Corrosion inhibition of iota-carrageenan natural polymer on aluminum in presence of zwitterion mediator in HCl media Mohammad M. Fares ⇑ , A.K. Maayta, Jamil A. Al-Mustafa Department of Applied Chemical Sciences, Jordan University of Science & Technology, P.O. Box 3030, 22110 Irbid, Jordan article info Article history: Received 11 June 2012 Accepted 9 August 2012 Available online 17 August 2012 Keywords: i-Carrageenan Pefloxacin mesylate Zwitterion mediator Aluminum Corrosion abstract i-Carrageenan a natural polymer has been used as corrosion inhibitor of aluminum in presence of peflox- acin mesylate, acting as zwitterionic mediator, in acidic medium. Considerable improvement in inhibition efficiency occurred in the presence of the mediator. Activation energy of corrosion and other thermody- namic parameters such as standard free energy, standard enthalpy, and standard entropy of the adsorp- tion process revealed better and well-ordered physical adsorption layers in presence of pefloxacin. Adsorption isotherms in absence or presence of pefloxacin mediator appropriately fit in the Langmuir iso- therms. The scanning electron microscope (SEM) images demonstrated smooth, glossy, and relatively coherent adsorption layers of the inhibitor on the metal surface in aqueous solution. After the exposure to 2.0 M HCl for 2 h, a smaller but consistent regular shaped fractured layer is obtained. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The use of environmentally-friendly compounds in various applications gained wide interest in the recent years due to the global awareness of the impact of the human activities on the ecol- ogy of our planet. This had diverted the attention of researchers to re-consider using natural products, natural polymers, plants and plant extracts as green corrosion inhibitors. Such green compounds are less harmful to the environment and to humans and can act effectively as successful corrosion inhibitors. They have many advantages over chemical inhibitors such as being natural, cheap, readily available and in general nontoxic with optimal biodegrad- ability profile. Many green inhibitors have been studied such as plant seeds [1,2], leaves [3–5], plants aqueous extract, peels, fruits [6–11], pectin [12], lignin [13], oil extracts [14–16] and plant alka- loids extracts [17,18]. Carrageenan is a gel-forming polysaccharide obtained by extraction of Rhodophyceae red seaweeds, in particular from Chondrus crispus, Euchema, Gigartina stellata and Iridaea. The major constituents of such compound are the so-called carrageen- ans, co-polysaccharides with a linear backbone of b-D-galactose and 3,6-anhydro-a-D-galactose partially sulfated (Fig. 1). Various degrees of sulfation (between 15% and 40%) identified by a Greek prefix: the three commercially available carrageenans are called Iota i-(mono-sulfate), Kappa j-(di-sulfate), and Lambda k-carra- geenan (three-sulfate). The first two are gel-forming systems, whereas k-carrageenan is a thickening agent [19]. On the other hand, pefloxacin mesylate is a fluoroquinolone antibiotic that offers a broad spectrum of antibacterial activity against some microorganisms [20]. Pefloxacin contains three ter- tiary amine groups and a carboxylic acid group within its structure (Fig. 2). These acidic and basic groups allow pefloxacin to exist in a zwitterionic form at some pH values and to undergo the protolytic equilibria summarized in Fig. 2. At low pH value, the amine groups are protonated and pefloxa- cin is positively charged. At higher pH value the carboxylic acid groups are deprotonated and pefloxacin is negatively charged. The interesting acid–base properties of pefloxacin could be of great importance in corrosion studies as it could act as mediator in the adsorption of polymeric molecules to metallic surfaces, which in some cases are weakly or cannot be adsorbed at all due to config- urational or steric factors. Aluminum and its alloys numerously used as vessels, pipes, machinery and chemical batteries could form passive oxide layer against corrosion. Upon pickling using hydrochloric acid, this pas- sive layer readily dissolves leaving the aluminum surface unpro- tected [21,22]. In this report, i-carrageenan is studied as green corrosion inhibitor mediated by pefloxacin mesylate on the alumi- num surface. The formation of well-oriented synergistic adsorption layers at the solid/liquid interface at different temperatures and in acidic media was investigated via the kinetic and thermodynamic parameters. Morphology of the adsorption layers also checked and investigated by scanning electron microscopic technique (SEM). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.08.018 ⇑ Corresponding author. Tel.: +962 2 7201000x23583; fax: +962 2 7201071. E-mail address: fares@just.edu.jo (M.M. Fares). Corrosion Science 65 (2012) 223–230 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci