This study was carried out to examine a possibility of evaluating the SCC susceptibility of stainless steels in high-temperature water by EPR test (Electrochemical Potentiokinetic Reactivation method). The effect of alloying elements of carbon, nitrogen and molybdenum on the extent of sensitization was evaluated by EPR test, and the correlation between EPR test result and SCC susceptibility for the steels was examined. The EPR test was conducted in 0.01M KSCN+0.5N H₂SO₄ solution, and critical passivation current density (I_a), maximum reactivation current density (I_r) and reactivation ratio (I_r/I_a) were measured. The SCC susceptibility was evaluated in terms of the crack depth observed after the CBB test (Creviced bent beam test) in oxygenized high-temperature water at 250°C for 310h. The conclusions obtained were: (i) the correlation between the SCC susceptibility and I_r or I_r/I_a is not affected by the content of carbon or nitrogen. (ii) molybdenum suppresses the active dissolution of the steel and influences I_a and I_r, for while the ratio I_r/I_a is well correlated with the SCC susceptibility.

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In order to study effects of temperature and applied stress on failure time, stress corrosion process and fracture mode, stress corrosion tests of SUS 304 stainless steel have been carried out in 28% sodium chloride solution at 106° to 250°C. The results obtained are as follows:
1) Time to failure is longer than that in 42% magnesium chloride solution, since stress corrosion cracks initiate at growing pits in 28% sodium chloride solution.
2) Apparent activation energies Q_i and Q_c for the crack initiation and propagation process are estimated to be 2.9 to 4.3kcal/mol and 6.2 to 8.5kcal/mol respectively, by which it is concluded that the stress corrosion process is controlled by the diffusion of reactive species in solution.
3) According to Ohtani's mechanochemical model, apparent activation energy ΔH for the anodic dissolution process of newly formed slip planes is evaluated to be 6kcal/mol which agrees well with the values already reported in regard to stress corrosion cracking of 18-8 stainless steel in magnesium chloride solution.
4) Regardless of temperature and applied stress, the fan shaped fracture is found as the general feature of the fracture mode on the initial stage of crack growth. This fracture mode shows that cracks grow due to the preferentially anodic dissolution along a paticular crystallographic orientation and tearing by stress. Consequently it is concluded that the mechanism of stress corrosion cracking in 28% sodium chloride solution is almost as same as that in concentrated magnesium chloride solution.

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Cathodic protection with an external current was applied to prevent stress corrosion cracking (SCC) in a reactor vessel made of SUS 316L stainless steel containing an organic acid with 0.5%Cl^-, pH 3 at 95°C. Transgranular SCC occurred mainly along the welded zone on the vessel wall after about 1 year operation without the protection. Initiation of the SCC was assisted with pitting and crevice corrosion caused by the deposition of an organic scale. A protection potential of -0.42V vs SCE and a required current density of about 10μA/cm² were obtained by electrochemical measurements both in laboratory and field experiment. A magnetite and a pure zinc were used as the anode and the reference electrode, respectively. The SCC has been prevented successfully over 4.5 years after application of the catholic protection.

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Using plasma jet spray technique, several spinel ferrites (magnetite, nickel- and cobalt-ferrites) were coated to about 50μm thick on titanium for the purpose of developing an insoluble anode for catholic protection. Anodic behavior of the anodes was measured in sodium chloride solutions. Magnetite showed the lowest polarization at gas evolution potential, while its dissolution rate was higher than that of other ferrites. Chemical analysis of anodic gas showed preferential chlorine evolution, similar to RuO₂ electrode. The dissolution rate of the ferrite coated anodes ranged 0.1 to 8.7g/A·Y, comparable to that reported for sintered ferrites. Tantalum undercoating on titanium surface gave improved adhesion between ferrite layer and the substrate.

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