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.     

Protection of the opposite reinforcement layer of RC‐structures by CP – results of numerical simulations

If cathodic protection (CP) is applied to reinforced concrete structures the anode is usually attached at the concrete surface closest to the corroding reinforcement in order to minimise voltage and protection current that is required. In specific circumstances an anode installation at the concrete surface next to the corroding reinforcement is not practicable, i.e. due to difficult access. In such cases it would be helpful to know whether it is possible to protect the reinforcement by an anode being installed at the opposite surface of the structural element. The question is: does the second reinforcement layer obtain sufficient current to be effectively protected? Since a general answer to this question is not possible because the distribution of protection current depends on several parameters like the geometry and amount of the reinforcement, concrete resistivity, the polarisation behaviour and geometry of corroding and passive zones, a 3 D‐FEM model of the current and potential distribution was developed and its accuracy was verified by laboratory results. This paper presents the model itself as well as the results of parameter studies carried out to show the impact of the reinforcement distribution, the concrete resistivity and the polarisation behaviour of the reinforcement on the current distribution within the reinforcement.     

Numerical design for cathodic protection systems for concrete

Cathodic protection (CP) has become a successful method for the rehabilitation of concrete structures affected by chloride-induced corrosion of reinforcing steel. CP involves applying an electrical current from an external anode through the concrete to the reinforcement. The current causes steel polarisation, electrochemical reactions and ion transport. Normally the anode is placed over relatively large surface areas, including those where the steel is passive. Conventional views assume that protection current will not significantly flow outside the anode area. In many cases this results in a conservative design. This paper presents principles and first results of numerical calculations for design of an example CP system by finite element modelling. The final objective is to develop a tool for more economical CP system design. In particular, a CP system for the protection of local damage in bridges (e.g. at leaking joints) has been simulated. The corroding area with respect to the size of the anode is varied. Current and potential distributions and depolarisation values are predicted, both close to and more distant from the anode. It appears that current densities required to achieve sufficient polarisation are much higher than those usually found in the field. Neglecting time-dependent repassivation processes is likely to be the main cause and further work is needed to include them. The present model can be used with reasonable confidence for preventive application to passive steel.     

Effect of pulse cathodic protection on corrosion mitigation and electrochemical conditions under a simulated coating disbondment on pipeline steel

Coating disbondments on pipeline steels are regions with high resistivity where conventional cathodic protection (CP) could not fully protect. Therefore, in an attempt to mitigate this challenge, this study investigates the effect of pulse CP on corrosion mitigation and electrochemical conditions under a simulated coating disbondment on X-52 pipeline steel. In this regard, conventional and pulse CP of ?870 mV_SCE were applied to the open mouth of a simulated coating disbondment. For pulse CP, frequencies of 1, 5, and 10 kHz were used. Results showed while the conventional CP was not able to fully protect the 20 cm simulated coating disbondment, for the pulse CP with increase in frequency from 1 to 5 kHz, and from 5 to 10 kHz, improve in CP potential protection under the simulated coating disbondment was achieved. This was accompanied by considerably lower corrosion and a more uniform pH distribution under the simulated coating disbondment.     

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.     

Effect of cathodic protection on macrocell currents: A viable method to monitor and control the efficiency of CP?

Corrosion protection of steel reinforcement in concrete structures by cathodic protection (CP) is a cost effective, reliable, and widely accepted method to stop and prevent the corrosion of the steel reinforcement. The efficiency of CP is usually monitored by the “24 h, 100 mV depolarization criterion,” a purely empirical criterion whose implementation is cost and labor intensive and that does not allow online control of CP. Within an extended research project on CP applied to concrete members of a highway bridge exposed to penetrating moisture, three sets of macrocells (MC), each composed of five MC sensors, were installed in conjunction with concrete resistance sensors and silver/silver chloride reference cells. Chloride profiles were determined from the cored or drilled powder originating from the installation of sensors and from drilling cores. Corrosion currents, steel potentials, and concrete resistance were monitored over a period of 1 year before, after installing the CP systems (which remained switched off for half a year for evaluating the effect of the conductive coating), and after start-up of the CP systems. The CP systems applied consist of a moisture resistant conductive coating. As expected CP has a pronounced effect on local MC currents: Anodic MC currents were reduced or changed into cathodic currents, whereas cathodic currents were only weakly influenced. By adjusting the applied protection current all anodic MC currents may be changed into cathodic MC currents. Results indicate that the 100 mV depolarization criterion is a conservative criterion in atmospherically exposed concrete; it is not reliable in strongly wetted concrete. There was no consistent correlation between the CP induced changes in the local MC currents and 24 h depolarization values indicating that large potential shifts induced by CP do not necessarily imply overprotection. Results show that monitoring MC currents before, during, and after CP operation allows to demonstrate in a transparent way the effect of CP on the corrosion of the steel reinforcement. Online monitoring of MC currents is proposed as a viable and comprehensible method to monitor and control the efficiency of CP.     

Korrosion und kathodischer Korrosionsschutz von unlegiertem Stahl in Sandböden

Corrosion and cathodic protection of unalloyed steel in sand soil Cathodic protection takes place if the potential criterion is satisfied. That means that the pipe-to-soil potential is less than the protection potential. This is to be controlled in the case of pipelines which are required to be inspected regularly by law. Protection potentials are described in technical standards (e.g. DIN 30676), and must be determined experimentally in doubtful cases. In general the protection potential is referred to as U = ?0.85 V for soils. In practical applications of this protection potential difficulties arise in the case of sand soils with a minute amount of salt and a high resistivity resulting from a very low humidity. These difficulties are related to a high protection current demand due to good aeration and a poor current distribution due to the high resistivity. On the other hand, sand soils are known to be less corrosive. Thus, one can think of using special protection potentials for sand soils which are more positive than ?0.85 V. The results of laboratory and field tests show that protection potentials in the range of ?0.75 V to ?0.65 V can be applied depending on soil resistivity. In these cases even micro cell action can be prevented.     

Early stage beneficial effects of cathodic protection in concrete structures

Over the last 25 years, cathodic protection (CP) of reinforced concrete structures suffering from chloride induced reinforcement corrosion has shown to be successful and durable. CP current causes steel polarisation, electrochemical reactions and ion transport in the concrete. CP systems are designed based on experience, which results in conservative designs and their performance is a matter of wait‐and‐see. CP systems can be designed for critical aspects and made more economical using numerical models for current and polarisation distribution. Previously, principles of numerical calculations for design of CP systems were reported. The results were satisfactory, except in terms of current density for active corroding systems. This was suggested to be due to neglecting beneficial effects of CP current flow. One of the beneficial effects is pH increase at the steel surface due to oxygen reduction. As the pH increases, the corrosion rate decreases and the current demand decreases. A simple model was set up for this transient process, suggesting that such effects should occur on the time scale of hours to days. This model was validated from start up data of a CP field trial system on part of a bridge. Field results confirmed the modelling proposed here.     

Investigations on the re-passivation of carbon steel in chloride containing concrete in consequence of cathodic polarisation

Although the complex changes at the steel–concrete interface due to cathodic polarisation are widely acknowledged to have a beneficial influence concerning the cathodic protection (CP) of steel in concrete, some questions concerning the repassivation of carbon steel in consequence of cathodic polarisation are still not satisfactorily clarified. In the recent literature, some indications are presented that repassivation occurs after a certain time of polarisation. Therefore, the investigations discussed in this paper aim to clarify, to what extent the re-passivation of carbon steel due to cathodic polarisation occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step, the corrosion state of five nominal equal test specimens was determined by electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarisation resistance, respectively, the specimens were polarised cathodically. Impedance data were recorded before, during and after polarisation. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and the impact of diffusion on the corrosion and polarisation behaviour. The results indicate that the reduction of oxides and oxygen diffusion during cathodic polarisation has strong impact on the systems behaviour and that repassivation effects occur after switching off the polarisation current and during depolarisation, respectively.     

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.     

Corrosion behavior of API 5L X‐60 pipeline steel exposed to near‐neutral pH soil simulating solution

Steel gas pipelines are exposed externally to damage by surface corrosion and cracking phenomena. They are the main deterioration mechanism under coating failure and cathodic protection (CP) that can reduce the structural integrity of buried gas transmission pipelines where the soil aggressiveness and bacterial activity appear. Corrosion phenomenon is accentuated by the soil parameters influence such resistivity, pH, temperature, moisture content and chemical composition of electrolytes contained in the soil. Soil parameters influence on pipeline steel corrosion behaviour exposed in near‐neutral pH soil simulating solution has been investigated by potentiodynamic polarisation and EIS method. Results showed that the steel corrosion increases, corrosion current density increases with temperature in the range from 20 to 60 °C. The associated activation energy has been determined. Impedance curves showed that the charge transfer resistance (R_t) increases with increasing immersion duration. Parameters such as corrosion current density (I_corr), polarization resistance (R_p), and soil resistivity (ρ) can serve as the parameters for evaluation of soil corrosivity.     

阴极保护在“引碧入连”供水工程中的应用

介绍了牺牲阳极阴极保护技术在大连市输水钢管上的应用情况,针对钢管管径粗达1820mm,土壤电阻率偏高,杂散电流的影响等具体工况,采取了有针对性的技术措施,电位测量结果证实了此项工程中阴极保护的合理、有效.     

酸性土壤中接地网牺牲阳极阴极保护法研究

目的提高牺牲阳极的阴极保护法在酸性土壤中对接地网的防腐能力,分析牺牲阳极阴极保护法在酸性土壤中应用的技术要点,总结保护效果优化措施。方法设计牺牲阳极模拟系统,模拟地网面积为3.52 m2,保护电流设计为35.2 m A,对Q235碳钢和镀锌钢两种常用接地材料的接地电阻、保护电位及保护电流进行研究。结果该方法对镀锌钢保护较好,保护电位均低于-0.95 V;对Q235碳钢保护较差,保护电位部分高于-750 m V,且波动较大,最大波幅可达201 m V。系统运行中,计算得出保护电流在降雨量较大时最高可达30.75 m A,降雨量较小时最低为11.89 m A,均低于设计值。结论由于阳极处砂石较多、土壤电阻率高,阳极不能完全释放电流。其次,土壤保水性差,电阻率波动大,系统运行不稳定也抑制了保护效果。酸性土壤盐基性离子大量淋失,土壤电阻率普遍较高,且受降雨扰动较大,牺牲阳极工作效率较低且稳定性差。需采用适当提高保护电流、降低阳极区土壤电阻率、优化阳极设计工艺参数等措施以达到良好的保护效果。     

The influence of iodate ion additions to the bath on the deposition of electroless nickel on mild steel

An alkaline hypophosphite bath (0.1?M nickel sulphate, 0.2?M sodium hypophosphite, 0.2?M sodium acetate and 0.1?M malic acid, adjusted to pH 5) was used to produce Ni–P coatings on uncoated and electroless nickel pre-plated mild steel. The deposition was monitored by open-circuit potential-time monitoring vs. a saturated calomel reference electrode and potentiostatic current–time monitoring together with anodic and cathodic polarisation. Classical mixed potential theory was applied to the polarisation data to calculate the effect of controlled iodate ion additions (0–1000?ppm) as an accelerator to the electrolyte on the plating rate. The mixed potential and deposition current density increased gradually with potassium iodate concentration. The use of electrochemical data allowed the optimum iodate additive concentration to be established using simple instrumentation.     

Effects of SRB on cathodic protection of Q235 steel in soils

The effects of sulfate reducing bacteria (SRB) on cathodic protection (CP) of the Q235 steel in the soils have been studied by bacterial analyses, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and energy‐dispersive X‐ray analysis (EDX). The results showed that the pH value of the soil around the steel gradually increased, the number of SRB and the corrosion rate of the steel decreased, and the CP efficiency increased with the increasing of applied cathodic potential. At the cathodic polarization potential of ?1050 mV, SRB still survived in the soils. At the same potential, the CP efficiency in the soil without SRB was higher than that with SRB, and the corrosion rate of the steel in the soil with SRB was much higher than that without SRB. The cathodic current density applied for the steel in the soil with SRB was bigger than that without SRB at the same cathodic potential.     

Electrochemical measurements of cathodic protection for reinforced concrete piles in a marine environment using embedded corrosion monitoring sensors

This study developed a sensor to monitor the corrosion of reinforced concrete structures. Concrete pile specimens with embedded sensors were used to obtain data on corrosion and cathodic protection for bridge columns in a real marine environment. Corrosion potential, cathodic protection current density, concrete resistivity, and the degree of depolarization potential were measured with the embedded sensors in concrete pile specimens. The cathodic protection (CP) state was accurately monitored by sensors installed in underwater, tidal, splash, and atmospheric zones. The protection potential measurements confirmed that the CP by Zn-mesh sacrificial anode was fairly effective in the marine pile environment. The protection current densities in the tidal, splash zones were 2–3 times higher than those in underwater and atmospheric zones. The concrete resistivity in the tidal and splash zones was decreased through the installation of both mortar-embedded Zn-mesh (sacrificial anode) and outside an FRP jacket (cover). Considering the CP, the cathodic prevention was more effective than cathodic protection.     

Local environment under simulated disbonded coating on steel pipelines in soil solution

Taking advantage of microelectrode technique, the local potential and pH in a crevice simulating disbonded coating on X70 pipeline steel were investigated as a function of cathodic protection (CP) in a near neutral pH soil bulk solution bubbled with 5% CO₂/N₂ gas. The experimental potential–pH (E–pH) diagrams were established for the steel in the crevice. Stress corrosion cracking (SCC) susceptibility of the steel in the local environment in the crevice was analyzed based on the experimental E–pH diagrams. The results showed that the local steel potential in the deep of the crevice was independent on CP potential applied at the opening. Due to the effect of the atmospheric CO₂, a near-neutral pH local environment promoting near-neutral pH SCC (also known as transgranular SCC, TGSCC) might be harbored in the crevice even with normal CP at the opening. During CP interruption, the steel potential decay and CO₂ absorption (pH decrease) might shift E–pH points into a susceptibility region of near-neutral pH SCC.     

Cathodic Protection of Steel Embedded in Porous Concrete

Abstract

The feasibility of applying cathodic protection to carbon steel strands embedded inporous concrete exposed to a chloride-containing environment was investigated. In-situ polarisation experiments were conducted on the embedded steel using a current-interrupt technique. This enabled the magnitude of the iR drop to be determined, and the actual (or corrected) potential of the embedded steel to be monitored throughout the experiments. The results suggested that, for the porous concretes which were examined, the minimum cathodic potential required to protect the embedded steel depended on the concentration of chlorides in the environment. These potentials are ?260 mV SHE (?578 mV CSE) for a 0·02 M NaCl environment and ?410 mV SHE (?728 mV CSE)for 0·7 M NaCl or sea water solutions.     

Mechanistic aspect of near-neutral pH stress corrosion cracking of pipelines under cathodic polarization

This paper investigates mechanistically stress corrosion cracking (SCC) of an X70 pipeline steel that is under cathodic protection (CP) in a near-neutral pH solution. It was found that there is a critical potential range, i.e., ?730 and ?920 mV_SCE, where the steel is in a non-equilibrium electrochemical state, and anodic dissolution (AD) reaction may occur when the steel is polarized cathodically. When the applied potential is more positive than this range, SCC is AD-based; while the applied potential is more negative, SCC of pipelines is under hydrogen embrittlement (HE) mechanism. When the polarization potential is within the range, SCC of the steel is under the combined effect of AD and HE. Therefore, AD may still occur on pipeline steel that is under CP with the potential within this critical range, contributing to the cracking process.     

Corrosion protection studies of CeO2–TiO2 nanocomposite coatings on mild steel

Nanocomposite coatings have evolved as corrosion-resistant materials to protect metals and alloys in various environments. The need for development of corrosion-resistant materials for mild steel in marine environment is still in demand. The CeO₂–TiO₂ nanocomposite powders were produced via hydrothermal synthesis and the corrosion resistance behaviour of the nanocomposite coatings were evaluated in 3.5% NaCl solution using Tafel polarisation and electrochemical impedance spectroscopy techniques. The trends of open-circuit potential curves provided clear evidence that the incorporation of CeO₂ in TiO₂ nanostructures is beneficial, as it introduces potential shift towards noble positive potential for nanocomposite coatings. Also, the corrosion resistance was enhanced with increase in the CeO₂ content in TiO₂ nanocomposite coatings. Almost 22 times decrease in the corrosion current densities of mild steel were attained for 15?wt-% CeO₂–TiO₂, which demonstrated the advantage of CeO₂–TiO₂ nanocomposite coatings for corrosion protection of mild steel.