Zn-Mg 합금코팅 강재의 흑화 및 내식 특성

Abstract

Demands from the consumers for the steel having better design and appearance are increasing as the industry develops. Also the demand for steel materials having a black colored appearance is increasing in various industrial fields requiring advanced design. Therefore black paint is most widely used when making black colored appearance. However external impacts can cause the paint peeling and deterioration ect., which may adversely affect decorativeness and corrosion resistance. Therefore, recently, a new method of blackening the plating layer itself has been studied. In this study, a Zn-Mg alloy coated steel material having different Mg contents was manufactured and a film with a blackened coating layer was prepared through steam treatment. And the steam treatment was performed at the temperature of 120 ℃ and 150 ℃, and it was confirmed that a more uniform black film was formed at 150 ℃. The morphology and elemental composition of the film were analyzed through FE-SEM and EDS, and the crystal structure and component phase of the coating film were analyzed both through XRD and XPS. In addition, in order to evaluate the blackening characteristics of the blackened film, brightness analysis, UV-VIS-NIR spectroscopy and PL spectroscopy were performed. To evaluate the corrosion resistance, natural immersion test, electrochemical polarization test and electrochemical impedance spectroscopic test were performed. From the results, it was confirmed that the surface of ZM10 and ZM20 was more uniformly blackened. And in the case of the coating layer having Mg content of 2 %(ZM2) and 5 %(ZM5), it was confirmed that a stripe pattern appeared on the surface. The blackening of the coating layer was partly observed due to defects in the Zn and Mg oxides during the steam treatment process, and it is believed that the defects created a new energy level in the forbidden band, resulting in light absorption in the visible wavelength region. As a result of corrosion resistance evaluation, ZM10 showed the best performance, followed by ZM20, ZM5 and ZM2 in order. In particular, in the case of ZM10 and ZM20, the oxide film showed high surface resistance, which is considered to be roused from the structural difference of the oxide film formed on the surface. ZM10 and ZM20 also showed excellent corrosion resistance due to their dense oxide films, whereas in the case of ZM2 and ZM5, a porous oxide film was formed and it is assumed that the corrosion rate is relatively fast. In this study, through the above results, the black color of the Zn-Mg coating after steam treatment is due to oxides and is thought to be due to defects occurring during the steam surface treatment process. The corrosion resistance was found to be the highest. Through the above study, it was possible to confirm the blackening mechanism of the Zn-Mg coating film and the corrosion resistance according to steam treatment, and to suggest a basic design guideline for the blackening film production.