Self-healing coatings: An alternative route for anticorrosion protection

Abstract

Polymer coating systems are classically applied on a metal surface to provide a dense barrier against the corrosive species. Cathodic protection is used for many applications at the same time than coatings to protect the metal structures from corrosive attack when the coating is damaged. However, the current demand will increase with the disbonded areas; moreover, the reactions that take place at the cathode can cause a progressive enlargement of the unbonded area. Self-healing coatings were considered as an alternative route for efficient anticorrosion protection while maintaining a low demand in cathodic protection. Such coatings typically incorporate micro or nanocapsules that contain film-formers and repair the coating damage when the coating is scratched. Selfhealing systems were developed for metal structures under cathodic protection using specific-film formers sensitive to the electrical field and pH encountered in the vicinity of a default on a coated structure under cathodic protection. The present paper describes the principle of this novel self-healing concept and discusses the healing efficiency on the basis of laboratory results. Electrochemical Impedance Spectroscopy was used to evaluate the performance of the barrier efficiency and continuous current demand monitoring assessed the ability of specific film-formers to provide self-healing and repair defects generated through the coating to the metal.

Self-healing coatings in anti-corrosion applications

“…Halloysite nanotubes and boehmite nanoparticles are employed as reservoirs for corrosion-inhibiting compounds [57][58][59]. Cerium cations [58,[60][61][62], 8-hydroxyquinoline [48,57], benzotriazole [45,48,49,63,64], mercaptobenzimidazole (MBI) [63], dicyclopentadiene (DCPD), alkoxysilanes [65], silyl ester [24], linseed oil [37,44,66], hexamethylene diisocyanate (HDI) [40], triethanolamine (TEA) [41], monomers and catalysts for ROMP [67], magnesium ions [68], chromium(III) and cerium(IV) oxides [34] and cerium(III) chloride [60,69] are used as corrosion inhibitors. The action mechanisms connected with the particular compounds are presented in Table 1.…”

“…In the considered example, CaCO 3 microbeads were used as containers [54]. Micro/nanocontainers can be incorporated into a polymeric coating [35,54,68,71], less often into a sol-gel coating [27,72], and sporadically into a metallic coating, e.g. one based on electrochemical nickel [73].…”

“…A similar solution is the encapsulation of magnesium ions. Then, the self-repair process is possible owing to Mg(OH) 2 precipitation [68].…”

Self-healing polymers and composites

“…50,51 While corrosion inhibition can indeed be recovered by applying conventional, polymerisation based healing to polymer coatings, many have also investigated the deposition of corrosion and rust inhibiting film formers into areas of damage. Similar to the previously discussed self-healing systems, microcapsules have been shown to successfully deliver these film forming compounds (which have included organic molecules, organic salts 52 and inorganic salts 53 ) to the damaged regions of coatings and subsequently inhibit metal corrosion and rusting. Another intuitive delivery approach was recently developed by Shchukin and coworkers where the film forming compounds were incorporated into thin layers of polyelectrolytes, which were either directly coated on the metal surface or layered on SiO 2 nanoparticles embedded in a coating.…”

“…[30][31][32] The second approach, developed by Lewis and co-workers 33,34 and shown in equation (7), is similar to the first, but instead utilises the mean fatigue crack propagation rate (FCPR) 49,162,163 The efficiencies of many self-healing systems that show promise for coating applications have been measured by depositing the polymer as a film over a metal electrode and measuring the current that can flow through healed polymers. [53][54][55][56] Wilson and co-workers developed an approach to calculate the average retention of healing capabilities r avg of polymers and composites that were initially fabricated at different temperatures. 82 This calculation is shown in equation (8) are equal to the average peak load value on loaddisplacement curves of healed specimens whose polymer matrices' were previously post-cured at 35 and 125uC respectively.…”