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99506 PROTECTING ALUMINUM IN ATMOSPHERE THROUGH GALVANIC COUPLING TO ZINC

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Product Number: 51300-99506-SG
ISBN: 99506 1999 CP
Author: R Diwan, A. Raman and X. Huang
Industry: Chemical Inhibitors
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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. Keywords: Atmospheric corrosion sensors, corrosion monitoring, silver reference electrode, thin film electrodes, passivation, cathodic protection of Al with Zn, corrosion tests, zinc coating on aluminum.
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. Keywords: Atmospheric corrosion sensors, corrosion monitoring, silver reference electrode, thin film electrodes, passivation, cathodic protection of Al with Zn, corrosion tests, zinc coating on aluminum.
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