Electrochimica Acta 54 (2008) 162–167 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Effect of hydrodynamic conditions on the inhibition performance of l-methionine as a “green” inhibitor H. Ashassi-Sorkhabi ∗ , E. Asghari 1 Electrochemistry Research Laboratory, Physical Chemistry Department, Chemistry Faculty, University of Tabriz, Tabriz, Iran article info Article history: Received 10 March 2008 Received in revised form 13 August 2008 Accepted 14 August 2008 Available online 22 August 2008 Keywords: Corrosion Inhibition RDE l-Methionine Hydrodynamic conditions abstract Influence of hydrodynamic conditions on the behavior of l-methionine (l-MTI) as a green organic inhibitor was studied using mild steel rotating disc electrode. Electrochemical impedance spectroscopy (EIS) and polarization measurements were carried out in the absence and presence of inhibitor in 1 M H 2 SO 4 solu- tion under static conditions and at different rotation speeds; the inhibitor concentration was 5 × 10 -3 M in all tests. The open circuit potentials (OCP) were measured versus time in all studied rotation speeds; it was found that the OCP shifted toward more positive potentials as the rotation speed increased. It was also found that the inhibition efficiencies for still electrode and low speed rotations did not have noticeable changes, but in much higher rotation speeds, increased efficiencies were observed. This behavior could be attributed to the enhanced mass transport of inhibitor molecules from bulk to the metal surface in high rotation rates. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction Excessive corrosion attack is known to occur on mild steel in acidic aqueous media. Acid solutions are widely used in different industrial processes, for example, in acid pickling of iron and steel, scale removal in metallurgy, acid cleaning of boilers and oil-well acidizing, therefore the study of corrosion of steel in acid solutions and its inhibition have practical importance. Different corrosion inhibitors are used to control and reduce corrosion of metals in cor- rosive media. Due to toxicity of most inorganic corrosion inhibitors such as chromates and nitrites and also because of restrictive envi- ronmental regulations, these inhibitors are being replaced by new environment-friendly organic compounds. Steel corrosion in dif- ferent solutions has also been effectively controlled using organic inhibitors containing nitrogen, oxygen or sulphur [1–10]. Amino acids are non-toxic, relatively cheap and easy to produce with puri- ties greater than 99% which have been used in corrosion inhibition of different metals such as nickel [11], cobalt [12], iron and steel [13–15], aluminium [16,17] and Pb–Sb–Se–As alloy [18] in H 2 SO 4 and HCl solutions. In our previous work [16] methionine (an amino acid containing sulphur atom) was used for inhibition of aluminium corrosion in acidic solution. Oguzie et al. [14] studied corrosion ∗ Corresponding author. Tel.: +98 411 33931316; fax: +98 411 3340191. E-mail addresses: habib ashassi@yahoo.com, ashassi@tabrizu.ac.ir (H. Ashassi-Sorkhabi). 1 ISE member. inhibition and adsorption behavior of methionine on mild steel in H 2 SO 4 solution and synergistic effect of iodide ion in static condi- tions. They showed that methionine acts as a corrosion inhibitor for mild steel in acid solution and its performance improves in the pres- ence of iodide ion. Most of the investigations on organic inhibitors have been made in static conditions. There are few studies in the literatures on the effect of flow and hydrodynamic conditions on performance of inhibitors under laminar or turbulent flow. Some authors investigated the effect of flow on corrosion of metals and alloys such as nickel–aluminium–bronze (NAB) [19,20], stainless steel [21,22], Cu–Ni alloys [23–27] and carbon steel [28] in different corrosive solutions. Huet et al. [29] have investigated the electro- chemistry of pit generation-repassivation sequences of Fe–Cr and 304 stainless steel in a sulphuric acid/NaCl mixture by time resolved rotating ring disc electrode (RRDE) experiments. The modeling of passivation mechanism of Fe–Cr binary alloys from ac impedance and frequency resolved RRDE have also been reported in the litera- tures [30,31]. There are other studies about corrosion inhibition of some metals under flow conditions. The influence of flow on for- mation and stability of an inhibitive layer using EIS measurements and AFM analysis have been studied by Bommersbach et al. [32]. They showed that the formation of protective film is faster under high rotation rates. They also observed that for immersion times more than 140h hydrodynamic stresses on the sample surface are responsible for the protective layer modification. Matos et al. have reported the use of rotating disc and hemispherical electrodes for copper dissolution in HCl in the presence of benzotriazole [33]. They showed that the effect of benzotriazole depends on the elec- 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.024