Comparative Study of Corrosion Resistance and Corrosion Products in Hot Spring, Seaside, and Other Environments Between Zn and Zn-7Al Alloy-Coated Steel

The corrosion resistance of double, hot-dipped Zn-7Al alloy coating developed for structural steel was studied in a hot spring environment where hot water vapor, including ionic and gaseous components such as sulfate (⁠$SO42−$⁠), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), and carbon dioxide (CO₂) spouted out. These results were then compared with those obtained from exposure sites at seaside and rural environments. Additionally, the alloy was compared with conventional Zn coating in all locations. The corrosion loss of the Zn-7Al alloy-coated steel exposed in the hot spring environment for 3 years was less than half of that of conventional Zn coating, while the surface of Zn-7Al was kept largely intact, and the surface of the Zn-coated steel was covered with red rust. When both coatings were exposed to the seaside environment for 10 years or to the salt spray test, the corrosion loss of the Zn-7Al alloy-coated steel was less than a fifth of that of the Zn one. Through x-ray diffraction (XRD) analysis of the corrosion products, ultralow-loaded hardness tests of the coating layer cross section and transmission electron microscopy (TEM) analysis of the surface layer and interfacial structures, the strengths of the corrosion resistance of the Zn-7Al alloy coating were investigated.