Corrosion Inhibition of Nickel in Sulfuric Acid Using Tween Surfactants

Portugaliae Electrochimica Acta 21 (2003) 315-326 PORTUGALIAE ELECTROCHIMICA ACTA Corrosion Inhibition of Nickel in Sulfuric Acid Using Tween Surfactants M. Abdallah, A.Y. El-Etre* Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Received 3 December 2002; accepted in revised form 1 April 2003 Abstract Four commercial non-ionic surfactant compounds, namely tween 80, 60, 40 and 20, were tested as inhibitors for corrosion of nickel in 1.0 M H2SO4 solution. Weight loss measurements, potentiostatic polarization and cyclic voltammetry techniques were used in this study. It was found that all the four used compounds act as good inhibitors for acid corrosion of nickel. The inhibition efficiencies obtained by the three techniques were almost the same, and increase with increasing the hydrocarbon chain length, the presence of a double bond in the chemical structure of the surfactant and with increasing the surfactant concentration. The polarization studies show that tween compounds act as mixed inhibitors. The inhibition action of these surfactants is interpreted in view of their adsorption on the metal surface making a barrier to mass and charge transfer. It was found that the adsorption of only tween 20 and 40 follows Langmuir adsorption isotherm. The values of free energy of adsorption for them were calculated. It was found that the adsorption process is spontaneous and increases, for different surfactants, in the same direction as inhibition efficiency. The cyclic voltammetry shows that there is only one anodic peak corresponding to the dissolution reaction of nickel electrode. The current of this dissolution peak was used also for corrosion rate measurements and in evaluation of inhibition efficiencies of the used compounds. Keywords: nickel, corrosion inhibition, tween, surfactant. Introduction Nickel is one of the most important metals and is used in a large number of applications. The pure nickel has a good corrosion resistance and is frequently used as a protective coat for other metals and alloys. For the same reason, nickel is used as alloying element with other metals. Nickel based alloys show a * Corresponding author. E-mail address: M. Abdallah and A.Y. El-Etre / Portugaliae Electrochimica Acta 21 (2003) 315-326 considerable resistance against different types of corrosion. Even the addition of small quantity of nickel to an alloy improves its corrosion resistance character. The corrosion resistance of nickel is due to the formation of a passive film on its surface upon exposure to the corrosive media. Nevertheless, nickel could be attacked by acidic media in a considerable rate. Many works were conducted to study the passivation of nickel in different acidic solutions [1-5]. It was reported that the passive film formed on nickel surface in low concentrations of sulfuric acid is NiOOH [3] or NiO and Ni2O3 [4]. On the other hand, β-NiSO4.6H2O was suggested as the passive compound formed in very high concentrations of sulfuric acid [5]. However, the obtained results showed that nickel establishes a kind of passivity in acidic solutions in which the corrosion current, in the passive potential range, is somewhat higher than those recorded by other passive metals. Because nickel is frequently used in contact with acidic solutions, its corrosion rate must be controlled. One of the useful methods of controlling the corrosion process is the addition of corrosion inhibitor. Many researches were published in the literature concerning the usage of inhibitors for nickel corrosion in acidic solutions [6-12]. Most of the tested inhibitors are organic compounds containing sulfur or nitrogen in their chemical structures. It was found that this kind of compounds is chemically adsorbed on the nickel surface forming a barrier for mass and charge transfer and consequently decreasing the rate of corrosion. Unfortunately, most of these compounds are harmful for human and environment. Therefore, additional work should be conducted to find safe and cheap corrosion inhibitors for nickel in acidic solutions. This work is devoted to test a series of tween surfactants as inhibitors for nickel corrosion in sulfuric acid solution. Tween compounds can be easily synthesized from relatively cheap raw materials. In addition, tween compounds are non toxic and have surface active property. Weight loss measurements, cyclic voltammetry and polarization techniques were used in the study to evaluate the inhibition efficiency of the tested tween compounds. 316 M. Abdallah and A.Y. El-Etre / Portugaliae Electrochimica Acta 21 (2003) 315-326 Experimental Coupons of pure nickel with dimensions of 1 x 2 x 0.2 cm were used in weight loss experiments. For potentiostatic polarization technique, a cylindrical rod of nickel embedded in araldite with an exposed bottom area of 0.5 cm2 was used. Before each experiment, the electrode was polished to a mirror finish with different grades of emery papers, degreased with acetone and finally rinsed with distilled water. BDH grade sulfuric acid was used for the preparation of the test solutions. Weight loss measurements were carried out by the same method as described elsewhere [13]. Each of the nickel sheets was immersed, for 6.0 hours, in 50 mL of 1.0 M H2SO4 solutions containing different concentrations of tween compounds, at 25 (± 1) o C. A three electrodes cell, with saturated calomel reference electrode (SCE) and platinum foil counter electrode was used in polarization experiments. Both potentiostatic polarization and cyclic volammetry technique were carried out using a PS remot potentiostat with zum PS6 software for calculation of electrochemical parameters. The used tween surfactants are organic compounds which have surface active properties and commercially available with relatively low price. Four different tween compounds are used in the present study, namely, tween 20, 40, 60 and 80. Tween compounds are basically polyoxyethelene sorbitan combined with different fatty acids. The number associated with the tween name determines the predominant fatty acid contained in the tween structure. Thus, for tween 20, 40, 60 and 80, the acids are, respectively, monolaurate, monopalmitate, monostearate and monooleate. Results and Discussion Weight loss measurements The losses of weight of nickel sheets due to their immersion in solutions of 1.0 M H2SO4 containing different concentrations of tween compounds were measured. It was found that the addition of any of the used four tween compounds lowers the weight loss of the nickel sheet than its value in the free acid solution. This 317 M. Abdallah and A.Y. El-Etre / Portugaliae Electrochimica Acta 21 (2003) 315-326 result indicates that the four tween compounds act as inhibitors for nickel corrosion in sulfuric acid solution. The inhibitive action of tween compounds could be attributed to the adsorption of their molecules on the nickel surface, forming a barrier between the bar metal (...truncated)