Abstract
To achieve a deeper understanding of the mechanism of protection of aluminum alloys by magnesium-rich primer (MgRP), the behavior of MgRP on aluminum alloy (AA)2024-T3 (UNS A92024) was investigated using a Kelvin probe and electrochemical methods. Different magnesium-rich coating systems were compared and galvanic corrosion experiments were performed with bare AA2024-T3 and MgRP-coated AA2024-T3. The electrochemical properties of magnesium-rich coatings primarily depend on the polymer matrix, whereas the source of magnesium pigment plays a secondary role in the systems studied. MgRP acts like an insulator under dry conditions. Water has to penetrate the polymer-pigment network for the coating to act as a sacrificial anode. This results in an activation time for cathodic protection. The ability of MgRP to protect the aluminum alloy substrate in a bulk solution depends on the coated/bare-substrate area ratio. Basic or cathodic corrosion of AA2024-T3 is possible for samples in contact with MgRP. Thin electrolyte layer experiments and cathodic polarization curves in solutions equilibrated with different gases showed that carbon dioxide (CO2) in high concentration was able to buffer the pH on the AA2024-T3 surface so that no basic corrosion occurred. However, the amount of CO2 available in the air was not enough to prevent cathodic or basic corrosion in an air-exposed AA2024-T3 sample polarized cathodically.
