The corrosion behavior of Type 304 SS in formic acid was investigated by measuring the corrosion weight loss, the polarization curve and the impedance at the steel-solution interface. The results are summarized as follows. (1) The corrosion rate of the steel was found to change with the remarkable dependence on the concentration of formic acid and the temperature. (2) The corrosion potential (E_corr) showed a linear relationship against pH of formic acid solution, and the cathodic reaction was approximately first order with respect to the activity of proton. (3) The selective corrosion dissolution of Fe, Cr and Ni of this steel was not observed.

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The dissolved ozone effect on metal corrosion behavior in water was investigated by immersion tests at stagnant state, slow flow state and fast flow state, and by electrochemical measurements at stagnant state. Corrosion rates for low alloy steels, copper and brass increased with increasing ozone concentration at various water flow states. Carbon steel also showed the same behavior at stagnant state. This is because ozone acts as an accepter of electrons produced by the metal dissolution reaction. On the contrary, carbon steel in deionized water at fast flow state was passvvated by ozone. Ozone seems to have passivation ability, as oxygen. Stainless steels also gained stronger passivation state by the contact with ozone.

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In order to study the effect of Cr and Mo on the crevice corrosion of stainless steel in high temperature high purity water, both crevice corrosion tests in deionized water and electrochemical tests in dilute Na₂SO₄ solution have been conducted at 285°C on solution and sensitization heat-treated stainless steels with different Cr and Mo contents. Characterization of specimen surface has also been carried out using ESCA after crevice corrosion test in high temperature high purity water. Those results are summarized as follows:
(1) The corrosion mode at the crevice of solution heat-treated materials was uniform corrosion and the corrosion was reduced with the increase of Cr and Mo content in steel.
(2) The corrosion mode at the crevice of sensitization heat-treated materials was not only uniform but also intergranular, and the susceptibility to intergranular corrosion was eliminated by lowering the C content in stainless steel.
(3) Both the crevice corrosion and electrochemical behavior on solution and sensitization heat-treated materials suggested that the solution inside the crevice was acidic, and the existence of MoO₃ in the surface film at the crevice area of a specimen after exposed to high purity water sustained the above presumption.

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Eighty six ERW pipes that failed in practice by grooving corrosion have been examined in order to obtain metallurgical and environmental factors involved in the occurrence of this type of corrosion. The penetration rate is 12mm/y at maximum and 2mm/y in average. Higher penetration rates are associated with low specific resistance of the environment. However, the specific resistance is not a “go”-or-“no go” criterion because grooving corrosion still occures in environments of very high specific resistance, e. g., 10, 000Ω-cm. Chemical composition of steel has no apparent bearing with susceptibility to grooving corrosion. Galvanizing retards the initiation of grooving corrosion, but its beneficial effect is limited because the protective film is lost eventually by corrosion. The present knowledge does not allow to predict the occurrence of grooving corrosion in ordinary ERW pipes.

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The corrosion rate of pure iron in aqueous solutions has been examined by a square wave linear polarization method as functions of solution pH, amount of oxygen dissolved in solution, and salt concentration. A square wave current with the pulse width of Δt=30s (frequency f=0.017Hz) was adapted to measure the apparent polarization resistance, R_p+R_s, and it was interrupted for a short time less than 0.1ms at an interval of 1ms to estimate the solution resistance, R_s. From the polarization resistance R_p the corrosion current i_c of iron was estimated within 15% error in sulphate solutions in a wide range of pH from acid to alkali by using the following equation, i_c=K/R_p with K=0.025(V). In sulphate solutions of 0.5mol kg^-1[SO₄^2-] under the air-opened condition, the corrosion current of iron decreases with pH in acidic solutions (pH<4), whereas in neutral solutions (4<pH<10) the corrosion current is independent of pH. In alkaline solutions (pH>10) iron becomes passivated. Both salt concentration and dissolved oxygen concentration in sodium sulphate solution influence the corrosion current. With increasing the concentration of sodium sulphate, the corrosion rate decreases in aerated and oxygenated solutions but it increases in deaerated solutions.

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We already developed the nickel plating method. In order to anodize plated nickel to NiOOH, it needs more than 10 hours to reach less than 0.1μA/cm of residual current which is thought to be passive current of plated nickel. Such longer time of potentiostatic technique is due to the intense passivation by higher anodic current at early stage which disturbs furthermore passivation of the interior of plated nickel. The two steps method, i. e., the galvanostatic of 10μA/cm followed by the potentiostatic of 0V vs SCE, was found to shorten the passivation period up to 30 minutes and also to improve the detection sensitivity of hydrogen by one order compared with the previous method. In the fracture experiment to be submitted later, hydrogen content in steel to be measured is needed almost not to be affected by the flow rate of electrolyte and variation in cathodic current density. Such requirements were filled by the selection of 10mA/cm in a boric acidpotassium chloride solution.

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