Effect of alternating current on cathodic protection on pipelines

In this work, the effects of alternating current (AC) on the performance of cathodic protection (CP) and the CP potential readings were investigated on a 16Mn pipeline steel in a simulated soil solution. The presence of AC interference decreases the CP effectiveness to protect the steel from corrosion. Only when CP potential is sufficiently negative, the steel is under a complete protection even when the AC current density is 400 A/m². Moreover, the AC would shift CP potential from the designed value. The effect of AC on the CP performance depends on the cathodic potential applied on the steel.     

Study of criterion for assuring the effectiveness of cathodic protection of buried steel pipelines being interfered with alternative current

Interference of alternative current (AC) on corrosion of X65 steel was investigated in soil. It was observed that the unfavorable effect of interfering AC was able to be effectively inhibited by increasing the direct current density of the cathodic protection (CP) system. A clear correlation was established between the CP current density and the tolerable AC current density. This led to a new criterion for assuring the effectiveness of CP of buried pipelines being interfered with AC. Field experimental results on a buried pipeline running below a 500 kV transmission line showed that the criterion could satisfactorily predict the risk of AC interfering on the CP system.     

Effect of AC interference on cathodic protection monitoring

Abstract

Cathodic protection (CP) monitoring in the presence of simultaneous AC and DC interference could lead to erroneous measurements, since IR drop contribution due to both DC and AC could heavily affect potential readings. Therefore, to know the true potential (or true polarisation level), the ohmic drop contribution has to be eliminated. In literature, there is lack of agreement about the proper procedure to measure the true potential in the presence of AC. Laboratory tests on carbon steel specimens in soil simulating conditions were carried out focusing on potential measurement problem in the presence of AC through standard potential measurement procedure and the use of a potential probe. Results suggest the need of a proper methodology for potential measurement to determine reliably CP conditions.     

A study on cathodic protection against crevice corrosion in dilute NaCl solutions

Potential and current distributions in a cathodically protected crevice between a simulated coating and segmented mild steel electrodes were measured in dilute NaCl solutions. The distributions became more uniform with time due to an increase in solution conductivity and depletion of dissolved oxygen in the crevice. Generally, a negative shift of control potential and an increase in initial solution conductivity and crevice thickness resulted in a higher polarization level on the steel. However, if the control potential is too negative, the polarization level may be lower than that under a suitable control potential because of hydrogen evolution. On the basis of these results, a mechanism of cathodic protection against crevice corrosion in high-resistivity environments was proposed.     

Mathematical modeling on the corrosion of unprotected structure due to stray current resulting from cathodic protection system

Mathematical modeling on the corrosion of unprotected structures due to a stray current resulting from a nearby cathodic protection system was carried out using the boundary element method. The model consists Laplace’s equation with non-linear boundary conditions (Tafel equations) and the iterative technique to determine the mixed potential of the unprotected structure. The model is applied to an unprotected bare structure as well as a coated structure with several defects. The amount and the location of corrosion along the unprotected structure correlate strongly with experimental results within the experimental conditions studied.     

Effect of cathodic protection on corrosion of pipeline steel under disbonded coating

In this work, a test rig was developed to study the effect of cathodic protection (CP) on corrosion of X70 pipeline steel in the crevice area under disbonded coating through the measurements of local potential, solution pH and dissolved oxygen concentration. Results demonstrated that, in the early stage of corrosion of steel, CP cannot reach the crevice bottom to protect steel from corrosion due to the geometrical limitation. Corrosion of steel occurs preferentially inside crevice due to a separation of anodic and cathodic reaction with the depletion of dissolved oxygen in the crevice solution. The main role of CP in mitigation of sequential corrosion of steel in crevice under disbonded coating is to enhance the local solution alkalinity. With the increase of distance from the open holiday, a high cathodic polarization is required to achieve appropriate CP level at crevice bottom. A potential difference always exists between the open holiday area and inside crevice, reducing the CP effectiveness.     

Stray current control by a new approach based on current monitoring on a potential probe

To control stray current interference due to DC transit systems, cathodic protection at the so-called constant potential condition is used. Pipeline potential is continuously measured by means of a fixed reference electrode, and the measured value is used to control dynamically the feeder in order to match the protection condition. An accurate analysis of this technique reveals that during interference what is maintained constant is the IR drop in soil rather than the true potential of the structure. A new approach was investigated: the innovation consists of the use of the current, rather than the potential, to control the feeder on the basis of an effective range of the current the probe should receive. Laboratory tests confirmed the feasibility of such an approach. Modelling and test results are presented and discussed.     

The cathodic protection potential criteria: Evaluation of the evidence

Recent publications have ascribed the mechanism of cathodic protection (CP) in soils to the development of passivity at the steel surface. This view has, in turn, prompted discussion on the need to re-evaluate the long-established criteria for protection. This paper presents a contribution to that discussion. It advances, or rather it restates, the proposition that neither immunity nor passivity is necessarily relevant. It argues that protection is simply a consequence of a potential-driven lowering of the anodic dissolution rate. However, it also emerges that elucidating any mechanism convincingly requires dependable field or laboratory data on the very low corrosion rates occurring on cathodically polarised specimens. A critical review of published data highlights a lack of reliability in this area.     

Alternating current corrosion of cathodically protected pipelines: Discussion of the involved processes and their consequences on the critical interference values

Based on laboratory studies and model concepts, a profound understanding of the involved processes in ac corrosion and the required limits has been obtained in the last years. But there was no information whether these thresholds can be effectively applied to pipelines or whether operational constraints make their implementation impossible. Therefore, an extensive field test was carried out. Thereby, the relevance of the laboratory tests for field application could be demonstrated and all threshold values were confirmed. Detailed analysis made it possible to explain the observed threshold values based on thermodynamic and kinetic considerations. The results summarized in the present work are the basis for the normative work defining the thresholds for the operation conditions of cathodically protected pipelines.     

Effects of Tafel slope,exchange current density and electrode potential on the corrosion of steel in concrete

A parametric study is carried out to investigate the effect of variations in anodic and cathodic Tafel slopes, exchange current densities and electrode potentials on the rate of steel corrosion in concrete. The main goal of this investigation is to identify the parameters that have significant influence on steel corrosion rate. Since there is a degree of uncertainty associated with the selection of these parameters, particularly during modelling exercises, it is intended that the results of this study will provide valuable information to engineers and researchers who simulate steel corrosion in concrete. To achieve this goal, the effect of a parameter on the corrosion rate of steel is studied while all other parameters are kept constant at a predefined base case. For each parameter, two extreme cases of anode‐to‐cathode ratio are studied. The investigation revealed that the variations in the anodic electrode potential have the greatest impact on the corrosion rate, followed by the variations in the cathodic Tafel slope.     

Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O₂) in soil ground water, a differential O₂ concentration cell may develop in the crevice because O₂ diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O₂ concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O₂, and that of dissolved carbon dioxide (CO₂) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O₂ on the crevice corrosion rate and the added effect of dissolved CO₂ are mathematically modeled.     

Critical questions and answers about cathodic protection

ABSTRACT

Cathodic protection has been in use for almost two hundred years, yet there still exist a number of misunderstandings surrounding its application. In this paper, we question some common statements and beliefs to determine whether they are based on fact or fiction. Our questions include: What is the origin of the ubiquitous ?0.85?V_CSE cathodic protection criterion and why does it work? Does cathodic protection really stop corrosion or simply reduce it to negligible levels? What does the instant off potential tell us and is it really the true polarised potential? Does hydrogen gas cause cathodic disbonding? Do the potential shifts measured during interference testing indicate real interference? The answers to these questions are important because they allow the correct interpretation of field and experimental data, based on a true understanding of the electrochemical and thermodynamic basis of cathodic protection.     

Disbonding of underwater-cured epoxy coating caused by cathodic protection current

Cathodic disbonding of the underwater-applied, ultra-thick, solvent free epoxy coating subjected to various levels of cathodic protection was investigated during the period of the coating cure. The results indicate that the partially cured coating was of low resistivity, between 10³ and 10⁵ Ω cm² for the cathodic polarization of on-potentials between −0.98 and −1.4 V_Ag/AgCl/sw. The coating was shown to be capable of withstanding normal levels of cathodic protection between off-potentials of −0.8 and −1.1 V_Ag/AgCl/sw while the IR drop, introduced by the coating in the same potential range, increased from 0.06 to 0.1 V and has to be taken into account at the design stage of the cathodic protection system. Beneficial influence of calcareous deposit formation on the cathodic protection current was confirmed, particularly for the failed coating. The initial period (1 week) of coating cure was shown as the most critical for disbonding processes caused by the excessive cathodic polarization.     

Use of the polarisation shift criterion to evaluate cathodic protection of on-grade steel storage tanks

The cathodic protection (CP) of on-grade coated steel storage tanks deployed on reinforced concrete slabs in soils of 20–30?kΩ?cm resistivity was evaluated for two different time periods 9 years apart. The required CP current was calculated and the ohmic drop was measured using the instant-off potential method. The resulting polarisation shift is a useful indicator of the level of protection afforded and whether the potential criterion or the polarisation shift criterion is fulfilled. It was also noted that the greater the ground resistivity, the less corrosive the environment for an on-grade steel structure.     

The limits of the Pourbaix diagram in the interpretation of the kinetics of corrosion and cathodic protection of underground pipelines

This article deals with the basics of corrosion and corrosion protection, with the focus on equilibrium thermodynamics. Particular attention is paid to the easy-to-understand creation of Pourbaix diagrams, their uses, and limitations of this tool. In addition, the basics of electrochemical kinetics are dealt with to provide an overview of the possibilities and the interaction of both methods. Finally, it is argued that the globally recognized and practice-proven protection potential criteria remain valid.     

SVET and SECM imaging of cathodic protection of aluminium by a Mg-rich coating

The mechanism of cathodic protection of an aluminium substrate by a Mg-rich coating was investigated using localized techniques. Both scanning vibrating electrode techniques (SVET) and scanning electrochemical microscopy (SECM) were used to investigate the processes occurring at the surface of exposed metal when electrically connected or disconnected in a galvanic couple with the Mg-rich coating. The SVET has shown the evolution of the pit activity with time under conditions of sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. It was shown that the cathodic protection provided by magnesium to aluminium substrates acts by both preventing pit nucleation and inhibiting the growth of the pre-existing pits.     

Assessment of cathodic protection effect on long-distance gas transportation pipelines based on buried steel specimens

Different severe corrosions are found at the damaged point of anticorrosive coating on natural gas pipelines, even though the cathodic protection (CP) potential is between −0.85 and −1.15 V. Therefore, there are totally 168 buried steel specimens (BSSs) and 28 soil samples at 28 buried points that are taken out in situ and then analyzed in the lab to assess the CP effectiveness. First, the CP status is simulated by BSSs at the damaged points, and the corrosion products on BSSs are analyzed microscopically. Second, the corrosion rates of the testing BSSs are calculated by the weight-loss method to perform the quantitative assessment. Third, the specific reasons for external corrosion are explored via analyzing the CP potential attenuation tendency and the corrosiveness of soil. The corrosion growth trend is also investigated by comparing the test results among three test cycles within 8 years. Finally, the control measures for external corrosion of pipelines are put forward. The study is beneficial to evaluate the CP system and determine the causes of external corrosion of pipelines, thereby formulating corresponding protective measures.     

Research on corrosion rate assessment of buried pipelines under dynamic metro stray current

To provide a reference for the evaluation of corrosion rate under dynamic metro stray current, both simulated experiments in laboratory and coupon tests in the field were conducted to explore the corrosion behavior and the relationship between corrosion rates and dynamic DC interference parameters of buried pipelines. The results show that the dynamic period has a significant influence on the corrosion rate and the ratio of actual corrosion rate to the theoretical corrosion rate. The statistical data of pipe to soil potentials or current densities of coupons electrically connected to pipelines obtained from the field in many cities of China show that the dynamic fluctuation periods under metro stray current are within the range of 0–300 s, among which the majority of fluctuation periods distribute from 50 to 200 s. The dynamic fluctuation of pipe to soil potentials and current densities of coupons exhibits symmetric or asymmetric characteristics. Under the situation of symmetric positive and negative current density, the ratio of the actual corrosion rate to the theoretical corrosion rate ranges from about 4% to 22% under the typical dynamic periods range from 50 to 200 s based on the simulated experiments results in the laboratory. Besides, two corrosion rate assessment methods, including the positive shifting and time ratios of off-potentials and symmetrical or asymmetric characteristics of DC current density of coupons were proposed. In addition, the evaluation of the on-potential criteria to assess the corrosion risk of time-variant anodic interference was also discussed in the paper.     

A mathematical model for the cathodic protection of tank bottoms

A mathematical model previously developed for predicting the cathodic protection of pipeline networks is extended to treat cathodic protection of the bottoms of cylindrical above-ground storage tanks. A single tank was modeled for which protection was provided by an anode located infinitely far from the tank bottom, by a series of anodes distributed around the circumference of the tank, and by an anode grid laid directly underneath the tank bottom. The influence of an insulating barrier associated with secondary spill containment was also modeled for the arrangements in which anodes are placed adjacent to the tank bottom. The performance predictions are strongly dependent on anode placement, the oxygen content of the soil, and the presence of insulating barriers associated with secondary spill containment.     

Estimation of residual corrosion rates of steel under cathodic protection in soils via voltammetry

Steel coupons were buried in soil for 2 months under cathodic protection. Their residual corrosion rates were deduced from voltammetry and weight loss measurements. In aerated soils, the current density due to O₂ reduction, j_K,O2, was modelled with a mixed activation–diffusion controlled kinetic. The anodic part j_A of the current density j, computed as j_A = j − j_K,O2, obeyed Tafel law. Its extrapolation to the protection potential gave a corrosion rate (∼7 μm yr⁻¹) consistent with that obtained from weight loss measurements. With a deficient protection, corrosion rates remained at ∼80 μm yr⁻¹, a value given by both methods.