A Revisit to the Corrosion Inhibition of Aluminum in Aqueous Alkaline Solutions by Water-Soluble Alginates and Pectates as Anionic Polyelectrolyte Inhibitors

The corrosion behavior of aluminum (Al) in alkaline media in presence of some natural polymer inhibitors has been reinvestigated. The inhibition action of the tested inhibitors was found to obey both Langmuir and Freundlich isotherms models. The inhibition efficiency was found to increase with increasing the inhibitors concentration and decrease with increasing the temperature, suggesting physical adsorption mechanism. Factors such as the concentration and geometrical structure of the inhibitor, concentration of the corrosive medium, and temperature affecting the corrosion rates were examined. The kinetic parameters were evaluated, and a suitable corrosion mechanism consistent with the kinetic results obtained is suggested and discussed. 1. Introduction Aluminum and its alloys are known as attractive materials which have a wide application in engineering technology owing to their low cost, light weights, and high thermal and electrical conductivities [1]. They also showed high resistivity toward a wide variety of corrosion environments [2]. This behavior may be attributed to the formation of a protective, tightly adhered invisible oxide film on the metal surface. The remarkable potential applications of aluminum in industrial technology particularly in aluminum-air industry encouraged many investigators to study the influence of various inhibitors on Al dissolution in both acidic [1–14] and alkaline [15–20] media since the kinetics of dissolution is reduced by the presence of a thick oxide-hydroxide film at the anode surface. Most of workers have directed their research towards concentrated alkaline media, which permit optimum performance of the air cathode and a low level of aluminum polarization during normal operation [20]. Unfortunately, most of the inhibitors available in the markets are not only expensive but also toxic to living beings. Again, the safety of environmental issues of corrosion inhibitors which may be arise in industry has always been a global concern. Therefore, the cheap, nontoxic, ecological, and friendly corrosion inhibitors are of more importance. Consequently, many alternative ecofriendly corrosion inhibitors have been developed [17]. Recently, the influence of alginate and pectate anionic polyelectrolyte as natural polymers inhibitors on the behavior of dissolution of Al metal in alkaline solutions has been reported elsewhere [21]. Unfortunately, the adsorption isotherm models have not been tested to the experimental results. Hence, the information on the corrosion mechanism still remains incomplete. In view of the above