Low-alloy steels containing a few percent chromium corrode less than plain carbon steels when exposed to aerated neutral waters including sea water, in spite of the general concept that the corrosion rate of carbon and low-alloy steels in natural waters is independent of their heat treatment or alloy composition. The superiority of Cr-bearing steels as tested in artificial sea water vanishes when part of the steel surface is plated with copper. This indicates that even if the rate of diffusion of oxygen toward the surface is the same, the rate of oxygen reduction thereon depends on the nature of the surface. The observed fact that the shift of the corrosion potential on stirring the water decreases with increased chromium content demonstrates that the corrosion product films formed on Cr-bearing steels provide a better barrier to oxygen diffusion. The diffusion coefficients of oxygen in the corrosion product films have been estimated using the timelag method, where time-lag was determined from the transient potential change of corroded steels upon stirring the water. D thus found is in the range of 10^-6-10^-7cm²/sec, the value being smaller for Cr-bearing steels.

抄録全体を表示

The Influence of steel composition and insulating gasket materials on the crevice corrosion of stainless steel piping has been investigated through the loop test and the polarization measurements in 5% NaCl (pH=3.0) at 50°C.
The result of the loop test showed that the crevice corrosion tendency was markedly varied with the composition of stainless steels and of gasket materials attaching to the steel. For the stainless steel composition, the tendency decreased in the order: SUS 24, SUS 28, SUS 33 and 18Cr-16Ni-5Mo. For the gasket material, it decreased in the order: chrysotile asbestos, blue asbestos, SBR and Teflon.
The polarization measurements were also made on the stainless steel specimen (crevice-free) and that coupled with gasket materials (crevice-containing). The current density ratio-potential curves showed the same tendency obtained from the loop test. The larger the ratio in the potential range from passivation to pitting potential, the larger the crevice corrosion tendency of the couple of a stainless steel and a gasket material.
The variation of the crevice corrosion tendency contributed by the gasket materials can be explained mainly on the swelling nature when immersed in the environmental solution, and on the adhesional wetting nature of the surface.

抄録全体を表示