Studying the cathodic disbondment of coatings using electrochemical impedance spectroscopy

The disbondment of protective organic coatings is a widely reported pipeline coating failure mode in the oil and gas industry. Traditional methods of evaluating cathodic disbondment of pipeline coatings are based on visual inspection of pipeline conditions, and laboratory testing of cathodic disbondment resistance (CDR) using standard methods such as ASTM G8. Although some other laboratory-based techniques, such as scanning kelvin probe and scanning acoustic microscopy have been used to study the cathodic disbondment (CD) of coatings, these are often difficult to apply in practical testing. Over the past decade, electrochemical impedance spectroscopy (EIS) has been employed as a potential method for measuring CD. This paper reports preliminary results from an EIS study designed to characterise CD behaviour of epoxy coatings under excessive cathodic protection. EIS data correlated well with the area of disbonded coating. Analysis of EIS data can provide valuable information on the initiation and rates of CD.

A review of techniques for the monitoring of cathodic shielding and corrosion under disbonded coatings

The mitigation of external corrosion of energy pipelines by a combination of barrier coatings and Cathodic Protection (CP) is not always effective. Even when design specifications are properly met, the shielding of cathodic protection current from reaching steel surface by disbonded barrier coatings, often referred to as cathodic shielding, may lead to severe corrosion problems such as deep pitting, high and near neutral pH Stress Corrosion Cracking (SCC) and Microbiologically Induced Corrosion (MIC). Unfortunately, current indirect assessment methods used in the pipeline industry have serious difficulties in detecting such corrosion problems. This paper provides a brief review of current techniques and their limitations when being applied under complex buried pipeline environmental conditions. The main purpose is to identify potential methods that could be utilised in the design of new monitoring probes specific for the monitoring of cathodic shielding and corrosion of disbonded coatings in the pipeline industry.

Sliding wear behavior of plasma sprayed Fe3Al–Al2O3 graded coatings

Fe3Al–Al2O3 double-layer coatings (DC), Fe3Al-Fe3Al/50%Al2O3–Al2O3 triple-layer coatings (TC) and Fe3Al-Al2O3 graded coatings (GC) were produced from a series of Fe3Al/Al2O3 composite powders with different compositions on low carbon steel substrate using PLAXAIR plasma spraying equipment. Friction behaviors and wear resistance of the three kinds of coatings have been investigated under different loads. Tests were carried out using an MRH-3 standard machine, in lineal contact sliding under dry condition against hardmetal, at a sliding velocity of about 1.57 ms−1. Wear rates under different loads were measured and the friction coefficients were recorded. SEM analysis was carried out to identify the wear mechanisms. The results show that the GC has higher wear-resistance than DC and TC. The tribological characteristics of graded coating were different along the depth of the coatings, and the surface of coatings with pure Al2O3 does not show the best wear resistance. The wear rate and friction coefficients were also different under different loads. The failure types of plasma-sprayed Fe3Al-Al2O3 graded coatings in lineal contact were: loosening of ceramic particles, crack nucleation and propagation, brittle fracture, plastic deformation, and adhesive wear.

A novel approach for monitoring pipeline corrosion under disbonded coatings

Underground pipeline corrosion monitoring is a complex technical challenge. Currently there is no corrosion monitoring probe that is able to provide in situ information on corrosion under disbonded coatings. This paper presents a proof of concept of a novel corrosion monitoring probe intended to simulate corrosion under disbonded pipeline coatings and monitor its rate under Cathodic Protection (CP). The probe's capabilities to measure corrosion rates and simulate disbonded coating conditions are illustrated by a typical experiment that involved testing of the probe in 0.1M NaCl at -850mVCSE. Estimated metal thickness losses based on results measured by the probe were compared against corrosion patterns and profilometry measurements of control specimens exposed to the same conditions.

Finite element analysis of residual stresses in metallic coatings through a compound casting

Al and Mg alloys are widely used in industry as main lightweight alloys. They have excellent properties, such as low density, high ductility, and high specific strength, and so on. Generally speaking, Mg alloys are better than Al alloys. However the corrosion of Mg alloys is much more difficult to control compared Al alloys. Therefore to combine these two lightweight alloys, a composite-like structure is an ideal solution since Al alloys can be used as protective coatings for Mg alloys. Compound casting is a realistic technique to get this coating system. In the current study, we numerically study the compound casting using finite element method (FEM) to make these two alloys, a composite-like structure, satisfy requirements to resist corrosion required from industry, in which the aluminum layer is acting as a protective coating for the magnesium substrate. Several finite element models have been developed by using the birth and death element technique and we focus on compound casting-induced residual stresses in the compounded structure. The numerical results obtained from the proposed finite element models show the distribution profiles of thermal residual stresses. We found the major factors influencing the residual stresses are the temperature to pre-heating the Al substrate and the thickness of Mg deposits. © (2014) Trans Tech Publications, Switzerland.

Monitoring cathodic shielding and corrosion under disbonded coatings

This work presents a novel corrosion monitoring probe designed for simulating the conditions developed under disbonded coatings and for measuring current densities and their distribution over a simulated pipeline surface. The probe’s concept was experimentally evaluated via immersion tests under Cathodic Protection (CP) in high resistivity aqueous solution. Under the disbonded area, anodic currents as well as cathodic currents were both measured. Anodic current densities were used to calculate metal losses by means of Faraday’s law. Calculated corrosion patterns were compared with corrosion damage observed at the probe’s surface after a period of test. The probe’s working principles are explained in terms of simple electrochemistry.

Evaluation of coatings for mg alloys for biomedical applications

The aim of this work was to assess a number of coatings developed for Mg for biomedical applications. The Mg substrates were high-purity (HP) Mg and ME10, an alloy recently developed for improved extrudability. The research utilized the new fishing-line specimen configuration to allow direct comparison to our recent in vivo and in vitro measurements. The in vitro measurements were immersion tests of fishing-line specimens immersed in Nor's solution at 37 °C. Tests of substantial duration are needed because the corrosion rates of uncoated samples are low. Nor's solution is the designation given to Hank's solution through which CO2 is bubbled at a partial pressure of 0.009 atm. In this solution, pH is maintained constant by the interaction of CO2 and the bicarbonate ions in the solution. This is the same buffer as that which maintains the pH of blood. Coatings examined were: (i) an anodization using a bio-friendly alkaline electrolyte consisting of phosphate, borate, and metasilicate, (ii) octyltrimethoxysilane (OSi), (iii) 1,2-bis[triethoxysilyl]ethane (BTSE), (iv) anodization+OSi, and (v) anodization + BTSE. The performance of coated samples was comparable to or better than that of the uncoated samples, and there was a substantially better performance for the ME10 samples after anodization+OSi. Reasons for the various performances are discussed.

Facile fabrication of superhydrophobic conductive graphite nanoplatelet/vapor-grown carbon fiber/polypropylene composite coatings

The fabrication of superhydrophobic surfaces with mechanical durability is challenging because the surface microstructure is easily damaged. Herein, we report superhydrophobic conductive graphite nanoplatelet (GNP)/vapor-grown carbon fiber (VGCF)/polypropylene (PP) composite coatings with mechanical durability by a hot-pressing method. The as-prepared GNP/VGCF/PP composite coatings showed water contact angle (WCA) above 150° and sliding angle (SA) less than 5°. The superhydrophobicity was improved with the increase of VGCF content in the hybrid GNP and VGCF fillers. The more VGCFs added in the GNP/VGCF/PP composite coating, the higher porosity on the surface was formed. Compared to the GNP/PP and VGCF/PP composite coatings, the GNP and VGCF hybrid fillers exhibited more remarkable synergistic effect on the electrical conductivity of the GNP/VGCF/PP composite coatings. The GNP/VGCF/PP composite coating with GNP:VGCF = 2:1 possessed a sheet resistance of 1 Ω/sq. After abrasion test, the rough microstructure of the GNP/VGCF/PP (2:1) composite coating was mostly restored and the composite coating retained superhydrophobicity, but not for the VGCF/PP composite coating. When the superhydrophobic surface is mechanically damaged with a loss of superhydrophobicity, it can be easily repaired by a simple way with adhesive tapes. Moreover, the oil-fouled composite surface can regenerate superhydrophobicity by wetting the surface with alcohol and subsequently burning off alcohol.

Electrochemical method for studying localized corrosion beneath disbonded coatings under cathodic protection

This paper presents a new method for measuring localized corrosion under disbonded coatings by means of an electrochemical sensor, denoted differential aeration sensor (DAS). It measures the distribution of electrochemical currents over an electrode array surface partially covered by a crevice that simulates a disbonded coating. The DAS has been evaluated using immersion tests at open circuit and under cathodic protection (CP) conditions. Under both conditions, anodic as well as cathodic current densities were detected within the crevice. A fundamental understanding for the detection of anodic currents under CP has been explained in terms of basic electrochemistry. Based on the current distribution data provided by the sensor, two different analysis methods have been used to estimate corrosion and its distribution. These methods consisted of a direct application of Faraday's Law to the anodic currents detected by the array, and on a sensor-specific method denoted corrected currents' method. It has been demonstrated that under diffusion controlled conditions this latter method produces a better corrosion estimation than the direct application of Faraday's Law. The corrected currents' method allowed the estimation of corrosion patterns outside the crevice under CP. Good correlation between electrochemical calculations and surface profilometry results has been obtained.

Understanding the effectiveness of cathodic protection under disbonded coatings

© 2015 Elsevier Ltd. All rights reserved. Factors affecting the effectiveness of cathodic protection under disbonded coatings were studied using a partially covered two dimensional electrode array that simulates a crevice under a disbonded coating and allows for the mapping of electrochemical currents under the influence of cathodic protection (CP). This technique enabled the study of the effects of major factors, including crevice gap size, solution conductivity and applied CP potential on the distribution and evolution of CP currents over the electrode array surface. The effect of each of these factors on the overall current distribution profile has been explained using a new electrochemical model. This model suggests that, despite the detrimental effect of cathodic shielding, the steel under disbonded areas could still be protected, independent of the crevice geometry and solution resistivity, by means of concentration polarization instead of direct electrochemical polarization. A set of conditions for maintaining CP potentials more negative than -850 mVCSE along disbonded coating areas was deduced from the proposed model and validated against potential profiles available in the literature. The possible practical implications of the current mapping technique used in this study for developing in-situ CP monitoring sensors has also been discussed.

Electrochemically monitoring localized corrosion patterns and CP effectiveness under disbonded coatings

© 2015 by Nace International. This paper presents new experimental evidences on the capability of a novel electrochemical corrosion monitoring sensor, which was recently conceived, for measuring localized corrosion under disbonded pipeline coatings. The sensor's design includes an artificial crevice for simulating the conditions developed under disbonded coatings and an electrode array for measuring current density distribution over its surface. The sensor capabilities were further evaluated by studying the dependency of corrosion patterns and current density distribution on the Cathodic Protection (CP) potential applied upon immersion in an aqueous environment. At the less negative CP potential, a good correlation was found between the inhomogeneous corrosion distribution under the disbonded coating as measured by the sensor and actual metal loss and corrosion attack observed on its surface at the end of the test. At more negative CP potentials no corrosion was detected or observed on the sensor's surface. In addition, characteristic changes in the cathodic current distribution at different CP potentials illustrated the possibility of employing the sensor to obtain valuable feedback on the performance of a given CP setup, without requiring its interruption or compensation of IR-drops. Furthermore, the sensor's capability to detect some of the effects of overprotection were shown at the most negative CP potential applied.

Considerations associated with the testing of external pipeline coatings

Reliable testing methodologies for the assessment of protective coatings are critical for ensuring the integrity and durability of pipeline coatings (such as field joint coatings) and the mitigation of pipeline corrosion. Currently the failure of joint coatings is one of the major concerns in corrosion protection of pipelines, although they represent only approximately 5% of the coated area in a pipeline system. This paper presents an overview of major testing methodologies currently used in the pipeline industry for the selection, testing, and life prediction of coatings, in particular field joint coatings. Particular focus is on the discussion of difficulties and limitations in testing methods for assessing pipeline coating cracking, cathodic disbondment and loss of adhesion. It is shown that there are limitations in current methodologies in evaluating the coating flexibility - a key parameter for avoiding coatings cracking during hydrostatic testing, cyclic pressure operation and field bending. Methodologies for assessing the effect of holidays in coatings on the cathodic disbondment of pipeline coating under excessively negative cathodic protection (CP) voltages also require improvement. Furthermore, methods for understanding the effects of coating wet adhesion on pipeline coating, cracking and disbondment also need attention. Some preliminary results for addressing some of these issues are also presented in this paper.

Experimental and stochastic approaches to assessing the strain demand of pipelines and flexibility requirements for coatings

A criterion for selecting a coating for an energy pipeline is that the coating should have a suitable flexibility to meet the high strain demand during hydrostatic testing and during field bending. This requires knowledge of the level of strain demand for the pipeline, and also the maximum strain that could be
tolerated by the coating system. Whereas average strains imposed during manufacturing and construction are reasonably well predicted, there is insufficient understanding on the factors leading to localised deformation of the pipe. Significant work has been carried out in the past to develop tests for assessing
the coatings’ ability to handle a certain amount of strain based on bend testing, tensile testing and burst testing. However, there is a concern as to whether these tests properly represent localised micro-strains associated with construction activities including field bending and pressure testing, particularly pressure testing of pipelines designed for operation at 80% of specified minimum yield strength (SMYS). Consequently coatings considered "suitable" for modern pipelines may fail. The first issue discussed in this paper is main factors affecting strain localisation. The non-deterministic distributions of heterogeneities over the pipe provide a ground to consider the mechanisms of localisation as a stochastic process. An approach is proposed to quantify the maximum localised strain demand through cold field bending and hydrostatic experiments. Another issue discussed in this paper is the experimental assessment of coating flexibility under the effects of localised strains. Preliminary mandrel tests have been carried out to assess the uniformity of the imposed strain. Although mandrel testing has been shown to be a useful method for relative comparison of coating flexibility, it has several weaknesses that could significantly affect the reliability and reproducibility of the results.