ABSTRACT
The problems, objectives and need of the acid cleaning of a steam generator are briefly discussed and the main concepts explaining the phenomenon of corrosion inhibition by means of nitrogen-containing organic compounds are reviewed. The main characteristics of the corrosion inhibitors to be used for performing this operation are examined and some methods for evaluating their effectiveness are described.
The techniques based on the capacitance of the metal-solution interphase, the polarization resistance in the low-frequency region and the area method are introduced in an intuitive way using an approximate electric representation of the electrochemical process and Kramers-Kroning’s relations.
These techniques have been employed to study corrosion inhibition of iron and of two low alloy steels in 5% by weight HCl solutions containing different concentrations of a commercial product at temperatures of 65 °C, 75 °C, 80 °C and 90 °C. Electrochemical measurements were performed over the frequency interval [0.08, 20000] Hz using the galvanostatic configuration. The computerized system was controlled by suitable software ensuring that the potential difference between the working and reference electrodes would always be inside the interval [4, 6] mV RMS.
Experimental results have shown that this approach can prove to be a valid aid for a corrosionist. It can also characterize the behaviour of iron very faithfully, because of the close connection existing between double-layer capacitance, polarization resistance and mean corrosion rate. In the case of the two low alloy steels, however, the situation was more complex, also because the mean corrosion rate was higher and in some instances localized attacks were observed.
