Effects of compressive stresses on the breakdown and self-healing of the surface passivation films on Ti-35421 alloy by in-situ electrochemical monitoring

Abstract:

The effects of compressive stresses on the breakdown and self-healing of the surface passivation films on Ti-35421 (Ti-3Al-5Mo-4Cr-2Zr-1Fe) alloy after damaging by the Al2O3 ceramic needles have been investigated on a self-made scratching tester by using the in-situ electrochemical monitoring. The passivation potential of Ti-35421 alloy in 3.5% NaCl solution is determined to be -0.27~0.01 V. Without the compressive stress, the potential recovers to the initial leverl shortly. When the scratching experiments carry out under the passivation polarization of -0.13 V, the increase of compressive stress leads to the decrease of the atomic binding forces, the increase of metallic dissolution rates, and the slow down of repassivation rates. In-situ electrochemical data show that the potential drops and the currents rise, and the self-healing duration times become longer. The growth and sel-healing of the passive films in the transient stage basically conform to the linear high-field model. Observation of scratch morphology shows that the scratching generates the higher residual stresses at the bottom of the grooves. It is found that microcracks appear in the scratch grooves under the compressive stresses basing on the scratch morphology, indicating that the damages of the scratches under high compressive stresses are partialy unrecoverable, which results from the synergetic effects of the residual stresses and environmental corrosion.

Key words:Ti-35421 alloy Potentiostatic polarization Scratch test Passivation film self-healing In-situ electrochemistry monitoring