Influence of magnesium nitrate on the corrosion performance of sol–gel coated AA2024-T3 aluminium alloy

Traditional anticorrosion technology has relied heavily on using reducible metal species, predominantly hexavalent chromium (Cr(VI)), for protecting reactive metal alloys such as aluminium which is extensively used in the aerospace sector. However, the impending changes in the use of Cr(VI) in Europe and the United States have forced aerospace manufacturers to examine alternative materials for protecting aluminium. One of the most promising alternatives being investigated are organosilane based sol–gels containing anticorrosion additives. In this work the anticorrosion properties of magnesium (II) nitrate (Mg(NO3)2) as an inhibitor were investigated at different concentrations (0.1%–1.0 wt.%) in a methyltrimethoxysilane (MTEOS) sol–gel on the aluminium alloy AA 2024-T3 and compared to AlodineTM 1200 (the established Cr(VI) pre-treatment). Electrochemical evaluation of the coating system by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) measurements correlated strongly with results obtained from Neutral Salt Spray (NSS) exposure data. The surface morphology of the coating was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS). The results indicated that the optimum performance was achieved with a Mg (NO3)2 level of 0.7% w/w. It is proposed that the superior anticorrosion properties of the Mg2+ rich sol–gel are due to the pore blocking mechanism of insoluble magnesium precipitates formed during the hydrolysis process.