Iraqi Journal of Chemical and Petroleum Engineering Vol.14 No.4 (December 2013) 19- 25 ISSN: 1997-4884 Corrosion Inhibition of Galvanic Couple Copper Alloy/Mild Steel in Cooling Water System Aprael S. Yaro and Munaf A. Idan Chemical Engineering Department-College of Engineering-University of Baghdad-Iraq Abstract The driving idea for the present work was to combine the effect of polyvinyl alcohol (PVA) as corrosion inhibitor with the distance between the anodic and cathodic elements of the galvanic cell, beside their area ratio, in scope of synergistic suppression of galvanic corrosion on Cu/Fe model couple, using weight loss method. The performance affecting galvanic corrosion process has been tested for three major factors affect the process: 1. Four PVA inhibitor concentrations were selected to be (0, 1000, 4000 and 7000 ppm) in simulated cooling water. 2. Two cathode: anode area ratios as 1:1 and 2.4:1. 3. Two distances apart cathode – anode as 3 and 7 cm. Maximum corrosion inhibition achieved was 86% which indicates that increasing inhibitor concentration leads to decrease dissolution process followed hydrogen evaluation Cu electrode as cathode element in galvanic cell. Keywords: Galvanic Corrosion, Mild steel, Polarization, Galvanic Current Introduction The study of inhibition mechanism, electrochemical, and kinetic behavior of water-soluble polymers such as poly vinyl alcohol as a corrosion inhibitor to protect Cu-CS galvanic couple in aqueous media, contributes to the prevention of corrosion, particularly in industrial equipment which inevitably requires joining pieces of different metals for its construction [1–6]. Cooling water systems are often constructed by dissimilar metals such as copper fins that cool the fluid by convection, internal copper tubes and mild steel shells. The conditions of the cleaning process during the manufacture of heavy duty heat exchangers promote the dissolution of the anodic metal in a galvanic couple especially with an unfavorable cathode-anode area ratio of 2.4 to 1.0. The use of polyvinyl alcohol as corrosion inhibitor is related to its outstanding properties. The film forming and adhesive qualities enable nearly all water-soluble polymers to find uses as binders [7, 8, 9]. Galvanic corrosion, resulting from a metal contacting another conducting material in a corrosive medium, is one of the most common types of corrosion. In many cases, galvanic corrosion may result in quick deterioration, but in other cases, the galvanic corrosion of one metal may Iraqi Journal of Chemical and Petroleum Engineering University of Baghdad College of Engineering Corrosion Inhibition of Galvanic Couple Copper Alloy/Mild Steel in Cooling Water System 20 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net result in the corrosion protection of an attacked metal, which is the basis of cathodic protection by sacrificial anode [10]. When two dissimilar conducting materials in electrical contact with each other are exposed to an electrolyte, a current, the galvanic current, flows from one to the other, galvanic corrosion is that part of the corrosion that occurs at the anodic member of such a couple and is directly related to the galvanic current by Faraday’s law [11]. Many factors play a role in galvanic corrosion in addition to the potential difference between the two coupled metals. Depending on the circumstance. Compared to normal corrosion, galvanic corrosion is generally more complex because, in addition to material and environmental factors, it involves geometrical factors [12]. Experimental Work 1. Materials The materials of electrodes used in this investigation were 2 coupons from mild steel type SA 515 GR 60 having the following dimensions:  Big coupon ( 4.9cm length ,3cm width and 0.3 cm thickness).  Small coupon ( 2.83cm length , 3cm width and 0.3 cm thickness). These specimens having the following chemical compositions (% wt) (were supplied by Al-Dura refinery): Table 1, The chemical composition of mild steel coupon (%wt) Carbon manganese Phosphorus sulfur silicon Fe 0.24 0.9 0.035 0.035 0.15 -0.4 Remainder The second electrode was copper type ASTM B-111-443 with 3.5 cm length, 4.43 cm width and 0.2 cm thickness having the chemical compositions as follows: Table 2, The chemical composition of copper coupons (%wt) Copper Lead Iron Zinc Arsenic 70 – 73 0.07 0.06 Reminder 0.02- 0.06 2. Solution The chemical composition of water solution used throughout the experiments was actually same as the chemical composition of water used in the cooling system of Al-Dura Refinery Iraq as follows: Table 3, Chemical composition of tested solution Component Concentration, ppm Na + 441 Cl - 303 SO4 -2 352 HCO3 - 123 CO3 -2 37 The tested solution was prepared by dissolving 500 ppm NaCl, 520 ppm Na2SO4, 170 ppm anhydrous NaHCO3, and 66 ppm Na2CO3 in one liter of distilled water. Inhibitor solution was prepared by dissolving appropriate amount of polyvinyl alcohol (PVA). 3. Chemicals The table below lists the compounds and chemical used in this investigation: Table 4, Compound used in this investigation Compounds Formula Purity% Acetone C3H6O 99.5 Benzene C6H6 - Hydrochloric acid HCl 98.9 Poly vinyl alcohol C2H3OR* - Sodium chloride NaCl 95.5 Sodium carbonate NaCO3 96 Sodium bicarbonate NaHCO3 97.9 Sodium sulfide Na2SO4 90 * where R : H or COCH3 Aprael S. Yaro and Munaf A. Idan -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 21 4. Weight Loss Method Specimens were abraded in sequence under running tap water using emery papers of grade numbers; 220, 320, 400, and 600 respectively, washed with running tap water followed by distilled water, dried on a clean tissue, immersed in benzene for five seconds and dried with clean tissue, immersed in acetone for five seconds and dried with clean tissue, kept in a desiccator over silica gel for one hour before each run. Procedure 1. The dimensions of each specimen were measured with vernire to the 2 nd decimal of millimeter and accurately weight to the 4 th decimal of gram before using. 2. Before each test, the cell was washed with running tap water followed by distilled water and test solution. 3. Specimens were completely immersed in 1000 cm 3 solution of corroding contained in the cell. They were exposed for period of 24 hours, desired concentration of inhibitor and the coupons were apart 3 and 7 cm from each other. 4. All the experiments were done with an area ratio of C/A (2.4:1 and 1:1), the working electrode was constructed joining metal coupons with above mentioned area ratio for mild steel, connected to an insulated copper wire. 5. After each test, the mild steel specimen was washed with running tap water, scrubbed with a brush to remove corrosion products, then washed with tap water followed by distilled water and dried on a clean tissue, immersed in benzene, dried, immersed in acetone, dried and left in a desiccators over silica gel for one hour before weighting then accurately weight to the 4 th decimal. Results and Discussion A total of 16 runs for weight loss measurements were made expressing rate of two area ratio of mild steel couple to Copper in simulated cooling water system containing different concentration of PVA as corrosion inhibitor. Two levels for both the distance and area ratio of (Ac/Aa) for electrode were adopted, while four levels for inhibitor concentration as independent variables. Corrosion rate calculations of mild steel (anode) coupled to copper (cathode) from weight loss data were performed using the following equation: ( ) ( ) ( ) ( ) …(1) Table 5, corrosion rate of mild steel coupled to copper in simulated cooling water under different operating conditions Corrosion rate (gmd) Distance between Fe-Cu (3 cm) Distance between Fe-Cu (7 cm) Inhibitors concentrations (ppm) Area ratio Cu/Fe(1:1) Area ratio Cu/Fe(2.4:1) Area ratio Cu/Fe(1:1) Area ratio Cu/Fe(2.4:1) Blank 11.77 20.95 7.698 15.47 1000 7.756(54) 11.32(46) 5.707(34.1) 9.987(35.5) 4000 3.541(70.7) 6.04(71.1) 2.254(51.5) 5.627(63.6) 7000 1.668(82.9) 4.15(80.2) 1.317(86) 3.306(78.6) ( ) indicates % inhibition in presence of PVA as corrosion inhibitor. The quantitative description of the physical condition effect on corrosion rate of mild steel coupled to copper in simulated cooling water was performed. An empirical modeling technique called response surface Corrosion Inhibition of Galvanic Couple Copper Alloy/Mild Steel in Cooling Water System 22 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net methodology is used to evaluate the relationship between the controllable experimental variables and observed results. [13]. The results were analyzed using the analysis of variance (ANOVA) as appropriate to experimental design used. The regression equations obtained gives the corrosion rate of mild steel coupled to copper as cathodic element in galvanic corrosion, as function of area ratio (X1) and distance between the electrodes (X2), in absence and presence of corrosion inhibitor (0,1000,4000and7000)ppm. Regression analysis was utilized by using Statistica program version 10.1 to generate four models for given inhibitor concentration with correlation coefficient of R2=1.0. CRBlank = 7.5 + 7.32 X1- 0.765 X2 - 0.25 X1X2 …(2) CR1000 ppm = 7.13+ 2.16 X1- 0.64 X2 + 0.13 X1X2 …(3) CR4000 ppm= 3.18 + 1.33 X1 - 0.48 X2 + 0.15 X1X2 …(4) CR7000 ppm= -0.11 + 2.04 X1+0.0003 X2 - 0.09 X1X2 …(5) Where X1, X2 are the area ratio and distance between electrodes respectively and CR is the corrosion rate in (gmd). Equations (2 through 5) showed suitable models to describe the response of the mild steel coupled to copper under investigation. A high values of R2 =1 justified excellent correlation between the independent variables. This indicates a good agreement between the predicted and experimental values of the corrosion rates of mild steel coupled to copper as shown in table (5). Thus the effect of distance between the anodic and cathodic elements in galvanic couple and the area ratio of (C/A) on the response can be obtained at fixed levels of inhibitor concentrations (0,1000,4000and7000 ppm). The darker the red color means higher corrosion rate of mild steel coupled to copper, while the darker the green color means the lower the corrosion rate. Figures (1 through 5) corroborate the fact that minimization of corrosion rate of mild steel coupled to copper as cathodic element is possible in simulated cooling water only at low C/A area ratio and large anode to cathode distance apart. The corresponding analysis of variance (ANOVA) is represented in tables (6 through 9) in absence of inhibitor and at different inhibitor concentrations. The result obtained from this analysis indicate the significance of variables studied through the p-value (i.e., p-value is less than 0.05). Fig. 1, Three dimensional surface plot showing corrosion rate of Carbon steel coupled to copper in SCW in absence of PVA as corrosion inhibitor at different area ratios and distance between anode and cathode Aprael S. Yaro and Munaf A. Idan -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 23 Table 6, ANOVA for corrosion of mild steel coupled to copper in simulated cooling water in absence of inhibitors, at different area ratios and distances between electrodes Source of variation Sum of squares Degree of freedom MS effect Fₒ P-value X1 72.0801 1 72.0801 147.102 0.000065 X2 22.9441 1 22.9441 46.825 0.00082 X1X2 0.5027 1 0.5027 1.0259 0.00825 Error 1.96 4 0.4900 Fig. 2, Three dimensional surface plot showing corrosion rate of Carbon steel coupled to copper in SCW containing 1000 ppm PVA as corrosion inhibitor at different area ratios and distance between anode and cathode Table 7, ANOVA for corrosion of mild steel coupled to copper in simulated cooling water in 1000 ppm PVA, at different area ratios and distances between electrodes Source of variation Sum of squares Degree of freedom MS effect Fₒ P-value X1 15.3821 1 15.8321 114.725 0.000074 X2 2.8595 1 2.8595 20.721 0.00093 X1X2 3.3282 1 3.3282 24.1173 0.00221 Error 0.552 4 0.138 Fig. 3, Three dimensional surface plot showing corrosion rate of Carbon steel coupled to copper in SCW containing 4000 ppm PVA as corrosion inhibitor at different area ratios and distance between anode and cathode Corrosion Inhibition of Galvanic Couple Copper Alloy/Mild Steel in Cooling Water System 24 IJCPE Vol.14 No.4 (December 2013) -Available online at: www.iasj.net Table 8, ANOVA for corrosion of mild steel coupled to copper in simulated cooling water in 4000 ppm PVA, at different area ratios and distances between electrodes Source of variation Sum of squares Degree of freedom MS effect Fₒ P-value X1 0.73017 1 0.73017 3.9035 0.000037 X2 8.64654 1 8.64654 46.2246 0.00078 X1X2 0.18076 1 0.37405 1.9997 0.00482 Error 0.74822 4 0.187055 Fig. 4, Three dimensional surface plot showing corrosion rate of Carbon steel coupled to copper in SCW containing 7000 ppm PVA as corrosion inhibitor at different area ratios and distance between anode and cathode Table 9, ANOVA for corrosion of mild steel coupled to copper in simulated cooling water in 7000 ppm PVA, at different area ratios and distances between electrodes Source of variation Sum of squares Degree of freedom MS effect Fₒ P-value X1 4.99746 1 4.99746 82.3305 0.000013 X2 0.35701 1 0.35701 5.8757 0.00035 X1X2 0. 6076 1 0. 6076 10.0099 0.00591 Error 0.2428 4 0.0607 Conclusions On the basis of the results presented, the following conclusions can be drawn: 1. The inhibition action of (PVA) increases with the increase of inhibitor concentration and distance between anode and cathode. 2. The methodology of three level factorial design was shown to be very useful for the process variables evaluation as main and combined effect on the response. 3. The multi-variable regression describes the behavior of the corrosion inhibition process with high accuracy (R 2 =1.0). 4. The importance of galvanic phenomenon is greater than as the ratio of cathode/anode increases, however the results show compatibility of both materials at both area ratios and (PVA) concentrations except at 7000 ppm the couple not severing from galvanic corrosion. References 1- C. M. Mustafa and S. M. S. I. Dulal, “Molybdate and nitrite as corrosion Iinhibitors for copper-coupled steel in simulated cooling water,” Corrosion, vol. 52, no. 1, pp. 16–22, 1996. Aprael S. Yaro and Munaf A. Idan -Available online at: www.iasj.net IJCPE Vol.14 No.4 (December 2013) 25 2- E. J. 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