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Ion chromatographic analysis of corrosion inhibitors in concrete
Construction and Building Materials 16 (2002) 73–81
Ion chromatographic analysis of corrosion inhibitors in concrete M.M. Pagea,*, C.L. Pagea, V.T. Ngalab, D.J. Ansticec b
a School of Civil Engineering, University of Leeds, Leeds LS2 9JT, UK W.S. Atkins Consultants Ltd., Auchinleck House, Fiveways, Birmingham B15 1DJ, UK c Maunsell Ltd., Imperial House, 31 Temple Street, Birmingham B2 5DB, UK
Received 23 April 2001; received in revised form 27 November 2001; accepted 9 January 2002
Abstract A method employing ion chromatography for identifying and quantifying ions present in the aqueous phase of concrete after surface treatment with corrosion inhibitors is reported in this paper. With this technique, a broad range of ions including ethanolamine, nitrite and monofluorophosphate, present in the inhibitors tested, could be identified in solution. When ethanolamine and nitrite were applied to the surface of concrete, they were readily detected in samples into which they had penetrated. In the case of monofluorophosphate-treated concrete, only the hydrolysis products, fluoride and phosphate were detected and not the monofluorophosphate ion itself. Furthermore, concentrations of other important ions, such as chloride, were also quantifiable by this technique. One of the main advantages of ion chromatography for this type of application is its ability to analyse a wide range of ions in a given sample. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ion chromatography; Corrosion inhibitors; Surface treatment; Reinforced concrete
1. Introduction In recent years, various corrosion inhibitors have been applied to the surface of reinforced concrete structures with a view to reducing the rate of corrosion of embedded steel to acceptable levels, in cases where carbonation or chloride contamination of the concrete has occurred w1x. In order to determine whether such substances actually reach the reinforcing bars and, if so, in what concentration, a reliable method is needed for analysing them. In this paper, a simple technique is described for detecting and determining concentrations of a range of ions present in commercially available inhibitors, their hydrolysis products and also other ions, which might affect their action. 2. Methods Three different substances used in solution as active components of corrosion inhibitors, viz. ethanolamine, nitrite and monofluorophosphate, were applied to laboratory-prepared concrete specimens, both carbonated and *Corresponding author. Fax: q44-113-343-2243. E-mail address: [email protected] (M.M. Page).
non-carbonated, and to a naturally carbonated reinforced concrete slab obtained from a 60-year-old power station in mid-Wales. In addition, solutions of inhibitors based on ethanolamine and monofluorophosphate were applied on site, by the suppliers, to the surface of a 25-year-old concrete structure (carbonated to a depth of between 10 and 15 mm), which was located at a power station in northeast England w2x. 2.1. Preparation of concrete specimens in the laboratory The laboratory-prepared concrete specimens were made using ordinary portland cement (OPC) of high waterycement ratios (0.65 and 0.8) to simulate poor quality concrete. The composition of the OPC is given in Table 1. The specimens were all 100=100=90 mm, and contained three mild steel bars of uniform diameter of 6.35 mm, at cover depths of 5, 12 and 20 mm from one face. Some were artificially carbonated by exposing them for several months to an atmosphere of 65% RH in a sealed tank, through which pure CO2 gas was passed for approximately 30 min twice a day, in order to stimulate carbonation-induced corrosion of the reinforcement. Others were contaminated by the addition of
0950-0618/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 2 . 0 0 0 1 7 - X
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
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Table 1 Chemical analysis of ordinary portland cement (OPC) Oxide:
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
Na2O
K2O
LOI
% of OPC by mass:
63.58
21.20
5.34
2.62
3.38
1.30
0.09
0.75
1.53
various quantities of analytical grade NaCl (0.3–2.4% chloride by weight of cement) to the concrete mix before casting in order to stimulate chloride-induced corrosion. In the case of the naturally carbonated reinforced concrete slab, core specimens, 75 mm in diameter with a depth of 110 mm, were obtained. One face of each core was cut such that the depth of cover to the reinforcing bars was approximately 12 mm. The sets of specimens were exposed to a wetting and drying regime which consisted of a period of immersion in deionized water for 48 h, followed by storage in air over saturated magnesium chloride solution (approx. 33% RH) for at least 28 days. The corrosion activity of the embedded steel bars was monitored as described elsewhere w3x. After 6 months, when the corrosion measurements were reproducible, inhibitor treatment was carried out.
solution (approx. 0.7 mol dmy3) were carried out by the supplier w2x. 2.3. Exposure conditions All specimens treated with inhibitors in the laboratory were subsequently stored over water in a tank for 14 days before being exposed for a further 18 months to the wetting and drying regime described above. The site-treated concrete structure was exposed to natural weather conditions in an unsheltered location on the coast in northeast England for 12 months (October 1998–October 1999), in the case of the ethanolaminebased inhibitor, and 15 months (July 1998–October 1999), in the case of sodium monofluorophosphate. 2.4. Extraction of inhibitors
2.2. Application of inhibitors Inhibitors were applied to the clean, dry concrete surface as recommended by the manufacturers. The following proprietary inhibitors were investigated: a an ethanolamine-based solution, found by means of ion chromatography to contain 2.3 mol dmy3 of ethanolamine and also 1.2 mol dmy3 of phosphate. This was applied five times to the concrete specimens, ensuring that the surfaces were dry before each application. In the case of the 25-year-old concrete structure in northeast England, the surface was cleaned thoroughly by scrubbing with a wire brush before application of the inhibitor by the supplier on site w2x; b a calcium nitrite-based inhibitor, found by ion chromatography to contain 2.7 mol dmy3 nitrite. This was applied three times with intervening periods of drying of approximately 4 h and then overlaid with 25 mm of a cement-based mortar (waterycement ratio of 0.4), containing a recommended dosage of 20 dm3 of approximately 6 M calcium nitrite solutiony m3 of concrete w3,4x; and c. sodium monofluorophosphate in aqueous solution applied 10 times with intervening periods of drying, followed by 10 applications of deionized water again with intervening periods of drying. In the case of laboratory-treated specimens, a 15% by weight solution (approx. 1 mol dmy3) was applied. For the sitetreated concrete, four applications of a 5% solution (approx. 0.35 mol dmy3) followed by six of a 10%
After exposure of the specimens, cylindrical cores, 75 mm in diameter, were taken so that one end of the cylinder was derived from the inhibitor-treated face. A small amount of water, designed to have minimal effect on sample composition, was added as a lubricant during coring. The cores were then ground so that the constituents of the concrete, in 2-mm-thick layers parallel to the treated surface, were collected and stored in sealed polythene bags. In the early stages of the research, the powdered layers were then dried at 105 8C for 24 h to remove evaporable water. In later work, however, the drying stage was omitted. This was considered preferable as it avoided possible loss of volatile inhibitors, such as ethanolamine, and, although it meant that evaporable water could not be eliminated, the latter generally did not constitute more than approximately 5–10% of the specimen weight. In practice, it was found that the results obtained for the ethanolamine-treated samples did not vary substantially irrespective of whether the drying stage was omitted. A 2-g sample was taken from each layer of powdered concrete and placed in a 100-cm3 beaker. Either 20 cm3 of 2% HNO3 with approximately 60cm3 of deionized water, in the case of some of the ethanolaminetreated specimens, or approximately 80cm3 of deionized water, in the case of all other specimens were added at 20 8C. The mixture was vigorously agitated and placed in an ultrasonic water-bath for 5 min, after which it was again agitated. After the concrete particles had settled, the extract was filtered through Whatman no. 3 filter paper into a 250-cm3 volumetric flask. The concrete
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
particles were washed with 80-cm3 portions of deionized water, shaken thoroughly, allowed to settle and the washings poured through the filter paper into the flask. This was repeated until there was approximately 250 cm3 of filtrate in the flask. The total volume was then made up to 250 cm3 with deionized water. 2.5. Extraction efficiencies In order to determine the proportion of inhibitor that could be extracted from concrete, specimens similar to those described above were cast with known quantities of inhibitor added to the concrete mix and cured for 1 month. Representative pieces of the specimens were ground, dried and 2-g samples treated, as described above, in order to extract the inhibitor. 2.6. Stability of sodium monofluorophosphate
a Previous work has indicated that sodium monofluorophosphate is liable to suffer hydrolysis at rates dependent on solution pH w5x. In order to test whether water (rather than acid) extraction was preferable for obtaining sodium monofluorophosphate samples from concrete, a series of preliminary experiments was undertaken to assess the effect of solution pH on sodium monofluorophosphate stability. Aqueous solutions were prepared containing 290 mg of sodium monofluorophosphate per cm3 (2 mM) at various pH values: (i) pH 13.7 (using NaOH) to simulate the pH of non-carbonated concrete; (ii) pH 9.3 (using NaOH) to simulate the pH of carbonated concrete; (iii) pH 7 (using deionized water); (iv) pH 1.6 (using HNO3) to reproduce the pH of the acid extracts made from concrete; and (v) pH 0.5 (using HNO3), to represent the concentration of acid added to powdered concrete samples before dilution with water. The quantities of the monofluorophosphate anion, PO3F2y, and its hydrolysis products, Fy and PO3y 4 , were monitored over a period of 40 days using ion chromatography as described in Section 2.7. b Two further experiments were performed in order to check whether the extraction procedure described in Section 2.4 affected the stability of sodium monofluorophosphate. It has been suggested that sodium monofluorophosphate may be unstable under conditions of high pH ()12) in the presence of excess Ca2q ions w6x. It seems unlikely that excess Ca2q ions account for the differences claimed to arise between laboratory specimens a few months old and site specimens of several years w6x, since it has generally been observed that the concentration of Ca2q ions in the pore solutions of a range of cements drops to only a few micromoles per cm3 within 1 day of curing, and becomes relatively constant soon
75
thereafter w7,8x. However, it was important to test whether possible exposure of sodium monofluorophosphate to Ca2q ions or any other species produced during the extraction procedure could be the reason for its absence from extracts. i. Ten samples of cement paste made from OPC with a waterycement ratio of 0.6 were cured for 7 weeks at 100% RH (3 weeks at 20 8C and 4 weeks at 38 8C), and then carbonated in 100% CO2 for 7 months at 65% RH. Five of the specimens were then placed in a 5 mm depth of 15% (approx. 1 mol dmy3) sodium monofluorophosphate solution and all specimens were stored at 100% RH. After 28 days, a 5-mm disc was sliced from the submerged end of each of the five specimens, which had been placed in sodium monofluorophosphate solution. These discs were combined and pore liquid expressed using a pore press as described elsewhere w8x. This avoided any possibility that sodium monofluorophosphate might be broken down as a consequence of the extraction procedure normally used (Section 2.4). The five specimens of carbonated cement paste, not treated with sodium monofluorophosphate, were similarly sliced and expressed to provide a control sample of pore solution. ii. Two different samples of concrete, one 15-yearold naturally carbonated and one non-carbonated, were ground to a powder. Various amounts (10 mg, 100 mg and 1 g) of solid sodium monofluorophosphate were added to 2-g samples of the powdered concretes. A replicate of each of these samples was dried in an oven at 105 8C for 24 h. The dry weights were recorded. To determine whether aspects of the extraction procedure, such as drying or exposure of the extract to powdered concrete, resulted in the breakdown of sodium monofluorophosphate, the dried and undried samples were subjected to the same method of extracting corrosion inhibitors as described in Section 2.4. The solvent used was deionized water. 2.7. Ion chromatography Solutions and pore liquids, obtained as described in Sections 2.4, 2.5 and 2.6, were diluted as necessary. Twenty-five microlitre samples were analysed quantitatively for inhibitor ions, their hydrolysis products and other interacting ions using a Dionex DX500 ion chromatography system fitted with a GP40 gradient pump, a self-regenerating suppression system and an ED40 electrochemical detector w9,10x. Samples of anions were analysed in auto-suppression recycle mode after being passed through IonPac AS14 analytical and AG14 guard columns using a solution containing 2 mmol dmy3
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
76
Fig. 1. Ion chromatogram showing conductivity peak of ethanolamine.
Na2CO3q2.5 mmol dmy3 NaHCO3 as eluent at a flow rate of 2 cm3 miny1. Cation samples were passed through IonPac CS14 analytical and CG14 guard columns. They were analysed in auto-suppression recycle mode using a 7.5 mmol dmy3 solution of H2SO4 as eluent, at a flow rate of 1 cm3 miny1. The conductivity profiles and concentrations of ions were recorded using PeakNet data handling software w11x. 3. Results and discussion All three inhibitors could be readily detected by means of ion chromatography (see Figs. 1–3). 3.1. Ethanolamine-based inhibitor Originally, the inhibitor was extracted using dilute nitric acid. Acids are commonly used for the extraction of substances from concrete w12x and do not adversely affect the stability of ethanolamine. Penetration profiles, obtained using ion chromatography, for ethanolamine in various concretes treated with ethanolamine in the lab-
oratory are shown in Fig. 4. Later work on concrete, surface-treated with ethanolamine on site, showed similar profiles for both acid-extracted and water-extracted samples, as shown in Fig. 5. Ion chromatography was also used to show that the recovery of ethanolamine from concrete, in which it had been incorporated in the original concrete mix, was approximately 45% at concentrations of ethanolamine up to 8 mgyg concrete. Ethanolamine therefore, is readily detectable by means of ion chromatography in extracts of concrete obtained using either acid or water as the solvent. It was shown to penetrate to more than 10 mm in all the concretes tested, with the greatest amount entering the laboratory non-carbonated concrete and the least found in site concrete (Figs. 4 and 5). Its effect on the corrosion of the reinforcing bars is discussed elsewhere w1x. A further application of ion chromatography was in the detection of other ions originally present in the commercially supplied ethanolamine-based solution. For example, the solution was found to contain 1.2 mol dmy3 PO3y ions and 1.2 mol dmy3 Kq ions, suggesting 4 the presence of KH2PO4 as well as ethanolamine. No
Fig. 2. Ion chromatogram showing conductivity peaks of a range of anions including nitrite.
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
77
Fig. 3. Ion chromatogram showing conductivity peaks of monofluorophosphate (with fluoride and phosphate). Table 2 Amounts of nitrite detected at 12-mm cover depth in various concretes Type of concrete
Concentration of nitrite (mgyg concrete)
Laboratory non-carbonated Laboratory carbonated Site carbonated
1.63 0.21 0.24
ions such as PO3y 4 , which are potential corrosion inhibitors w13x, in addition to the specified inhibitor. 3.2. Calcium nitrite-based inhibitor Fig. 4. Penetration profiles of ethanolamine in concrete surface-treated with ethanolamine in the laboratory.
Fig. 5. Penetration profiles of ethanolamine in concrete surface-treated with ethanolamine on site.
PO3y ions were detected in water-extracted samples of 4 inhibitor-treated concrete. In acid extracts, however, some PO3y was detected in the carbonated samples of 4 concrete but only within the first few millimetres and not to the level of the reinforcing bars. Thus, ion chromatography can be used to detect the presence of
In the cases of nitrite-based inhibitors, the use of acid in their extraction from concrete was not feasible as they are unstable at low pH, NOy 2 being converted to NOy 3 . Therefore, extraction of the calcium nitrite-based inhibitor was performed using deionized water. Extraction efficiencies in the region of 90%, using water as a solvent, have been obtained for NOy 2 incorporated into the concrete mix w14x. As in the case of ethanolamine, ion chromatography has shown that NOy 2 has penetrated to more than 10 mm in all the concrete specimens tested, with significantly greater amounts found in the laboratory noncarbonated (chloride-contaminated) specimens than in either the site or laboratory carbonated samples. For example, at 12 mm cover depth, the amounts of nitrite detected (in mgyg concrete) are shown in Table 2. The larger degree of penetration of nitrite into non-carbonated concrete might be due to the counter-diffusion of chloride and hydroxyl ions present at higher concentrations in this concrete. Ion chromatography can also be used to detect and quantify chloride ions present in the same samples (Fig. 6). This is a useful application since it has been y suggested that the ratio of NOy ions is important 2 yCl in determining the effectiveness of NOy 2 as an inhibitor
78
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
Fig. 6. Ratio of water-soluble nitrite to chloride ion concentrations in non-carbonated concrete.
Fig. 7. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 0.5 M NaOH.
Fig. 8. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 20 mM NaOH.
w15x. Further discussion of the effect of NOy 2 on the corrosion of embedded steel is given elsewhere w1,3x. 3.3. Sodium monofluorophosphate-based inhibitor
1. The results of experiments designed to test the stability of sodium monofluorophosphate under various
Fig. 9. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in deionized water.
Fig. 10. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 0.16% HNO3.
Fig. 11. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 2% HNO3.
conditions, as described in Section 2.6 (a) and (b), are discussed below. a In order to investigate the stability of sodium monofluorophosphate in solutions of different pH values, ion chromatography was carried out over a period of 40 days to detect the amounts of PO3F2y, Fy and PO43y ions. The results are shown
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81 Table 3 The amounts of monofluorophosphate detected in extracts compared with the amounts added to concrete powder State of carbonation of concrete
With or without drying of concrete powder
Conc. of PO3F2y added (ppm of extract)
Conc. of PO3F2y detected (ppm of extract)
Carbonated Carbonated Carbonated Carbonated Carbonated Carbonated Carbonated Carbonated
Undried Undried Undried Undried Dried Dried Dried Dried
0 29.9 272 2724 0 29.3 274 2724
0 22.8 277 2458 0 24.3 260 2783
Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated
Undried Undried Undried Undried Dried Dried Dried Dried
0 30 273 2724 0 30 274 2722
0 19.0 193 2767 0 14.0 167 2639
in Figs. 7–11. It may be seen that the PO3F2y ion remains relatively stable over 40 days in alkaline and neutral solutions (covering the pH range found in the concrete samples and in the solutions used for extraction), and indeed was still stable 1 year later. Under acid conditions such as those used in the extraction of ethanolamine, the sodium monofluorophosphate undergoes rapid hydrolysis at pH 0.5 (2% HNO3), the PO3F2y ion becoming undetectable after 4 days; at pH 1.6 (0.16% HNO3), hydrolysis is slower with approximately 19% of the original PO3F2y ion concentration remaining after 40 days. Thus, in order to avoid hydrolysis of sodium monofluorophosphate during extraction, water was used as the solvent. i. Analysis of pore liquid expressed from specimens of cement paste, which had been stored in sodium monofluorophosphate solution, revealed the presence of a little additional dissolved fluoride (5.1 mg Fy yg cement paste) compared with untreated specimens (1.7 mg Fy yg cement paste). However, neither PO3y 4 nor PO3F2y was detected in the pore solution. Since the extraction procedure described in Section 2.4 was not used to produce this pore liquid, the absence of sodium monofluorophosphate cannot be explained by its breakdown during the extraction procedure. ii. The amounts of PO3F2y extracted from the dried and undried samples of powdered concrete, with various quantities of PO3F2y added to them and subjected to the same method of
79
extracting corrosion inhibitors as described in Section 2.4, are shown in Table 3. The concentrations of PO3F2y detected in the solutions after extraction may be compared directly with the concentrations calculated from the quantities of PO3F2y added before extraction. In all cases where sodium monofluorophosphate was added to the concrete powders (carbonated and noncarbonated, dried and undried), the PO3F2y ion was clearly detectable in the extracts made from them. There was no detectable loss of sodium monofluorophosphate in carbonated samples. Although some loss was observed in non-carbonated samples, significant quantities of sodium monofluorophosphate were detectable even when the specimens were left in contact with water for more than a week. It therefore appears that the drying regime and the normal extraction procedure, which involved contact between powdered concrete and water for 24 h, are not the causes of failure to detect sodium monofluorophosphate in extracts made from sodium monofluorophosphate-treated concrete. 2. Although PO3F2y is stable at the pH of the concrete specimens tested, easily extractable when added directly to concrete powder (as shown in the previous section) and readily detectable by means of ion chromatography (see Fig. 3), no trace of the ion was found in the samples extracted, using water as the solvent, from any of the concrete specimens tested, which had been surface-treated with the compound. However, Fy, a hydrolysis product of PO3F2y, was readily detectable and shown to penetrate to depths in excess of 10 mm in both carbonated and noncarbonated concrete samples (Fig. 12). The other hydrolysis product of sodium monofluorophosphate, the PO3y ion, was also found to penetrate carbonated 4 concrete (Fig. 13), but was not detected in soluble form beyond 4 mm depth in non-carbonated concrete.
Fig. 12. Penetration profiles of water-soluble fluoride in concrete surface-treated with sodium monofluorophosphate.
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M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
4. Conclusions
Fig. 13. Penetration profiles of water-soluble phosphate in concrete surface-treated with sodium monofluorophosphate.
This might be due to the removal from solution of various low solubility phosphate-containing compounds, such as fluorapatite (Ca5(PO4)3F), under the highly alkaline conditions of non-carbonated concrete. Water-soluble PO3F2y and PO43y could not be detected in extracts of concrete made with up to 60 mg sodium monofluorophosphate per gram of cement incorporated into the original concrete mix. Fy was found to be present at concentrations equivalent to approximately 25% of that added as sodium monofluorophosphate. Thus, there is evidence for hydrolysis of sodium monofluorophosphate and significant penetration of water-soluble Fy and PO43y derived from sodium monofluorophosphate into carbonated concrete, and of Fy into non-carbonated concrete. Similar degrees of penetration of Fy and PO3y were observed for laboratory 4 and site-carbonated concrete samples treated in the laboratory. There was less penetration for laboratory non-carbonated concrete, in which the solubilities of various fluoride and phosphate-containing compounds are very low, and site-treated concrete, to which less sodium monofluorophosphate was applied. It therefore appears that sodium monofluorophosphate was not stable under any of the conditions of surface application investigated in the work reported here. Discussion of the effect of sodium monofluorophosphate on corrosion of the reinforcing bars is presented elsewhere w1x. It should be pointed out that ion chromatography has an advantage over a number of commonly used techniques, such as EDXA and XRF, which involve the identification of a constituent element of the inhibitor and not the complete ion. These latter methods may lead to erroneous conclusions about the presence of ions in concrete. For example, the detection of phosphorus has been taken to indicate that sodium monofluorophosphate is present w6,16,17x when any other phosphoruscontaining species, such as phosphate, might be the actual source of the element.
The research described in this paper has demonstrated that, when used with appropriate extraction techniques, ion chromatography provides a reliable means of characterising the distribution within concrete of certain soluble corrosion inhibitors applied in concrete repair systems. One of the main advantages of ion chromatography for this type of application is its ability to analyse a wide range of ions in a given sample. When ethanolamine and nitrite were applied to the surface of concrete, their penetration into concrete samples was readily detected by means of ion chromatography. In the case of monofluorophosphate-treated concrete, only the hydrolysis products, fluoride and phosphate, were detected in solution and not the monofluorophosphate ion itself. Further work is being undertaken to extend the ion chromatographic techniques to the analysis of a wider range of inhibitors. Acknowledgments Most of the work described in this paper was undertaken while the authors were based at Aston University. The research was supported by the EPSRC through a grant (GRyK 52638) under the Materials for Better Construction Programme, and by the Electricity Research Co-funding Scheme (ERCOS) with a contribution from British Energy plc. The authors wish to acknowledge this financial support, and are grateful to Dr S. Khan (British Energy) and Mr D. Warne (ERCOS) for their advice. References w1x Page CL, Ngala VT, Page MM. Corrosion inhibitors in concrete repair systems. Mag Concr Res 2000;52(1):25 –37. w2x Anstice DJ. Corrosion inhibitors for the rehabilitation of reinforced concrete. Ph.D. thesis. Aston University, 2000. w3x Ngala VT, Page CL, Page MM. Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 1: calcium nitrite. Corros Sci (in press). w4x Grace Construction Products. Application guide: ‘Postrite’. Cambridge, MA, USA: W.R. Grace & Co, 1994. w5x Van Wazer JR. Phosphorus and its compounds, vol. 1. New York: Interscience, 1958. w6x Raharinaivo A, Bouzanne M, Malric B. Influence on concrete ageing on the effectiveness of monofluorophosphate for mitigating the corrosion of embedded steel. Proc EuroCorr 97 1997;585 –90. w7x Diamond S. Effects of two Danish flyashes on alkali contents of pore solutions of cement–flyash pastes. Cem Concr Res 1981;11:383 –94. w8x Page CL, Vennesland Ø. Pore solution composition and chloride binding capacity of silica-fume cement pastes. Mater Struct 1983;16:19 –25.
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81 w9x GP50 gradient pump operator’s manual. Document no. 031377, Dionex Corporation, 1998. w10x ED40 electrochemical detector operator’s manual. Document no. 034855, Dionex Corporation. 1995. w11x PeakNet software user’s guide. Document no. 034914, Dionex Corporation, 1998. w12x British Standard: testing concrete — methods for analysis of hardened concrete. BS 1881-124, 1988. w13x Mayne JEO, Menter JW. The mechanism of inhibition of corrosion of iron by solutions of sodium phosphate, borate and carbonate. J Chem Soc 1954;103–107.
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w14x Jeknavorian AA, Chin D, Saidha L. Determination of a nitritebased corrosion inhibitor in plastic and hardened concrete. Cem Concr Aggregates 1995;17:48 –54. w15x Gaidis JM, Rosenberg AM. The inhibition of chloride-induced corrosion in reinforced concrete by calcium nitrite. Cem Concr Aggr 1987;9:30 –3. w16x Raharinaivo A. Action des monofluorophosphates sur la cor´ rosion des armatures dans le beton. Laboratoire Central des ´ Ponts et Chaussees: rapport DTyOAMyAR 81–96, 1996, 16 pp. w17x MFP technical information. Balvac, BICC Group, 1998, 8 pp.
Ion chromatographic analysis of corrosion inhibitors in concrete M.M. Pagea,*, C.L. Pagea, V.T. Ngalab, D.J. Ansticec b
a School of Civil Engineering, University of Leeds, Leeds LS2 9JT, UK W.S. Atkins Consultants Ltd., Auchinleck House, Fiveways, Birmingham B15 1DJ, UK c Maunsell Ltd., Imperial House, 31 Temple Street, Birmingham B2 5DB, UK
Received 23 April 2001; received in revised form 27 November 2001; accepted 9 January 2002
Abstract A method employing ion chromatography for identifying and quantifying ions present in the aqueous phase of concrete after surface treatment with corrosion inhibitors is reported in this paper. With this technique, a broad range of ions including ethanolamine, nitrite and monofluorophosphate, present in the inhibitors tested, could be identified in solution. When ethanolamine and nitrite were applied to the surface of concrete, they were readily detected in samples into which they had penetrated. In the case of monofluorophosphate-treated concrete, only the hydrolysis products, fluoride and phosphate were detected and not the monofluorophosphate ion itself. Furthermore, concentrations of other important ions, such as chloride, were also quantifiable by this technique. One of the main advantages of ion chromatography for this type of application is its ability to analyse a wide range of ions in a given sample. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ion chromatography; Corrosion inhibitors; Surface treatment; Reinforced concrete
1. Introduction In recent years, various corrosion inhibitors have been applied to the surface of reinforced concrete structures with a view to reducing the rate of corrosion of embedded steel to acceptable levels, in cases where carbonation or chloride contamination of the concrete has occurred w1x. In order to determine whether such substances actually reach the reinforcing bars and, if so, in what concentration, a reliable method is needed for analysing them. In this paper, a simple technique is described for detecting and determining concentrations of a range of ions present in commercially available inhibitors, their hydrolysis products and also other ions, which might affect their action. 2. Methods Three different substances used in solution as active components of corrosion inhibitors, viz. ethanolamine, nitrite and monofluorophosphate, were applied to laboratory-prepared concrete specimens, both carbonated and *Corresponding author. Fax: q44-113-343-2243. E-mail address: [email protected] (M.M. Page).
non-carbonated, and to a naturally carbonated reinforced concrete slab obtained from a 60-year-old power station in mid-Wales. In addition, solutions of inhibitors based on ethanolamine and monofluorophosphate were applied on site, by the suppliers, to the surface of a 25-year-old concrete structure (carbonated to a depth of between 10 and 15 mm), which was located at a power station in northeast England w2x. 2.1. Preparation of concrete specimens in the laboratory The laboratory-prepared concrete specimens were made using ordinary portland cement (OPC) of high waterycement ratios (0.65 and 0.8) to simulate poor quality concrete. The composition of the OPC is given in Table 1. The specimens were all 100=100=90 mm, and contained three mild steel bars of uniform diameter of 6.35 mm, at cover depths of 5, 12 and 20 mm from one face. Some were artificially carbonated by exposing them for several months to an atmosphere of 65% RH in a sealed tank, through which pure CO2 gas was passed for approximately 30 min twice a day, in order to stimulate carbonation-induced corrosion of the reinforcement. Others were contaminated by the addition of
0950-0618/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 2 . 0 0 0 1 7 - X
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
74
Table 1 Chemical analysis of ordinary portland cement (OPC) Oxide:
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
Na2O
K2O
LOI
% of OPC by mass:
63.58
21.20
5.34
2.62
3.38
1.30
0.09
0.75
1.53
various quantities of analytical grade NaCl (0.3–2.4% chloride by weight of cement) to the concrete mix before casting in order to stimulate chloride-induced corrosion. In the case of the naturally carbonated reinforced concrete slab, core specimens, 75 mm in diameter with a depth of 110 mm, were obtained. One face of each core was cut such that the depth of cover to the reinforcing bars was approximately 12 mm. The sets of specimens were exposed to a wetting and drying regime which consisted of a period of immersion in deionized water for 48 h, followed by storage in air over saturated magnesium chloride solution (approx. 33% RH) for at least 28 days. The corrosion activity of the embedded steel bars was monitored as described elsewhere w3x. After 6 months, when the corrosion measurements were reproducible, inhibitor treatment was carried out.
solution (approx. 0.7 mol dmy3) were carried out by the supplier w2x. 2.3. Exposure conditions All specimens treated with inhibitors in the laboratory were subsequently stored over water in a tank for 14 days before being exposed for a further 18 months to the wetting and drying regime described above. The site-treated concrete structure was exposed to natural weather conditions in an unsheltered location on the coast in northeast England for 12 months (October 1998–October 1999), in the case of the ethanolaminebased inhibitor, and 15 months (July 1998–October 1999), in the case of sodium monofluorophosphate. 2.4. Extraction of inhibitors
2.2. Application of inhibitors Inhibitors were applied to the clean, dry concrete surface as recommended by the manufacturers. The following proprietary inhibitors were investigated: a an ethanolamine-based solution, found by means of ion chromatography to contain 2.3 mol dmy3 of ethanolamine and also 1.2 mol dmy3 of phosphate. This was applied five times to the concrete specimens, ensuring that the surfaces were dry before each application. In the case of the 25-year-old concrete structure in northeast England, the surface was cleaned thoroughly by scrubbing with a wire brush before application of the inhibitor by the supplier on site w2x; b a calcium nitrite-based inhibitor, found by ion chromatography to contain 2.7 mol dmy3 nitrite. This was applied three times with intervening periods of drying of approximately 4 h and then overlaid with 25 mm of a cement-based mortar (waterycement ratio of 0.4), containing a recommended dosage of 20 dm3 of approximately 6 M calcium nitrite solutiony m3 of concrete w3,4x; and c. sodium monofluorophosphate in aqueous solution applied 10 times with intervening periods of drying, followed by 10 applications of deionized water again with intervening periods of drying. In the case of laboratory-treated specimens, a 15% by weight solution (approx. 1 mol dmy3) was applied. For the sitetreated concrete, four applications of a 5% solution (approx. 0.35 mol dmy3) followed by six of a 10%
After exposure of the specimens, cylindrical cores, 75 mm in diameter, were taken so that one end of the cylinder was derived from the inhibitor-treated face. A small amount of water, designed to have minimal effect on sample composition, was added as a lubricant during coring. The cores were then ground so that the constituents of the concrete, in 2-mm-thick layers parallel to the treated surface, were collected and stored in sealed polythene bags. In the early stages of the research, the powdered layers were then dried at 105 8C for 24 h to remove evaporable water. In later work, however, the drying stage was omitted. This was considered preferable as it avoided possible loss of volatile inhibitors, such as ethanolamine, and, although it meant that evaporable water could not be eliminated, the latter generally did not constitute more than approximately 5–10% of the specimen weight. In practice, it was found that the results obtained for the ethanolamine-treated samples did not vary substantially irrespective of whether the drying stage was omitted. A 2-g sample was taken from each layer of powdered concrete and placed in a 100-cm3 beaker. Either 20 cm3 of 2% HNO3 with approximately 60cm3 of deionized water, in the case of some of the ethanolaminetreated specimens, or approximately 80cm3 of deionized water, in the case of all other specimens were added at 20 8C. The mixture was vigorously agitated and placed in an ultrasonic water-bath for 5 min, after which it was again agitated. After the concrete particles had settled, the extract was filtered through Whatman no. 3 filter paper into a 250-cm3 volumetric flask. The concrete
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
particles were washed with 80-cm3 portions of deionized water, shaken thoroughly, allowed to settle and the washings poured through the filter paper into the flask. This was repeated until there was approximately 250 cm3 of filtrate in the flask. The total volume was then made up to 250 cm3 with deionized water. 2.5. Extraction efficiencies In order to determine the proportion of inhibitor that could be extracted from concrete, specimens similar to those described above were cast with known quantities of inhibitor added to the concrete mix and cured for 1 month. Representative pieces of the specimens were ground, dried and 2-g samples treated, as described above, in order to extract the inhibitor. 2.6. Stability of sodium monofluorophosphate
a Previous work has indicated that sodium monofluorophosphate is liable to suffer hydrolysis at rates dependent on solution pH w5x. In order to test whether water (rather than acid) extraction was preferable for obtaining sodium monofluorophosphate samples from concrete, a series of preliminary experiments was undertaken to assess the effect of solution pH on sodium monofluorophosphate stability. Aqueous solutions were prepared containing 290 mg of sodium monofluorophosphate per cm3 (2 mM) at various pH values: (i) pH 13.7 (using NaOH) to simulate the pH of non-carbonated concrete; (ii) pH 9.3 (using NaOH) to simulate the pH of carbonated concrete; (iii) pH 7 (using deionized water); (iv) pH 1.6 (using HNO3) to reproduce the pH of the acid extracts made from concrete; and (v) pH 0.5 (using HNO3), to represent the concentration of acid added to powdered concrete samples before dilution with water. The quantities of the monofluorophosphate anion, PO3F2y, and its hydrolysis products, Fy and PO3y 4 , were monitored over a period of 40 days using ion chromatography as described in Section 2.7. b Two further experiments were performed in order to check whether the extraction procedure described in Section 2.4 affected the stability of sodium monofluorophosphate. It has been suggested that sodium monofluorophosphate may be unstable under conditions of high pH ()12) in the presence of excess Ca2q ions w6x. It seems unlikely that excess Ca2q ions account for the differences claimed to arise between laboratory specimens a few months old and site specimens of several years w6x, since it has generally been observed that the concentration of Ca2q ions in the pore solutions of a range of cements drops to only a few micromoles per cm3 within 1 day of curing, and becomes relatively constant soon
75
thereafter w7,8x. However, it was important to test whether possible exposure of sodium monofluorophosphate to Ca2q ions or any other species produced during the extraction procedure could be the reason for its absence from extracts. i. Ten samples of cement paste made from OPC with a waterycement ratio of 0.6 were cured for 7 weeks at 100% RH (3 weeks at 20 8C and 4 weeks at 38 8C), and then carbonated in 100% CO2 for 7 months at 65% RH. Five of the specimens were then placed in a 5 mm depth of 15% (approx. 1 mol dmy3) sodium monofluorophosphate solution and all specimens were stored at 100% RH. After 28 days, a 5-mm disc was sliced from the submerged end of each of the five specimens, which had been placed in sodium monofluorophosphate solution. These discs were combined and pore liquid expressed using a pore press as described elsewhere w8x. This avoided any possibility that sodium monofluorophosphate might be broken down as a consequence of the extraction procedure normally used (Section 2.4). The five specimens of carbonated cement paste, not treated with sodium monofluorophosphate, were similarly sliced and expressed to provide a control sample of pore solution. ii. Two different samples of concrete, one 15-yearold naturally carbonated and one non-carbonated, were ground to a powder. Various amounts (10 mg, 100 mg and 1 g) of solid sodium monofluorophosphate were added to 2-g samples of the powdered concretes. A replicate of each of these samples was dried in an oven at 105 8C for 24 h. The dry weights were recorded. To determine whether aspects of the extraction procedure, such as drying or exposure of the extract to powdered concrete, resulted in the breakdown of sodium monofluorophosphate, the dried and undried samples were subjected to the same method of extracting corrosion inhibitors as described in Section 2.4. The solvent used was deionized water. 2.7. Ion chromatography Solutions and pore liquids, obtained as described in Sections 2.4, 2.5 and 2.6, were diluted as necessary. Twenty-five microlitre samples were analysed quantitatively for inhibitor ions, their hydrolysis products and other interacting ions using a Dionex DX500 ion chromatography system fitted with a GP40 gradient pump, a self-regenerating suppression system and an ED40 electrochemical detector w9,10x. Samples of anions were analysed in auto-suppression recycle mode after being passed through IonPac AS14 analytical and AG14 guard columns using a solution containing 2 mmol dmy3
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Fig. 1. Ion chromatogram showing conductivity peak of ethanolamine.
Na2CO3q2.5 mmol dmy3 NaHCO3 as eluent at a flow rate of 2 cm3 miny1. Cation samples were passed through IonPac CS14 analytical and CG14 guard columns. They were analysed in auto-suppression recycle mode using a 7.5 mmol dmy3 solution of H2SO4 as eluent, at a flow rate of 1 cm3 miny1. The conductivity profiles and concentrations of ions were recorded using PeakNet data handling software w11x. 3. Results and discussion All three inhibitors could be readily detected by means of ion chromatography (see Figs. 1–3). 3.1. Ethanolamine-based inhibitor Originally, the inhibitor was extracted using dilute nitric acid. Acids are commonly used for the extraction of substances from concrete w12x and do not adversely affect the stability of ethanolamine. Penetration profiles, obtained using ion chromatography, for ethanolamine in various concretes treated with ethanolamine in the lab-
oratory are shown in Fig. 4. Later work on concrete, surface-treated with ethanolamine on site, showed similar profiles for both acid-extracted and water-extracted samples, as shown in Fig. 5. Ion chromatography was also used to show that the recovery of ethanolamine from concrete, in which it had been incorporated in the original concrete mix, was approximately 45% at concentrations of ethanolamine up to 8 mgyg concrete. Ethanolamine therefore, is readily detectable by means of ion chromatography in extracts of concrete obtained using either acid or water as the solvent. It was shown to penetrate to more than 10 mm in all the concretes tested, with the greatest amount entering the laboratory non-carbonated concrete and the least found in site concrete (Figs. 4 and 5). Its effect on the corrosion of the reinforcing bars is discussed elsewhere w1x. A further application of ion chromatography was in the detection of other ions originally present in the commercially supplied ethanolamine-based solution. For example, the solution was found to contain 1.2 mol dmy3 PO3y ions and 1.2 mol dmy3 Kq ions, suggesting 4 the presence of KH2PO4 as well as ethanolamine. No
Fig. 2. Ion chromatogram showing conductivity peaks of a range of anions including nitrite.
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81
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Fig. 3. Ion chromatogram showing conductivity peaks of monofluorophosphate (with fluoride and phosphate). Table 2 Amounts of nitrite detected at 12-mm cover depth in various concretes Type of concrete
Concentration of nitrite (mgyg concrete)
Laboratory non-carbonated Laboratory carbonated Site carbonated
1.63 0.21 0.24
ions such as PO3y 4 , which are potential corrosion inhibitors w13x, in addition to the specified inhibitor. 3.2. Calcium nitrite-based inhibitor Fig. 4. Penetration profiles of ethanolamine in concrete surface-treated with ethanolamine in the laboratory.
Fig. 5. Penetration profiles of ethanolamine in concrete surface-treated with ethanolamine on site.
PO3y ions were detected in water-extracted samples of 4 inhibitor-treated concrete. In acid extracts, however, some PO3y was detected in the carbonated samples of 4 concrete but only within the first few millimetres and not to the level of the reinforcing bars. Thus, ion chromatography can be used to detect the presence of
In the cases of nitrite-based inhibitors, the use of acid in their extraction from concrete was not feasible as they are unstable at low pH, NOy 2 being converted to NOy 3 . Therefore, extraction of the calcium nitrite-based inhibitor was performed using deionized water. Extraction efficiencies in the region of 90%, using water as a solvent, have been obtained for NOy 2 incorporated into the concrete mix w14x. As in the case of ethanolamine, ion chromatography has shown that NOy 2 has penetrated to more than 10 mm in all the concrete specimens tested, with significantly greater amounts found in the laboratory noncarbonated (chloride-contaminated) specimens than in either the site or laboratory carbonated samples. For example, at 12 mm cover depth, the amounts of nitrite detected (in mgyg concrete) are shown in Table 2. The larger degree of penetration of nitrite into non-carbonated concrete might be due to the counter-diffusion of chloride and hydroxyl ions present at higher concentrations in this concrete. Ion chromatography can also be used to detect and quantify chloride ions present in the same samples (Fig. 6). This is a useful application since it has been y suggested that the ratio of NOy ions is important 2 yCl in determining the effectiveness of NOy 2 as an inhibitor
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Fig. 6. Ratio of water-soluble nitrite to chloride ion concentrations in non-carbonated concrete.
Fig. 7. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 0.5 M NaOH.
Fig. 8. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 20 mM NaOH.
w15x. Further discussion of the effect of NOy 2 on the corrosion of embedded steel is given elsewhere w1,3x. 3.3. Sodium monofluorophosphate-based inhibitor
1. The results of experiments designed to test the stability of sodium monofluorophosphate under various
Fig. 9. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in deionized water.
Fig. 10. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 0.16% HNO3.
Fig. 11. Changes in concentrations of monofluorophosphate, fluoride and phosphate ions with time in 2% HNO3.
conditions, as described in Section 2.6 (a) and (b), are discussed below. a In order to investigate the stability of sodium monofluorophosphate in solutions of different pH values, ion chromatography was carried out over a period of 40 days to detect the amounts of PO3F2y, Fy and PO43y ions. The results are shown
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81 Table 3 The amounts of monofluorophosphate detected in extracts compared with the amounts added to concrete powder State of carbonation of concrete
With or without drying of concrete powder
Conc. of PO3F2y added (ppm of extract)
Conc. of PO3F2y detected (ppm of extract)
Carbonated Carbonated Carbonated Carbonated Carbonated Carbonated Carbonated Carbonated
Undried Undried Undried Undried Dried Dried Dried Dried
0 29.9 272 2724 0 29.3 274 2724
0 22.8 277 2458 0 24.3 260 2783
Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated Non-carbonated
Undried Undried Undried Undried Dried Dried Dried Dried
0 30 273 2724 0 30 274 2722
0 19.0 193 2767 0 14.0 167 2639
in Figs. 7–11. It may be seen that the PO3F2y ion remains relatively stable over 40 days in alkaline and neutral solutions (covering the pH range found in the concrete samples and in the solutions used for extraction), and indeed was still stable 1 year later. Under acid conditions such as those used in the extraction of ethanolamine, the sodium monofluorophosphate undergoes rapid hydrolysis at pH 0.5 (2% HNO3), the PO3F2y ion becoming undetectable after 4 days; at pH 1.6 (0.16% HNO3), hydrolysis is slower with approximately 19% of the original PO3F2y ion concentration remaining after 40 days. Thus, in order to avoid hydrolysis of sodium monofluorophosphate during extraction, water was used as the solvent. i. Analysis of pore liquid expressed from specimens of cement paste, which had been stored in sodium monofluorophosphate solution, revealed the presence of a little additional dissolved fluoride (5.1 mg Fy yg cement paste) compared with untreated specimens (1.7 mg Fy yg cement paste). However, neither PO3y 4 nor PO3F2y was detected in the pore solution. Since the extraction procedure described in Section 2.4 was not used to produce this pore liquid, the absence of sodium monofluorophosphate cannot be explained by its breakdown during the extraction procedure. ii. The amounts of PO3F2y extracted from the dried and undried samples of powdered concrete, with various quantities of PO3F2y added to them and subjected to the same method of
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extracting corrosion inhibitors as described in Section 2.4, are shown in Table 3. The concentrations of PO3F2y detected in the solutions after extraction may be compared directly with the concentrations calculated from the quantities of PO3F2y added before extraction. In all cases where sodium monofluorophosphate was added to the concrete powders (carbonated and noncarbonated, dried and undried), the PO3F2y ion was clearly detectable in the extracts made from them. There was no detectable loss of sodium monofluorophosphate in carbonated samples. Although some loss was observed in non-carbonated samples, significant quantities of sodium monofluorophosphate were detectable even when the specimens were left in contact with water for more than a week. It therefore appears that the drying regime and the normal extraction procedure, which involved contact between powdered concrete and water for 24 h, are not the causes of failure to detect sodium monofluorophosphate in extracts made from sodium monofluorophosphate-treated concrete. 2. Although PO3F2y is stable at the pH of the concrete specimens tested, easily extractable when added directly to concrete powder (as shown in the previous section) and readily detectable by means of ion chromatography (see Fig. 3), no trace of the ion was found in the samples extracted, using water as the solvent, from any of the concrete specimens tested, which had been surface-treated with the compound. However, Fy, a hydrolysis product of PO3F2y, was readily detectable and shown to penetrate to depths in excess of 10 mm in both carbonated and noncarbonated concrete samples (Fig. 12). The other hydrolysis product of sodium monofluorophosphate, the PO3y ion, was also found to penetrate carbonated 4 concrete (Fig. 13), but was not detected in soluble form beyond 4 mm depth in non-carbonated concrete.
Fig. 12. Penetration profiles of water-soluble fluoride in concrete surface-treated with sodium monofluorophosphate.
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4. Conclusions
Fig. 13. Penetration profiles of water-soluble phosphate in concrete surface-treated with sodium monofluorophosphate.
This might be due to the removal from solution of various low solubility phosphate-containing compounds, such as fluorapatite (Ca5(PO4)3F), under the highly alkaline conditions of non-carbonated concrete. Water-soluble PO3F2y and PO43y could not be detected in extracts of concrete made with up to 60 mg sodium monofluorophosphate per gram of cement incorporated into the original concrete mix. Fy was found to be present at concentrations equivalent to approximately 25% of that added as sodium monofluorophosphate. Thus, there is evidence for hydrolysis of sodium monofluorophosphate and significant penetration of water-soluble Fy and PO43y derived from sodium monofluorophosphate into carbonated concrete, and of Fy into non-carbonated concrete. Similar degrees of penetration of Fy and PO3y were observed for laboratory 4 and site-carbonated concrete samples treated in the laboratory. There was less penetration for laboratory non-carbonated concrete, in which the solubilities of various fluoride and phosphate-containing compounds are very low, and site-treated concrete, to which less sodium monofluorophosphate was applied. It therefore appears that sodium monofluorophosphate was not stable under any of the conditions of surface application investigated in the work reported here. Discussion of the effect of sodium monofluorophosphate on corrosion of the reinforcing bars is presented elsewhere w1x. It should be pointed out that ion chromatography has an advantage over a number of commonly used techniques, such as EDXA and XRF, which involve the identification of a constituent element of the inhibitor and not the complete ion. These latter methods may lead to erroneous conclusions about the presence of ions in concrete. For example, the detection of phosphorus has been taken to indicate that sodium monofluorophosphate is present w6,16,17x when any other phosphoruscontaining species, such as phosphate, might be the actual source of the element.
The research described in this paper has demonstrated that, when used with appropriate extraction techniques, ion chromatography provides a reliable means of characterising the distribution within concrete of certain soluble corrosion inhibitors applied in concrete repair systems. One of the main advantages of ion chromatography for this type of application is its ability to analyse a wide range of ions in a given sample. When ethanolamine and nitrite were applied to the surface of concrete, their penetration into concrete samples was readily detected by means of ion chromatography. In the case of monofluorophosphate-treated concrete, only the hydrolysis products, fluoride and phosphate, were detected in solution and not the monofluorophosphate ion itself. Further work is being undertaken to extend the ion chromatographic techniques to the analysis of a wider range of inhibitors. Acknowledgments Most of the work described in this paper was undertaken while the authors were based at Aston University. The research was supported by the EPSRC through a grant (GRyK 52638) under the Materials for Better Construction Programme, and by the Electricity Research Co-funding Scheme (ERCOS) with a contribution from British Energy plc. The authors wish to acknowledge this financial support, and are grateful to Dr S. Khan (British Energy) and Mr D. Warne (ERCOS) for their advice. References w1x Page CL, Ngala VT, Page MM. Corrosion inhibitors in concrete repair systems. Mag Concr Res 2000;52(1):25 –37. w2x Anstice DJ. Corrosion inhibitors for the rehabilitation of reinforced concrete. Ph.D. thesis. Aston University, 2000. w3x Ngala VT, Page CL, Page MM. Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 1: calcium nitrite. Corros Sci (in press). w4x Grace Construction Products. Application guide: ‘Postrite’. Cambridge, MA, USA: W.R. Grace & Co, 1994. w5x Van Wazer JR. Phosphorus and its compounds, vol. 1. New York: Interscience, 1958. w6x Raharinaivo A, Bouzanne M, Malric B. Influence on concrete ageing on the effectiveness of monofluorophosphate for mitigating the corrosion of embedded steel. Proc EuroCorr 97 1997;585 –90. w7x Diamond S. Effects of two Danish flyashes on alkali contents of pore solutions of cement–flyash pastes. Cem Concr Res 1981;11:383 –94. w8x Page CL, Vennesland Ø. Pore solution composition and chloride binding capacity of silica-fume cement pastes. Mater Struct 1983;16:19 –25.
M.M. Page et al. / Construction and Building Materials 16 (2002) 73–81 w9x GP50 gradient pump operator’s manual. Document no. 031377, Dionex Corporation, 1998. w10x ED40 electrochemical detector operator’s manual. Document no. 034855, Dionex Corporation. 1995. w11x PeakNet software user’s guide. Document no. 034914, Dionex Corporation, 1998. w12x British Standard: testing concrete — methods for analysis of hardened concrete. BS 1881-124, 1988. w13x Mayne JEO, Menter JW. The mechanism of inhibition of corrosion of iron by solutions of sodium phosphate, borate and carbonate. J Chem Soc 1954;103–107.
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w14x Jeknavorian AA, Chin D, Saidha L. Determination of a nitritebased corrosion inhibitor in plastic and hardened concrete. Cem Concr Aggregates 1995;17:48 –54. w15x Gaidis JM, Rosenberg AM. The inhibition of chloride-induced corrosion in reinforced concrete by calcium nitrite. Cem Concr Aggr 1987;9:30 –3. w16x Raharinaivo A. Action des monofluorophosphates sur la cor´ rosion des armatures dans le beton. Laboratoire Central des ´ Ponts et Chaussees: rapport DTyOAMyAR 81–96, 1996, 16 pp. w17x MFP technical information. Balvac, BICC Group, 1998, 8 pp.