Corrosion and Corrosion Crack Resistance of the PT3V Titanium Alloy in Aqueous Solutions of Ammonia

Abstract

We describe some results of experimental investigations into the influence of the structure of the PT3V titanium pseudo-α-alloy, formed by plastic deformation and heat treatment, on its mechanical properties, corrosion, and cyclic crack resistance in aqueous ammonia media of various concentrations and temperatures. The kinetics of electrode processes on a smooth surface and at the tip of a fatigue crack is studied. By using the methods of the dropping mercury electrode and a disk rotating electrode, we identified the main partial electrochemical reactions and determined the limiting stage of the cathodic process (diffusion of molecular oxygen), the hydrodynamic conditions of the course of these reactions, and the diffusivity of oxygen in aqueous ammonia media at 298 K (D = 4.8·10−8m2/sec). The kinetics of fatigue and corrosion-fatigue fracture of the alloy under study (crack initiation and growth) was investigated as well. We have established that, at the stage till crack initiation in air, alloys with a globular structure have the maximum lifetime, which is 1.5–2 times as much as that of lamellar structures. In corrosive media, the time to crack initiation increases with the ammonia concentration and solution temperature. A corrosive medium strongly affects fatigue crack growth in the alloy for ΔK below 5–7 MPa m1/2, i.e., if V > (2·10−8–8·10−9) m/cycle, especially for the initial (thin-lamellar) and β-transformed structures. With the help of investigations of electrochemical processes in a corrosion-fatigue crack, we established the linear dependence of the stabilized value of the electrolytic potential Est on ΔK. Specific features of the fatigue fracture of alloys having different structures in air and aqueous ammonia media were discovered on the basis of metallographic analysis. Finally, we analyzed the kinetics and the mechanisms of corrosion and corrosion-fatigue crack growth.