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.     

Numerical evaluation of the capacity of galvanic anode systems for patch repair of reinforced concrete structures

Cathodic protection (CP) is an electrochemical repair or corrosion prevention technique for steel structures exposed to a corrosive environment. For reinforced concrete (RC) usually impressed current CP is used, due to the comparably high resistivity of the concrete, serving as electrolyte. Nevertheless, the market provides a wide range of galvanic anode systems for RC structures. Their most common use is the application within the framework of partial concrete replacement due to chloride-induced corrosion. This patch repair is often accompanied by the so-called anode ring effect, causing accelerated corrosion of the rebar in the substrate concrete in the vicinity of repair patches. This is caused by the cathodic capabilities of the repassivated rebar. Galvanic anodes are reported to prevent this effect. In this paper, a numerical model is proposed, which is capable of determining the effectiveness of the method dependent on, for example, the type and quantity of anodes, rebar content, and geometry or climatic conditions. The method is presented for a specific set of input parameters and the applicability is discussed against the background of different protection criteria.     

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.     

碳化混凝土再碱化过程中钢筋在Ca（OH）2溶液中的电极过程

研究了碳化混凝土再碱化过程中钢筋在Ca(OH)2溶液中的电极表面电化学反应,采用电化学测量技术结合模拟试验考察了电极表面发生各类反应的可能性.结果表明:再碱化过程中,钢筋电极表面的电化学反应与钢筋表面的状态有密切关系,当电极表面存在锈蚀层时,再碱化过程析氢反应和氧化铁的还原同时发生,并使表面氧化层的价态逐渐降低,在电极表面形成金属铁,析氢过程产生的OH-使钢筋周围混凝土的碱性提高.     

Conventional and pulse cathodic protection of reinforced concrete: Electrochemical behavior of the steel reinforcement after corrosion and protection

The applicability and efficiency of an alternative for impressed current cathodic protection (CP) for reinforced concrete, based on pulse technology, was investigated. The technique, denoted as pulse CP (pCP), was evaluated on the basis of a comparative analysis to reference (non-corroding), corroding, and conventional CP conditions, in terms of long-term monitoring of electrochemical parameters for the embedded steel with time of corrosion and protection. The hereby reported results are for the total duration of the experiment, i.e., 460 days of conditioning, also presented in comparison with earlier stages. Protection was applied after corrosion was initiated (using corrosion medium of 5% NaCl), at different time intervals (here reported are starting points 60 and 150 days of age). Both CP regimes used current density of 5 or 20 mA/m² steel surface. The pulse CP was applied as a pulse-shaped block current (square wave) with the current itself being the feedback control signal, using 12.5–50% duty cycle at 500 Hz to 1 kHz frequency. Under equal environmental conditions and for a comparatively long period of application, the pulse CP was found to perform as effectively as the conventional CP with regard to electrochemical behavior of the steel reinforcement. Furthermore, the pulse CP technique was found to achieve more rapidly the so called “open circuit potential (OCP) passivity” as a result of an enhanced ion transport (chloride withdrawal) and more favorable cement chemistry (increased alkalinity around the steel reinforcement).     

Long-term factors influencing the adhesion of metallized zinc to concrete

Metallized zinc cathodic protection often is used to mitigate steel reinforcement corrosion in concrete. The zinc is flame or arc sprayed to the surface of the concrete structure to be protected. The metallized coating should be applied such that its adhesion to concrete is maximized. Factors believed to affect adhesion were investigated for pure zinc and a 85Zn-15Al alloy. On reinforced concrete samples polarized for more than 800 days at three different current densities, adhesion decreased. The reduction increased as a function of time and of the current density applied. When no current was applied, there was a small initial increase of adhesion with time. For the metallized samples that were freeze-thaw cycled, no loss of adhesion was found after 70 cycles.     

热镀锌层在饱和Ca(OH)2溶液中的腐蚀行为

热浸镀锌是保护混凝土钢筋的有效措施之一。用饱和Ca(OH)2溶液模拟混凝土孔隙液,研究热镀锌层在该模拟液中的腐蚀行为。采用电化学测试、SEM、EDS和XRD手段分别分析了试样在不同浸泡阶段的电化学参数、表面微观形貌以及腐蚀产物的化学成分和相组成,结果表明:在浸泡腐蚀过程中,锌晶粒的晶界处优先出现腐蚀小孔,然后在小孔附近开始生成锌酸钙晶体Ca[Zn(OH)3]2.2H2O,随着锌酸钙晶体不断生长,逐渐覆盖锌层表面,形成耐蚀性逐渐增强的保护层。当锌基底被锌酸钙晶体完全覆盖后,腐蚀电流密度Jcorr降至临界钝化值附近,锌层处于钝化状态。     

钢筋混凝土结构实施阴极保护的几个问题

1混凝土结构中钢筋腐蚀的原因 钢筋混凝土中钢筋的腐蚀是一个电化学过程,腐蚀的原因有内因和外因.内因:钢筋本身的不均匀性,如成分不均匀、钝化膜不连续、受应力作用不同等.外因:环境因素,如混凝土的物理化学性质不均匀性,介质中含氧、水、二氧化碳、各种盐类及不同pH等会在钢筋表面不同部位形成电位差;混凝土作为介质能在电位不同...     

Supercapacitor characteristic of La-doped Ni(OH)_2 prepared by electrodeposition

Samples of lanthanum-doped nickel hydroxide were prepared by electrodeposition method. The structure and electrochemical properties of the samples were studied by X-ray diffraction and a home-made open three-electrode cell system,respectively. The results show that the deposition process of Ni(OH)2 and La(OH)3 is mainly controlled by electrochemical polarization,which makes it easy to form uniform fine crystals. In addition,La(OH)3 is not a separate phase and lanthanum ions are doped into Ni(OH)2 crystal la...     

导管架平台外加电流阴极保护系统缩比模型模拟研究

本文采用搭建导管架平台缩比模型,对比研究了电流密度、辅助阳极数量和距离对远地式及拉伸式外加电流阴极保护系统保护效果的影响.结果表明远地式和拉伸式外加电流系统均可以实现对海洋平台的有效保护,屏蔽效应是导致保护电位分布不均匀的主要原因,增加阳极数量可以大幅提高平台保护电位分布的均匀性.由于拉伸式外加电流系统辅助阳极分布更均...     

Stray current-induced corrosion in cathodic protection installations of steel-reinforced concrete structures: FEM study of the critical parameters

A wide range of parameters was investigated by numerical calculations concerning their impact on DC stray current corrosion of reinforced concrete (RC) structures. A simplified model geometry was used to extract the relevant parameters and their interaction in terms of stray current-affected structures. This study mainly focuses on RC structures that are fitted with cathodic protection installations. The findings reveal a complex interaction between the investigated parameters. The possible relevance of further parameters, which is not the subject of this study, was emphasised.     

数值方法在阴极保护中的应用及进展

在阴极保护工程设计中,为了更准确地获得被保护体的电位分布,需要探索一种比传统设计方法更有效的方法来解决工程实际问题。随着计算机技术的兴起,数值计算方法开始被运用于阴极保护领域,极大地促进了阴极保护的发展。本文综述了数值方法在阴极保护中的应用状况、发展情况,进而对未来发展作了展望。     

Activity coefficient calculation model for NaAl（OH）4-NaOH-H2O system

Resorting to Debye-Hückel equation, the conception of the apparent dielectric constant ε' of sodium aluminate solution is introduced. By supposing that all the influences are attributed to it, ε' is successfully related to caustic ratio αK, concentration mNaoH, T and temperature T. Then an activity coefficient calculation model for NaAl (OH)4-NaOH-H2O system from 25 ℃ to 100 ℃ is established, which can be used in much wider αK and mNaoH, T ranges than those covered by the equilibrium solubility data.     

Thermal sprayed titanium anode for cathodic protection of reinforced concrete bridges

Stable operation of cobalt catalyzed thermal sprayed titanium anodes for cathodic protection (CP) of bridge reinforcing steel was maintained in accelerated tests for a period equivalent to 23 years service at Oregon Department of Transportation (Oregon DOT) bridge CP conditions with no evidence that operation would degrade with further aging. The cobalt catalyst dispersed into the concrete near the anodeconcrete interface with electrochemical aging to produce a more diffuse anode reaction zone. The titanium anode had a porous heterogeneous structure composed of α-titanium containing interstitial oxygen and nitrogen, and a fee phase thought to be Ti(O,N). Splat cooling rates were 10 to 150 K/s, and microstructures were produced by equilibrium processes at the splat solidification front. Nitrogen gas atomization during thermal spraying produced a coating with more uniform composition, less cracking, and lower resistivity than using air atomization.     

Inhibitory effects of 5-aminouracil on cathodic reactions of steels in saturated Ca(OH)2 solutions

The inhibitory effects of 5-aminouracil on cathodic reactions of steels were investigated using saturated Ca(OH)₂ solution to simulate the environment in pores in concrete. Polarization measurements showed that 5-aminouracil effectively inhibited these cathodic reactions, indicating the potential of 5-aminouracil as an inhibitor for macro-cell corrosion in concrete/soil systems, which are caused by cathodic reactions in concrete. Cathodic reactions were inhibited in accordance with the Langmuir adsorption isotherm. FT-IR and XPS measurements suggested that 5-aminouracil was adsorbed onto cathodic portions of the steel through the formation of a coordination bond with the amino group, mainly in the form of dimer.