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Effect of concrete cover thickness on hydrogen permeation through cathodically protected steel membranes
ELSEVIER
Materials Chemistry and Physics 49 (1997) 16-21
Effect of concrete cover thickness on hydrogen permeation through cathodically protected steel membranes Jiang-Jhy Chang, Ran Huang, Weichung Yeih Department’of
Harbor
and River Engineering,
National
Taiwan Ocean University,
Keelung,
Taiwan, ROC
Received 17 May 1996; accepted 4 September 1996
Abstract Hydrogenpermeationthroughcathodicallyprotectedsteelmembranes with variousconcretecover thicknesses wasmeasured usingthe electrochemical permeationtechnique.Experimentalresultsshowed&hatthe permeationcurrentthroughdathodicallyprotectedsteelmembranesincreased with the concretecoverthickness.In addition,rich calciumdeposits werefoundon thecathodicsideof the membrane. Keywords:
Hydrogen permeation; Concrete cover thickness; Calcium deposits
1. Introduction Although cathodic protection is a suitableandwidely used method for corrosion prevention of reinforcing steelin concrete, it still presents someproblems which remain to be solved. Physical and chemical inhomogeneitiesand instabilities inherent to the concrete material may leadto nonuniform distribution of the cathodic protection currents and result in localized overprotected areas [ 1,2]. It wasreported [ 31 that the overprotection cathodic current softened the C-S-H gel in concrete, so that both the compressivestrength and the durability of concrete could be reduced. Also, it was found [ 31 that steel-concrete interface debondmentoccurred, and that the integrity of the structure could be affected. In addition to the above-mentioned problems, hydrogen embrittlement of reinforcing steel might result from overprotection [2]. Therefore, there is a need to study hydrogen permeation through ferrous materials embeddedin concrete. A number of studieson hydrogen permeationthrough cathodically protected steelor steelmembranesin various environmentshave beenconducted I:4-91; however, only hydrogen permeation through cathodically protected steel in mortar [ 9] has been reported. There is still no available data on hydrogen permeation through steelin concrete. Hence, this study aimedto investigate the concrete cover effect on hydrogen permeation through cathodically protected steelmembranes
analyzed usinganX-ray fluorescencespectrometer.Also, the microstructure of the steel is illustrated in Fig. 1. The steel used in this investigation had a ‘bandedpearlite and ferrite structure. The steelwas cut into 10 cm diametercircular membranes with thickness from 1.3 mm to 1.6 mm. Both sidesof the steelmembranewere ground using sandpaper(up to MOO). The steel membranewas immersedand cleaned in acetone solution and cleanedby an ultrasoniccleanerfor five minutes. After all the surfacetreatment wa:scompleted,to the cathodic sideof the steelmembranewas attachedanacrylic mold using epoxy glue, and the anodic side of the steel membranewas Table 1 Chemical properties of steel Element Wt.%
C 0.47
Si 0.25
Mn 0.83
P 0.018
s 0.011
Cr 0.25
Fe bal.
2. Experimental The steel usedin this study wasAISI 1045mediumcarbon steel, and the chemical composition, shown in Table 1, was 0254-0584/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PUSO254-0584(96)01903-7
Fig. 1. Micrograph of the steel showing a ferrite/pearlite structure ( X 100).
J.-J. Chang et al. /Materials
Chemistry
sealed to prevent corrosion during the curing process. The concrete mix design is shown in Table 2. The concrete was introduced into the acrylic mold after mixing, and the specimens were vibrated by a mechanical vibration table to minimize the entrapped air and to homogenize the concrete-steel membrane interface [9]. The specimens were cured for 28 days under a relative humidity of 95 + 2% and a temperature of 23 f 1°C. In order to study the effect of the concrete cover thickness on the hydrogen permeation current density, three different cover thicknesses (2, 4 and 6 cm) were selected. To reduce the background current density, the anodic side of the steel membrane was electrochemically plated with nickel film. The specimens w&e immersed into electrolyte. The steel membrane was connected as the cathode, and a 10 cm X 10 cm square titanium mesh was connected as the anode. The composition of the electrolyte and its operational environment are shown in Table 3. The permeation was measured according to the method developed by McCright [lo]. The experimental apparatus included two 7 1 cubic electrolytic cells, reference electrodes (SCE electrodes were used), counter-electrodes (titanium mesh was used), a potentiostat-galvanostat combination and specimens. The test set-up is shown in Fig. 2. The specimens were sealed with silicone sealant, rubber O-rings and screws. The steel membrane acted as a common electrode for both cells. The nickel-coated surface was the hydrogen exit side and was connected to the anode. The other side was the hydrogen entry side and was connected to the cathode. One of the cells (the anode cell) was filled with 0.1 N NaOH solution. The residual hydrogen was extracted by applying a 250 mV (SCE) constant potential in the membrane electrode for 10 h, and a low background current density (0.6 PA cme2) was achieved. Artificial seawater was then introduced into the other cell (the cathodic cell), and the cathodic current was applied afterwards. During the test, hydrogen was cathodically generated at the cathodic side of the membrane electrode, transportedthroughmembrane, and wasanodically oxidized at the anodic side. A 250 mV (SCE) constantpotential was applied to make the permeated hydrogen atoms oxidize into water. The electrochemical reaction was as follow [ll]: I&+OH-+HzO+e-
(1)
The oxidizing current was equal to the hydrogen permeation current moving through the membrane. The cathodic applied currents were 60, 200, 600 and 1500 p,A cm-* for the steel membrane (without concrete
and Physics 49 (1997)
17
16-21
Table 3 The composition of electrolyte and its operational condition for the Watt bath method NiSOl. 6H,O NiClz. 6H,O WO4 pH value
24og1-’ 45g1-’ 3og1-’ 4.5-5.5
Temperature Stirring Bath potential
45-65°C necessary 6-12 v
1. concrete-covermembrane 2. saturatedcalomelelectiode(SCE) 3. counter electrode
membrane--l I I determination cell permeationcell ardfioisl seawater 0.1N NaOH Fig. 2. The test set-up for the hydrogen permeation experiments.
cover). For the 2 cm concrete-covered steel membrane, 20, 60 and 200 PA cm-’ cathodically charging currents were applied. For the 4 cm and 6 cm concrete-covered steel membranes, only a20 PA cm-* cathodically charging current was applied to evaluate the effect of the concrete cover thickness. 3. Data analysis
3.1. Permeation In the experiment, the flux of hydrogen through the specimen was obtained by measuring the current density, i,, which was converted to hydrogen permeation flux using the following equation [ 121:
Table 2 Concrete mix design
(2)
Cement
Water
Fine aggregate
Coarse aggregate
350
204
745
1024
Mix proportions (kg me3) W/C=O.583.
The permeation rate was defined [ 121 as
18
J.-J. Chang et al. /Materials
Chemistry
In the equations, $ is the steady-state permeation current density, n is the number of electrons transferred, F is Faraday’s constant, L is the thickness of the steel membrane and 1, is the steady-state flux. For lattice diffusion control, the expression for the coefficient of permeation, 4, is similar in form to that for gas permeation with the exception that the permeation rate is linear in terms of iE’2, the square root of the constant cathodic charge current density, rather thanpLy, the square root of the constant inlet hydrogen pressure. 3.2. Difision For diffusion as the rate limiting step, Deff is related to the time lag [ 131 and (4) where Deff is the effective diffusion coefficient, and tL is the time lag.
4. Results and discussion
4.1. Hydrogen permeation for the steel membrane without concrete cover The hydrogen permeation current densities for the steel membrane (without concrete cover) protected by various cathodic applied current densities (60, 200, 600, 1500 PA cmm2) are shown in Fig. 3. The hydrogen permeation density is obtained by subtracting the background current density from the measured value. It appears that the hydrogen permeation current density reached a steady value under an applied 60 PA cmU2 cathodic current density. After
.a
0
,“
...,...l,,,,,,,,,,,,,,,
100
Fig. 3. Permeation Table 4 The steady-state
potentials
200
transient
300 Time (min)
for steel in artificial
under various
Cathodic current density ( p,A cm-‘) Steady-state potential (V, SCE)
400
cathodic
current
500
seawater.
densities 20 -0.88
for uncovered
and Physics 49 (1997)
16-21
that, the steady-state hydrogen permeation current density increased as the cathodic current density increased. Two mechanisms are proposed in this case: ( 1) the increasing cathodic current density induced more absorbed hydrogen atoms on the cathodic side of the steel membrane, so that the hydrogen permeation current density increased; (2) on the other hand, the calcium deposit on the cathodic side reduced the adsorbed area of hydrogen atoms, so that the permeation current density decreased. Furthermore, the local cathodic current density increased under a constant applied cathodic current density, which increased the permeation current density. As reported by previous researches [ 9,14,15], the calcium deposit formed on the cathodic side when the cathodically protected current was applied to the steel membrane in the solution containing Ca2+, Mg2+ and Sr2+. In this study, the hydrogen plermeation current density increased as the cathodic current density increased, which implies that the first mechanism dominated. The final steady-state potentials corresponding to the steady hydrogen permeation current density are indicated in Table 4. The iR drop effect is neglected since it is not apparent for steel membrane immersed in the artificial seawater. Before the cathodic current was applied, the initial potential was - 0.54 V (SCE) . As the cathodic currentdensity became higher, a lower corresponding steady-state potential was observed. Hamazah and Robinson [ 161 studied hydrogen permeation for cathodically protected steel in artificial seawater by applying a constant cathodic potential. They found that the hydrogen atom permeated into steel muchmore easily when the applied cathodic potential was lower and the hydrogen permeation current density became higher. The test results in this study correlate fai:rly well with their findings. Although two methods (constant cathodic potential and.constant cathodic current density) are applicable, the constant cathodic current density method is more popular than the constant potential method [ 4-101. Fig. 4 illustrates the relationship between the steady-state hydrogen permeation current density (~7) and the cathodic current density (i,) . The linear regression equation is 1; = 0.021ii’2 + 0.385, in which the linear response coincides with previous research results [9,17], However, it appears that the steady hydrogen permeation current density exists even without applying cathodic current density. This violates physical intuition. Therefore, taking the constraint condition into account, an alternative regression model is proposed and shown in Fig. 4. The firs& regression curve does not pass the origin due to the calcium deposit. As explained above, the calcium deposit increases the local cathodic current density and forms a potential barrier which reduces the permeation current density. It seems that the increasing effect of the local steel membrane 200 - 1.08
600 -1.21
1500 - 1.41
J.-J. Chang et al. /Materials
Chemistry
and Physics 49 (1997)
16-21
19
2.5
N-
$2
y(x)=O.OZlx+O.385
00
20
40
60
80
100
Fig. 4. Permeation current density vs. the square root of charging density for the uncovered steel membrane electrode.
0
current
current density is more significant in causing the linear regression line to shift from the origin. 4.2. The concrete cover effect on the hydrogen permeation For a steel membrane with a 2 cm concrete cover, various cathodic current densities were applied in order to distinguish the effect of the concrete cover. The test results are shown in Fig. 5. It appears that the hydrogen permeation current density declined after passing a peak value and then reached a steady value at 20 FA cm-’ applied cathodic current density. This phenomenon was quite different from that for the steel specimens without concrete cover. This phenomenon can be explained as follows: (1) After 28 days of standard curing, the concrete was almost mature, and the variation in pore size was negligible. The hydrated products containing calcium products and the soluble calcium ions in the pore solution were attracted and migrated to the cathodic side to produce a calcium-rich deposit. The calcium deposit affected the hydrogen permeation, and this effect has been explained in the above paragraph. (2) The dense concrete cover may have affected the effective hydrogen permeation area and conductive area of the cathodic current on the steel membrane. As with the calcium deposit, the concrete cover influenced the hydrogen permeation positively or negatively. (3) The pore solution in the concrete was highly alkaline, so that a passive protection film was formed on the steel surface, which resulted in a barrier for hydrogen adsorption [ 91. However, this protective film may have accelerated the charging-discharging reaction for the hydrogen permeation [ 181, and the hydrogen permeation may have speeded up. (4) During the test, debondment between the interface of the steel membrane and the concrete was observed, and rich calcium deposits on the cathodic side of the steel membrane was found. The interface debondment occurred because the high local cathodic current increased the hydrogen generation, and hydrogen atoms reacted into hydrogen molecules, producing hydrogen pressure in the interface. The interface debondment expanded
100
200 300 Time (min)
Fig. 5. Permeation transient for steel with 2 cm concrete different applied cathodic current density.
400
SO0
cover thickness
at
the surface area of hydrogen adsorption and the conductive area for the cathodic applied current; the former increased the hydrogen permeation current, and the later decreased the hydrogen permeation current by means of a lower local cathodic current density. However, the hydrogen pressure increased the hydrogen permeation rate [9,11]. A scanning electron micrograph of the calcium deposit is shown in Fig. 6, and the energy dispersive X-ray spectrum of the calcium deposit is illustrated in Fig. 7. In this study, 20, 60 and 200 PA cmm2 cathodic current densities were sequentially applied. As indicated in Fig. 5, the permeation current density increased as the applied cathodic current density increased, and no apparent drop occurred under high cathodic current densities (60 and 200 PA cm-‘). This was possibly because that calcium ions nearby the steel-concrete interface reacted during the application of 20 p,A cmm2 cathodic current density, and because the mass transfer of calcium ions from far field to the interface was so slow that the hydrogen permeation current density did not drop further.
Fig. 6. SEM of the interface
showing
the presence of calcium
deposit.
20
J.-J. Chang et al. /Materials
Chemistry
Energy
and Physics
49 (1997)
162i
(k.9)
Fig. 7. Energy dispersive X-ray spectrum of the calcium deposit.
4.3. Cover thicknesseffect on hydrogen permeation
Fig. 8 illustrates the hydrogen permeation current densities for various concrete cover thicknesses under 20 p,A cm-* cathodic current density. It appears that for 2 cm concrete cover steel membrane, the hydrogen permeation current density dropped after passing the peak value. However, specimens with other cover thicknesses had different results. The effect of cover thickness can be explained as follows: when the hydrogen atoms accumulated on the cathodic side, some of the hydrogen atoms penetrated into the steel membrane, and some hydrogen atoms reacted into hydrogen molecules. They may have diffused out of the interface and concrete cover, and the diffusion may have depended on the cover thickness. This is why the sharp drop in the permeation current density after passing the peak value was only observed for the 2 cm concrete cover specimen in which the
-
1.40 -
“5 3
1.20 -
P d 3
calcium deposit effect was more significant than the hydrogen pressure effect (most of the hydrogen molecules diffused out of interface, so that fewer hydrogen molecules accumulated in the interface). For specimens with 4 and 6 cm concrete covers, the hydrogen pressure effect became more significant because the amount of hydrogen molecules in the interface became larger as the concrete cover thickness is thicker. Fig. 9 shows that the hydrogen permeation current density increased as the cover thickness increased as indicated by ElSherik et al. [ 91. They reported that there existed a linear relationship between the steady-state hydrogen permeation current densities ,and mortar cover thicknesses. The mortar cover thickness ranged from 2 to 10 mm. It should be noticed that in engineering practice, a 2 mm to 10 mm concrete cover is not taken into account. Therefore, the concrete thickness considered in this study is greater than 2 cm. Test results show that the hydrogen current densities increased faster in concrete specimens with cover thicknesses from 2 cm to
1.00 0.80 -
$ I
0.60 -
35
0.40 -
&
0.20 -
0.00 .$$1 0
100
200
300 Time (min)
400
500
600
Fig. 8. Permeation transient for steel with different concrete cover thicknessesat an applied cathodic current density of 20 p,A cm-‘.
0
2 Cover
4 thickness
6 (cm)
-1 a
Fig. 9. Permeation current density vs. concrete coverthickness at an applied cathodic current density of 20 p,A crnm2-
J.-J. Chang et al. /Materials
Chemistry
Table 5 Permeation rates and diffusion coefficients for different concrete cover thicknesses Concrete cover thickness
J& (mol H m-r s-‘)
D eff
(cm)
0 2 4 6
5.15x lo-” 5.24X10-” 1.89X 10-r’ 1.97 x lo-‘0
1.49x lo-lo 3.29x lo-r0 2.31 X10-l’ 1.74x lo-lo
( mZs-‘)
4 cm. However, a small increment in hydrogen permeation current density could be found for specimenswith cover thicknessesfrom 4 cm to 6 cm. A nonlinear relationship between the hydrogen current density and the concrete cover thicknessimplies that the concrete cover thicknessbecamea controlling factor for hydrogen permeation. The differences betweenthe concrete and mortar were in the aggregatephase. Compared with mortar, concrete was a more heterogeneous material. The aggregatesacted as potential barriers for the diffusion of hydrogen molecules. The hydrogen molecules on the steelmembrane-concrete interface may have diffused out of the concrete, and the aggregatesalong the interface could effectively interfere with the molecule movement. The maximum aggregate size used in the concrete usually was about 2 cm. Therefore, for specimenswith a concrete cover of 2-4 cm, the aggregate effect significantly affected the hydrogen permeation current density. However, for a concrete cover between 4 cm and 6 cm, thehydrogen permeation current density remainedalmost the same. The hydrogen permeation rates and effective diffusion coefficients were computed and are listed in Table 5. It appearsthat the hydrogen permeation rate increasedas the cover thickness increased, and that the effective diffusion coefficient decreasedasthe cover thicknessincreased.However, the effective diffusion coefficient for the specimenwithout a concrete cover was smallestasindicated in Table 5. The permeation environment for the specimenswith concrete covers was pore solution rather than artificial seawater. 5. Conclusions The nonlinear relationship between the hydrogen permeation current density and cover thickness suggeststhat the
and Physics49
(1997)
i6-21
21
aggregateeffect was more prominent as long as the aggregateswere along the steel-concrete interface. An increasein the concrete cover thicknessresultedin higher hydrogen pressure and accelerated the hydrogen permeation through the steel membrane.The hydrogen permeation current density also increased as the cathodic applied current density increased.The deposititself was apotential barrier for hydrogen permeation, so that the calcium deposit on the cathodic side performed asan inhibitor. Debondment of the interface between the steeland concrete occurred due to the existence of the rich calcium deposit and hydrogen pressure.The calcium deposit increasedthe cathodic current density 1ocalIy and allowed lesshydrogen to permeateinto the steelbyreducing the effective area.
Acknowledgements The authorswant to expresstheir sincerethanks to Dr J.K. Wu for many constructive discussions.
References [l] B.S. Wyatt and D.J. Irvine, Mu&r. Perform., 26 (1987) 12. [2] J.A. Lehmann, Mater. Peform., 26 (1987) 32. [3] Rasheeduzzafar, Mohammad Golam Ali and Ghazi J. Al-Sulaimani, ACI Mater. J., 90 (1993) 8. [4] T. Zakroczmski, Ser. Metail., 19 (1985) 521. [5] T. Zakroczmski and Z.S. Smialowska, J. Electrochem. Sot., 132 (1985) 2548. [6] JR. Scully and P.J. Moran, J. Electrochem. SOL, I35 (1988) 1337. 171 R.B. Hamazah and M.J. Robinson, Corros. Sci., 27 (1987) 971. [S] K.A. Lucas and M.J. Robinson, Corros. Sci., 26 (1986) 705. [9] A.M. El-She& and M. Shirkhanzadah, Corrosion, 48 (1992) 1001. [IO] R.D. McCright, Ph.D. Thesis, The Ohio State Univ., Columbus, OH, USA, 1971. [ 1 l] R.F. Blundy and L.L. Shreir, Corros. Sci., 17 (1977) 509. [12] CC. Juang and J.K. Wu, Corros. Sci., 36 (1994) 1727. [ 131 J. Crank, The Mathematics of DifJsion, Oxford University Press, Oxford, 1977, p. 44. 1141 W.H. Thomason, Mater. Perform., 28 (1988) 63. [ 151 R.U. Lee and J.R. Ambrose, Corrosion, 44 (12) (1988) 887. [ 161 R.B. Hamazah and M.J. Robinson, Corros. Sci., 27 (1987) 971. [ 171 J.M. Chen and J.K. Wu, Corrosion Sci., 33 ( 1992) 657. [18] C.J. Yang, Master’s Thesis, National Taiwan Ocean Univ., Keelung, Taiwan, ROC, 1996.
Materials Chemistry and Physics 49 (1997) 16-21
Effect of concrete cover thickness on hydrogen permeation through cathodically protected steel membranes Jiang-Jhy Chang, Ran Huang, Weichung Yeih Department’of
Harbor
and River Engineering,
National
Taiwan Ocean University,
Keelung,
Taiwan, ROC
Received 17 May 1996; accepted 4 September 1996
Abstract Hydrogenpermeationthroughcathodicallyprotectedsteelmembranes with variousconcretecover thicknesses wasmeasured usingthe electrochemical permeationtechnique.Experimentalresultsshowed&hatthe permeationcurrentthroughdathodicallyprotectedsteelmembranesincreased with the concretecoverthickness.In addition,rich calciumdeposits werefoundon thecathodicsideof the membrane. Keywords:
Hydrogen permeation; Concrete cover thickness; Calcium deposits
1. Introduction Although cathodic protection is a suitableandwidely used method for corrosion prevention of reinforcing steelin concrete, it still presents someproblems which remain to be solved. Physical and chemical inhomogeneitiesand instabilities inherent to the concrete material may leadto nonuniform distribution of the cathodic protection currents and result in localized overprotected areas [ 1,2]. It wasreported [ 31 that the overprotection cathodic current softened the C-S-H gel in concrete, so that both the compressivestrength and the durability of concrete could be reduced. Also, it was found [ 31 that steel-concrete interface debondmentoccurred, and that the integrity of the structure could be affected. In addition to the above-mentioned problems, hydrogen embrittlement of reinforcing steel might result from overprotection [2]. Therefore, there is a need to study hydrogen permeation through ferrous materials embeddedin concrete. A number of studieson hydrogen permeationthrough cathodically protected steelor steelmembranesin various environmentshave beenconducted I:4-91; however, only hydrogen permeation through cathodically protected steel in mortar [ 9] has been reported. There is still no available data on hydrogen permeation through steelin concrete. Hence, this study aimedto investigate the concrete cover effect on hydrogen permeation through cathodically protected steelmembranes
analyzed usinganX-ray fluorescencespectrometer.Also, the microstructure of the steel is illustrated in Fig. 1. The steel used in this investigation had a ‘bandedpearlite and ferrite structure. The steelwas cut into 10 cm diametercircular membranes with thickness from 1.3 mm to 1.6 mm. Both sidesof the steelmembranewere ground using sandpaper(up to MOO). The steel membranewas immersedand cleaned in acetone solution and cleanedby an ultrasoniccleanerfor five minutes. After all the surfacetreatment wa:scompleted,to the cathodic sideof the steelmembranewas attachedanacrylic mold using epoxy glue, and the anodic side of the steel membranewas Table 1 Chemical properties of steel Element Wt.%
C 0.47
Si 0.25
Mn 0.83
P 0.018
s 0.011
Cr 0.25
Fe bal.
2. Experimental The steel usedin this study wasAISI 1045mediumcarbon steel, and the chemical composition, shown in Table 1, was 0254-0584/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PUSO254-0584(96)01903-7
Fig. 1. Micrograph of the steel showing a ferrite/pearlite structure ( X 100).
J.-J. Chang et al. /Materials
Chemistry
sealed to prevent corrosion during the curing process. The concrete mix design is shown in Table 2. The concrete was introduced into the acrylic mold after mixing, and the specimens were vibrated by a mechanical vibration table to minimize the entrapped air and to homogenize the concrete-steel membrane interface [9]. The specimens were cured for 28 days under a relative humidity of 95 + 2% and a temperature of 23 f 1°C. In order to study the effect of the concrete cover thickness on the hydrogen permeation current density, three different cover thicknesses (2, 4 and 6 cm) were selected. To reduce the background current density, the anodic side of the steel membrane was electrochemically plated with nickel film. The specimens w&e immersed into electrolyte. The steel membrane was connected as the cathode, and a 10 cm X 10 cm square titanium mesh was connected as the anode. The composition of the electrolyte and its operational environment are shown in Table 3. The permeation was measured according to the method developed by McCright [lo]. The experimental apparatus included two 7 1 cubic electrolytic cells, reference electrodes (SCE electrodes were used), counter-electrodes (titanium mesh was used), a potentiostat-galvanostat combination and specimens. The test set-up is shown in Fig. 2. The specimens were sealed with silicone sealant, rubber O-rings and screws. The steel membrane acted as a common electrode for both cells. The nickel-coated surface was the hydrogen exit side and was connected to the anode. The other side was the hydrogen entry side and was connected to the cathode. One of the cells (the anode cell) was filled with 0.1 N NaOH solution. The residual hydrogen was extracted by applying a 250 mV (SCE) constant potential in the membrane electrode for 10 h, and a low background current density (0.6 PA cme2) was achieved. Artificial seawater was then introduced into the other cell (the cathodic cell), and the cathodic current was applied afterwards. During the test, hydrogen was cathodically generated at the cathodic side of the membrane electrode, transportedthroughmembrane, and wasanodically oxidized at the anodic side. A 250 mV (SCE) constantpotential was applied to make the permeated hydrogen atoms oxidize into water. The electrochemical reaction was as follow [ll]: I&+OH-+HzO+e-
(1)
The oxidizing current was equal to the hydrogen permeation current moving through the membrane. The cathodic applied currents were 60, 200, 600 and 1500 p,A cm-* for the steel membrane (without concrete
and Physics 49 (1997)
17
16-21
Table 3 The composition of electrolyte and its operational condition for the Watt bath method NiSOl. 6H,O NiClz. 6H,O WO4 pH value
24og1-’ 45g1-’ 3og1-’ 4.5-5.5
Temperature Stirring Bath potential
45-65°C necessary 6-12 v
1. concrete-covermembrane 2. saturatedcalomelelectiode(SCE) 3. counter electrode
membrane--l I I determination cell permeationcell ardfioisl seawater 0.1N NaOH Fig. 2. The test set-up for the hydrogen permeation experiments.
cover). For the 2 cm concrete-covered steel membrane, 20, 60 and 200 PA cm-’ cathodically charging currents were applied. For the 4 cm and 6 cm concrete-covered steel membranes, only a20 PA cm-* cathodically charging current was applied to evaluate the effect of the concrete cover thickness. 3. Data analysis
3.1. Permeation In the experiment, the flux of hydrogen through the specimen was obtained by measuring the current density, i,, which was converted to hydrogen permeation flux using the following equation [ 121:
Table 2 Concrete mix design
(2)
Cement
Water
Fine aggregate
Coarse aggregate
350
204
745
1024
Mix proportions (kg me3) W/C=O.583.
The permeation rate was defined [ 121 as
18
J.-J. Chang et al. /Materials
Chemistry
In the equations, $ is the steady-state permeation current density, n is the number of electrons transferred, F is Faraday’s constant, L is the thickness of the steel membrane and 1, is the steady-state flux. For lattice diffusion control, the expression for the coefficient of permeation, 4, is similar in form to that for gas permeation with the exception that the permeation rate is linear in terms of iE’2, the square root of the constant cathodic charge current density, rather thanpLy, the square root of the constant inlet hydrogen pressure. 3.2. Difision For diffusion as the rate limiting step, Deff is related to the time lag [ 131 and (4) where Deff is the effective diffusion coefficient, and tL is the time lag.
4. Results and discussion
4.1. Hydrogen permeation for the steel membrane without concrete cover The hydrogen permeation current densities for the steel membrane (without concrete cover) protected by various cathodic applied current densities (60, 200, 600, 1500 PA cmm2) are shown in Fig. 3. The hydrogen permeation density is obtained by subtracting the background current density from the measured value. It appears that the hydrogen permeation current density reached a steady value under an applied 60 PA cmU2 cathodic current density. After
.a
0
,“
...,...l,,,,,,,,,,,,,,,
100
Fig. 3. Permeation Table 4 The steady-state
potentials
200
transient
300 Time (min)
for steel in artificial
under various
Cathodic current density ( p,A cm-‘) Steady-state potential (V, SCE)
400
cathodic
current
500
seawater.
densities 20 -0.88
for uncovered
and Physics 49 (1997)
16-21
that, the steady-state hydrogen permeation current density increased as the cathodic current density increased. Two mechanisms are proposed in this case: ( 1) the increasing cathodic current density induced more absorbed hydrogen atoms on the cathodic side of the steel membrane, so that the hydrogen permeation current density increased; (2) on the other hand, the calcium deposit on the cathodic side reduced the adsorbed area of hydrogen atoms, so that the permeation current density decreased. Furthermore, the local cathodic current density increased under a constant applied cathodic current density, which increased the permeation current density. As reported by previous researches [ 9,14,15], the calcium deposit formed on the cathodic side when the cathodically protected current was applied to the steel membrane in the solution containing Ca2+, Mg2+ and Sr2+. In this study, the hydrogen plermeation current density increased as the cathodic current density increased, which implies that the first mechanism dominated. The final steady-state potentials corresponding to the steady hydrogen permeation current density are indicated in Table 4. The iR drop effect is neglected since it is not apparent for steel membrane immersed in the artificial seawater. Before the cathodic current was applied, the initial potential was - 0.54 V (SCE) . As the cathodic currentdensity became higher, a lower corresponding steady-state potential was observed. Hamazah and Robinson [ 161 studied hydrogen permeation for cathodically protected steel in artificial seawater by applying a constant cathodic potential. They found that the hydrogen atom permeated into steel muchmore easily when the applied cathodic potential was lower and the hydrogen permeation current density became higher. The test results in this study correlate fai:rly well with their findings. Although two methods (constant cathodic potential and.constant cathodic current density) are applicable, the constant cathodic current density method is more popular than the constant potential method [ 4-101. Fig. 4 illustrates the relationship between the steady-state hydrogen permeation current density (~7) and the cathodic current density (i,) . The linear regression equation is 1; = 0.021ii’2 + 0.385, in which the linear response coincides with previous research results [9,17], However, it appears that the steady hydrogen permeation current density exists even without applying cathodic current density. This violates physical intuition. Therefore, taking the constraint condition into account, an alternative regression model is proposed and shown in Fig. 4. The firs& regression curve does not pass the origin due to the calcium deposit. As explained above, the calcium deposit increases the local cathodic current density and forms a potential barrier which reduces the permeation current density. It seems that the increasing effect of the local steel membrane 200 - 1.08
600 -1.21
1500 - 1.41
J.-J. Chang et al. /Materials
Chemistry
and Physics 49 (1997)
16-21
19
2.5
N-
$2
y(x)=O.OZlx+O.385
00
20
40
60
80
100
Fig. 4. Permeation current density vs. the square root of charging density for the uncovered steel membrane electrode.
0
current
current density is more significant in causing the linear regression line to shift from the origin. 4.2. The concrete cover effect on the hydrogen permeation For a steel membrane with a 2 cm concrete cover, various cathodic current densities were applied in order to distinguish the effect of the concrete cover. The test results are shown in Fig. 5. It appears that the hydrogen permeation current density declined after passing a peak value and then reached a steady value at 20 FA cm-’ applied cathodic current density. This phenomenon was quite different from that for the steel specimens without concrete cover. This phenomenon can be explained as follows: (1) After 28 days of standard curing, the concrete was almost mature, and the variation in pore size was negligible. The hydrated products containing calcium products and the soluble calcium ions in the pore solution were attracted and migrated to the cathodic side to produce a calcium-rich deposit. The calcium deposit affected the hydrogen permeation, and this effect has been explained in the above paragraph. (2) The dense concrete cover may have affected the effective hydrogen permeation area and conductive area of the cathodic current on the steel membrane. As with the calcium deposit, the concrete cover influenced the hydrogen permeation positively or negatively. (3) The pore solution in the concrete was highly alkaline, so that a passive protection film was formed on the steel surface, which resulted in a barrier for hydrogen adsorption [ 91. However, this protective film may have accelerated the charging-discharging reaction for the hydrogen permeation [ 181, and the hydrogen permeation may have speeded up. (4) During the test, debondment between the interface of the steel membrane and the concrete was observed, and rich calcium deposits on the cathodic side of the steel membrane was found. The interface debondment occurred because the high local cathodic current increased the hydrogen generation, and hydrogen atoms reacted into hydrogen molecules, producing hydrogen pressure in the interface. The interface debondment expanded
100
200 300 Time (min)
Fig. 5. Permeation transient for steel with 2 cm concrete different applied cathodic current density.
400
SO0
cover thickness
at
the surface area of hydrogen adsorption and the conductive area for the cathodic applied current; the former increased the hydrogen permeation current, and the later decreased the hydrogen permeation current by means of a lower local cathodic current density. However, the hydrogen pressure increased the hydrogen permeation rate [9,11]. A scanning electron micrograph of the calcium deposit is shown in Fig. 6, and the energy dispersive X-ray spectrum of the calcium deposit is illustrated in Fig. 7. In this study, 20, 60 and 200 PA cmm2 cathodic current densities were sequentially applied. As indicated in Fig. 5, the permeation current density increased as the applied cathodic current density increased, and no apparent drop occurred under high cathodic current densities (60 and 200 PA cm-‘). This was possibly because that calcium ions nearby the steel-concrete interface reacted during the application of 20 p,A cmm2 cathodic current density, and because the mass transfer of calcium ions from far field to the interface was so slow that the hydrogen permeation current density did not drop further.
Fig. 6. SEM of the interface
showing
the presence of calcium
deposit.
20
J.-J. Chang et al. /Materials
Chemistry
Energy
and Physics
49 (1997)
162i
(k.9)
Fig. 7. Energy dispersive X-ray spectrum of the calcium deposit.
4.3. Cover thicknesseffect on hydrogen permeation
Fig. 8 illustrates the hydrogen permeation current densities for various concrete cover thicknesses under 20 p,A cm-* cathodic current density. It appears that for 2 cm concrete cover steel membrane, the hydrogen permeation current density dropped after passing the peak value. However, specimens with other cover thicknesses had different results. The effect of cover thickness can be explained as follows: when the hydrogen atoms accumulated on the cathodic side, some of the hydrogen atoms penetrated into the steel membrane, and some hydrogen atoms reacted into hydrogen molecules. They may have diffused out of the interface and concrete cover, and the diffusion may have depended on the cover thickness. This is why the sharp drop in the permeation current density after passing the peak value was only observed for the 2 cm concrete cover specimen in which the
-
1.40 -
“5 3
1.20 -
P d 3
calcium deposit effect was more significant than the hydrogen pressure effect (most of the hydrogen molecules diffused out of interface, so that fewer hydrogen molecules accumulated in the interface). For specimens with 4 and 6 cm concrete covers, the hydrogen pressure effect became more significant because the amount of hydrogen molecules in the interface became larger as the concrete cover thickness is thicker. Fig. 9 shows that the hydrogen permeation current density increased as the cover thickness increased as indicated by ElSherik et al. [ 91. They reported that there existed a linear relationship between the steady-state hydrogen permeation current densities ,and mortar cover thicknesses. The mortar cover thickness ranged from 2 to 10 mm. It should be noticed that in engineering practice, a 2 mm to 10 mm concrete cover is not taken into account. Therefore, the concrete thickness considered in this study is greater than 2 cm. Test results show that the hydrogen current densities increased faster in concrete specimens with cover thicknesses from 2 cm to
1.00 0.80 -
$ I
0.60 -
35
0.40 -
&
0.20 -
0.00 .$$1 0
100
200
300 Time (min)
400
500
600
Fig. 8. Permeation transient for steel with different concrete cover thicknessesat an applied cathodic current density of 20 p,A cm-‘.
0
2 Cover
4 thickness
6 (cm)
-1 a
Fig. 9. Permeation current density vs. concrete coverthickness at an applied cathodic current density of 20 p,A crnm2-
J.-J. Chang et al. /Materials
Chemistry
Table 5 Permeation rates and diffusion coefficients for different concrete cover thicknesses Concrete cover thickness
J& (mol H m-r s-‘)
D eff
(cm)
0 2 4 6
5.15x lo-” 5.24X10-” 1.89X 10-r’ 1.97 x lo-‘0
1.49x lo-lo 3.29x lo-r0 2.31 X10-l’ 1.74x lo-lo
( mZs-‘)
4 cm. However, a small increment in hydrogen permeation current density could be found for specimenswith cover thicknessesfrom 4 cm to 6 cm. A nonlinear relationship between the hydrogen current density and the concrete cover thicknessimplies that the concrete cover thicknessbecamea controlling factor for hydrogen permeation. The differences betweenthe concrete and mortar were in the aggregatephase. Compared with mortar, concrete was a more heterogeneous material. The aggregatesacted as potential barriers for the diffusion of hydrogen molecules. The hydrogen molecules on the steelmembrane-concrete interface may have diffused out of the concrete, and the aggregatesalong the interface could effectively interfere with the molecule movement. The maximum aggregate size used in the concrete usually was about 2 cm. Therefore, for specimenswith a concrete cover of 2-4 cm, the aggregate effect significantly affected the hydrogen permeation current density. However, for a concrete cover between 4 cm and 6 cm, thehydrogen permeation current density remainedalmost the same. The hydrogen permeation rates and effective diffusion coefficients were computed and are listed in Table 5. It appearsthat the hydrogen permeation rate increasedas the cover thickness increased, and that the effective diffusion coefficient decreasedasthe cover thicknessincreased.However, the effective diffusion coefficient for the specimenwithout a concrete cover was smallestasindicated in Table 5. The permeation environment for the specimenswith concrete covers was pore solution rather than artificial seawater. 5. Conclusions The nonlinear relationship between the hydrogen permeation current density and cover thickness suggeststhat the
and Physics49
(1997)
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21
aggregateeffect was more prominent as long as the aggregateswere along the steel-concrete interface. An increasein the concrete cover thicknessresultedin higher hydrogen pressure and accelerated the hydrogen permeation through the steel membrane.The hydrogen permeation current density also increased as the cathodic applied current density increased.The deposititself was apotential barrier for hydrogen permeation, so that the calcium deposit on the cathodic side performed asan inhibitor. Debondment of the interface between the steeland concrete occurred due to the existence of the rich calcium deposit and hydrogen pressure.The calcium deposit increasedthe cathodic current density 1ocalIy and allowed lesshydrogen to permeateinto the steelbyreducing the effective area.
Acknowledgements The authorswant to expresstheir sincerethanks to Dr J.K. Wu for many constructive discussions.
References [l] B.S. Wyatt and D.J. Irvine, Mu&r. Perform., 26 (1987) 12. [2] J.A. Lehmann, Mater. Peform., 26 (1987) 32. [3] Rasheeduzzafar, Mohammad Golam Ali and Ghazi J. Al-Sulaimani, ACI Mater. J., 90 (1993) 8. [4] T. Zakroczmski, Ser. Metail., 19 (1985) 521. [5] T. Zakroczmski and Z.S. Smialowska, J. Electrochem. Sot., 132 (1985) 2548. [6] JR. Scully and P.J. Moran, J. Electrochem. SOL, I35 (1988) 1337. 171 R.B. Hamazah and M.J. Robinson, Corros. Sci., 27 (1987) 971. [S] K.A. Lucas and M.J. Robinson, Corros. Sci., 26 (1986) 705. [9] A.M. El-She& and M. Shirkhanzadah, Corrosion, 48 (1992) 1001. [IO] R.D. McCright, Ph.D. Thesis, The Ohio State Univ., Columbus, OH, USA, 1971. [ 1 l] R.F. Blundy and L.L. Shreir, Corros. Sci., 17 (1977) 509. [12] CC. Juang and J.K. Wu, Corros. Sci., 36 (1994) 1727. [ 131 J. Crank, The Mathematics of DifJsion, Oxford University Press, Oxford, 1977, p. 44. 1141 W.H. Thomason, Mater. Perform., 28 (1988) 63. [ 151 R.U. Lee and J.R. Ambrose, Corrosion, 44 (12) (1988) 887. [ 161 R.B. Hamazah and M.J. Robinson, Corros. Sci., 27 (1987) 971. [ 171 J.M. Chen and J.K. Wu, Corrosion Sci., 33 ( 1992) 657. [18] C.J. Yang, Master’s Thesis, National Taiwan Ocean Univ., Keelung, Taiwan, ROC, 1996.