* M. El-Batouti, M. A. Khalifa Chemistry Department, Faculty of Science, Alexandria Univer- sity, P.O. 426, Alexandria 21321 (Egypt) F. Mahgoub, A. Bakr Aknish University of Alexandria Research Center, Alexandria (Egypt) Corrosion inhibition of steel in crude oil storage tanks Korrosionsinhibierung von Stahl in Roho ¨ llagertanks M. A. Khalifa, M. El-Batouti*, F. Mahgoub, A. Bakr Aknish The corrosion behavior of steel in water from certain oil fields with various organic inhibitors was studied by applying potentio- dynamic technique at pH 5.9. The inhibitors which were used in- clude 2-methylbenzimidazole (I), 2-mercaptobenzimidazole (II), 2- mercapto-5-methylbenzimidazole (III), and 2-mercaptothiazole (IV). The inhibiting efficiency of the different additives was eval- uated from their anodic and cathodic polarization curves at different temperatures. A comparative study of curve fitting procedures using the kinetic thermodynamic model and those utilizing well-known adsorption isotherms was undertaken. Three types of isotherms were used, Langmuir, Frumkin and Flory-Huggins isotherm. The results show that the kinetic-thermodynamic model and Flory-Huggins isotherms are more suitable to fit the data for the four inhibitors, at all applied temperatures. From the values of binding constants, K, the order of inhibitor efficiency is: III > IV > II > I and thus 2-mercapto-5-methylbenzimidazole was the most effec- tive inhibitor. The thermodynamic parameters obtained for all the compounds used showed low values of H ads , which indicates that the action of these inhibitors is physically adsorptive in nature. Mit Hilfe potentiodynamischer Messungen bei pH 5,9 wurde das Korrosionsverhalten von Stahl in Wasser aus bestimmen O ¨ lfeldern bei Zugabe verschiedener organischer Inhibitoren untersucht. Die eingesetzten Inhibitoren beinhalten 2-Methylbenzimidazole (I), 2- Mercaptobenzimidazole (II), 2-Mercapto-5-Methylbenzimidazole (III), and 2-Mercaptothiazole (IV). Die Inhibitorwirksamkeit der verschiedenen Zusa ¨tze wurde aus deren anodischen und kathodi- schen Polarisationskurven bei verschiedenen Temperaturen ermit- telt. Eine vergleichende Untersuchung der Kurvenanpassungen mit Hilfe des kinetisch-thermodynamischen Modells sowie bekannter Adsorptionsisotherme wurde durchgefu ¨hrt. Dabei wurden drei Ar- ten von Isothermen verwendet, die Langmuir-, die Frumkin- und die Flory-Huggins-Isotherme. Die Ergebnisse zeigen, dass das ki- netisch-thermodynamische Modell und die Flory-Huggings-Iso- therme bei allen untersuchten Temperaturen besser geeignet sind, die Daten fu ¨r die vier Inhibitoren anzupassen. Aus den Werten der Bindungskonstanten K ergibt sich die Rei- henfolge der Inhibitorwirksamkeit: III > IV > II > I Das bedeutet, dass 2-Mercapto-5-Methylbenzimidazol der wirk- samste Inhibitor war. Die fu ¨ r alle eingesetzten Inhibitoren ermittel- ten thermodynamischen Parameter zeigten niedrige Werte fu ¨rH ads , was darauf schließen la ¨sst, dass die Wirkung dieser Inhibitoren phy- sikalisch-adsorptiver Natur ist. 1 Introduction Corrosion is a serious problems in oil and gas productions. Both general and localized corrosion types are encountered in this industry. The extent of the problems depends on the fluid composition, e.g. oil types, oil/water ratio, water salinity and gas type and content [1]. Water is one of the main factors af- fecting the corrosion rates. It is found in some degree with all the petroleum produced [2 – 4]. The use of corrosion inhibitors in petroleum production is the most common method for cor- rosion control in these highly corrosive environments. The compounds most often used as corrosion inhibitors are nitrogen containing organic compounds (aliphatic and aro- matic amines) [5 – 7], oxygen, and sulfur-containing com- pounds [8 – 9]. Several of these compounds have been studied as corrosion inhibitors for steel in different environments [10, 11]. Heterocyclic compounds, such as 2-mercaptobenzothia- zole [12, 13] and 2-mercaptobenzimidazoles [14, 15] have been used to inhibit corrosion in neutral aqueous solutions. The choice of inhibitors was based upon three considerations. The first is that these derivatives of the investigated imidazole are organic compounds whose composition and molecular structure provide good conditions for inhibition. Secondly, they are intermediary or secondary products of reactions in the pharmaceutic industry and, therefore, were relatively cheap. Thirdly, they are environmentally friendly [16]. The imidazole molecule shows three different anchoring sites sui- table for surface bonding, the nitrogen atom with a lone Sp 2 electron pair, the C(4)H-C(5)H „edge“, and the aromatic ring [17]. The present study aims to throw more light on the effect of temperature on the corrosion rate of mild steel in produced water in the presence and absence of the inhibitors. Polariza- tion technique is used in order to calculate the thermodynamic and activation parameters of the dissolution process of mild steel. The application of kinetic-thermodynamic model to ob- tain thermodynamic (adsorption equilibrium values and hence DG ads , DH ads , and DS ads ) parameters is used to describe the corrosion characteristics of steel surface and the effectivensss of these compounds as inhibitors from a quantitative point of view. Materials and Corrosion 54, 251–258 (2003) Crude oil storage tanks 251 F 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0947-5117/03/0404-0251$17.50þ.50/0