SECM investigation of corrosion inhibition by tungstate- and vanadate-doped polypyrrole/aluminum flake composite coatings on AA2024-T3

• Polymers decrease O2 consumption and prevent corrosion product buildup in defects.
• Results support a corrosion control mechanism that combines several components.
• Mechanism includes galvanic coupling, anodic passivation, and inhibitor release.

Scanning electrochemical microscopy (SECM) experiments were performed over defects on AA2024-T3 samples coated with an epoxy-based composite containing aluminum flakes covered with either tungstate- or vanadate-doped polypyrrole. The SECM probe was scanned laterally over the defect with the probe potential set to either reduce dissolved oxygen (redox competition mode) or oxidize ferrocenemethanol (feedback mode). Scans were performed over several days of immersion in naturally aerated KCl/ferrocenemethanol solution and results from three different composite coatings were obtained. Coatings containing the as-received aluminum flakes with no polypyrrole showed high levels of corrosion and corrosion product buildup, while those coatings containing polypyrrole-covered flakes showed depletion of dissolved O2 over the scribe, but with no concomitant buildup of corrosion product. The results support a corrosion control mechanism that combines galvanic coupling between the polypyrrole and the aluminum flakes, anodic passivation of the substrate, and inhibitor release into the defect. SEM/EDX was utilized to provide evidence of corrosion product formation and the release of dopant ions.