Surface protection of steel in acid medium by Tabernaemontana divaricata extract: Physicochemical evidence for adsorption of inhibitor Kavitha Rose b,c , Byoung-Suhk Kim d,e , Kalyanaraman Rajagopal f , Sankar Arumugam b, ⁎, Kesavan Devarayan a, ⁎ a Department of Basic Sciences, College of Fisheries Engineering, Tamil Nadu Fisheries University, Nagapattinam 611001, India b Department of Chemistry, Kandasamy Kandar's College, Salem 638182, India c Department of Chemistry, Dhirajlal Gandhi College of Technology, Salem 636 005, India d Department of Organic Materials & Fiber Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea e Department of BIN Convergence Technology, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea f Department of Biotechnology, Vels University, Chennai 600117, India abstract article info Article history: Received 8 August 2015 Received in revised form 25 November 2015 Accepted 2 December 2015 Available online xxxx For the first time, the homogeneous adsorption of a green inhibitor on the surface of steel was evidenced using energy dispersive X-ray spectral elemental mapping. Inhibition of steel corrosion in 1.0 M hydrochloric acid by Tabernaemontana divaricata plant extract and the evidences for adsorption of the inhibitor are described in this study. The inhibition characteristics of the green inhibitor were evaluated by means of weight loss and electro- chemical measurements. A maximum of 95% inhibition efficiency was achieved by using 500 ppm of inhibitor. The adsorption of green inhibitor was found to obey the Langmuir's isotherm model. Further, the energy disper- sive spectral mapping revealed the homogeneous distribution of the nitrogen, which indicated for the presence of inhibitor on the steel surface. © 2015 Elsevier B.V. All rights reserved. Keywords: Corrosion Green inhibitor Adsorption Langmuir isotherm 1. Introduction Steel is a widely used iron-carbon alloy for construction of articles, structures, and vessels of everyday use. The preference for the use of steel is mainly because of its high tensile strength and economy. Steels tend to deteriorate in acid medium due to corrosion. It was reported that the loss due to metallic corrosion is greater than the loss due to any natural disasters [1]. Hence, the corrosion inhibition of steels in acid medium becomes an intensive field of research [2]. Since the first report for the use of an organic inhibitor against corrosion of steel in acid medium [3], numerous number inhibitors were reported as poten- tial candidates for inhibition of steel corrosion [4–14]. Among them, chromium-based inhibitors were found to be very useful. Drawbacks of using chromium inhibitors included severe health issues and non- eco-friendliness. Therefore, organic inhibitors were studied more in the past decade as a replacement of chromium inhibitors [4]. Especially, inhibitors derived from natural sources, such as plant extracts were more attractive for this purpose. The advantages for using plant based inhibitors include eco-friendliness and economy. In recent years, the authors so far have been exploring several organic inhibitors for inhibition of steel corrosion in different corrosive environments [6–10,12–14]. Tabernaemontana divaricata (TD) is a tropical plant, commonly available in countries such as India, Sri Lanka, and other South East Asian countries [15]. It is commonly known as Crape Jasmine, East India Rosebay, or Nandiyavattai in Tamil. TD belongs to Apocynaceae family and flowers appear sporadically throughout the year. In view of the abundance, eco-friendliness, and economy, in the present study, the ethanolic extract of TD was eval- uated as a potential steel corrosion inhibitor in hydrochloric acid medi- um. The inhibition efficiency of TD extract was studied by means of weight loss measurements and electrochemical studies. Energy disper- sive X-ray spectroscopy was employed to analyze the surface of the inhibitor adsorbed-steel. The results are discussed in detail in the fol- lowing sections. 2. Experimental 2.1. Preparation of inhibitor The TD extraction was performed by means of a conventional distillation method similar to the procedure described in our previous studies [10,12]. Fresh leaves of TD (1 kg, fresh weight) were collected from Salem, Tamil Nadu, India. The leaves were left for drying under shade in air atmosphere for about 5 days. The dried leaves were crushed into fine powder and soaked in 95% ethanol for 3 days. Then the extract was filtered using a Whatman 20 filter paper and the solvent was re- moved under vacuo to yield a green colored oily product (28 g). Journal of Molecular Liquids 214 (2016) 111–116 ⁎ Corresponding authors. E-mail address: kesavannitt@gmail.com (K. Devarayan). http://dx.doi.org/10.1016/j.molliq.2015.12.008 0167-7322/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq