential decreases with increase in the surface area of specimen used, but approaches to the constant potential of E_crit at an infinite surface area. In this experiment, the numerical value of kα is so large that no significant surface area dependence is observed as far as a slow potential sweep velocity is used. But in other cases for different materials or environments, a small value of kα could be expected so that this surface area effect on the pitting potential could not be neglected.

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Repassivation potential, E_R, could be regarded to be well reproducible crevice characteristic which cover repassivation as well as initiation of crevice corrosion as reported in our previous papers. The cyclic polarization procedure to determine E_R has to be conducted under sufficiently slow rates of potential sweep and requires a long time. This paper describes the development of two automatic data acquisition and control systems for E_R measurement which can eliminate disadvantages of manual operations which are laborious and tedious. The system A has microcomputer, PS-80 (TEAC), with digital magnetic tapes, A/D and D/A converters, printer and real time clock (RTC) to control 3 potentiostats. The system B has single boad computer, TK-85 (NEC), with A/D and D/A converters, miniprinter and RTC to control 2 potentiostats. E_R values obtained by the systems A and B for a nut/nut-crevice of Type 316 steel in 3% NaCl solution at 25°C measured -0.29N--0.30V, SCE which coincided with those manually determined as reported earlier. Those systems were also applied to crevices of Type 444 steel. Each system has been running for more than 5 months.

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X-ray sterss measurement is an excellent way to trace non-destructively the process of stress generation at the oxide/steel interface and its relation to the structural and compositional changes of the oxide and steel during high temperature oxidation.
Mild steel (1.16wt% Mn, 0.2wt% C) was oxidized in air over the temperature range of 250°C-500°C, the stress and the lattice constants of steel and oxide were measured by X-ray stress measurement with Cr-K_α characteristic X-ray.
A gradual decrease of lattice constants was obsvered in steel over 100hrs oxidation at 500°C, while those of pure iron, and of steel measured by Co-K_α X-ray, under the same oxidation conditions, were nearly constant. The oxidation stresses of steel measured by Cr-K_α X-ray were found compressive after prolonged time oxidation in air at 500°C, but those of pure iron, and of steel measured by Co-K_α X-ray, remained tensile. The differences may be attributed to the compositional changes of minor constituents in steel near oxide/steel interface.

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