versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003
J. Chil. Chem. Soc., 48, N 3 (2003) ISSN 0717-9324
CORROSION INHIBITION OF COPPER IN 0.5 M HYDROCHLORIC ACID BY
H. BAEZA, M. GUZMÁN*, P. ORTEGA AND L. VERA.
Departamento de Química, Facultad de Ciencias, Universidad Católica del Norte,
( Received: August 8, 2002 Accepted: April 14, 2003)
The inhibition of copper corrosion by 1,3,4-thiadiazole-2,5-dithiol (bismuthiol), under 0.5 M HCl at 25 C, was studied by means of the potentiodynamic polarization technique. It was found that bismuthiol was chemically adsorbed on the copper surface, and follows a Langmuir isotherm. The inhibition mechanism was found to be mixed, and it was originated through the formation of bismutiol-copper complexes on the surface of the metal. Bismuthiol shows good inhibition efficiency under these conditions.
Due to its good corrosion resistance in water and its excellent heat conductivity, copper and its alloys are broadly used in heating systems and condensers. However, these systems should be regularly cleaned due to inlays of carbonates and oxides that diminish their heating transmission. Diluted hydrochloric acid is normally used to clean these surfaces; a corrosion inhibitor is added to avoid the action of this acid on the copper. A corrosion inhibitor is a chemical substance that can diminish de effect of a corrosion media through decreasing its rate of attack. The inhibition mechanisms are, the inhibitor is adsorbed on the surface of the metal; the inhibitor provokes a small corrosion on the surface of the metal and also it is adsorbed forming a compact protective thin layer; the inhibitor forms a precipitate on the surface of the metal; acting on the aggressive medium in such a way to forms protective precipitates or removing aggressive agents. Some inhibitors acts on the cathode (cathodic inhibitor) or in the anode (anodic inhibitor) or in both (mixed mechanism inhibitor).
Several inhibitors of copper corrosion in aqueous solutions at neutral pH can be found in literature, but there are few data for more aggressive environments 1-9). Generally speaking, an organic compound that contains a functional group with heteroatoms, behaves fairly well as a corrosion inhibitor 10, 11). The strength of coordination bond of heteroatoms (O < N < S < P) parallels its efficiency as corrosion inhibitor. As an example of this behavior, it can be mentioned that 1,3,4-thiadiazole-2,5-dithiol, bismuthiol, forms polymers with metals 12) and there are several patents of this compound as corrosion inhibitor 13, 14). In this article the efficiency of bismuthiol as inhibitor of copper corrosion in HCl (0.5 M), is presented.
Bismuthiol was Aldrich (DI2900-3), analytical grade, figure 1. The copper used for probes was 99.98 % purity.
|Fig. 1. Bismuthiol (1,3,4-thiadiazole-2,5-dithiol) structure.|
Electrodes. The working electrode was made upon a copper cylinder which was connected to a copper conductor wire. The cylindrical part was isolated with epoxy resin, leaving an exposed area of 0.21 cm2. Before each experiment the exposed area of the working electrode was treated with a soft sand paper; polished with an alumina powder of 0.3-0.05 mm of granulometry until a metallic shine; rubbed with a graphite cloth; brushed carefully; washed with distilled water; degreased with ethanol and finally dried with a soft paper.
The reference electrode was saturated calomel (SCE), which was cleaned before each experiment to avoid contamination from inhibitor. Distilled water was used to clean it externally, dried with soft paper and some KCl crystals were kept inside. The auxiliary electrode was a platinum cylinder with 1.77 cm2 of exposed surface. Before each experiment it was cleaned with distilled water, degreased with ethanol, rinsed again with bi-distilled water and dried with soft paper.
The working system. Fifty mL of electrolyte, 0.5 M HCl or 0.5 M HCl with inhibitor, are introduced into the electrochemical cell and kept at constant temperature; the working electrode was maintained inside the cell for 30 or 60 min. After this time the auxiliary electrode was introduced into the cell in such a way that it faced (at same high) the working electrode at 2 - 3 mm apart. The electrolyte was sucked into the Luggins' capillary and the reference electrode was disposed in its position.
Methodology. The potentiodynamic measures were conducted at 25+0.1 C after a 60-minutes period of immersion of the working electrode into 0.5 M MCI with different bismuthiol concentrations (5 x 10-5 M; 10 x 10-5 M; 20 x 10-5 M; 40 x 10-5 M; 80 x 10-5 M). The cell was connected to a Radiometer PGZ 301 Potentiostat, and the scanned speed was fixed at 0.166 mV/s. Cyclic voltametries were conducted at same temperature after a 15 minute-period of immersion of the working electrode in HCl 0.5 M or HCl 0.5 M with bismuthiol, 80 x 10-5 M, at 50 mV/s scanning speed. Bismuthiol and the product, obtained at surface of the working electrode, were analyzed by IR spectroscopy.
RESULTS AND DISCUSSION
The polarization curves (under Tafel's format) for the working electrode in 0.5 M HCl and bismuthiol, are displayed in Figure 2 and Table I. It can be observed that as the inhibitor concentration is increased, the corrosion current density is decreased, and the zero-current potential is displaced to a more anodic region.
Fig. 2. Potenciodynamic polarization curves for copper in HCl 0,5 M y 25 ± 0,1 C at different bismuthiol concentration.
It is observed small electrode passivity between -100 and 0 mV (SCE), which can be attributed to the inhibitor adsorption in the electrode surface. The polarization resistance is increased with inhibitor concentration, correspondently with the decreased corrosion current density. Tafel's constants change with the inhibitor concentration which is an indication of its effect on the anodic and cathodic reactions (mixed mechanism). The percent inhibition efficiency (EF (%)) is defined by eq. (1), Icorr(inh) and icorr are the corrosion current density with and without inhibitor, respectively. Table 1 shows that the efficiency is increased as the inhibitor concentration is increased, and vcorr (corrosion rate, mm per year) is decreased. Last column of Table I show the corrosion rate as penetration in mm/year, it is assumed that cuprous ion is formed.
Figure 3 shows the cyclic voltammograms obtained in 0.5 M HCl with (80x10-4 M) and without bismuthiol at 50 mV/s sweeping speed. In the absence of bismuthiol, case (A), two oxidation peaks are observed at 130 (Cu (I) formation15) from Cu) and 370 (Cu(II) formation from Cu(I)) mV. The reverse sweep shows a strong peak at -270 mV corresponding to the reduction of soluble species and Cu(I)16). Voltammograms obtained in the presence of bismuthiol, case (B), show the presence of two peaks, a strong one at 448 mV which probably correspond to the formation of a bismuthiol - Cu (I) complex, and a weak one at 628 mV related to the bismuthiol - Cu (II) complex formation. On the other hand, in the reverse branch of the voltammogram are again two peaks observed. A weak one appears at 94 mV which correspond to the reduction of the bismuthiol - Cu complex, and a more intense one at -300 mV corresponding to all the soluble species and to the bismuthiol - Cu (I) complex at the electrode surface.
| ||Fig. 3. Cyclic voltammograms for copper in HCl 0,5 M with (B) and without bismuthiol (A).|
Figures 4 and 5 show the IR spectra of bismuthiol and its copper complex at high (4000 - 600 cm-1) and low wavenumber (500 - 200 cm-1). In the second region the bismuthiol - Cu complex is very different than the spectrum of pure bismuthiol; it is almost flat and without resolution compared with other bismuthiol - metal complexes17). This complex shows peaks at 305, 312 and 347 cm-1 but the peaks at 220 and 371 cm-1, attributed to pure bismuthiol, are not present. More pronounced differences can be observed in the copper complex at 4000 - 600 cm-1 zone; the peak at 1505 cm-1, attributed to bismuthiol, is absent and an intense peak at 1629 cm-1 is present.
|Fig. 4. IR spectra of bismuthiol and its copper complex, at high wavenumber.||Fig. 5. IR spectra of bismuthiol and its copper complex, at low wavenumber.|
The interaction of surface - inhibitor can be estimated from the experimental data. The inhibitor efficiency depends on the type and number of active sites at metal surface, charge density, the molecular size of inhibitor, the metal - inhibitor interaction and the metallic complex formation18). The adsorption isotherm can give information of the metal - inhibitor interaction. If the adsorption of the inhibitor follows Langmuir isotherm, then the surface coverage degree, , is given by eq. 2. The adsorption coefficient, b, depends on temperature and its normal adsorption free energy19), eq. 3.
The surface coverage degrees of the inhibitor, ,correspond to one hundredth of the percent efficiency. Figure 6 (obtained from data on Table 1) shows that the bismuthiol adsorption on cooper surface corresponds to a Langmuir isotherm behavior. This isotherm assumes that the adsorbed molecule occupy only one site and there are not interactions with other molecules adsorbed.
| ||Fig. 6. Langmuir adsorption isotherm for bismuthiol on copper surface in 0.5 M HCl solution at 25 ± 0,1 C.|
Limitations of the Langmuir isotherm can be detected through the analysis of data by other isotherm systems. The multisite Langmuir isotherm20), eq. 4, does not considers interactions between adsorbed molecules. The number of sites occupied by one molecule on the surface is represented by n It was found that n =1, in this case. Interactions between adsorbed molecules (but not multisite adsorption) can be obtained through the Frumkin21) isotherm, eq. 5. The interaction parameter, a, can be positive or negative. Positive values indicate that the adsorption energy is increased by the interaction of adsorbed molecules. Data shows that a = 0, in this case. The inhibitor adsorption process can be considered as the substitution22) of x water molecules by an inhibitor molecule, eq. 6. This substitution process (interactions between neighbor adsorbed molecules are not considered) is described by the Flory - Huggins isotherm23), eq.7. Data shows in this case that x 1, indicating that one water molecule is substituted by one inhibitor molecule. Through the Langmuir-Freundlich24, 25) isotherm, eq. 8, where h heterogeneity parameter (0 < h < 1), a measure of the adsorption energy in different sites on the surface, can be calculated. Data gives h = 1, in this case.
Results of isotherm calculations using data of Table I are summarized in Table II. In this Table it can be observed that the standard free energy adsorption is around -31 kJ mol-1. As the values reported 26) for this parameter is in the range -21 to - 42 kJ mol-1 for majority organic inhibitors in aqueous media, it can be inferred that the process under study is spontaneous, and the inhibitor is chemically adsorbed on metal surface.
Results obtained support the transformation of every isotherm equation (eq. 4 to eq. 8) into the Langmuir isotherm.
Isotherm b DGadsorption Parameter R2
From experimental data and the general discussion about adsorption isotherms it can be concluded that bismuthiol is a good inhibitor of the copper corrosion in 0.5 M HCl at 25 °C. Its inhibition process corresponds to a mixed mechanism, (i.e. affect the anodic and cathodic process) which can be explained as a consequence of the spontaneous substitution of a water molecule by bismuthiol in just one site on the copper surface, forming a bismuthiol-copper complex. The produced adsorption is better described by a Langmuir isotherm with a normal adsorption free energy of -31 kJ mol-1 corresponding to a chemical adsorption process.
Authors would like to thanks the contribution of projects FONDEF D97F-1084-Chile and "QSAR modeling through neural network of new copper corrosion inhibitors" from Universidad Católica del Norte, CHILE.
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