Abstract
The main aspects of steam generator acid cleaning and the developments of chelating process technology are examined and a brief description is given of a pilot plant which was designed for the improvement and optimization of the composition of EDTA solutions to be used for cleaning the low-carbon steels and for the characterization and qualification of commercial systems. The inner surface of the plant, which can be run directly by a computer and can operate at temperatures up to 150 °C under a pressure of 5 atm, is chemically inert to a wide spectrum of electrolytic solutions. The tests, which were mostly carried out at 100 °C, using a base solution containing 100 g/l of EDTA at room temperature and varying its pH value from 6 to 9, were intended to investigate the corrosion behaviour of SA 106 grade B carbon steel and SA 213 low alloy steels, such as T11, T22, T5 and T9, at flow rates of 0.5 ms-1, 1.0 ms-1 and 1.5 ms-1.
Test results have shown that the pH of the solution and the dissolved oxygen content may exert a beneficial effect on the corrosion rate of these steels. Under suitable conditions the free corrosion of these steels may exhibit a transition from the active to passive state. The change being determined also by the solution flow rate.
Electrochemical measurements were performed with direct and alternating current by means of a Solartron computerized system, consisting of an electrochemical interface and a frequency response analyser, and experimental data were processed in real time using different mathematical approaches for evaluating the polarization resistance and the corrosion current density. The results were rather critical, owing to the a priori choice to eliminate or reduce the ohmic drop contribution to the potential difference between the working and reference electrodes. In fact, all EDTA solutions had, with respect to the geometry of our electrochemical cells, a value of the electric resistance of about 1Ω, which is fairly high as compared with that observed in other acid solutions.
The difficulties, that were encountered in the real time numerical analysis of potentiostatic or galvanostatic polarization curves for obtaining the values of the electrochemical parameters, can be overcome if a parametric study is carried out during information retrieval or the contribution of the electrolyte ohmic drop to the electrode overvoltage is not eliminated completely.
