Utilization of watermelon rind extract as a green corrosion inhibitor for mild steel in acidic media N.A. Odewunmi *, S.A. Umoren, Z.M. Gasem Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, 31261, Saudi Arabia 1. Introduction Mild steels are frequently used for a wide range of applications in construction industry due to their very good mechanical properties and low cost compared with many metallic materials. The poor corrosion resistance of mild steel in acid [1–3] is a major constraint in its applications. The use of inhibitors in controlling its corrosion processes has proven to be efficient and most practicable [4,5]. However, most of the compounds that constitute these inhibitors are toxic to human and its environments. The toxicity of these organic and inorganic inhibitors has paved way to explore the use of non-toxic natural products inhibitors [6–9] that are environmentally friendly. The use of extracts of some plants to inhibit the corrosion of steel in acid media has been reported in the literature [10–27]. Watermelon is vine-like flowering plant which belongs to the family of Cucurbitaceae with varieties of seeded and seedless species. It is widely grown due to its large edible fruits that contain a hard green rind and a watering reddish, yellowish or pink pulp. It is an essential agricultural product utilized in the production of many food products such as fruit cocktails, juices and nectars [28,29]. The most useful part of the watermelon in the food industry is its pulp. Watermelon, Citrullus lanatus has been reported by Johnson et al. [30] to contain valuable amount of anti-nutrients namely saponin, alkaloids, hydrogen cyanide, tannins, phytate, phenol, oxalate and flavonoids in fresh and dried watermelon peel (WMP), watermelon pulp (WMPu) and water- melon rind (WMR). In addition, WMR being an agricultural waste product has been reported to be a natural source of citrulline, a non-essential amino acid [31]. The chemical structure of citrulline (major component of WMRE) is shown in Fig. 1. These compounds contain heteroatoms (N, O) and aromatic ring which are regarded as centers of adsorption hence the extracts obtained from watermelon waste products could function as a potential corrosion inhibitor. There are few reports on the use of watermelon parts as corrosion inhibitor for metals in some corrosive environments. For instance watermelon peel extract (WMPE) and watermelon leaf have been used for corrosion protection of zinc in natural sea water [32]. However, there is no report to the best of our knowledge on the use of watermelon rind as corrosion inhibitor for any metal in any corrosive environment. Therefore our main objective in the present work is to utilize extract of WMR as a corrosion inhibitor for mild steel in acidic media (HCl and H 2 SO 4 ) using electrochemi- cal impedance spectroscopy (EIS), potentiodynamic polarization (PDP) and linear polarization (LPR) methods. 2. Experimental 2.1. Materials preparation 1 M HCl and 0.5 M H 2 SO 4 solutions as corrosive media were prepared with A-R grade 37% HCl (Sigma–Aldrich) and 98% H 2 SO 4 Journal of Industrial and Engineering Chemistry 21 (2015) 239–247 ARTICLE INFO Article history: Received 17 December 2013 Accepted 17 February 2014 Available online 26 February 2014 Keywords: Watermelon Acid corrosion Corrosion inhibition Mild steel Biomaterials ABSTRACT Electrochemical techniques were used to investigate the efficacy of watermelon rind extract (WMRE) as corrosion inhibitor for mild steel in HCl and H 2 SO 4 solutions. The inhibition efficiency increased as the concentration of the extract increases. Potentiodynamic polarization results indicate that WMRE act as a mixed-type inhibitor. The values of effective capacitance obtained by utilizing the impedance parameters in both acidic media are in the range of double layer capacitance. Corrosion inhibition effect of WMRE can be attributed to the adsorption of constituents of WMRE onto the mild steel surface which can be approximated by Temkin adsorption isotherm model. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +966 13 860 7895; fax: +966 13 860 3996. E-mail address: adewalen@kfupm.edu.sa (N.A. Odewunmi). Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec http://dx.doi.org/10.1016/j.jiec.2014.02.030 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.