![]() For a further mechanistic approach, the polarization behaviors were analyzed by a numerical model based on the mixed potential theory. Therefore, this study examined the hydrogen evolution, absorption, and hydrogen-induced cracking behaviors of the GI and GA-coated steel sheets during the surface degradation of coatings in a neutral aqueous environment, using a combination of electrochemical polarization, electrochemical permeation, and metallographic observation of coating dissolution. These suggest that the corrosion damage and even the hydrogen evolution and penetration behaviors can be different in GI and GA coatings. ![]() Furthermore, the extent and kinetics of the coating damage caused by the corrosion reaction can depend significantly on the composition and structure of the coatings. Nevertheless, the hydrogen evolution and absorption mechanism under a neutral aqueous corrosion system are unclear. Once the coatings are damaged locally, however, a galvanic couple between the coatings and steel substrate can be formed, and the applied large cathodic polarization on the exposed steel substrate can promote hydrogen evolution. Theoretically, the exchange current density for hydrogen evolution reaction on Zn, and the diffusion coefficient of hydrogen in Zn with a hexagonal close-packed (HCP) structure are much lower than the case of Fe 7, 19, 20, 21, and the danger of hydrogen infusion would not be expected. On the other hand, the sacrificial dissolution characteristics of Zn and Zn–Fe coatings, providing an effective anti-corrosion function, can be the primary driving force for hydrogen evolution on a steel surface. The primary function of both coatings is based on galvanic protection of the underlying steel substrates from corrosion because the coatings have much lower electrochemical potentials under most corrosive conditions 17, 18. ![]() For additional improvements in weldability and paintability, a galvannealed (GA) coating, composed of several distinctive Zn–Fe intermetallic layers, is also used by annealing the GI coating at 500~600 ☌ 14, 15, 16. Some auto-parts are coated with Zn-based alloys to provide an anti-corrosion function, and they have been employed under the name of galvanized steels with a hot-dip Zn coating (GI). Some of them are infused into the steel matrix and are trapped at metallurgical defects, leading to hydrogen embrittlement 12, 13. When steel is exposed to atmospheric/aqueous environments during the service, hydrogen atoms can be generated by cathodic reduction 8, 9, 10 or hydrolysis reactions 10, 11 on the surface. On the other hand, the higher strength of the steel sheet raises significantly a concern regarding hydrogen embrittlement and pre-mature cracking failures 1, 4, 5, 6, 7. Ultra-high-strength steel sheets with a tensile strength of >1500 MPa have been developed and employed in some auto-parts. Among them, the application of lightweight materials centered on ultra-high-strength steel sheets is considered one of the major technical issues 1, 2, 3, 4. With the growing environmental awareness and stricter regulations on emissions, the automotive industry is facing significant challenges. These result in significant differences in hydrogen penetration and cracking behaviors between the two coated ultrastrong steels. In contrast, the corrosive species can only penetrate through the pre-existing cracks in the brittle Fe-Zn intermetallic phases composed of the GA coating, and the driving force for hydrogen evolution becomes smaller. The higher absorption rate of hydrogen was more pronounced in corroded GI-coated steel caused by the larger cathodic polarization applied to the exposed substrate, and a more severe form of coating dissolution by aqueous corrosion in a 3.5% NaCl + 0.3% NH 4SN solution. The hydrogen evolution and absorption behaviors are controlled primarily by the potential differences between the coating and exposed steel substrate, and the corrosion-induced damage pattern of the coating. ![]() Various experimental analyses on hydrogen evolution, absorption, and cracking behaviors were conducted to gain a fundamental understanding of the hydrogen embrittlement of ultrastrong steel sheets with galvanized (GI) and galvannealed (GA) coatings. ![]()
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