The Influence of CaCO3 Scale Formation on AC Corrosion Rates of Pipeline Steel Under Cathodic Protection

In this paper, the influence of CaCO3 scale formation on the AC corrosion rate of pipeline steel under cathodic protection was investigated. The CaCO3 layer was deposited on X65 steel electrochemically and then studied in a soil simulant. A method for determining the fractional surface coverage of the scale based on either deposition current or electrochemical impedance spectroscopy data is presented. After the deposition process, samples were exposed to 3 V root mean square (RMS) AC potential at 60 Hz and varying degrees of cathodic protection at DC potentials of −770, −700, −650, and −600 mV vs. saturated calomel electrode. It was demonstrated that the scale acts as a simple barrier and the corrosion rate of steel, exposed at the base of pores in the scale, is equal to that of boldly exposed samples. These results are discussed in the light of the previously developed theoretical model for AC corrosion.