Erosion-corrosion attack of stainless steels was investigated in aqueous solution containing chloride and sulfide, and effects of material factors on the relation between electrochemical characteristic and erosion-corrosion resistance were elucidated. Erosion-corrosion attack of solution treated steels was influenced by pH of solution and alloy composition. This attack depended not on hardness but on corrosion resistance. Although an excellent relation is obtained between erosion-corrosion weight loss and pitting potential at constant pH, the relation varied with pH. No dependency on pH was observed for the relation between the weight loss and the repassivation potential, thus this potential is considered to be valid for evaluation of erosion-corrosion resistance. For sensitized steels, this evalutation by repassivation potential was valid to N bearing stable austenitic steels, but it is not applicable for the steels susceptible to selective attack due to carbide or σ phase precipitation.

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An automatic measuring system of SCC crack propagation rate was developed. Crack length was obtained as a variation of an electric resistance of the specimen, in which the electric resistance was obtained from the voltage measurement under the constane current of 1A. Whole system for the measurement is composed of a personal computer system (PC-8801, PC-8049N, PC-80S 31, PC-8023C), interface system (8255, ADC) and test machines (4 slow strain rate machines). A variation of electric resistance of an 1mm thick Type 304 stainless steel exposed in air at room temperature without stress was within 0.5 micro-ohm. This corresponds to 30-40 micro meter of crack length of the specimen. Stress corrosion test of Type 304 stainless steel was carried out in boilling 42% MgCl₂ solution by using a slow strain rate technique. The results showed a good agreement with those obtained in previous studies.

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In order to discuss the probability distribution of intergranular stress corrsion carcking life for sensitized 304 stainless steels, a series of the creviced bent beem (CBB) and the uni-axial constant load tests were carried out in oxygenated high temperature, high purity water. The following concludions were resulted; (1) The initiation process of intergranular sterss corrosion cracking has been assumed to be approximated by the Poisson stochastic process, based on the CBB test results. (2) The probability distribution of intergranular stress corrosion cracking life may consequently be approximated by the exponential probability distribution. (3) The experimental data could be fitted to the exponential probability distribution.

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Repassivation potentials, E_R, were measured in 0.5mol/l NaCl solution at 30°C for PTFE/metal-crevice of surface treated stainless steels which had been dipped for 6h in 61% nitric acid containing 40g/l Na₂Cr₂O₇·2H₂O at 50°C. Under usual measuring conditions, or in case of ordinary extent of previous growth of crevice corrosion, the E_R values obtained were constant and coincided with critical crevice corrosion potential, V_crev, above which crevice corrosion could initiate and continue to grow. These facts confirm that the concept of E_R as a well defined characteristic can be applied also to surface treated stainless steels. As the second stage of the study effects of variables in surface treatment procedures on E_R values were determined. Most noble values of E_R were obtained for steels which had been dipped for 6h in 61% HNO₃+(20-40)g/l Na₂Cr₂O₇·2H₂O solutions at 50°C. Such optimum conditions as evaluated through E_R measurements would be effective to improve crevice corrosion resistance for the steels.

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As corrosion failures of 316L in urea synthesis solutions are due to either the active region dissolution or the corroison at the transition zone from the passive to the transpassive region, effects of process variables of the urea reactor on these two types of corrosion were electrochemically examined in order to advance the reliability of corrosion preventive methods. For making simple methods to evaluate resistance of 316L to the corrosion at the transition zone from the passive to the transpassive region, seven 316L specimens were immersed in an urea synthesis solution and the results were discussed by comparison with the results of Huey test and chemical compositions of the test specimens. The conclusions obtained are as followws.
(1) 316L for the urea reactor required more amount of air to be injected for preventing active dissolution as the temperature of the reactor rose or the molar ratio of H₂O/CO₂ increased.
(2) Sulfide, an impurity in CO₂ gas, had a very detrimental effect on passivation of 316L. As it is not practical to compensate the harmfulness of sulfide by increase of the amount of air injection, the content of sulfide in CO₂ gas should be lowered before feeding to the urea plant.
(3) For reducing the corrosion at the transition zone from the passive to the transpassive region, higher molar ratio of NH₃/CO₂, lower ratio of H₂O/CO₂ and lower temperature were advantageous.
(4) Sulfide was also harmful to this corrosion.
(5) Annealed 316L was subject to weight loss owing to general corrosion and intergranular corrosion simultaneously at the transition zone from the passive to the transpassive region. Resistance of 316L to these two types of corrosion were correlated with Cr content in 316L and results of Huey test, respectively.

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On the practical procedure to measure the repassivation potential E_R of crevice corrosion proposed by professor Tsujikawa and his groups, anodic reaction in the crevice at the E_R and mass transport from the crevice during the measurement were discussed. It was pointed out that anodic dissolution still continued at the E_R for a severe crevice condition and the E_R was affected by mass transport and dilution of the solution inside the crevice.

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