Influence of mass transport on the competition between corrosion and passivation by inhibitor release after coating breakdown

• Inhibition competes with the triggering of localized microstructural corrosion.
• Transport in solution of inhibitors imaged using a coated cut-edge electrode.
• Validation by in situ pH imaging combined to FEM modelling.

Interactive combination of the properties of storage, release of pigments and transport of inhibitors are controlling the efficacy of inhibitive coatings deposited as primer on 2024 alloys subject to environments with a risk of atmospheric corrosion. A test method is proposed to characterize the competition between the triggering of localized microstructural corrosion and the passivation of a bare metal surface. The behaviour of a cut-edge electrode machined from a coated AA2024-T3 plate has been used to mimic the electrochemical response after a breakdown in the surface coating. In this work, the calibration of the described method is based on conventional chromate/dichromate inhibitors, assumed to be cathodic inhibitor. In situ pH imaging with an ion-selective microprobe was used to confirm the effect of the inhibiting pigment depletion inside the primer coating on the competition between corrosion and passivation. This test method is intended as useful for selection of chromate replacement.