Electrochimica Acta xxx (2005) xxx–xxx Electrochemical study on magnesium anodes in NaCl and CaSO 4 –Mg(OH) 2 aqueous solutions Fidel Guadarrama-Mu˜ noz a , Juan Mendoza-Flores a , Ruben Duran-Romero a , J. Genesca b,∗ a Mexican Petroleum Institute, Exploration and Production Corrosion, Eje Central L. Cardenas, 152, Mexico 07730 DF, Mexico b Corrosion Laboratory, Department of Metallurgical Engineering, Faculty of Chemistry, UNAM, 04510 Mexico DF, Mexico Received 23 June 2004; received in revised form 29 November 2004; accepted 25 February 2005 Abstract The anodic dissolution behavior of three samples of magnesium anodes, with different electrochemical efficiency, was studied in two test aqueous solutions: 3% NaCl and saturated CaSO 4 ·2H 2 O–Mg(OH) 2 . Polarization curves and electrochemical impedance spectroscopy (EIS) were used. It was found that activation processes takes place at the metal–electrolyte interface. These processes can be represented by two parallel electric equivalent circuits, arranged in series, and in series with the resistance of the solution. It is proposed that the physical meaning of the two parallel circuits can be associated to a charge transfer process and to the presence of a layer of corrosion products on the surface of the metal. © 2005 Elsevier Ltd. All rights reserved. Keywords: Cathodic protection; Current efficiency; EIS; Magnesium anodes 1. Introduction At the time, there are two laboratory tests methods for the evaluation of magnesium anodes used in the cathodic pro- tection industry of Mexico. These methods are based on the mass loss of magnesium test samples, immersed in a defined test environment, during a determined period of time and at a constant anodic current density. Both methods intend to pro- vide a way of comparing the performance of different anode alloys in a controlled environment. One method is the ASTM G-97-97, “standard tests method for laboratory evaluation of magnesium sacrificial anode test specimens for underground applications” [1]. The second method is described by the Mexican standard NMX–K–109–1977, “magnesium anodes used in cathodic protection” [2]. The efficiency of Mg anodes usually is determined using the ASTM G97-97 [1] test method, which involves cutting and machining five 12.7 by 152 mm specimens from an anode ∗ Corresponding author. Fax: +52 55 56225228. E-mail address: genesca@servidor.unam.mx (J. Genesca). casting. The specimens are placed in separate containers with a calcium sulfate, CaSO 4 /magnesium hydroxide, Mg(OH) 2 electrolyte, connected in series with coulometers and pow- ered at a constant current density of 0.039 mA cm -2 for 14 days. The weight loss during the test period is compared to the theoretical weight loss calculated based on coulometer measurements. The Mexican test method indicates a test envi- ronment made of artificial sea water, a testing current density of 1.25 mA cm -2 and a testing time between 96 and 120 h. Both testing methods are in fact galvanostatic tests, in which a known direct current is passed through test cells connected in series in order to determine efficiency of sacrificial anode materials. To a considerable extent anode efficiencies have been reported only for the 36 (0.039 mA cm -2 ) and 72 mA ft -2 current densities since these correspond most closely to the current densities encountered in actual underground ser- vice [3]. The Mexican NOM K 109–1977 is a more severe test than its ASTM counterpart and anode current density, 1.25 mA cm -2 exceeds normal underground service, but with short testing period. 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.144 EA-11008; No. of Pages 11