Orazem Underground Pipeline Corrosion

1. Auflage 2014
ISBN: 978-0-85709-926-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band 63, 340 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Underground pipelines transporting liquid petroleum products and natural gas are critical components of civil infrastructure, making corrosion prevention an essential part of asset-protection strategy. Underground Pipeline Corrosion provides a basic understanding of the problems associated with corrosion detection and mitigation, and of the state of the art in corrosion prevention. The topics covered in part one include: basic principles for corrosion in underground pipelines, AC-induced corrosion of underground pipelines, significance of corrosion in onshore oil and gas pipelines, numerical simulations for cathodic protection of pipelines, and use of corrosion inhibitors in managing corrosion in underground pipelines. The methods described in part two for detecting corrosion in underground pipelines include: magnetic flux leakage, close interval potential surveys (CIS/CIPS), Pearson surveys, in-line inspection, and use of both electrochemical and optical probes. While the emphasis is on pipelines transporting fossil fuels, the concepts apply as well to metallic pipes for delivery of water and other liquids. Underground Pipeline Corrosion is a comprehensive resource for corrosion, materials, chemical, petroleum, and civil engineers constructing or managing both onshore and offshore pipeline assets; professionals in steel and coating companies; and academic researchers and professors with an interest in corrosion and pipeline engineering. - Reviews the causes and considers the detection and prevention of corrosion to underground pipes - Addresses a lack of current, readily available information on the subject - Case studies demonstrate how corrosion is managed in the underground pipeline industry

Autoren/Hrsg.

Orazem, Mark

Leseproben

2 AC-induced corrosion of underground pipelines
B. Tribollet,    LISE/CNRS, France M. Meyer,    GDF-Suez, France Abstract:
Pipelines buried in soil are protected by a thick organic coating complemented by cathodic protection (CP). In spite of this double protection, when these pipelines are in the vicinity of a high voltage AC electrical field, such as a power line or an electrical railway for instance, corrosion may occur at the location of coating holidays (or defects). This phenomenon may be explained by a faradic rectification due to a non-linearity of interface behaviour, and by the AC field effect, as well as the impact of the AC current transfer on the chemistry of the electrolyte near the interface in the presence of CP. This chapter presents a critical review of the fundamental understanding of the AC-assisted corrosion phenomenon, and also some laboratory investigation, such as the analysis of corrosion products obtained under carefully controlled AC-corrosion tests using Raman spectroscopy. The particular role of the green rust at the interface is highlighted. Key words
faradic current; faradic rectification; AC corrosion; cathodic protection; green rust 2.1 Introduction
Electromagnetic fields created by high level operational alternating currents in high voltage electric power lines or in AC-powered railway systems sharing in parallel, or crossing, underground steel pipeline right-of-ways, induce electromotive forces into these pipelines. Following several occurrences of deep pitting and leaks of pipelines associated with this induction effect, it has been recognized that it may lead to specific steel corrosion where there are defects in coatings due to the alternating current flowing between these coating weaknesses and the surrounding soil.1–4 This phenomenon has some similarities with, but also differences from, the phenomenon of corrosion of metals in electrolytes when they are subjected to alternating current transfer at the electrolyte/metal interface. This type of metal corrosion has been known for some time5 but was traditionally not considered to be a major corrosion process for steel. The term AC corrosion described the enhanced corrosion caused by an externally applied AC current, with a frequency typically between 15 and 60 Hz. This type of corrosion is also called AC-enhanced corrosion, or AC-induced corrosion, usually contracted to AC corrosion. With the fast development of industry and urbanization, the demand for energy requires the construction of an increasing number of high-voltage, high-power transmission lines and the laying of large diameter, high-pressure pipelines. These pipelines are set underground to preserve environmental conditions and their laying down in the soil follows secure procedures. Since underground structures are not easy to inspect under operational conditions, there is a need for reliable above-ground monitoring techniques and assessment criteria that can confirm the preservation of their integrity with regard to any potential damage threat, including corrosion damage (Fig. 2.1). 2.1 Pipelines in the vicinity of high voltage electric power lines. External damage due to soil corrosiveness may threaten the integrity of underground pipelines. Protection of these assets against external corrosion is insured by a dual system comprising an anticorrosion coating, aimed at preserving the steel from contact with the soil, and a CP system to ensure electrochemical protection of the steel at coating defects. Control of external corrosion through this dual system has a very long industrial track record and, since decades, CP criteria have been elaborated by pipeline operators for ensuring an effective protection in the absence of any electromagnetic-induction effect. However, research works in the 1990s showed that, when the AC influence occurs on thick polymer coated underground steel pipelines in conjunction with CP polarization, AC-induced corrosion may occur on carbon and low alloy steels, even when the pipeline steel is polarized to a DC potential satisfying the classical CP potential criteria of - 0.85 V/Cu/CuSO4 electrode (copper sulfate electrode, CSE) (see for example References 5 and 6). In this chapter, only carbon steel material will be considered even if the problem exists also for stainless steel.7 This corrosion problem is not yet totally solved, as shown by the number of recent papers published on this topic.8–13 Indeed, there is no consensus on the mechanism of the phenomenon, particularly as it applies to corrosion in soils, and more specifically for underground coated pipelines subjected to CP. There is no more consensus on the extent of the effect of alternating current on underground metallic structures, i.e. on the kinetics on the corrosion process. It is well known that when AC current is transferred, or an AC voltage is applied, to a metal/electrolyte interface, under charge transfer control or mixed control, a faradic rectification occurs, due to a non-linear relationship of the current–voltage characteristics of the interface. The rectification effect leads to a shift of the free corrosion potential, as well as to an increase of the interfacial kinetics.14,15 This particular enhanced corrosion can be explained at least in two ways: • By virtue of the faradic rectification at the carbon steel/electrolyte (natural soil water) interface: by shifting the free corrosion potential of the steel towards more negatives values, it is assumed that the rectification effect decreases the cathodic polarization level at a given polarized potential value. • The alternative excursion of AC signals in anodic and cathodic domains due to the AC interference: excessive excursion of the interfacial electrochemical potential in the anodic domain may lead to steel corrosion even though the DC CP level fulfils the CP criteria recommended for CP protection of non-AC interfered pipelines. The first phenomenon may be examined by harmonic analyses of the electrode interface. In fact, the non-linear response of the electrode interface may be fully expressed by the sum of high order harmonic responses. The second process may be apprehended by simultaneous recording of the current and the potential on an electrode subjected to an AC voltage, or else to an AC current, perturbation. From the potential signal (including the ohmic drop due to the electrolyte resistance in series with the electrode interface characteristics), the ‘true’, i.e. IR-free, electrode potential and its time-wise variation will be evaluated. Using, in addition, electrochemical impedance spectroscopy (EIS), together with an equivalent electrical circuit model of the metal/electrolyte interface, this true potential allows the evaluation of the instantaneous current density used by the charge of the double layer capacitance. As a result, the faradic current, passing through the electrode interface under the AC perturbing signal will be evaluated. A tentative explanation of AC corrosion was given by Büchler and Schöneich16 according to the schematic representation given in Fig. 2.2. The authors suggested that the AC-corrosion phenomenon, on the cathodically polarized underground pipeline steels, may be attributed to destabilization of the pseudo-passive films that normally form at the external surface exposed to the ground at the coating defect, upon application of DC CP polarization of the steel. More precisely, as a consequence of the cyclic excursions of the steel electrochemical potential at the fundamental frequency of the AC perturbation, and assuming that the ‘positive’ excursion drives the steel surface potential in the so-called ‘passivity domain’, build-up of a protecting passive film occurs in the first anodic cycle while, assuming that the ‘negative’ excursion, in conjunction with the cathodic polarization, drives the steel surface potential below the stability domain of the passive film, it results in the reduction of the passive film. If, owing to the DC CP polarization of the steel surface, an alkaline environment is assumed at steel surface, the Fe(II) formed during the reduction of the passive film has only a limited solubility. As a consequence, it is accumulated on the metal surface, forming a porous rust layer. At each consecutive cycle a new passive film is formed underneath the rust layer, upon the ‘positive’ excursion of the steel surface potential, while the Fe(II) formed during the dissolution of the passive film, upon the ‘negative’ excursion of the steel surface potential, is added to the rust layer. Therefore, the thickness of the rust layer is increased with every oxidation/reduction cycle. A metal loss is taking place due to such successive AC cycles, giving rise to passivation and dissolution of this passive film with every cycle. Fig.2.2 Schematic representation of the processes taking place on steel under AC interference. Actually, it has been suspected, from several earlier research works on AC corrosion,17,18 that the faradic current transferred at the steel/electrolyte interface, when subjected to an AC voltage, or else to an AC current, perturbation may be constituted of many processes. During the anodic excursion, the faradic current may be composed of the oxidation of part or all of the hydrogen adsorbed during the...