Corrosion inhibitors suppressing generalized corrosion of metals have been reviewed in the earlier articles, “Action of Corrosion Inhibitors (Part 1) and (Part 2)” [Zairyo-to-Kankyo, 56, 243 and 292 (2007)]. Actions of corrosion inhibitors for local corrosion, stress corrosion cracking, hydrogen embrittlement, corrosion-wear, corrosion of rebar in concrete and corrosion in non-aqueous solutions, and also of additives in rust preventing oils, scale inhibitors, oxygen scavengers, and volatile corrosion inhibitors are described in this review.
We developed a new heat-resistant optical fiber AE sensor and a homodyne Mach-Zender type laser interferometer to monitor the acoustic emission (AE) from hot members. The optical fiber coated with copper was used as a heat resistant sensor, and AEs from hot pipes were measured up to 600°C for 108 ks. The sensor was found to monitor the longitudinal mode cylinder wave which conventional PZT sensor can not monitor, when the sensing fiber was wound on the pipe. The system was utilized for monitoring AEs from crack and exfoliation of non-protective scales produced by catastrophic oxidation of Type 304 stainless steel pipe at 700°C. The cylindrical AEs were classified into two fracture types, i.e., Mode-I fracture with a crack opening vector in the circumferential direction (scale fracture) and in the radial directions (exfoliation). A number of AEs detected during cooling was found to be produced by Mode-I crack of the scales. Their sources were located using arrival time differences of L(0,2) and F(1,1) modes at 80 kHz and found to be in the zone with non-protective scales.
In order to make clear the effects of residual stress and hardening on intergranular stress corrosion cracking (IGSCC) behavior in the welds of Type 316L low-carbon austenitic stainless steel with surface hardening, the residual stress and hardness in the butt-joint of pipes as a typical example of the actual structure were estimated and the grain boundary sliding was analyzed from the viewpoint of micro-deformation. On the basis of these results, the mechanism of IGSCC was discussed by the integrated knowledge between metallurgy and mechanics. The relationship between plastic strain and hardness in hard-machined surface near welds was clarified from the experimented relationship and the analysis method by the thermal elastic-plastic analysis. The distributions of hardness and residual stress with the actual surface machining could be simulated. It was made clear that grain boundary sliding occurred in the steel at 561K by a constant strain rate tensile test. From the comparison of grain boundary sliding behavior between solution treated specimen and cold-rolled one, it was found that the grain boundary sliding in cold-rolled one occurs in smaller strain conditions than that in as received one, and the amount of grain boundary sliding in cold-rolled one increases remarkably with increase in rolling reduction. In addition, it was clarified that the grain boundary energy is raised by the grain boundary sliding. On the basis of these results, it was concluded that the cause of IGSCC in the welds of Type 316L low-carbon austenitic stainless steel with surface hardening is the increase in grain boundary energy due to grain boundary sliding induced by residual stress of multi pass welding and surface hardening.
The long term hydrogen absorption behavior and the possibility of hydrogen embrittlement were studied for titanium overpack for high level radioactive waste disposal. The results of galvanostatic cathodic polarization tests showed that as the cathodic current density was decreased, the amount of absorbed hydrogen for a constant amount of cathodic charge was increased and hydrogen permeated into inside of titanium specimen. The results of mechanical property tests of hydrogen-charged titanium specimens showed that the most remarkable embrittlement was observed in the titanium specimens in which hydrogen was distributed uniformly. The amount of absorbed hydrogen for 1000 years in titanium overpack of 6 mm thickness was estimated to be about 400 ppm. It was expected that the rupture of titanium overpack with uniform hydrogen distribution would be initiated in case that the crack size in titanium was over about 2-3 mm under the stress condition correspond to yield strength.