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 ...

The mechanism of corrosion protection of the widely used 2024-T3 aluminum alloy by cerium and lanthanum inhibitors in chloride media is described in detail in the present work. The corrosion process was investigated by means of scanning Kelvin probe force microscopy (SKPFM), in situ atomic force microscopy, and scanning electron microscopy coupled with energy dispersive spectroscopy. Employment of the high-resolution and in situ techniques results in a deep understanding of the details of the physical chemistry and mechanisms of the corrosion processes. The applicability of the SKPFM for mechanistic analysis of the effect of different corrosion inhibitors is demonstrated for the first time. The inhibitors under study show sufficient hindering of the localized corrosion processes especially in the case of pitting formation located around the intermetallic S-phase particles. The main role of Ce(3+) and La(3+) in the corrosion protection is formation of hydroxide deposits on S-phase inclusions buffering the local increase of pH, which is responsible for the acceleration of the intermetallics dealloying. The formed hydroxide precipitates can also act as a diffusion barrier hindering the corrosion processes in active zones. Cerium nitrate exhibits higher inhibition efficiency in comparison with lanthanum nitrate. The higher effect in the case of cerium is obtained due to lower solubility of the respective hydroxide. A detailed mechanism of the corrosion process and its inhibition is proposed based on thermodynamic analysis.

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This work contributes to the development of a new generation of active corrosion protection coatings composed of hybrid sol−gel films doped with halloysite nanotubes able to release entrapped corrosion inhibitors in a controllable way. A silica-zirconia-based hybrid film was used in this work as an anticorrosion coating deposited on 2024 aluminum alloy. Halloysite nanotubes with inner voids loaded by corrosion inhibitors (2-mercaptobenzothiazole) and outer surfaces layer-by-layer covered with polyelectrolyte multilayers were introduced into the hybrid films. The sol−gel film with the nanocontainers reveals enhanced long-term corrosion protection in comparison with the undoped sol−gel film. This effect is obtained because of the self-controlled release of the corrosion inhibitor triggered by the corrosion processes. Utilization of the inner halloysite nanotube lumen as a storage medium for the corrosion inhibitor offers a novel way of fabricating composite core−shell type nanomaterials with their further application as a main component of feedback-active coatings.