Corrosion and Cathodic Protection
Corrosion of Steel in Concrete
In recent years it has been found that many reinforced concrete structures have suffered from premature degradation. The most widespread cause of this degradation has been found to be chloride contamination of the concrete at the level of the steel reinforcement. There are several sources of these chlorides. Some of the most common sources of these chlorides are additions to the mix water, such as set accelerators, alternatively the chloride has come from the structures environment such as marine conditions, de-icing salts or swimming pool disinfectant. The presence of chloride does not normally directly affect the concrete but allows corrosion of the steel reinforcement to occur. This corrosion is normally prevented by the alkaline nature of the concrete. Corrosion of the reinforcement reduces the load bearing capacity of the structure and also it is common that rust forming on the steel surface stains the concrete surface and later cracks or spalls the surface layer. The chloride in the corrosion reaction on the steel surface is not consumed, acting only as a catalyst for the formation of iron hydroxide and later iron oxide, thus the reaction will continue until all the raw materials for the corrosion process, namely steel and oxygen, are consumed.
There are several approaches to stop ongoing corrosion of steel reinforcement in concrete which is contaminated with chloride. The simplest and most direct solution is to demolish the structure and replace with a new, and hopefully, improved construction. Another approach is to remove the areas of concrete where the chloride concentration is greatest and replace with uncontaminated mortar or concrete. This often requires that substantial volumes of concrete are removed with significant cost, utilisation and structural stability implications. Cathodic protection is now commonly used to prevent chloride contaminated concrete from corroding by passing small amounts of electrical current through the concrete on to the steel. Another method, which is less common than cathodic protection, is the electrochemical removal of chloride. In this process chloride is attracted from the concrete surrounding the reinforcement through the concrete to the surface by impressing substantial quantities of electrical current.
Each of the techniques described above has advantages and drawbacks and the most appropriate technique to use requires careful study of the particular requirements of an individual structure. Sometimes a single structure might require two or even three different repair techniques.
Cathodic protection is an established renovation method for reinforced concrete, which has in recent years become widely used on rehabilitation projects for reinforced concrete and steel in masonry worldwide. As with most electochemical processes some uncertainty exists as to the way it works and this is outlined below.
When reinforcement steel corrodes, the process is similar to taking power from an ordinary battery. In a battery, and when steel corrodes, a metal dissolves and this leads to the production of a current between the pole and the Өpole. For steel reinforcement that corrodes in concrete, one very small area is the pole (anode) and another much bigger area is the Өpole (cathode). The corrosion current flows out of the steel at the anode, the part corroding, through the concrete and into another part of the steel where there is no corrosion occurring, i.e. the cathode. This flow is called a corrosion circuit. Steel is dissolved at the anode and forms iron oxide.
For a battery the electrical connection between and Ө can be disconnected. The circuit is then broken with the result that current is stopped and thus the dissolution of metal stops.
For steel reinforcement in concrete the current running through the concrete cannot be disconnected as the corrosion circuit is buried in the structure. Instead it is possible by using an "artificial" anode to add a new and higher current to the original corrosion circuit which runs in the opposite direction of the corrosion current. This makes all the previous poles (anodes) now into current receivers. Thus the complete reinforcement is made into a negative pole, i.e. cathodic, hence the name "cathodic protection".
In a normal cathodic protection system for reinforced concrete which has chloride contamination the currents used are very small (typically 5-20 mA/m2 of steel reinforcement) and typically the voltage required is around 2-6 volt. A schematic of a corrosion cell and the effect of applying cathodic protection is given below.
Cathodic Protection of Reinforced Concrete
Cathodic protection is not a new invention having been used on ships for over a hundred years and is now virtually universally applied on oilrigs, onshore and offshore pipelines, tanks and many other metal, principally steel, objects buried in the ground or water. In the last twenty years it has also become popular for protecting exposed above ground reinforced concrete structures which have suffered from chloride induced corrosion of the rebar. Over this time the benefits and limitations of cathodic protection and the various forms it can take have become more clearly defined.
The principal difference between cathodic protection systems which are traditionally employed and those used for protecting exposed concrete structures is that in the former case a single or small number of anodes remote from the steel will provide adequate protection whereas in the latter case there needs to be a large number of anodes or a continuous anode 'sheet' over the concrete. The reason for this is that the concrete surrounding the steel is quite thin and has a relatively high resistance to the passage of current.
For concrete cathodic protection systems the anode, which is the part that injects electrical current into the concrete, has been continuously modified and evolved over the past thirty years with several anodes becoming extinct and other new species constantly being tested and evaluated. At present there are two leading generic types, namely:
- a mesh or ribbon (normally titanium, coated with a mixed metal oxide) which is covered by a cementituous overlay such as gunite;
- a discrete anode system where anodes are individually placed on or in the concrete surface such as the durAnode system.