ABSTRACT Adsorption isotherm of N,N′-bis(2,4,6-trihydroxyacetophenone)-2,2-dimethylpropandiimine (THAPDP) as an environmentally friendly Salen Ligand corrosion inhibitor on mild steel was studied in alkaline solution (pH=10) containing 3% NaCl. Measurements were carried out using electrochemical and surface techniques. Potentiodynamic polarization curves suggested that this compound was an effective corrosion inhibitor for mild steel and the protection efficiency was increased with the increase in inhibitor concentration. The experimental results indicated that this organic compound was a mixed-type inhibitor. Adsorption on the mild steel surface followed the Langmuir isotherm model. Activation parameters and thermodynamic adsorption parameters of the corrosion process such as E a , Δ H , Δ S , K ads , and Δ G ads were calculated at different temperatures.Keywords: Corrosion, Inhibitor, Adsorption isotherm, Langmuir, Salen ligand. 1. INTRODUCTION Carbon steel has been widely used under different conditions in various industries [1]. One of the known applications of mild steel is to being used in the steam boiler of the power plants [2]. The corrosion rate of mild steel is remarkably dependent on the chemical composition of the aqueous media in which the metal surface is exposed. Feedwater pH is one of the chemistry parameters that affects corrosion, and for an all-steel feedwater system, typically the more alkaline solution, pH > 9.2 up to about 10, is convenient. In practice corrosion can never be stopped but hindered to a reasonable level. Corrosion prevention systems favor the use of corrosion inhibitors with low or zero environmental impacts. A corrosion inhibitor is a chemical substance that, upon addition to a corrosive environment, results in the decrease of corrosion rate to an acceptable level. Most of the well-known inhibitors are organic compounds which mainly contain heteroatoms such as oxygen, nitrogen or sulphur, and multiple bonds, which allow an adsorption on the metal surface [3-8]. The resultant film of chemisorbed inhibitor is responsible for protection either by physically blocking the surface from the corrosion environment or by retarding the electrochemical processes. These compounds are often simultaneously affected on both anodic and cathodic corrosion reactions. Therefore, they are sometimes referred to as mixed-type corrosion inhibitors [8-11]. The planarity and the lone electron pairs in the heteroatoms, are important features that determine the adsorption of these molecules on the metallic surface [12].The aim of this work is to investigate the use of THAPDP as an environmentally friendly inhibitor for the corrosion of mild steel in alkaline solution (pH=10) containing 3% NaCl evaluated by Tafel polarization data. The effect of temperature and concentration on corrosion inhibition were studied. Thermodynamic parameters were calculated from experimental data of the inhibition process at different temperatures.