Role of oxygen in the galvanic interaction between polypyrrole and aluminum alloy

Conjugated polymers continue to be of interest as possible corrosion-control coatings for metal alloys. In this work, electrochemical interactions between polypyrrole (PPy) films and the aluminum alloy 2024-T3 (AA) were investigated by electrochemical polarization and galvanic coupling techniques in dilute Harrison's solution (0.35 wt% (NH4)2SO4, 0.05 wt% NaCl). A two-compartment electrochemical cell was used for the galvanic coupling measurements which permitted assessment of the role of dissolved oxygen by segregating the PPy film and AA in individual compartments. Electroactive 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (BDA or Tiron) and electroinactive benzene-1,3-disulfonic acid, disodium salt (BDS) were used as dopants to study the mediating effect (if any) of BDA on the coupling behavior. The results indicate that oxygen reduction mediated by the PPy coating dominates the coupling interaction. The electrochemical polarization state of AA 2024-T3 in a pinhole simulating a PPy coating defect was characterized by the potential measured immediately after decoupling, by the anodic hydrogen evolution, and by galvanodynamic polarization. The results support the conclusion that the alloy in the coating defect was electrochemically active, not passive, during coupling with PPy. The anodic protection model commonly accepted for ferrous metals does not appear to apply to AA 2024-T3.