Electrochemical Measurements on Previously Conditioned in Concrete Reinforcing Steel in Cement Extract Solutions

210th ECS Meeting , Abstract #817, copyright ECS

Electrochemical measurements on previously conditioned in concrete reinforcing steel in cement extract solutions.

D.Koleva1, K.van Breugel1, J.H.W.de Wit2, V.Bachvarov3

1_{Delft University of Technology, Dept.Civil Engineering}

&Geosciences, Stevinweg 1, 2628 CN Delft, Netherlands

2_{Delft University of Technology, Dept.Materials Science}

&Engineering, Mekelweg 2, 2628 CD Delft, Netherlands

3_{BAS, IPC, Acad.G.Bonchev,11, Sofia 1000 , Bulgaria}

The objective of the present work is investigation of the electrochemical behavior of reinforcing steel in cement extract (CE) solutions. The reinforcing steel was previously embedded in concrete and maintained in conditions of corrosion and two regimes of cathodic protection (pulse CP and conventional CP). The evaluation was made on the basis of correlating electrochemical parameters, derived from impedance spectroscopy (EIS), cyclic voltammetry (CVA) and potentio-dynamic polarization (PDP). Furthermore, morphology and composition of these layers were determined using scanning electron microscopy (ESEM) and energy-dispersive X-ray analysis. Using CE as electrolyte medium and combination of methods, information for the properties of the previously formed on the steel surface product layers was derived and further allowed evaluation of the CP effectiveness, which normally is assessed using empirical criteria only. The materials used were steel electrodes from the reinforcing steel, comprising groups R – non-corroding, C – corroding, A – pulse CP protected and B – CP protected. All measurements were performed in a three-electrode cell, using Pt as counter electrode and SCE as reference, electrolyte CE, pH 12.6 at room t°. EIS was carried out by superimposing an AC voltage of 10 mV in the frequency range of 50 kHz to 10 mHz. Scan rate of 0.5 mV/s was used for PDP and 200 mV/s for CVA measurements.

As the steel was previously maintained in different technical conditions in the concrete specimens, the product layers present certain morphology and composition, related to the relevant conditions. Fig.1 depicts the steel surface for specimen R (a), specimen C(b) and specimens A and B (c,d).

Fig.1 Surface morphology, specimens R(a),C(b),A(c)&B (d)

Fig.2 PD curves for all specimens in CE solution.

The product layers in specimens R are enriched in calcium, specimen C depicts significant extent of localized corrosion, the steel surface in specimens A and B is relatively clean and mainly magnetite and hematite are present. The different composition and morphology was expected to bring about different electrochemical behavior of the steel electrodes in the CE solution. Fig. 2 presents PD curves for all specimens, revealing the similar behavior for A and B with external polarization. Table 1 presents the calculated polarization resistance (Rp) values, proving the CP efficiency.

Table 1. Rp values calculated from PDP measurements. After prolonged CP, the corrosion rate for protected steel (A,B) (being considered as relatively bare steel) is higher (lower Rp values respectively) than non-corroding steel (R). However, Rp values for A and B are higher than corroding conditions (specimen C). The latter observations are additionally proved by EIS response – Fig.3, best fit parameters – Table 2 and CVA measurements – Fig.4.

Fig.3 EIS response in Nyquist(left) and Bode(right) format

Table 2 Best fit parameters for the EIS response.

Fig. 4 CVA curves: cycles 1 to 15 for R and B (top) and overlay of cycle 15th for all specimens (bottom).

The research demonstrates that with external polarization specimens A and B have better performance than specimen C. Further, the CP efficiency is evaluated in a fundamental way and additionally, based on the similar electrochemical behavior of specimens A and B it can be concluded that the pulse CP is as effective as the conventional CP regime.

Rp A B C R kΩ.cm2 _{13.07 18.93 1.79 85.06} Cell Rel Ω Rox.l kΩ.cm2 _µF/cmCox.l 2 _k Rp Ω.cm2 _µF/cmCf 2 A (pCP) 3.1 0.003 312.2 18.5 32.4 B (CP) 3.0 0.009 46.3 24.4 50.2 Cell Rel Ω Rox.l kΩ.cm2 Qox.l [Yo], n 1/Ω Rp kΩ.cm2 Cf µF/cm2 C (corrod.) 3.0 0.24 0.4358e-04 n=0.5243 9.02 63.7 R (refer.) 2.9 0.13 0.3716e-04 n=0.5376 116.8 123.6

Overlay, specimens R,C,A and B, scan rate 0.5 mV/s, range -200 to +1800 vx OCP

-1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 1.00 -8 1.0x10 -7 1.0x10 -6 1.0x10 -5 1.0x10 -4 1.0x10 -3 1.0x10 -2 1.0x10 E / V (SCE i / A R C A B 0 1K 1K 2K 2K 3K 3K 4K 4K 0 1K 1K 2K 2K 3K 3K 4K 4K Z' / ohm -Z'' / ohm R A B -2 -1 0 1 2 3 4 5 0 1 1 2 2 3 3 4 4 0 10 20 30 40 50 60 70 log(f) log(Z)(o) -p h a s e / d e g (+ )

cR, CE, 200mV sc rate, scan 1 - 15

-1.678-1.428 -1.178-0.928-0.678 -0.428-0.178 0.072 0.322 0.572 0.822 -1 -0.668x10 -1 -0.418x10 -1 -0.168x10 -1 0.082x10 -1 0.332x10 -1 0.582x10 E / V i / A

cB, May 23'06, CE, 200mV sc rate

-1.668 -1.418 -1.168 -0.918 -0.668 -0.418 -0.168 0.08 2 0.33 2 0.582 0.83 2 -1 -0.852x10 -1 -0.602x10 -1 -0.352x10 -1 -0.102x10 -1 0.148x10 -1 0.398x10 E / V i / A -1.750 -1.250 -0.750 -0.250 0.250 0.750 -0.100 -0.075 -0.050 -0.025 0 0.025 0.050 0.075 E / V(SCE) i / A I I III IV V VI A B C R 15th _scan III’

IV’ I,II, VI: Fe
Fe(OH)2;

Fe(OH)2
Fe(OH)3; Fe3O4
Fe(OH)2

III, V: Fe(OH)2/Fe(OH)3
γ-FeOOH

Fe(OH)2
Fe3O4

IV: Fe3O4
γ-FeOOH

Fe2O3 (Ca2+,OH-)
CaFeO4.H2O

III, III’, IV’ : Fe3O4
FeOOH/Fe2O3

a b