Passivation behavior of titanium-6al-4v alloy in phosphoric-acid solution

Titanium metal, because of its light weight, good mechanical properties, and excellent corrosion resistance, has been increasingly considered for various applications in the process industries through laboratory investigat-ions. The corrosion resistance of the metal is due to its inherent nature to form stable titanium dioxide. The potential- pH diagram of titanium (1) indicates that the formation of oxide film on the metal becomes thermodynami-cally unstable in nonoxidizing acidic aqueous environ-ments, which results in a general dissolution of the metal. This behavior limits the more widespread use of the metal in many nonoxidizing acids such as sulfuric, hydro-chloric, phosphoric, and oxalic in different process indus-tries. The stability of the metal in such environments can be restored by the adjustment of potential by the use of anodic passivation either by means of impressed current or by alloying with other metals, such as palladium or nickel, or by introducing a redox system in the enviro-nment (2). Out of all these methods, anodic passivation is considered superior because of high range of passivation potential observed for titanium in most of the enviro-nments. For a metal to be protected anodically in an envi-ronment, the necessary condition is a low order of pasSive dissolution current and passive potential