The inhibitory effect of diethanolamine (DEA) on corrosion of mild steel in 0.5 M H 2 SO 4 was investigated by various corrosion monitoring techniques. Galvanostatic polarization study revealed that this compound is a very good inhibitor. The inhibition efficiency (I%) varies in the range of 88.7% to 55.3 % for a concentration range of 10 -3 M to 10 -7 M at 303 K, respectively. A study of corrosion potential (E corr ) reveals that DEA is a mixed type inhibitor. DEA inhibited mild steel corrosion due to physical adsorption of the inhibitor on the metal surface. The study at higher temperatures indicates that the inhibition efficiency decreases with the increase in temperature. The adsorption of DEA on the mild steel surface in 0.5 M H 2 SO 4 follows the Frumkin's adsorption isotherm. The results of potentiostatic polarization study revealed that DEA is a strong passivating additive. The results of infra red (IR) spectroscopy, scanning electron microscopy (SEM) and quantum chemical study supplement the results of the electrochemical techniques.

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The inhibitive effect of benzyl triphenyl phosphonium chloride (BTPPC) on the corrosion of mild steel in aerated 1N sulphuric acid at different temperatures was investigated using galvanostatic polarisation, potentiostatic polarisation, infrared spectroscopy, scanning electron microscopy and quantum chemical studies. Corrosion potential, corrosion currents, cathodic and anodic Tafel slopes, heat of adsorption and effective activation energies were calculated in the presence and absence of the BTPPC. Inhibition efficiencies of BTPPC at different concentrations and temperatures were studied. The nature of the adsorption of BTPPC on metal surface was also examined. It is concluded that the passivation parameters depend on the presence of the already adsorbed anions. BTPPC is one of the rare inhibitors that only passivate the metal at lower concentrations while passivation at higher concentrations is not observed. The electrochemical data fit Temkin's isotherm. The results of surface morphology and quantum chemical analysis are in agreement with the electrochemical results.