No abstract

To investigate the corrosion mechanism of magnesium alloys under atmospheric conditions, field exposure tests were performed. In a marine environment, AZ31B and AZ91D alloys experienced localized corrosion, including pitting and filiform corrosion. In order to reproduce such actual atmospheric corrosion in a laboratory experiment, we employed a wet-dry cyclic test at a constant dew point. From the results of the wet-dry cyclic tests, the propagation mechanism of localized corrosion on magnesium alloys has been discussed. Localized corrosion occurred during the wetting period, and the corroded sites thus formed were repassivated during the drying period. In the next wetting period, new localized corrosion took place, but the repassivated sites were not reactivated. The repassivated sites have higher corrosion resistance than the non-corroded part of the surface because of the barrier effect of the corrosion products. In the case of AZ91D, Al enrichment in the surface region of the metal matrix is responsible for this corrosion resistance.

Nanotechnology has attracted increasing interest in various research fields for fabricating functional nanomaterials. In this study, we investigated the effect of poly(vinyl alcohol) (PVA) addition on the formation and thermoresponsive properties of poly(N-isopropyl acrylamide)-based nanogels in aqueous dispersion polymerizations. During dispersion polymerization, PVA appears to play three roles: (i) it bridges the generated polymer chains during polymerization, (ii) it stabilizes the formed polymer nanogels, and (iii) it regulates the thermoresponsive properties of the polymer nanogels. By regulating the bridging effect of PVA via changing the PVA concentration and chain length, the size of the obtained polymer gel particles was maintained in the nanometer range. Furthermore, we found that the clouding-point temperature increased when using low-molecular weight PVA. We believe that the knowledge gained in this study regarding the effect of PVA concentration and chain length on nanogel formation will aid in the future fabrication of functional polymer nanogels.

To clarify the mechanism of atmospheric corrosion of aluminum alloys in practical environments, a cyclic wet-dry corrosion test, in which the dew point of the circulating air was maintained at a constant level, has been conducted. The atmospheric corrosion of AA1100-O and AA6061-T6 aluminum alloys was randomly initiated around dispersed intermetallics due to the galvanic coupling between the aluminum matrix and intermetallics. The corrosion rate of aluminum alloys in the first cycle was higher than in the subsequent cycles due to the initiation of localized corrosion. As the number of wet-dry cycles increased, the increase of the amount of corrosion loss on aluminum alloys gradually tapered off, which was mainly attributed to the propagation of localized corrosion initiated in the first cycle. In the wet-dry conditions employed in our test, once localized corrosion was initiated in the first wetting period, no new sites of corrosion occurred in the subsequent cycles.