The functional life of paint in DoD is defined by the barrier properties and protective corrosion inhibiting capacity of the coating system. The time until corrosion initiation of an underlying substrate can thus be defined in general terms as 1) the time for an external environment to reach the coating/substrate interface through coating porosity, coating cracking, or mechanical damage and 2) inhibitor exhaustion/depletion. In other words, once a coating system has been breached, protection is afforded by the availability of inhibitors at the defect site. In this work, we seek to understand the parameters that influence inhibitor exhaustion. An aluminum alloy Multi-Electrode Array (MEA) probe is used to measure corrosion currents directly as a function of exposure time. Both inhibitor leach rate and residual coating protectiveness are measured as a function of time and compared for four aerospace primer systems. Leaching of inhibitors is measured using Ion Coupled Plasma (ICP) to quantify the amount of inhibitor in a 50 mL exposure cell at fixed exposure intervals. At these fixed exposure intervals, the partially depleted coating systems are then expose in a 1.2 mL cell and the inhibition kinetics measured using the MEA probe. Inhibitor exhaustion times are then defined as the time where there is no longer enough inhibitor leaching from a coating system to protect the MEA. Integration of inhibitor exhaustion test results into modeling approaches is briefly discussed.
Once a coating system has been breached, protection is afforded by the availability of inhibitors at the defect site. The onset of subsequent corrosion, therefore, is a function of inhibitor release rate, the ratio of inhibitor to aggressive ions, and consumption of inhibitor in the vicinity of the defect. The parameters affecting each of these processes must be understood in order to appropriately model the onset of corrosion for either a first principles or an empirical approach. The influence of coating properties and environmental conditions on inhibitor exhaustion is discussed below.