Re-alkalisation technology applied to corrosion damaged concrete

Transcription

1 Concrete Repair, Rehabilitation and Retrofitting II Alexander et al (eds) 2009 Taylor & Francis Group, London, ISBN Re-alkalisation technology applied to corrosion damaged concrete G.K. Glass FaberMaunsell, Birmingham, UK A.C. Roberts & N. Davison Concrete Preservation Technologies, Notts, UK ABSTRACT: This work examines the processes of steel corrosion initiation and arrest in chloride contaminated concrete. It is noted that the local production of acid is an essential feature in chloride induced corrosion damage. An acidification-realkalisation model of chloride induced corrosion has been developed to improve the explanation of some experimental observations. A simplified electrochemical treatment consisting of a hybrid of a pit re-alkalisation process to arrest corrosion followed by supplementary galvanic protection to maintain a high ph and steel passivity has been applied to concrete structures. Both the pit-realkalisation and supplementary galvanic treatments are delivered from a permanently installed sacrificial anode system. Risk management includes monitoring and a strategy to deal with any future risk of corrosion. An identified risk may be treated using a 2 week pit re-alkalisation treatment from a permanently installed sacrificial anode system with an impressed current connection. 1 INTRODUCTION Chloride induced corrosion is a major cause of damage to steel reinforced concrete structures. This work reviews the processes of steel corrosion initiation and arrest in chloride contaminated concrete and mechanisms and methods are improved. 2 CHLORIDE INDUCED CORROSION Chloride induced corrosion is often explained using a pitting potential model (Pedeferri 1996). In this model the presence of chloride affects the potential gradient that may be tolerated across the passive file before passive breakdown occurs. At high chloride contents passive film breakdown occurs more easily. The important factors in the model are potential, aggressive ion and inhibitor content. However at least equally as important as potential is the local ph reduction. Acid production is an essential feature if significant corrosion damage to passive steel is to occur (Szklarska-Smialowska 1986). Chloride induced corrosion damage of passive steel in concrete requires the local production of acid. This concept is not well appreciated for reinforced concrete because textbooks tend to distinguish chloride induced corrosion from carbonation induced corrosion by noting that chloride induced corrosion occurs despite the ph of the concrete cover remaining high. However a local ph reduction occurs in what is otherwise a highly alkaline concrete surrounding. ph values as low as 4 have been measured on corroding steel in what is otherwise a very alkaline concrete environment (Hartt & Nam 2008). Steel in concrete is normally in a highly alkaline environment where the steel passive film is stable (Pourbaix 1990). However passive films are not perfect. Chloride contamination combined with some local dissolution of iron through an imperfection results in a local ph reduction. Without the ph reduction re-passivation occurs. With the ph reduction, damage occurs. 3 SOLID PHASE EFFECTS It has been postulated that the calcium hydroxide phase in hydrated cement paste acts to inhibit corrosion initiation, improving the resistance to corrosion in chloride contaminated concrete (Page 1975, Sykes & Balkwill 1988, Yonezawa et al. 1988). It achieves this by inhibiting the reduction in ph associated with corrosion initiation. This introduces the concept that acid soluble solid phases participate in the process of corrosion initiation. 831

2 Resistance to ph reduction (mol/kg) Soluble Chloride ph 10.5 Resistance to ph reduction Figure 1. The resistance to a reduction in ph and soluble chloride content determined on chloride contaminated OPC concrete (Glass et al. 2000). Soluble Chloride (% acid soluble) ChlorideThreshold (% cement) OPC SRPC 30% PFA 65% GGBS Millscale on steel % Interfacial Voids Figure 2. Published relationship between interfacial voids and chloride threshold level (Glass & Reddy 2002). The model has been extended to include all solids with ph dependent dissolution behaviour (Glass et al. 2000). Figure 1 shows the results of acid neutralisation analysis carried out on a sample of chloride contaminated concrete. The resistance to acid neutralisation (quantity of acid per unit of ph reduction) shows that there are many phases that inhibit a reduction in ph. In addition, chloride is released as the ph reduces. Thus solid phases release both inhibitive and aggressive ions during the process of corrosion initiation. The concept that bound chloride (chloride removed from the pore solution by the cement paste) can participate in the process of corrosion initiation is controversial. However, an analysis of experimental data has provided strong evidence to support this (Glass & Buenfeld 1997). Bound chloride does not remain bound in acidic conditions and enters solution at surprisingly high ph values (Figure 1). One implication of this is that it provides a technical justification for using the ratio of the acid soluble chloride content to the cement content as an index of the corrosion risk in concrete. This counters the argument for using chloride to hydroxide pore solution concentration ratios (Sergi & Glass 2000). 4 PREVENTING CORROSION There are many known ways to improve concrete durability. These are usually associated with preventing chloride ingress by for example, using coatings or improving the barrier properties of the concrete cover. However, the above discussion has introduced the concept of solid phase corrosion inhibitors. Solid phases that release hydroxyl ions act to inhibit corrosion damage. This effect is most evident at voids at the steel concrete interface (Hartt & Nam 2008). Figure 2 shows the relationship between the percentage of interfacial voids and the chloride threshold level for corrosion initiation. When interfacial voids are not present high chloride threshold levels can be achieved. Indeed high chloride threshold levels are often achieved in well compacted laboratory concrete specimens (Glass et al. 2007). Electrochemical treatments result in the generation of hydroxide on a steel cathode and a significant increase in the tolerance of steel to the presence of chloride ions in concrete may be achieved using these treatments to increase the reservoir of hydroxide ions at the steel-concrete interface. Other potential methods of increasing the chloride threshold level include coating the steel with compounds that will promote the formation of inhibitive solid phases at the steel interface after the concrete is cast, and removing electrolyte from the pore solution. 5 CORROSION MODEL Figure 3 illustrates the processes occurring in a corrosion cell in concrete. Dissolving iron reacts with water to form iron hydroxides and hydrogen ions. The positively charged hydrogen ions are balanced by the presence of negatively charge chloride ions producing hydrochloric acid (Figure 3(a)). This stabilizes the local ph fall and promotes further dissolution of iron (Glass et al. 2006). The dissolution of iron at a corroding site (anode), is at least in part supported by the consumption of oxygen (oxygen reduction) at a location (the cathode) away from the anode. This process was modelled using a 2 dimensional numerical model that was governed by a description of the reactions occurring at the anode and the cathode as well as by ensuring conservation of current within the concrete (Glass et al. 2006). It was assumed that 5% of the steel was corroding and the concrete resistivity was 200 Ωm. The model predicted an average steel corrosion rate of approximately 15 ma/m 2, while the local steel corrosion rate at the anode was 300 ma/m 2. (1 ma/m 2 is 832

3 Concrete 200 Steel Potential (mv-sce) H 2 O HCl Fe Cl - Cl e - O OH - 2 Passive Film Open Circuit Steel Interface Concrete Surface Steel Distance (mm) Potential (mv vs SCE) Fe Fe 2 O Corrosion Fe 2+ Passive Fe 3 O Fe Immune Figure 4. Model of corrosion initiation and arrest showing the stability of iron and its corrosion products. ph Figure 3. An illustration of the processes occurring in a corrosion cell on steel in concrete (a) together with the potential field (contours at 50 mv intervals) between the concrete and steel surface (b) and the potentials on the steel and concrete surface (c) (Glass et al. 2006). equivalent to 1μm of steel section loss per year) The potential contours within the concrete cover are given in Figure 3(b) and the potential as a function of distance along the steel surface and along the concrete surface are given in Figure 3(c). The corroding anode on the steel shifted the potential of the adjacent steel cathode in the negative direction by more than 250 mv. In practice this also represents the next location on steel in concrete that starts to corrode. A model that has been used to describe chloride induced steel corrosion initiation is the pitting potential/repassivation potential model (Pedeferri 1996). In this model corrosion initiates because the steel potential rises above the pitting potential. Protection is achieved with a large negative potential shift that lowers the steel potential below the repassivation potential. However in Figure 3, the cathodic area of steel that was shifted in the negative direction by the largest amount was the most likely to corrode next. A new model that provides a better explanation of this observation is the acidification/pit realkalisation model (Glass et al. 2006). In this model, steel corrosion adjacent to the anode occurs because the local ph of the environment is lowered by the presence of the adjacent anode. Corrosion arrest is induced by raising the ph of the local environment. This acidification-realkalisation model is illustrated in Figure 4. Evidence supporting this model is presented in the cathodic protection data provided in Figure 5 Steel Potential (mv vs SCE) Elapsed Time (Hours) Figure 5. Potential decays determined on steel in a Portland cement concrete containing 3% chloride after various periods of cathodic protection (Glass et al. 2004). (Glass et al. 2004). This laboratory study showed that it is difficult to achieve large potential shifts on actively corroding steel using practical applied current densities in the short term. However after sufficient period of cathodic protection the open circuit potential of the steel shifts to more positive values indicating that the steel passive film had formed. Substantial steel potential shifts were then achieved. Inducing steel passivity was a prerequisite to achieving the potential shift. The primary factor inducing steel passivity is pit realkalisation. 6 ARRESTING CORROSION 47 days 27 days 4 days Prior to Treatment There are number of ways to arrest an active corrosion process. However, only electrochemical treatments are considered here. These are widely regarded as the most powerful techniques. 833

4 Electrochemical treatments include cathodic protection (CP) chloride extraction and re-alkalisation. CP is a permanent treatment and may be applied using impressed current of using sacrificial anodes. Sacrificial anode systems are not as powerful as impressed current anode systems, but are much simpler to install. Chloride extraction and realkalisation are temporary treatments but it is difficult to remove all the chloride. Another electrochemical treatment is to use a combination of a temporary treatment with galvanic protection. This is referred to a hybrid treatment. The temporary treatment is applied to arrest corrosion with the principle mechanism being pit realkasation. It will typically last less than 2 weeks. The galvanic treatment is provided as supplementary protection (Roberts et al. 2008). The commercialized hybrid treatment uses a sacrificial anode with an impressed current connection to deliver both the temporary and the galvanic treatments. A single anode system is installed for both processes. The absence of a permanent power supply makes the system much simpler and allows the components to be embedded within the concrete cover. Figure 6 shows the current delivered by one Duoguard anode to 0.25 m 2 of steel in a concrete block Current per anode (ma) Impressed current (pit re-alkalisation) Galvanic current (maintaining a high ph) Time (days) Figure 6. The current delivered by one anode in a hybrid treatment applied to 0.25 m 2 of steel in concrete containing 4% chloride by weight of cement. Steel Potential (mv vs SCE) Potential Decay After 60 days Potential Prior To Treatment (4% Cl - ) Elapsed Time (min) Figure 7. Steel potential prior to the hybrid treatment and the potential decay on interrupting the treatment after 60 days. containing 4% chloride by weight of cement in a dry laboratory environment. The current driven off the anode by a 12V power supply was approximately 50 ma (8500 ma/m 2 of anode surface) in this aggressive environment. In galvanic mode, the current decayed to approximately 1 ma (170 ma/m 2 of anode surface). The steel potential prior to applying the hybrid treatment and a steel potential decay measured 60 days after the application of the impressed current phase in the concrete specimen described above is shown in Figure 7. The open circuit potential of the steel shifted to significantly more positive values indicating that the corrosion was arrested by the treatment. 7 CORROSION RISK MANAGEMENT Corrosion risk may be assessed non-destructively using corrosion potential and corrosion rate measurements. Corrosion rates can be calculated from a potential shift and an applied current density. Figure 8 shows an example of the calculated corrosion rate as a function of the change in potential (potential shift) induced by an applied current density of 2 ma/m 2. Similar curves can be calculated for other applied current densities. Corrosion rates below 1 ma/m 2 represent a steel section loss of less than 1 mm every 0 years and are generally considered to represent passive steel. Corrosion rates rising above 2 ma/m 2 are considered to represent and increasing risk of localised corrosion activity. The potential of the galvanic anode steel couple and the measured corrosion rates recorded during the first year after the initial impressed current phase of a hybrid treatment applied to a bridge structure is shown in Figure 9. The bridge suffered from chloride 2 ) Corrosion Rate (ma/m Active Corrosion Rates Passive Corrosion Rates i corr iappl = Δ 2.3 E 2.3ΔE exp exp β c β a Passive Steel 2 ma/m Potential Shift (mv) Figure 8. The corrosion rate plotted as a function of potential shift and current density together with an example of its interpretation. 834

5 Potential (mv vs Ag/AgCl) P Steel corrosion rate ( ma/m²) Time (days) Figure 9. Measured potentials and corrosion rates in a bridge structure. Figure 10. Potential map 1 month after switching a hybrid system to galvanic mode in a sheltered concrete column. induced corrosion arising from the use of de-icing salts and approximately 300 m 2 of steel was treated. The data suggests that the steel is passive. The corrosion rates are negligible and the potentials are moving to more passive values. Another method of monitoring uses potential mapping. A detailed potential map obtained on a section of a concrete column containing galvanic anodes is shown in Figure 10. The potential data was obtained by measuring the potential of a manganese dioxide reference electrode on the concrete surface relative to the embedded steel on a 50 mm grid. The presence of strong anodes is indicated by strong peaks in the potential map and this indicates that the system is functioning. The absence of smaller peaks between the strong installed anodes indicates that there is negligible corrosion risk. The use of such a non-invasive potential mapping technique is more compatible with low maintenance electrochemical treatments. Risk management includes monitoring and a strategy to deal with any future risk of corrosion. Such a strategy may be included within the design of a hybrid electrochemical treatment by connecting the anodes to the steel at accessible locations. This allows the temporary impressed current treatment to be applied in the future using the existing anode system. This will typically involve the use of a temporary power supply for a period of 2 weeks. 8 CONCLUSIONS The local production of acid is an essential feature in chloride induced corrosion damage to reinforced concrete. Solid cement phases with ph dependent dissolution behaviour affect the process of corrosion initiation. Solid phase inhibitors release hydroxide to prevent corrosion damage. Bound chloride presents a corrosion risk. A reservoir of hydroxide may be formed at the concrete interface using electrochemical treatment. The acidification realkalisation model has been developed to improve the description of chloride induced corrosion of steel in concrete. It explains why corrosion spreads across a steel surface from a point of corrosion initiation to adjacent surfaces receiving a significant level of galvanic protection and why cathodic protection of actively corroding steel in concrete is associated with a positive shift in steel potential. A hybrid electrochemical treatment consisting of a pit realkalisation process to arrest corrosion followed by supplementary galvanic protection to maintain the steel passivity has been developed. Both the pit realkalisation and supplementary galvanic treatments are delivered from a permanently installed sacrificial anode system. The treatment has the power to arrest an aggressive corrosion process and simpler than impressed current technologies. Risk management includes monitoring and a strategy to deal with any future risk of corrosion. Corrosion risk may be assessed non-destructively using corrosion potential and corrosion rate measurements. An identified risk may be treated using a temporary pit realkalisation treatment from a permanently installed sacrificial anode system with an impressed current connection. REFERENCES Glass, G.K. & Reddy, B. 2002, The influence of the steel concrete interface on the risk of chloride induced corrosion initiation, COST 521: Final Single Project Reports, UK6, pp Glass, G.K. & Buenfeld, N.R. 1997, The presentation of the chloride threshold level for corrosion of steel in concrete, Corrosion Science, 39 pp Glass, G.K. & Reddy, B., Clark, L.A. 2007, Towards rendering steel-reinforced concrete immune to chloride induced corrosion Proceedings of the Institution of Civil Engineers, Construction Materials, 160(4), pp Glass, G.K., Reddy, B. & Buenfeld, N.R. 2000, The participation of bound chloride in passive film breakdown on steel in concrete, Corrosion Science, 42, pp Glass, G.K., Davison, N. & Roberts, A.C. 2006, Pit realkalisation and its role in the electrochemical repair of reinforced concrete, G.K. Journal of Corrosion Science and Engineering, Volume 8, Paper 10,

6 Glass, G.K., Roberts, A.C. & Davison, N. 2004, Achieving high chloride threshold levels on steel in concrete, Corrosion 2004, NACE, Paper No Hartt, W.H. & Nam, J. 2008, Effect of Cement Alkalinity and Concrete Microstructure Upon Chloride Threshold for Corrosion Initiation of Reinforcing Steel in Concrete, Corrosion 2008, Paper No Page, C.L. 1975, Mechanism of corrosion protection in reinforced concrete marine structures, Nature, 258(5535): pp Pedeferri, P. 1996, Cathodic protection and cathodic prevention, Construction and Building Materials, Vol 10(5), pp Pourbaix, M. 1990, Thermodynamics and Corrosion, Corrosion Science, 30 pp Roberts, A.C. Davison, N. & Glass, G.K Arresting and Preventing Corrosion of Steel in Concrete, Corrosion 2008, NACE, Paper No Sergi, G. & Glass, G.K. 2000, A method of ranking the aggressive nature of chloride contaminated concrete, Corrosion Science, 42(12), pp Sykes, J.M. & Balkwill, P.H. 1988, The use of synthetic environments for corrosion testing (eds. P.E.Francis and T.S.Lee), p. 255, ASTM STP 970 (1988). Szklarska-Smialowska, Z. 1986, Pitting corrosion of metals, National Association of Corrosion Engineers, Houston, Texas, pp Yonezawa, T., Ashworth, V. & Procter, R.P.M. 1988, Pore solution composition and chloride effects on the corrosion of steel in concrete, Corrosion, 44(7) pp