Cerium ions are introduced into mixed oxide nanopar

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Cerium ions are introduced into mixed-oxide nanopar- ticles (e.g., ZrO 2 /CeO 2 ) which play the role of nanoreser- voirs storing and slowly releasing cerium ions when a defect is formed in the sol–gel film [ 75 ]. Cyclodextrin–organic inhibitor complexes [ 54 , 72 ], hollow polypropylene fibers [ 76 ], and conducting polyan- iline [ 77 ] have been explored as prospective reservoirs for corrosion inhibitors to be incorporated in the protective film. 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 [ 78 ]. Encapsulation of organic inhibitors by means of plasma polymerization and following incorporation to the coatings was found to be good for slow release of inhibitor [ 79 ]. Among all the nanocontainers of corrosion inhibitors the prospective techniques are cyclodextrin/organic inhibitor inclusion complexes and layer by layer assembly of organic corrosion inhibitors in nanocontainers, which are reported to show superior corrosion protection. 4.1 Cyclodextrin/inhibitor inclusion complexes Organic corrosion inhibitors can be incorporated into the coating material as inclusion complexes with b -cyclodex- trin. Cyclodextrins are cyclic oligosaccharides that possess a unique molecular cup-shaped structure with a hydrophilic exterior surface and a hydrophobic interior cavity and are effective complexation agents with the ability to form inclusion complexes with various organic guest molecules that fit within the cyclodextrin cavity. Organic aromatic and heterocyclic compounds are usually predominant candidates for the inclusion complexation reaction. Non-ionizable Mercaptobenzothiazole (MBT), mercap- tobenzimidazole (MBI) are effective corrosion inhibitors, which substantially reduce the localized anodic corrosion currents. MBT and MBI are encapsulated within the hybrid coatings both as their free molecular forms and as inclusion host–guest complexes with cyclodextrin. Schematic rep- resentation of this concept is shown in Fig. 1 . Good corrosion protection properties have been showed by the slow release of the non-ionizable inhibitors from the cyclodextrin/inhibitor inclusion complexes and by the self- healing of corrosion defects for long-term protection of the high strength aluminum alloys against atmospheric corro- sion [ 72 ]. The results of PDS tests shown in Table 2 illustrate corrosion reduction trends for both groups of inhibitors as indicated by a shift to more noble corrosion potentials along with a significant drop in corrosion current values as compared with the inhibitor-free SNAP coating. However, the effect of slowly release of inhibitors by cyclodextrin/inhibitor inclusion complexes does not appear in Table 1 due to the properties of PDS tests self, which has been proved by scanning vibrating electrode technique (SVET).
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