The effect of an artificially synthesized simonkolleite layer on the corrosion of electrogalvanized steel J.D. Yoo a , P. Volovitch a,⇑ , A. Abdel Aal a , C. Allely b , K. Ogle a a Chimie ParisTech, ENSCP – CNRS (UMR 7045), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b ArcelorMittal Research SA, 57283 Maizières-lès-Metz, France article info Article history: Received 25 May 2012 Accepted 29 October 2012 Available online 7 November 2012 Keywords: A. Zinc A. Steel B. Polarization B. X-ray diffraction C. Oxygen reduction abstract Electrochemical behavior of low alloy steel covered by a synthetic layer of simonkolleite was studied alone and in galvanic couple with zinc. Simonkolleite on steel inhibits cathodic oxygen reduction in 5 wt.% NaCl at pH = 7 and 9 but it is unstable at pH = 11. In NaCl solution the galvanic current density between Zn and steel is reduced by more than twice if the synthetic simonkolleite layer is deposited on steel. The polarity between Zn and steel is inverted after several hours in NaHCO 3 solutions. X-ray dif- fraction confirmed the instability of simonkolleite in this conditions and its transformation into carbon- ate containing salts. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The corrosion of galvanized steel is governed by a complex mech- anism in which the sacrificial protection by zinc is combined with the formation of a corrosion product layer which can enhance protection by a barrier effect [1,2]. The best strategy to improve the life time of the product is to maximize the barrier resistance while maintaining a sufficient galvanic effect. In such a strategy the understanding of the role of the corrosion product on the cathodic and anodic reactivity of steel and zinc and on the galvanic couple is fundamental. Despite more than 50 years of use of galvanized steel, the role played by indi- vidual corrosion products in the corrosion mechanism of galvanized steel is still under discussion. Several authors have demonstrated for long-term exposure [3] and for laboratory tests [4] that formation of some corrosion products correlates with lower corrosion rate, while others have little blocking or enhancing effect on corrosion reactions. Different functions of iron-containing corrosion products for steel corrosion were recently reviewed by Tamura [5]. To our knowledge no similar work exists for Zn based corrosion products. Different basic salts of zinc can form on zinc coated steel: hydrozincite (HZ, Zn 5 (CO 3 ) 2 (OH) 6 ), simonkolleite (ZHC, Zn 5 (OH) 8 Cl 2 H 2 O), gordaite (NaZn 4 Cl(OH) 6 SO 4 6H 2 O), zinc hydroxysulfate (ZHS, Zn 4 SO 4 (OH) 6 nH 2 O) [6–9]. Another zinc based carbonate cor- rosion product is sodium zinc carbonate hydrate (SZC, Na 2 Zn 3 (CO 3 ) 4 3H 2 O) [10]. The layered hydroxide salts of zinc (LHS) have layered structure in which octahedral and tetrahedral coordinated zinc atoms in the layer are bonded with OH  from the layer and with anions such as CO 2 3 , Cl  , and SO 2 4 placed in the interlayer space [11,12]. In hydrozincite, CO 2 3 ions form near covalent bonds between the layers of zinc atoms whereas hydrogen bonds of OH– Cl in simonkolleite (ZHC) and of OH–O between SO 2 4 coordinating to Zn atoms in ZHS maintain the structure. Water molecules are also intercalated between the two layers. The layered structured of LHS allows intercalation and anion-exchange. The presence of basic salts formed on zinc [13–17] and steel [1] can influence both cathodic and anodic reactions on zinc and steel due to mass and charge transport. For anodic reactions, the concentration of Cl  can be critical which can explain the importance of possible ion exchange properties of basic zinc salts. The inhibiting effect of the corrosion product layer on the ability of Cl  ions to reach the anodic sites on Zn was previously reported [18]. The cathodic reaction in atmospheric corrosion is usually the reduction of oxygen which can pass by 2e  (reactions (1) and (2)) or 4e  (reaction (3)) process [15–16,19,20]. O 2 þ H 2 O þ 2e  ! HO  2 þ OH  ðrate determining stepÞ ð1Þ HO  2 þ H 2 O þ 2e  ! 3OH  ð2Þ O 2 þ 2H 2 O þ 4e  ! 4OH  ð3Þ For Zn under its corrosion product the main cathodic reaction is an oxygen reduction for potentials until 1.3 V vs. SCE while at more negative potentials zinc corrosion products are also reduced and the water reduction becomes significant [14]. 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.10.024 ⇑ Corresponding author. Tel.: +33 1 44 27 67 36; fax: +33 1 46 34 07 53. E-mail address: polina-volovitch@chimie-paristech.fr (P. Volovitch). Corrosion Science 70 (2013) 1–10 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci