Effect of Thermal Oxidation on SCC Resistance of Ti- ۴AI-۲V in Chloride Environments

High corrosion resistance of titanium and its alloys is one of the important factors in materials selection and increasing their application in different industries, such as chemical reactors and marine industries. Ahhough this resistance is mainly due to the spontaneous formation of a thin, stable, protective and strongly adherent passive oxide fibn in the presence of water and oxygen, but the growth of this layer in some heat treatment operations in high temperatures could change its positive role to a negative parameter. Welded components of titanium alloys may be used in the fabricated conditions; or either after the stress relieving or solution annealing and air-cooling. The stress relief is generally performed at ۵۳۸°C and the solution annealing at ۷۰۴°C. It was important to characterize the effect of the high temperature air formed oxide :fihn on the stress corrosion resistance of material in chloride-containjng solutions and to compare the results with the behavior of non-heat treated material In this research, the efficiency of thermal oxidation in temperatures between sooDe to ۸۰۰۰e on see resistance behavior of this alloy was studied by SSRT method in different NaCl concentrations and different strain rates. The results approved that formation of this oxidized layer will increase the sensibility of this alloy to see in chloride environments in some conditions. sec resistance examination performed by SSRT method, and different macroscopic and microscopic investigations used for studying crack propagation. Results shows that the specimens without any thermal oxidation process didn't show any sensibility to see in chloride solution, but after thermal oxidation in temperatures higher than ۶۰۰۰e (when the color of surface changes to dark blue) mechanical property parameters such as UTS point, elongation and reduction of cross section area reduces to lower degrees. On the other hand SEM images show non-ductile fracture surmces near the surfB.ce of specimens. Compatibility of resuhs on the see crack growth mechanjsms of titanium alloys in chloride solutions, especially attention to the role of ahmrinum and oxygen diffusion zone under the oxide surface, show that see crack nucleation surely occurs after the oxide Jayer breakdown.

Keywords:

Stress corrosion , atmospheric oxidation , titanium alloys , chloride