Letter Zn–Al layered double hydroxides as chloride nanotraps in active protective coatings J. Tedim a,⇑ , A. Kuznetsova a , A.N. Salak a , F. Montemor b , D. Snihirova b , M. Pilz c , M.L. Zheludkevich a , M.G.S. Ferreira a a CICECO, Department of Ceramics and Glass Engineering, University of Aveiro, 3810-193 Aveiro, Portugal b ICEMS, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal c SINTEF Materials & Chemistry, Forskningsveien 1, 0373 Oslo, Norway article info Article history: Received 21 September 2011 Accepted 4 October 2011 Available online 10 October 2011 Keywords: A. Organic coatings B. X-ray diffraction abstract Zn–Al layered double hydroxides (LDHs) intercalated with nitrate anions are suggested as chloride nano- traps for organic polymeric coatings. The addition of such nanotraps to a polymer layer drastically reduces the permeability of corrosive chloride anions through the protective coatings. In solution, Zn(2)–Al–NO 3 LDHs are responsive to the concentration of chlorides and the release of nitrates is accom- panied by entrapment of chlorides, with the process governed by ion-exchange equilibrium. In particular, a coating modified with LDH–NO 3 was found to exhibit significantly lower permeability to chlorides when compared to both unmodified and LDH–Cl-containing coatings, which proves the applicability of LDHs in delaying coating degradation and corrosion initiation. Ó 2011 Elsevier Ltd. All rights reserved. The development of protective coatings based on nanostruc- tures has been a route to obtain the so-called ‘smart’ coatings capa- ble of responding to stimuli from environment [1]. In the field of corrosion protection there are several studies reporting the appli- cation of micro and nanoreservoirs for intercalation or encapsula- tion of corrosion inhibitors and controlled release, depending on local conditions such as pH [2], presence of aggressive species [3] and electrochemical activity [4]. Layered double hydroxides (LDHs), also called hydrotalcite-like compounds, are anion-exchangers. Structurally, LDHs consist of mixed-metal, positively-charged hydroxide layers separated by an- ions and water molecules [5]. Since the pioneering work of Buchheit et al. [3], several groups have focused on the anticorrosion properties of LDH-derived materials, including Buchheit’s [6], McMurray’s [7–9], Kendig’s [10] and more recently our group [11,12]. In some of these studies, active corrosion protection was attributed to the release of either inorganic or organic inhibitors from LDHs, triggered by the presence of corrosion-relevant anions such as chlorides [3,11,12]. The main role of LDHs is storage and release of inhibitors on demand, as a result of anion exchange between inhibiting species and chloride anions from the electrolyte solution. However, in parallel to the release of inhibitor an equivalent amount of harmful chlorides is adsorbed by LDHs. The amount of chloride ions penetrat- ing the coating, at least in early stages of immersion, is small and the entrapment of chlorides can assume an important role in delay- ing coating degradation, as well as the consequent initiation of corrosion-related phenomena. LDHs without any corrosion inhibi- tor have already been reported to improve corrosion protection when added to coating formulations. In particular, Williams and McMurray showed that LDHs loaded with carbonates and nitrates rendered a positive effect to the coated metallic substrates under filiform corrosion tests, when compared to the blank coating [8]. The decrease of delamination rate in coatings modified with LDHs was explained by the decrease in concentration of chlorides pene- trating along the metal coating interface in the filiform defect zone, resulting from displacement of LDH-intercalating anions by chlorides. Nevertheless, one of the main disadvantages of using CO 3 -loaded LDHs is the relative low anion-exchange equilibrium constant, due to the high affinity of CO 3 2 to stick to the layered structure, thus limiting the amount of anions that can be exchanged [5]. This partially explains the better performance of nitrate-loaded LDHs with respect to carbonate-loaded ones [8], together with pos- sible inhibiting effects provided by the nitrate anions as reported elsewhere [13]. In this work, Zn(2)–Al (Zn/Al atomic ratio 2:1) LDHs loaded with nitrate are investigated as possible chloride nanotraps, with their effect on the permeability of an organic-based coating, typically used in automotive industry for corrosion protection, being also surveyed. The effect of LDHs on (decreasing) coating permeability could arise from either entrapment of species (active role), or from increase in tortuosity of the system, which leads to a large decrease in the diffusion coefficient of species (passive role) [14]. Nonethe- less, to achieve a truly enhancement of coating barrier properties by increase of tortuosity, the polymer–LDH interface should be maximized. Such effect requires a very good dispersion with exfo- liation of the layered clay into individual lamellae, which is not the 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.10.003 ⇑ Corresponding author. Tel./fax: +351 234378146. E-mail address: joao.tedim@ua.pt (J. Tedim). Corrosion Science 55 (2012) 1–4 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci