Assessing the influence of coating thickness on its mechanically induced loss of integrity

© 2017 by NACE International. Coatings form an integral part of the corrosion protection system of oil and gas pipelines that are often operating under high mechanical stress and corrosive environmental conditions. It is necessary to assess coatings for potential loss of integrity, such as mechanically induced holidays and cracks, before they are put into operation. The standard method of assessing coating's behavior and flexibility under mechanical strain is the mandrel bending test, wherein high voltage holiday test is normally used to determine the existences of discontinuities in coating films. A limitation of the holiday detector is that it can only detect coating defects ex situ, i.e. after a mandrel bending test; it cannot monitor the loss of coating integrity under sustained mechanical stress and environmental effects. A new experimental method has been proposed to perform in situ measurement of the loss of coating integrity under the combined effect of mechanical strain and environmental exposure. The experimental setup consists of a small scale tensile rig capable of applying a mechanical strain, coupled with an electrochemical cell that is used to expose the coating to a corrosive environment. Electrochemical impedance spectroscopy (EIS) has been used as a method to gauge the changes in the electrochemical properties of the coating as a result of the applied strain and extended environmental exposure. Tensile samples coated with pipeline coatings of thicknesses ranging from 250 to 2000 μm were subjected to tensile elongation, with the electrochemical impedance being measured before and after application of the mechanical strain. The mechanical strain that led to a holiday in the coating large enough to completely expose the underlying metal substrate to the electrolyte, as gauged by impedance changes, has been considered to be the critical strain limit for a coating of a certain thickness. At this strain, the Bode impedance at the lowest frequency dropped to a low value of around 104- 106ohm.cm2. Lower critical strain limits were found for higher thickness coatings. The presence of the holidays and morphological changes in the coating surface were confirmed using surface characterization techniques such as optical microscopy.