The corrosion protection of a coating system incorporating a pre-treatment, Mg-rich primer (MgRP) and topcoat for a AA2024-T351, as illustrated in Figure 1(i), was studied in immersion conditions. The spatial distribution of net anodic and cathodic current densities over a scribed coated sample was mapped using a scanning vibrating electrode technique (SVET). The MgRP was studied in the presence of three different pretreatments (Non-film forming (NFF), trivalent chromium pretreatment (TCP) and anodization with chromate seal (ACS). Experiments were conducted with two coating/scribe area ratios (0.2 and 5) to examine the effect of galvanic coupling between the AA2024-T351 substrate and the MgRP (as demonstrated in Figure 1(ii)). These studies were repeated with the full system including the top-coat. To assess the corrosion inhibition properties of each system and configuration, the pit initiation period, number of pits and area-averaged integrated current density values of areas in the scribe region moving progressively away from the coating/scribe interface were monitored.

An NFF/MgRP system with a coating/scribe ratio of 0.2 was shown to reduce localized anodic activity (corresponding to pits) in the scribe region adjacent to the scribe/coating interface by 2-3 orders of magnitude (Figure 1(ii) (a) and (b)) when compared to the control experiment,. A quasi-steady-state galvanic current distribution was detected in the region adjacent to coating interface indicating enhanced cathodic activity. E_corr values substantially lower than the pitting potential (E_pit) were measured suggesting that­ the MgRP offers effective sacrificial anode-based cathodic protection. Area-averaged anodic current density values increased with increasing distance from the coating edge.

Experiments conducted for the more resistive TCP and ACS pretreated MgRP, with coating/scribe area ratios of 0.2, indicated moderate sacrificial protection and no significant scribe protection, respectively. However, when this ratio was increased to 5, a delayed onset of anodic activity was observed for NFF, TCP and ACS pretreatments indicating an alternative mode of inhibition in the form of anionic species leaching from pretreatment or by the leaching of Mg²⁺. In the presence of a top coat, no significant sacrificial protection was observed due to a more positive galvanic couple potential suggesting limited electrolyte penetration resulting in insufficient conductive pathways for the MgRP to couple to 2024-T351 scribe.

Acknowledgements: This material is based on research sponsored by the US Air Force Academy under agreement number FA7000-13-2-0020. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes not withstanding any copyright notation thereon. This work was supported by the Office of the Undersecretary of Defense Corrosion University Pilot Program, directed by Mr. Daniel Dunmire with insights and guidance from Dr. John R. Scully (Advisor, UVA), Dr. Craig Matzdorf (NAVAIR), Dr. William Abbott (Battelle), Mr. R.J. Santucci and Dr. Andrew King (UVA). The authors recognise the financial support of the EPSRC, Welsh Government and Innovate UK for the SPECIFIC Innovation and Knowledge Centre.

Figure 1 i) Schematic showing a cross-section of a scribed coating system comprising a pretreatment, Mg-rich primer and top-coat on a AA2024-T351 substrate immersed in aqueous NaCl.

ii)(a) and (c) Surface plots showing the distribution of normal current density j_z above a 2024-T351 alloy adjacent to 2024-T351/NFF/MgRP in aerated 2 M NaCl solution. Data were obtained from SVET scans 24 h after sample immersion. (b) and (d) show the visual appearance of the samples after 36 h immersion.