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Electrochemical impedance, cyclic voltammetry, and chronoamperometry have been used for characterization of the medium-temperature half-cells Ce 0.8 Gd 0.2 O 1.9 ͉La 0.6 Sr 0.4 CoO 3−␦ ͑Sys 1͒, Ce 0.8 Gd 0.2 O 1.9 ͉Pr 0.6 Sr 0.4 CoO 3−␦ ͑Sys 2͒, and Ce 0.8 Gd 0.2 O 1.9 ͉Gd 0.6 Sr 0.4 CoO 3−␦ ͑Sys 3͒ at various electrode potentials and temperatures. Analysis of the impedance data shows that the kinetically mixed process is probable, characterized by the slow electron transfer to an adsorbed and thereafter dissociated oxygen atom O ads , as well as by slow mass transfer of electroactive species inside the cathode or O ads at the internal surface of the porous cathode. The total polarization resistance increases with rising the atom mass of the A-site cation in the porous perovskite structure. The values of activation energy decrease slightly with increasing negative polarization and in the order of half-cells Sys 3 Ͼ Sys 2 Ͼ Sys 1. The transfer coefficient ␣ c over 0.5 indicates the deviation of the mainly charge-transfer-limited process toward the mass-transfer-limited process in the porous cathode with decreasing temperature. The electrochemical characteristics of half-cells Sys 2 and Sys 1 are stable under repetitive potential and thermocycling during long operation times ͑t Ͼ 1200 h for Sys 2 and t Ͼ 4800 h for Sys 1͒.

This work contributes to the development of new feedback-active anticorrosion systems. Inhibitor-doped hydroxyapatite microparticles (HAP) are used as reservoirs, storing corrosion inhibitor to be released on demand. Release of the entrapped inhibitor is triggered by redox reactions associated with the corrosion process. HAP were used as reservoirs for several inhibiting species: cerium(III), lanthanum(III), salicylaldoxime, and 8-hydroxyquinoline. These species are effective corrosion inhibitors for a 2024 aluminum alloy (AA2024), used here as a model metallic substrate. Dissolution of the microparticles and release of the inhibitor are triggered by local acidification resulting from the anodic half-reaction during corrosion of AA2024. Calculated values and experimentally measured local acidification over the aluminum anode (down to pH = 3.65) are presented. The anticorrosion properties of inhibitor-doped HAP were assessed using electrochemical impedance spectroscopy. The microparticles impregnated with the corrosion inhibitors were introduced into a hybrid silica-zirconia sol-gel film, acting as a thin protective coating for AA2024, an alloy used for aeronautical applications. The protective properties of the sol-gel films were improved by the addition of HAP, proving their applicability as submicrometer-sized reservoirs of corrosion inhibitors for active anticorrosion coatings.

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