In a previous study, self-healing polyurethane coatings with microcapsules containing healing agents have been developed for the protection of metal from corrosion. At the initial stage of the development of these coatings, we synthesized the microcapsules containing healing agents by adding organic solvents to inhibit the solidification of the whole capsule. In this study, the structure of the capsules formed with different solvents, such as chlorobenzene, toluene and xylene was examined. We also evaluated the self-healing ability of coatings with these capsules after damaging the coating by scratching.
The amount of healing agents contained in capsules depends on the kind and the amount of solvents added. In the capsule formed with 20 or 50 wt%-chlorobenzene, there is almost no healing agent of liquid phase, while in the capsule formed with 70 wt%-chlorobenzen healing agents are contained. Toluene and xylene are more effective inhibitors than chlorobenzene. The capsule formed with 70 wt%-toluene or xylene contains much more amounts of the healing agent of liquid phase than that formed with 70 %-chlorobenzene, and the coatings with the former capsules have an appreciable self-healing ability.
Effects of sensitization, relative humidity （RH）, and chloride concentration on stress corrosion cracking （SCC） behaviors at low temperature in atmospheric environments were studied by using U-bend specimens of as-received （cold-worked） and sensitized Type 304 steels. NaCl and MgCl2 salts were put on the specimens, and the specimens were set in the constant temperature and humidity chamber with 40～70% RH at 40°C for 720h. The results showed that the Type 304 steel became more susceptive to SCC when increasing humidity and chloride concentration. MgCl2 was more effective to SCC while NaCl was less effective. In the sensitized specimens, deep cracks were observed with small branches and the crack propagated through grain boundaries.
Cyclic corrosion tests, consisting of 0.86 kmol･m‒3 NaCl solution spraying, drying, and wetting process, were conducted using a crevice-structured spot-weld specimen of ferritic stainless steels in order to investigate the effect of alloying elements on the corrosion behavior. The addition of Ni to 18Cr1Mo ferritic stainless steels was the most effective in decreasing the maximum corrosion depth in the crevice. Mo was an effective alloying element and the increase of Cr was not effective. Ni and Mo acted as effective elements decreasing the critical passive current densities and the addition of Ni was the most effective in lowering repassivation pH in acidic NaCl aqueous solutions. The result that the maximum corrosion depth decreases remarkably in the crevice could be attributed to the benefitical effect of Ni that inhibit anodic dissolution and accelerate repassivation.