Corrosion resistance of ferritic and austenitic stainless steels has been investigated by using spot-welded specimens and TIG-welded specimens by long term corrosion tests at 80°C at the interface of air and chloride solutions containing 1, 000 and 21, 000ppm Cl^-. The corrosion activity of the steels during the test period was followed by galvanic couple current between the test specimens and the crevice-free specimens prepared from the same kind of the steels. Corrosion of stainless steels at air-chloride solution interface was significantly accelerated in comparison with that completely immersed in solution. Pitting corrosion of ferritic stainless steels, Ti stabilized 17Cr steel and Ti stabilized 17Cr-1 Mo steels, which had become inactive in early period at total immersion tests, continued throughout the testing period to lead to severe failure at the interface of air and 1, 000ppm Cl^- solution interface. 18Cr-2 Mo steel and 26Cr-1 Mo steel, however, remained almost free from attack under the same conditions as described above. At the interface of air and 21, 000ppm Cl^- solution only 30Cr-2 Mo steel showed corrosion resistance. In austenitic stainless steels, type 304, type 304L and RXM 15 containing 3% Si showed a pronounced corrosion throughout the test period of 31 weeks. Typical stress corrosion cracking occurred in all runs of type 304 steel, but slight stress corrosion cracking took place in type 304L steel only on a run at the interface of air and 1, 000ppm Cl^- solution. No stress corrosion cracking occurred on RXM 15 steels, while there were typical crevice corrosion failures. A slight stress corrosion cracking was also observed at spot-welded specimens of type 316 steel, but no stress corrosion cracking except slight pitting attack was observed in type 316L and type 310S steels.

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A new sand-erosion testing method has been developed which uses a magnetostriction vibratory unit commonly used for the accelerated cavitation erosion testing of materials. The test specimen is vibrated axially with a high frequency in a sand-bed instead of liquid. Two types of sand-bed have been devised; one is a fluidized-bed in which sand particles are suspended uniformly in an upward flow of water. Experiments showed that the erosion in this test was due to particle impact and not cavitation. The other is a fixed-bed of wet particles, which is pressed down to prevent the particles mutual movement. Regardless of the type of the sand-bed adopted, the vibratory sand-erosion testing facilities have the following features. (1) The amount of sand used, the specimen size and the power consumption are all small. (2) The damage develops only on the testing surface of the specimen. No other part of the facility is damaged. (3) The reproducibility of the test results is excellent. (4) The erosion rate can be controlled. Accelerated erosion testing is possible. (5) The entire testing surface is uniformly damaged.

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In order to evaluate the effect of temperature on the thickness and structure of passive films on a Mo-bearing austenitic stainless steel, ellipsometric measurements coupled with electrochemical techniques have been performed on a 17Cr-13Ni-3 Mo steel, and also on a 17Cr-13Ni steel for comparison, in a neutral 0.3mol/l-Na₂SO₄ solution at various temperatures ranging from 274 to 348K. From the measurements under anodic polarization, it was found that with an increase in temperature of the solution and/or Mo content of the steel, the thickness and growth rate of passive film increased, whereas the real and imaginary parts of complex refractive index of the film decreased. The analysis of variations in ellipsometric parameters during the cathodic reduction of the film, and also during the dissolution of the film in acid, revealed that the film was a two-layer structure; the outer layer of a precipitated Fe (III) hydroxide and the inner layer of an ordinary passive film. It is concluded that the increase in thickness of the film with rising temperature is mainly due to growth of the outer layer. These experimental results are discussed with reference to the corrosion resistance of the stainless steel at elevated temperatures.

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The oxidation behavior of Ni-20 Cr-3 Si alloys containing 0.004, 0.012 or 0.12% Ce was studied for periods up to 200h in air at 1273 and 1473K by weight-change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The oxides predominantly formed on the alloys were Cr₂O₃ and SiO₂. The surface oxides formed on Ni-20 Cr-3 Si and Ni-20 Cr-3 Si alloys containing 0.004 or 0.012% Ce spalled initially at the grain boundaries. Spalling of the surface oxide of these alloys occurred mainly at the oxide layer/alloy interface for the oxidation time longer than 30h at 1473K during cooling. On the other hand, the weight gain and weight of spalled oxide of Ni-20 Cr-3 Si alloy with 0.12% Ce markedly decreased compared with those of the other alloys.

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Some questions of repassivation potential, E_R, were considered by using a nut/nut crevice of Type 316 steel which is similar in design to the geometry of a real crevice. The first is the variation of E_R with the extent of previous growth as has been confirmed in the case of E_R against pit propagation. Cyclic polarization procedure to determine E_R was conducted in such a way that potentials were changed in steps of 10mV every 10 minutes at relatively noble potentials and in steps of 10mV every 1 hour in the potential range where anodic currents decreased less than 5μA. The E_R for the crevice in 3% NaCl solution at 25°C measured -0.30±0.01V, SCE and did not depend on the amount of electricity passed during previous growth processes. This fact suggests that there exists well reproducible repassivation potential for any crevice. The second question is the relation between “repassivation” and “initiation” potentials. The E_R obtained above coincided with critical potential, V_crev, below which crevice corrosion does not initiate within 10³min. The latter result cannot exclude the possibility of repassivation potential as a crevice characteristic which covers initiation of crevice corrosion. Effects of surface roughness on crevice corrosion were also examined. Variation of surface finish of nuts in the range of SiC grit numbers from 80 to 1200 made no significant difference in the E_R values obtained.

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