Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, a-D-Glucose and Tannic acid) A. Ostovari a, * , S.M. Hoseinieh a , M. Peikari a , S.R. Shadizadeh b , S.J. Hashemi a a Technical Inspection Engineering Department, Petroleum University of Technology, Abadan, Iran b Petroleum Engineering Department, Petroleum University of Technology, Abadan, Iran article info Article history: Received 17 February 2009 Accepted 14 May 2009 Available online 23 May 2009 Keywords: A. Acid solutions B. EIS B. SEM C. Acid inhibition C. Pitting corrosion abstract The inhibitive action of henna extract (Lawsonia inermis) and its main constituents (lawsone, gallic acid, a-D-Glucose and tannic acid) on corrosion of mild steel in 1 M HCl solution was investigated through electrochemical techniques and surface analysis (SEM/EDS). Polarization measurements indicate that all the examined compounds act as a mixed inhibitor and inhibition efficiency increases with inhibitor concentration. Maximum inhibition efficiency (92.06%) is obtained at 1.2 g/l henna extract. Inhibition efficiency increases in the order: lawsone > henna extract > gallic acid > a-D-Glucose > tannic acid. Also, inhibition mechanism and thermodynamic parameters are discussed. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The study of mild steel corrosion phenomena has become important particularly in acidic media because of the increased industrial applications of acid solutions. As an example, the refin- ing of crude oil results in a variety of corrosive conditions. Refinery corrosion is generally caused by a strong acid attacking the equip- ment surface [1]. The other important fields of application are acid pickling, industrial cleaning, acid descaling, oil-well acid in oil recovery and the petrochemical processes [2]. Among the acid solutions, hydrochloric acid is one of the most widely used agents. Due to the exposure of mild steel to corrosive environments, they are susceptible to different types of corrosion mechanisms; there- fore, the use of corrosion inhibitors to prevent metal dissolution will be inevitable. A great number of scientific studies have been devoted to the subject of corrosion inhibitors for mild steel in acidic media [3–12]. The majority of well-known inhibitors are or- ganic compounds containing heteroatoms, such as O, N or S, and multiple bonds, which allow an adsorption on the metal surface [13]. These compounds can adsorb on metal surface and block the active surface sites to reduce the corrosion rate. Four types of adsorption may take place by organic molecules at metal/solution interface: (a) electrostatic attraction between the charged mole- cules and charged metal, (b) interaction of uncharged electron pairs in the molecule with the metal, (c) interaction of p-electrons with the metal and (d) combination of (a) and (c) [14]. However, the stability of the inhibitor film on the metal surface depends on some physicochemical characteristics of the molecule, related to its functional groups, aromaticity, possible steric effects, elec- tronic density of donors, type of corrosive medium, structure, charge of metal surface and nature of interaction between the p- orbital of inhibitors with the d-orbital of iron [10,15]. Although many synthetic compounds show good anticorrosive action, most of them are highly toxic to both human beings and environment. These inhibitors may cause temporary or permanent damage to organ system such as kidneys or liver, or to disturb a biochemical process or to disturb an enzyme system at some site in the body [16]. The toxicity may manifest either during the syn- thesis of the compound or during its applications. These lead inves- tigations to focus on the use of naturally occurring substances in order to find low-cost and non-hazardous inhibitors. De Souza and Spinelli have studied the inhibitive action of naturally occur- ring biological molecule caffeic acid on the corrosion of mild steel in acidic solution [17]. The extracts of Chamomile, Halfabar, Black cumin and kidney bean were studied for their corrosion inhibition of steel in acidic media by Abdel-Gaber et al. [18]. Plant extracts have became important as an environmentally acceptable, readily available and renewable source of materials for wide range of cor- rosion prevention; therefore, finding naturally occurring sub- stances as corrosion inhibitors is a subject of great practical significance [19–24]. 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.05.024 * Corresponding author. Tel.: +98 912 3056552; fax: +98 631 4429937. E-mail address: A.Ostovari@gmail.com (A. Ostovari). Corrosion Science 51 (2009) 1935–1949 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci