Corrosion inhibition of a carbon steel by the association of phosphonate-derived products with zinc salts
In this study, electrochemical measurements were carried out to characterize the inhibitive efficiency of the association of phosphonate-derived products with zinc salts, employed for water treatment in cooling circuits. The influence of parameters such as the concentration of the formulation and the effect of the solutions pH on the protection of the metal were studied. Steady-state current-voltage curves and electrochemical impedance diagrams have been obtained for a carbon steel rotating disc in a 200 mg · l-1 NaCl solution. The choice of this medium is based upon the following criteria : (i) its low electrical conductivity is close to that encountered in natural saline waters ; (ii) its corrosivity is fairly high ; (iii) it Is formed by an easily reproducible baseline solution. The inhibitive efficiency has been determined tram the plotting of the cathodic potentiostatic curves. It is important to note that in low conductivity media, the current-voltage curves are distorted by a non-negligible uncompensated ohmic drop (I.Re), Re being the electrolyte resistance between the working and the reference electrodes. The interpretation of these curves (i.e. corrosion rate evaluation, Tafel slopes determination) is only possible if they have been corrected according to the following relationship. Eapparent = Etrue + ReI In this study, the electrolytic resistance value has been determined from electrochemical Impedance measurements : the high frequency limit of the impedance gives Re.The ohmic drop is then automatically compensated by the potentiostat to obtain the corrected steady-state curve. The inhibitive efficiency is evaluated by the relationship : Ei = (icorr0-icorr / icorr0) x 100 in which iocorr and icorr represent the current densities without and with inhibitor, respectively. In addition to the determination of the current densities obtained from the steady-state current-voltage curves, the polarization resistance, Rp, has been measured from the impedance diagram : the low frequency limit of the impedance gives the sum Re + Rp ; Rp is equal to the slope of the steady-state current voltage curve. In this study, only the evolution of this value as a function of concentration has been examined. It has been shown from the steady-state technique that the compound presents a very good efficiency for low concentrations (50 mg · l-1). Between 50 and 200 mg · l-1, the inhibitive efficiency increases from 95 to 98 %. For 100 mg · l-1 it is effective over a wide pH range (5.5 to 9) with a better protection for pH = 7 and pH = 8. The measurements of the polarization resistance corroborate these results. In a second part, the inhibitive efficiency of the compound has been compared to different inhibitors used for similar uses and tested in the laboratory. Thus, zinc chloride, zinc fluorophosphate and the association of fatty amine and phosphonic acid present a lower efficiency than the product tested here.