The role of corrosion product layers in the pitting resistance of powder-processed Al–Cr–Co–Mn–Zr alloys containing icosahedral quasicrystalline dispersoids

The impact of the surface corrosion product layers on the pitting corrosion behavior of three Al–Cr–Mn–Co–Zr alloys has been studied using a combination of electron microscopy, X-ray and electrochemical techniques. These alloys contain an FCC Al matrix, different volume fractions of icosahedral quasicrystalline (I-phase) dispersoids and minority grain boundary phases. The I-phase dispersoids act as cathodic sites, and alloys with higher I-phase volume fractions have lower open circuit potentials and higher corrosion current densities before salt fog exposure. Upon ASTM salt fog exposure for 6 weeks, these trends reverse due to the development of a corrosion product bilayer. The bilayer consists of a dense outer layer with angular crystallites of bayerite, and an inner boehmite layer with a fibrillar morphology and a gradient structure with the highest density at the interface with the alloy. This bilayer was more well developed at higher I-phase volume fractions and after upset forging. The layer was found to improve the resistance of the alloys to pitting corrosion, with the pitting potentials increasing dramatically to values as high as 1432 mV (with respect to saturated calomel electrode) after the salt fog exposure. These values were significantly higher than those for the conventional high-strength Al alloys Al 6061 (− 126 mV) and Al 7075 (− 706 mV) tested under identical conditions.