A room temperature cured sol–gel anticorrosion pre-treatment for Al 2024-T3 alloys

The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion protection system. Historically these corrosion protection systems have been based upon the use of chemicals containing Cr(VI) compounds. Organic–inorganic hybrid silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In the present study, a hybrid epoxy–silica–alumina coating with or without doped cerium nitrate has been prepared using a sol–gel method. The hybrid coatings were applied by a dip-technique to an Al–Cu alloy, Al 2024-T3, and subsequently cured at room temperature. The anticorrosion properties of the coatings within 3.5% NaCl were studied using electrochemical impedance spectroscopy (EIS), and conventional DC polarisation. An exfoliation test method involving immersion in a solution of 4 M NaCl, 0.5 M KNO3 and 0.1 M HNO3 was also used. The cerium nitrate doped sol–gel coating exhibited excellent anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The resistance to corrosion of the sol–gel coating was also evaluated by analysing the morphology of the coating before and after corrosion testing using scanning electron microscopy.