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Duplex coatings combine an "active" protection provided by a metal deposit, playing the role of sacrificial anode to cathodically protect the substrate with "passive" protection of an organic layer, playing the role of a diffusional barrier to hinder transport of ionic species. The corrosion behavior of these duplex coatings was studied in a chlorinated solution for long immersion times. The electrochemical impedance spectroscopy was measured during the immersion and an original physical model was proposed. This model considers first, for short immersion times, a porous electrode behavior corresponding to impedance proportional to the square root of plane electrode impedance. For longer immersion times, the development of secondary pores in the walls of the initial pores was assumed. Consequently, an impedance proportional to Z 0.25 was obtained, where Z is the impedance of a plane electrode. This model agrees with the experiment and needs only three unknown parameters for short immersion times and five for long immersion times.Duplex coatings were originally developed as a protection against atmospheric corrosion and have also been applied to mitigate corrosion in the soil. Duplex coatings combine an "active" protection provided by a metal deposit, playing the role of sacrificial anode to cathodically protect the substrate with "passive" protection of an organic layer, playing the role of a diffusional barrier to hinder transport of ionic species. The protection provided by duplex coatings must be conceived as being the result of a synergy between the individual protections offered by the metal deposit and the organic coating in the sense that total protection is higher than the sum of individual protections of the two coatings.The metal deposit is a Zn-Al alloy which combines the high corrosion resistance of aluminum oxides with the excellent cathodic protection of zinc. 1 With a composition close to eutectic, a rich aluminum network mechanically connects zinc oxides, slowing down corrosion by the formation of a more compact physical barrier. 2,3 These oxides, in aerated neutral medium, have the composition Zn 6 Al 2 ͑OH͒ 16 CO 3 · 4H 2 O. 4 The role of traditional anticorrosion paints was extensively studied in literature, 5 particularly by electrochemical impedance spectroscopy. [6][7][8][9][10][11] The main parameters are C f , the coating capacity, and R po , the pore resistance. Usually, for these paints, porosity must be as low as possible in order to isolate the substrate. For the duplex coatings under investigation, the paint is present to slow down the corrosion of the metallic coating by decreasing the exchange with the external medium, but the active protection of the metallic coating must be effective. Thus, the paint must be sufficiently porous to allow electrochemical reactivity of the metallic coating.In the present work, the corrosion behavior of duplex coatings was studied in an aerated chlorinated solution ͑NaCl 6 g L −1 ͒ for long immersion times ͑ϳ200 days͒. The study of artificial defects o...

SummaryPure AISI 316L steel is investigated after solution heat treatment (1050 °C/H 2 O) and structural sensitization (650 °C). Two quite different intergranular corrosion tests are used to determine the degree of structural sensitization due to the precipitation of secondary phases along the grain boundaries (mainly the M 23 C 6 and σ-phase): the oxalic acid etch test and the electrochemical potentio-kinetic reactivation test. Generally, the dissolution of chromium-rich carbides (M 23 C 6 ) is provoked by oxalic acid etch tests, whereas the chromiumdepleted zones, in the vicinity of chromium-rich carbides (M 23 C 6 ), are attacked by electrochemical potentio-kinetic reactivation tests. Both intergranular corrosion tests are used to determine the maximum degree of structural sensitization. Thus structural analysis by carbon replicas reveals the Laves phase, and both the M 23 C 6 and (Cr,Mo) x (Fe,Ni) y phases. The results of intergranular corrosion tests are related to the findings of the structural analysis.