ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2015, Vol. 51, No. 2, pp. 285–294. © Pleiades Publishing, Ltd., 2015. 285 1 1. INTRODUCTION Acids are deployed in many service environments such as pickling, cleaning of boilers, de-scaling and acidization of oil well. In order to reduce the undesir- able base metal dissolution by these processes, corro- sion inhibitors are usually added. Inhibitors allow the reduction of corrosion rate by influencing the kinetics of corrosion processes which are consistent with anodic and cathodic conjugate reactions. Many organic compounds containing oxygen, nitrogen and sulphur have been studied as corrosion inhibitors for metal. Corrosion inhibitors are of great practical importance, being extensively employed in minimiz- ing metallic waste in engineering material [1]. Hetero atoms such N, O, S and some cases Se and P are capable of forming coordinate-covalent bond with metals owing to their free electron pairs [2]. Compounds with π-bonds also generally exhibit good inhibitive properties due to interaction of π-orbital with the metal surface [3]. Schiff base with –CH=N– linkage (azomethine) have both the above features combined with their structure which make them effec- tive potential corrosion inhibitors [4, 5]. Several Schiff base metal complexes have been investigated as corro- sion inhibitors [6, 7]. The existing data show that these 1 The article is published in the original. inhibitors act by adsorption on the metal/solution interface. This phenomenon could take place via: (i) electrostatic attraction between the charged metal and the charged inhibitor molecules, (ii) dipole-type interaction between unshared electron pairs in the inhibitor with the metal, (iii) π electrons-interaction with the metal, and (iv) combination of all of the above [8]. If the adsorption process involves overlap of occu- pied ligand non-bonding orbital with metal empty inner d or f orbital, a coordinate type bond formed and the process is termed chemisorption [9]. This situation can arise in cases where the inhibitor molecules con- tain lone pairs of electrons, multiple bonds, or conju- gated p-type bond system [10, 11]. Furthermore Hydrogen bonding plays a key role in chemical, cata- lytic and biochemical processes, as well as in supramo- lecular chemistry and crystal engineering [12]. Mas- soud and coworker are synthesized and structurally characterized by X-ray single crystal structure analysis of two silver (I) pyrazine complexes [13]. The com- plexes are tested as corrosion inhibitors for mild steel in 0.1 M nitric acid medium using potentiodynamic polarization technique. Strong hydrogen bonds fur- ther connect the chains of complexes to extend the dimensionality to a 3D network structure. However, studies about the effect of metal complexes as corro- Electrochemical and X-ray Structural Study of Corrosion Inhibition and Adsorption Behavior for Mild Steel by a New Schiff-base Cobalt Complex in HCl 1 Mojtaba Nasr-Esfahani a , Mahmoud Zendehdel a , and Behroz Jafari a, b a Department of Chemistry, Najafabad Branch, Islamic Azad University, Isfahan 85141-43131, Iran b Department of Materials Science and Engineering, Najafabad Branch, Islamic Azad University, Isfahan 85141-43131, Iran e-mail: m-nasresfahani@iaun.ac.ir Received January 01, 2014 Abstract—A new cobalt(III) Schiff base omplex (SBCC), [Co(L) 2 ]Cl (where L = N-(2-(2-hydroxyethy- lamino)ethyl) 5-methoxysalicylideneimine), was synthesized and structurally characterized by X-ray single crystal structure analysis. Intramolecular C–H⋅⋅⋅O hydrogen bonds stack molecules on 020 directions and intramolecular hydrogen bonding with the chloride ions is the result of stacking the molecules on c direction in unit cell and 011 crystallographic sheets. More available surface area collect from 020 sheets and these sur- faces are useful for making a blocking layer of complex with adsorption on the surface of steel. Corrosion inhi- bition and adsorption behavior of mild steel in 0.5 M HCl by SBCC were investigated using electrochemical techniques. It was found that inhibition efficiency increased with SBCC concentration. The inhibition effi- ciency is 84% for 400 ppm concentration of the complex. Corrosion inhibition was afforded by adsorption of SBCC onto the metal following El-Awady kinetic–thermodynamic adsorption isotherm model. The results obtained show that SBCC acts as a mixed-type inhibitor and it could serve as an effective corrosion inhibitor of mild steel in aqueous solution. DOI: 10.1134/S2070205115020136 PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION