CHAPTER SIX

6) CHARACTERIZATION METHODS

Variations in the thickness of the concrete layer between 10 and 50 mm appear, according to Fl G 87, to affect the permeability more than do the W/C variations. One should, however, be aware that variations in the con-crete cover entail the following:

a thicker concrete cover gives better moisture curing and, therefore, a hi g her degree of hydration.

the quantity of open pores must decrease when the degree of hydra-tian increases and the thickness increases.

the degree of pore fillin g mig h t, perhaps, have been different for thicker concrete covers despite the fact that the specimens were per-mitted to dry out for about 6 months.

a thinner specimen would also have been more inclined to crack, particular ly at the phase limits between aggregate and paste.

FIG 87 thus presents the combined effect of a.large number of different mechanisms on the permeability of the concrete. On the other hand, the same conditions apply to a concrete cover in a normal structure.

The most important parameter with regard to the diffusion of oxygen in concrete is the moisture content or degree of pore filling of the concrete.

This agrees with the theoretical discussion presented previously, see Fl G 88. The greater the moisture content of the material the more difficult i t is for the oxygen to penetrate the concrete. Moisture curing is significant in this context since even as low a value as 80% relative humidity during the conditioning has considerable effects. The moisture curing during the first phase is thus a factor of greater importance for the permeability of the concrete cover and, consequently, for its resistance to penetration against co2 , than for the strength effects obtained from the moisture curing.

0eff. 02 [m2/s,. 10-e J

10 5 2

5 2 0,1

5 2 0,01

First 7 d in 10Ö% RH

curing condit i ens

r~alue for

L.!J

mm glass

o

10 20 30 40 50 60 70 80 90 100 RH.%

e

Portland cement, w/c 0,50, 7 d in 100.% RH

x 0,67, eonstant RH

6 0,42, eonstant RH

o 0,50, eonstant RH

o

0,50, 7 d in 80% RH

FIG 88. Measured effective diffusion coefficient for 02 with varying con-ditioning and moisture state. The cement typ e was. Slite Portland cement. The measurement was carried out at 20°C. Specimen age 6-12 months.

Finally, Fl G 89 shows the significance of the cement type for different diffusion coefficients. Slag cement gives a lower value throughout des pi te the f act that the porosity can be said to be of the same magnitude. The larger hygroscopicity of slag cement probably plays some part in this. Slag cement has a low permeable pore system at a relative humidity as low as 80% and a W l C

=

O, 40 as a result of pores sealed with liquid. 1t should also be noted that the strength was considerably lower throughout for the slag cement concrete when compared with pure Portland cement concrete with the same W l C.

FIG 89.

· Det f. 02 1m2/s 1e

10S

J

10 5 2

5 2 0,1

5 2 0,01

o

10 20 30 40 50 60 70 80 90 100 RH.%

Effective diffusion coefficient for 0 2 for Slite Portland cement anl} slag cement (65% slag + 35% Portrand). concrete. Temperature 20 C. Specimen age 6-12 months.

An inititial theoretical consideration of

o

2 diffusivity in completely water saturated and completely dry concrete respectively might be expected to result in a difference on an order of si z e of 3-4 time s the power of 10. 1t has not been possible to establish this in the .studies. De spi te increased

accuracy in the measurements, the content of nitrogen gas amounting to 5 ppm 0 2 may have been too high for the w ater saturated concrete. W ater satJrated specimens gave the same values as were obtained from measure-ments on a 10 mm thick plexiglass sheet. Furthermore, i t is possible t hat a liquid-filled pore system may have less physical effect than a gas-filled pore system since it is more difficult for the gas molecules to pass through the denser liquid medium, in other words the free medium wavelength of the molecules is different in the t wo media.

5.2 Corrosion investigations with corrosion cells

5.2.1 General

The atmospheric corrosion of various metals has been studied at the Swedish Corrosion Institute. A corrosion cell for measuring the corrosion current under different environmental conditions was developed for this purpose, see Kucera and Collin /1977/. The design of the cell is illustra-ted in FIG 90.

FIG 90.

1- Re

v

General arrangement of electrochemical device for measurement of atmospheric corrosion:

Am zero resistance anmeter, the circuit of which is shown to the right

B electrochemical cell of electrolytic type a electrodes b insulators V extern al emf.

v

Thin steel plates with a thickness of l mm were assembled and insulated from each other by means of a thin plastic film of polycarbonate with a thickness of O .l mm. The cell was then east in to epoxy, after w hi ch on e surface was exposed by means of grinding so that the edges of the steel plates came in direct contact with the corrosion medium. The corrosion process was controlied by means of an external voltage source so that a minor difference in potential occurred between adjacent metal surface s.

During the atmospheric corrosion investigation, the voltage supplied was such that the potential difference amounted to 200 m V. This abnormal difference in voltage was not regarded as having any noteworthy effect on the corrosion process. The corrosion current between the artificial an o die and cathodic areas was regarded as a measure of the actual rate of corro-sion in the metal which was studied.

-15V +15V

1/2 MC 1458 10k

-15V

FIG 91.

Diagram source for tor and source for (the bottom

showing voltage electronic

integra-external voltage electrolytic cells figure).

B 1?

leve l

_Q

~~ l ~F

+ 15 v

-15V Cell i100kl,J l,J l,J 1,1100 1.---A-L-) DC output

current

in )(1

1M balance

47

-:--=-

-15V

~y:9"""'-AC or DC time CX

supply coun!er

2k~• l

rO

~----1

220K

100

-=

Set point

ramp 2k

ch ange

-::-FIG 92. Diagram showing electronic integrator with two current ranges. l

High range counter

+15 v

Ramp change re la y

+24V

Low rmge counter

...

O>

o

The current was measured by means of a special integrator which acted as a zero ammeter, see FIGS 91 and 92. Naturally, not all the current mea-sured corresponded precisely to the weight loss obtained during the com-parative investigation. Particular ly in the c ase of dry elimates, the distance between the anodic area and the cathodic area became so small t hat the plastic film could not be bridged over, de spi te the f act t hat the areas h ad a potential difference of 2 00 m V. On the other hand, a too large potential difference could not be selected since this would have influenced the natural corrosion process to an excessively high degree. Consequently, only part of the corrosion current was recorded. This current, w hi ch is known as the cell factor and is the relation between the measured current converted to corrosion rate and the actual corrosion rate, varied between O. 05 and O .15 in the investigations carried out by Kucera and Collins /1977 l. The cell factor increased within increases in the degree of deposit of, for example,

so

2 on the ruetal surface. This indicated t hat the cells should function better in concrete in which there is always a thick con-crete cover with good conductivity in contact with the exposure surface.

5.2.2 Preliminary investigations and short-term experiments

A number of preliminary investigations were carried out to determine if this type of cell could be used for studying the corrosion of steel in concrete.

First of all, one cell was placed in a saturated Ca(OH) 2 solution and another cell was placed in distilled w ater. The difference in potential between the anodic area and the cathodic area was set at 100 m V. The currents which were recorded from these cells had roughly the same magnitude to begin with. The cell which was placed in water began, however, to corrode immediately. This could be noted d urin g the first in-spection after 10 ho urs. The corrosion current remained at the same lev el as at the start for this cell. The cell in the Ca(OH) 2 solution did not show any corrosion attacks during the time when the environment was strongly basic and di d not contain aggresive substances. The corrosion current al so decreased gradually so that after an exposure period of 7 days it was approximately O. 02 of the current first measured. This decrea se in the current shows that the ruetal surface was passivated by an oxide layer.

NaCl solution was added on. the seventh day so that the concentration

around the cell amounted to 1%. The solution was still sa tura te d with Ca(OF!) 2 . A corrosion attack on certain anadie surfaces could clearly be seen af ter 24 ho urs in this solution, see P hot os 6 and 7 . It could al so be noted that the corrosion current had increased to the same level as that at the start or to the level which the other cell in water had. The recorded current had thus increased markedly as a result of the fact that the corrosion process ha d start ed.

Against this background, this cell typ e was re garde d as suitable for at least providing an indication of when the corrosion process had been initiated.

Four corrosion cells were east in concrete as illustrated in FIG 93 in the next stage. The aim was to stud y the characteristics of the cells in standard concrete which not only has an alkaline pore solution but also act s as a physical and chemical barrier for certain substances. One thick and one thin reinforcement bar were placed on either side of each cell.

These permitted a visual assessment of the fact that the cells bellaved in the same way as the bars. The cover in this c ase was 7 mm for both cells and the bars. Two different concrete qualities called KC1-76 and KC2-76 were u sed in the stud y.

200

o ,-, u

o

,r

l

Co11crete beom

800 /

l / /

/

/ Electrochemicol

.-L--,

cell

l

l c======:::::--=- -=-l-_::-_+_-_-_---=--=---::...----==

::J

Cover 7 mm Bors ~ 5 011d Qi 11

FIG 93. Placing of cells and reference bars i11 specimen s.

After casting, the specimet1s were moisture cured for 10 day s. The experi-ments then began. H alf of the specimens were placed in a 3% NaCl solution and the other half were place d in ordinary t ap w ater. The arabient tempe-rature was 20+2°C. In order to increase the corrosivity of the environment adjacent to the embedded bars, the specimens were alternately dried and

wetted. The currents w hi ch flowed in to the cells we re measured from the moment when the samples were lowered into the vessels at the beginning of the experiment. The applied difference in potential was set at 50 mV in this case. The experiment continued for ab out 60 day s and show ed the results which can be seen in FIGS 94 and 95. In the middle of the experi- · mental period, the potential of the embedded reference bars was measured with a saturated calomel electrode. The result is presented in Table 18.

10

N

~ E

<(

~

.

....

c

Cl.l

... ...

;::)

u 0,1 Qj

o

0,01

O,OOl

w

D

l l

w

W4hw4h

D

i U

D

W= wetting D=drying .

.j, W4h = wetting. 4 ho,urs

O KC1 (w/c-.0.9) in 3% NaCI-solution O KC1 in water

500 1000 1500

Time, h

FIG 94. Preliminary experiment with corrosion cells which were east in concrete with W l C approximately O. 9. One of the specimens was dipped during a number of periods in an NaCl solution while the qther was dipped in w ater. The increase in current from the cell which was exposed to the chloride solution can clearly be seen.

The above values are potentials measured with a saturated calomel elec-trode. The measurements were carried· out 30 days before the experiments were discontinued. All speciiTJens were moist since they were taken up for drying out. The potentials are so low t hat they indicate t hat the steel is in an active state.

o 10

~

1..)

'8

c 1 o

N

~

~

0,1

...

c '

Q) L..

L.. 0,01

:J 1..)

u Q)

0,001

w D w

500

O KC2 (w/c-05) in 3% NaCI solution O KC2 in water

1000

Time, hours 1500

FIG 95. Prelir.~inary experir.~ent with corrosion cells which were east in eoneretc with W l C approximately O. 5. One of the specimens was dipped in an NaCI solution during a number of periods while the other was dipped in w ater. The increase in the current from the cell which was exposed to the chloride solution can clearly be seen.

FIG 94 can be interpreted in the following manner:

During the first period in the salt solution, a sufficient quantity of acti-vated substances - in this case, chloride - penetrate to th'e cell. The corrosion process is initia te d. This can be seen through the marked in-crease in current at the end of the period.

During the drying procedure, the current intensity continues to increas1e until it reaches a maximum in conjunction with the next wetting operation.

The reason for this may be an increase in or the formation of new anodic areas on the cell.

During the next wetting period, the current intensity decreases assymp-totically towards a limit value. The relation between the anodic areas and the cathodic areas has now been stabilized and wetting does not initiate an y furthe r corrosion areas. The cathodic areas, on the other hand,

con-sume the 0 2 which occurs in the immediate surroundings. The process

be-come~:> mo re and mo re cathodically controlied.

T AB. 18 . Result of measurement of potential on bars exposed adjacent to corrosion cells KC1-KC2. Saturated calomelectrode was used.

Potentials indicated in volts.

X mm V

100 200 300 KC1 ;H20 6 5 -0,565 -0,565 -0,565

6ll

-0,600 -0,600 -0,595 KC1 ;H20 6 5 -0,600 -o;6oo -0,600 + Cl

6ll

-0,595 -0,596 -0,596 KC2;H 20 6 5 -0,385 -0,365 -0,360 611 -0,610 -0,610 -0,610 KC2;H 20 6 5 -0,560 ~0,560 -0,560 + Cl

6ll

-0,658 -0,655 -0,655

400 500 -0,565 -0,560 -0,595 -0,590 -0,600 -0,600 -0,595 -0,596 -0,360 -0,365 -0,610 -0,610 -0,555 -0,552 -0,654 -0,652

600 -0,565 -0,590 -0,602 -0,594 -0,360 -0,610 -0,550 -0,650

Concrete cover 7 mm

700 -0,560 -0,590 -0,603 -0,598 -0,360 -0,605 -0,549 -0,646

A marked increase in the corrosion current occurs immediately in conjunc-tion with the next drying process and reaches a higher absolute value t han before. The drying process eauses w ater to evaporate at the concrete surface. This, in turn, disturbs the entire liquid balance in the specimen.

The catodically controlied process ceases since 0 2 is no longer forced to diffuse to the corrosion areas. The transport is now mainly carried out by the liquid movement which was started by the drying process. At the same ti!lle, further anodic areas are formed since the effectiveness of the cathodic areas has increased at the same time as the chloride concentration around the cell has increased.

Shortly after the drying process was started for the seeond time, the corrosion process reached its highest val u e so far. The rate of corrosion t hen decreased in pace with the drying out of the specimen. The optimum conditions were disturbed by the fact that the electric contact was gradually impaired between the anodic and cathodic areas.

The same development pattern, in principle, was followed du ring continued eyeles of wetting and drying.

The passivated cell, in other words, the specimens which had been expo-sed to ordinary t ap w ater, show ed a small corrosion current throughout the entire experiment. The drying and wetting eyeles disturbed the con-ditians, however. We can assume that the passive state changes over to an active state throughout the seeond long wetting period but that the rate of corrosion remains very low due to complete water saturation and a lack of 0 2 . This is also indicated by the results from the potential measurements.

W hen the specimen was later dried, the potential and the rate of corrosion increased to such an extent that the active state changed back to a passive state. The longer the experiment t hen continued with a satis-factory supply of 0 2 , the smaller became the recorded current. This indi-cates a very stab le passive state.

The results presented in FIG 95 follow the results presented in FIG 94 to a considerable extent. The better concrete qp.ality, i. e. the less permeable p o re systems with all t hat entails, the higher threshold values, the mo re difficult transport conditions etc giv e ris e to the following reflections:

During the first wetting process in NaCl solution, the chlorides are not capable of initiating corrosion attacks. A not insignificant quantity of C l-has, on the other hand, penetrate d to the pore system. This manifests it-self during the subsequent drying period. The water evaporation from the concrete surface eauses the pore system to be emptied of w ater. The sub-stances which occur in the pore solution thus increase in concentration.

The chemical equilibrium reactions do, admittedly, cause the larger quau-tities to be fixed in the solid phase when the concentration in the solution increases but the high concentration of activating substances in the solu-tion becomes the decisive factor. In this c ase, it is chloride w hi ch, w hen the concentration on the surface is increased, eauses an increase in the

diffusion rate towards the steel. This transport is, how ev er, inhibited samewhat by the liquid moving in the opposite direction. By way of summary, it can be said that the concentration of chlorides will increase adjacent to the steel surface in conjunction with drying as well as wetting.

Consequently, initiation also occurs in this case during the drying period.

Another factor which also influences the intiation procedure in this case consist s of the mutually chan ge d anodic and cathodic reactions. D urin g the drying, the reaction is displaced, thus entailing an increase in the equi-librium potential. This can al so be an accelerating factor. W hen the corro-sion process has been initia te d, we can see that the cathodic control in conjunction with wetting becomes more noticeable in a concrete which has a low W l C. This al so applies to the current control in conjunction with a shortage of electrolyte or a drying out.

As far as the potential measurements are concerned, the results seem to indicate that all the steel bars are in an active state. The experiment cannot, however, ten u s anything ab out the corrosion rate.

The potential measurements thus support what has been said above con-cerning the passive cells, i. e. t hat they alternate between an active and a passive state bu t t hat the rate of corrosion is very low all the time.

The experiments were concluded by breaking apart the specimens so that any corrosion attack on the cells or on the referent:e steel could be studied ocularly. Photos 8-13 are intended to mustrate the results.

All of the reference bars showed small corrosion attacks at the end sur-faces due to the minor adhesion failures which always occur around a smooth bar w hi ch projects from concrete. The er ack had given rise to a local earbonaHon with corrosion attacks as a result. The fact t hat the end surfaces of the steel bars were sealed with silicone rubber also contributed in all certainity to the corrosion of the end surface s.

Apart from t hat, visible corrosion attacks could only be not ed ·on steel specimens which had been exposed to a chloride. solution. This applies both to cells and to reference bars.

Against this background, the corrosion cells we re deemed to be suitable for:

studies of the mechanism involved in conjunction with changes in environmental conditions or other comparative studies

indications of corrosion initiation

On the other hand, the experiments tell us nothing about the suitability of the cells for absolute measurements of the rate of corrosion.

5.2.3 Long-term experiments and mechanisms studies

General

Three different types of cell were east in concrete in order to study corrosion mechanisms and the influence of a number of important para-meters on the corrosion process. The three typ e s of cell we re:

A. The packa ge mo del w hi ch consiste d of t hin plates in accordance with FIG 90.

B. Cells of smooth, cold-worked reinforcement steel, see FIG 96.

C. Cells of thin plates, in which each plate had a wire connector of its own, see FIG 97.

At the time in question here, moisture measuring equipment which was, in principle, new and which made it possible to record the relative humidity in concrete, had been developed at Lund University of Technology, see Nilsson l 1977 l. Since the moisture state closest to the corrosion areas is of decisive significance, specimens were also p repared for this measuring proeecture, which entailed placing a plastic tu be at the same level as that of the corrosion cells. Du ring this latter measurement, a p robe could be guided down into the tube for measuring the relative humidity in the pore system which was closest to the tub e opening at the bottom. See FIG 98.

All of the material data is presente d in Appendix 4.

In document New catalytic systems based on nanodispersed metals in inorganic matrices prepared using the sol-gel techniques (Page 68-82)