CORROSION BEHAVIOUR OF FRICTION STIR PROCESSED QE22 MAGNESIUM ALLOY

Abstract

Effect of the microstructural changes after friction stir processing on the corrosion behaviour of rare earth added QE22 magnesium alloy is studied. The composition of QE22 alloy is 2.60 wt% Ag, 1.73 wt% Nd, and 0.68 wt% Zr, and balance magnesium. Its hardness varied from 90 VHN for FSP zone to 76 VHN for the base metal zone. Electrochemical tests performed in 3.5wt% NaCl solution showed that FSPed alloy with ultrafine grained magnesium matrix with dissolved (Mg12Nd2Ag) precipitates and refined second phase (Mg12Nd) precipitates displays a significantly lower corrosion resistance. There are large number of micro-galvanic cells and it weakens the barrier to corrosion propagation in magnesium matrix. Higher weight loss is observed in immersion test, larger corrosion current values in polarization studies and lower charge transfer resistance and capacitance values in EIS plots. The inferior corrosion resistance of FSPed alloy is attributed to three factors. Firstly, deformation induced crystal defects made the magnesium matrix more active. Secondly, the refined second phase (Mg12Nd) precipitates formed a large number of micro-galvanic cells and promoted corrosion in the magnesium matrix. Lastly the dissolution of continuous net like precipitates (Mg12Nd2Ag) in magnesium matrix promoted corrosion in FSPed alloy