Surface Potential Distribution in Corrosion Degradation Process of Organic Coated Steel
Authors: Itou Sayaka , Katayama Hideki , Akashi Takaya 
Published: February 2020

Organic coated steels are generally used in many applications such as cars, appliances and construction. Various methods to evaluate corrosion performance of organic coated metal have been suggested in the past, and for example, visual observation and gross measurement are well known as typical evaluation methods. Electrochemical impedance technique has been widely used for evaluation of under-film corrosion as subjective and quantitative evaluation method. However, the normal technique provides little information on the shape and the location of deteriorated area of organic coated metals. In this work, we focused attention on surface potential measurement which can detect surface reactivity. The purpose of this study is to evaluate the usefulness of surface potential measurement as a non-destructive evaluation method for degradation of coated steels.
Carbon steel plates (50mm×50mm×2mm) were used as substrate metals. The metals were coated with clear acrylic lacquer spray. The surface potential was measured by surface reaction measurement device (RM201, Hirocon) having a X-Y stage. The surface potential of the sample with the coating thickness of approximately 10µm was measured at different distance between the sample and Au-probe in order to examine an influence of the distance.
Surface potential distribution measurement was performed on the model sample for under-film corrosion. Wet-dry cyclic corrosion test was carried out on the substrate metal by alternating conditions of adding 100µL droplet of 0.5M NaCl solution and drying at 293K and 20%R.H. After 8 cycles, the surface was coated. The coating thickness was approximately 90µm. Surface potential distribution was measured on the sample in laboratory environment (293K, 30%R.H.). The measurement range was 30mm×30mm including the corroded area. The surface profile was also measured with Laser Displacement Sensor (LK-G3000, KEYENCE).
Organic coated samples were prepared with about 20µm in thickness. The wet-dry cyclic corrosion test was carried out to these samples. The degradation degree of each sample was different by the repeated cycle number of the corrosion test. The surface potential and profile distribution were measured after corrosion test. For comparison, electrochemical impedance (EIS) measurement was conducted.

The sample surface exposed to corrosive environment changed to lower surface potential than the other area. The size and position of the area where surface potential changed corresponded to degradation area. It means that surface potential measurement can detect degradation areas of coated steel even under organic coating.
The wet-dry cyclic corrosion test of the organic coated samples was carried out. After several cycles, a part of the surface showed lower potential than the surrounding area. There were a good correlation between the surface potential change area and the organic coating degradation area. The surface potential changed in two stages in degradation process. The first stage corresponds to the gradual change to -0.1V, and the surface potential decreased down to -0.6V. EIS result showed that coating degradation and under-film corrosion occur in the first and second stage, respectively.

This result suggests that the degradation process of coated steels can be monitored by surface potential measurement. It is concluded that surface potential measurement is an useful tool to evaluate degradation of coated steels.