Effect of impact angle on hot corrosion resistance of abrasive water jet peened Ti-6Al-4V alloy

The effect of peening angle (10° to 40°) on NaCl induced hot corrosion behavior of Ti-6Al-4V alloy was evaluated at 750 °C for 100 h. Peening at higher impact angles progressively increases the surface roughness, number of sub-grains, magnitude of compressive residual stresses and hardness of the alloy. During hot corrosion, the surfaces of Ti-6Al-4Al, irrespective of the peened condition, develops oxide scales that contain oxides of Ti, Al and V. Hot corrosion rate, measured in terms of rate of mass gain and the rate constant, K_p, was highest for the unpeened alloy but decreased for surfaces subjected to AWJ peening. Surfaces peened at increasing impact angles have smaller mass gain rate and the one peened at the impact angle of 30° exhibited the lowest corrosion rate and a lowest K_p of ∼0.08 mg²/cm⁴/h. The surface peened at 40° is, however, not as corrosion resistant. The mechanism of corrosion was discussed in the context of corrosion products formed and the opposing influences of initial roughness and compressive residual stresses. While higher roughness promotes hot corrosion by facilitating sites for corrosive attack, higher compressive residual stresses retard the diffusion of corrosive species into the Ti-6Al-4V surface. Based on these discussions, the optimum impact angle for AWJ peening for imparting maximum hot corrosion resistance on Ti-6Al-4V was determined to be ∼30°.