The comparative study between the inhibitive effect of sodium molibdate and sodium vanadate on the corrosion behaviour of aluminium alloy in chloride medium
1Meryem ACILA/ Material-Environement Interactions Laboratory (LIME);University of Jijel – Algeria.
Email:acmeriem@gmail.com
2Hakim BENSABRA/ Material-Environement Interactions Laboratory (LIME);University of Jijel-Algeria
3Noureddine AZZOUZ/ Material-Environement Interactions Laboratory (LIME);University of Jijel – Algeria.
4Somia BELKHIR/ Material-Environement Interactions Laboratory (LIME); University of Jijel – Algeria.
Abstract:
The effect of molybdate and vanadate inhibitor ions on the corrosion of aluminium alloy in near-neutral chloride solution has been studied using measurements of the open circuit potential and its variation with time, electrochemical impedance measurements and polarization curves. In addition, scanning electron microscopy coupled with energy dispersive spectroscopy and X-ray photoelectrons were used for surface analysis.
The results show that both inhibitors present an interesting protective effect against pitting corrosion of aluminum.
However the effect of molybdate is more significant than the one of vanadate. This inhibitive effect is reflected through the substantial reduction of both the rate of pit nucleation and the current rise characterizing the pit propagation progress.
Keywords— Corrosion; aluminum; chlorides; inhibitors;, molybdate; vanadate.
I. INTRODUCTION:
Although aluminium is a reactive metal (E°= -1.66 V vs SHE), it is resistant to corrosion in solutions of pH between 4 and 9, whenever aggressive ions, such as chloride, are not present. This resistance is attributed to the presence of a thin, adherent and protective surface oxide film [1]. Above and below this pH range, solubility of the oxide film increases, and aluminium exhibits uniform attack [3]. Most of the electrochemical corrosion behavior carried on aluminum studies were conducted in chloride media and inhibitors used to fight against this phenomenon are generally chromate, harmful to health and the environment [2].
In this paper, we report the inhibition ability of two inhibitors, sodium molybdate and sodium vanadate against AA5083 aluminium alloy corrosion in 0.1M NaCl solution. Their presence in the solution showed an effect on absorption ability and corrosion inhibition to AA5083. By measuring the polarization and electro-chemical impedance spectroscopy (EIS), we found that the inhibitors acted as anodic inhibitor and sodium molybdate had a better inhibition than sodium vanadate.
II. EXPERIMANTAL PROCEDURES:
II.1. Sample preparation
Al-Mg series 5083 Al alloy (Fe 0.24%, Mn 0.74%, Si 0.11%, Cu 0.06%, Cr 0.09%, Mg 4.76%, Ti 0.01%, Zn 0.13% and balance Al, mass fraction) was used as hull material. In general, the Al-Mg series alloys are used in applications that require corrosion resistance,
The corrosion experiments were performed on rectangular samples, which were cut from plate-shaped 5083 aluminum alloy. The WE was enclosed with epoxy resin, to allow the cross-section (0.5 cm2) to expose to the solution. Before each experiment, the exposed surfaces were with silicon carbide paper to 1200 grit in presence of aqueous alumina suspension and then degreased with acetone, cleaned with distilled water and finally dried in air. The corrosion tests were carried out immediately after drying.
In all measurements, the counter electrode was a platinum gauze and the reference electrode was Ag/AgCl. All potentials are referred to the Ag/AgCl.
The polarization E against I curves were obtained by means of the linear potential sweep technique with sweep rates of 1 mV going from cathodic to anodic side. Impedance measurements were performed in the frequency range from 100 kHz to 100 mHz with an a.c. voltage amplitude 10 mV.
II.2. OCP measurements
Fig. 1 presents a typical variation of aluminium alloy AA5083 open circuit potentials (OCP) with time in 0.1M NaCl solution , in the absence and presence of 0.1M sodium molybdate and sodium vanadate. The OCP values in the absence of inhibitors as expected, in the region from -0.765 to -0.768 v vs. Ag/AgCl [4,5,6].
However, when molybdate and vanadate ions are added to the solution, the OCP values are displaced to the more positive direction (especially for molybdate , where this value is -0.541 v vs. Ag/AgCl).
Fig. 1. Evolution of the open circuit potential (OCP) vs. exposure time in 0.1M NaCl solution for samples of AA5083 alloy in the absence and in the presence of molybdate and vanadate ions 0.1M.
For vanadate at the same concentration, the EOCP was −0.678 Vvs.Ag/AgCl. The positive shift in EOCP indicated that the molecules inhibited the anodic corrosion of AA5083.
II.3.
EIS studiesThe EIS tests were implemented at OCP (open circuit potential) with a frequency range of 100KHz to 100 mHz with 10 points per decade and a sinusoidal potential perturbation 10 mV in amplitude. The sinusoidal potential fluctuation was within 5 mV in amplitude. The impedance plots of aluminium as Nyquist plots and the equivalent circuit model fit well the impedance spectrum with a small error and are represented in Fig.3. The Nyquist plots showed one capacitive semi-circle at high frequency region for all the examined solutions. These capacitive semi-circles indicate that the aluminium dissolution is mainly controlled by charge transfer process across the aluminium/solution interface. This capacitive loop is related to the dielectric properties of the oxide film [7,8].
Fig. 2 shows the Nyquist plots of AA5083 aluminium alloy after1 h immersion in 0.1M NaCl solution.
Fig. 2. results of Nyquist plot for AA5083 alu-minium alloy in 0.1 M NaCl solution with and without inhibitors.
The appearance of two semicircles in the impedance diagram was common to all systems. The high frequency semicircle
corresponds to a capacitive loop and the low frequency semicircle corresponds to an inductive loop. The semicircle radii were dependent on the inhibitor used and its concentration. Nyquist plots for aluminium in 0.1M NaCl, alone and in the presence of sodium molybdate and sodium vanadate are presented in Figures 4. The diameter of the high frequency capacitive semicircle markedly increased with the addition of the inhibitors, especially in the presence of molybdate ions indicating its superior inhibiting properties.
II.4.Potentiodynamic polarization measurements :
Fig. 3 represents the potentiodynamic polarization curves of AA5083 alloy in 0.1M NaCl with and without inhibitors.
Fig. 3. Potentiodynamic polarization curves for AA5083 alloy in 0.01M NaCl solution in the absence and in the presence of the tow inhibitors.
Fig. 3 indicates that the addition of the inhibitors reduces the current density. It is well known that the major corrosion problem with aluminium and its alloys in the presence of chloride ions, Cl_ is the localized breakdown of the passive film, leading to the initiation and growth of corrosion pits.
Laboratory studies of pitting inhibition are usually performed by comparing the course of anodic polarization curves taken in the presence and absence of inhibitors [9,10]. The pitting potential, Epit can be defined, in a potentiodynamic polarization curve, as the potential below which the metal surface remains passive and above which pitting corrosion starts to grow on the metal surface [11,12] It can be seen from Fig.3 that the differences between corrosion potential, Ecorr and pitting potential Epit with molybdate in contrast with vanadate ion, what indicates that molybdate ion has better protective effect against pitting corrosion of aluminum alloys in NaCl solution than vanadate ions.
Although molybdate ions shows better corrosion inhibition effect than vanadate ions against pitting corrosion in 0.1M NaCl solution.
The values of corrosion potential (Ecorr), pitting potential (Epit), their difference (Epit e Ecorr), corrosion current (Icorr), are summarized in Table 1.
-0,9 -0,8 -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0
0 1000 2000
E/ V vs Ag/AgCl
t / S
without with Mo with vanad
-1,5 -1 -0,5 0 0,5 1 1,5
1E-10 0,0000001 0,0001 0,1
E[ V/Ag/AgCl]
I [A.cm-2]
without with Mo with vanad
-100000 0 100000 200000 300000 400000 500000 600000
0 200000 400000 600000
Zi (ohm cm2)
Zr (ohm cm2)
without with Mo with vanad
inhibitor Ecorr(v) Icorr(µA) Epit(v) Ecorr-Epit without -0.686 1.266*10-1 -0.580 0.106
MoO4
- -0.678 3.393*10-2 -0.020 0.658
VO3- -0.652 5.026*10-2 -0.440 0.212
Table 1: Electrochemical parameters of corrosion obtained from potentiodynamic polarization curves for AA5083 alloy in 0.1M NaCl solution in absence and presence of different inhibitors.
III. CONCLUSION:
The inhibiting action and the adsorption behavior of sodium molybdate and sodium vanadate on AA 5083alloy was investigated in 0.1M NaCl solution by means of potentiody- namic and impedance measurements.
Polarization measurements show that the addition of inhibitors induces a decrease in the anodic current density. Molybdate ions inhibits the pitting corrosion of AA5083 alloy more effectively than the vanadate ions in the same range of concentrations. EIS studies also show the diameter of the high frequency capacitive semicircle markedly increased with the addition of the inhibitors, especially in the presence of molybdate ions indicating its superior inhibiting properties.
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