A NOVEL METHOD FOR STUDYING THE CORROSION RESISTANCE AND MICROSTRUCTURE OF ENAMEL COATING MODIFIED ON HIGH-STRENGTH STEEL IN 3.5 wt.% NaCl SOLUTION

A new method was introduced to research the corrosion resistance of enamel coatings which were sintered at different temperatures in this paper.1–4 Scanning electrochemical microscopy (SECM) conjunction with capacitance-potential test and Mott–Schottky analysis technique were used to study the coating failure process and semiconductor conduction behavior of enamel coating in 3.5wt.% NaCl solution. The high strength steel (SAPH440) was selected as the metal substrate. The coatings which were sintered at different temperatures on the high strength steel were analyzed and their corrosion behaviors and microstructures were also measured by potentiodynamic polarization curves, Mott–Schottky analysis technique and scanning electrochemical microscopy.5–10 In this paper, five kinds of enamel coatings were prepared on high strength steel. The SECM results show that the Faraday current of the microprobe tip gradually decreases with the soaking time increasing, which indicates that the coatings become failure slowly with the invasion of water and aggressive ions. At the same time, the tip current was homogeneous during the immersion process, which reflected that the surface of the sample remained stable, no obvious bulges appeared. Capacitance–potential test and Mott–Schottky analysis7,8 show that the external ions and water molecules gradually penetrate into the coating; coating showed characteristics of n-type semiconductor as time increases, the capacitance increases gradually, the space charge layer thickness decreases, slope of Mott–Schottky curve decreased gradually which indicates that the failure of coating in the process of soaking slowly with the charge carrier density increases.