A.M. Altwaiq, S.J. Khouri, R.A. Abdel-Rahem
Keywords: conductivity, corrosion, inhibitor, lignosulfonate.
Abstract:
The conductivity method was successfully applied to monitor the corrosion and inhibition processes. Measurements of electrical conductivity of three different corrosive solutions (HCl, NaOH, and NaCl) were performed at 20 oC with two different concentrations containing zinc sheets in the absence and presence of four different concentrations of sodium lignosulfonate (1.0, 5.0, 10.0, and 20.0 mM). The analysis of curves that illustrates the changes of conductivity of these solutions provides qualitative information about the strength of corrosion as well as the extent of corrosion inhibition behaviour. The results obtained from conductivity measurements revealed that sodium lignosulfonate was an effective corrosion inhibitor in acidic medium (for both 0.10 and 1.0 M HCl) but was less effective in salt and alkaline media.
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Conductivity Method as a Monitoring Technique for Corrosion and Corrosion Inhibition Processes A.M. Altwaiqa, , S.J. Khourib, R.A. Abdel-Rahema,* Department of Chemistry, College of Arts and Sciences, University of Petra, P.O. Box: 961343, Amman 11196, Jordan. a b Department of Basic Sciences, American University of Madaba, Madaba, Jordan. Corresponding Author E-mail: rabdelrahem@uop.edu.jo * Abstract The conductivity method was successfully applied to monitor the corrosion and inhibition processes. Measurements of electrical conductivity of three different corrosive solutions (HCl, NaOH, and NaCl) were performed at 20 oC with two different concentrations containing zinc sheets in the absence and presence of four different concentrations of sodium lignosulfonate (1.0, 5.0, 10.0, and 20.0 mM). The analysis of curves that illustrates the changes of conductivity of these solutions provides qualitative information about the strength of corrosion as well as the extent of corrosion inhibition behaviour. The results obtained from conductivity measurements revealed that sodium lignosulfonate was an effective corrosion inhibitor in acidic medium (for both 0.10 and 1.0 M HCl) but was less effective in salt and alkaline media. Keywords: conductivity, corrosion, inhibitor, lignosulfonate. 1 Introduction Corrosion monitoring has been routinely practiced by many researchers in the last decades to determine the amount of corrosion and the rate of metal loss in the environment [1]. Corrosion measurement techniques play a significant role in determining the critical factors of corrosion and in reducing their effects. These techniques fall into six categories; Electrical resistance monitoring, electrochemical methods, hydrogen monitoring, weight loss coupons, non-destructive testing techniques and analytical techniques [2]. Some of analytical techniques include the drawing off fluid samples for analysis in laboratories where the areas of interest under this item are the concentration of metal ions, oxygen counts, conductivity and pH measurements [3]. The electrochemical corrosion monitoring techniques such as polarization techniques, potentiometric methods and galvanic sensors are complicated and require special expertise where isolation from oxygen by using nitrogen is necessary [1]. Unlike the previous electrochemical techniques, the conductivity method used in this study is available and simple method and not affected by pressure of oxygen as well as its results are easy to interpret [4]. Due to its simplicity and availability, this study focuses only on the conductivity measurements to monitor the corrosion inhibition behaviour of sodium lignosulfonate for the first time. Other corrosion monitoring techniques such as weight loss and potentiodynamic polarization were studied before by the same research group [5]. This study aims at measuring the variations of electrical conductivity of three different corrosive media containing zinc sheets in the absence and presence of four different concentrations of sodium lignosulfonate in order to study corrosion and corrosion inhibition processes. The three selected corrosive media were HCl (1.0 M and 0.10 M), NaOH (1.0 M and 0.10 M) and NaCl solution (5.0% and 0.50% w/w). As shown in Figure 1, sodium lignosulfonate contains both hydrophilic groups like sulfonic, hydroxyl groups hydrophobic groups (carbon chains). It is an anionic surfactant, processing a certain degree of surface activity, which may promote surface adsorption and hence have application in corrosion inhibition process [6]. CH3 O O _ O + Na S HCl O O O H3c + Na O S O _ O Figure 1: the typical chemical structure of sodium lignosulfonate. The typical chemical structure of sodium lignosulfonate . 2 1. Material and methods 1.1. Materials used Pure zinc sheets (99.0%) were purchased from Sigma-Aldrich (UK). Sodium lignosulfonate was provided in powder form from Gainland Chemical (Deeside, UK). Solutions of 0.10 M HCl and 1.0 M HCl were prepared from concentrated HCl 37% (w/w) that was purchased from Merck (UK). Solutions of 0.10 M and 1.0 M NaOH and solutions of 0.50% and 5.0% (w/w) NaCl were prepared from extra pure NaOH and NaCl that were provided from S.C. chemical company (New Berlin, USA). All solutions were prepared using deionized water. 2.2 Electrical conductivity measurements Zinc sheets (2.0 cm × 2.0 cm × 0.025 cm) were polished down by emery papers of 100–800 grit and degreased with ethanol, then they were immersed inside a 100 mL beaker covered with paraffin film contains different corrosive solutions (HCl, NaOH, and NaCl) in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonate at 20 oC. All experiments were made under stirring conditions of 450 revolutions per minute (rpm), and the values of electrical conductivity were followed with time. The experiment was done 2 times and the average of conductivity readings were used. The electrical conductivity of the investigated solutions was measured using CC-501 conductometer (Elmetron, Witosa, Poland). The meter is equipped with custom LCD display, which enabled simultaneous observing of the measured function and temperature. The conductivity meter contains temperature compensation mode and internal data logger for 200 measurements with time date and temperature. The conductometer co-operates with Pt-1000 temperature probe with Chinch connector. 2. Results and discussion In order to investigate zinc corrosion process and its inhibition by sodium lignosulfonate, two different concentrations with three different corrosive media were selected. The results of these experiments are summarized in the following sections. 3.1. Electrical conductance measurements in HCl solutions When zinc sheet is immersed in HCl solution, it corrodes according to the following chemical reaction [7]: Zn(s) + 2H+(aq) + 2Cl-(aq) → Zn2+(aq) + 2Cl-(aq) + H2(g) (1) As shown in Figure 2, the value of electrical conductivity per mS/cm of 1.0 M HCl solution containing zinc sheet decreases with time. Generally, the electrical conductivity depends on many factors like ionic charge and ionic mobility, in addition to the ionic concentration when dealing with molar conductivity. According to equation 1, it is clear that H + ions are replaced by Zn2+ ions, a comparison between the ionic molar conductance of H + ( 349.65 S cm2 -1 2+ 1 2+ 2 -1 mol ) and the ionic molar conductance of Zn (( /2 Zn =52.5 S cm mol ) [8] gives us an explanation of why this decrease in values of electrical conductivity with time of 1.0 M HCl solution that containing the zinc sheet. 3 As shown in Figure 2, the decrease in electrical conductivity with time is more pronounced in the solution of 1.0 M HCl compared to the solutions of 1.0 M HCl containing different concentrations of sodium lignosulfonate that demonstrates a significant inhibition property of lignosulfonate in the corrosion process. In order to compare the slopes of the changes in conductivity with time of one solution with another the conductivity needs to be linearized so that the square root of time was used in the figures instead of time. Figure 2: the changes in electrical conductivity with time for Zn sheets immersed in 1.0 M HCl solutions in the presence and absence of 1.0, 10.0 and 20.0 mM of sodium lignosulfonate (LS) at 20 oC. The slope values of the changes of electrical conductivity of 1.0 M HCl solutions with and without sodium lignosulfonate (LS) are listed in Table 1. The slope value was taken from the linear portion of the plot. The significant difference between the slope value of 1.0 M HCl solution without LS (slope= -8.7071) and solutions containing 1.0, 5.0, 10.0 and 20.0 mM LS indicates the inhibition behaviour of LS against the corrosion of zinc sheets in the 1.0 M HCl solution. 4 Table 1: the slope values of changes of electrical conductivity in different solutions of 1.0 M and 0.10 M HCl containing zinc sheets and 0, 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonates (LS). Solution description Slope value in Slope value in 1.0 M HCl (mS cm-1 min.-1/2) 0.1 M HCl (mS cm-1 min.-1/2) Without LS -8.7071± 0.2719 -0.5801±0.0093 With 1.0 mM LS -0.1938±0.0073 -0.018±0.0014 With 5.0 mM LS -0.2145+0.083 -0.064±0.0011 With 10.0 mM LS -0.2725±0.0115 -0.0802±0.0008 With 20.0 mM LS -0.0210±0.0130 -0.1309±0.0013 Figure 3 shows the variation of electrical conductivity of 0.10 M HCl solutions in the presence and absence of sodium lignosulfonate. An obvious difference between the decrease of electrical conductivity of 0.10 M HCl solutions containing zinc sheets in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of LS that is shown in Figure 3. Figure 3: the changes in electrical conductivity with time for Zn sheets immersed in 0.10 M HCl solutions in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonate (LS) at 20 oC. 5 The slope values of the variation of electrical conductivity of 0.10 M HCl solutions with and without LS are listed in Table 1. As in the previous case, the obvious difference of the conductivity-time slope value of the solution without LS (slope= -0.5801) and, for instance, the solution containing 5.0 mM LS (slope= -0.0064) indicates the inhibition property of LS against the corrosion process in acidic medium. It is noticed that there was no systematic trends of variations in slopes with increasing the concentration of lignosulfonate. The ratio of the two slopes for the 0.1M solutions of HCl is 32.2 whereas the 1.0 M HCl solutions have a ratio of 44.9 strongly indicates that the lignosulfate is working as an inhibitor and appears to be more effective in a stronger acid solution. Other information can be deduced from the above measurements are that the corrosion of zinc metal in 0.10 M HCl is weaker than that in 1.0 M HCl, which will be in a good agreement with the slope values of variation of electrical conductivity with time of the solutions of 1.0 M HCl and 0.10 M HCl (Table 1). This means that the conductivity measurement provides qualitative information about the strength of corrosion as well as the extent of the inhibition behaviour. Addition of low concentrations of lignosulfonate (1, 5, 10 and 20 mM) did not lead to a change in the pH value or a critical increase of conductivity readings of the corrosive solutions. The corrosion behaviour of zinc in acidic medium is efficiently inhibited by compounds containing nitrogen, oxygen and sulfur atoms [9, 10]. Generally, such compounds increase the hydrogen overvoltage on zinc metal. Hydrogen overvoltage can be described as the difference between the hydrogen’s equilibrium reactions in a solution and the hydrogen itself present in the solution when it begins to form a corrosive reaction with metal [11]. Most corrosion inhibitors are organic compounds that mainly contain nitrogen, sulfur or oxygen atoms and multiple bonds or aromatic rings in their structures, and they reduce the corrosion rate by blocking the active sites on the metal surfaces [5, 12-17]. The Pourbaix diagram of zinc is shown in Figure 4. The diagram for zinc undergoes dissolution in acidic solutions (as Zn+2 ions) and in basic solutions (as zincate ions ZnO2 -2) as well as passivation of the zinc sheet as it is corroding in oxygenated alkaline solutions. Figure 4: the Pourbaix diagram for zinc at 20 oC. 6 3.2 Electrical conductance measurements in NaOH solutions: When zinc sheet is immersed in NaOH solution, it corrodes according to the following chemical reaction [18]: Zn (s) + 2OH- (aq) → ZnO2 -2 (aq) + H2 (g) (2) As shown in Figure 5, the value of electrical conductivity of NaOH solution containing zinc sheet decreases with time. According to chemical reaction 2, it is clearly that the total ionic mobilities in the products side are less than that in the reactants side. The difference of electrical conductivity was unnoticeable between a solution of 1.0 M NaOH in the absence of LS and solutions of 1.0 M NaOH in the presence of 1.0, 5.0, 10.0 and 20.0 mM of LS as shown in Figure 5. Figure 5: the changes in electrical conductivity with time for Zn sheets immersed in 1.0 M NaOH solutions in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonate (LS) at 20 oC. As indicated in Table 2, all of the slope values of the conductivity changes with time of the solutions of 1.0 M NaOH with 1.0, 5.0, 10.0 and 20.0 mM LS are less than in the case of 1.0 M NaOH without LS. 7 Table 2: the slope values of changes of electrical conductivity in different solutions of 1.0 M and 0.10 M NaOH containing zinc sheets and 0, 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonates (LS). Solution description Slope value in Slope value in 1.0 M NaOH (mS cm-1 min.-1/2) 0.10 M NaOH (mS cm-1 min.-1/2) Without LS -0.2234±0.0051 -0.1621±0.0006 With 1.0 mM LS -0.1075±0.0007 -0.1265±0.0007 With 5.0 mM LS -0.2025±0.0022 -0.1152±0.0007 With 10.0 mM LS -0.2011±0.0005 -0.1305±0.0004 With 20.0 mM LS -0.1321±0.0009 -0.0921±0.0003 Figure 6 shows the changes of electrical conductivity readings of 0.10 M NaOH solutions containing zinc sheets with and without LS. It is found that the corrosion process in this solution is slow as indicated by the slope values of the curves that shows the variation of electrical conductivity of these solutions with time (see Table 2). In this case, the corrosion inhibition behaviour of LS is less obvious than in basic medium at 1.0 M NaOH due to the weakness of corrosion of zinc metal in 0.10 M NaOH solution. The weakness of corrosion of zinc in alkaline solution could be considered to the possibility of passivation process of the zinc as shown in the Pourbaix diagram (Figure 4). As indicated in Table 2, the difference in the slope values of the 0.10 M NaOH solution without LS (slope= - 0.2234) and the same solution containing 1.0 mM LS (slope= - 0.1075) indicate again the corrosion inhibition behaviour of LS. The variation of conductivity readings of corrosive media containing zinc sheets with and without inhibitor indicates the strength of corrosion process and the inhibition behaviour. The inhibition behaviour of LS will be clear if the corrosion is strong, so that in the case of testing 0.10 M NaOH as corrosive medium, the inhibition behavior of LS was less visible than in acidic medium. 8 Figure 6: the changes in electrical conductivity with time for Zn sheets immersed in 0.10 M NaOH solutions in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonate (LS) at 20 oC. 3.3 Electrical conductance measurements in NaCl solutions: Figure 7 shows the variation of electrical conductivity of 5.0% (w/w) NaCl solutions in the presence and absence of LS. As shown in Figure 7, the conductivity of the solution without LS decreases with time, while the same solution with different concentrations of LS, the conductivity increases slightly with time. The slope values of the variation of electrical conductivity of NaCl solutions in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of LS with time that are listed in Table 3. Because the slope value of electrical conductivity was less in case of presence of LS than in case of absence of LS, again we could notice the inhibition behaviour of LS. 9 Figure 7: the changes in electrical conductivity with time for Zn sheets immersed in 5.0% (w/w) NaCl solutions in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonate (LS) at 20 oC. Table 3: the slope values of changes of electrical conductivity in different solutions of 5.0% and 0.50% (w/w) NaCl containing zinc sheets and 0, 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonates (LS). Solution description Slope value in 5.0% (w/w) NaCl (mS cm-1 min.-1/2) Slope value in 0.50% (w/w) NaCl (mS cm-1 min.-1/2) Without LS 0. 287±0.0255 0.0114±0.0002 With 1.0 mM LS 0.1147±0.0029 0.0072±0.0003 With 5.0 mM LS 0.1650±0.0043 0.0087±0.0001 With 10.0 mM LS 0.0346±0.0014 0.0072±0.0001 With 20.0 mM LS 0.0391±0.0020 0.0082±0.0001 Figure 8 shows the variation of electrical conductivity of 0.50% (w/w) NaCl solutions with and without LS. As shown in Figure 8, the values of electrical conductivity of NaCl solution containing zinc sheet were increasing with time. This increasing of electrical conductivity was 10 due to the conversion of Zn to Zn+2. In case of NaCl solutions, there is no replacing of Zn+2 ions with H+ as occurring in the acidic medium or with OH- as occurring in the basic medium. Figure 8: the changes in electrical conductivity with time for Zn sheets immersed in 0.50% (w/w) NaCl solutions in the presence and absence of 1.0, 5.0, 10.0 and 20.0 mM of sodium lignosulfonate (LS) at 20 oC. The slope values of the change of electrical conductivity of 0.50 (w/w) NaCl solutions with and without LS are listed in Table 3. Again, the difference of the slope values of the solution without LS (slope= 0.0114) and, for example, the solution contains 1.0 mM LS (slope= 0.0072) indicates the inhibition property of LS against the corrosion process in the salt medium. The inhibition behaviour of lignosulfonate is already affirmed [5, 9], but the new in this study that HCl (1.0 M and 0.10 M), NaOH (1.0 M and 0.10 M) and NaCl solution (5.0% and 0.50% w/w) were tested as corrosive media. A result from this study tests the conductivity method to be a simple monitoring tool of corrosion and corrosion inhibition processes. Conclusion This study succeeds in using the conductivity method as a new monitoring tool to provide qualitative information about the strength of corrosion as well as the extent of the inhibition behaviour. The variation of conductivity readings of three different corrosive media containing zinc sheets in the absence and presence of four different concentrations of sodium lignosulfonate indicates the inhibition behaviour of sodium lignosulfonate in the three selected corrosive media. The inhibition behaviour of sodium lignosulfonate was very pronounced in acidic medium and less visible in a basic medium (0.10 M NaOH) and neutral medium (0.50%NaCl). 11 Acknowledgements The authors are grateful to the faculty of Scientific Research at Petra University for supporting financially. Mustafa Mohamad and Miss Alaa Qtaishat are acknowledged for their effective technical help. 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