P R O T E C T I O N O F STEEL REBARS IN L I G H T W E I G H T C O N C R E T E W I T H THE USE O F CORROSION INHIBITORS G Batis G Grigoriadis National Technical University of Athens C A Meletiou TITAN Cement Co S.A. Greece A B S T R A C T . The use of corrosion inhibitors (CI) is a well-established method for protecting steel rebars against corrosion at almost all types of constructions, made of reinforced concrete. The scope of this study was to investigate the effect of CI on the durability of lightweight concrete made with pumice. Two types of inhibitors were used, based on calcium nitrite and aminoalcohols, respectively. Specimens made with the use of Cement Kiln Dust (CKD) as an additive, were also tested and their performance was compared to the respective one obtained with the use of inhibitors. It has been shown in previous investigations that, conventional concrete made with cement containing 6% CKD exhibited a reduced corrosion of steel rebars. All specimens were exposed to a severe, corrosive environment containing chlorides. The results indicated that, corrosion and carbonation rates were significantly reduced in all specimens treated with the two inhibitors. They also indicated that CKD performed remarkably well under highly corrosive conditions and, in some cases, provided better protection of rebars than the CI's. This last indication provides new potential for further utilization of CKD in cement and concrete, which may lead to considerable economic and environmental benefits. Keywords: Concrete, Durability, Corrosion, Carbonation, Inhibitors, Protection, Calcium nitrite, Aminoalcohols, Cement kiln Dust (CKD), Steel rebars D r George Batis is Associate Professor of National Technical University of Athens, Department of Chemical Engineering, Section of Materials Science and Engineering, Greece. Grigorios Grigoriadis is a PhD Student at Section of Materials Science and Engineering, Department of Chemical Engineering, NTUA, Greece. D r C A Meletiou is a Research Fellow in Concrete Technology Laboratory, Division of Research & Technology, Department of R&D, TITAN Cement Co., S.A., Greece. Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. 494 Batis, Grigoriadis, Meletiou INTRODUCTION Lightweight concrete (LC) is a widely known type of concrete with many applications in modern construction technology, such as insulating materials, prefabricated structural elements, wall panels, grouts, etc. The use of this type of concrete is often quite suitable, because reduces significantly the overall construction cost while, at the same time, provides satisfactory mechanical properties. The main disadvantage of LC is that performs poorly against corrosion. The use of porous aggregates (e.g. volcanic pumice) for the production of LC, results in a dramatic increase of the final porosity. Thus, the diffusion of both atmospheric 0 and C 0 through the open pores is increased, along with the absorption of other corrosive factors, mainly chlorides, due to capillary action. Consequently, reinforced LC exhibits higher corrosion and carbonation rates than normal concrete . 2 2 The most widely used method to protect steel rebars against corrosion, is the application of corrosion inhibitors (CI). Since their first use in 1978, their utilization in the construction of reinforced concrete is being continuously increasing. Nowadays, their use is considered to be one of the most preferable methods of corrosion protection, because it is easy to apply, it costs less than all the other methods with no maintenance required and increases significantly the service life of a typical construction - in some cases even up to 100%. In addition, treatment with CI, normally, does not affect the mechanical properties of the concrete, while loss of bonding between rebars and the concrete is avoided - a problem which frequently occurs when rebars are treated with epoxy coatings, Zn or PVC . In this investigation, the effectiveness of using CI in reinforced LC made with volcanic pumice is evaluated. Two types of inhibitors were used, an aqueous solution of calcium nitrite and an organic, nitrogenous solution based on aminoalcohols. Specimens were prepared by using type II cement and volcanic pumice at a high W/C ratio, in order to increase the already high porosity of the concrete and, thus, to accelerate the corrosion of the rebars. The experimental tests were carried out under a severe, corrosive environment containing chlorides at, approximately, sea-water concentration. For comparative reasons, the performance of Cement Kiln Dust (CKD) as a protective additive, was also investigated. This is a by-product of low-alkali cement manufacturing process, mainly containing calcium carbonate. Previous investigations [3, 4] have shown that CKD, can improve the durability of concrete by stabilizing pH at higher values (>11.9), due to its relatively high alkalinity, thus preventing the dissolution of the protective passive film surrounding the steel. Table 1 Chemical composition of cement, CKD and pumice, % COMPONENT Si0 A1 0 Fe 0 CaO MgO K 0 Na 0 S0 Others 2 2 2 2 2 3 3 3 CEMENT 1135 CKD PUMICE 27.38 9.10 5.65 45.39 2.73 0.94 0.56 2.71 7.71 13.68 4.36 2.30 42.59 1.23 0.79 0.28 0.10 34.67 70.55 12.24 0.89 2.36 0.10 4.21 3.49 0.03 6.13 Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Use of C o r r o s i o n I n h i b i t o r s 495 MATERIALS AND E X P E R I M E N T A L P R O C E D U R E The materials used in this investigation were: cement (type 1135), volcanic pumice from Yali island in SE Greece, as an aggregate and CKD. The chemical composition of these materials is shown in Table 1. Two types of corrosion inhibitors were used, one based on calcium nitrite (CN) and the other based on aminoalcohols (AM). Inhibitor AM was applied, both as an admixture (AMI) and as an impregnation (AM3) by surface spraying. The mix proportions for the preparation of all series of test specimens, are shown in Table 2. Each series was consisted of three prismatic specimens (80 mm 80 mm 100 mm) with four cylindrical steel bars (100 mm 12 mm) embedded in each one, as shown in Fig. 1. On each steel bar was properly attached a copper wire. The top surface of all specimens and the part of steel bars which protrudes over the concrete were coated with an epoxy glue to protect them from atmospheric corrosion. x x x The specimens were partially immersed in a 3% wt NaCl solution, up to a height of 25mm. After immersion, the following measurements were carried out at specified time intervals: Corrrosion half-cell potential, versus Saturated Calomel Electrode (SCE). Gravimetric mass loss of the rebars, according to ISO/DIS 8407.3 method. • Mean depth of carbonation, usingg phenolpthalein indicator (RILEM CPC-18 method). • Porosity after 9 months of exposure, using Hg-intrusion porosimetry method (MIP). # # Table 2 Mix proportions of all series of test specimens SERIES CODES MATERIALS H35 (g) Pumice (kg) Water (ml) CKD (g) Inhibitor CN (ml) Inhibitor AM (ml) Inhibitor AM (ml)* RF BD CN3 CN6 AMI AM3 AM3/1* 500 1.5 500 500 1.5 500 30 500 1.5 400 500 1.5 300 500 1.5 485 500 1.5 500 500 1.5 500 100 — — — — 200 — — — — — — — — — — — — — — — — — — 15 15 15 (*) Used externally, as impregnation by surface spraying, before immersing in NaCl solution. (**) The specimens were treated with inhibitor AM, after 1 month of exposure. MEASUREMENTS AND RESULTS C o r r o s i o n Potential M e a s u r e m e n t s Figures 2-5 present the measurements of corrosion potential versus time of exposure, for all series of specimens. Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. 496 Batis, Grigoriadis, Meletiou mm 100 mm 25 mm 20 mm "80mm" Figure 1 Specimens' shape and dimensions E X P O S U R E TIME, days Figure 2 Corrosion potential of CN specimens w u (/) > > •RF •AMI a 100 200 300 400 500 E X P O S U R E TIME, days Figure 3 Corrosion potential of A M I specimens Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Use of Corrosion Inhibitors 497 ' 0 100 200 300 400 500 E X P O S U R E T I M E , days Figure 4 Corrosion potential of AM3 specimens E X P O S U R E T I M E , days Figure 5 Corrosion potential of CKD specimens From the potential measurements shown in Fig. 2, it is clear that the reference exhibits a rapid decrease of potential to a level of -650 mV, which takes place within about 30 days of exposure. With the addition of the CN inhibitor at 3% wt of cement in the mixture, the time required to obtain this level of potential is extended to about 100 days, thus indicating the protecting effect which is provided. In the case where this inhibitor is added at a 6% wt of cement in the mixture, the reinforcement of steel remains in the passive state (> -350 mV) for up to 400 days as the results show in Fig. 2. It is important to note that, this extraordinary protection against corrosion obtained by the use of CN inhibitor at that % level, took place in spite of the fact that a number of cracks were present in the test specimens. These cracks may have been caused by the accelerating effect produced by the excessive level of NO2" in concrete (the recommended maximum addition level of CN inhibitor is about 3.5% wt of cement). The application of inhibitor AM, either as an admixture, or as surface impregnation, drives the corrosion potential to more electropositive values, as shown in Fig. 3, 4. This effect is more clearly shown in the case where AM is applied as impregnation by surface spraying 30 days after initial exposure (specimens AM3/1). The initial corrosion potential before the spraying is -650 mV, while under application the potential goes to -350 mV within the next 30 days, indicating the inhibiting effect. Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. 498 B a t i s , G r i g o r i a d i s , Meletiou The addition of 6% CKD in the mixture, presented similar performance with the reference, as far as corrosion potential is concerned. This performance can be seen in Figure 5. M a s s Loss A n d C a r b o n a t i o n Results From the test measurements obtained after the initial 9 months of exposure, it can be seen that, relative to the reference, either the CI or CKD resulted in lower % mass loss of steel and depth of carbonation, as shown in Figures 6-7. It is noted that, significantly lower % mass loss and depth of carbonation were obtained with the addition of CN inhibitor at 6% wt. of cement in the mixture. However, the lowest depth of carbonation was obtained in the specimens made with the addition of 6% CKD, probably due to the relatively higher alkalinity, as compared to the other additions. The above performance was also observed throughout the total test duration of 15 months. It is important to note that, the specimens made with the addition of CKD exhibited the best overall performance throughout the total test duration. 2,50 tn 2,00 s eg • 9 months 1,50 • 12 months • 15 months 0,00 RF BD CN3 CN6 AMI AM3 AM3/1 S E R I E S OF S P E C I M E N S Figure 6 Mass loss of the rebars, % •9 months m 12 m o n t h s d 15 months CN3 CN6 AMI AM3 AM3/1 S E R I E S OF S P E C I M E N S Figure 7 Carbonation depth of all specimens Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Use of Corrosion Inhibitors 499 Porosity Measurements Porosity measurements show that all series of test specimens have high porosity values, between 34.0 to 38.0. DISCUSSION The experiments indicated that all the additions used in this investigation provided protection against corrosion in all specimens made of lightweight concrete with pumice. The use of anodic inhibitors (such as calcium nitrite) and anodic-cathodic ones (such as aminoalcohols) at a rate of 3% appears to provide about the same degree of protection. At this rate of addition, the CI exhibited a level of corrosion, similar to that normally obtained in conventional reinforced concrete made without the use of any inhibitors for corrosion protection. The use of CN inhibitor at the increased rate of 6% is prohibited, due to the observed crack formation. Comments On CKD Anti-corrosion Action On the other hand, the addition of 6% CKD, with the given composition, surprisingly exhibited the best protection against corrosion, although this material is not classified as an inhibitor. The protective action against corrosion of the CKD addition, is mainly attributed to its fineness and relatively higher alkalinity, which cause a reduction of total porosity and carbonation, respectively, as shown in previous investigations [3,4]. The effect of nitrite ions The inhibiting effect of nitrite ions on steel corrosion induced by chlorides has been reported by other investigations. Several hypotheses have been formulated, concerning steel passivation by nitrite ions. One among them, suggests that nitrite ions are easily oxidized in alkaline environments, consuming oxygen according to the following total reaction : 2N0 +0 2 >2N0 2 3 Another hypothesis [6, 7] suggests that nitrite ions are sacrificially reduced to nitrogen with a simultaneous oxidation of ferrous to ferric ion, according to the following total reaction: 6Fe(OH) 2 + 2N0 2 + 3H 0 2 > 6Fe(OH) 3 + N + OFT 2 The formation of iron oxyhydroxide or other similar products, would decrease the formation of ferrous or ferric chlorocomplexes and, consequently, would supress iron anodic dissolution. According to the most universally accepted hypothesis [8, 12], a stable passive layer could be formed on the reinforcement, avoiding all the metastable intermediate forms, according to the following reaction: 2+ 2Fe + 20H +2N0 ~ 2 ->2NOt+2y-FeOOH Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. 500 Batis, G r i g o r i a d i s , Meletiou Thus, the nitrite ions compete with chlorides for the ferrous ions at the anode. Nitrite ions must be present in sufficient quantities and come in immediate contact, by diffusion, with the structure in order to provide protection. T h e Use O f Amino-alcohols As C o r r o s i o n I n h i b i t o r s Amines and alkanolamines and their salts with organic and inorganic acids, have been described and patented for various applications, such as for the protection of steel in cementitious matrices [5, 9]. The effectiveness of this class of compounds, has been shown by means of various experiments [6, 7, 10]. The new technology of spraying CI can be used in lightweight concrete. Earlier experiments [8, 11] have shown that aminoalcohols chemisorbs from aqueous solutions onto oxidized steel surfaces. The thickness and the composition of the absorbate phase, depend upon the composition and the concentration of the aminoalcohols solutions. The absorbate phase is formed even if chlorides are present. The corrosion inhibition effect is explained by the fact that aminoalcohols displaces, due to its strong bonding, ionic species from the oxidized steel surface, in particular chlorides which cause corrosion and forms a durable passivating film . CONCLUSIONS Based on all the measurements and results obtained in this investigation, the following main conclusions can be drawn: • • • • Corrosion inhibitors based on CN and AM can be effectively used for corrosion protection of lightweight concrete made with pumice Inhibitors applied as admixture exhibited better corrosion protection than as an impregnation by surface spraying on lightweight concrete The use of 6% CKD as an addition in lightweight concrete, provides a significant level of protection against chloride-induced corrosion The possibility of using CKD for protecting against corrosion constructions made of lightweight concrete, might have significant economic and environmental consequences. REFERENCES 1. BATIS, G., AIDINI, A., LOUVARIS, G., NICOLAIDES, A., «Reinforcement Corrosion in Pumice Lightweight Concrete)), Concrete 2000, Dundee U.K., 1993, ppl53-162. 2. NUERBERGER, U., «Korrosionsschutz im Massivebau)), Expert Verlag, Boebligen, 1992, pp 93-102. 3. BATIS, G., KATSIAMBOULAS, A., MELETIOU, C.A., CHANIOTAKIS, E., «Durability of Reinforced Concrete Made with Composite Cement Containing Kiln Dust)), Concrete in the Service of the Mankind, Vol. 1, Edited R.K. Dhir and T.D. Dyer, 1996, pp 67-72. 4. TASIOS, TH., ALIGIZAKI, K., «Durability of Reinforced Concrete)), Athens, 1993. Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved. Use of C o r r o s i o n I n h i b i t o r s 501 5. JUSTNES, H., NYGAARD, E.C., «The Influence of Technical Calcium Nitrate Additions on the Chloride Binding Capacity of Cement and the Rate of Chloride Induced Corrosion of Steel Embedded in Mortars», Corrosion and Corrosion Protection of Steel in Concrete, Edited R.N. Swamy, Vol II, 1994, pp 491-502. 6. SAGOE-GRENTSIL, K.K., YILMAZ, V.T., GLASSER, F.P., «Properties of Inorganic Corrosion Inhibition Admixtures in Steel-Containing OPC Mortars. Part 1: Chemical Properties)), Advances in Cement Research, Vol. 4, No 15, 1991/1992, pp 91-96 7. YILMAZ, V.T., SAGOE-CRENTSIL, K.K., GLASSER, F.P., «Properties of Inorganic Corrosion Inhibition Admixtures in Steel Containing OPC Mortars. Part 2: Electrochemical Properties)), ibit, pp 97-102. 8. PROWELL, B.D., WEYERS, R.E., AL-QUADI, I.L., «Evaluation of Corrosion Inhibitors for the Rehabilitation of RC Structures)), Concrete 2000, Edited R.K. Dhir, M.R. Jones, Dundee, UK, 1993, ppl223-1238. 9. MAEDER U., «A New Class of Corrosion Inhibitors)), Corrosion and Corrosio Protection of Steel in Concrete, Vol II, Edited R.N. Swamy, 1994, pp851-864. 10. MAEDER, U., «A New Class of Corrosion Inhibitors for Reinforced Concrete)), Third CAMNET/ACI Int. Conf, Edited Malhotra, St. Andrews, Canada, 1996, pp 215-232. 11. WELLE, A., LIAO, J.D., KAIZER, K., GRUNZE, M., MAEDER, U., BLANK, N., «Interactions of N,N'-dimethylaminoethanol with Steel Surfaces in Alkaline and Chlorine Containing Solutions)), Applied Surface Science, 119, 1997, pp 185-198. 12. BERKE, N.S., HICKS, M.C., "Protection Mechanism of Calcium Nitrite", Corrosion Conference on Understanding Corrosion Mechanism in Concrete: A Key to Improving Infrastructure Durability - MIT-Cambridge University, Massachusetts, USA, 27-31 July 1997. Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.