42 lbl assembly of inhibitors in nanocontainers the

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4.2 LBL assembly of inhibitors in nanocontainers The layer by layer (LBL) technique, based on the adsorp- tion of oppositely charged layers, is particularly advanta- geous for industrial applications to prepare multilayered ultra-thin films due to low cost, simplicity and versatility of its experimental procedure. This technique provides a way 180 J Sol-Gel Sci Technol (2010) 54:174–187 123
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to control the construction of ultra-thin films at nanoscale, and a long-term stability of the film can be obtained easily. The storage of corrosion inhibitors in polyelectrolyte multi-layers can confer several advantages: they can pre- vent a negative effect of the corrosion inhibitor on the stability of the coating, decrease the influence of the coating polymerization on the inhibitor, and provide intelligent release of the corrosion inhibitor, as the per- meability of the polyelectrolyte assemblies can be regu- lated by pH, humidity, and light [ 80 82 ]. A change of pH is a more preferable trigger for corrosion protection systems since corrosion activity leads to local changes of pH in the cathodic and anodic areas. Halloysite, a naturally occurring negatively charged alumosilicate, exhibits in the hydrated state a tubular morphology with an average diameter of 50 nm and length of 500–700 nm. Halloysite nanotubes with inner voids loaded by MBT and outer surfaces LBL covered with polyelectrolyte multi-layers are introduced into the hybrid films. Because self-controlled release of corrosion inhibi- tors is triggered by corrosion processes, the hybrid coating actually acts as a main component of feedback-active coatings [ 83 ]. Hybrid coatings comprising an inhibitor-doped LBL clay-polyelectrolyte over-coated with a dense organically modified silicate are investigated as corrosion protectors for aluminum. Stratification of this system arises during the drying process. A preferential segregation of inhibitor- doped LBL clay-polyelectrolyte onto the surface of the substrate and sol–gel moving to the surface of the coating occurs. A combination of LBL assembly of corrosion inhibitors and self-stratification of components of hybrid coatings is found to produce a synergistic enhancement of corrosion resistance. Moreover, the sol gel hybrid stratum seals the inhibitor ions into the polyelectrolyte layer, thereby producing a reservoir of corrosion inhibitors in the direct vicinity of the metal. Importantly, the LBL multi- layers are not affected by sol–gel promoting lateral diffu- sion of an active corrosion inhibitor along the surface. The combination of strong barrier functions, efficient corrosion inhibitor delivery, low cost of the components and eco- logical compliance is difficult to attain in traditional cor- rosion protection approaches [ 84 ]. This hybrid coating has electrochemically shown superior corrosion protection compared to hexavalent chromium coatings [ 85 ].
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