Exposure Test of Various Zinc-Coated Steel Sheets and Corrosion Resistance Evaluation of Zinc-Based Corrosion Products in Japan
Authors: Fujihashi Kenta , Oshikawa Wataru 
Published: December 2024

Hot-dip zinc-coated steel sheets are widely used because of their high corrosion resistance, long-lasting effects, and low cost. In addition, aluminum and magnesium have been added to hot-dip zinc-coated steel sheets to improve corrosion resistance, and their use is expanding to supported stand of photovoltaic modules and gratings for infrastructure facilities. However, the corrosion resistance of each plated steel sheet has not been evaluated and only a few studies have compared them. Therefore, 10-year exposure tests were performed at six locations in Japan and the corrosion weight loss of each plated steel sheet was measured and compared with those of other plated steel sheets. In addition, the polarization curves of unexposed plated steel sheets and those exposed for five years were measured by linear sweep voltammetry (LSV), and the changes in corrosion resistance due to the corrosion products were verified.
Exposure sites were selected based on airborne salinity, the presence or absence of highways (in industrial areas), and the presence or absence of volcanoes. Exposure began in April 2013; specimens were collected 1, 2, 3, 5, and 10 years later. The exposed specimens were hot-dip zinc-coated (Zn), hot-dip zinc-5% aluminum alloy-coated (5% AlZn), hot-dip 55% aluminum-zinc alloy-coated (55% AlZn), and hot-dip zinc-aluminum-magnesium alloy-coated steel sheets (ZnAlMg). The collected specimens were observed externally, photographed, and their corrosion weight losses and electrochemical properties measured. The corrosion weight loss was measured by immersing the collected specimen in a chromic acid-based rust-removal solution to remove the corrosion products, and subsequently comparing the weight of the specimen before and after the removal of the corrosion products. However, when the corrosion products of carbon steel as the matrix (red rust) were obtained over the entire area of the specimens, the plating layer was regarded as almost dissolved, and the corrosion weight loss was not measured. For electrochemical measurements, LSV of the collected specimens was performed in a 0.1 M Na2SO4 solution. For this measurement, an O-ring with a diameter of 1 cm was used to expose only the part of the specimen in contact with the solution.
The corrosion weight loss in the various specimens increased with increasing airborne salinity; However, the corrosion weight loss at the exposure site where the volcano was located was the largest. Corrosion weight loss is assumed to be affected more by volcanoes than by airborne salinity. In addition, almost no effect of the presence or absence of highways was present (in industrial areas). The corrosion weight loss increased in the order of Zn, 55% AlZn, 5% AlZn, and ZnAlMg at several exposure sites, but increased in the order of Zn, 5% AlZn, ZnAlMg, and 55% AlZn at the volcanic exposure site. Therefore, the specimens at the exposure sites with and without volcanoes were compared where the average annual airborne salinity was 1 mg dm-2 day-1 or more. The results of LSV of these specimens in 0.1 M Na2SO4 solution are shown in Fig. 1. These results indicate that the corrosion products created at the volcanic exposure site inhibited the dissolution of the plating layer. In addition, aluminum is required to obtain an inhibitory effect on the dissolution of the plating layer owing to corrosion products, and this inhibitory effect improves with increasing aluminum content in the plating layer.

Figure 1