Hydrogen Permeation Behavior And Embrittlement Of Hot-dip Coated Steels Exposed To Seawater

The security of hot-dip coated steels was imperiled by the synergy of hydrogen permeation and stress in marine environment. In this thesis, three kinds of hot-dip coated steels (Galvanized, Galfan and Galvalume) were used as investigative object. Self hydrogen permeation curves of three hot-dip coated steels were measured using a Devanathan-Stachurski bipolar cell. The effect of the ratio of the area of the exposed steel substrate versus coating surface was investigated. Hydrogen permeation resistance and self hydrogen permeablity when defect existed were compared among three materials. Also, the hydrogen permeation mechanism was analyzed combined with these coatings'chemical composition and structure and their corrosion mechanism in seawater. The stress-strain curves of these materials with different hydrogen charging conditions were obtained by SSRT, and their mechanical properties was compared. Fractography were also carried out to investigate the effect of hydrogen permeation on the mechanical properties of steel substrate. The results showed that:(1) Hydrogen permeation resistance of Galvanized coating is the lowest, and that of Galfan coating is higher. And when these two kinds of coating have defect, hydrogen absorption and permeation were accelerated. On the contrary, Galvalume coating showed excellent hydrogen permeation resistance, and showed almost no self hydrogen permeation even with defect. Hydrogen permeation flux decreased with the ratio of the area of the exposed steel substrate versus coating surface. With the higher content of aluminum and compacter structure and better corrosion resistance, the coating showed better hydrogen permeation resistance and less self hydrogen permeability.(2) Self hydrogen permeation of Galvanized and Galfan coating in seawater reduced their percentage elongation after fracture and density energy and shift their fracture characteristics from ductile to quasicleavage. The percentage elongation after fracture and density energy of Galvalume coating were also reduced and its fracture characteristics was shift from ductile to locally quasicleavage. With longer hydrogen charging time, the materials showed higher susceptibility to hydrogen embrittlement. With higher content of aluminum and better corrosion resistance, the coating showed lower susceptibility to hydrogen embrittlement in seawater.