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Assessing the Influence of Coating Thickness on its Mechanically Induced Loss of Integrity

A new method for in situ measurement of loss of coating integrity.  Tensile samples with pipeline coatings of thicknesses from 250 to 2000 μm were subjected to tensile elongation, electrochemical impedance measured before and after.  Confirmed with techniques such as optical microscopy.

Product Number: 51317--9294-SG
ISBN: 9294 2017 CP
Author: Shyama Ranade
Publication Date: 2017
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$20.00

Coatings form an integral part of the corrosion protection system of oil and gas pipelines. It is necessary to assess and analyse the coatings for loss of integrity such as mechanically induced holidays and cracks before they are put into operation. The standard method of assessing coating’s behaviour 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 mandrel bending. A new experimental method has been proposed to perform in-situ measurement of the loss of coating integrity under the effect of combined 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 has been used as a method to gauge the changes in the barrier properties of the coating as a result of the applied strain. Tensile samples coated with pipeline coatings of thicknesses ranging from 250 to 3000 ?m 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 bare the underlying metal substrate to the electrolyte was considered to be the critical strain limit. At this strain the Bode impedance at the lowest frequency dropped to a low value of around 10^5 – 10^6 ohm.cm^2. Such a drop for higher thickness of coatings was seen to occur at lower strain values. Morphological changes in the coating surface were observed using characterisation techniques. The presence of the holidays in the coating surface was confirmed using the optical microscopy.

Key words: EIS, mechanical strain, environment, cracking

Coatings form an integral part of the corrosion protection system of oil and gas pipelines. It is necessary to assess and analyse the coatings for loss of integrity such as mechanically induced holidays and cracks before they are put into operation. The standard method of assessing coating’s behaviour 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 mandrel bending. A new experimental method has been proposed to perform in-situ measurement of the loss of coating integrity under the effect of combined 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 has been used as a method to gauge the changes in the barrier properties of the coating as a result of the applied strain. Tensile samples coated with pipeline coatings of thicknesses ranging from 250 to 3000 ?m 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 bare the underlying metal substrate to the electrolyte was considered to be the critical strain limit. At this strain the Bode impedance at the lowest frequency dropped to a low value of around 10^5 – 10^6 ohm.cm^2. Such a drop for higher thickness of coatings was seen to occur at lower strain values. Morphological changes in the coating surface were observed using characterisation techniques. The presence of the holidays in the coating surface was confirmed using the optical microscopy.

Key words: EIS, mechanical strain, environment, cracking

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