M. SANGEETHA et al … GREEN CORROSION INHIBITORS-AN OVERVIEW ZAŠTITA MATERIJALA 52 (2011) broj 1 3 M. SANGEETHA 1 , S. RAJENDRAN 1 , paper T. S.MUTHUMEGALA 1 , A.KRISHNAVENI 2 UDC: Green corrosion inhibitors-An Overview To replace the environmentally hazardous chromates, several non –chromates have been used as corrosion inhibitors. Extracts of plant materials top the list. The plant extracts are environmentally friendly, non- toxic and readily available. These extracts contain many ingredients. They contain several organic compounds which have polar atoms such as O, N, P and S. They are adsorbed onto the metal surface through these polar atoms; protective films are formed. Adsorptions of these ingredients obey various adsorption isotherms. The films have been analyzed by many surface analysis techniques such as AFM, FTIR, UV, Fluorescence spectra and SEM. Key words: green inhibitors, environmentally friendly corrosion inhibitors, natural product, plant materials. Use of some inhibitors, such as chromates, has been banned because of toxicity and the environ- mental hazards they create. Hence there is a strive to make use of environmental friendly, non toxic /less toxic, extracts of naturally occurring plant materials as corrosion inhibitors[1-111]. Extracts of plant materials contain a wide variety of organic compounds. Most of them contain hetero- atoms such as P, N, S, O. These atoms coordinate with the corroding metal atom (their ions), through their electrons. Hence protective films are formed on the metal surface and hence corrosion is prevented. Metals: Extracts of plant materials have been used to control the corrosion of various metals such as mild steel [2, 5-10, 13-18, 20, 23, 24, 26, 28-45, 47, 48, 50, 52-55, 59, 60, 62-65, 67-72, 74-76, 81, 83- 85, 88-98, 100-105, 107, 108, 111], Zinc [1, 76], various types of steel [3, 4, 19, 21, 51, 78, 80], Aluminium and its alloys [11, 12, 22, 25, 27, 46, 49, 56-58, 61, 73, 77, 79,82, 86, 87, 106, 109, 110 ], Copper [49, 66], nickel [76], and tin [99]. Medium: The inhibition efficiency of plant ext- racts, in controlling corrosion of metals in various environments has been investigated. Acidic medium, [1, 2, 5-9, 16-34, 36-44, 47-56, 60, 62-65, 67-69, 71,77-85, 88-91, 94-105,108-111], alkaline medium [3, 11, 12, 45, 46, 61, 73, 86, 106], and neutral medium [10, 57, 59, 66, 74, 75, 87], have been used for this purpose. Additives: Plants extracts have been used as corrosion inhibitors, alone or in combination with other inhibitors such as Zn 2+ [46, 72, 73], HEDP [13], KI [56, 27], KSCN [27], KCl, KBr[36]. Address authors: 1 Corrosion Research centre, P.G. and Research Department of Chemistry, GTN Arts col- lege, Dindigul – 624005, India, 2 Corrosion Research centre, Department of Chemistry, Yadava college, Madurai-India. Temperature: The inhibition efficiency of plant extracts have been evaluated at room temperature [1, 3-11, 13-16, 23-26, 28, 29, 31, 33, 36-39, 42-46, 49- 52, 54-57, 59, 60, 66-68, 70-77, 80-82, 84, 86, 90, 92, 95-101, 103-105, 111], and also at high temperatures [2, 12, 17, 19, 20, 22, 27, 30, 32, 35, 41, 47, 48, 53, 58, 61-65, 69, 78, 79, 81, 94, 102, 107-110]. Methods: Various methods have been used to evaluate the inhibition efficiency of plant extracts, usually , weight loss method [3, 5-7, 13, 15-18, 27- 35, 71-75, 81-86, 88-92, 94-98, 102-106, 109-111], electrochemical studies (polarization and AC impe- dance) [6, 7, 13, 21, 26, 29, 38, 56-60, 87, 100-107], and gasometric studies have been employed.[2, 8, 17, 22, 28, 47, 52, 55]. Adsorption isotherms: The adsorption behavior of the ingredients present in the plant extract onto the metal surface has been investigated, the type of adsorption isotherm has been proposed. Langmuir adsorption isotherm [1, 12, 18, 19, 21, 22, 25, 27, 29, 33, 34, 44, 49, 52, 61-64, 67, 82, 92, 100, 103-105], Freundlich adsorption isotherm [28, 29, 44, 79, 87, 92, 95, 109], Temkin adsorption isotherm [23, 29, 32, 44, 55, 58, 60, 79, 86, 110], Frumkin adsorption isotherm [41, 44, 69, 78,108], and Flory-Huggins adsorption isotherm [29, 35, 44, 68, 69], have been proposed, various thermodynamic parameters such as changes in free energy, enthalpy and entropy have been calculated inorder to support the proposed adsorption isotherms. The ingredients are adsorbed onto the metal surface, through the polar atoms such as O, N, P, S present in the active ingredients of the extract. Surface analysis: The protective films formed on metal surface, during the process of corrosion protection of metals by plant extracts, have been analyzed by various surface analysis techniques such as AFM [6, 34], ESCA [6, 34], Raman spectroscopy [7], SEM [17, 18, 26, 30, 32, 33, 48, 50, 60, 64, 101],