Paper reviews author's research results on the Acoustic Emission (AE) source wave analysis of delayed fracture and stress corrosion cracking (SCC), and discusses what contribution has the AE made to the corrosion research. Both the dynamics and progression of micro-fractures in hydrogen assisted fracture of high and low tension steels were quantitatively revealed by the source inversion or source wave analysis of AE signals. No theory or hypothesis proposed so far, however, appears to explain the extremely fast fracture and dilatation kinetics observed in hydrogen blistering of low strength steels. Transgranular SCC of austenitic stainless steel does not produce any AE, suggesting an anodic dissolution of metal along active paths, but intergranular SCC in solution and molten salts does produce AEs. In order to study the mechanism of environmental cracking, hybrid technique of AE and potential noise analyses is needed.
There are about 80, 000 above ground tanks for oil storage in Japan. Most tanks are constructed in a coastal industrial zone, therefore, corrosive environment is not mild. More than 50 leakage accidents in these tanks were reported in 2003 and about 60% of them are due to corrosion. In this report, recent trends of accident due to corrosion are described as follows. (1) Tendency of leakage accidents due to corrosion. (2) Relationship between corrosion rate and used years. (3) Introduction of recent research on evaluation of corrosion damage and protection for bottom plates.
The corrosion behavior of Si3N4 SiC, mullite, alumina and sapphire was investigated in supercritical water at 450°C and 45MPa for 2 to 50h. Corrosion resistance for the ceramics was as follows in the order; Si3N4<SiC<mullite<alumina<sapphire. Pitting corrosion with formation of amorphous layer and intergranular corrosion due to dissolution of additives were observed in PLS-Si3N4 and PLS-SiC, respectively. The corrosion behavior of mullite was characterized by dissolution of SiO2 and formation of boehmite residue layer. High purity alumina ceramics showed the highest corrosion resistance in the ceramics. Intergranular corrosion proceeded in the alumina ceramics and its corrosion rate was strongly dependent on the impurity content.
In order to understand the corrosion phenomenon of a 304 type stainless steel pipe that was used in a water filtration plant, the water inside the pipe was analyzed, and the distribution of corrosion location, the shape of corrosion pit as well as the corrosion morphology were observed. Besides, the potential of the corroded pipe in fresh water was measured. It was found that crevice corrosion occurred not only near welds but also at base metal and the flange of the pipe, although the chloride ion concentration in the water was less than 30ppm. Manganese oxide was observed on the inner surface of the pipe, which made the potential of the pipe higher than ER, CREV of 304 stainless steel, and thus induced the corrosion of the pipe. The manganese oxide was believed to result from oxidizing of Mn2+ in the water by chloric disinfectant. The shape of corrosion pit and its corrosion morphology in this investigation are similar to those observed at corrosion pit of MIC (microbiological induced corrosion) of stainless steels, which suggests that it is difficult to determine MIC from only the pitting shape and corrosion morphology.