1、 Item No. 24241 NACE International Publication 01110 This Technical Committee Report has been prepared by NACE International Task Group 356* on Reinforced Concrete: Stray-Current-Induced Corrosion. Stray-Current-Induced Corrosion in Reinforced and Prestressed Concrete Structures February 2010, NACE
2、International This NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, process
3、es, or procedures not included in this report. Nothing contained in this NACE International report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or
4、 protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circum
5、stances may negate the usefulness of this report in specific instances. NACE International assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE International report are responsible for reviewing appropriate health, safety, and regulatory documen
6、ts and for determining their applicability in relation to this report prior to its use. This NACE International report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred
7、 to within this report. Users of this NACE International report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulator
8、y requirements prior to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn
9、if more than 10 years old. Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1-281-228-6200). FOREWORD This techni
10、cal committee report reviews the corrosion of reinforcing and prestressing steel in concrete structures caused by stray currents. It provides information on the history of stray-current corrosion, the sources of stray currents, the mechanism of corrosion, the effects on structures, and detection and
11、 mitigation of stray-current-induced corrosion on steel in concrete. Also covered are measures taken during the design phase and modelling of the stray-current effects. The report is intended for use by designers of reinforced concrete (RC) structures, professionals dealing with electrochemical tech
12、niques (e.g., cathodic protection CP, realkalization, and electrochemical chloride removal), owners of structures with the potential for reinforcement corrosion caused by stray currents, owners of systems that could generate stray currents to concrete structures, and electrical engineers. Even thoug
13、h much of the information is applicable to metallic structures, this report focuses only on reinforced and prestressed concrete structures. _ *Chair Kalliopi K. Aligizaki, Aedificat Institute Freiburg, Freiburg, Germany. NACE International 2 The report makes reference to galvanic corrosion; however,
14、 corrosion because of galvanic coupling between reinforcing steel and other metals is not part of this report. This report was prepared by NACE Task Group (TG) 356, “Reinforced Concrete: Stray-Current-Induced Corrosion.” This TG is administered by Specific Technology Group (STG) 01, “Reinforced Conc
15、rete,” and is sponsored by STG 05, “Cathodic/Anodic Protection.” It is issued by NACE International under the auspices of STG 01. NACE technical committee reports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In many cases, they discuss specific appl
16、ications of corrosion mitigation technology, whether considered successful or not. Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration when applying this technology in the future. However, these statements are not intended to
17、be recommendations for general application of this technology, and must not be construed as such. INTRODUCTION State of Steel in Concrete Steel reinforcing bars embedded in chloride-free, high-quality concrete are in a passive condition with a negligible corrosion rate. The highly alkaline pore solu
18、tion in portland cement pastes (pH is greater than 12.5)1allows a stable, protective oxide film to form on the surface of the encased steel. The protective oxide film on the steel surface is not formed or can be destroyed if (1) the cement paste is not in contact with the reinforcing steel, such as
19、at voids and cracks; (2) alkalinity is lost by reaction with certain gases and liquids; or (3) excessive amounts of chloride or other aggressive ions are present in the vicinity of the steel. It has been shown2that chloride ion content as low as approximately 0.2 percent by mass of cement (or approx
20、imately 0.6 kg/m31 lb/yd3 of concrete, depending on the cement content of the mix) at the steel depth can initiate the corrosion process. If any of these conditions occurs, and both sufficient moisture and oxygen are in contact with the steel, an electrochemical cell forms, resulting in corrosion. C
21、orrosion most commonly proceeds by the formation of an electrochemical cell. This electrochemical cell is composed of four elements: (1) an anode; (2) a cathode; (3) an electrical connection between the two (steel); and (4) an ionic connection provided by an electrolyte (concrete pore solution).3,4A
22、 current flows from the anodic area through the electrolyte to the cathodic area and then through the metal to complete the circuit. The anodic area, which has the most negative potential, is the area that becomes corroded through loss of metal ions to the electrolyte. The cathodic area (to which el
23、ectrons flow) is protected from corrosion. If any one of the elements of the electrochemical cell is eliminated, corrosion can be prevented. Dissimilar metal couples and externally applied electric currents can also initiate or accelerate corrosion by forcing the current flow, resulting in galvanic
24、or electrolytic corrosion, respectively. Galvanic corrosion occurs when the current is self-generated, such as when dissimilar metals are in contact (galvanic coupling), when variations in conditions take place on a single metal surface, or when differences exist within the electrolyte. The galvanic
25、 cell creates an electrical energy source similar to that of a battery cell, capable of supplying current flow from a more active metal (anode) to a more noble metal (cathode) using a common electrolyte. This may occur when steel reinforcement is bonded to a grounding system using bare copper compon
26、ents. The principles of galvanic coupling and corrosion are used for CP of steel in concrete and are discussed in NACE Publication 01105.5Corrosion induced by galvanic coupling is outside the scope of this report. Electrolytic corrosion takes place when electric current from an outside source leaves
27、 the metal through the electrolyte. In electrolytic corrosion, metal corrodes where the current leaves the metal (the metal is the anode) but no corrosion occurs where the current enters the metal (the metal is the cathode).6,7This mechanism is discussed in detail in Principles of Stray-Current Corr
28、osion below. NACE International 3 Electrolytic corrosion can occur in concrete structures as a result of currents from external current sources leaving the reinforcing steel and is referred to as “stray-current” corrosion. Note that in the technical literature, the terms “electrolytic corrosion” or
29、“electrolysis” were used in early studies of externally imposed current on metal structures (and reinforcing steel) on site and in laboratory examinations of the phenomenon. In recent literature, the term “stray current” has been used for on-site studies of current from outside sources into structur
30、es, and is also the term used in this report. Stray Currents Stray currents are defined as electrical currents flowing through electrical paths other than the intended paths. Stray currents can arise from railways, CP systems, high-voltage power lines, or other sources (such as welding equipment). S
31、tray currents can deviate from their intended path because they find a lower resistance, parallel and alternative route to flow, for example, through metallic structures buried in the soil such as pipelines, tanks, and industrial and marine structures. Subway systems consist of many miles of steel-r
32、einforced concrete tunnel networks. Modern subway systems are designed to restrict the generation of stray current through the use of isolated track fasteners. However, some traction current might inevitably stray into the concrete electrolyte from the running rails in the vicinity of the load (tran
33、sit vehicle) and return to the running rails. Stray currents can be a serious problem for the corrosion engineer, not only for the damage they can cause, but also for the difficulties encountered in the solution of the problem. Stray currents may be direct currents (DC) or alternating currents (AC),
34、 depending on the source. Stray-current interference can result in localized corrosion of reinforcing bars where current leaves the steel and in hydrogen embrittlement of prestressing steel where current enters the steel if the potential is negative enough to generate hydrogen gas (see Polarity of S
35、teel below). Reinforcing steel in concrete is susceptible to stray current because concrete is an electrolyte, thus a conductive medium, that supports the pickup onto and subsequent discharge of stray current from the embedded steel. History of Stray-Current Corrosion The first reported scientific o
36、bservation of external current-induced corrosion was made by Sir H. Davy in 1812, who deduced from laboratory experiments that iron corrodes more rapidly when it is positively charged.8M. Faraday quantified Davys work, and in 1833 formed his laws (see Amount of Mass Reacted below) that express a pro
37、portion between the mass of metal reacted and the electric current applied. In 1887, the first serious case of stray-current corrosion was reported in the U.S. from a tramway system operating in Brooklyn affecting iron pipes.9Similar cases of stray-current corrosion caused by tramway operation were
38、reported in 1893 in Great Britain and in 1916 in Melbourne, Australia. The problems from stray-current corrosion became increasingly important around that time because of the rapid development of electric railway lines and the increase in traffic.10In 1904, the effects of stray currents resulting fr
39、om electric railways on corrosion of buried structures were reported in Germany,11and in 1910, the first guidelines came into effect for limiting stray currents from DC railways in order to protect gas and water pipes.12In 1906 and 1907, attention was given to the potential damage of RC structures c
40、aused by stray currents from electric railways and other power sources in the U.S. In 1910, the Bureau of Standards undertook a thorough investigation into the nature of corrosion of steel in concrete from the influence of stray currents.13It was reported that among the RC structures that came to th
41、e Bureaus attention, stray current had been a contributing cause of steel corrosion only in chloride-contaminated concrete. At low and moderate potentials and in the absence of chloride, steel corrosion caused by stray current was negligible, but with chloride present, rapid deterioration may be exp
42、ected even at low potentials.13NACE International 4 In 1975, the International Federation of Prestressed Concrete (FIP)(1)conducted an international investigation among its members asking for cases of stray-current-induced corrosion on concrete structures and what measures were taken against such ef
43、fects. From the 23 responses received,14four cases of damage caused by stray currents were reported in Western Europe, and one case that required further investigation. As a result of that study, in 1980 FIP prepared a report on the influence of stray currents on the durability of prestressed concre
44、te structures, and the measures taken to curb such effects.15Since then, corrosion of reinforcing steel by stray currents has received attention from corrosion engineers worldwide. Currently, international standards and recommendations for durable structures give guidance for eliminating the effects
45、 of stray currents to reinforced and prestressed concrete structures.16,17,18Appendix A provides selected examples of laboratory and on-site studies on corrosion of reinforcing and prestressing steel in concrete structures caused by stray currents. Amount of Mass Reacted The amount of metal mass rea
46、cting at the anode of an electrochemical cell as expressed by Faradays Law in Equation (1) is proportional to the current flowing, the duration of the current, and the electrochemical equivalent of the substance or substances concerned.3FntIm= (1) Where: m = mass reacted, (g) I = current, (A) t = ti
47、me, (s) = atomic weight (expressed in g) n = metal valence (number of equivalents exchanged) F = Faradays constant (= 96,485 C/equivalent) From Faradays law, a current of 1 A for one year consumes approximately 9 kg (20 lb) of steel (using = 55.8 g and n = 2). Although most corrosion reactions may h
48、ave a driving potential of hundreds of millivolts, stray-current corrosion may have driving potentials of tens of volts. Therefore, stray-current corrosion reactions can be typically 100 to 1,000 times faster than other forms of corrosion. However, it is well known that in cases of corrosion of rein
49、forcing steel by electric currents, the amount of corrosion is in many cases considerably less than the theoretical amount predicted by Faradays Law. In the case of concrete, actual steel corrosion is expected to be less than the corrosion predicted by Faradays Law because of other processes that occur at the steel/concrete interface.13,19,20DEFINITIONS Alkali-Silica Reaction: The reaction between the alkalies (sodium and potassium) in portland cement and certain siliceous rocks or minerals, such as opaline
