Pitting potential V′_c·100 of austenite-ferrite duplex phase stainless steel SUS 329 J1 and austenitic stainless steels SUS 316, 317 and 317 J1 were measured in 3.5% NaCl solution under heat transfer condition with parallel flow system. A direction of the heat transfer from the back surface to the front surface of the specimen which contacts with the 3.5% NaCl solution was specified as (+) heat transfer in this experiment.
V′_c·100 of every stainless steel shifted to a less noble potential with increasing flux of the (+) heat transfer. The values of V′_c·100 under a flux of 10°C/2mm showed marked less noble ones at the flow velocity of 0.1m/s and became more and more noble with increasing flow velocity toward the values obtained under a flux of zero.
Alloying elements Cr and Mo were effective to resist to the lowering of V′_c·100 value under the heat transfer condition.
The results obtained above do not extrude the practical experience of SUS 329 J1 for sea water heat exchanger.

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The rusting of ball bearings caused by vapour from the insulating varnish in electric motors was investigated by measuring the corrosive acid vapour using ion chromatographic analyser. It was found that formic acid evolved depending on the kind of varnishes, and that the amount had good correlation with the extent of rusting of bearings tested in a relative humidity of 70 percent. Based on the limit of formic acid present, a quantitative evaluation of varnish-induced rusting and classification of varnishes are presented.

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Stress corrosion cracking tests of Type 310S steel were conducted potentiostatically in neutral 15-35% MgCl₂ solutions at 80°C using smooth or notched round-bar specimens. The object of this study is to compare crevices with pits as initiation sites for SCC cracks and consider the meaning of higher concentration of MgCl₂ solution more easily to occur SCC. Before SCC tests, repassivation potential for pit propagation, E_{R, PIT}, and that for the notch as a corrosion crevice, E_{R, CREV}, were determined as functions of MgCl₂ concentration. Even the value of E_{R, PIT} did not depend on pit radius as an extent of preceding growth of pitting corrosion in such higher concentrations of MgCl₂ solutions. Dissolution rate at pit-bottom, I_{h, PIT} was found to be expressed as
E=α+βlog I_{h, PIT}
where α and β are constants which depend on MgCl₂ concentration. Dissolution rate at crevice, I_{h, CREV}, was taken to be equal to that of I_{h, PIT} at corresponding electrode potential and MgCl₂ concentration. SCC in neutral chloride solution required local anodes as initiation sites which are pits for smooth specimens and crevices for notched specimens. Crack growth rate, l, did not depend on MgCl₂ concentration, electrode potential and types of crack initiation. SCC region in terms of dissolution rate, I_h, at local anode to initiate cracks was written as
I*_{h, i}<I_h<l
or in terms of electrode potential, E, as
E_{R, i}<E<E^V
where I*_{h, i}(i=PIT, CREV) is the value of I_h at just above E_{R, i} and E^V is the potential where I_h equal to l. E_{R, CREV} which was less noble than E_{R, PIT} enlarged potential range for SCC of notched specimen to much less noble potentials than that of smooth specimen. This is one of marked features of crevices compared with pits. There existed critical MgCl₂ concentration, C_i*(i=PIT, CREV), below which SCC cracks would not occur. C*_CREV was much lower than C*_PIT, which is another marked feature of crevices compared with pits.

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Active dissolution of stainless steels in an urea plant solution was investigated by immersion test and electrochemical measurement, since the corrosion causes proposed up to now are in conflict with some important behaviors of corrosion. The corrosion rates of stainless steels in a deaerated urea solution at 195°C increased with nickel content and decreased with chromium content, and particularly 316L steel corroded at the rate of 60.6mm/y. The detrimental effect of nickel on the active dissolution was ascribed to the effect of ammonia in the urea solution, because a nickelbearing austenitic steel had higher critical passivation current than a nickel-free ferritic steel and also nickel never passivated in an ammonia solution. A potential-pH diagram was calculated for nickel-ammonia-water system. The diagram demonstrated that nickel was potentially oxidized by hydrogen evolution reaction and dissolved as amine complex ions in pressurized solutions with high concentration of ammonia. Cathodic currents were measured in ammonium sulfate solutions and potassium hydrogencarbonate solutions, since urea solution was estimated to be rich in ammonium ion and hydrogencarbonate ion. The cathodic current increased not with ammonium ion concentration, but with hydrogencarbonate ion concentration. It was supposed that hydrogencarbonate ion would be involved in cathodic reaction of corrosion in the urea solution.

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Corrosion and its inhibition of oil and gas wells are briefly discussed forcusing on sour wells, concerning inhibition mechanisms of corrosion inhibitors and the several case histories of inhibitor treatment of hostile environment wells.
Sulfur compounds as potential inhibitors for sulfide stress cracking are proposed.

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