Exploring long-term protection at large damage sites in aerospace coatings using inhibitor and carrier synergies
The search for chromate replacements for aluminum alloys began nearly 50 year ago. During this period the regulatory restrictions on the use of Cr(VI) kept increasing along with the demand for alternatives with lower ecological footprint but outperforming currently used chromate-based systems. The most studied alternative so far is the use of less harmful corrosion inhibitors stored inside delivery carriers that account for reduced unwanted inhibitor-coating side reactions and that enable ‘on-demand’ release. While studies using these concepts have shown good protection at mid-size damages (100 µm width or diameter) for relatively long immersion periods, larger damages remain a challenge.
 P.J. Denissen, S.J. Garcia, Corros. Sci. 128 (2017) 164–175.
In this work we explore the potential and underlying mechanisms of inhibitor synergies and different release carriers responsible for long-term local protection at large damages. For the proof of concept we focused on epoxy coatings as matrices on aluminum alloy 2024-T3 with 1±0.05 mm wide damages exposed to 0.05M NaCl for several weeks. Different inorganic and organic inhibitor combinations (e.g. DMTA, DMDT, DEDTC, and Ce(NO3)3) and delivery systems (diatom frustules and zeolites) were used. The combinations selected allowed for a range of inhibitor release mechanisms, kinetics, and local amounts delivered. Besides the more traditional evaluation methods, the degree of corrosion protection was investigated by newly developed techniques using high spatiotemporal resolution such as Opto-Electrochemistry , Local Electrochemical Impedance Mapping (LEIM)  and Laser Speckle Interferometry (LSI) . These techniques enabled to locally study the inhibitor release mechanisms, synergies and effect on the coating and exposed metallic surface. The approached here followed helped gaining a major understanding of the relevant factors to consider in future developments of delivery strategies using new environmentally friendly inhibitors.
 V. Shkirskiy, P. Volovitch, V. Vivier, Electrochim. Acta 235 (2017) 442–452.
 H.M. van der Kooij, A. Susa, S.J. García, S. van der Zwaag, J. Sprakel, Adv. Mater. 29 (2017) 1–6.