A.M. Hassanein

Cathodic protection afforded by an intermittent current applied to reinforced concrete

Abstract In this work, intermittent cathodic protection was applied to chloride contaminated reinforced concrete exposed to simulated tidal zone conditions. It was observed that an integrated protection current of just 6 mA/m 2 induced the passivation of steel exhibiting an initial corrosion rate of 60 mA/m 2 in conditions characterised by weakly polarised cathodic reaction kinetics. This provides the first direct laboratory evidence that protection may be achieved with a cathodic current that is small compared to the corrosion rate. In this case the protective effects of a negative potential shift may be ignored; it is the changes in the environment at the cathode that induce passivation and provide the basis for cathodic protection. Such changes occur slowly and do not give the instantaneous protection offered by a large negative potential shift. A large potential shift combined with a reducing environment may be generated in saturated conditions characterised by strongly polarised cathodic reaction kinetics. Both passivating and reducing environments at the steel will persist following current interruption. The dominance of these persistent effects suggests that the integrated value of the current required for protection will be insensitive to current variations. Positive trends in open circuit potentials or sustained negative potentials are indicative of passivating and reducing conditions respectively, although an intermittent current complicates non-destructive performance assessment.

Protection current distribution in reinforced concrete cathodic protection systems

Current distribution from a surface mounted anode to steel reinforcement in atmospherically exposed concrete is modelled as a function of the condition of the steel, the resistivity of the concrete and anode-steel geometry. The boundary conditions at the steel have a significant effect on current distribution with more uniform distribution arising at low steel corrosion rates. In a typical situation the surface of a steel bar facing the anode may receive 50% more current than the opposite surface. As cathodic protection has proved to be effective in these cases, a basis for many design decisions that influence current distribution is that their effect is small by comparison. When more than one layer of reinforcement is present the current distribution is significantly worse. In this case a surface anode may not be enough and discrete anodes may be necessary to improve current distribution. An increase in the concrete resistivity, cover and the anode to cathode area ratio at a constant anode current density will increase the voltage drop through the concrete inducing an improvement in the environment at the steel that promotes steel passivity.

A Mathematical Model for Electrochemical Removal of Chloride from Concrete Structures

Abstract An examination of the literature suggests that the electrochemical removal of chloride (ECR) from reinforced concrete is affected strongly by chloride binding (i.e., by removal of chloride...

Obtaining impedance information on the steel-concrete interface

Abstract Corrosion of steel in concrete can be assessed using a number of techniques. In the present work, impedance spectra were obtained by transforming time domain coulostatic transient data int...

Chloride Removal by Intermittent Cathodic Protection Applied to Reinforced Concrete in the Tidal Zone

A basis for protecting steel in concrete using intermittent cathodic protection (CP) is provided by the removal of chloride ions from the surface of the steel. The present study reported on parallel, experimental, and theoretical investigations undertaken to study chloride removal effects induced by an intermittent CP current in a tidal zone situation. Factors that affected the short-term chloride removal efficiency included the resistivity of concrete, charge passed, treatment duration, initial chloride content, concrete cover, and chloride diffusion coefficient. Very high extraction efficiencies initially were observed sometimes, possibly as the result of pressure-induced flow of the pore solution resulting from the generation of gas at the cathode. Chloride removal appeared to be limited by the rate of release of bound chloride within the period of treatment if a high charge was applied in a limited period of time. However, in the long term, the chloride profile reached a steady state governed only by the concentration gradients and the average applied electric field determined by the charge passed in a given time and the resistivity. This may be used to select the steady-state design current density.

Effect of intermittent cathodic protection on chloride and hydroxyl concentration profiles in reinforced concrete

AbstractThis work quantifies the changes induced by intermittent cathodic protection (CP) in the pore solution composition of cement paste and concrete specimens exposed to sea water. In most cases the pulsed current resulted in a significant reduction in the Cl-/OH-ratio of the pore solution in the vicinity of the cathode. Although the increase in the hydroxyl concentration at the cathode was limited at low currents, the current was still important in maintaining a high pH value and has a similar effect to the presence of precipitated Ca(OH)2 . Significant differences were noted between the concrete and the cement paste specimens. The cement paste specimens had a higher initial pH value, a lower resistivity, and a greater free chloride content corresponding to the greater volume of pore solution at a higher cement content. Thus, more significant protective effects were induced in the concrete specimens treated with the same applied charge. A beneficial increase in resistivity, resulting in an increase in...

The use of small electrochemical perturbations to assess the corrosion of steel in concrete

In this work an experimental and theoretical comparison has been made between the use of the potentiostatic and coulostatic methods to assess the corrosion rate of steel in concrete. Numerical Laplace transformation was used for preliminary impedance analysis of the coulostatic transients to aid the understanding of the experimental data. The corrosion tests were carried out on a range of reinforced concrete specimens made with various types of cements (plain, flyash and slag blended cements), different water to cement ratios (0.4 and 0.5) and various admixed chloride concentrations varying from 0 to 3% by weight of cement. Despite the inherent variability of concrete, which usually results in a large scatter of results, similar trends in the performance of the different concrete mixes were predicted by the two methods. Curve fitting analysis of the transients obtained from both methods was found to result in a lower estimate of the polarisation resistance parameter than other conventional methods. However, this analysis may represent an improvement over the other estimates of its value as it provides a consistent methodology of excluding the very fast or slow transient processes which may not result from the corroding interface. The differences in the results obtained from the various methods of analysis of the two methods, together with their advantages and disadvantages, are discussed.

Low frequency impedance data by Laplace transformation of coulostatically induced transients

Abstract Measuring low frequency data using conventional methods has presented problems in some systems with the values of key parameters often being obtained by extrapolation. Laplace transformation of coulostatic transients has potential advantages in this frequency range, although difficulties may arise with the resolution of the long-term transient decay. In this work the effect of measurement resolution is examined. It is shown that the low-frequency impedance response of a corroding system may be obtained using coulostatically induced transients determined at achievable levels of measurement resolution. If the unresolved transient decay is negligible, as is usually the case, no lower bound is placed on the frequency range of the impedance spectrum. On the other hand, when a very slow long-term decay makes a significant contribution to the impedance data determined, the lower bound of the frequency range is limited by the duration of the transient. However, in this case, the adverse effect on the frequency range is minimised by the longer duration of the resolvable transient describing a given percentage of the potential decay. A key feature of the analysis of coulostatic transients is that the response is recorded after the application of a perturbation of very short duration and therefore only information on the interfacial impedance is obtained. Thus the effects of a possibly dominant bulk electrolyte impedance are eliminated.

Comparison Of Methods For Measuring The Electrochemical Corrosion Of Steel in Concrete

In this work an experimental and theoretical assessment is made of the use of various electrochemical techniques to obtain information on the corrosion state of steel in concrete. The transient response produced by potentiostatic, galvanostatic and coulostatic perturbations together with the AC impedance response may all be analysed to give information on the interfacial capacitance and polarisation resistance describing the steel-concrete interface. With the exception of the coulostatically induced transient, these responses also contain some information on the electrolyte resistance. Because of the slow transient behaviour of steel in concrete, the measurement of AC impedance data in the frequency domain may require a very long measurement period (hours for passive specimens). Near complete galvanostatic and coulostatic transients may also require long measurement periods (typically 15 minutes for passive specimens). However in the case of the coulostatically induced transient the duration of the perturbation is very small and any perturbation induced changes in the corrosion state of the steel are therefore limited. Potentiostatic transients are generally recorded in the shortest time period (typically 30 seconds), the perturbation again being applied while the transient is recorded; unlike the other methods this measurement period is relatively insensitive to the corrosion rate. In theory the data obtained by all the transient methods may be converted from the time domain into the frequency domain to give the impedance spectrum. Coulostatically induced transients offer the most advantages in this respect.