The inhibitory effect of glycerol on copper corrosion in aerated NaCl (0.5 M) solutions at three pH values (4, 7, and 10) was evaluated. Inhibition efficiency was assessed with conventional electrochemical techniques: open circuit potential, potentiodynamic polarization, and electrochemical impedance analysis. Glycerol reduced the corrosion rate of copper in NaCl solutions. The best inhibition effect (
As worldwide biodiesel production increases so does production of the byproduct glycerol [
Chemical structure of glycerol.
Deionized water and reagent grade NaCl, HCl, and NaOH were used to prepare NaCl solutions with three different pH values. Glycerol (99.5%) was purchased from Sigma-Aldrich (CAS 56-81-5). Samples were cut from a pure (99.999%) copper rod (Goodfellow, 5.0 mm diameter) and embedded in epoxy resin. A bar cross section was exposed to the NaCl solution. Before each exposure, the sample was abraded with a series of different grade emery papers (up to 1200), rinsed with water and ethanol, and dried with hot air.
All electrochemical measurements were done using a standard three-electrode cell configuration. Electrochemical experiments were run in the following order: open corrosion potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic scan. All experiments were carried out using a Gamry PCI4 Potentiostat/Galvanostat/ZRA instrument with CMS 100 and 300 software. OCP was measured for 15 min prior to each impedance experiment, which was done at 10 kHz to 20 mHz with a 10 mV peak-to-peak amplitude using ac signals at OCP. The potentiodynamic scan was begun immediately after EIS by scanning from −0.25 to 0.25 V versus OCP at a 1 mVs−1 sweep rate. A standard calomel electrode (SCE) was used as a reference in all electrochemical experiments. Impedance data were examined for causality, stability, and linearity with the Kramer-Kronig relationship using the method described by Boukamp [
A scanning electron microscope (SEM, Philips XL30 ESEM) was used to examine any inhibitory effect of glycerol on copper corrosion. Samples examined with SEM were abraded with emery paper (up to 2000), polished with a 0.25
The OCP (
Open circuit potential (OCP) for Cu in NaCl (0.5 M) solutions with and without glycerol at three pH values: (a) pH 4, (b) pH 7, and (c) pH 10.
The polarization curves for copper in NaCl (0.5 M) solutions with and without glycerol in different concentrations at three pH values showed how the Cu corrosion potential (
Polarization curves for Cu in NaCl (0.5 M) solutions with and without glycerol at three pH values: (a) pH 4, (b) pH 7, and (c) pH 10.
No significant change was observed in the polarization curve anodic current at pH 4, indicating that the anodic reaction was unaffected by addition of glycerol. Glycerol’s effect became apparent at pH 7 and even more obvious at pH 10. In all cases, a linear portion of the polarization curve was not easily determined. Extrapolation of both sides of the polarization curves to calculate corrosion parameters is preferable, although occasionally just one side is used, mainly when a Tafel linear region is too small or is difficult to observe [
Electrochemical parameters determined by Tafel extrapolation of the cathodic branch of the potentiodynamic polarization curves for copper in aerated NaCl (0.5 M) with and without glycerol at three pH values.
pH | Glycerol |
|
|
|
|
|
---|---|---|---|---|---|---|
pH 4 | 0.0 | −243 | 5.27 | −335 | 61.9 | Blank |
0.1 | −270 | 2.97 | −201 | 66.7 | 43.6 | |
0.5 | −275 | 2.69 | −207 | 65.1 | 48.9 | |
1 | −277 | 2.89 | −208 | 65.9 | 45.1 | |
2 | −281 | 1.98 | −209 | 62.6 | 62.4 | |
|
||||||
pH 7 | 0.0 | −212 | 11.2 | −575 | 64.1 | Blank |
0.1 | −206 | 6.57 | −442 | 72.8 | 41.3 | |
0.5 | −228 | 5.88 | −313 | 103.0 | 47.5 | |
1 | −217 | 5.19 | −384 | 82.7 | 53.6 | |
2 | −215 | 3.03 | −215 | 93.2 | 72.9 | |
|
||||||
pH 10 | 0.0 | −208 | 9.7 | −750 | 81.3 | Blank |
0.1 | −186 | 3.9 | −261 | 161.3 | 60 | |
0.5 | −203 | 2.3 | −203 | 206.4 | 76.2 | |
1 | −185 | 2.1 | −208 | 205.0 | 78.1 | |
2 | −213 | 1.6 | −209 | 200.0 | 83.2 |
The best inhibition efficiency was attained at pH 10, possibly due to formation of a layer of copper-glycerol complexes near the copper surface, effectively reducing the reaction area and producing an inhibitory effect [
In the example shown in Figures
Typical Kramer-Kronig analysis results for Cu in NaCl (0.5 M, pH 10) solution containing glycerol (2 M). (a) Nyquist diagram, (b) Bode diagram, and (c) residual error (
The Nyquist impedance plots for copper in NaCl (0.5 M) solutions with and without glycerol at three pH values show a depressed semicircle the diameter of which can be obtained by extrapolating toward the low frequency value limit (
Electrochemical impedance parameters obtained by fitting the Nyquist plots for Cu in aerated NaCl (0.5 M) with and without glycerol at three pH values.
0.5 M NaCl |
GC |
|
Rp1 |
CPE1 |
|
CPE2 |
|
|
||
---|---|---|---|---|---|---|---|---|---|---|
|
|
|
| |||||||
4 | 0 | 15.0 | 106 | 3.9 × 10−5 | 0.88 | 60.6 | 8.9 × 10−4 | 0.45 | 6.0 × 10−5 | bk |
0.1 | 17.7 | 105 | 6.9 × 10−5 | 0.82 | 81.9 | 8.4 × 10−4 | 0.46 | 5.3 × 10−5 | 10 | |
0.5 | 17.4 | 175 | 7.1 × 10−5 | 0.83 | 56.1 | 7.2 × 10−4 | 0.46 | 3.4 × 10−5 | 28 | |
1 | 23.2 | 200 | 2.6 × 10−5 | 0.93 | 15.7 | 4.6 × 10−4 | 0.48 | 2.8 × 10−4 | 28 | |
2 | 16.6 | 247 | 4.2 × 10−5 | 0.88 | 62.9 | 5.6 × 10−4 | 0.48 | 1.4 × 10−4 | 46 | |
|
||||||||||
7 | 0 | 16.7 | 5002 | 6.6 × 10−6 | 0.989 | 3.27 × 10−3 | 1.2 × 10−4 | 0.49 | 5.1 × 10−2 | bk |
0.1 | 15.2 | 6569 | 8.2 × 10−6 | 0.980 | 2.0 × 10−3 | 7.9 × 10−5 | 0.51 | 3.1 × 10−2 | 23 | |
0.5 | 16.1 | 7871 | 7.4 × 10−6 | 0.980 | 4.9 × 10−3 | 3.9 × 10−5 | 0.55 | 5.4 × 10−2 | 36 | |
1 | 16.0 | 13590 | 1.0 × 10−5 | 0.934 | 2.4 × 10−3 | 3.3 × 10−5 | 0.47 | 1.1 × 10−1 | 63 | |
2 | 16.6 | 14660 | 6.2 × 10−6 | 0.980 | 3.0 × 10−3 | 2.5 × 10−5 | 0.20 | 1.2 × 10−1 | 65 | |
|
||||||||||
10 | 0 | 15.8 | 6190 | 47.9 × 10−6 | 0.60 | 835.5 × 10−3 | 5.9 × 10−6 | 0.92 | 123.9 × 10−3 | bk |
0.1 | 15.0 | 13560 | 37.3 × 10−6 | 0.38 | 292.5 × 10−3 | 13.9 × 10−6 | 0.82 | 12.8 × 10−3 | 54 | |
0.5 | 15.4 | 22540 | 35.8 × 10−6 | 0.32 | 15.6 × 10−3 | 13.6 × 10−6 | 0.91 | 1.3 × 10−3 | 72 | |
1 | 19.6 | 27530 | 30.6 × 10−6 | 0.30 | 2.65 × 10−3 | 12.96 × 10−6 | 0.90 | 0.1 × 10−3 | 77 | |
2 | 19.7 | 36750 | 29.0 × 10−6 | 0.32 | 2.2 × 10−3 | 11.6 × 10−6 | 0.90 | 0.2 × 10−3 | 83 |
GC = glycerol concentration.
Nyquist plot for Cu in NaCl (0.5 M) at four glycerol concentrations as a function of three pH levels: (a) pH 4, (b) pH 7, and (c) pH 10.
Equivalent circuit model used to fit the impedance data from the copper/NaCl (0.5 M) interface with and without glycerol.
The fit was generated using the equivalent circuit in Figure
Equivalent circuit fit for Cu in 0.5 M NaCl (pH 10) + 0.5 M glycerol; (a) Nyquist, (b) phase, and (c) modulus.
The most significant parameters are total polarization resistance (
Comparison of the inhibition efficiency obtained by potentiodynamic polarization and the impedance data shows that both results follow the same trend. The very slight difference in values was probably caused by differences in measurement time [
Micrographs (SEM) of copper samples exposed to 0.5 M NaCl at pH 7 with and without added glycerol showed no visible indications of ion chloride attack on the copper surface in the presence of glycerol (Figure
SEM micrographs of copper surface after 120 hours of immersion at room temperature (a) in aerated 0.5 M NaCl (pH 10) + 2 M glycerol and (b) in aerated 0.5 M NaCl (pH 10).
Glycerol inhibits corrosion of copper in aerated NaCl (0.5 M) solutions. Inhibition efficiency increases with increasing glycerol concentration and is especially notable at high pH values. Glycerol does not affect the anodic or cathodic reactions, suggesting that increased viscosity reducing mass transport apparently explains the reduction in corrosion rate from acid to neutral solutions. The reduced anodic corrosion rate at pH 10 is probably due to two causes: an increase in solution viscosity and presence of a film on the copper surface, most likely composed of copper-glycerol complexes. SEM images confirm the effectiveness of glycerol as an inhibitor of copper corrosion in NaCl solutions.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the Consejo Nacional de Ciencia y Tecnología for financial support via Grants no. 205050, FOMIX-Yucatán 2008-108160, and CONACYT LAB-2009-01 no. 123913. The authors also thank Biol. Ana Ruth Cristobal Ramos for her technical assistance.