Electrochemical polarization data for the individual thin film electrodes Al and Zn as well as in galvanic couple connecting the two together have been generated using atmospheric corrosion sensors developed recently. Thin films of these metals were subjected to cyclic fog testing and to continuous immersion in selected electrolytes corresponding to atmospheric condensates and were tested also in actual atmospheric exposure under bold (totally open and subject to direct hit by rain and sun light) and sheltered (not hit by rain and sun light) conditions. Results generally indicate that zinc corrodes by very large amount tending to protect aluminum in the process. Aluminum corrodes by the least and remains passivated under test conditions involving only plain water. In the presence of chloride ion~ pitting is activated, though zinc provides initial protection and tends to stabilize passivation. Under open bold conditions, zinc electrode experiences the largest corrosion current densities and passivates itself and Al remains also in the passive state. The potentials of both electrodes converge to nearly the same value around +100 mV measured against an open silver film. Under the sheltered locations, both the corrosion potentials and current densities oscillate, possibly due to repeated tendencies of passivation and depassivation and attainment of a totally protected state for Al may require secondary protection measures along with the use of zinc.
Can articles and structures made out of aluminum alloys be protected efficiently in the open atmosphere by the application of the cathodic protection principle, particularly by coupling to zinc or to the common zinc anodes? An answer to this question would certainly require that the electrochemical behavior of the aluminum - zinc galvanic couple be known precisely as a function of exterior atmospheric variables, such as temperature, humidity and chemistry of the condensed layer of water. Atmospheric corrosion sensors that have been developed recently at LSU/SU [1-3] enable electrochemical characterization of such couples not only in the open atmosphere, but under accelerated laboratory test conditions as well. Through these tests the kinetics of corrosion processes in galvanic couples can be followed as a function of time in the presence of corrosion products on the corroding metals. Using such a sensor, electrochemical polarization tests have been undertaken on aluminum ? zinc couples in the open~ natural atmosphere, as well as in two kinds of laboratory tests, cyclic fog (plain and salt-laden) test and continuous immersion. The results obtained from the polarization of electrodes, Al and Zn separately and the A1-Zngalvanic couple, are presented, discussed and interpreted in this paper to answer the question posed at the beginning of this section.
Aluminum articles that are prone to corrosion are generally protected by the application of coatings and/or corrosion inhibitors. Cladding corrosion resistant aluminum layers on corrosion prone aluminum alloys (alcladding) is a well developed practice. Some of the alloys are anodized and colored or impregnated with a corrosion inhibitor in the oxide layer for protection. Alternatively, corrosion inhibitors are added to the corrosive to form corrosion protective films on the surface of alloys. Commonly used coatings are of two kinds: organic (polymeric) and chemical conversion-type. In the former category the fluoro-polymers are dominant, while chromate conversion coatings are the most effective ones for the aluminum alloys. The chromate coatings used on aluminum articles, such as solar collectors and solar heat exchangers, etc. have since been discontinued owing to their carcinogen
