Corrosion behavior of steel in high alumina cement mortar samples: Effect of chloride

CEMENT and CONCRETE RESEARCH. Vol. 21, pp. 635-646, 1991. Printed in the USA. 0008-88216/91. $3.00+00. Copyright (c)1991 Pergamon Press plc.

CORROSION BEHAVIOUR OF STEEL IN SAMPLES: EFFECT OF C H L O R I D E

HIGH

ALUMINA CEMENT

MORTAR

S.Gofii*, C. Andrade" and C.L. Page*" * Institute of C o n s t r u c t i o n Science "Eduardo Torroja", Madrid. Spain. ** Department of Civil Engineering, Aston University, Birmingham. England. (Refereed) (ReceivedJan.4;infinalformMarch22,1991) ABSTRACT The influence of chloride ion on the corrosion of steel in high alumina cement m o r t a r samples has been i n v e s t i g a t e d by means of the polarization r e s i s t a n c e technique. X-ray d i f f r a c t i o n (XRD) has been employed to evaluate the evolution w i t h time of the crystalline phases. The hexagonal to cubic c o n v e r s i o n of the hydrated aluminates was clearly detected after prolonged storage of all samples being favoured by the presence of chloride ion. The w a t e r liberated from this c o n v e r s i o n produced a dramatic decrease in the resistivity values of the mortars and an increase in c o r r o s i o n intensity for the higher content of chloride tested (1% by w e i g h t of cement). INTRODUCTION The employment of high alumina cement in building construction is severely restricted owing mainly to the loss of strength that the h y d r a t e d material exhibits w h e n stored in certain environments. Many authors have studied the causes of this loss of strength and there is agreement that the principal one is the c o n v e r s i o n of hexagonal to cubic c a l c i u m aluminate hydrate (i-8). In this reaction, water molecules are liberated causing an increase of the capillary porosity and therefore a loss of strength of the material. Factors such as temperature, w a t e r / c e m e n t ratio and C02 strongly influence the stability of the h y d r a t e d aluminates and, at temperatures higher than 30QC, the cubic variety can be obtained directly rather than as a result of c o n v e r s i o n (9-10). On the other hand the subsequent carbonation of this compound leads to more stable carbonated products (11-13). At temperatures < 10~C conversion is not produced and the hexagonal hydrate is formed, remaining u n c h a n g e d for long times (3). 635

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In regard to the corrosion behaviour of reinforcement embedded in mortars made of high alumina cement, very little research appears to have been done, although relatively numerous failures have been reported to result from c a r b o n a t i o n of the cover (14-15). Andrade and co-workers (14), however, systematically studied the c o r r o s i o n process in the presence of several carbonated phases, examining the influence of the hydrated aluminate conversion. Corrosion investigations involving high alumina cement are of interest for several reasons. On the one hand, the pH of the pore solution is lower than that of hydrated P o r t l a n d cement and thus the passive state of the steel may be less stable; on the other hand, the p o s s i b i l i t y of Friedel's salt formation leading to the elimination of free chloride ion from the pore solution phase may be favoured. Another i n t e r e s t i n g question concerns the role that conversion plays in the corrosion process, in the absence of carbonation. The aim of this paper is to report the i n f l u e n c e of the aforementioned factors on reinforcement corrosion behaviour. The chloride ion was i n t r o d u c e d as NaCI during the mixing process at dosages of 0.4 and 1% by weight of cement. Samples were cured at 25~C in a saturated atmosphere for 139 days. The electrochemical study has been undertaken by polarization resistance measurements, and the structural c o m p o s i t i o n of the solid phase hydrates has been d e t e r m i n e d by X-ray diffraction (XRD) .

EXPERIMENTAL PROCEDURE Materials and Specimen Preparation British high alumina cement with the c o m p o s i t i o n given in Table I, was employed to prepare mortar specimens of 2 x 5.5 x 8 cm 3, with w a t e r / c e m e n t ratio of 0.5 and c e m e n t / s a n d ratio of I/3. Chloride additions were made by d i s s o l v i n g the required quantities of AR Grade NaCI in the mix water to yield total chloride ion concentrations of 0.4 or 1.0% by w e i g h t of cement. These levels were chosen to compare the c o r r o s i o n data with those expected for OPC. A c c o r d i n g to previous studies (16), the risk of significant c h l o r i d e - i n d u c e d corrosion in OPC concrete is generally low for total chloride contents b e l o w 0.4 and high for total chloride contents in excess of 1% by weight of cement. Table I A n a l y s i s

of a B r i t i s h

High Alumina

Cement,

CaO

SiO=

AI=O3

Fe203

MgO

37.8

2.72

42.9

15.7

0.75

wt%

The corrosion cell was similar to that used in previous studies (17-18). As shown in Figure i, two identical steel bars and a carbon a u x i l i a r y electrode were embedded in the mortar

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637

specimen. The steel bars of 6mm nominal d i a m e t e r had previously been mechanically polished, d e g r e a s e d in a c e t o n e and weighed. In order to standardise the exposed area (4.2 cm2), ends of the steel bars were coated with insulating tape. A saturated calomel electrode was employed as reference. Samples were kept in a saturated atmosphere at 25~C throughout the experiment.

POTENTIOSTAT WOrkin9

i c \

AA Mor,0r

Fig. 1 Mortar cell employed for the electrochemical measurements

Rebar

8cm

In order to study the solid phases by X - r a y diffraction, a series of samples of identical composition to those used for the corrosion study were prepared and k e p t under similar experimental conditions. The hydration was stopped w i t h alcohol at the moment that each corrosion m e a s u r e m e n t was made. The solid sample was then filtered and dried in a d e s i c c a t o r with silica gel. Techniques Electrochemical measurements of the corrosion potential (Ecorr) and corrosion rate (Icorr) of embedded steel electrodes were obtained from polarization resistance measurements, following the procedures used in previous studies (17-18). An Amel Model 551 p o t e n t i o s t a t with electronic c o m p e n s a t i o n of the ohmic drop (R~) between reference and w o r k i n g electrode was used. The Rp value was determined by Stern and Geary's method (19). A p o l a r i z a t i o n sweep from -i0 to i0 mV about Ecorr was applied to the steel electrodes at a rate of i0 mV.min -~. Icorr was c a l c u l a t e d assuming values of B = 26 mV for corroding steel or 52 mV for passive steel in the S t e r n - G e a r y equation: I Icorr =

E

B x B = .....

Rp

The e l e c t r o c h e m i c a l loss estimated from the integration of the Icorr time curves has been compared with the corresponding gravimetric losses. The good a g r e e m e n t obtained between the two w e i g h t losses validates the B values employed.

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The distance between electrodes was always the same every measurement. The reference electrode was located on surface of the mortar specimen, at 0.7 cm from the counter at 2 cm from the working electrode, as Figure 1 shows.

for the and

X-ray diffraction patterns were obtained using a Philips diffractometer, graphite monochromator. PW-1730~00, CuK~ radiation and analytical software, Philips A P D - 1 7 0 0 (V.S.3.O).

RESULTS Electrochemical Analysis The evolution with time of Ecorr and Icorr for steel bars in mortar specimens with 0.0, 0.4 and 1% CIadditions are plotted in Figure 2. Each point on the graphs is the average of two identical tests. The dashed zone r e p r e s e n t s the boundary between active corrosion (Icorr > 0.2 ~ A . c m -~) and passivity (Icorr < 0.i pA.cm-=), as was justified in p r e v i o u s papers (1718). As can be seen, the results obtained for the samples without CIand with 0.4% CIion show a similar trend in behaviour. The Icorr values are always b e l o w the d a s h e d zone, which indicates that the steels remained p a s s i v a t e d throughout the experiment. This is confirmed by the fact that Ecorr ,value of -200 mV obtained at the beginning of the experiment gradually became more positive, reaching a stable value of about 0 mV.

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CORROSION, STEEL ALUMINOUSCEMENT MORTAR, CHLORIDE

639

In the case of the m o r t a r sample c o n t a i n i n g i% CI- ion, the behaviour is quite different. During the first few days of curing, the Icorr values were above the d a s h e d zone, decreasing gradually towards stable minima, w h i c h i n d i c a t e d a tendency for the steel to repassivate. Nevertheless, after 31 days, the Icorr values started to increase to the limit of the dashed zone and this maximum value was stable for a period of 41 days before starting to decrease again to the p a s s i v e zone. The trends in the Ecorr measurements confirmed the implications of the Icorr data. At the beginning of the experiment values of -300 mV were obtained, which gradually became more positive. After 31 days, however, Ecorr started to decrease signifying that the corrosion was becoming more active, subsequently changing to more positive values which were indicative of repassivation. The corrosion b e h a v i o u r of steel bars embedded in OPC mortar specimens w i t h 0 or 1% CI- as NaCI was characterised by lower Icorr values (below the d a s h e d zone) than those of HAC throughout the experiments. This seems to indicate that a somewhat less aggressive pore electrolyte phase was present in the Portland cement. A notable difference between the b e h a v i o u r of HAC and OPC was observed on comparing the e v o l u t i o n of their electrical resistances (ohmic drop R ~ ) with time, as shown in Figure 3. For HAC, the R ~ values increased rapidly w i t h the progress of the hydration reaction, but then suffered a drastic decrease after a certain period of time, w h i c h was different for the samples with and w i t h o u t chloride ion. This behaviour was not o b s e r v e d in mortar specimens made of P o r t l a n d cement, in which the Pu~ values were virtually stable after a substantial degree of hydration had occurred. It should be mentioned, however, that w h e n OPC was employed, the R ~ values ranged from 500 to i000 ohm., whereas those obtained in the case of HAC were s u b s t a n t i a l l y higher (maximum value 7000 ohm). X - r a y Diffraction Analysis The evolution of the X-ray d i f f r a c t i o n patterns with time is shown in both Figure 4 (without chloride) and Figure .5 (with 1% chloride). In both cases the intensity of the X-ray reflections corresponding to the u n h y d r a t e d compounds decreased progressively as the hydration reaction increased, almost d i s a p p e a r i n g at 131 days. *

Samples w i t h o u t chloride

The hexagonal varieties of the h y d r a t e d aluminate with two d i f f e r e n t stoichiometries (CaAI204. 10H20 and Ca2AI205. 8H=0) were d e t e c t e d at 1 day and the i n t e n s i t y of their X-ray reflections increased up to 6 days (Fig. 4 (b) and (c)).'. T h e r e a f t e r they d e c r e a s e d progressively, a l m o s t disappearing by the end of the experiment (Fig. 4 (d), (e) and (f)).

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S. Gofii et al.

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The cubic variety of the hydrated aluminate (Ca3AI=06. 6H=0) together with Ca3AI=06. 8-12H=0 was d e t e c t e d at 6 days, increasing with time as the hexagonal hydrates decreased (Fig.4 (c), (d), (e) and (f)). Another hydrated compound, aluminium hydroxide, was detected at 26 days, its X-ray reflections increasing with time in parallel with those of the cubic aluminate (Fig., 4 ( d ) , (e) and (f)). * Samples w i t h chloride The presence of the chloride ion r e t a r d e d hydration as can be seen by comparing the intensity of the unoverlapped r e f l e c t i o n of anhydrous CaAI204, w h i c h appears at 30.2~ in the 28 zone; this intensity is always higher in the samples with chloride (Compare Fig.4 versus Fig.5). F r i e d e l ' s salt appears at 1 d a y (See Fig. 5 (b)). It would seem that the intensity of the X-ray reflections of this compound d e c r e a s e thereafter with time (See Fig. 5 (d) and (e)) but increase again at 131 days (Fig. 5 (f)). The hexagonal Ca2Al20s. 8H=O was detected only at 6 days (Fig. 5 (c)). DISCUSSION

two

The XRD results shown in Figures hexagonal varieties of calcium

4 and 5 indicate that the aluminate hydrate are the

641

CORROSION, STEEL, ALUMINOUS CEMENT MORTAR, CHLORIDE

Vo. 21, No. 4

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Vol. 21, No.

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4

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CORROSION. STEEL, ALUMINOUSCEMENT MORTAR. CHLORIDE

643

first compounds formed during the hydration reaction. Nevertheless, these are metastable, converting with time into the cubic variety of c a l c i u m a l u m i n a t e hydrate a c c o r d i n g to the following reactions: 3 (CaAI=04. Ca=Al=0s.

10H20)

~ .......

8H20 + Ca(OH)2

CaaAI=06.

6H=O + 4 AI(OH)a

~==e CaaAI206.

+ 18 H=O

6H=O + 3H=O

In these conversions, the water m o l e c u l e s liberated (32 and 16% by weight of hexagonal hydrated aluminate), are incorporated into the interstitial pore solution. This phenomenon, as is well known, produces an increase of porosity in the material and therefore a loss of strength. The presence of chloride conversion which is produced comparing the intensity of reflections, at 26 and 71 days in Figs, 4 and 5). The CI- ion is bound to the new hydrated (Ca4AI=06CI=. 10H=O). However, not stable over long times, reaction: Ca4Al=06Cla.

ion seems to enhance the at early ages, as can be seen the hexagonal and cubic X-ray (See d i f f r a c t o g r a m s (d) and (e) added during the mixing process, solid phase as F r i e d e l ' s salt it seems that this compound is decomposing a c c o r d i n g to the

10H=O ~=m 4Ca ÷2 + 2AIO=- + 4OH- + 2CI- + 8H=0

If this d e c o m p o s i t i o n is produced, both water m o l e c u l e s and free Cl-ion will be i n c o r p o r a t e d into the pore solution, and therefore embedded r e i n f o r c e m e n t s will be affected from the point of view of their corrosion. According to previous study (20), the decomposition of Friedel's salt may be p r o d u c e d ~f the pH of the pore solution decreases, as is the case if silica fume is added to the cement pastes made of OPC. The decrease of pH, in the p r e s e n t case, is probably due to alkali dilution by the water molecules liberated during conversion. In fact, the electrical resistivity of the mortars drastically decreases when conversion and Friedel's salt d e c o m p o s i t i o n take place (Fig.3) This trend in resistivity could form the basis of another instrumental method for detecting conversion during HAC hydration. With regard to the corrosion behaviour of steel, in the case of mortar specimens w i t h o u t chloride or with 0.4% chloride ion, p a s s i v i t y was m a i n t a i n e d throughout the e x p e r i m e n t (in spite of the somewhat reduced pH values of the pore solution, ranging from 11.4 to 12.5, in comparison with those c h a r a c t e r i s t i c of e q u i v a l e n t OPC pore solution). A possible e x p l a n a t i o n for this b e h a v i o u r is that the [CI-]/[OH-] threshold remains lower than that r e q u i r e d to d e s t r o y the p a s s i v e film. This point is being e x a m i n e d by means of further experiments. In the case of 1% of CI-, Friedel's salt d e c o m p o s i t i o n produces a clear increase of the corrosion i n t e n s i t y values

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(Fig.2), which are higher than those obtained in e q u i v a l e n t OPC mortar specimens. This d i f f e r e n t b e h a v i o u r may be due to the lower pH value of the pore solution and therefore h i g h e r [CI-]/[OH-] ratios. The corrosion activity then decreases reaching negligible values of Icorr after 1-2 months, due to the further binding of chloride ions as Friedel's salt, as can be deduced from the XRD data (Fig. 5 (f)), where the intensity of X-ray reflections of Friedel's salt increase again. As was mentioned in the introduction, the influence of conversion on the corrosion b e h a v i o u r of steel e m b e d d e d in HAC mortar specimens has been p r e v i o u s l y studied (14). The results showed an increase of corrosion intensity (in the absence of chloride ion), associated with the conversion. It should be mentioned, however, that the t e m p e r a t u r e employed to produce the conversion was 40gC, and so carbonated solid phases together with the cubic aluminate hydrate were formed. This simultaneous carbonation probably d e c r e a s e d strongly the pH of the pore solution, thus causing the observed increase of Icorr values.

CONCLUSIONS From deduced:

the present results,

the

following c o n c l u s i o n s can be

The hexagonal to cubic c o n v e r s i o n of the h y d r a t e d aluminates was produced by prolonged storage of all the HAC samples studied and was favoured by the presence of chloride ion. The occurrence of c o n v e r s i o n was d e t e c t a b l e by monitoring ohmic drop values, which d r a s t i c a l l y decreased owing to changes in the r e s i s t i v i t y of the mortars. Friedel's salt was formed in the presence of chloride ions but this compound was relatively unstable decomposing a p p a r e n t l y in response to changes of pore solution composition with time. The conversion of aluminates does not by itself produce d e p a s s i v a t i o n of steel r e i n f o r c e m e n t in mortars containing up to 0.4% CI- by w e i g h t of cement. However, Friedel's salt d e c o m p o s i t i o n and Clliberation to the pore solution leads to the depassivation of steel in HAC mortars c o n t a i n i n g CI-.

1%

HAC is more agressive than OPC for similar chloride content.

ACKNOWLEDGEMENTS S. Gofii g r a t e f u l l y acknowledges: financial support received from the Royal Society, the D e p a r t m e n t of Civil E n g i n e e r i n g of

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CORROSION, STEEL, ALUMINOUSCEMENT MORTAR, CHLORIDE

University of Aston and the C o n s t r u c t i o n Science Institute Eduardo Torroja, for the provision of l a b o r a t o r y facilities.

645

of

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i0

ii

I. Jawed, J. Skalny and J.F. Young: in "Hydration of Portland Cement" (ed. P. Barners) Chapter 6, pp 237317, (1983). R. Alegre: "Etude des effects sur les ciments a l u m i n e u x hydrates de la t r a n s f o r m a t i o n de CaOAI203. IOH20 dans l'action de la temperature" Revue Mat. de Const., 630 (1968) 101-08. M. Ferez, T. Vazquez and F. Trivi~o: "Study of s t a b i l i z e d phases in high alumina cement mortars. Part I H y d r a t i o n at elevated temperatures followed by carbonation", Cem. Concr. Res., 13 (1983) 759-70.

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12

M. Perez, T. Vazquez and F. Trivi~o: "Study of stabilized phases in high alumina cement mortars. Part II Effect of CaCO3 added to high alumina cement mortar subjected to elevated temperature curing and carbonation", Cem. Concr. Res., 14 (1984) i-I0.

13

M. Perez Mendez and F. Trivi~o Vazquez: "Study of the strength developed by stable c a r b o n a t e d phases in high alumina cement", Cem. Concr. Res., 14 (1984) 161-69.

14

M. Perez, F. Trivi~o y C. Andrade: "Corrosi6n de armaduras galvanizadas y sin p r o t e g e r embebidas en cemento aluminoso estabilizado", Mater. de Constr., 182 (1981) 49-68.

15

J. Naumann et H. Baumed: "Rupture par absorption d'hydrogene d'armatures precontraites de beton de ciment alumineux", Archir fur das Einsenhnuttenweser, (1961) pp 89-94. Reference no. 3 appears in no. 15 of this paper.

16

C.L. Page and electrochemistry (1982) 109.

17

C. Andrade and J.A. Gonzalez: "Quantitative measurements of corrosion rate of r e i n f o r c i n g steels embedded in concrete using polarization resistance measurements", Werkst. Korros. 29 (1978) 515.

18

J.A. Gonzalez, S. Algaba J., 15 (1980) 135.

19

M. Stern and A.L. Geary: "A theoretical analysis the shape of p o l a r i z a t i o n curves", J. Elect. Soc., (I) (1957) 56.

20

C.L. Page and O. Vennesland: "Pore solution composition and chloride binding c a p a c i t y of silicafume cement pastes", Mater. et Constr., 16 (91) (1983) 19-25.

K.W.J. Treadaway: "Aspects of of steel in concrete", Nature,

and C. Andrade,

the 297

Br. Corros. of 104