クロメート皮膜の自己補修作用による亜鉛めっき鋼材の防食挙動とその機構

抄録

Although the exact mechanism of self-healing effect of chromate coating is still not clearly understood, this mechanism is known to prevent not only the corrosion of metal substrate under the chromate coatings but also from the coating defects. This research focuses on investigating the corrosion behavior of chromated electro-galvanized steels by using both the a.c. impedance method and the scanning vibrating electrode technique (SVET). The corrosion protection mechanism of the self healing effect was estimated by using XPS analysis.
The self-healing effect was clearly identified with the observed increase of charge transfer resistance when conducting the impedance measurement and the markedly decrease of both the corrosion area and corrosion current during the SVET experiments.
The results of XPS analysis indicated that the chemical conversion type chromate coating contains both Cr(III) and Cr(VI). It was found that the water-soluble constituent consisted of mostly Cr(VI). Moreover, the chromate film was observed on non-coated metal surfaces in aqueous solution also. This was because when the Cr(VI) ions were reduced at corrosion catholic region, the corrosion current increased at anodic region as the consequences thus rendering the passivation of the metal surface. The Cr (VI), was reduced at cathode, became Cr(III) which then reforming the chromate, the passivity film. Based upon these observations, the self-healing effect was confirmed via the effect of anode-inhibitor and filming type inhibitor.