Investigation on Effect of Pulse Cathodic Protection on CP Penetration Depth Under the Tape Coating Disbondment on Pipeline Steel

The effect of pulse cathodic protection (CP) on CP penetration depth under tape coating disbondment of pipeline steel has been investigated in this study and results have been compared with conventional CP. In this regard, a laboratory cell with the length of 2000 mm that can simulate the underlying conditions by applying CP was designed and made. For investigation of crevice width on CP penetration depth coupons with dimensions of 2000×10 mm and 2000×50 mm made from API X-52 pipeline steel were placed under coating disbondment with the gap size of 3 mm and different widths. By placing the cell in a simulated soil solution C2, the potential of - 1000mVSCE through two methods of conventional & pulse CP with the frequency of 10000 Hz was applied to the open mouth (OM) of the simulated coating disbondment. Result showed by applying conventional CP for both the crevice widths, potential failed sharply to Open Circuit Potential (OCP) near to the OM while for pulse CP, in contrast to conventional CP, which CP penetration depth was increased. At the crevice with the width of 50 mm CP level was measured more than the width of 10 mm, such that the entire disbondment length was protected by CP. The lower CP level for the crevice with the width of 10 mm could be due to concentration polarization resistance which restricted the penetration of the positive current within the crevice. Also, pH value under the coating disbondment with applying pulse CP at the crevice with the width of 50 mm was measured more than the pH value for the disbondment with 30 mm width other due to higher CP level under this disbondment. In summary, results showed despite the conventional CP, applying pulse CP with high frequency could be an effective method in increase of the CP penetration depth under tape coating disbondments.