THE USE OF GALVANOSTATIC PULSE MEASUREMENTS TO DETERMINE CORROSION PARAMETERS Galvanostatic Pulse Measurement of Corrosion

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1 THE USE OF GALVANOSTATIC PULSE MEASUREMENTS TO DETERMINE CORROSION PARAMETERS Galvanostatic Pulse Measurement of Corrosion D.W.LAW, S.G.MILLARD and J H BUNGEY Department of Civil Engineering, Liverpool University, Liverpool, UK Durability of Building Materials and Components 8. (1999) Edited by M.A. Lacasse and D.J. Vanier. Institute for Research in Construction, Ottawa ON, K1A 0R6, Canada, pp National Research Council Canada 1999 Abstract This paper reports the results of galvanostatic pulse experiments to determine the corrosion parameters associated with passive and active reinforcing steel in concrete. Galvanostatic pulse measurements have been conducted on a number of short sections of steel bar embedded in concrete. The dynamic response to an applied galvanostatic pulse is measured and used to evaluate the instantaneous rate of corrosion. The bars have displayed a range of corrosion rates, from passive steel to highly active corrosion. Analysis of the galvanostatic pulse response has enabled the separation of components that make up the charge transfer resistance to be determined. By undertaking galvanostatic pulse experiments at a range of lateral distances from the reinforcing bar it has been possible to determine which of the components are associated with the corrosion process and which are due to diffusion effects within the concrete. From these results it is possible to calculate the corrosion rate of the reinforcing steel specimens monitored and to determine suitable equilibration times for linear polarisation resistance measurements for reinforcing steel in concrete in general. Keywords : Concrete, Corrosion Rates, Galvanostatic Pulse, Steel Reinforcement, Structural Assessment, Dynamic Response 1 Introduction The problem of accurately and rapidly assessing the condition of corroding steel in reinforced concrete structures has long been a problem for the Civil Engineering industry. A number of electrochemical techniques have been developed to assess the corrosion equilibrium and corrosion rate of the reinforcing steel and to enable an estimate of the service life remaining to a reinforced concrete structure.

2 The most established of these techniques is half-cell potential mapping. The steel potentials relative to a stable reference half cell are recorded and are related in the guidelines given in ASTM C to the probabilities of corrosion occurring. However, these are only broad guidelines and the different environmental conditions in individual structures can significantly effect the likelihood of finding corrosion. Other techniques have been developed to give a direct measure of the instantaneous corrosion rate and include the Linear Polarisation Resistance (LPR) (Gowers et al., 1992) and AC Impedance methods (John et al. 1991). The typical classification of the corrosion rates is given in Table 1. Table 1: Typical corrosion rates Rate R ct (Ω.cm 2 ) i corr (µacm -2 ) High Medium Low Passive above 250 less than 0.1 LPR has been developed based on Stern-Geary theory where the corrosion current, I corr, is given by : I corr = β a β c. 1 = B (1) 2.3 (β a + β c ) R ct R ct β a, β c - Tafel constants; B - Stearn-Geary Constant; R ct - Charge Transfer Resistance. To calculate the R ct value the reinforcing steel is polarised from its equilibrium potential by a fixed amount. This is usually in the range 10 or 20 mv to ensure that for active corrosion the shift lies within the linear region of the Stearn-Geary plot. The resulting current is then monitored at the end of a selected time period, usually between 30 seconds to 5 minutes. At the end of this period a value for R ct is calculated from the potential shift applied divided by the current induced. For greater accuracy an average value for R ct is taken using both a positive and negative potential shift. The corrosion current density, i corr, is determined by dividing the total corrosion current, I corr, by the surface area of the steel being polarised. From the corrosion current density, i corr, it is possible to calculate the annual steel section loss, which enables an estimation of the service life. A number of uncertainties arise with the use of LPR measurements. The area of bar that is being polarised is unknown, due to an uncertain degree of lateral spread of the applied current. The development of a guard ring to limit the spread of the polarisation current has gone some way to addressing this problem (Feliu et al. 1996). Another problem is the choice of a suitable equilibration time at which to monitor the current response to the applied potential shift. This is due to the fact that R ct is made up from a number of different mechanisms occurring within the concrete. These include those interfacial resistances at the surface of the bar which are directly related to corrosion and those resistances associated with the bulk concrete. To calculate the corrosion rate

3 accurately it is necessary to select the measurement time required to evaluate those processes directly associated with corrosion but to exclude those processes associated with the bulk concrete. The AC impedance technique is a laboratory method which enables greater information to be derived relating to the mechanisms occurring within the system. However, this technique is particularly time consuming and can often provide data which is difficult to interpret. A technique that has recently been developed to study the dynamic response to an applied current has been that of Galvanostatic Pulse (Newton & Sykes 1988, Gowers et al 1996, Millard et al. 1995). In this technique a small current perturbation is applied to a steel bar using an auxiliary electrode on the surface of the concrete in a similar manner to a LPR measurement. The resulting potential transient is then monitored with respect to a reference electrode. Analysis of this transient response allows the different resistance and capacitance components of the system to be resolved, enabling an accurate corrosion rate to be determined. 2 Experimental 2.1 Galvanostatic Pulse The galvanostatic pulse experiments were conducted using a brass 15 cm x 10 cm auxiliary electrode placed on the surface of the concrete. A wetted carbon foam pad was used to ensure a good electrical contact between the auxiliary electrode and the concrete surface. This auxiliary electrode had a Silver/Silver Chloride reference electrode located in the centre, Figure 1. Fig 1: Galvanostatic Pulse Monitoring Equipment A purpose built, battery powered, constant current source was used to apply the current perturbation. The current source allowed a stable 1, 0.1 or 0.01 ma current to

4 be applied. The resultant transient voltage response was monitored using a 16-bit data acquisition program, DT-Vee. A pulse duration of between 45 to 180 seconds was used with a sampling rate of 1000 Hz selected to collect the dynamic response data. This sampling rate allows those system components with small time constants to be resolved. At higher sampling rates it was observed that the resolution is not improved but the data processing times are greatly extended. A current perturbation was selected to give an initial potential shift of 20 mv to 30 mv, which would fall within the linear region of the Stearn-Geary plot. To this end initial tests were always conducted with a 0.01 ma pulse. If this did not sufficiently perturb the system the steel was allowed to re-equilibrate back to the rest potential, and the current pulse increased, as required, to achieve the desired potential shift. 2.2 Data Analysis The data collected was analysed to obtain an equivalent electronic circuit, Figure 2, (Millard et al. 1995, Gowers et al., 1996, Gowers & Millard, in press). Using a simple Randles circuit at time t the transient potential response, V t, to a constant current perturbation, I, is given by V t = R Ω.I + R ct.i[ 1 - exp( -t /R ct C dl ) ] (2) where I is the current perturbation C dl R Ω Fig. 2: Simple Randles Circuit R ct In this circuit R Ω represents the ohmic resistance of the concrete between the surface electrode and the steel bar. C dl represents a double layer capacitance at the surface of the steel bar and R ct represents the charge transfer resistance at the surface of the steel bar. As seen in Equation 1, the rate of corrosion is proportional to R ct. After a suitable equilibration time the transient response reaches a steady-state potential V max, where log e (V max - V t ) = log e (IR ct ) - t / R ct C dl (3) Thus a plot of log e (V max - V t ) against t gives a linear graph with gradien/ R ct C dl and intercept log e (IR ct ). In more complex system with several separate components there will be a succession of linear sections of the graph, which can be represented by a succession of resistors and capacitors in series. By extracting each successive section

5 the separate components can be resolved and the R ct and C dl determined for each component. Knowledge of the R ct and C dl enables the time constant for those components to be calculated as the product of the capacitance and resistance. 2.3 Concrete Specimens Galvanostatic pulse experiments were conducted on two concrete specimens. Both specimens were made of OPC concrete with a nominal 28 day compressive cube strength of 50 Nmm -2. Specimen #1 was a slab of dimension 1000 x 1000 x 150 mm, with two mild steel bars, 10 mm diameter, 100 mm long, denoted MS1 and MS2. The bars had a cover of 25 mm, were positioned parallel to each other 50 mm apart, and located 500 mm from the edge of the slab. Specimen #2 was a slab of dimensions 750 x 750 x 150 mm, with one mild steel bar, denoted MS3, and two stainless steel, denoted SS1 and SS2. Each bar was 10 mm diameter, 100 mm long. The bars had a cover of 25 mm, were positioned parallel to each other 25 mm apart, and located 250 mm from the edge of the slab. The surface area of all the bars was taken as cm 2. Each specimen was subjected to the an external environment and intermittent ponding with 1 Molar sodium chloride solution to promote corrosion over a period two years. Potential transients were recorded over time periods of between 45 and 180 seconds and at lateral distances ranging from 0 to 300 mm from the location of the bar. 3 Results The potential transients determined had different characteristic shape for the active (mild steel) and passive (stainless steel) systems. A typical transient for an active system is given in Figure 3 and for a passive system in Figure 4. The data shows that a significantly higher potential shift, E, is obtained from a passive system than an active system for the same current perturbation. Fig. 3: Potential Transient, MS3, Active Corrosion, Pulse 0.01 ma

6 Fig. 4: Potential Transient, SS2, Passive Corrosion, Pulse 0.01 ma For the passive systems measured the potential shift, E for the lowest current perturbation, 0.01 ma, was larger than that in the linear region of the Stearn-Geary plot. This may result in some inaccuracies in the resolution of the components of the transient, however, these will not be significant when converted to corrosion rates. For the active corrosion systems at higher corrosion rates a current pulse was selected to obtain the mv shift required to fall within the Stearn-Geary linear region. This shift was achieved in all the systems measured Analysis of the potential transients enabled a number of separate components with different time constants to be resolved. The resistance, capacitance and associated time constant for each component are given in Tables 2-6. The resistances in bold are those attributed to the charge transfer resistance. 4 Discussion When investigating the condition of reinforcing steel in concrete the corrosion rates are generally classified as passive, low, medium or high based on the magnitude of corrosion current density i corr, Table 1. In order to calculate the corrosion current density from the galvanostatic pulse data acquired, the charge transfer resistance, R ct must be determined. This value can be calculated by summing the individual resistances of the separate components, associated with corrosion, resolved from the galvanostatic pulse experiments. The analysis of the transient response from the stainless steel bars, SS1 and SS2, gave components with resistance in the range 400 to 500 Ω cm -2, which are consistent with a passive system. This is to be expected as the stainless steel bars will not be corroding. As the distance from the bar increased there was no discernible increase in the magnitude of any of the individual components. This would indicate that the components resolved are associated with corrosion at the steel/concrete interface and are not due to bulk effects in the concrete, which would be expected to increase with an increase in lateral distance. The capacitances associated with these components are all

7 Table 2 : Results for bar MS1, 90 seconds pulse Distanc e (Ω cm - 2 ) (Ω cm - 2 ) (µfcm -2 ) ( Ω cm -2 ) (µfcm -2 ) R 3 Cd 3 (µfcm -2 ) t 3 R 4 Cd 4 (µfcm -2 ) Table 3 : Results for bar MS2, 90 seconds pulse Distance R 3 Cd 3 t Table 4 : Results for bar SS1, 90 seconds pulse Distance R 3 Cd 3 t t 4

8 Table 5 : Results for bar SS2, 90 seconds pulse Distance R 3 Cd 3 t Table 6 : Results for bar MS3, 45 and 90 seconds pulse Pulse (sec) Distance (Ω cm - 2 ) (Ω cm - 2 ) R 3 Cd 3 t 3 R 4 (Ω cm - 2 ) Cd Table 7: Results for bars SS1 and SS2, 180 seconds pulse Bar Distance SS SS SS SS SS SS t 4

9 less than 100 µf/cm 2 and their time constants are all less than 30 seconds and would confirm that a measurement period of at least 30 seconds should be adopted for LPR measurements. Indeed for passive systems a measurement period of 120 to 300 seconds is often employed. Additional galvanostatic pulse data acquired using a pulse duration of 180 seconds, Table 7, again showed no significant increase in resistivity with distance for the components associated with the longer time constants. In the active systems studied there was a marked difference between the data for the specimens MS1 and MS2 and specimen MS3. Specimen MS1 displayed an increase in resistance with distance for that component with the largest time constant. Specimen MS 2 displayed an increase in resistance for the components with the two longest time constants. However specimen MS3 displayed no increase in resistance with distance. The capacitances associated with the components displaying this increase in specimens MS1 and MS2 were greater than 1000 µf/cm 2, for those with time constants in the region of twenty five seconds, for bar MS2 the capacitances associated with the second time constant, 2.75 seconds, were between 609 and 239 µf/cm 2. All the capacitances in specimen MS3 were less than 1000 µf/cm 2. Overall, those components in the region µf/cm 2 appear to be related to either corrosion or bulk diffusion, those above 1000 µf/cm 2 more likely related to bulk diffusion, while those less than 100 µf/cm 2 are almost certainly related to corrosion. This is in general agreement with previous work (Newton & Sykes 1988, Gowers et al 1996, Millard et al 1995, Glass et al 1997, Glass et al 1998). In addition there is evidence that those resistive components relating to the dielectric properties of the concrete may be resolved with time constants in the millisecond range. In the data collected components within these bounds generally display capacitances of less than 1 µf/cm 2. In calculating corrosion rates from galvanostatic pulse data components associated with very small capacitances should not be included in the calculations. A comparison of the corrosion rate calculated for all components with time constants less than 30 seconds, corresponding to a LPR measurement with a 30 second equilibrium period, and the corrosion rate calculated when the components associated with the bulk diffusion effects are discounted, is given in Table 8. Table 8: Corrosion Rates for Mild Steel, 90 seconds pulse, Lateral distance 0 mm Bar ΣR ct Corrosion Rate (R ct ) (µacm -2 ) ΣR Total Corrosion Rate(R Total ) (µacm -2 ) MS MS MS The data shows that a significant underestimate of the corrosion rate may occur for those where a 30 second equilibrium period is used. Hence, the selection of an appropriate LPR equilibrium time is important when monitoring actively corroding structures.

10 5 Conclusions 1. The galvanostatic pulse method enables the separate components of the charge transfer resistance to be resolved. This in turn enables those components associated with the corrosion process to be identified and a corrosion rate calculated. 2. The technique can readily differentiate between active and passive systems. 3. Components with capacitances in the range µf/cm 2 are associated with corrosion. Capacitances over 100 µf/cm 2 may be attributable to either the corrosion process or bulk diffusion effects, though capacitances greater than 1000 µf/cm 2 appear more likely to be associated with bulk effects. Each system must be individually investigated to determine which resistance components should be used to evaluate the rate of corrosion. 4. Components with capacitance less than 1 µf/cm 2 may be attributed to the dielectric properties of the concrete. 5. In structures with actively corroding reinforcing steel, the equilibrium period used for LPR measurements should be carefully selected to prevent an underestimate of the corrosion rate. 6 References Gowers, K.R., Millard. S.G. & Gill, J.S. (1992) Techniques for increasing the accuracy of linear polarisation resistance measurement in concrete structures. Corrosion92, NACE Houston Texas, Paper 205 John, D.G., Searson, P.C. & Dawson, J.L. (1981) Use of AC impedance technique in studies on steel in concrete in immersed conditions. British Corrosion Journal, Vol. 16, No. 2, pp Feliu, S., Gonzales, J.A. & Andrade, C. (1996) Multiple-electrode method for estimating the polarisation resistance in large structures. Journal of Applied Electrochemistry, Vol. 26, pp Newton, C.J. & Sykes, J.M. (1988) A galvanostatic pulse technique for investigation of steel corrosion in concrete. Corrosion Science, Vol. 28, No. 11, pp Gower, K.R., Bungey, J.H. & Millard, S.G. (1996) Galvanostatic pulse transient for determining concrete reinforcement corrosion rates. Proceedings Corrosion Reinforced Concrete Construction, Cambridge, pp Millard, S.G., Gowers, K.R. & Bungey, J.H. (1995) Galvanostatic pulse techniques: A rapid method of assessing corrosion rates of steel in concrete structures. Corrosion95, NACE, Houston Texas, Paper 525 Gowers, K.R. & Millard, S.G. (in press) Electrochemical techniques for corrosion assessment of reinforced concrete structures Glass, G.K., Page, C.L., Short, N.R. & Zhang, J-Z. (1997) The analysis of potentiostatic transients applied to the corrosion of steel in concrete. Corrosion Science, Vol. 39, pp Glass, G.K., Hassanein, A.M. & Buenfeld, N.R. (1998) Small perturbation electrochemical techniques used to asses the corrosion of steel in concrete. Proceedings Corrosion and the Environment, Bath, pp