Electrochemical characteristics of polythionic acid stress corrosion cracking have been examined mainly on sensitized Type 304 stainless steel. The test solution was prepared by passing sulfur dioxide gas into deionized water for 4 hours and then bubbling hydrogen sulfide for 3 hours. The rest potential of the steel lies approximately at -100mV (SCE) showing little change during initiation and propagation of cracks. When tested potentiostatically at various potentials, the susceptibility to cracking is greatest at -200 to +100mV, on which potential region the rest potential falls. Above 250mV the susceptibility decreases and beyond 950mV general corrosion accompanied by pitting is the main consequence. At a base potential of -400mV, on the other hand, the susceptibility still persists. Annealed Type 304 does not corrode intergranularly whether stressed or not. The sensitized steel shows intergranular attack which is accelerated by stressing. Evidences indicate that tests in the polythionic acid and in the standard acid copper sulfate solution are electrochemically equivalent.

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The similarity between results of dew-cycle corrosion tests and the results of field exposure has been evaluated by comparing the corrosion rate of mild steel with that of weathering steel at several dew-cycle conditions in 20ppm SO₂. It was known that both the similarity and acceleration of tests are, particularly in the case of steel, largely dependent on the drying process.
An attempt was made to convert the corrosion quantity vs. testing time curve, which was generally used, into the curve of corrosion quantity vs. total dewing time, that is testing time minus drying time. If Fe (II) rust, formed during wet or dew period, is sufficiently oxidized to Fe (III) rust in the process of drying, the slope of corrosion quantity vs. total dewind time should become sharper. This is because the reduction of Fe (III) rust to Fe (II) during the succeeding dew period induces the corrosion of steel. The period 30 minutes of light dewing (temp. of specimen: 35°C; atmosphere: 37°C, 100% RH) showed a good similarity between the test results and the results of field exposure. And the corresponding optimum drying period seemed to exist between 30 to 60 minutes.

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