ФИЗИКОХИМИЯ ПОВЕРХНОСТИ И ЗАЩИТА МАТЕРИАЛОВ, 2009, том 45, № 2, с. 248-256
ФИЗИКО-ХИМИЧЕСКИЕ ^^^^^^^^
ПРОБЛЕМЫ ЗАЩИТЫ МАТЕРИАЛОВ
УДК 620.197.3
ELECTROCHEMICAL BEHAVIOUR OF ZINC IN CHLORIDE AND ACETATE SOLUTIONS
© 2009 r. Gtiray Kilin^^eker*, Hasan Gal ip**
*Cukurova University, Department of Chemistry, Adana, Turkey **Eastern Mediterranean University, Department of Chemistry, G. Magusa, T.R.N.C.
E-mail: gkilinc@cu.edu.tr Поступила в редакцию 14.08.2007 г.
The corrosion behaviour of zinc has been investigated in chloride medium, in the absence and presence of acetate ions. The temperature effect has also been investigated by varying the temperature in a range between 298 and 328 K, while the pH was kept constant at 8.5. Polarization resistance (Rp) and activation energy (Ea) values were determined, by means of potentiodynamic and EIS measurements. Thermodynamical properties have been evaluated with help of current-potential measurement results. The role of temperature, ionic species and the formation of oxide film on anodic and cathodic processes are discussed. It was shown that zinc acetate [Zn(CH3COO)2] formation could provide important protection with other zinc corrosion products, against the attack of corrosive environment.
PACS: 82.45.Hk
1. INTRODUCTION
Zinc and its alloys, in addition to their widely use in making machine parts, also are being successfully used especially in plating steel materials by hot immersion method. The reason for preferring these alloys in many applications lies in the fact that in addition to their superior mechanical properties, they are economically feasible. Predicting the effects of different ions on zinc stability is important in designing machine parts. Actually, zinc corrosion gives rise to highly protective oxide and hydroxide products. However, the electrochemical behaviour of zinc in alkaline and weakly alkaline environment have been frequently studied. Because, it is known that zinc could undergo corrosion under alkaline conditions, as a result of passive film degradation. The zinc hydroxide complexes formed on the surface, make the anodic dissolution of metal very complicated [1-4].
The formation of zinc oxide adsorbed to the surface by (ZnO)ads makes up the layer on the surface;
Zn + Zn(OH)2 + 2OH-
2ZnO + 2H2O + 2e-
The corrosion of zinc could occur within the open pores of zinc oxide layer via the following reactions [5]:
anodic branch:
Zn
cathodic branch:
Zn2+ + 2e-
O2 + 2H2O + 4e 4OH-diffusion control
2H+ + 2e H2.
The pH dependence of these two reactions constituting the cathodic process is the same. But it can proceed only at different overvoltages. From -1.200 V to -2.000 V H+ ions are reduced; between -1.100 V-1.200 V oxygen reduction takes place [6].
In general, the polarization behaviour of a metal varies with pH, with concentration of the solution and with temperature [7]. For example, while the corrosion rate of
zinc is high in S O^- solutions of lower corrosion rate is observed under pH 4. In chloride solutions on the other hand, depending on pH (pH = 7-9) as the concentration of chloride (Cl) ion increases, it is said that the dissolution rate of zinc increases. From the kinetic parameters, it is deduced that the dissolution takes place via the formation of ZnCl2 first, and then ZnCl^- ion. Exceeding the solubility product of ZnCl2 on the surface, causes the onset of first passivity [8-10]. It is stated that this ion causes the wearing of fitting elements (gate valves, check valves, nipples etc) made from zinc and its alloys which are used in heating-cooling systems. This is explained with its higher adsorptive interaction with surface with respect to oxygen and hydroxide ions due to its high electronegativity. These adsorbed chloride ions, combine with the zinc ions formed as a result of corrosion and pass into solution as zinc chloride. Thus, the precipitation Zn(OH)2 and the formation of the passive film on the metal surface is prevented. Consequently, the dissolution of zinc goes on as an autocatalytic process. Because, while the ZnCl2 entering the solution combines with oxygen and water forming Zn(OH)2, the chloride
Table 1. The Corrosion Potentials (Ecorr), Polarization Resistances (Rp), the current densities and Percent Inhibitions (I%) measured at -1.400 V and -0.500 V for zinc in different solutions having pH value of 8.5 under different temperatures
Metal Medium (T/K) Ecorr (V/SCE) Rp (ohm) -1.400 V x 103 (mA cm-2) -0.500 V x 103 (mA cm-2) I%
Zinc Cl- 298 -1.107 331 -1.8б0 70.8 *
308 -1.1б8 315 -2.754 77.б *
318 -1.2б3 245 -3.398 101.б *
328 -1.325 210 -4.570 182.0 *
CH3COO- 298 -0.900 15849 -0.080 7.8 97.9
308 -0.950 8120 -0.158 10.5 96.1
318 -0.993 287б -0.бб5 l7.O 91.5
328 -1.025 1б43 -1.004 18.2 87.2
Cl- + CH3COO- 298 -0.995 б4б -0.398 81.3 48.8
308 -1.0б8 б05 -0.513 83.2 47.9
318 -1.14б 553 -1.288 105.2 55.7
328 -1.195 401 -1.778 103.3 47.б
ion passing into solution is being adsorbed again by metal surface:
2ZnCl2 + O2 + 2H2O + 4e- 2Zn(OH)2 + 4Cl-.
This event increases the rate of corrosion just like a catalyst [11-13].
The data present in the literature related to the corrosion behaviour of zinc and its alloys are important for material selection and part designing. However the variety and complexity of corrosive environments favours the studies subjecting zinc corrosion. In order to study the effect of CH3COO- ions on the corrosion of zinc in aqueous media, potentiokinetic current-potential curves have been obtained in 3.5% NaCl, in 0.1 M CH3COONa and in 3.5% NaCl + 0.1M CH3COONa solutions at pH = 8.5 at temperatures 298 K, 308 K, 318 K and 328 K.
In order to explain the results of the experiment, corrosion rate and thermodynamical properties have been evaluated with help of current-potential measurement results and corrosion behaviours of zinc have been studied.
2. EXPERIMENTAL
The electrochemical behavior of zinc was studied at different temperatures (298 K, 308 K, 318 K and 328 K), under conditions of conditioned boiler water (pH 8.5) solutions containing 3.5 % NaCl, in 0.1 M CH3COONa and in 3.5% NaCl + 0.1 M CH3COONa. The pH value of the solution was adjusted by HCl and NaOH. The chemicals used in the experiments were of analytical grade (Merck). The constant temperature conditions were provided by thermostat (NUVE BM101).
The working electrodes were made of zinc bars (99.98% Zn) coated with polyester block, leaving a surface area of 0.82 cm2 open for exposure, the counter electrode consisted of a platinum plate having an area of 1 cm2 and the reference electrode was saturated calomel elec-
trode (SCE). The current density-potential curves were obtained potentiokinetically by using three electrode technique (Potentiostat: Type Prt 10-0.5 Tacussel). Before each measurement the surface of the working electrode was polished by mechanical grinding using successively no: 400, 600, 800 and 1000 emery paper (Dap-Struers), then washed with distilled water and dried. During the experiment the aqueous solution was continuously stirred at constant speed with a magnetic stirrer and the pH value was checked by a pH meter (Edt Gp 353 Act pH). After the corrosion cell was set up, it was left for 2.5 hours to reach equilibrium. The working electrode was then polarized first in cathodic and then in anodic direction at a rate of 0.01 V s-1 tarting from the equilibrium potential measured against SCE, and current-potential curves were drawn (in -1.8 V...+0.2 V). Also the polarization resistance values (RP) were determined, with linear polarization method in a potential range of Ecorr + 8 mV. The current values measured were converted to current density taking into account the electrode surface and then current-potential curves were drawn.
3. RESULTS AND DISCUSSION
The experimental results of zinc obtained at pH 8.5 in different solutions and under different temperatures are given in figures 1-10 and in Table 1-3. Figures 1-3, 8-10 are the polarization curves obtained in 3.5% NaCl, in 0.1M CH3COONa and in 3.5% NaCl + 0.1 M CH3COONa respectively. Figures 4, 5 and 6 represented the Nyquist and Bode plots obtained in the same solutions. Figure 7 summarizes the corrosion rate variation with temperature. Figures 8-10 summarizes the corrosion rate, Gibbs Energy and enthalpy values variation with temperature. Within these Figures the temperature conditions were named as; a: 298, b: 308, c: 318, and d: 328 K, respectively.
Table 2. The corrosion potentials (Ecorr), Corrosion Current (/corr) and values of polarization resistance Rp values for zinc in the 1.0, 0.5, 0.25 and 0.10 M NaCl and 1.0, 0.5, 0.25 and 0.10 M NaCl + 0.1 M CH3COONa solution at pH 8.5 (298 K)
Medium E ^corr (V/SCE) Icorr X 102 (A cm-2) Rp (ohm)
1.00 M NaCl -1.033 10.40 434
0.50 M NaCl -0.998 6.109 454
0.25 M NaCl -0.985 3.176 454
0.10 M NaCl -0.939 1.275 2000
1.00 M NaCl + + 0.1 M CH3COONa -1.026 8.927 526
0.50 M NaCl + + 0.1 M CH3COONa -1.014 5.762 909
0.25 M NaCl + + 0.1 M CH3COONa -1.023 3.834 1110
0.10 M NaCl + + 0.1 M CH3COONa -0.976 1.909 3333
3.1. The electrochemical behaviour of Zinc in 3.5% NaCl solution
As seen in Figure 1, the corrosion potentials of zinc in chloride containing medium are between -1.107 V and -1.325V (Table 1) when the temperature is varied between 298 and 328 K. At pH 8.5 and 298 K, while the current densities determined at anodic (-0.500 V) and at cathodic (-1.400 V) potentials are 70.8 mA/cm2, and -1.860 mA/cm2 respectively, when the temperature is raised, the current value at -0.500 V increases to 182 mA/cm2, and the one at -1.400 V goes up to -4.570 mA/cm2, but this dependence on temperature is not systematic.
By comparing the values obtained from Figure 1 and given in Table 1, the variation of corrosion potential with temperature (dEcoJdT)pH c has been determined to be approximately =8 x 10-3 V K-1.
According to the data obtained, at 298 K (pH 8.5) the selective adsorption of
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