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LONG-TERM PERFORMANCE OF ZnO

AS A REBAR CORROSION INHIBITOR

Oladis Troconis de Rincn, Orlando Prez, Edgar Paredes, Yaxcelys CalderaCarolina Urdaneta and Isabel Sandoval

Centro de Estudios de Corrosin, Facultad de IngenieraUniversidad del Zulia, A10.482. Maracaibo, Venezuela

. INTRODUCTION

Proper design and preparation of concrete in accordance with ACI(1) standards and tmaintenance of the structures under those conditions would guarantee them a long and efficienn aggressive media. However, these requirements are not always met and adhered to. It is thehe case that, in civil construction, there have always been corrosion problems in reinforcem

both in structures erected near coasts, quays, ports, bridges, etc., and in structures in whicdesign and preparation of the concrete did not follow the appropriate standards and are expoaggressive conditions or lack of timely maintenance, reducing their useful life, generating

epair and/or rehabilitation costs; in some cases, even the loss of these structures, endangerinife and safety of persons using them.

With current knowledge, the protection methods now available to neutralize the aggressive eof chlorides introduced while mixing the concrete, are reduced to cathodic protection and the unhibitors, as galvanizing does not seem to offer sufficient guarantees for high chontamination. This situation arises in cases where it is necessary to carry out repairs on strun the splash/wave zone, where appropriate water is not available to prepare the reinforced conither because it is far away from these sources or roads are in bad condition, and/or becau

water or aggregates available are contaminated by chlorides, scenarios that would bring abouse of these contaminated components, with the consequent deterioration of the reinforcing storrosion. A viable alternative for controlling the corrosion phenomenon is the use of cornhibitors when the concrete is prepared, so that they may reduce corrosion rates to technolerable levels.

Corrosion inhibitors are compounds that, added or supplied to concrete in small proportions,be compatible with it, not cause unwanted reactions nor negatively alter its physical propbesides, they must compete economically with other control methods. From the action standnhibitors affect the rate of corrosion processes, since they influence the kinetics o

lectrochemical reactions that cause these processes by simultaneously retarding either the a


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Corrosion inhibitors(2-21) are presented as an attractive option for preventing the corrosion on concrete in the presence of chloride ions. The first patent was awarded to a company in 19ommercial application of nitrite-based inhibitors(16).

Of the different inhibiting substances evaluated, appropriate for the alkaline medium in conalcium nitrite has produced the best results, even for cracked concrete(10), so it has beenommercially with relative success. However, calcium nitrite is very costly and difficult to obtome countries, so studies of other additives have been undertaken in an effort to replace it,

partially or totally. It has been found that Ca(NO2)2 improves the corrosion resistance of sthloride-ion attacks for w/c ratios < 0.5(5,12), the w/c ratio being considered as the factor

affecting corrosion. On the contrary, other authors(20) found that corrosion rate varies significwith the cement type, the minerals used and fine aggregate and, to a lesser degree, witwater/cement ratio. It has also been indicated, that if the durability of the structures is guaranteed, the inhibitor must be used in good quality concrete because the characteristics different components in the concrete influence durability(13).

E. Ramirez(17) studied other inhibitors (gluconate, phosphate, urotropine, chromates, resorluoroglucinol, and zinc oxide). He did not find any with efficiency comparable to that o

nitrites. He points out, however, that the protection afforded by nitrites is temporary when mmersion, requiring very good quality concrete and very thick covering. In the case of sevorroded reinforcements, nitrite fails to passivate them, so repeated spraying of pre-cortructures with inhibiting solutions cannot be considered a method of protecting einforcements. It has also been shown(17) that NO2

-/Cl- is ineffective when the ratio is <

Ca(NO2)2 is an anodic inhibitor, it could become hazardous when its concentration is reducevels that would increase the corrosive attack. It has also been indicated that, with immersion

Cl- and NO2- would also leach out of the concrete because they have similar diffusion coeffic

eaving the same relative proportion of both ions that were present before immersion.

Other investigations(18) developed rehabilitation processes for reinforced concrete structures,our inhibitor formulations: i) organic inhibitor, which is thought to form a protective filransformation into a metallic soap; ii) a mixture of surfactant and amino salts in water, or voorrosion inhibitor; iii) alcanolamine, an additive that would function by migrating and inhibit

he same way as iii); and iv) Ca(NO2)2. In field application to chloride-contaminated concrepre-corroded reinforcements, the results indicate that ii) and iii) showed a significant reductorrosion rate, and reduction in i) was lower than ii) and iii), whereas Ca(NO2)2 produubstantial increase in corrosion rate. The latter result has been obtained in other investigations

On the basis of the foregoing discussion, a good case may be made for the use of Ca(NO2)orrosion inhibitor in reinforced concrete structures, given the series of advantages it pre

advantages that make it reliable within the initial and borderline conditions considered. Howor other conditions where nitrite performance is unsatisfactory, further evaluation and a mo

depth study of other inhibitors and application methods are needed. Such is the caseZnO(4 8 19-21) which has been under study to try and replace nitrite either partially or to


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he reason why all the evaluations carried out in Centro de Estudios de Corrosin (CEC) useype of mixture.

In view of these results and of those obtained with Ca(NO2)2, field use of the latter is cur

pecified together with concrete of excellent quality (w/c = 0.4), it was decided to evaluaffect of w/c ratio in the behavior of ZnO and its mixture with Ca(NO2)2. This work thepresents a summary of the experimental procedure and the results obtained during the pasyears evaluation.

2. EXPERIMENTAL PROCEDURE

A total of 243 concrete probes with w/c ratios of 0.45, 0.5 and 0.6 were prepared, using Lakeo mix 27 pattern probes (probes with no inhibitor), 162 probes with inhibitor concentrations

or Ca(NO2)2) at 2, 3 and 4% on the basis of cement weight, mixture at 2% (on the same bas

both inhibitors, and tap water in 27 pattern probes. The nomenclature is as follow:

Figure 1. Details of Probes for

Electrochemical Tests

The pattern probes and those with nitrite were cured for 28 days, counted as of the dapreparation. The probes with inhibitors mixed in were cured for a longer period because zinc etards setting, so they were taken out after several days, depending on the zinc oxide concentr

Curing was done in tap water. Compressive strength was determined for 81 of these probes awere used to determine porosity.

Figure 1 shows the arrangement of the probes used to evaluate the electrochemical proppotentials, Rp and potentiodynamic cyclic polarization curves at 0.28 mV/s), of the dif

mixtures tested


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were sprayed with salt water at 3.5% until the 147th day with respect to the preparation of thprobes and, finally, they were placed in a salt chamber with saline solution at 5%, 25C and kPa. This exposure was done for 10.5 h/day, for 500 h. The data for corrosion potentials, corrate, resistivity and ohmic drop were evaluated statistically by the Qi method in order to d

non-representative values in the calculation of the median.

3. DISCUSSION OF RESULTS

.1Physical Tests.

After 28 days, the compressive strength test was applied to the test probes (with tap water andwater), the probes with calcium nitrite and those with the calcium nitrite and zinc oxide mixt2%; this test was carried out on the probes with zinc oxide at 3% after 73 days and 69 days ase of the zinc oxide probes at 4%. This time difference with the zinc oxide probes was due

etarding effect on setting exerted by zinc oxide, as indicated above2.

Figure 2 shows that the results for all probes (with different w/c ratios and different inhoncentrations) were higher than the design strength, the best behavior being that of calcium n

at 4%. Resistance values grow as concentration of both inhibitors is increased. It should be hat the zinc oxide probes have greater strength values than the calcium nitrite at w/c ratio =

This results has been observed in previous studies(2).

Porosity increases with w/c ratio (Figure 3) except for probes prepared with zinc oxide at 2%

Figure 2. Compressive Strength of Probes

with different w/c ratios and different

Inhibitor Concentrations

Figure 3. Porosity of Evaluated

Probes

Strength at 28 days: P, L, M y N Strength at 69 days: Z4

Strength at 73 days: Z

PL

N2N3

N4Z3

Z4M 0,45

0,50,6

0

50100

150

200

250

300

350

400

Comp

ressiveStrength

(Kg/cm2)

Probe

w/c

P LN2

N3N4

MZ2

Z3Z4 0,45

0,50,6

0

3

6

9

12

15

Probe

w/


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ones prepared with Ca(NO2)2. The lower the w/c ratio, the better the inhibitor behavior, withbeing the best. After approximately 300 days exposure to the marine environment, (w/c = 0.5teel was still passive in all probes prepared with ZnO but not with Ca(NO 2)2, where the ste

been activated, regardless of inhibitor content.

This work presents the results for the probes prepared with the best mixture (w/c ratio = Figure 4 shows the best behavior for ZnO and the mixtures, no difference being observed behe tap water and ZnO probes after more than 400 days exposure to the saline medium.

-800

-700

-600

-500

-400

-300

-200

-100

0

514

20

26

44

51

58

65

86

93

100

107

114

122

128

135

142

163

268

304

384

404

Time (days)

POTENTIALvs

.Cu/CuSO4

(mV

)

L04

P0

M2

N2

N3

N4

Z2

Z3

Z4

Figure 4. Average Corrosion Potential of Reinforcing Steel in Concrete with w/c = 0.45,

during the Evaluation Period.

This behavior is also observed in Figure 5, which shows the variation in the corrosion cdensity during the evaluation period. However, greater steel-corrosion rates were always obsn the ZnO probes, even though they decreased drastically as time went on, especially for

0.45, where they have remained constant during the last days of evaluation (300 days).

0.2

0.4

0.6

0.8

1

1.2

1.4

Icorr

(A/cm

2)


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Figures 610 show the cyclic curves for the different conditions under evaluation. They nhibitor behavior under three different conditions: i) Post-cure (approximately 28 days), ii)he steel in some probes started to become activated (approximately 172 days), and iii) afte

days of evaluation. Only the curves for the w/c ratio = 0.45 and 4 % inhibitor concentration.

Figure 6.PotentiodynamicCyclic Polarization Curves for Steel at 0.28 mV/s, at differentvaluation times. Ca(NO2)2 at 4%, w/c ratio = 0.45.

Note the behavior of Ca(NO2)2 (Figure 6), where loss of passivity is observed after 407 days. F7 shows that the corrosion rate for the steel in the concrete with ZnO decreases, maintainipassivity.


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Figures 8 and 9 show behavior, depending on inhibitor concentration for the same w/c ratio407 days evaluation.

Figure 8. Potentiodynamic Cyclic Polarization Curves for Steel at 0.28 mV/s, inCa(NO2)2 as F Inhibitor, w/c ratio = 0.45, after 407 days evaluation.


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Note that the steel has lost its passivity for all Ca(NO2)2 inhibitor concentrations. This is nase for ZnO, where passivity is lost only when concentration is at 2%. Note that there is very

difference between 3 and 4 %.

Figure 10 compares inhibitors with the mixture and the test probe prepared with Lake watxposed to the saline medium. The lowest corrosion rate is observed in the steel in the pprepared with the mixture. Although the steel in the probes prepared with ZnO and Ca(NO2)2he same corrosion rate, in the latter there is a hysteresis behavior in its inverse scan, indic

pitting in the sample. As already stated above, the potential is more negative.

Figure 10. Comparison between Potentiodynamic Cyclic Curves for Steel in different

nhibitor mixtures used at w/c ratio = 0.45, after 407 days evaluation.

.1 Chloride Content at Reinforcement Level in the Concrete

Table 1 shows the results of the determination of free chloride ions at reinforcement level, aftperiods indicated above. Note the effect of ZnO, where concentrations are very low, espewhen the w/c ratio is 0.5 and 0.45; the same effect is observed in the mixtures. This may be dudecrease in concrete porosity and, possibly, to ZnO reaction with the chloride ions.

As is well known, ZnO acts as a cathodic inhibitor when used in water because it can preciompounds in the cathode because of its high alkalinity in comparison with the anode. Howoncrete has high alkalinity, so it is therefore expected that ZnO precipitates compounds bothhe cathodic and anodic areas, as well as within the concrete itself, which helps to reduce po

and maintain the passivity of the steel in the concrete when it is exposed to a medium with

hloride-ion content It is also possible that ZnO reacts with the chloride ions in the con


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4. CONCLUSIONS

. Chloride diffusion and, therefore, corrosion rate, is controlled by reducing the w/c ratio tregardless of calcium nitrite content.

2. Using ZnO reduces concrete porosity and chloride diffusion.

. After the exposure period ( 500 days), the potential values (> -200 mV vs. Cu/CuSOcyclic curves of the steel in the probes with ZnO as inhibitor show passivation, even thcorrosion rates are high (> 0.1 A/cm2).

4. When ZnO is used, an appreciable change in corrosion rate is observed when the inhconcentration is changed from 2% to 3%.

. After approximately 500 days exposure to the saline medium, all the probes preparedCa(NO2)2showed breaks in the passive film on the reinforcement.

6. The ZnO/Ca(NO2)2 mixture at 2% showed the best results throughout the study period.

Table 1. Chloride Concentration at Reinforcement Level (ppm for cement weight)PROBE A B C

P04 327 261 394P05 346 207 432

P06 383 194 778M24 1571 1833 784M25 2214 4419 756M26 2759 14090 5186Z24 2355 2612 2353Z25 1938 3729 1081Z26 1842 3683 63283Z34 1571 1964 589Z35 1938 1522 3677Z36 2148 4143 18674Z44 1310 3273 784Z45 1386 12870 4543Z46 2454 13964 15562N24 523 523 5887N25 484 4845 11294N26 844 9348 69505N34 327 1833 7456N35 553 829 14059N36 1764 5062 46165N44 720 654 8240N45 484 691 18277

N46 690 613 42359L04 589 3272 18050


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REFERENCES

. American Concrete Institute. Building Code Requirements for Reinforcements Conreported by ACI Committe 318. Inc: ACI Manual of Concrete Practice. Detroit, v.3, (1992

2. O. Trocnis de R., Uso de los Inhibidores para el Control de la Corrosin del AceRefuerzo del Concreto. Memorias VIII Reunin Latinoamericana de ElectroqumCorrosin. (Argentina, 1988). p. 444-446.

. R.J. Craig, L. Wood, "Effectiveness of Corrosion Inhibitors and their Influence on the PhProperties of Portland Cement Mortars". Highway Research Record.N 328, HighwayBoard, (1970), p 77/88

4. N. Martnez de F., O. Trocnis de R. "Efecto de los Inhibidores ZnO y Ca(NO2)2y del AgLago en las Propiedades Fsico-qumicas del Concreto". Memorias del II ConIberoamericano de Corrosin y Proteccin. Tomo I. (Maracaibo- Venezuela, 1986) p. 397-

. N.S. Berke, W.R. Grace. Corrosion/85. Paper No. 273. (Houston, TX: NACE Internat1985)

6. A.M. Rosemberg, J.M. Gaydis, et al, "A Corrosion Inhibitor Formulated with Calcium Nfor used in Reinforced Concrete". ASTM-STP 629. (1977).

7. O. Trocnis de R., O. Prez, Y. Longa, J. Ludovic. Corrosion/93, paper no. 339. (HoustonNACE International 1993)

8. A.M. Rosemberg, J.M. Gaidis, Materials Performance 18, 11(1979): p.45.

9. F. Alvarez, C. Surez, "Estudio de Inhibidores de Corrosin en Concreto Contaminado conTheses. LUZ. Facultad de Ingeniera. (Maracaibo-Venezuela, 1982).

0.N.S. Berke, M.P. Dallaire, M.C. Hicks Corrosion/91. paper No. 550. (Houston, TX: NInternational, 1991)

1.P. Tourney, N. Berke, A Call for Standardized Test for Corrosion-Inhibiting AdmixtConcrete International. (April 1993), p. 57-62.

2.C.A. Loto, Corrosion 48, 9(1992): p. 759.

3.N.S. Berke, W.R. Grace. Corrosion/87. paper No. 132. (Houston, TX: NACE Internat1987)

4

O Trocnis de R A Survey on Research in Corrosion Control Systems for ReinfConcrete Structures Proceedings 1st Pan-American Corrosion and Protection Con


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7.N. Thompson, et. al., Effect of Concrete Mix Components on Corrosion of Steel in ConcCorrosion/96. Paper no. 334. (Houston, TX: NACE International, 1996)

8.E. Ramrez C., Corrosin de las Armaduras Embebidas en Hormign en Ambient

Extrema Agresividad. Posibilidad de Proteccin Mediante Inhibidores. PhD. Theses DocUniversidad Complutense de Madrid. Facultad de Ciencias Qumicas. Centro NacionInvestigaciones Metalrgicas. (Madrid, 1994).

9.B.D.Prowell. valuation of Corrosion Inhibitors for the Repair and Rehabilitation of ReinfConcrete Bridge Compoets. M.Sc. Theses. Virginia Polytechnic Institute and State UnivBlacksburg. (VA. USA, 1992).

20.O. Prez, Estudio del ZnO, Ca(NO2)2 y del Ca5(PO4)3OH, como Inhibidores de CorrosiAcero de Refuerzo en Concreto Preparado con Agua del Lago de Maracaibo. MSc. T

Universidad del Zulia. (Maracaibo-Venezuela 1993).

21.Y. Caldera, E. Paredes, Estudio de la Calidad del Concreto sobre el Efecto Inhibidor deen el Acero de Refuerzo.Theses. Universidad del Zulia. Facultad de Ingeniera. (MaracVenezuela. 1996).

22.I. Sandoval, C. Urdaneta, Estudio de la Relacin Agua/Cemento en el ComportamienZnO y Ca(NO2)2 como Inhibidores de Armaduras Theses. Universidad del Zulia. FacultIngeniera. (Maracaibo-Venezuela, 1997).


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CURRICULUM VITAE

OLADIS T. DE RINCON

Oladis Trocnis de Rincn is a Professor of Chemical Engineering at Universidad del (University of Zulia), where she graduated as a Chemical Engineer in 1972. She obtaine

Master degree in Chemical Engineer from the University of Oklahoma at Norman in 1975.

O.T. de Rincn is the founder and head of the Corrosion Research Center, of the Engine

Faculty. She has been leader of several projects which are supported by different local and na

esearch fund agencies. She was awarded a Research Fellowship from the National Coun

Scientific Research Venezuela (CONICIT). She has obtained various research awards, distinand plaques for her original research activities and her effort to form research groups

University. She was awarded the JESUS ENRIQUE LOSSADA ORDER, in First Class in Professor Oladis is a prolific writer in her research field, having published 33 pape

nternational Journals.

Professor Rincn was Director at Large of NACE International (1993-1995) and now

Director of the Latin American Region (1996-1999). She was one of the founders of the VenezSections. In addition, she has been active in NACE committees for a number of years, servi

member of Group Committee 3T-K, International Relations (IRC), International Rel

Management (IRMC), Award and Research Committees. At the present time, she is still a m

of Education Committee. Oladis de Rincn is one of the Venezuelan representatives onternational Corrosion Council.

She is the International Coordinator of DURAR (Durability of Rebars) and the VenezCoordinator of PATINA (Anticorrosive Atmospheric Protection of Materials), the Iberoam

Networks from CYTED (Science and Technology for Development Program).

Professor Oladis de Rincn was the Editor-in-chief of a research journal Revista Tcni

ngeniera from 1984 to 1991. The Revista Tcnica is an International Journal, andawarded as the best Venezuela Research in 1988 by the CONICIT. She has organized

esearch and technical seminars, conferences, symposia and congresses in the corrosion and ngineering fields. As one of her main research interest is corrosion in concrete reinforcemen

s brokering an agreement between the University of Zulia and the Venezuelan Governm

onnection with corrosion and rehabilitation of the Bridge on Lake Maracaibo (one of the lo

oncrete bridge in the world). She has published and served as adviser on several proCorrosion problems and rehabilitation of the University Hospital Maracaibo, Corrosion pro

and rehabilitation of Buildings exposed to rural environments, Corrosion problems

ehabilitation of Docks located in a marine environment, Evaluation of mortars based on A

and Epoxy Resins to Repair Reinforced Concrete Structures

41 de rincon paper

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