Garlic Powder as a Safe Environment Green Corrosion Inhibitor for Mild Steel in Acidic Media; Adsorption and Quantum Chemical Studies Aprael S. Yaro, a Anees A. Khadom b, * and Rafal K. Wael a a Chemical Engineering Department, College of Engineering, University of Baghdad, Aljadrea, Baghdad, Iraq b Chemical Engineering Department, College of Engineering, University of Diyala, Baquba City, Daiyla, Iraq (Received: Oct. 26, 2013; Accepted: Feb. 15, 2014; Published Online: Apr. 14, 2014; DOI: 10.1002/jccs.201300480) The corrosion inhibition of mild steel in H 3 PO 4 solution by garlic powder was investigated using weight loss and polarization techniques. The adsorption of garlic powder was found to obey Langmuir adsorption isotherm. Maximum inhibition efficiency was 75% at 50 o C and 250 ppm inhibitor concentration. The val- ues of heat of adsorption were negative indicating the spontaneous adsorption process. Quantum chemical calculations were used successfully to evaluate the theoretical inhibitor efficiency. Mathematical and sta- tistical analyses were also used to represent the corrosion rate data with high correlation coefficients. Po- larization measurements showed that garlic powder was a mixed – type inhibitor. Keywords: Adsorption; Corrosion; Mild steel; Polarization; Kinetics; Quantum chemistry. INTRODUCTION The corrosion of mild steel is a fundamental aca- demic and industrial concern that has received a consider- able amount of attention. 1,2 The isolation of mild steel from corrosive agents is the most effective way to prevent elec- trochemical corrosion. Among the different methods avail- able, 3,4 the use of corrosion inhibitors is usually the most appropriate way to achieve this objective. Nowadays, natu- rally occurring molecules exhibiting a strong affinity for metal surfaces are the focus of research oriented toward the development of environmentally friendly corrosion inhibi- tors; compounds showing good inhibition efficiency and low environmental risk. Among the so-called ‘‘green cor- rosion inhibitors” are organic compounds that act by ad- sorption on the metal surface, such as ascorbic acid, 5 suc- cinic acid, 6 tryptamine, 7 caffeine, 8 and extracts of natural substances. 9–13 The efficiency of these organic corrosion inhibitors is related to the presence of polar functions with S, O or N atoms in the molecule, heterocyclic compounds and p electrons. The polar function is usually regarded as the reaction centre for the establishment of the adsorption process. 14 One of natural material is the garlic powder which contains many of Phytochemicals and Nutients com- ponents. Allicin and Alliin are the most important compo- nent which consist 72% of garlic powder sulfur compo- nents. 15 The chemical structure of these component were shown below: The presence of these organic components made us to select this natural material as a corrosion inhibitor. In this work the inhibitory effect of garlic powder on mild steel corrosion in 1 M H 3 PO 4 at different temperatures was stud- ied. EXPERIMENTAL Mild steel specimens were used as the working electrodes throughout the study. The composition (wt.%) of the mild steel was: Fe, 98.9; C, 0.199; Si, 0.124; Mn, 0.053; Al, 0.0514; Cr, 0.009; Cu, 0.0468; Ti, 0.0089; V, 0.0076; Ni, 0.00352; Co, 0.0091; Mo, 0.005; Pb, 0.05 and with an exposed area of 10.8 cm 2 . The specimens were cleaned according to ASTM standard G1- 03. 16 High purity garlic power produced locally by Al-Mlwya factory – Iraq, for production and packing food powders was used as corrosion inhibitor. For weight loss tests the specimens were fully immersed for two hours in 250 cm 3 corrosive solution at 1 M H 3 PO 4 , 30, 40, 50, 60 o C, and 50, 100, 150, 200, 250 ppm garlic concentration. For polarization tests, corrosion cell has four necks was used, one was fitted with working electrode, one for immersing a thermometer in order to observe the test temperature and the other one had a spherical joint for manipulating the lug- ging capillary probe. The probe was adjusted to be in a distance not more than 2 mm from the working electrode. The fourth necks input platinum electrode. All potential values were measured in reference to saturated calomel electrode (SCE). DC power supply which is equipped with current and voltage limiters offers better stability and control, its type is 6236B, triple output supply Hewlet Packard, USA. All polarization carried out at 30 o C and J. Chin. Chem. Soc. 2014, 61, 615-623 © 2014 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 615 JOURNAL OF THE CHINESE CHEMICAL SOCIETY Article * Corresponding author. E-mail: aneesdr@gmail.com