Corrosion-fatigue behavior of micro-arc oxidation coated 6061-T6 Al alloy

Abstract

The corrosion-fatigue response of Al alloys becomes an important consideration for qualifying them for aerospace and other structural applications, the MAO coatings with three different thicknesses namely 10 µm, 50 µm and 90 µm were provided on peened (SP + MAO) and un-peened (Plain MAO) substrates. Further, the corrosion (potentiodynamic polarization) tests in 3.5 wt% NaCl solution and corrosion-fatigue tests at 5-different maximum alternating stress levels were evaluated for each material condition. In addition, the surface roughness, surface and cross-sectional coating morphologies in as deposited and corroded conditions; fractured morphology and corrosion-fatigue crack propagation features were evaluated; correlated with the corresponding corrosion rates and corrosion-fatigue life. The results obtained highlight the role of corrosion medium in significant degradation of corrosion-fatigue life as compared to the plain-fatigue life of both bare substrate and MAO coated conditions. A new parameter namely the normalized corrosion-fatigue life index (NCFI) provides a quicker insight and possible benchmarking of the coating’s ability to withstand the damaging action of simultaneous corrosion and fatigue. A 50 µm thick SP + MAO coating was identified as a candidate choice to withstand diverse degradation modes namely the simple corrosion, plain-fatigue and corrosion-fatigue.