CATHODIC PROTECTION OF REINFORCEMENT
If an investigation shows that the principal reason for the corrosion of reinforcement is the presence of chlorides in the concrete, then normal repair methods will not be durable, resulting in the need for a continuous programme of repair. This leads to the question of what can be done to ensure a satisfactory and durable repair. The main method to deal with this problem is the use of cathodic protection.
2. General principles of cathodic protection
Cathodic protection has been used for many years for the protection of steel and iron structures such as jetties, storage tanks, and underground pipelines.
|Cathodic protection of reinforcement (Image courtesy: Conrehab)|
In the early 1960s highway engineers in the USA and Canada started trials on the use of cathodic protection for the reinforcement in bridge decks which had been severely damaged by chloride attack from deicing salts. It is now used extensively in both USA and Canada for this purpose, mainly the impressed current system. In the UK cathodic protection is being increasingly specified to combat chloride corrosion of rebars, mainly in highway bridges.
Corrosion of steel is an electro-chemical process and exposed steel in a moist environment will corrode due to differences in electrical potential on the surface of the metal itself. These areas form anodes and cathodes, and this allows an electric current to flow from anode to cathode; the metal suffers corrosion at the anodic areas. The objective in cathodic protection is to ensure that the steel to be protected forms the cathode.
There are two practical ways of introducing cathodic protection to steel:
1. by connecting the steel to a metal which is ‘less noble’ in the electrochemical series (this forms the anode); this is known as sacrificial anode protection;
2. by the application of an external electric current of sufficient intensity to ‘swamp’ the corrosion current; this is the impressed current technique.
A. Sacrificial anodes
The following list shows part of a basic series in which the metals high on the list become anodic to those lower down and hence provide protection. This can be described as sacrificial anode protection and depends on on metal being designed to corrode and so prevent another metal connected to it from corroding.
• mild steel
• cast iron
• stainless steel (cromium-based)
When two dissimilar metals are in contact in an electrolyte, a current is produced as a function of the electrochemical series of metals. For example, if steel is in electrical contact with zinc in an electrolyte, a current will flow from the zinc to the steel because zinc is anodic to steel and the zinc will corrode but the steel will not. This is the principle of the use of sacrificial anodes.
B. Impressed current
Protection can be provided by the introduction of an impressed current (dc). The structure to be protected is connected to the negative supply (the cathode) and the positive to an introduced anode which is specially selected to have semi-inert or non-corrodable properties. The range of suitable anodes is limited and is designed to last very much longer than the ‘sacrificial’ anodes referred to in the previous paragraph, and a useful life of 20 years or more can be expected. The materials available are graphite, platinized titanium, high-silicon iron, tantalum or niobium; lead/silver alloys are often used in marine installations.
The relevant British Code of Practice is BS 7361: Cathodic Protection; the USA Code is ASTM B.843. In considering cathodic protection to an existing structure the following matters need careful attention:
1. The type of protection which will be the most suitable in the given circumstances, i.e. sacrificial anodes or impressed current.
2. The use of cathodic protection to reinforcement encased in concrete introduces special problems. The pore water in the concrete acts as the electrolyte of the corrosion cells.
3. The impressed current must be adequate to suppress the corrosion current. Concrete has a high resistance to the flow of electric current and this has to be overcome.
4. It has been claimed that cathodic protection can cause hydrogen embrittlement but I have not seen a reliable report of such a case.
5. If cathodic protection by means of impressed current is used in a building, care must be taken to ensure that corrosion of unprotected ferrous metals in the building does not occur.
The use of a cathodic protection system for a reinforced concrete structure requires special knowledge and experience in concrete technology in addition to that required for ‘normal’ cp systems used for steel structures. For the system to be successful in a reinforced concrete structure a number of factors should be taken into consideration, the principal two being:
1. the possibility of initiating alkali-silica reaction;
2. the existence of significant discontinuities in the reinforcing steel.