Electrochemical behaviour of some transition metal ...

Corrosion Science 51 (2009) 409–414

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Electrochemical behaviour of some transition metal acetylacetonate complexes as corrosion inhibitors for mild steel M. Mahdavian, M.M. Attar * Polymer Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413 Tehran, Iran

a r t i c l e

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Article history: Received 6 September 2008 Accepted 11 November 2008 Available online 20 November 2008 Keywords: A. Mild steel C. Neutral inhibition B. EIS B. SEM Transition metal complexes

a b s t r a c t Corrosion inhibition of some metal acetylacetonate complexes including Co(acac)2, Cu(acac)2, Mn(acac)2 and Zn(acac)2 was evaluated using electrochemical impedance spectroscopy (EIS) in 3.5% NaCl for mild steel. The results were compared to zinc potassium chromate (ZPC) solution in 3.5% NaCl. Corrosion inhibition of these metal complexes followed the order: ZPC > Co(acac)2 > Zn(acac)2 > Mn(acac)2 while Cu(acac)2 displayed corrosion catalytic activity. Solutions containing metal acetylacetonate complexes had an increase in pH compared to Blank and ZPC solutions, which indicated partial dissociation of ligand and metallic cations. However, after 24-h contact with the mild steel samples the solutions pH were dropped which implied decrease of the complex concentration in the test solutions. SEM images showed no detectable deposited film on the surface exposed Co(acac)2 solution while EDX analysis revealed precipitation of a layer containing 4.14% Co. SEM-EDX results for samples immersed in Zn(acac)2 and Mn(acac)2 solution showed precipitation of Zn and Na components and Mn complex on the surface, respectively. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Metal acetylacetonate complexes are widely used as catalyst of chemical reactions e.g. oxidative dehydrogenation (ODH) of ethane and epoxidation of geraniol [1–3] and as stabilizer or precursor in sol–gel processes [4–6]. They are used in polymeric coatings as stabilizer of cross-linking reaction [7], catalyst of drying process [8,9] and toughening agent [10]. Very few works have been performed to study anticorrosive behaviour of metal acetylacetonate complexes. Harms et al. [11] proposed corrosion inhibition through precipitation of Fe (II) phosphate and Fe (III) phosphate in presence of Fe (III) acetylacetonate and Fe (II) acetylacetonate, respectively, where mild steel was immersed in phosphate containing solution. Palladium acetylacetonate is suggested as an effective corrosion inhibitor for water-cooled nuclear reactor [12]. It is reported that palladium acetylacetonate decomposes and deposits palladium on the oxide surface. Cerium, terbium, praseodymium acetylacetonate complexes are used to design non-toxic corrosion protection pigments [13]. Interaction of transition metal complexes with mild steel is greatly affected by their standard electrode potentials, their reactivity and the nature of the ligand that could stabilize the metallic complexes. Standard electrode potential of divalent cations follows the order: Cu(II)/Cu (+0.34 V) > Co(II)/Co (0.277 V) > Fe(II) > Fe (0.44 V) > Zn(II)/Zn (0.76 V) > Mn(II)/Mn (1.18 V) [14]. Reduc* Corresponding author. Tel.: +98 21 64542404; fax: +98 21 66468243. E-mail address: [email protected] (M.M. Attar). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.11.010

tion of Cu(II) and Co(II) species on the mild steel surface is possible due to their noble standard electrode potential compared to Fe(II). However, it should be noted that negative charged ligands like nitro, thiocyanate, oxalato, glycinato and acetylacetonate could stabilize the higher oxidation states [15]. Hence, the reduction of Cu(II) and Co(II) on the steel surface could be affected by the ligands surrounded them. Electrochemical impedance spectroscopy (EIS) as powerful nondestructive test could be used to extract electrochemical parameters involved in corrosion process. Extracted parameters are useful to evaluate corrosion protection performance of inhibitors, organic and inorganic coatings [16–19]. The aim of the present work is to evaluate corrosion inhibitive performance of some transition metal acetylacetonate complexes via EIS. In this regard, SEM-EDX and pH-metry were utilized to get more information about the mechanism of inhibition. 2. Experimental A 3.5% solution of NaCl was prepared from laboratory grade NaCl and distilled water. Transition metal acetylacetonate complexes including Co(acac)2, Cu(acac)2, Mn(acac)2, Zn(acac)2 were obtained from Merck and used without further purification. Metal complexes were separately dissolved in 3.5% NaCl solution. Solubility of the metal complexes in 3.5% NaCl were measured and the obtained values are listed in Table 1. To make experimental condition corresponding for all samples, the test solutions were prepared at 3.1 104 M. The mentioned concentration is the sol-

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M. Mahdavian, M.M. Attar / Corrosion Science 51 (2009) 409–414

Table 1 Solubility of metal complexes in 3.5% NaCl and the prepared concentration of the complexes in test solutions. Metal complex (MC)

Concentration of MC in test solution

Solubility of MC in 3.5% NaCl

Co(acac)2 Zn(acac)2 Mn(acac)2 Cu(acac)2 ZPC

3.1 104 M 3.1 104 M 3.1 104 M 3.1 104 M 3.1 104 M

9.2 103 M 7.3 103 M 3.1 104 M 5.6 104 M 4.2 103 M

Table 2 Composition of the mild steel panels. Elements

At %

C Si S P Mn Ni Cr V Cu W Ti Sn Co Al Nb Fe

0.0377 0.0161 0.0086 0.0056 0.5805 0.0137 0.0159 0.0018 0.065 0.0066 0.0034 0.0188 0.0053 0.0227 0.0037 99.195

ubility of manganese acetylacetonate, which has the minimum solubility among studied complexes (Table 1). Isothermal water bath was used in order to maintain the temperature precisely at 30 °C within ±0.1 °C variance. Mild steel sheets of 2 mm thickness obtained from Mobarakeh Steel Company and cut down to 2.5 7 cm2 samples. The composition of the mild steel panels is recorded in Table 2. Samples were polished with a magnetic polisher to remove mill-scale of the surface, followed by degreasing with xylene and acetone. Surface roughness in the range of 1–5 microns peak-to-valley was measured using Elcometer 223. An area of 1 cm2 of samples was exposed to the electrolytes whilst other area of the plates were sealed with beeswax-colophony mixture. Three replications were made to ensure repeatability. Each specimen was immersed in

100 ml of prepared electrolytes and EIS measurements were carried out using Autolab PGSTAT12 after 24 h of immersion. In order to compare electrochemical behaviour of metal acetylacetonate complexes Blank solution and zinc potassium chromate pigment extract at concentration of 3.1 104 M in 3.5% NaCl solution were used as references. EIS was implemented at open circuit potential in the frequency range of 102–10+4 Hz with 10 mV perturbation. Reference electrode and counter electrode were silver–silver chloride and platinum, respectively. Dissociation of acetylacetonate complexes in water was assessed by pH-meter of WTW model pH-315i before and after 24hr contact with the mild steel samples. Morphology and surface analysis of the samples immersed in test solutions for 24 h were studied using SEM-EDX (Philips XL30). 3. Results and discussion EIS diagrams for samples immersed in electrolytes containing ZPC, Co(acac)2, Cu(acac)2, Mn(acac)2, Zn(acac)2 as well as Blank solution are presented in Fig. 1a and b. These diagrams show two time constants for sample exposed in ZPC solution and one relaxation time for the rest of samples. The simple electrical circuit models of Fig. 2 were used to fit EIS results. The electrical elements in Fig. 2a are Rs, Rct and CPEdl, which represent electrolyte resistance, charge transfer resistance at metal-electrolyte interface and non-ideal double layer capacitance, respectively. Taking advantage of circuit model of Fig. 2a electrochemical behavior of samples immersed in solutions containing Co(acac)2, Cu(acac)2, Mn(acac)2, Zn(acac)2 was studied. Fig. 2b includes extra elements namely Rf and CPEf which represent film resistance and non-ideal film capacitance, respectively. Exploiting electrical circuit model of Fig. 2b, EIS results of samples exposed to ZPC solution was assessed. The capacitance values were calculated according to the Eq. (1) [20]:

C dl ¼

ðY 0 Rct Þ1=n : Rct

ð1Þ

In Eq. (1), Cdl represents double layer capacitance, Y0 and n are CPE parameters and Rct is the charge transfer resistance. On top of extracted parameters of fitted equivalent circuit, two parameter of bode plot including phase angle at 10 kHz and impedance magnitude at 100 mHz are listed in Table 3 as mean values of three replications.

Fig. 1. Bode diagrams of samples after 24 h of immersion in test solutions showing frequency dependency of (a) impedance magnitude and (b) phase angle.

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Fig. 2. Electrical circuit model used to fit EIS results with (a) one and (b) two time constant(s).

Table 3 Electrochemical parameters extracted from EIS results fitting and bode plots.

Co(acac)2 Cu(acac)2 Mn(acac)2 Zn(acac)2 ZPCa Blank

Rct (kX cm2)

Y0 (lF cm2)

n

Cdl (lF cm2)

h (deg.) at 10 kHz

log Z at 100 mHz

4.37 0.88 1.24 2.43 6.63 0.90

33.27 261.5 34.46 31.94 28.78 89.60

0.80 0.83 0.81 0.81 0.62 0.82

20.67 191.3 16.52 17.27 10.47 50.92

8.36 0.57 3.03 3.22 45.09 1.33

3.47 2.81 3.06 3.31 3.76 2.91

a Rf, Y0f and nf took the values of 165.3 X cm2, 1.12 lF cm2 and 0.82, respectively.

Except Cu(acac)2, all compounds resulted in increase of charge transfer resistance and decrease of double layer capacitance (Table 3) which could be connected to adsorption or film formation. Samples immersed in Mn(acac)2 solution showed negligible increase of charge transfer resistance compared to Blank (Table 3) which could be related to its physical adsorption (physisorption) on the surface. After exposure to Mn(acac)2 solution, surface color changed to dark gray. Samples exposed to Co(acac)2 solution exhibited the greatest impact among the other acetylacetonate complexes on corrosion inhibition (Table 3). After exposure to Co(acac)2 solution, surface color altered to pale brownish green at near 90 observation angles, however at near zero angle the appearance of the surface was just the same as an unexposed surface. Despite reduction of cobalt as an element due to its less negative standard electrode potential compared to Fe(II), it seems that a complex film of cobalt is formed. This process might be occurred for Co(II) acetylacetonate, because acetylacetonate ligand could prevent Co(II)/Co reduction on the mild steel surface. In presence of Co(acac)2 charge transfer resistance was increased from 0.9 kX m2 in Blank to 4.37 kX cm2. Some authors proposed ‘‘oxygen-scavenging” phenomenon for corrosion inhibition of Co(II) complexes [21]. Divalent cobalt could be oxidized to trivalent state in presence of dissolved oxygen. They proposed that sparingly soluble trivalent cobalt complex is responsible for inhibitive activity [21]. Samples immersed in Zn (acac)2 revealed increase of charge transfer resistance from 0.9 kX cm2 in Blank to 2.43 kX cm2 (Table 3). Some possible mechanisms could be imagined for corrosion inhibitive effect of Zn acetylacetonate complexes including complex formation between Zn(acac)2 and Fe oxide and hydroxide species and precipitation of insoluble Zn(II) compounds on the surface. Surface analysis provided some information of possible mechanism, which is discussed subsequently. Samples immersed in ZPC displayed the highest inhibition compared to other test solution (Table 3). ZPC improved charge transfer resistance from 0.9 kX cm2 in Blank to 6.63 kX cm2. In addition, ZPC forms a complex film on the surface with 165.3 X cm2. ZPC can enhance corrosion protection with further oxidation of previously formed oxide/hydroxide layers resulting in the formation of Cr2O3 and Fe2O3 hybrid on the surface [22]. During exposure of the specimens to the solution of Cu(acac)2 surface color was altered from polished iron to copper which

indicates reduction of Cu(II) on the surface. Reduction of Cu(II) on steel could occur because of much more positive standard electrode potential of Cu(II)/Cu (+0.34 V SHE) compared to Fe(II)/Fe (0.44 V SHE). Although negative charged ligands like acetylacetonate could stabilize the higher oxidation states [15], it seems that the change can not overcome the big difference between standard electrode potentials of Fe(II)/Fe and Cu(II)/Cu. Deposition of copper on the steel surface could lead to galvanic coupling which in turn results in decrease of charge transfer resistance. Appearance of Bode plots provides some information about the electrochemical behaviour of studied systems. Bode plots (Fig. 1a) display shift of frequency at maximum phase angle to higher frequencies in presence of Zn, Co, Mn acetylacetonate complexes and ZPC solution. However, in the case of Cu acetylacetonate this shift is toward lower frequencies. Impedance magnitude at 100 mHz and phase angle at 10 kHz give qualitative rough image of corrosion protection. In a case where a capacitor and a resistor are parallel, phase difference between current and voltage is a criterion for current to pass through either capacitor or resistor. The electrochemical behavior of metal-electrolyte interface is capacitive if charge transfer resistance and/or double layer capacitance are high. In such a case, current would mostly pass through capacitor; therefore, phase angle would be near 90. Moreover, the electrochemical behavior of metal-electrolyte interface is resistive if resistance and/or capacitance are low. Consequently, current would mostly pass through resistor; therefore, phase angle would be near zero [23]. As surface coverage takes place due to formation of protective film on the surface the charge transfer resistance increases and the double layer capacitance decreases. Decrease of the double layer capacitance has almost negligible impact on the capacitor impedance at high frequencies (Z = 1/jCx); however, the charge transfer resistance increment is quite influential (Z = R), never mind what frequency is. Hence, phase angle at 10 kHz shifts to more negative values as film formation takes place, showing increase of capacitive behavior at metal-electrolyte interface. According to Table 3, the most negative value of phase angle at 10 kHz (h) is obtained for ZPC solution which is around 45°. It was shown that for intact coatings the value of h is close to 90° [23]. Acetylacetonate complex of Co(II) takes the second rank having h value around 8°. The least negative h is obtained in the case of acetylacetonate complex of Cu(II). Impedance magnitude at 100 mHz almost goes in the same direction with phase angle at 10 kHz. The highest impedance at 100 mHz was obtained for samples exposed to ZPC solution and the lowest was observed for electrolyte composed of acetylacetonate complex of Cu (II). The results of pH-metry of the test solutions before and after contact with the mild steel samples are recorded in Table 4. Solutions containing metal acetylacetonate complexes had an increase in pH compared to Blank and ZPC solutions, which indicates partial dissociation of the anionic ligand and the metallic cations as depicted in Eq. (2). With increase of atomic number, the metallic complexes introduced increase of pH following the sequence: Mn < Co > Cu < Zn. In other words, stability of the complexes for pH variations follows the order: Mn > Co < Cu > Zn. Except Mn(II), the order of stabilities is in good agreement with the Irving-Wil-

Table 4 The pH of test solutions before and after immersion of mild steel samples.

Co(acac)2 Cu(acac)2 Mn(acac)2 Zn(acac)2 ZPC Blank

pH (before immersion)

pH (after immersion)

8.39 6.88 7.06 7.61 6.08 6.01

6.96 6.81 6.66 5.95 6.25 6.12

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liams [24] series (K(Mn) < K(Co) < K(Cu) > K(Zn)) which describes stability of transition metal complexes. Exemption of Mn(II) from the Irving-Williams stability series could be attributed to ligand substitution reaction which results in replacement of acac with OH and consequently decrease of the pH from what is expected,

MðacacÞ2 þ 3H2 O $ ½MðacacÞðH2 OÞ2 Þþ þ HðacacÞ þ OH

ð2Þ

In Eq. (2), M represents the transition metal cations. Generally corrosion results in formation of Fe+2 and OH at anodic and cathodic sites, respectively. So, one could expect the pH increment after immersion. Increase of pH is occurred for Blank and ZPC test solutions. Nevertheless, after 24-h exposure to the mild steel pH drop was occurred for the solutions containing acetylacetonate complexes, this implies decrease of the concentration of the complexes in the test solutions. Hence, decrease of pH after immersion could be related to precipitation of metallic complexes on the mild steel surface. The pH drop for Zn(acac)2 solution was so that the final pH was considerably acidic compared to the Blank solution. Such a pH drop could be related to consumption of OH or production of H+ at the interface.

Fig. 3 displays surface morphology of the exposed samples to the test electrolytes. ZPC solution (Fig. 3a) resulted in almost smooth surface without considerable difference with unexposed steel sample. Similar to ZPC, Co(acac)2 resulted in almost smooth surface (Fig. 3b). It may be related to formation of very thin layer on the surface, which could not be detected by SEM. The micrograph of the sample immersed in Cu(acac)2 solution (Fig. 3c) revealed formation of fine Cu particles rather than deposition of a uniform Cu film on the mild steel surface. Presence of Cu islands on the steel surface can increase the effective electrode area, which accompanied by galvanic coupling results in increase of double layer capacitance and charge transfer resistance decline. In addition, fresh Cu surface results in more electrochemically resistive behaviour of the sample, which influences h value to get closer to zero (Table 3). The micrographs of the samples immersed Mn(acac)2 (Fig. 3d) and Zn(acac)2 (Fig. 3f) solution show different kind of pattern on the surface which could be attributed to different nature of the films on the surface. The surface composition of the exposed samples was assessed using EDX analyzer and the results are presented in Fig. 4. Compar-

Fig. 3. SEM micrograph of samples exposed to 3.5% NaCl solution containing (a) ZPC, (b) Co(acac)2, (c) Cu(acac)2, (d) Mn(acac)2, (e) blank and (f) Zn(acac)2.


413

Fig. 4. EDX surface analysis results of samples immersed in different electrolytes-surface compositions are based on atomic percentage and M represents metallic cation of acetylacetonate complexes.

ing the composition of steel samples (Table 2) and EDX results (Fig. 4) one could hypothesize the process of inhibition. The results demonstrate formation of a layer on the samples immersed in Co(acac)2, Cu(acac)2, Mn(acac)2 and Zn(acac)2 electrolytes composed of Co (4.14%atm), Cu (19.07%atm), Mn (1.29%atm) and Zn (7.21%atm), respectively. As listed in Table 2 atomic percentage of Co, Cu, Mn and Zn in the unexposed mild steel is 0.005%, 0.057%, 0.589%, and 0%, respectively. In other words, each of metallic complexes forms a layer on the mild steel surface. It should be noted that EDX examines fluorescent X-ray generated from the sample by electron or X-ray beam excitation. The escape depths of these X-rays are much deeper (of order of 104 m) than those of the Auger electrons and photo-electrons (about 1010 m). Therefore, the EDX data are more representative of bulk constituent present at high concentrations [25]. Appearance of Na peaks in EDX results of some samples could be the result of specific interaction of the acetylacetonate complexes with the mild steel surface. Percentage of Na+ in film composition resulted by the acetylacetonate complexes were found to decrease in the order: Zn > Cu > Co=Mn=0 (Fig. 4). Presence of Na at higher concentrations than Cl could be related to formation of anionic complexes on the surface, which is neutralized by Na+. A schematic interaction mechanism of zinc acetylacetonate complexes with mild steel surface is displayed in Fig. 5. As stated previously, the interaction of Mn(acac)2 and Co(acac)2 with the mild steel surface is probably due to physical adsorption and deposition of trivalent cobalt on the surface, respectively. Hence, the percentage of deposited Na on the samples immersed in Mn and Co complexes is zero which means no ionic complex forms on the surface. Stability of the [Fe–O–Zn(acac)2] is greatly affected by Zn ionic radius of the metal complex. Stable anionic complexes need

Fig. 5. (a) Mild steel surface hydroxide interaction with zinc acetylacetonate complexes. (b) Formation of anionic complexes on the mild steel surface.

to be neutralized by electrolyte cations. This may be the reason of highest Na content of protective film in of Zn(acac)2 solution. Formation of [Fe–O–Zn(acac)2] on the surface suggests anchoring of Zn(acac)2 complexes to the surface and production of H+ at the electrode–electrolyte interface which is responsible for surprising pH drop of the Zn(acac)2 solution after contact with the mild steel samples (Table 4). Murgia et al. [26] proposed anchoring of metal acetylacetonate complex to the surface. They suggested anchoring of V(acac)3 to silica particles having OH functionality where they studied formation of new catalyst by sol–gel process. Presence of Na on the films formed by Cu(acac)2 on the steel surface could not be related to formation of [Fe–O–Cu(acac)2] since this complex is not thermodynamically stable; however, Cu(acac)2 could be thermodynamically stable on the previously deposited Cu. Reduction of Cu(II)/Cu leads to release of acetylacetonate anions. Hence, presence of Na at higher concentration than Cl in the films deposited from Cu(acac)2 solution could be related to formation of sodium acetylacetonate. 4. Conclusion Evaluation of EIS, SEM-EDX and pH-metry results lead to following conclusions: 1. Co, Zn, Mn(II) acetylacetonate complexes show corrosion inhibition behaviour for mild steel in 3.5% NaCl solution. Their inhibition follows the sequence: Co(II) > Zn(II) > Mn(II). 2. Cu(II) acetylacetonate complex reveals no inhibitive action which could be related to reduction of Cu(II) on the surface. Copper is formed not as a uniform film, but as a separated islands on the mild steel surface. Deposition of Cu islands on the mild steel surface leads to galvanic coupling and increase of effective electrode surface area, which consequently results in lower charge transfer resistance. 3. After exposure of the mild steel to the test solutions, the pH of the solutions dropped, which implies decrease of the complex concentration in the test solutions indicating precipitation of metallic complexes on the mild steel surface. 4. SEM-EDX results displayed precipitation or film formation of the metallic complexes on the mild steel surface.

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