It was experienced that the critical current density for passivation, i_crit, in anodic polarization curves for stainless steel in 5% sulfuric acid solution became significantly high and the pitting potentials in 3.5% sodium chloride solution became low when these solutions were deaerated by nitrogen pretreated with active copper at 170°C. It has been found that the nitrogen was contaminated by sulfur dioxide generated from active copper having a small quantity of impurity sulfur. The value of i_crit of Type 304 stainless steel observed in a 5% sulfuric acid solution contaminated with SO₂ is more than two orders of magnitude higher than that measured in the solution free from SO₂. Similar influence of sulfur dioxide was observed for Type 316, Type 430 and 18 Cr-2 Mo stainless steels. The addition of a small amount of hydrogen sulfide to 5% sulfuric acid solution caused a similar effect, i. e., it increases i_crit of Type 304 stainless steel significantly. In the measurement of pitting potential in acidified 3.5% sodium chloride solutions, hydrogen sulfide was also found to accelerate the active dissolution of stainless steels in the pH region near 1. It is suggested that the lowering of pitting potential by sulfur dioxide can be attributed to the lowering of pH by the formation of sulfurous acid and to the acceleration of active dissolution by the action of hydrogen sulfide formed.

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The principle of the corrosion monitor is based on the mathematical analysis and the experimental results on the impednaces of various corrosion systems reported previously. Following to this, the substruction of the impedance at infinite frequency from that at a low frequency yields the faradaic resistance R_c which is readily reduced to the corrosion rate of the electrode metal, if an appropriate selection was made on the frquencies.
Using a superposed signal of the sinusoidal waves of 10k and 0.01Hz, the corrosion monitor is so desined to give R_c or the corrosion rate dirctly on a recorder in logarithmic scale. Since a pair of parallel plates of specimens are used as electrodes without a reference electrode, the moniter is not only easy to operate but also it permits to measure the corrosion rate in less conductive solutions such as tap water or distillcd water. The corrosion rates hitherto tested has covered from 1.3mA (15mmpy) to 0.06μA (0.001mmpy). The error evaluated by weight loss measurement was less than 40% in the entire range of corrosion rate tested for various combination of metals and environments. Since the monitor gives either the corrosion rate of one specimen continuously or the corrosion rates of 10 specimens for a interrupted sampling time, it can be used for on-line monitoring of the corrosion rate in actual environment.

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Initiation of stress cracking of a solutionized Type 304 stainless steel was studied in 25%MgCl₂ solution at 80°C. At first, critical pitting potential (V_C), dissolution rates of pit-bottoms (I_h) at different potentials and repassivation potential for growing pits (E_R) were determined without applied stress. Under a static stress of 20kg/mm², stress corrosion cracks issued only from corrosion pits which continued to grow at potentials between V_C and E_R. In this potential range it has been found that crack growth rate, i, is larger than I_h, which had been found to be a requirement for crack initiation of this steel in 1N H₂SO₄ contatining 0.5M NaCl at 25°C under pulsating load condition.

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Properties of various kinds of hardly consumable electrode for electrolytic protection are explained. The relations between the composition of electrode material, current density, specific resistance of the electrolyte and the consumption rate of electrode material, etc, are discussed in detail.

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This review describes corrosion protection by metallic coatings. The method includes electroplating, electroless plating, hot dipping, vacuum plating, metal spraying, diffusion coating and facing. Combination of basis metal and coated metal is important to get effective protection.

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