Synergistic effect of tolutriazol and sodium carboxylates on zinc corrosion in atmospheric conditions

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Electrochimica Acta 53 (2008) 4839–4845

Synergistic effect of tolutriazol and sodium carboxylates on zinc corrosion in atmospheric conditions C. Georges, E. Rocca ∗ , P. Steinmetz Laboratoire de Chimie du Solide Min´eral UMR CNRS 7555, Nancy Universit´e, BP 239, 54506 Vandoeuvre les Nancy, France Received 22 November 2007; received in revised form 23 January 2008; accepted 26 January 2008 Available online 5 February 2008

Abstract During the transport or the storage, galvanized steel products undergo the wet storage stain phenomenon, leading to the formation of unsightly and extensive white rust of zinc. In the context of development of temporary protection to replace mineral oils, the present study is focused on the combined action of sodium heptanoate (CH3 (CH2 )5 COONa, noted NaC7 ) and 3-methyl benzotriazol (named tolutriazol, TTA) on zinc corrosion. Stationary and dynamic electrochemical measurements have shown that the association of NaC7 and TTA provokes the formation of insoluble and hydrophobic passive layer, and lead to a positive synergy between the anodic and cathodic inhibiting actions of the two respective compounds. According to XPS and MEB analysis, the protective material is a layer containing zinc cation, heptanoate anion and TTA molecules, formed by a mechanism of precipitation on the zinc surface. In accordance with the coordination properties between Zn2+ and TTA determined by pH titration, the TTA molecules is trapped in a zinc soap, probably in a compound, Zn(TTA)2 (C7 )2 , which explains the efficiency of this formulation in the climatic chamber test. © 2008 Elsevier Ltd. All rights reserved. Keywords: Tolutriazol; Carboxylate; Corrosion inhibition; Zinc corrosion

1. Introduction Atmospheric corrosion is one of the most prevalent types of corrosion for zinc, owing to the extensive use of galvanized steels. Among the atmospheric corrosion forms of zinc, the “wet storage stain” appearing on galvanized steel products during the transport or the storage can seriously affect the appearance and the quality of the metal surface and lead to the formation of an extensive “white rust” [1,2]. Moreover, an incomplete drying or a close packing of the articles can attract and absorb moisture by capillary action and induce a local attack of the metal surface. To avoid this kind of corrosion, a temporary protection is necessary. In practice, applications of mineral oil or chromate coatings are used in the galvanizing industry as a surface treatment to prevent this phenomenon [3]. To fulfil the environmental standard concerning the wastewater rejects in industries, several surface treatment based on anionic inhibitors, more or less toxic, have been considered [4–6]: silicate, phosphate, phosphonate, benzoate, amine derivatives.

∗

Corresponding author. Fax: +33 3 83 68 46 11. E-mail address: [email protected] (E. Rocca).

0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.01.073

Over the past 10 years, non-toxic linear sodium monocarboxylates extracted from vegetable oils with a general formula CH3 (CH2 )n−2 COONa were investigated with success to inhibit corrosion of many metals (Cu, Zn, Pb and Mg) in aerated aqueous solutions [7–11]. As proved by X-ray diffraction, EcAFM and different spectroscopy, the protection is generally attributed to the presence of a thin metallic soap film, which can be made up of both metallic carboxylate and hydroxide [8]. In the case of zinc corrosion, previous study has shown that a thin layer of zinc hydroxycarboxylate, Zn5 (CH3 (CH2 )5 COO)2 (OH)8 , can be an environment-friendly anticorrosion alternative by creating an efficient barrier in some conditions of temporary protections [11]. Nevertheless, in conditions of closed packing of galvanized packing, the use of sodium carboxylate is not enough efficient to completely inhibit the zinc corrosion. Indeed, the local diffusion of oxygen in micro-environments between two metallic parts induces a rapid and important local attack of the metal surface by a differential aeration between the external surface and the interstice, which provokes the formation of pain killing white and voluminous corrosion products also named “cosmetic corrosion”. Then, the zinc oxidation provokes the formation of white materials on the surface, mainly composed by zinc layered

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hydroxide: Zn5 Cl2 (OH)8 , H2 O and Zn5 (CO3 )2 (OH)6 [12,13]. The crystallization of these materials leads to voluminous and powdery deposits on surfaces. In addition, these materials on zinc surfaces are susceptible to absorb chloride and water by intercalation reactions or dissolution/co-precipitation mechanisms, and constitute tanks to retain and maintain the humidity and the corrosive anions. So, the purpose of this work is to study the possibility to reinforce the protection of zinc by combining carboxylate compound with triazol compound. Triazol compounds have already been studied as corrosion inhibitors of zinc by some authors, but in practice, these compounds are easily washable from zinc surface after application [14,15]. So, the study of the addition of 3-methyl benzotriazol also named tolutriazol (noted TTA) to sodium carboxylate solution was carried out to improve the protection of zinc carboxylate layer. The effect of TTA concentration on the inhibition efficiency has been studied in an aerated corrosive reference water (ASTM D1384 [16]) by using different stationary electrochemical methods and electrochemical impedance spectroscopy (EIS) to characterize the electrochemical behaviour of the passive film. Electrochemical quartz crystal microbalance (EQCM) was used to evaluate in situ the amounts of zinc corrosion products with or without inhibitors. The morphology of the passive layers was examined by scanning electron microscopy (SEM) and the composition of the passive layer was analyzed by X-ray photoelectron spectroscopy (XPS). Zinc/tolutriazol complex with different TTA proportions were prepared in aqueous solution by acid-base titration to understand the surface chemistry between the cation Zn2+ and the TTA molecules. Finally, galvanized steel sheets were treated in TTA/carboxylate solution, and finally allowed to corrode in a climatic chamber. 2. Experimental Linear sodium heptanoate CH3 (CH2 )5 COONa noted NaC7 was obtained through the neutralisation of heptanoic acid by sodium hydroxide. The corrosive medium used as reference in electrochemical experiments was the ASTM D 1384-87 solution (noted ASTM [16,18]) with following composition: 148 mg l−1 Na2 SO4 , 138 mg l−1 NaHCO3 and 165 mg l−1 NaCl. The 0.05 mol l−1 of sodium heptanoate with different amounts of tolutriazol noted TTA (or 3-methyl-benzotriazol) were made in the ASTM medium and their pH was adjusted to 8. The electrochemical tests were performed, in aerated conditions, with a three-electrode electrochemical cell, connected to an EGG PAR 273A potentiostat and a frequency response analyzer (Solartron FRA 1255), driven by a computer. In this configuration, the circular working electrode surface (pure zinc: 99.99%) is vertical, facing the Pt-disk counter electrode. The reference electrode was a KCl-saturated calomel electrode (Hg/Hg2 Cl2 , E = +0.242 V/SHE) and all the working electrode potentials were given versus this reference. Before the experiments, the working electrode was mechanically polished with successively finer grades of SiC emery papers up to 1200. Samples were finally rinsed with distilled water, ethanol and dried.

The following experimental sequence was used: (i) Measurements of the corrosion potential (Ecor ) and the polarisation resistance (Rp ) every 2 h for a duration of 20 h, with a scan rate of 0.166 mV s−1 for a range of 20 mV (Ecor ± 10 mV). The error on the Rp assessment was evaluated at less than 10%. (ii) Measurements of the electrochemical impedance spectrum from 105 to 4 × 10−3 Hz with a 10 mV amplitude. (iii) Recording of the potentiodynamic curve, i = f(E), from −300 mV versus Ecor to 1400 mV, with a sweep rate of 1 mV s−1 . Electrochemical impedance data were fitted with Zsimpwin software using non-linear least square fit techniques [17]. An EQCM was used for in situ measurement of the mass change of the electrode during the electrochemical measurements. The EQCM equipment was an AMETEK QCA-922 plating monitor with 9 MHz quartz crystals of 5 mm in diameter. The crystal quartz was coated with a platinum film on the two sides. The exposed area of the platinum film was 0.196 cm2 . Then, a zinc film was deposited by galvanostatic reduction (i = −5 mA cm2 ) on the crystal quartz from a solution containing 360 g/l ZnSO4 ·7H2 O and 30 g/l NH4 Cl with 120 g/l of glucose during 250 s. During the experiments, the mass change of the platinum-coated quartz is the function of the shift of its resonance frequency. Over a wide mass range, the relationship between the shift of the resonance frequency (f) and the mass change (m in ␮g cm−2 ) is described by the Sauerbrey equation: f = −K m, where K is a positive constant. In our case, the Sauerbrey coefficient K was measured to 180 Hz ␮g−1 cm2 during the zinc electrodeposition by applying the Faraday law. The powder of zinc hydroxyheptanoate (Zn5 (CH3 –(CH2 )5 –COO)2 (OH)8 , noted Zn5 (C7 )2 (OH)8 ) was obtained by precipitation after mixing of sodium heptanoate, zinc nitrate aqueous solutions at pH 8. The white precipitates were filtered, rinsed with distilled water, and then dried under vacuum in a desiccator. To highlight the different complexes formed in the presence of zinc cations and TTA in function of pH, acid-base titration experiments were realised by adding, under stirring and nitrogen bubbling, 14 ␮l of a 0.1 mol l−1 NaOH solution every 100 s into different Zn(ClO4 )2 /TTA melt, by the help of 721 NET Titrino Metrohm. The initial pH of the mixing was adjusted to 2.5 with HClO4 concentrated solution and 0.1 mol l−1 NaClO4 solution was used as ionic buffer. Finally, electrogalvanized sheets provided by Arcelor (France) were treated by inhibiting solutions, before they underwent the following corrosion cycle in a climatic chamber (KBEA 300, LIEBISCH): 8 h at 100% humidity, using twice-distilled water heated to 40 ◦ C, and then 16 h under the room conditions. 3. Results and discussion 3.1. Corrosion behaviour of zinc in TTA/NaC7 solution In ASTM corrosive water, the Rp and Ecor of zinc in 0.05 mol l−1 NaC7 solution containing different TTA concen-

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Fig. 2. Mass variation (m in ␮g cm2 ) of zinc versus immersion time for free inhibitor ASTM solution (curve A) and ASTM solutions with 1 g l−1 TTA (curve B), with 0.05 mol l−1 NaC7 (curve C) and with 0.05 mol l−1 NaC7 + 1 g l−1 TTA (curve E).

Fig. 1. Polarisation resistance, Rp (a) and corrosion potential, Ecor (b) of zinc versus immersion time for free inhibitor ASTM solution (*, A) and ASTM solutions with 1 g l−1 TTA (, B), with 0.05 mol l−1 NaC7 (, C), with 0.05 mol l−1 NaC7 + 0.25 g l−1 TTA (, D) and with 0.05 mol l−1 NaC7 + 1 g l−1 TTA (䊉, E).

trations versus the immersion time were shown in Fig. 1. The lowest values of Rp were observed for the ASTM solution without inhibitor (A: Rp < 10 k cm2 ). In the heptanoate solution without TTA (curve C), the values of Rp remains relatively low (30 < Rp < 70 k cm2 ); whereas in presence of TTA (curves B, D and E), the Rp values are two orders of magnitude higher in comparison with the values measured in the inhibitor free ASTM water (Rp > 1000 k cm2 ). However, whatever the TTA concentration, the Ecor of zinc in the NaC7 solution are 200–400 mV higher than in the inhibitor free ASTM water (A) as noted in the Fig. 1b. Moreover, we can note that the addition of single TTA (curve B) is not sufficient to maintain a high Ecor of zinc with time in the ASTM water. At open-circuit potential, EQCM experiments allow in situ monitoring the mass change of the zinc-coated quartz in presence of different inhibiting compositions (Fig. 2). The highest value of the mass gain rate was observed in the ASTM solution without inhibitor because of the growth of zinc corrosion products (curve A: m = +65 ␮g cm2 after 4 h immersion). In the 0.05 mol l−1 NaC7 solution (curve C), the value of the mass gain rate is relatively important despite the fact that the inhibition efficiency is significant according to the Rp measurements. In presence of TTA (curves B and E), the mass gain rate remains very low and is negligible for the solution containing only TTA (curve B), compared to the others.

Over all the potential range, the electrochemical behaviour of zinc in the inhibiting solutions was evaluated by recording the voltammograms after 24 h of immersion, displayed in Fig. 3. The comparison of the voltammograms reveals that the presence of 0.05 mol l−1 NaC7 (curve C) induces a decrease of the anodic current on zinc, and the formation of passivation plateau until −100 mV. However, the NaC7 addition has no effect on the cathodic kinetic of oxygen reduction on zinc. Concerning the TTA addition, we can observe a drastic decrease of the corrosion current density in the Tafel region of the voltammogram; nevertheless the pitting phenomenon is very rapid with the potential increase and no passivation plateau is observed (curve B). The corrosion current density of zinc with TTA is drastically reduced because of an important inhibition of the oxygen reduction in the cathodic region of the voltammogram. By combining NaC7 and TTA in the ASTM solution (curves D and E), the corrosion current density of zinc decrease of more than two orders of magnitude and the passive layer is stable until 1400 mV. In these conditions, it is remarkable to note that the addition of 0.25 g l−1 TTA to 0.05 mol l−1 NaC7 is sufficient to maintain the cathodic inhibition and suppress the anodic pitting phenomenon as shown in the Fig. 3 (curve D).

Fig. 3. Voltammograms of zinc in free inhibitor ASTM solution (curve A) and ASTM solutions with 1 g l−1 TTA (curve B), with 0.05 mol l−1 NaC7 (curve C), with 0.05 mol l−1 NaC7 + 0.25 g l−1 TTA (curve D) and with 0.05 mol l−1 NaC7 + 1 g l−1 TTA (curve E).

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Fig. 6. Electrical equivalent circuit used to fit the EIS data of the Fig. 5.

composed by: Fig. 4. Mass variation of zinc in free inhibitor ASTM solution (curve A) and ASTM solutions with 1 g l−1 TTA (curve B), with 0.05 mol l−1 NaC7 (curve C), and with 0.05 mol l−1 NaC7 + 1 g l−1 TTA (curve E) recorded during the voltammograms.

The recording of the mass change with EQCM during the voltammogram confirms the electrochemical behaviour of zinc in these different mediums, as displayed in Fig. 4. So, the addition of single NaC7 in ASTM water provokes a very small mass change until 0 to +100 mV, which corresponds to the formation of the small passivation plateau in the voltammograms. After +100 mV, the large dissolution of zinc to Zn2+ cations in the solution provokes a drastic mass loss of the zinc-coated quartz. By comparing curves A and B, we confirm that the TTA addition has a very small effect on the anodic reaction of zinc dissolution in these conditions. On the other hand, the mass change of zinc in presence of the both compounds (curve E) is very small during the potential scan, which underlines a very efficient inhibition of zinc dissolution. 3.2. EIS study of passive layers In order to characterise the passive layer formed in the different solutions, EIS measurements were carried out at the Ecor after 20 h of immersion. The impedance data are displayed as Bode plots in the Fig. 5 for three different solutions: free inhibitor ASTM solution, ASTM with 0.05 mol l−1 NaC7 and ASTM with 0.05 mol l−1 NaC7 and 1 g l−1 TTA. In the three cases, the electrochemical interface can be interpreted with two time constants by using the same simple equivalent circuit (Fig. 6),

• Re , the electrolyte resistance, • (Rt , CPEfilm ) which is assigned to the charge transfer process across the electrochemical interface and the capacity of the passive layer (the admittance of the CPE is defined as 1/Z = CPE(jωn ), • (Rd , CPEd ) which is assigned to the diffusion process of oxygen at the interface. According to the fitting results noted in the Table 1, the constant phase element noted CPEd can be interpreted as a Warburg-like component with a n coefficient close to 0.5. The choice of this circuit is a compromise between a reasonable fitting of the experimental values and a good description of the electrochemical system, by keeping the number of circuit elements at a minimum. In Table 1, we can note that the passive layer induces an increase of the charge transfer resistance Rt in presence of single NaC7 . Moreover, the decrease of the passive layer capacity (CPEfilm ) in presence of NaC7 indicates also the growth of compact and more protective film on zinc. However, the diffusion resistance, Rd , slightly changes with single NaC7 , which indicates that this passive film is not sufficient to reduce the diffusion processes at the zinc surface. With addition of TTA, Rt faintly increases, but we can observe a drastic increase of the diffusion resistance, Rd . These results confirm that heptanoate has mainly an inhibiting effect on the anodic reaction and the charge transfer process, whereas TTA brings an inhibiting effect on the cathodic reaction and the diffusion of oxygen.

Fig. 5. Bode plots of impedance data of zinc after 20 h of immersion in ASTM solutions (䊉), with 0.05 mol l−1 NaC7 (), with 0.05 mol l−1 NaC7 + 1 g l−1 TTA ().

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Table 1 Value of electrical elements of equivalent circuit fitting the EIS data of the Fig. 5

Re ( cm2 ) CPEfilm (S sn cm−2 ) n Rt ( cm2 ) Rd ( cm2 ) CPEd (S sn cm−2 ) n

ASTM water

ASTM water (NaC7 0.05 mol l−1 )

ASTM water (0.05 mol l−1 NaC7 , 1 g l−1 TTA)

380 2.3 × 10−6 0.88 1.57 × 104 2.66 × 104 8.4 × 10−5 0.45

145 2.9 × 10−7 0.96 9.76 × 104 3.39 × 104 2.5 × 10−6 0.50

136 5.4 × 10−7 0.98 4.34 × 105 5.63 × 106 6.3 × 10−7 0.55

3.3. Climatic chamber test To simulate the real conditions of differential aeration of closed packing, the electrogalvanized steel sheets were piled up and pressed in an experimental set up displayed in Fig. 7a. The treated electrogalvanized sheets were immersed during 10 min in the inhibiting solution containing 0.1 mol l−1 NaC7 and 1 g l−1 of TTA. After 25 days of test (25 cycles), the macroscopic aspect of sheets was represented in Fig. 7b and c. The non-treated sheets were covered by white rust responsible for the problems of “cosmetic” corrosion of zinc, and mainly composed by hydrozincite compound (Zn5 (CO3 )2 (OH)6 ) according to XRD analysis. In contrary, the treated sheets remain unchanged after 25 days of test. 3.4. Analysis of passive layers The zinc surface after immersion for a long immersion time (48 h) in solution containing 0.1 mol l−1 of NaC7 at pH 8 with or without 1 g l−1 TTA was observed by SEM. Some typical images, displayed in the Fig. 8, clearly reveals that the zinc surface is well covered by a thin layer of a precipitated com-

pounds. EDX spectroscopy performed on these layers exhibit the presence of carbon, oxygen and zinc. In the case of the layer formed in NaC7 solution, XRD measurements on the layer show the presence of a crystallised material with three peaks at low angles, characteristic of a lamellar compound as illustrated by periodic diffraction peaks (Fig. 9). This layered compound is identified as zinc hydroxyheptanoate Zn5 (C7 )2 (OH)8 by comparison with the powder compound synthesized by pH-controlled precipitation and studied in a previous study [11]. When TTA is added to the NaC7 solution, the passive layer seems to be less porous, more homogeneous than the one formed in the NaC7 solution without TTA (Fig. 8b). But, in this case, the layer is not enough crystallized to be analyzed by XRD. Nevertheless XPS analysis performed on the TTA/C7 layer reveal bring to light the presence of a high concentration of nitrogen, which corresponds to the incorporation of the TTA molecule in the layer (Fig. 10). According to literature, many coordination compounds exist with divalent metal such as Zn2+ , or Cu2+ and triazol derivatives as ligands [18,19]. One of the hypotheses to explain the incorporation of TTA molecules in the zinc hydroxycarboxy-

Fig. 7. (a) Experimental set-up of the packing test in the climatic chamber. Macroscopic aspect of galvanized sheet after 20 days in a climatic chamber in packing conditions: non-inhibited (b) and inhibited by a formulation based on NaC7 /TTA (c).

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Fig. 8. SEM observations of zinc surfaces immersed for 48 h in solution containing 0.1 mol l−1 NaC7 + 2 g l−1 TTA (a), and in solution containing 0.1 mol l−1 NaC7 (b).

Fig. 9. XRD patterns of passive layer treated by 10−1 mol l−1 NaC7 solution 48 h (1), and zinc hydroxyheptanoate Zn5 (C7 )2 (OH)8 synthesised at pH 8 (2).

Fig. 10. XPS spectra of zinc surfaces immersed for 48 h in ASTM solutions with 0.05 mol l−1 NaC7 (a), with 1 g l−1 TTA (b) and with 0.05 mol l−1 NaC7 + 1 g l−1 TTA (c).

Fig. 11. Acid base titration curves obtained by using NaOH 0.1 mol l−1 as titrating solution.

late thin layer is the formation of metallic complex between the Zn2+ cation and TTA. To determine the possible coordination compounds between Zn2+ and TTA, the pH of Zn2+ /TTA mixture was monitored along a slow addition of NaOH. Typical pH = f(VNaOH ) titration curves of the Zn2+ solutions with different Zn2+ /TTA ratio are displayed in Fig. 11. In all the curves, the first pH-drop before pH 4 corresponds to the titration of the HClO4 excess. The titration of the pure Zn2+ solution shows one pH-plateau corresponding in these conditions to the precipitation of zinc hydroxide around pH 7.5. The final step at pH 11–12 corresponds to the dissolution of the Zn(OH)2 compounds into zincate ions (ZnO2 − ) The titration of the solution with a low Zn2+ /TTA ratio (1/1) reveals two distinctive pH-plateaus. The second plateau at pH 7.5 correspond to the precipitation of Zn(OH)2 , but the first plateau at pH 4–5 is assigned to the formation of a Zn/TTA mixed compound which inhibits the precipitation of zinc hydroxide. For Zn2+ /TTA = 1/2 ratio, the precipitation of Zn(OH)2 at pH 7.5 is completely inhibited by the formation of Zn/TTA compound at pH 4–5. In presence of a large excess of TTA (Zn2+ /TTA = 1/4),

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we also observe no precipitation of Zn(OH)2 at pH 7.5, but the last pH-drop above pH 7.5 corresponds to the titration of the excess of TTA (pKa = 8.2 [20]). Consequently, according to these results in presence of TTA (Fig. 11), an addition of 2 mol of OH− is necessary to completely trap 1 mol of Zn2+ and avoid the precipitation of Zn(OH)2 . So, we can propose the formation a zinc coordination compound with TTA according to the reaction during the titration at pH 4–5: Zn2+ + 2(TTA-H)+ + 2OH− → Zn(TTA)2+ + 2H2 O Consequently it is reasonable to suggest that the thin passive layer formed on zinc in presence of TTA and NaC7 may be build up by Zn(TTA)2 2+ and C7 − anions in an insoluble compound Zn(TTA)2 (C7 )2 . 4. Conclusion From the results obtained through this study, the combination of sodium heptanoate and tolutriazol is an interesting alternative to mineral oils to form a very thin protective layer, and then to inhibit the formation of the “white rust” on zinc. A simple mechanism of film formation on zinc can be proposed: after an initial oxidation of zinc into Zn2+ cations by the oxygen dissolved in the solution, a thin protective layer is formed on the metal by co-precipitation of a TTA/C7 -based compound. According to the electrochemical results, the heptanoate anions has preferably an inhibiting effect on the anodic reaction by forming an insoluble and hydrophobic zinc soap layer, whereas the tolutriazol molecule has an inhibiting effect on the cathodic reaction of oxygen reduction on zinc. It is noteworthy that the corrosion current density fall by three orders of magnitude and to stabilize a passivation plateau beyond 1 V by the use of only 0.25 g l−1 of TTA in 0.05 mol l−1 of NaC7 . In practice, the precipitated layer on the surface inhibits almost totally the formation of “white rust” on zinc: the mass gain is negligible and the passive layer is stable over a large potential domain. Indeed, besides its insoluble and filming character, the TTA/C7 compound acts as an efficient barrier against oxygen on zinc and decreases drastically the rate of oxygen reduction.

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So the formation of insoluble and hydrophobic zinc soap layer, enriched in TTA molecules, leads to a positive synergy between the anodic and cathodic inhibiting actions of the two compounds. So, the TTA molecules is trapped in the zinc soap, probably in a compound, Zn(TTA)2 C7 , which explains the notable efficiency of this association, despite the conditions of streaming and differential aeration in climatic chamber. Further works will be devoted to the study of the compound Zn(TTA)2 (C7 )2 , and its action on the oxygen reduction on zinc. Indeed, the use of metallic soaps can be an interesting way to fix metallic complex as cathodic inhibitors on metal surface. References [1] X. Gregory Zhang, Corrosion and Electrochemistry of Zinc,Plenum Press, New York, 1996. [2] C.E. Bird, F.J. Strauss, Mater. Perform. 15 (11) (1976) 27. [3] A.T. El-Mallah, M.R.G.A. Magd, Met. Finish. 68 (1984) 54. [4] Y.I. Kuznetsov, L.P. Podgornova, Zashch. Met. 19 (1983) 98. [5] C.P. De Pauli, O.A.H. Derosa, M.C. Giordiano, J. Electroanal. Chem. 86 (1978) 335. [6] S.A. Awad, Kh.M. Kamel, J. Electroanal. Chem. 24 (1970) 217. [7] G.T. Hefter, N.A. North, S.H. Tan, Corrosion 53 (1997) 657. [8] E. Rocca, G. Bertrand, C. Rapin, J.C. Labrune, J. Electroanal. Chem. 503 (2001) 133. [9] E. Rocca, C. Rapin, F. Mirambet, Corros. Sci. 46 (2003) 653–665. [10] E. Rocca, J. Steinmetz, Corros. Sci. 43 (2001) 891. [11] E. Rocca, C. Caillet, A. Mesbah, M. Franc¸ois, J. Steinmetz, Chem. Mater. 18 (26) (2006) 6186. [12] F. Zhu, D. Peerson, D. Thierry, Corrosion 57 (2001) 582. [13] D. Peerson, A. Mikailov, D. Thierry, Proc. Eurocorr, Nice (France), 12–16 September, 2004. [14] K. Wang, H.W. Pickering, K.G. Weil, J. Electrochem. Soc. 150 (4) (2003) B176–B180. [15] K. Wippermann, J.W. Schultze, R. Kessel, J. Penninger, Corros. Sci. 32 (2) (1991) 205–230. [16] ASTM Standard D 1384, in: Standard test method for corrosion test engine coolants in glassware, ASTM, West Conshohocken, PA, 1988. [17] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [18] A.H. Yuan, H. Zhou, Acta Cryst. E E60 (11) (2004) m1565. [19] J. Lu, K. Zhao, Q.-R. Fang, X. Ji-Qing, J.-H. Yu, X. Zhang, H.-Y. Bie, T.-G. Wang, Cryst. Growth Des. 5 (3) (2005) 1091. [20] H. Wang, C. Burda, G. Persy, J. Wirz, J. Am. Chem. Soc. 122 (24) (2000) 5849.