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.     

Influence of AC interference to corrosion of Q235 carbon steel

Abstract

The alternating current (AC) has a strong influence on the corrosion of carbon steel Q235 in soil. AC corrosion^1–3 was investigated by morphology observation, weight loss analysis and electrochemical measurement. The results of morphology observation and weight loss analysis showed that AC increased the corrosion rate and the influence was weakened when a protective layer was built up. The electrochemical tests confirmed that in the presence of AC, the corrosion potential shifted toward the negative direction and the exchange current density increased.^4–9     

Effects of alternating current interference on cathodic protection potential and its effectiveness for corrosion protection of pipelines

In this work, the effect of alternating current (AC) interference on cathodic protection (CP) potential on a X65 steel in a near-neutral pH bicarbonate solution was investigated, and the CP performance under AC was evaluated by weight-loss measurements. The CP potential applied on the steel cannot be maintained in the presence of AC interference. The shift of the CP potential depends on the applied CP level and AC current density. No matter if the direct current potential of the steel is shifted negatively or positively upon application of AC, the steel suffers from increased corrosion. The AC decreases the effectiveness of CP for corrosion protection. The CP standard at ?0.850?V (copper sulphate electrode) that does not consider the AC interference is not appropriate for AC corrosion protection.     

Cathodic protection monitoring of buried carbon steel pipeline: measurement and interpretation of instant-off potential

The criterion used to verify the cathodic protection condition of a structure is based on structure-to-electrolyte potential measurement, which can include an ohmic drop contribution. Among the available techniques, the use of potential probes with internal reference electrode and of corrosion coupons with a so-called soil-access tube represents the most appropriate way to measure the IR-free potential, eliminating the ohmic drop contribution. An alternative is represented by on-off technique if equalising, galvanic or stray currents are not present. Laboratory tests have been carried out in soil-simulating solution on cathodically protected carbon steel in order to evaluate the effectiveness of off-potential for the assessment of cathodic protection. Instant-off potential is not reliable in overprotection condition; moreover, the accuracy of the measurement is strongly influenced by the presence of external current as galvanic or equalising currents. The effect of low-pass filter in the data acquisition system on the potential reading is discussed.     

Influence of activated carbon on crevice corrosion in adsorption tower of advanced liquid processing system

ABSTRACT

The adsorption tower made of type 316L stainless steel (SS) in Multi-nuclide Removal Equipment (Advanced Liquid Processing System) which uses Ag-impregnated activated carbon (Ag AC) as an adsorbent experienced crevice corrosion. The influence of Ag AC on the crevice corrosion susceptibility and E_sp of 316L SS was investigated by performing electrochemical experiments. Crevice corrosion was observed in the specimen in contact with the Ag AC. On the other hand, there was no crevice corrosion without the Ag AC in both pH 7.4 and pH 12 solutions. Clear ennoblement of spontaneous potential (E_sp) by in contact with activated carbon was observed and that was clearly higher than the repassivation potential for crevice corrosion (E_R,CREV). Thus, the presence of the AC notably increased E_sp of 316L SS and this resulted in increased crevice corrosion susceptibility by the galvanic effect.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

GdBaCo2O5+δ-Sm0.2Ce0.8O1.9 composite cathodes for intermediate temperature SOFCs

The GdBaCo₂O_5+δ-Sm_0.2Ce_0.8O_1.9 (GBCO-SDC) composite cathodes were prepared, their thermogravimetric measurement, thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance were studied as function of temperature and SDC content. The adjustment of TEC to electrolyte, which is one of the main problems of GBCO cathode, could be achieved to lower TEC values with more SDC addition, while maintaining reasonable high electrical conductivity. By means of DC polarization and AC impedance spectroscopy, the electrochemical performance of GBCO-SDC composite cathodes on SDC electrolyte was examined. Results indicated that the proper addition of SDC could improve the performance of GBCO cathode. The optimum content of SDC in the composite cathodes was 40 wt.% with the polarization resistance (Rp) of 1.39 Ω cm² at 500 °C, which was significantly lower than that of pure GBCO cathode (7.26 Ω cm²).     

埋地管道阴极保护电位IR降评估方法的研究

理论分析土壤电阻率和涂层缺陷电阻等参数对埋地管道阴极保护电位测量时IR降误差的影响，建立IR降电阻因子计算模型，并用实验数据验证，此外，还讨论了IR降误差的工程预测方法.     

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.     

Effect of AC interference on the corrosion behavior of cathodically protected mild steel in an artificial soil solution. Part I: Investigation on formed corrosion product layer

AC-induced corrosion is a big threat even for cathodically protected pipelines nowadays. While this phenomenon was intensively investigated in the last decades, the corrosion mechanisms due to AC interference remain unclear. In the present work, investigations on the surface processes on cathodically protected mild steel during AC polarization have been performed. They utilized high-speed potential measurements that have demonstrated the influence of the polarization parameters on the resulting alternating voltage. The corrosion product layer was characterized with scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy, which clearly show the effect of the parameters of the applied alternating current on the surface under different cathodic protection (CP) conditions. It was demonstrated that the properties of the formed corrosion product layer, meaning the layer thickness, amount of oxygen, and so on, is not only dependent on the AC polarization parameters but also on the CP potential itself.     

埋地钢质管道交流干扰测试与评价

高压交流输电线路及交流电气化铁路对与其临近铺设的埋地钢质管道会引起交流干扰,产生交流腐蚀,并对阴极保护系统的安全运行带来不利影响。因此,埋地钢质管道中交流干扰的检测与评估是必要的。国内外对于埋地金属管道受交流干扰的测试与分析,主要围绕交流干扰电压、交流电流密度、腐蚀速率等参数的测试来开展。在交流干扰及阴极保护有效性评价方面,国外以交流电流密度及交/直流电流密度比为主要参量。本工作通过对交流干扰下的管地电位信号时频特征研究,得出管地电位信号时频特征与交流干扰源频率存在一致性,为检测和判定埋地钢质管道上的交流干扰提供了一定的依据。     

Modelling of electromagnetic processes in system 'welding arc–evaporating anode' Part 2 – model of arc column and anode metal

Abstract

A mathematical model of electromagnetic processes occurring in the 'arc column–anode region–evaporating anode' system is suggested. Electric charge transfer processes in the multicomponent plasma of the electric arc and in the bulk of the metallic anode are described using the Ohm's law and charge conservation law generalised for a case of discontinuity of the electric field potential at the boundary between the arc plasma and the anode surface. A procedure was developed for finding the numerical solution of the stated problem by the shock capturing method, allowing modelling of the electromagnetic processes in the 'welding arc–evaporating anode' system in the presence of a reverse potential jump in the anode region (negative anode drop). Results of modelling of the said processes for gas metal arc and plasma transferred arc welding of steel in an inert gas (Ar) atmosphere are given.     

Characterizing corrosion behavior under atmospheric conditions using electrochemical techniques

AC and DC electrochemical experiments were performed as a function of humidity and contaminant concentration in an effort to identify the range of atmospheric environments where corrosion processes could be detected and possibly quantified. AC measurements exhibited two time constants at 25% relative humidity (RH), possibly indicating the ability to resolve both electrolyte resistance and interfacial impedance. Galvanic current measurements were sensitive to the presence of Cl_2(g) at 30% RH and electrochemical transients were detected at both 30% and 50% RH levels, also indicating sensitivity to interfacial processes. Higher humidity levels allowed better quantification due to decreasing electrolyte and interfacial impedances.     

Corrosion behaviour of zinc–nickel alloy coatings electrodeposited in additive free chloride baths

Abstract

Corrosion behaviour of nanocrystalline Zn–Ni alloy coatings (with 3 to 18 at-%Ni) electrodeposited on steel substrate from additive free chloride baths by DC plating has been investigated by measurement of open circuit potential with time and neutral salt spray test. The alloy coatings containing up to 16 at-%Ni exhibited an increase in resistance to the appearance of white and red rust with increasing Ni content. The resistance to the appearance of white rust and ability to protect the underlying steel substrate decreased with further increasing Ni content. This has been considered to be due to reduced passivating ability of the alloy coating with a significant rise in the grain size above 50 nm.     

埋地管道交流干扰与阴极保护相互作用研究进展

系统地综述了交流干扰对阴保电位、阴保电流密度、牺牲阳极电位、牺牲阳极消耗速率、牺牲阳极效率等参数的影响,同时阐述了交流干扰下阴保评价准则及交流腐蚀机理的最新研究成果。最后指出了目前研究存在的主要问题,展望了该研究领域的发展趋势。     

A comparison between the simulation and experimental results of cathodic protection of oil well casings

Metallic structures such as oil well casings in contact with soil or submerged under water are subject to corrosion. Leaks that develop in the casing of a well, constitute a serious problem and are expensive to repair. Cathodic protection (CP) is the cost‐effective control of external well casing corrosion. There are certain methods to ensure that the applied CP should meet the required criteria. In CP of well casings, monitoring of potential values is often based on surface E‐log I measurement which is a cost‐ and time‐consuming method. Simulation software and numerical methods often have been used to address this issue. In this paper, simulation of CP has been done by means of a computer software with finite element capability (ANSYS 11). Potential profiles were obtained by selecting different soil electrical conductivities (single‐ and multi‐layered media) and by changing applied current density values. Experimental procedure was established to determine the accuracy of simulation data. The results showed good agreement, especially in the single‐layered medium and at higher values of applied current.     

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.     

Coaxial TIG‐YAG & MIG‐YAG welding methods

Abstract

Arc stability in AC/DC pulsed MIG welding is mostly determined by the performance of the arc length control system. This system detects the arc voltage as arc length information for feedback purposes and varies the output of the welding power source so that the arc voltage is equal to the voltage set point. For example, at a high arc length, the arc voltage increases. The pulse frequency is reduced in response, the wire melting rate is reduced and the arc length is returned to normal.     

Relationships governing slip in the structure of the weld metal of welded joints in heat-resisting pearlitic steels

Summary

Previous papers have shown the high‐current AC‐MIG welding process to be superior to the conventional DC‐MIG process in terms of welding efficiency and welded joint performance. The authors have used the AC‐MIG process to weld heavy‐gauge steel plates at a tensile strength level of 980 MPa. Weld metal with a very low hydrogen content is obtained by the newly developed process, resulting in a reduced preheating temperature during welding of high‐strength steels. The AC‐MIG process also produces an arc climbing effect with electrode negative polarity ‐ a desirable effect in narrow‐gap welding. AC‐MIG weld metal further contains low oxygen, conferring high toughness. As a result, a narrow‐gap AC‐MIG welding process has been developed to ensure high efficiency in processing and metallurgically high‐quality welds in welding of 980 MPa high‐strength steels for penstocks (pressure shafts) in hydroelectric power stations. Welding wires with chemical compositions satisfying suitable strength and toughness criteria have been trial‐manufactured. Suitable welding parameters have also been determined for narrow‐gap welding of penstocks.     

Determination of the zero-charge ph value of a passive iron surface in aqueous solution

Two techniques of measurement of pH₀, which represents the pH of solution at which the total charge of passive metal electrode surface is zero have been developed. The procedures are based on the use of acid-base equilibria between oxide on electrode surface and solution. In the former technique, the pH of the drop of solution that is applied onto the electrode surface is measured. At pH₀, the pH of the drop does not change in time. In the latter procedure, the potential of a passive electrode that is immersed into solutions with various pH values is measured. pH₀ has been measured on a passive iron electrode. The pH₀ values measured by the two procedures are similar. Comparison of pH₀ of passive electrode and various oxides one to determine the composition of a passive film. The introduction of surface-active ions Ba²⁺ and Cl^? to the solution leads to a shift of pH₀ values and electrode potential in opposite directions.     

Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien Teil 1. Computerunterstützte Unterbrechertechnik zur Erfassung des Ohmschen Spannungsabfalls

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 1. IR drop measurement by a computer-assisted Interrupter technique An interrupter technique with computer-assisted calculation of the ohmic potential drop UΩ is described. It allows to measure UΩ while the working electrode is under potentiostatic control. As an additional result the differential double layer capacity C_D is calculated, which includes some informations about the state of the working electrode surface. Experiments with the presented interrupter technique show, that just using a Luggin capillary is absolutely insufficient to avoid IR drop, especially in low conductive media: steady state current-potential curves will yield erroneous results of the kinetic parameters of the electrode reactions if the IR drop is not carefully eliminated. The described interrupter technique gives the same results for the ohmic resistance RΩ as eg AC techniques or potential pulse measurements. The value of RΩ measured is always associated with the primary current distribution.