Electrochimica Acta 52 (2007) 2411–2422 Effect of Al on the galvanic ability of Zn–Al coating under thin layer of electrolyte A.P. Yadav a,b,∗ , H. Katayama a , K. Noda a , H. Masuda a , A. Nishikata b,1 , T. Tsuru b,1 a Corrosion Analysis Group, National Institute for Materials Science, Tsukuba, 305-0047 Ibaraki, Japan b Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 152-8552 Tokyo, Japan Received 24 June 2006; received in revised form 22 August 2006; accepted 23 August 2006 Available online 2 October 2006 Abstract The effect of Al on the galvanic ability of Zn–Al coating has been studied under thin electrolyte layers by measuring surface potential and surface pH. The changes of surface potential and surface pH over Zn–Al/steel galvanic couple corroding in artificial sea water (ASW) were measured at 60% and 90% RH at 298 K. In the initial stage of corrosion, Zn–55Al coating has shown better galvanic protection ability than Zn–5Al coating in both 60% and 90% RH. However, Zn–5Al coating was better in long term corrosion. The better galvanic ability of Zn–55Al coating in the initial stage of corrosion was related to the observation of pH as low as low as 2 on its surface. The low pH value was due to hydrolysis of Zn 2+ and Al 3+ ions. The low pH value was further confirmed by observing evolution of gas due to H + reduction on the Zn–55Al coating. With the progress of corrosion, the low pH region of coating layer extended towards the base steel. This helped expand the deposition of zinc corrosion products on the steel surface. The enhanced dissolution of zinc in Zn–55Al coating led to the formation of a barrier layer which limited the galvanic protection of remaining steel. This was not the case in Zn and Zn–5Al coating. The X-ray analyses of the corroded samples have shown the deposition of zinc corrosion products on the steel surface, which greatly depended on the RH value. The part of the steel surface covered with zinc corrosion products has shown relatively less noble potential than other part indicating that zinc corrosion products took a role to protect the base steel against corrosion. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test. © 2006 Elsevier Ltd. All rights reserved. Keywords: Galvanized steel; Zn–Al coating; Surface potential; Surface pH; Atmospheric corrosion; Galvanic corrosion 1. Introduction Zinc and aluminum protect steels through original barrier layer action of the coating, secondary barrier action of corrosion products, and galvanic action of coating layer at the exposed parts of underlying steel. Aluminum coating provides better protection than zinc coating since aluminum itself carrying pro- tective oxide is attacked very slowly [1]. However, under certain mild conditions the attack on aluminum is too slow to provide cathodic protection to steel and in such circumstances zinc coat- ing is preferred [2]. In view of limited galvanic protection by ∗ Corresponding author at: Corrosion Analysis Group, National Institute for Materials Science, Tsukuba, 305-0047 Ibaraki, Japan. Tel.: +81 3 5734 3146; fax: +81 3 5734 2835. E-mail address: amar2y@yahoo.com (A.P. Yadav). 1 ISE Member. aluminum, zinc alloyed with Al is used to enhance the corrosion resistance of zinc coating on the steel surfaces. The general corrosion behavior of Zn–Al alloy coating in atmospheric environment has been extensively studied [1,3]. However, there is limited study on the galvanic corrosion behav- ior of Zn–Al alloy coated steels, especially in atmospheric envi- ronment where corrosion occurs under a thin layer of electrolyte during wet–dry cycles, which limits the application of elec- trochemical methods. There are reports on the measurements of spatial distribution of Zn 2+ and OH - during galvanic cor- rosion of a model Zn/steel couple using a scanning tungsten pH electrode [4,5]. Anodic and cathodic regions on corroding metal surface have been detected by using a scanning ref- erence electrode technique (SRET) [6,7]. Similarly, Ishikawa and Isaacs have used a scanning vibrating electrode technique (SVET) to measure current distribution on a galvanic couple [8]. Nevertheless, most of these works have been made in bulk solution where the galvanic corrosion is different from 0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.08.050