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jieyang114金蟲(chóng) (初入文壇)
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[求助]
翻譯為中文，化學(xué)式可以不管
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SVET mapping at the scribes showed one large anode and one large cathode at the scribe in the first days of immersion. The fact that both the anode and the cathode are located at the scribe shows that the scribe itself constitutes a complete corrosion cell, without the need for further electrochemical reactions to balance the electrons. As already observed by Bierwagen and coworkers , for longer immersion times the cathodic currents cease to be detected by the SVET. This reveals that the cathodic reaction takes place away from the scribe, i.e., underneath the coating, and is thus a distinct sign of the delamination process. Since the cathodic and anodic re-actions have to balance each other, then a large ratio between the cathodic and the anodic areas will lead to a small density of the cathodic current. The cathodic current is essentially due to the reduction reaction of oxygen: O2 + 2H2 O + 4e−→4OH− and occurs at the metal–coating interface, thus leading to an increase of pH (responsible for the rupture of coating metal bonds and/or to saponification of the coating) but also to an accumulation of anions in a confined region. Compensation of this excess of anions can be achieved either by the out-ward flow of the hydroxyl anions themselves or, more easily by the inward flow of cations. As pointed out by Leidheiser and Wang[31] , the high concentration of Na+ in the outer solution makes them the most likely cations to migrate to the front of the delamination. Migration of sodium cations to the delamination front was confirmed by Stratmann, who found an excess of sodium in the delaminated area but also in a thin area considered to be beyond the delamination front[32] . This migration of cations can occur in two ways: either across the coating or from the scribe. This last possibility, however, would mean that other cations would migratefrom the bulk into the scribe, generating a significant ionic flow above the scribe, equivalent to that of Zn2+ions flowing away from the surface. Such a flow should be detected by the SVET, in the same way as anodic currents are detected. In contrast, for a flow of ions across the coating, the cross area for the flow is much larger, leading to very small potential gradients above the coating, which are necessarily difficult to detect. Protection of zinc by pigments of zinc chromate and zinc phosphate was monitored by EIS, SVET and OCP, with good agreement between the three techniques. Evaluation of the electrochemical parameters for the bare zinc in pigment extracts allowed the ranking of the decreasing corrosion resistance in the extracts in 0.1 M NaCl as: chro-mate > phosphate > plain salt solution. Interpretation of the impedance spectra of the scribed coatings has shown that the equivalent circuit is identical to that of the bare metal in a first stage, whereas the processes occurring underneath the coating start to appear in a second stage. A high frequency relaxation constant observed in the scribed, pigmented coat-ings, was assigned to the formation of a protective layer of pigment underneath the coating. The decreasing rate of de-lamination, taken as the rate of growth of the double layer capacitance, was the same, i.e., chromate > phosphate > clear coat. Mapping of the local ionic fluxes above the metal surface has shown that zinc corrosion in NaCl occurs in a localized way, whereas the presence of any of the two pigment extracts inhibits the establishment of this local activity. |
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