G.K. Glass

The influence of chloride binding on the chloride induced corrosion risk in reinforced concrete

Chloride binding by the cement in concrete may affect the rate of chloride ingress and chloride threshold level which in turn determine the time to chloride induced corrosion initiation. In this work, a theoretical assessment of the influence of binding when chloride ingress results from diffusion, is presented. While chloride binding reduces the free chloride content within the concrete, it may increase or decrease the total chloride content depending on the distance from the concrete surface. The total chloride content at the transition between these effects is independent of the period of diffusion and value of the diffusion coefficient. The time to corrosion initiation of embedded steel is dependent on the corrosion risk presented by bound chloride. Bound chloride may participate in corrosion initiation when the establishment of pH gradients are required to sustain passive film breakdown. This is the result of the effect on the pore solution chemistry of the pH dependent solubilities of solid phases containing bound chloride that are very similar to that of calcium hydroxide. In some circumstances, the time to corrosion initiation may be reduced by an increase in binding because of the possible corrosion risk presented by bound chloride.

On the corrosion risk presented by chloride bound in concrete

In previous work the influence of the solid phases of cement hydration on the pore solution chemistry during corrosion initiation has been discussed. It was noted that, because a fall in local pH is necessary for stable pits to develop on the passive steel, much of the chloride bound in concrete may participate in the process of corrosion initiation. At least two phases in hydrated ordinary Portland cement (OPC) will release such bound chloride before the pH falls to 11. In this work, these studies have been extended to include OPC blended with 10% calcium aluminate cement (CAC) and sulphate resisting Portland cement (SRPC). Evidence of a third phase that releases bound chloride was uncovered. Once again the data confirms that most of the bound chloride will be released by a relatively small reduction in pH. The release of chloride at such a high pH value compared to that required to sustain passive film breakdown suggests that the corrosion risk presented by bound chloride may be very similar to that presented by free chloride in concrete.

An investigation into the mechanisms of protection afforded by a cathodic current and the implications for advances in the field of cathodic protection

Abstract The influence of a cathodic current on the corrosion behaviour of mild steel exposed to both dilute sodium nitrate solutions and chloride contaminated concrete has been examined. In both cases the formation of passive films was promoted by stimulating a cathodic reaction on the metal surface, the cathodic kinetics being under activation control in the near passive region. The sodium nitrate solution was not capable of supporting a passive film on its own and it is suggested that, in this environment, film formation is promoted by the production of inhibitors (hydroxyl ions, free radical intermediates and metal oxide intermediates) resulting from the cathodic reduction reaction. In chloride-contaminated concrete an additional effect is the removal of aggressive anions via the flow of negative ionic current away from the metal surface. These passivating effects of the cathodic current provide persistent protection and may be harnessed to develop intermittent cathodic protection for use in areas such as the tidal zone where an electrolyte is not always present. The improvement in the local environment at the steel surface may facilitate a reduction in the protection current density, a factor which may be used to reduce the risk of overprotection when cathodic protection is applied to prestressing steel. The use of potential depolarisation and current density related criteria may be more appropriate than the achievement of a predetermined absolute potential in cases where the purpose of the cathodic protection system is to induce passivation.

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.

An assessment of the coulostatic method applied to the corrosion of steel in concrete

Abstract Potential-time transients were determined on active and passive steel in concrete following the application of a short current pulse. This resulted in the detection of two transient processes, the slower being strongly dependent on the condition of the steel. It is shown that this may be used to evaluate the corrosion rate of the steel and produces values which are similar to those obtained via DC methods of polarisation resistance determination. Furthermore, the method produces a value for the interfacial capacitance, it is insensitive to the electrolyte resistance and any effects of mass transfer during the perturbation can be minimised by ensuring that the duration of the perturbing pulse is less than 35 ms. Additional information is also obtained on the series of processes occurring at the corroding interface.

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...

The analysis of potentiostatic transients applied to the corrosion of steel in concrete

Potentiostatically induced current transients obtained on a range of reinforced concrete specimens were analysed to give estimates of the polarisation resistance and interfacial capacitance. The polarisation resistance was compared with the values obtained using more conventional DC methods of analysis and, while it was consistently lower, it was within the error normally attributed to the polarisation resistance method of corrosion rate determination. The interfacial capacitance values determined increased from 0.44 F m -2 for passive steel (polarisation resistance of 132 Ω m 2) to 26.5 F m -2 for active steel (polarisation resistance of 0.34 Ω m 2). This has a dominant effect on the time required for potentiostatically induced current transients to reach a steady state with a longer time being required by actively corroding steel. By contrast the potential decay time constants describing galvanostatically or coulostatically induced potential transients decrease with an increase in corrosion rate and values less than 25 s for active specimens and greater than 40 s for passive specimens were determined in this work.

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...

Theoretical assessment of the steady state diffusion cell test

The steady state diffusion cell test is often used to determine the chloride ion diffusion coefficient in cementitious materials. It involves the measurement of the flux of chloride ions through a specimen under near steady state conditions. It has been noted that such a test may also provide data which characterises the chloride binding capacity of the specimen. In this work a numerical model of chloride diffusion subject to the effects of chloride binding is used to assess the effect of deviations from the steady state on the data obtained from a diffusion cell test. It is noted that there will be a tendency to underestimate the diffusion coefficient, although good practice should limit this error. The predicted error in the chloride binding isotherm is smaller than that in the diffusion coefficient. Furthermore, the influence of errors in the effective porosity on model predictions is limited as the resulting effect on the values of the calculated parameters describing chloride diffusion and binding counteract each other.