Effect of minor elements on the intergranular stress-corrosion cracking (IGSCC) of carbon steel in an aqueous amine solution has been studied using slow strain rate testing (SSRT) technique at potentials in an active-passive transition range. A series of carbon steels with varying content of minor elements such as C, N, P, S and Si was used as specimens. The SSRT was carried out at a strain rate of 1.11×10-6/s in a 20mass%MEA solution of pH8 at 333K. It was found that the IGSCC of the carbon steels depended upon potentials and susceptibility maxima to the IGSCC were attained at potentials in the center of active-passive transition ranges. The susceptibility to the IGSCC increased with decreasing C and N content and with increasing P and Si content of the carbon steels. The susceptibility to the IGSCC hardly changed with increasing S content of the carbon steels.
Paid particular attention to the characteristics of rust layers, this study was conducted to research the corrosion behavior of low-alloy steel containing small amounts of Cu and P in the saline environment which composes cyclic variation between wet and dry periods. Corrosion resistance of the low-alloy steel depends on the relative humidity of atmosphere in dry periods. According to the results of the X-ray diffraction analysis of rust layers and polarization measurements, cathodic reaction of the low-alloy steel exposed to this environment is reduction of β and γ FeOOH to Fe3O4. EPMA analysis of rust layers indicates that localized corrosion of low-alloy steel proceeds in chloride nest to which chloride ions are concentrated. The tight rust layer containing Cu acts as effective barrier to the permeation of chloride ions.
Through the analyses of sensory evaluation of model panels simulating rusted stainless steels, it became clear that perception of human being is affected not only area ratio of spots but also their size and numbers. Based on this result, the quantitative evaluation method to obtain the level of dirtiness was developed using the plot of the density of rusty dots against their area ratio, in which the average size of rust can be calculated from the slope of a straight line connecting each datum point and the origin. Furthermore, it was found that the points of the same aesthetic category are plotted in a zone limited by straight lines with slope of ca. -1. By this approach, three stages of weathering process of stainless steels are first characterized as follows; (1) nucleation of pits from which increase in the number of small spots ensue, (2) expansion of individual rusty dots, and (3) coalescence. Aesthetic degradation of the stainless steels is appreciable up to the stage (2), while not so noticeable when in the process (3).
In this study, effects of alloying elements such as Mo, Cr and Ni on intergranular corrosion (IGC) resistance of Mo contained austenitic stainless steels in boiling 65% nitric acid solution (Huey Test) were studied. Obtained results are as follows; (1) Mo deteriorates IGC resistance of austenitic stainless steel in Huey test due to enhance precipitation of Laves ((Fe, Cr)2Mo) phase at grainboundary. (2) Cr improves IGC resistance of Mo contained austenitic stainless steel in Huey test, which is considered to reduce dissolution rate of Laves phase into nitric acid solution by increasing Cr content in Laves phase as increasing Cr content of stainless steel. (3) Ni suppresses occurrence of IGC of Mo contained austenitic stainless steel in Huey test.
Corrosion behavior of copper and adjacent stainless steel was evaluated in the test loop simulating cooling systems of an electronic instrument and others. Laboratory tests were conducted in low conductivity pure water at 40°C. These test results showed that the corrosion of copper was remarkably suppressed at 0.1μS/cm of water conductivity. This behavior was assumed to be due to stabilization of oxide films formed on copper in this conductivity region. Pitting was caused on adjacent stainless steel (Type 316L) in water containing more than 10ppm copper ions. As metallic copper was deposited around pits, it was assumed that the corrosion of stainless steel was progressed through the reduction process of Cu2+ ions. Based on the results, a water control procedure of cooling system was developed.