Effect of alternating current interference on coating disbondment and cathodic protection shielding on pipelines

Abstract

In this work, the effects of alternating current (AC) interference on coating disbondment on X65 pipeline steel and the cathodic protection (CP) permeation into the disbonding crevice were investigated. Cathodic disbondment tests showed that the AC is able to promote disbonding of coating from the steel, which is attributed to the solution alkalisation by the enhanced cathodic reduction reaction and generation of hydroxyl ions. In the absence and presence of AC, CP can be shielded by the geometry of coating disbondment. At small AC current densities, such as 100 A m⁻², the AC results in an enhancement of permeation of CP current into the crevice. However, with the increase in AC current density, corrosion product generates and deposits in the solution, blocking the ionic diffusion and CP permeation. When the AC current density is up to 500 A m⁻², the CP is blocked completely to reach the disbonding crevice.

Keywords:

• Coating disbondment
• Pipelines
• Alternating current
• Cathodic protection shielding