Colloids and Surfaces A: Physicochemical and Engineering Aspects
The adsorption, CMC determination and corrosion inhibition of some N-alkyl quaternary ammonium salts on carbon steel surface in 2 M H2SO4
Introduction
The adsorption of surfactants on solid/liquid interfaces is of central interest in colloid and surface science. Furthermore, the investigation of surfactants adsorbed on metal surfaces is extremely important in electrochemical studies such as corrosion inhibition, adhesion, lubrication, and detergency [1]. Handling of the aggressive fluids (e.g. 2 M H2SO4 at high temperature) is a problem that is very important from the standpoint of industrial applications, since it concerns the geothermal and oil industries as well as pickling, descaling, and acid cleaning of plants. Acid pickling baths are employed to remove undesirable scale (i.e. mill scale) from the surface of the metals. Once scale is removed, the acid is free for further attack the metal surface. These metal materials most often comprise stainless steels because of their wide applicability. Thus, the aggressiveness of the environment makes it necessary to add chemical compounds to the operating fluid, which will act as corrosion inhibitors to minimize the metal dissolution and the consumption of acid [2].
The application of surfactants as corrosion inhibitors has been extensively studied, and adsorption of the surfactant on the metal surface was found to be responsible for the corrosion inhibition of the metal surface. Most acid inhibitors are organic compounds containing nitrogen, sulphur and/or oxygen atoms [3], [4]. The corrosion inhibition of a metal may involve either physisorption or chemisorption of the inhibitor on the metal surface. Electrostatic attraction between the charged hydrophilic groups and the charge active centres on the metal surface leads to physisorption. Several authors showed that most organic inhibitors are adsorbed on the metal surface by displacing water molecules from the surface and forming a compact barrier film.
It is well known that surfactants are characterized by critical micelle concentrations (so called CMCs). The CMC is the concentration where surfactants in solution change their initial molecular solvated state. Most of the physical and chemical properties of surfactant solutions undergo an abrupt variation at this concentration. This effect is of interest for theoretical reasons, well as for practical applications.
The CMC is influenced by a number of factors that are dependent on the nature of the surfactant and the aqueous environment. The ionic strength of the solution is one of these influential factors, being responsible for the shift of the CMC value with respect to its primary value in pure water [5], [6], [7], [8]. For example, the CMC for sodium dodecylsulfate (SDS) in water is 8 × 10−3 mol L−1 and decreases in 0.5 mol L−1 H2SO4 to 8 × 10−4 mol L−1 and in 1 mol L−1 HCl to 1.6 × 10−5 mol L−1 [9], [10], [11]. Critical micelle concentrations can be determined by measuring any micelle influenced physical property as a function of surfactant concentration. In practice, however, the osmotic pressure, surface tension and electrical conductivity are the parameters most often measured [12].
The application of cationic surfactants as corrosion inhibitors of metal has been widely studied. Most frequent are studies in hydrochloric acid as medium, because of the better inhibition performance of surfactants in HCl than in sulphuric acid solutions [3], [13], [14]. Moreover, the corrosion inhibition of quaternary ammonium salts on metal surfaces in sulphuric acid of molarity higher than 1 mol L−1 has been rarely investigated until now.
The present study is an extension of our earlier work, where the possibility of determination of the CMC via kinetic parameters measured in corrosion processes was indicated [15]. The objective of the present work was to study the effect of cationic surfactants of the N-alkyl quaternary ammonium salt type as inhibitors of processes on stainless steel (SS), type X4Cr13. The inhibition of this SS in aqueous solutions of 2 mol L−1 H2SO4 was studied using three cationic surfactants; myristyltrimethylammonium chloride (MTACl), cetyldimethylbenzylammonium chloride (CDBACl) and trioctylmethylammonium chloride (TOMACl). On the basis of kinetic parameters measured in the corrosion processes, the shifts of CMCs of the chosen cationic surfactants in 2 mol L−1 H2SO4 were estimated. The surface tension of the surfactants in acid solution was also measured and used as an independent method of CMC determination.
Section snippets
Materials
The chosen cationic surfactants were Fluka products of pure quality (>97%) and used without purification. The critical micelle concentration of each surfactant in water was determined from measurements of the differential conductivity versus the concentration of the ionic surfactant, on the basis molar conductivity plots. The conductivity was measured at (25.0 ± 0.1) °C. These values for the CMC of N-alkyl ammonium chlorides were: (6.0 ± 0.5 × 103) M (M = mol L−1) for MTACl, (5.0 ± 0.2 × 10−4) M for CDBACl
Results and discussions
As mentioned above, physical adsorption is a result of electrostatic attractive forces between organic ions or dipoles (inhibitor) and the electrically charged surface of the metal. The surface charge of the metal is due to the electric field existing at the metal/solution interface.
The surface charge can be defined by the position of the corrosion potential Ecorr with respect to the respective potential of zero charge (PZC). When the difference Er = Ecorr − Eq=0 is negative, where Er is the
Conclusion
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Anodic and cathodic polarization curves for electrodes in 2.0 M sulphuric acid in the absence and presence of various concentrations of surfactants indicated that the selected surfactants act as mixed type inhibitors, i.e. promoting retardation of both anodic dissolution of the steel and the cathodic hydrogen discharge reaction. The best inhibition efficiency of all three surfactants used was achieved within a limited potential region. The inhibition efficiency of these inhibitors strongly
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