The corrosion protective property of various covering materials of steels for retaining wall was investigated by the electrochemical measurements and the weight loss test for five years. Moreover, the early evaluation of the corrosion protective property in various corrosion environments was also examined by the room test. In the field test, the aluminum spray coating on steel plate and the nontar epoxy paint on bolt nut showed the best corrosion protective property for five years, because their appearances were little change and the weight loss was small. The corrosion protective property of anchor was enhanced by using jointly together with the catholic protection. On the other hand, in the room test of various corrosion environments, the aluminum spray coating on steel plate showed the best corrosion protective property as well as the result obtained in the field test. But the cheapest hot dip galvanizing on steel plate did not last the excellent corrosion protective property in sea sand of the severest environment. The corrosion protective property of uni-chrome plating on bolt nut did not appreciably last in sea sand.

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In regard to the austenitic stainless steels, the interrelations among the corrosion resistance, pickling and nonmetallic inclusions such as MnS have been investigated metallographically in solution of L-ascorbic acid (C₆H₈O₆) as a food additive by an electrochemical method. In case of SUS 304 and SUS 316 stainless steels containing just a small quantity of MnS, the corrosion potential, E_corr, and the critical current density for passivation, i_crit, of each specimen were resulted in further noble value and smaller than those of the steels of the same kind containing a large quantity of MnS by pickling in the solution of 12%HNO₃+1%NH₄F, respectively. That is to say, the pickling inhibits the anodic dissolution reaction in the active region. On the other hand, as for SUS 303 and SUS 304 containing a large quantity of MnS, MnSe or Ca, E_corr and i_crit were resulted in the value of the most noble and smaller by pickling in 12%HNO₃ solution, respectively. In all of the specimens pickled in the solution of 12%HNO₃ or 12%HNO₃+1%NH₄F, the pitting potentials, Vc′₁₀₀, came to further noble value than those of the specimens pickled in 30%HNO₃ solution, because the surface inclusions such as MnS that act as the starting point of pitting were dissolved by the pickling in the solutions of the two varieties of the former.

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Impedance characteristics of corrosion at asphalt-sand/steel interface were studied on laboratory scale tests and model tanks of 2m in diameter. The equivalent circuit and impedance were interpreted in a similar manner to the under film corrosion of coated steels. Degradation of asphalt-sand layer was evaluated from decrement of film resistance R_f, and the corrosion rate was estimated qualitatively from the impedance R_cor measured at the lowest frequency range around or less than 10mHz. It was confirmed that the main factors to increase the corrosion rate of tank bottom were penetration of water precipitated and oxygen into the asphalt-sand/bottom plate interface. The two frequencies impedance method, proposed previously by the authours, can be applied as monitoring corrosion of fuel tanks, being supported by these impedance characteristics.

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It was theoretically shown in a previous paper (1) that ac impedance technique can be successfully utilized for determining and monitoring the optimum cathodic protection potential of the metals in a neutral solution. In this paper, the experimental studies have been made in order to support the theoretical analysis. The potential dependence of the faradaic impedance of carbon steels in 3.5% NaCl solution has been measured by ac impedance technique. The faradaic impedance showed a maximum at -0.70V with respect to sat. Ag/AgCl electrode. The cathodic protection efficiency obtained from weight loss also showed a maximum at the same potential, Therefore, the potential of faradaic impedance maximum corresponds well to the optimum cathodic protection potential, as the theoretical analysis has shown. It was concluded that the ac impedance technique is a promising method for determing as well as monitoring the optimum cathodic protection potential.

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Metallic materials may suffer various kinds of damage, depending on the using conditions and environments. Corrosion damage such as uniform corrosion, local corrosion like pitting or stress corrosion cracking can be observed in various manufacturing plants and is recognized to be one of the main reasons of their troubles. In the present paper the application of nondestructive testing methods, which are generally used, to the corrosion damage detection is reviewed. New ultrasonic testing techniques, although in an experimental stage, are also presented in relation to the detection of stress corrosion cracking.

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