Layer‐by‐Layer Assembled Nanocontainers for Self‐Healing Corrosion Protection

Abstract

The corrosion of metals is one of the main destructive processes that leads to huge economic losses. Polymer coating systems are normally applied on a metal surface to provide a dense barrier against the corrosive species in order to protect metal structures from corrosive attack. When the barrier is damaged and the corrosive agents penetrate to the metal surface the coating system can not stop the corrosion process. The most effective solution so far for designing anticorrosion coatings for active protection of metals is to employ chromate-containing conversion coatings.[1] However, hexavalent chromium species are responsible for several diseases, including DNA damage and cancer, [2] which is the main reason for banning Cr 6+ -containing anticorrosion coatings in Europe from 2007. The deposition of thin inorganic or hybrid films on metallic surfaces has been suggested as a pretreatment to provide an additional barrier against the corrosion species and mainly to improve adhesion between the metal and polymer coating system.[3] The films are usually deposited by the plasma polymerization technique or the sol-gel route. Sol-gel-derived thin films that contain either inorganic (phosphates, vanadates, borates, and cerium and molybdenum compounds) or organic (phenylphosphonic acid, mercaptobenzothiazole, mercaptobenzoimidazole, triazole) inhibitors are investigated as substitutes for chromates. [3a-e] Among them, the highest activity is shown for sol-gel coatings with a cerium dopant of a critical concentration in the 0.2-0.6 wt % range. However, the negative effect of the free inhibitor occluded in the sol-gel matrix on the stability of the protective film is observed for all types of inhibitors (for instance, a higher concentration of Ce leads to the formation of microholes in the sol-gel film [3f] ). This shortcoming calls for the development of nanometer-scale reservoirs to isolate an inhibitor inside and prevent its direct interaction with the sol-gel matrix. Nanoreservoirs should be homogeneously distributed in the film matrix and should possess controlled and corrosion-stimulated inhibitor release to cure corrosion defects.Mixed-oxide nanoparticles (e.g. ZrO 2 /CeO 2 ), [4] b-cyclodextrin-inhibitor complexes, [3c] hollow polypropylene fibers, [5] and conducting polyaniline [6] have been explored as prospective reservoirs for corrosion inhibitors to be incorporated in the protective film. The common mechanism of the nanoreservoir activity is based on the slow release of inhibitor triggered by corrosion processes. Ion exchangers have also been investigated as 'smart' reservoirs for corrosion inhibitors. Chemically synthesized hydrocalmite behaves as an anion exchanger: adsorbing corrosive chloride ions and releasing corrosion-inhibiting nitrite anions. [7] Despite considerable efforts devoted to the development of new, complex anticorrosion systems, practically no single solution is able to fulfill the requirements of sufficient corrosion protection while avoiding chromates in the coating, especially in the ...

Corrosion protection of AA 2024-T3 aluminium alloys using 3, 4-diaminobenzoic acid chelated zirconium–silane hybrid sol–gels

“…Similar approaches were investigated by European research groups which concentrated on epoxy functionalised silane-zirconium systems by adding cerium [24], azole based organic inhibitors in host molecules [25,26] and bis-silanes [27]. Further published data suggests that the route of zirconium inclusion is important, as the choice of chelating ligand may have an effect of the final coating performance [28].…”

Self-healing coatings in anti-corrosion applications

“…EIS can be used to estimate coating degradation and corrosion kinetics even when the time of coating exposure in a corrosive medium is not long. This technique is usually aided by SVET [45,50,62,96,121] or localized impedance spectroscopy (LEIS) [122]. In some cases, however, the properties of self-repairing coatings are assessed solely on the basis of the results obtained by EIS [55,102].…”

“…The performance of a self-healing coating in corrosive media can be investigated using various techniques, such as electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and the scanning ion-selective electrode technique (SIET) [45,50,53,57,58,62,63,96]. EIS can be used to estimate coating degradation and corrosion kinetics even when the time of coating exposure in a corrosive medium is not long.…”

“…Figure 6 schematically shows how a capsule is created using the particular methods [29]. As the building material, the following are used: poly(urea-formaldehyde) (PUF) [32][33][34][35], melamine-urea-formaldehyde (MUF) [35,36], phenol-formaldehyde [37], epoxy resin [38,39], methylene diphenyl diisocyanate (MDI) [40], poly(methyl methacrylate) (PMMA) [41,42], polystyrene [32,41], poly(allylamine) [43], polyvinyl alcohol (PVA) [43,44], polyurethane [40], polyphenol, amphiphilic block copolymers (polypyrrolidones, poly(ethylene oxide) [31], poly(caprolactone)) [31,32], cerium molybdate [45], zinc and aluminium nitrate [46,47], silica [27,32,[48][49][50], silver [42], gold [51], copper(II) sulphide [51], c-AlO(OH) [51], tin(IV) oxide [51] and titania oxides [42,52,53] and CaCO 3 microbeads [54]. The capsule material and the way of preparing capsules affect their shape and size.…”

Effect of organic chelates on the performance of hybrid sol–gel coated AA 2024-T3 aluminium alloys

“…Some United States Air Force studies investigated several routes towards improving performance including the use of rare-earth metal salts [18], amine cross-linkers [19][20][21], supramolecular inclusion and organic inhibitors [22,23]. Similar approaches were investigated by Ferreira's group in Portugal which concentrated on epoxy functionalised silane-zirconium systems by adding cerium [24], azole based organic inhibitors in host molecules [25,26] and bis-silanes [27]. This group has also published data which suggests the route of zirconium inclusion is important, as the choice of chelating ligand may have an effect of the final coating performance [28].…”