Monitoring of Coating Disbondment by a Scanning Kelvin Probe Technique

The coating disbondment and corrosion of a X65 pipeline steel under coating were studied by scanning Kelvin probe (SKP) measurements. The effects of immersion time and wet-dry cycle on the Kelvin potential profile and the corrosion behavior of the steel were investigated. Kelvin potential measured on “intact” area is shifted negatively with time, indicating an increasing water uptake under the “intact” coating. There is a more negative Kelvin potential on disbonded area than that on “intact” area, which is attributed to corrosion reaction of steel occurring under the disbonded coating. During wet-dry cycle, the thickness of solution layer trapped under disbonded coating decreases due to evaporation of water, causing a negative shift of Kelvin potential. It is associated with the reduction of oxygen solubility in the concentrated solution during drying of electrolyte.