The use of multiple electrochemical techniques to characterize Mg-rich primers for Al alloys

To insure that the corrosion protective behavior of Mg-rich primers over Al aircraft alloys is properly understood, multiple electrochemical techniques have been utilized to characterize the behavior of this class of Cr-free coatings. Many substitutes for Cr-containing pretreatments for Al alloys such as AA 2024 T-3 or AA7075 T-6 have been put forth as performing as well as the present Cr-containing materials, but none except Mg-rich primers (MRPs) have succeeded in passing the qualification testing required of these materials. The electrochemical testing was done in conjunction with the subjective “qualification” testing of the primer materials for US Air Force and Navy specifications so that as this new class of primers did not encounter any unexpected corrosion protection-related deficiencies. To confirm that the MRPs did indeed provide cathodic, sacrificial protection to these alloys, one measures the mixed corrosion potential of the Al–Mg system (Ecorrmixed), also know as the open circuit potential (OCP). This parameter indicates when the potential of the alloy substrate is modified by contact with the Mg to yield a potential cathodic to the Al Alloy (Ecorralloy). This was done by a steady state measurement of the total system OCP in immersion and also by measuring the potential of the minimum/free corrosion current (icorr) in a potentiodynamic scan of the mixed system. Both methods yielded values in excellent agreement, but initial studies of a system with a high quality topcoat gave problems in OCP measurement due to an exceedingly high impedance causing steady-state measurement difficulties. Electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM) were also used to characterize the system in immersion. Further, the scanning vibrating electrode technique (SVET) was used to examine local damage protection by the MRP about a deliberately induced scratch in the coating system. These results for the MRP system yield a very interesting, self-consistent set of data showing cathodic protection of one reactive metal (Al) by another more reactive metal (Mg).