Chloride induced rebar corrosion damage in reinforced concrete structures results mainly from the use of de-icing salts in cold climates and/or from the exposure to marine environments. It is clear that concrete use must be driven by considerations of durability as well as strength to build environmentally sustainable concrete structures. Embeddable reference electrodes are important in corrosion monitoring application of concrete structures. The data obtained from reference electrodes should be reliable and helpful to take an appropriate repair and rehabilitation measures in a proper time. Due to unavoidable potential fluctuations, the comparison between different types of reference electrodes and their long term stability in concrete are still under questionable.
The objective of the present investigation is the standardization of different materials for possible use as embeddable electrodes in concrete structures. Zinc, graphite, mixed-metal oxides(MMO) and MnO2 reference electrodes were fabricated in a laboratory as suitable as an embeddable use into a concrete structure. The corrosion characteristics, electrochemical stability and long term reliability of each reference electrode in various environments with different media and chloride concentration, have been investigated. The experiments were divided into three parts, in solutions(pH 4, 7, 10, 12.5, 13 & 13.5) containing different concentration of chloride, in mortars with & without chloride and in concretes with & without chloride.
Corrosion characteristics and electrochemical stability of Zinc, Graphite, MMO and MnO2 reference electrodes in concrete environment solutions
Zinc electrode showed their stability in concrete environments, however, the integrity of zinc surface is changed with time. Zinc electrode was found to be suitable as an embeddable electrode in concrete environments without chloride. The performance of graphite in concrete environment was good, however, thermodynamically, graphite was not a true reference electrode in concrete. Graphite electrode was found suitable for a short term monitoring application as a pseudo-reference. Self corrosion of MMO electrode was almost negligible in the solutions representing concrete environments. The MMO electrode showed better performance characteristics in concrete media and exhibited an excellent stability. The electrochemical stability in concrete environments exploited the MMO electrode as a suitable, embeddable reference electrode for corrosion monitoring in concrete structures. The MMO electrode was suitable both in the absence and presence of chloride ions. MnO2 electrode especially in concrete environments even in the presence of higher amount of chlorides was found to be good. Their electrochemical stability in concrete environments revealed that a good electrode for concrete structures.
Electrochemical stability of Graphite, MMO and MnO2 reference electrode embedded in mortar
Graphite electrode was less stable and more sensitive to the environmental changes in concrete. MMO electrode appeared a better performance in mortar medium. The electrochemical characteristics of MnO2 electrode embedded in mortar in three aqueous solutions showed a negligible polarization current and stable passive region. MnO2 electrode seems to be more stable and reliable electrode in mortar medium.
Very low polarization current has been observed in all three electrodes embedded in mortar. The addition of chloride did not show any influence on the performance of electrodes for one year exposed period.
Long term reliability of MnO2 reference electrode embedded in concrete
MnO2 electrode indicated to have a stable and reliable potential in concrete medium as well as in concrete structures. The uniformity of potential of MnO2 electrode was reproducible. The long term stability in concrete under outside exposure studies also showed an excellent performance of MnO2 electrode and a strong possibility as an embeddable use in concrete structures for corrosion monitoring applications. Unlike conventional reference electrodes, MnO2 electrode was free from harmful elements like mercury, sulphate and chloride. Advantageously MnO2 electrode can be used as an embeddable type into both new and old structures.