Several questions about electrochemical rehabilitation methods for reinforced concrete structures

Electrochemical realkalisation (ERA) and electrochemical chloride removal (ECR) have shown their capacity to eliminate, in just a few weeks, the causes of corrosion in reinforced concrete structures (RCS), and for this reason are currently receiving very special attention. Nevertheless, the present state of the art has not progressed enough to dissipate doubts about the efficiency of ERA and ECR as electrochemical rehabilitation methods (ERM) for corroding RCS. A series of highly important questions persist, and continue to be cause of controversy among specialists in the construction sector, which can be summed up in one all-encompassing question, namely:

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Is removing the sources of corrosion in RCS sufficient to stop rusting?To obtain a response to this capital question, electrochemical corrosion analysis techniques are used to study the responses of clean and precorroded steel electrodes embedded in small mortar specimens without chloride additions and large mortar slabs with and without chloride contamination. According to the results obtained, ERM are efficient in delaying the start of corrosion if used preventively. However, if applied too late they do not assure the repassivation of rebars with high precorrosion levels and consequently are not useful.

     

Considerations on reproducibility of potential and corrosion rate measurements in reinforced concrete

This communication analyses information supplied by the electrochemical parameters related to rebar corrosion in reinforced concrete structures (RCS). Corrosion potential and corrosion current density are determined for different sets of conditions. Tests are performed to gather data on conflictive aspects of the two electrochemical parameters, with regard to evaluating the results of RCS inspections. Consideration is made of the limitations of potential measurements if not accompanied by complementary indications such as concrete resistivity, rebar surface area involved in the measurements, or the instrumentally determined corrosion current. The capacity of galvanostatic pulses applied directly on large RCS to offer a reliable guide to corrosion rate of steel reinforcements is discussed.     

On the effectiveness of realkalisation as a rehabilitation method for corroded reinforced concrete structures

Based on available experience with the use of electrochemical realkalisation (ERA), a fairly recent method for rehabilitating reinforced concrete structures (RCS), carbonated concrete can undoubtedly be realkalised. To the authors' minds, however, the following questions remain unanswered: (a) does ERA always effectively repassivates reinforcements?; (b) can ERA be considered an end or just a means to ensuring RCS durability?; and (c) what is the use of ERA if it cannot stop corrosion? This work was aimed at answering the previous, controversial questions. To this aim, the behaviour in a Ca(OH)₂ saturated solution, and in a sound uncarbonated mortar consisting of 1:3:0.5 cement, sand and water, of reinforcing bars from an RCS that failed through carbonation after 29 years of service life was examined.     

Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion

The durability of reinforced concrete structures becomes a matter of concern, due primarily to the increase of damage by the corrosion of steel reinforcements. This corrosion is not only related to the composition and to the procedure of concrete manufacturing (water/cement, sand/cement, etc.), but also to the aggressive agents as chlorides, carbon dioxide, etc. present in the surrounding medium (Cl⁻, CO₂, etc.). It is well known that the first kind of rebar corrosion (chloride) is more detrimental and that this process contains three basic components: chloride diffusion, electrochemical corrosion and concrete fracture. Therefore the early detection of possible degradation of structures by means of non-destructive testing is essential in order to ensure the functionality of these structures.

This paper presents the results of an experimental investigation on the use of acoustic emission during the corrosion of steel rebars embedded in mortar and immersed in sodium chloride solution. The process of corrosion is accelerated by various imposed potentials and is followed by acoustic emission coupled to electrochemical techniques. The experimental results show that electrochemical techniques can evaluate the corrosive character of the medium used. The acoustic emission showed an activity characteristic of the corrosion initiation phase and the corrosion propagation phase. Thus, it was significantly possible to highlight the acoustic signature of the concrete damage related to the porosity of the mortar and to chloride concentration. The results also show a perfect correlation between the evolution of the acoustic activity and the current of corrosion density.     

Electrochemical rehabilitation methods for reinforced concrete structures: advantages and pitfalls

Abstract

There are still many unresolved issues concerning the efficiency of electrochemical realkalisation and electrochemical chloride removal as electrochemical rehabilitation methods for corroding reinforced concrete structures. The present paper seeks to answer a number of questions which, though seemingly elementary, continue to arouse controversy in scientific, technical and economic communities, despite the vast amount of work that has been devoted to research on corrosion in concrete embedded steel, such as whether corrosion can be stopped once it has started, whether corroded reinforced concrete structures can be repassivated, and whether it is sufficient to remove the sources of corrosion in order to stop rusting. A discussion is conducted on the relationship between the prerusting grade of rebars and the possibility of their repassivation; on whether electrochemical rehabilitation methods treatments are efficient, and if so, when and on whether a simple potential measurement can determine the passive or active state of a rebar. For this purpose an analysis is made, using electrochemical, gravimetric and metallographic techniques, of the response of clean and previously corroded steel electrodes in a Ca(OH)₂ saturated solution with and without a sodium nitrite corrosion inhibitor and in cement mortar. The effectiveness of electrochemical realkalisation and electrochemical chloride removal for repassivating prerusted steel in concrete is found to depend heavily on the prerusting grade.     

Corrosion of Carbon Steel and Corrosion-Resistant Rebars in Concrete Structures Under Chloride Ion Attack

Corrosion of reinforced concrete is the most challenging durability problem that threatens reinforced concrete structures, especially structures that are subject to severe environmental conditions (i.e., highway bridges, marine structures, etc.). Corrosion of reinforcing steel leads to cracking and spalling of the concrete cover and billions of dollars are spent every year on repairing such damaged structures. New types of reinforcements have been developed to avoid these high-cost repairs. Thus, it is important to study the corrosion behavior of these new types of reinforcements and compare them to the traditional carbon steel reinforcements. This study aimed at characterizing the corrosion behavior of three competing reinforcing steels; conventional carbon steel, micro-composite steel (MMFX-2) and 316LN stainless steel, through experiments in carbonated and non-carbonated concrete exposed to chloride-laden environments. Synthetic pore water solutions have been used to simulate both cases of sound and carbonated concrete under chloride ions attack. A three-electrode corrosion cell is used for determining the corrosion characteristics and rates. Multiple electrochemical techniques were applied using a Gamry PC4? potentiostat manufactured by Gamry Instruments (Warminster, PA). DC corrosion measurements were applied on samples subjected to fixed chloride concentration in the solution.     

Embeddable reference electrodes for corrosion monitoring of reinforced concrete structures

Mixed‐metal oxide (MMO), graphite and laboratory‐made Ag/AgCl electrodes were electrochemically characterized to be used as reference electrodes embedded in concrete structures. Electrodes were studied in both, aqueous solutions of pH ranging from 7 to 13.5 and embedded into cement mortars; and the electrochemical studies were carried out in the absence and presence of chloride ions. Potential evolution, polarisation behaviour, galvanostatic pulse response and impedance characteristics of the electrodes were carried out in aqueous solutions. Besides, the electrochemical stability of the electrodes embedded in mortar was studied for an exposure period of two years. It was found that the MMO pseudo‐reference electrode is pH‐sensitive, the graphite pseudo‐reference electrode is oxygen sensitive and the Ag/AgCl pseudo‐reference electrode is chloride sensitive. Then, regarding the corrosion monitoring of steel rebars, care should be taken to avoid misleading interpretations of the corrosion potential measurements. However, any of them can be used to measure the corrosion rate of the rebars by means of traditional electrochemical techniques.     

Kritischer korrosionsauslösender Chloridgehalt – Untersuchungen an Mörtelelektroden in chloridhaltigen alkalischen Lösungen

Critical chloride content – Investigations of mortar electrodes in alkaline chloride solutions Although the penetration of chloride ions into the concrete is obstructed by numerous protective mechanisms, the concrete thus a complex multi‐barrier protective system represents, the chloride ions nevertheless succeed it to break through the protecting passive oxide layer of the steel, like the numerous chloride‐induced corrosion damage from building practice clearly document. In a first part results have been outlined concerning the investigations of the reduced corrosion system of steel/concrete pore solution [1]. The present article comprises results of investigations into the critical corrosion‐inducing chloride content that have been obtained by electrochemical investigations of mortar electrodes in alkaline chloride solutions. Finally the findings gained are explained with a view to the relevance they imply for building practice, and particularly for the design of new buildings to be constructed and the assessment of existing structures.     

Effect of nitrite in corrosion of reinforcing steel in neutral and acid solutions simulating the electrolytic environments of micropores of concrete in the propagation period

Inhibitors in concrete are usually used to be effective in alkaline or neutral solutions, however, when corroding by chlorides, there is a local acidification and therefore it is necessary to study the effect of the inhibitor in acid pH values. Measurements of the corrosion rate of corrugated steel bars have been carried out in solutions simulating electrolytic chloride environments in the micropores of concrete in the propagation period. It has been studied the effect of sodium nitrite as a corrosion inhibitor when added to the mentioned solutions. The solutions considered consisted of sodium chloride for neutral condition and ferrous chloride for acid condition. This is the soluble compound produced during the corrosion of steel as a result of chloride attack. Comparison of the results of polarization resistance with gravimetrically determined weight losses are presented. Also, results of electrochemical impedance spectroscopy are presented here. It has not been observed a significative improvement in using nitrite as inhibiting agent in these systems. The corrosion seems to be related to the [Cl⁻]/[OH⁻] ratio in three different regions of pH identified from acid to alkaline pH values.     

2-hydroxyethilammonium oleate protic ionic liquid as corrosion inhibitor for aluminum in neutral medium

Protic ionic liquid (PIL) 2-hydroxyethylammonium oleate (2HEAOl) proved to be a good lubricant for aluminum-forming processes. However, with the aim of keeping the formed component integrity, it is interesting that the same substance employed during forming does not need to be removed and works out as corrosion inhibitor. Then, the aim of this study was to test the performance of 2HEAOl as corrosion inhibitor for aluminum in neutral 0.5 mol/L NaCl medium by electrochemical characterization. Results showed that the concentration of 5 × 10⁻⁴ mol/L was a suitable concentration to promote corrosion inhibition until 72 h at the high chloride concentration studied. The PIL worked out as mixed-type organic corrosion inhibitor, as it promoted the diminution of the oxygen reduction reaction rate and, in consequence, the pit initiation by its adsorption on the metal surface.     

矿物掺合料对钢筋氯盐腐蚀行为的影响及其作用机理

用水泥提取液模拟混凝土孔隙液,用自腐蚀电位、动电位极化和电化学阻抗技术,研究单掺粉煤灰或矿粉、复掺粉煤灰和矿粉对混凝土中钢筋氯盐腐蚀行为的影响.结果表明,粉煤灰和矿粉均能有效降低钢筋受氯离子腐蚀的风险,减小腐蚀速率,并能不同程度地提高钢筋腐蚀的临界氯离子浓度.抑制氯离子腐蚀能力由强到弱依次为:矿粉>矿粉+粉煤灰>粉煤灰>水泥.且随氯离子浓度的增大,矿粉和粉煤灰的阻锈效率会有不同程度的下降.用矿粉和粉煤灰按适当比例复掺的方法来对抗氯离子腐蚀不仅经济,而且有不错的阻锈效果.     

Influence of N,N′‐dimethylaminoethanol as an inhibitor on the chloride threshold level for corrosion of steel reinforcement

The aim of this study is to examine the influence of N,N′‐dimethylaminoethanol (DMEA) as an inhibitor on the chloride threshold level for corrosion of steel in a concrete contaminated by chlorides. The experiment has been carried out in a saturated Ca(OH)₂ solution and chloride contaminated concrete containing different chloride and DMEA level. The critical point of corrosion onset is concluded by combining the open‐circuit potential (E_corr) with corrosion current (I_corr), which is decided by electrochemical impedance spectra (EIS) in the solution. Besides, the EIS has also been applied to determinate the chloride threshold level in the chloride contaminated concrete. It has been found that the presence of DMEA represented as an amino‐alcohol inhibitor, exerts little influence on the chloride threshold level for corrosion of steel in the solution. Similarly, the effect of the DMEA on the chloride threshold level in the chloride contaminated concrete, is also negligible.     

A study on the passive state stability of steel embedded in activated fly ash mortars

The main reason for the premature failure of reinforced concrete structures (RCS) is corrosion of the reinforcements. The use of new mortars based on activated fly ash, an alternative to conventional portland cement, requires extensive research in order to check its passivating properties for reinforcements and the instability or permanence of the passive state achieved. The present paper studies this objective, analysing the passivating capacity of specimens manufactured with three different fly ash mortars treated with three activators and the stability of the passive state under the effect of 0%, 0.2%, 0.4% and 2% chloride additions in relation to the binder material, in an environment of apparently constant humidity and in humidity cycles. The evolution with time of corrosion potential (E_corr) and polarisation resistance (R_p) was studied. Polarisation curves and time constants (τ) were also assessed for some specimens using short duration galvanostatic pulses.     

Untersuchungen zum kritischen korrosionsauslösenden Chloridgehalt von Stahlfasern in künstlicher Betonporenlösung

Investigations into the critical corrosion‐inducing chloride content of steel fibres in artificial concrete pore solution It is well known, that reinforcement steel in concrete is normally protected against corrosion due to the high pH‐value of the pore solution of the concrete. This alkalinity leads to a passive layer on the steel surface, which prevents further corrosion. The passive layer can be destroyed by chloride ions diffusing into the concrete. The concentration of chloride in the concrete which leads to a destruction of the passive layer and therefore to corrosion of the steel is defined as the critical chloride content. Investigations in artificial concrete pore solutions show that the critical chloride content of black steel is strongly dependent on the pH‐value of the solution: the higher the concentration of the OH⁻‐ions the higher the critical chloride content. For steel fibres earlier investigations have shown, that steel fibres do not corrode in concrete even at high chloride contents. Therefore it could be assumed, that the critical corrosion‐inducing chloride content of steel fibres in concrete is distinctly higher than of conventional reinforcing steel. To verify this assumption the corrosion‐inducing chloride content of steel fibres is investigated in artificial chloride‐containing concrete pore solutions at different pH‐values. 5 different types of steel fibres, 1 lashing wire and as reference 1 reinforcing steel are investigated at 3 different pH‐value ranges. The concentration of chloride within the pore solution is gradually increased in time steps of 12 h. The beginning of corrosion is determined by current as well as potential measurements. Furthermore additional investigations are carried out with intermediate products of the fibre production (steel wires with different diameters) to investigate if the critical chloride content of the wires is increasing gradually with decreasing diameter. The investigations show, that steel fibres in artificial chloride‐containing pore solutions indicate an distinctly increased resistance against chloride‐inducing corrosion compared with conventional reinforcing steel for high pH‐values. With decreasing diameter of wires the critical chloride content increases gradually.     

Corrosion‐technical properties of high‐strength stainless steels for the application in prestressed concrete structures

Experience with prestressed concrete over about half a century has indicated that the corrosion resistance of conventional prestressing steel does not always satisfy, especially the prestressing steels are susceptible to chloride attack (de‐icing salts) and hydrogen (hydrogen‐induced stress corrosion cracking). On the other hand corrosion agents, such as chloride, condensation water, can penetrate in the concrete and arrive at the surface of steels. Hence, corrosion damage of prestressing steels can happen and, in the extreme cases, the prestressed concrete structure collapsed resulting from the failure of the tendon. In this paper, consideration is made to use high‐strength stainless steels as prestressing tendon with bond in concrete. The high‐strength stainless steels of qualities 1.4301 (X5CrNi18‐10), 1.4401 (X5CrNiMo17‐12‐2), 1.4436 (X3CrNiMo17‐13‐3) and 1.4439 (X3CrNiMoN17‐13‐5) with sequence of increasing austenite stability were investigated. For application in prestressing tendon with bond in concrete the cold‐drawn high‐strength stainless steel of quality 1.4401 is an optimal proposition regarding its satisfactory resistance against pitting corrosion and stress corrosion cracking (SCC) in structure‐related corrosive conditions. The lower alloyed steel 1.4301 has an insufficient resistance against the chloride‐induced corrosion because of the lack of molybdenum and the content of deformation martensite due to the strong cold‐drawing of its unstable austenitic structure.     

A field study of critical chloride content in reinforced concrete with blended binder

This paper presents the results from the investigation of chloride‐induced reinforcement corrosion in concrete slabs after over 13 years exposure in the marine environment. In the beginning of 1990s over 40 reinforced concrete slabs with different types of binder and water/binder ratios were exposed in a marine environment at Swedish west coast. In this study a new rapid technique was used for non‐destructive measurement of corrosion. Based on the results from the non‐destructive measurement, the actual corrosion of steel bars in five concrete slabs was visually examined and the chloride profiles in the penetrating direction as well as at the cover level were measured. The results show that the visible corrosion normally occurred about 10–20 cm under the seawater level, where the oxygen may be sufficiently available for initiating the corrosion. It is also found that chloride may easily penetrate through a poor interface between concrete and mortar spacer and initiate an early corrosion. As a conclusion, although the chloride level 1% by mass of binder may not be the same as the conventionally defined threshold value, it can be taken as the critical level for significant on‐going corrosion that is visible by destructive visual examination, despite types of binder.     

An Electrochemical Evaluation of Possible Non-Chromate Conversion Coating Treatments for Electrodeposited Zinc-Nickel Alloys

A number of possible non-chromate conversion coating formulations have been studied to examine their efficacy as replacements for chromate-based coating treatments on electrodeposited zinc-nickel alloys. Data have been collected from both electrochemical and conventional salt fog corrosion tests. In the latter tests all the alternative non-chromate systems studied delayed the onset of the corrosion of the zinc-nickel coating itself but only the simple molybdate and the permanganatelmolybdate dual treatment brought about improvements in times to red rusting of the steel substrate. The electrochemical evaluations suggested that simple molybdate-treated surfaces act as only moderate barriers to the chloride environments. Of the two dual treatments, permanganatel molybdate behaved similarly to the simple molybdate treated surfaces in the electrochemical tests, but the molybdatel permanganate was more protective, although this was not exhibited in the salt fog corrosion tests.     

Characterisation of the steel concrete interface submitted to chloride‐induced‐corrosion

This paper deals with the characterisation by means of electrochemical, gravimetric and analytical methods of chloride‐induced‐corrosion behaviour of steel coupons embedded in chloride‐containing‐cement pastes. Corrosion rates were estimated from electrochemical measurements as well as gravimetric ones. They vary from 2.6 to 5.7 µm/year for 5 and 10 g/L chloride‐containing‐cement pastes. Analytical characterisations (including optical and electronical microscopy and Raman micro‐spectroscopy) showed that corrosion patterns are not depending on the chloride content of the cement paste (5 and 10 g/L chloride in the interstitial solution). A localised corrosion pattern composed of pits growing inside the metallic substratum, a corrosion products layer (CPL) and a transformed medium (TM) was pointed out. CPL can be divided into two sub‐layers (CPL1 and CPL2), characterised by the presence or absence of calcium coming from the cement matrix.     

Untersuchungen über die Lochfraßkorrosion des passiven Eisens in chlorionenhaltiger Schwefelsäure

Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions.     

Analysis of corrosion resistance behavior of inhibitors in concrete using electrochemical techniques

Reinforced concrete is one of the most durable and cost effective construction materials. However, in high chloride environments, it can suffer from corrosion due to chloride induced breakdown of the normal passive layer protecting the reinforcing steel bars inside concrete. One means of protecting embedded steel reinforcement from chloride induced corrosion is the addition of corrosion inhibiting admixtures. In the present investigation, various inhibitors such as sodium nitrite, zinc oxide, mono ethanol amine, diethanolamine, and triethanol amine have been used in concrete in different percentages. Their effectiveness was then studied using various electrochemical techniques such as rapid chloride ion penetration test, open circuit potential measurement, electrochemical impedance measurement, potentiodynamic polarization measurement, and gravimetric weight loss measurement. The results thus obtained indicate that the addition of inhibitors enhances the corrosion resistance properties.