Effect of Superficial Oxides on Corrosion of Steel Reinforcement Embedded in Concrete

CORROSION ENGINEERING

879CORROSION–Vol. 50, No. 11

Submitted for publication October 1993; in revised form, May 1994.* Programa de Corrosión del Golfo de México, U. Autónoma de

Campeche, Ave. Agustín Melgar S/N, 24030 Campeche, Campeche,México. Present address: Mexinox SA de C.V., Av. Industrias 4100,Manzana 49-53, Zona Industrial, La Sección, San Luis Potosí, SLP,México.

** Instituto Mexicano del Petróleo, México, D.F.*** lnstituto de Investigaciones Eléctricas, México.

Effect of Superficial Oxideson Corrosion of Steel ReinforcementEmbedded in Concrete

J. Avila-Mendoza,* J.M. Flores,** and U.C. Castillo***

0010-9312/94/000201/$5.00+$0.50/0© 1994, NACE International

observation was explained in terms of a mechanisminvolving the self-reduction of Fe2O3 to Fe3O4.

KEY WORDS: chloride, concrete, hematite, iron oxide,magnetite, oxygen, rebars, rust, steel

INTRODUCTION

It is known widely that the protection afforded to steelby the normally passive environment of concrete canbe compromised seriously by the introduction ofaggressive ions, especially chlorides (Cl–). Mostresearch has studied clean steel surfaces preparedin the laboratory. However, natural oxides are formedon rebars during fabrication or exposure to theatmosphere and usually are present on rebarsurfaces. Their effect on corrosion behavior in thepresence and absence of Cl– ions is not understoodfully.

The performance of a particular concrete under agiven set of circumstances will be influenced bydesign, materials, and workmanship. Magnetite(Fe3O4, the grey-black oxide) and/or hematite (Fe2O3,the red-brown oxide) usually is present on the rebarsurface at the time a structure is cast. Fe3O4 may bepresent on the steel from the time the rebar ismanufactured originally.

Fe2O3, however, arises from prolonged atmos-pheric exposure of the bare iron (Fe) or from a slowtransformation of Fe3O4 in contact with wet air.

Fe forms three stable, solid compounds withoxygen (O): FeO (wüstite), Fe3O4, and Fe2O3.1 Todistinguish it from the cubic g-Fe2O3 modification,

ABSTRACT

The effect of superficial coverage with different iron oxideson the general corrosion resistance of steel embedded inconcrete was investigated. Electrochemical corrosion rateand potential measurements were made of rebars that hada bare surface (polished), an atmospherically rusted(hematite [Fe2O3]-rich) oxide-covered surface, and afurnace-produced (magnetite [Fe3O4]-rich) scale surface.Electrochemical characterization was made during andafter the curing period. After the curing period, specimenswere immersed partially and tested for 60 days in solutionswith no aggressive ions or of two different chloride (Cl–)concentrations to promote active corrosion. Results fromthe Cl–-free atmosphere of 100% relative humidityindicated similar passivation behavior for all surfaceconditions during the entire curing period. This suggestedthe anodic reaction acted as a controlling factor of thecorrosion rate. However, when the specimens wereimmersed partially in double-distilled water and in solutionsof two different Cl– concentrations, passivity deteriorateddifferently, depending upon the oxide present. The rustedcondition presented the highest corrosion rate. This ratedepended strongly upon Cl– concentration and wentbeyond the limiting value imposed by the assumedcathodic reaction of oxygen (O2) reduction. This


880 CORROSION–NOVEMBER 1994

film of Fe3O4 (gray-black oxide) covering all the steelsurface with a thickness of a few microns (achievedby exposing the specimens to 800°C for 20 s tosimulate the conditions of manufacture); and finally,rusted specimens in which the rust layer (red-brownoxide) was obtained by exposing the bare steel in ahumid location for several weeks.

The specimens were coated with an insulatinglacquer, except for a band ~ 5 cm long thatconstituted the test section and except for anallowance for external electrical contact.

Mortar specimens were made of ordinaryportland cement and had dimensions of 3 cm by 5 cmby 10 cm, with cement/sand = 1/3 and a water-to-cement (w/c) ratio of 0.5 by weight. The surface ofeach mortar specimen was coated with lacquer,except for one side (50 cm2 area), which was the onlysurface exposed to the solutions. The distance fromthe embedded steel specimen to the flat mortarsurface was 10 mm. Each mortar specimen containedthree steel bars with the same surface condition in arow along the center line. After a curing period of 30days in a chamber at 100% relative humidity (RH) at20°C, the mortar specimens were subjected to partialimmersion in double-distilled water and salt solutions,with the level of the solution kept below the top of themortar. The salt solutions used were made fromanalytical-grade sodium chloride (NaCI) atconcentrations of 1 wt%, 2 wt%, and 3 wt%.

The mortar specimens also contained anembedded graphite bar as a counter electrode,similar to the arrangement used by Gonzalez, et al.6

A standard calomel electrode (SCE) as the referenceelectrode was held in contact with the surface of themortar for measurement. Corrosion potentials (Ecorr)of each steel bar, under the different surfaceconditions, were monitored as a function of time andpolarization. Resistance values (Rp) were recordedduring the test, and the corrosion rate was calculatedfrom the Stern and Geary equation:

I = BRp

(1)

where B was taken as 26 mV for active specimensand 52 mV for passive ones.6 The potentiodynamicmeasurements were taken by polarizing cathodicallyat 20 mV/min from Ecorr up to –15 mVSCE and then upto 15 mVSCE to obtain the original slope. Potentio-dynamic anodic polarization tests also were carriedout. A potentiostat was used with electroniccompensation resistance for the ohmic drop (positivefeedback). Recorded values for the compensationwere taken as the electrical resistance values of themortar.

TABLE 1Composition of Mild Steel(A)

C Mn Si S P Cu Cr

wt% 0.13 0.52 0.17 0.032 0.014 0.24 0.12

(A) C, carbon; Mn, manganese; Si, silicon; S, sulfur; P, phosphorus; Cu,copper; Cr, chromium.

rhombohedric hematite also is called a-Fe2O3. Below570°C, FeO decomposes into Fe and Fe3O4.

Under open-air conditions, the prevalence ofgoethite (a-FeOOH) in atmospheric rusts has beenestablished. According to Misawa, et al., the mainphases in these rusts are a-, g-, and d-FeOOH andFe3O4.2

d-FeOOH also has been found to form witha-FeOOH during the initial stages, indicating thepresence of g-Fe2O3 in rusts.3 Mössbauer spectros-copy results of atmospheric rust phases has showng-FeOOH forms first and transforms later toa-FeOOH and g-Fe2O3.4 Infrared spectroscopy ofphase transformation processes in rust layers onweathering steels has shown that rusting proceedsthrough the formation first of g-FeOOH or ferrihydrite(depending on atmospheric conditions), evolution ofamorphous d-FeOOH, and final stabilization ofd-FeOOH containing traces of g-FeOOH.5 It wassuggested that g-Fe2O3·H2O forms in the intermediatestages of transformation of g-FeOOH to d-FeOOH,and that a-FeOOH might emerge ultimately as themost stable phase.5

After a 1985 earthquake in Mexico City revealedthe extent of corrosion on the reinforcement of somedamaged buildings, the civil engineering communityin Mexico demanded information on whetherembedded steel originally bearing surface oxideswould exhibit faster corrosion than rebars withoutsurface oxides.

The objective of the present work was toinvestigate the effect of superficial coverage withdifferent iron oxides on the general corrosionresistance of steel embedded in concrete.

EXPERIMENTAL METHOD

The steel specimens studied were cylinders15 cm high by 5 mm diam. A typical analysis of thesteel is given in Table 1.

All specimens in this study were ground using600-grit silicon carbide paper and degreased beforefurther treatment. Three types of rebar surfaceconditions were produced: specimens that hadsurfaces polished to a 1-µm alumina finish (mirrorfinish condition); specimens that had a continuous



Therefore, the limiting cathodic current density value(i) becomes:

i = 0.23 µA/cm2 (4)

Assuming O2 reduction was the only possiblecathodic process, rebar corrosion would be controlledfor the anodic reaction if the metal surface was in apassive state, or alternatively, controlled for thelimiting O2 reduction reaction when the metal wasactive. The maximum rate of the cathodic reactionO2 + 2H2O + 4e– = 4OH– would be sufficient tobalance the anodic reaction current density. For thepresent case, the limiting corrosion current density(icorr) would be 0.23 µA/cm2.

Results obtained during the 30-day curing periodwithout Cl– present showed the specimens had eithera polished surface or a Fe3O4 oxide present. The cor-rosion rate value fell far below 0.23 µA/cm2, remain-ing stable for the whole period. Results presented inFigure 1 showed bare steel and steel covered with aFe3O4 film in mortar were protected by a passive film,resulting from the buffering action of a lime-rich layerat the steel-mortar interface, as proposed by Page.8

For the specimens covered with rust, the value oficorr for the curing period was increased slightly, attimes reaching the ilim value of 0.23 µA/cm2 (Figure 1).This suggested passivity was achieved and main-tained, but not in such a stable manner as for thepolished and Fe3O4-covered specimens. The highericorr value of the roughened rusty surface probablyresulted from the change of the real-to-apparent arearatio.

Parallel measurements for the compensatedohmic resistance for the tested specimens showed a

RESULTS AND DISCUSSION

To compare the corrosion processes that takeplace on bare embedded steel with those that occurwhen a steel surface is covered by different oxidesunder the same exposure conditions, Ecorr andcorrosion rate (icorr) measurements with time werecarried out. The reported data represent averagevalues from triplicate specimens.

Curing PeriodFor all three samples, Ecorr values showed an

upward trend with time (Figure 1[a]), accompanied bya steady decrease in icorr (Figure 1[b]). This behaviorwas typical for the development of a passive oxidelayer with a decrease in anodic current as the filmthickens. With a decrease in anodic current, thepotential was driven protrusively upward.

It is believed generally that the access of O2 tothe metal constitutes the controlling factor for thecorrosion kinetics of rebars embedded in mortar.Thus, the limiting cathodic current density forreduction of O2 under planar diffusion (ilim) is:7

i lim = 2FJ (2)

where i = current density; F = Faraday constant; andJ = O2 flux.

For the diffusion-controlled cathodic reaction ofO2 plus water and free electrons to hydroxyl ions:

O2 + 2H2O + 4e– = 4OH– (3)

a mortar thickness of 10 mm, and w/c = 0.5, Gjorvcalculated the O2 flux (J) as 6 x 10–13 mol/cm2-s.7

FIGURE 1. (a) Ecorr and (b) icorr of steel electrodes presenting different surface conditions embedded in mortar.Curing time 30 days in the absence of Cl–, 100% RH.

(b)(a)



the surface condition presentation and is importantfor the understanding of any increase in the corrosionmagnitude of the process.

All three types of specimens, whether in distilledwater or Cl– solutions, showed a potential drop withincreasing corrosion rate. This downward shift couldhave been a result of stimulation of the anodicreaction or polarization of the cathodic reaction(Figure 5), as is observed with the onset of low-potential active corrosion. However, where diffusion-limited O2 reduction lowered the potential, this couldnot have led to an increase in corrosion rate. Theincrease in corrosion current linked to a downwardpotential shift could only have come through anincrease in the anodic kinetics.

(a) (b)

FIGURE 2. (a) Ecorr and (b) icorr of steel electrodes with different surface conditions embedded in mortar. Immersionperiod 60 days in double-distilled water.

FIGURE 3. (a) Ecorr and (b) icorr of steel electrodes with different surface conditions embedded in mortar. Immersionperiod 60 days in a 1% NaCl solution.

(a) (b)

similar tendency independent of the surface condi-tion: a fast increase in the R value during the first 3days, followed by a stable value of 700 Ω for the restof the test period.

Immersion PeriodFigures 2 through 4 represent the evolution over

time of Ecorr and icorr values for conditions of partialimmersion in different solutions and for rebarspresenting the three different surface conditions.

The measured ohmic resistance values of thepartially immersed specimens decreased slightlyfor the first few days, stabilizing within the range of500 Ω to 700 Ω afterward and for the rest of theimmersion period. This behavior was indistinct from



A surprising feature of these results, in the caseof the polished samples and those with Fe3O4 films,was that the fall in potential and increase in corrosioncurrent seen over the 60-day period were at mostonly slightly greater in the Cl– solutions than in puredistilled water (Figures 2 through 4). When theresults obtained at the end of the curing and immer-sion periods were plotted as a graph of log icorr vs E(Figure 6), roughly a linear behavior was evident, asnoted previously by Page and Havdahl.9 Theyexplained this behavior as due to anodic control ofthe corrosion reaction, with this plot providing adescription of the cathodic behavior as a function ofpotential. It was not clear from these experiments bywhat mechanism the anodic process was stimulated,particularly in the case of distilled water.

The result for rebar with the rusty surfaceshowed the most interesting behavior. For theseelectrodes embedded in mortar and partiallyimmersed in a 1% Cl– solution, the recorded Ecorr

values were the most negative for that condition(with fluctuations in the range of –500 mVSCE and–700 mVSCE at the end of the 60-day immersionperiod). High and increasing icorr values wereobserved throughout the test period, exceeding theilim value of 0.23 µA/cm2 by > 1 order of magnitude atthe end of the immersion period.

Results for the partial immersion in 3% NaClsolution (Figure 4) showed a similarly negative shiftin Ecorr of the rusted steel and a larger increase inmetal dissolution (≈ 4 µA/cm2) at the end of theperiod. The data summarized in Figures 3 and 4 didnot rule out the possibility that the rate of metaldissolution of rusted steel in mortar also dependedon Cl– concentration.

FIGURE 4. (a) Ecorr and (b) icorr of steel electrodes with different surface conditions embedded in mortar. Immersionperiod 60 days in a 3% NaCl solution.

FIGURE 5. Electrode potential vs current density schematicfor a corroding rusted steel electrode embedded in contami-nated mortar.

(a) (b)

It is the intimate contact between steel and thelime-rich interfacial zone of segregation (portlandite)from the cement that inhibits the aggressive action ofCl–, according to Page.9 As shown by Yonezawa, etal., pitting occurs more readily when another materialis present between the steel and the concrete.10

Thus, the rust layer may have interfered with theclose contact between the steel and the concrete,and pitting occurred in the two Cl– solutions.

The relationship between the log of icorr and Ecorr

values for the rusty surface again was linear, withbigger currents at more negative potentials (Figure6). However, the corrosion mechanism was quitedifferent for much bigger currents. This could be



The O2 reduction is the prevailing reaction on steelfree from oxide films and apparently occurs also onFe3O4-covered electrodes, but it also occursefficiently on steel covered with a Fe2O3 oxide.Bockris, et al., has shown recently that thedifferences in the kinetics and pathway for O2

reduction on bare Fe and passive Fe consist in theirhaving different electron transfer reaction pathways.12

The rate of O2 reduction on passive Fe is greater thanthat on bare Fe. This is because the O2 reductionsproceed through a 4e– pathway with little hydrogenperoxide (H2O2) as an intermediate on the bare Fe,and through a 2e– pathway with the formation of H2O2

as the product on the passive layer. The rateincreases for higher pH values.

Thus, the occurrence of high corrosion rates ofrusted steel partially immersed in solutions ofdifferent Cl– concentrations must be explained on thebasis of which of the two cathodic reactions predom-inates: the apparently efficient O2 reduction reactionon a passive layer of Fe2O3, or the cathodicallyinduced, and apparently easy, phase transformationreaction of Fe2O3-rich rust layer to Fe3O4.

Although the exact degree of Fe2O3 transforma-tion and O2 reduction were not known under thepresent experimental conditions, the corrosionreaction always was assumed to be limited by theavailability of O2. It is possible, therefore, that a Fe2O3

phase transformation predominated for the presentsystem, explaining the incremented magnitude ofcorrosion rates. Thus, the primary finding was that,even under limited O2 diffusion, the oxidized Fe upperoxides might oxidize the underlying metal, enhancingfurther corrosion.

The electrochemical behavior of rusted steelbars embedded in contaminated mortar matrices maybe proposed in terms of the mixed potential theory,as shown schematically in Figure 5. The presence ofincreased Cl– concentrations in the mortar markedlyincreases the rate of the anodic metal dissolutionprocess. Figure 5 also shows the expected kineticchanges upon introducing the cathodic reaction ofFe2O3 reduction. Thus, the behavior predicted by thesimplified model in Figure 5 generally was consistentwith the increased rates of metal dissolution andcorrosion potentials.

Results obtained from this study were in goodagreement with those recently reported by Stratmannand Hoffmann.11 They investigated the transforma-tions of atmospheric rust layers in situ by transmis-sion Mössbauer spectroscopy and suggested therewas a reduction of the rust layer to 3Fe3O4, allowing arusted steel specimen to corrode, even withoutreduction of O2.

Since it is widely recognized that the effective-ness of Cl– in the attack of steel embedded in mortar

accounted for by an alternative cathodic reaction. It iswell known that autoreduction of rust can take placein atmospheric corrosion, where metal oxidation issustained by reduction of the rust.11 This reactionmight have accounted for the high corrosion ratesseen on the rusty samples. Such a reaction wouldcease when all the rust was reduced to Fe3O4. Inpractice, if the concrete became dry again, the rustwould reoxidize, and the cycle could begin againupon wetting.

The experimental evidence presented for rustedspecimens suggested the presence of an efficientand parallel cathodic reaction, different from O2

reduction and coupled to the anodic reaction, whichwas interpreted as metal dissolution.

Since the maximum rate of the diffusion-controlled cathodic process O2 + 2H2O + 4e– = 4OH–

might be insufficient to balance the high anodicreaction i values of rusted steel when embedded incontaminated mortar, the following electrochemicalinterpretation was proposed for this specific type ofcorrosion:

Fe = Fe2+ + 2e– (5)(anode, base metal)

Fe2+ + 4Fe2O3 + 2e– = 3Fe3O4 (6)(cathode, existing rust)

Thus, the process would involve reduction of theexisting rust together with dissolution of the metal,especially when the surface was wet.

However, corrosion of rebars embedded inmortar also depends on ilim for the reduction of O2.

FIGURE 6. Comparison of observed electrode potentials andlog i values for the polished steel and the two oxide-coveredsteel electrodes embedded in mortar during curing and partialimmersion.



is governed by its special relationship with O2, itwould be of interest to carry out a more detailedinvestigation involving the effect of Cl– on rustedrebars in deaerated mortars.13-14

CONCLUSIONS

In results obtained during the 30-day curing periodfor all surface conditions with no Cl– present, thecorrosion rate fell below the calculated ilim value of0.23 µA/cm2. The anodic reaction (passivity) mayhave acted as the controlling factor of the corrosionrate. For partial immersion conditions in double-distilledwater, the corrosion process for specimens coveredwith Fe3O4 and for specimens with a polished steelsurface continued under anodic control. However, thebehavior for rusted steels was completely different. Acomplete activation of the electrode occurred. Ecorr

values fell abruptly, and icorr increased to values far inexcess of the calculated cathodic current density. When Cl– content of the solution was 1% and 3%,the corrosion rate values for the clean and Fe3O4-covered steel surfaces were low and similar. Thisindicated the electrodes were passive and underanodic control. For the rusted steel electrodes inmortar, very high corrosion rates were observedthroughout the test period. It was proposed that, even under limited O2

diffusion, the corrosion process of rusted steel

embedded in mortar involves the alternative cathodicreaction of reduction of rust in association with theanodic dissolution of the base metal. The Fe2O3 rich-oxide may oxidize the metal, enhancing the corrosionrate of the system.

ACKNOWLEDGMENTS

The authors acknowledge the assistance ofJ.M. Sykes of the Department of Materials, OxfordUniversity.

REFERENCES

1. L. Von Bogandy, H.J. Engell, The Reduction of Iron Ores, ScientificBasis and Technology (Berlin, Germany: Sprinqer-Verlag, 1971).

2. T. Misawa, K. Hashimoto, S. Shimodaira, Corros. Sci. 14 (1974):p. 131.

3. J.R. Johnson, P. Elliot, M.A. Winterbottom, G.C. Wood, Corros. Sci. 17(1977): p. 691.

4. H. Leidheiser Jr., l. Czako-Nagy, Corros. Sci. 24 (1984): p. 569.5. A. Raman, B. Kuban, Corrosion 44, 7 (1988): p. 483.6. J.A. Gonzalez, S. Algaba, C. Andrade, Brit. Corros. J. 15 (1980):

p. 135.7. O.E. Gjorv, O. Vennesland, A.H.S. El-Busaidy, “Diffusion of Dissolved

Oxygen Through Concrete,” CORROSION/76, paper no. 17 (Houston,TX: NACE, 1976), p. 13.

8. C.L. Page, Nature 258, 5,535 (1975): p. 514.9. C.L. Page, J. Havdahl, Mater. Constr. 18 (1985): p. 41.

10. T. Yonezawa, V. Asworth, R.P.M. Procter, Corrosion 44 (1988).11. M. Stratmann, K. Hoffmann, Corros. Sci. 29, 11/12 (1989): p. 1,329-

1352.12. V. Jovancicevic, J.O’M. Bockris, J. Electrochem. Soc. 133, 9 (1986):

p. 1,797-1,807.13. E. Escalante, M. Cohen, A.K.H. Kahn, “Measuring the Corrosion Rate

of Reinforcing Steel in Concrete,” NBSIR 84-2853, National Bureau ofStandards, Washington, D.C., April 1984.

14. D.A. Hansson, Cement Concrete Res. J. 14 (1984): p. 574.

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Effect of Superficial Oxides on Corrosion of Steel Reinforcement Embedded in Concrete