Reinforced Concrete Structures
Corrosion and Protection of Steel in Concrete
Deterioration of Steel in Concrete
Steel embedded in concrete is protected by forming stable corrosion products which stop further corrosion (also referred to as passive films of passivity) due to the high pH of cement pore water and the added benefit of barrier protection to the general environment provided by the cover concrete.
Two factors that cause corrosion of steel in concrete include chlorides and carbonation.
Chlorides from external environments like de-icing and marine salts can penetrate into concrete and break down the stable corrosion products allowing the steel to corrode.
Carbonation of the concrete is a reaction of carbon dioxide from the atmosphere with the calcium hydroxide in the concrete which caused corrosion to occur due to a reduced pH of the concrete pore water surrounding the embedded steel.
The result of corroding steel in concrete is significant as the corrosion product which can be up to 10 times greater than the original steel volume causes concrete cracking and spalling which can rapidly undermine the structural integrity and capacity.
Photo 1 - Steel Embedded in Concrete – Passive Film Intact
Photo 2 - Steel Embedded in Concrete – Chloride Induced Pitting Corrosion
Figure 1 - Steel Embedded in Concrete – Passive Film Intact
Figure 3 - Carbonation Induced Corrosion of Steel in Concrete
Figure 2 - Chloride Induced Pitting Corrosion of Steel in Concrete
Figure 4 - Effects of Cathodic Protection of Steel in Concrete
How Does Cathodic Protection Work?
The diagram of a basic corrosion cell below can aid in providing a better understanding of how cathodic protection works, from this diagram it can be seen that there are 4 distinct elements within a corrosion cell, as follows:
Electrolyte - Provide reactants.
- Remove waste.
Anode - Corrosion - metal loss, produces energy.
Cathode - No Corrosion Use up energy.
Metallic path - Moves energy.
Each of these four elements is critical for the corrosion process, removal of any one of these elements results in the corrosion stopping.
From the basic corrosion cell diagram, it can be seen that at the cathode no corrosion of the metal occurs, this is a result of an excess of available energy at this location which makes the metal surface negatively charged and causes a non-corrosion reaction to occur.
Figure 5 – Basic corrosion cell
Cathodic protection is a way of manipulating the corrosion cell by forcing only the cathodic reaction to occur on the whole surface of the metal as shown in figure 6.
This is achieved by providing energy to the metal from an external source.
The external energy sources can be based on:
galvanic anodes which is a material with more stored energy (e.g. zinc has more stored energy than steel).
impressed current cathodic protection, which is achieved by providing energy via an external power source (mains AC converted to DC) and using more noble or corrosion-resistant anode material like titanium coated with an active metal oxide.
Figure 6 – Basic Cathodic Protection Cell
On a typical metallic surface undergoing generalized corrosion (Figure 7), there will be a number of alternating anodes and cathodes, as energy is supplied to the metal surface the size of the cathodic areas increases with a corresponding decrease in the size of the anodic areas.
A minimum amount of energy is needed by the structure to ensure that all of the anode areas are changed to cathode areas. If this minimum amount of energy is not provided to the structure then some areas of the structure will remain anodic and continue to corrode as shown in the middle diagram of Figure 7 partial cathodic protection.
Once the minimum amount of energy has been supplied to the structure, all of the anode areas on the structure will be changed to cathode areas which will effectively stop any further corrosion on the structure surface from occurring.
This phenomenon of converting all anode sites to cathodes is commonly referred to as polarisation or that the structure has polarised (which means that all surfaces are the same voltage).
Figure 7 – Effects of Cathodic Protection Steel in Soil or Water
Effects of Cathodic Protection on Steel in Concrete
Cathodic protection of steel in concrete is a very effective method of stopping and controlling corrosion as the applied cathodic protection addresses both degradation processes namely Chloride induced pitting corrosion and Carbonation induced corrosion.
Cathodic protection can be used on contaminated undamaged concrete, without having to remove and replace the undamaged sound, but contaminated concrete. (Figure 8)
By making the steel negative, all chlorides which are negatively charged are repelled and migrate away from the steel surface to the anode surface. (Figure 9)
The cathodic reaction at the steel produces hydroxide which provides a localized pH increase at the steel surface.
The secondary benefits of cathodic protection of steel in concrete (chloride extraction and re-alkalization) which promote passive film formation, tend to provide an added service life after the cathodic protection systems have reached the end of life, depending on the exposure conditions this extra life can vary between 10-20 years.
Due to the reduced energy requirements, long-expected design lives, and secondary benefits, cathodic protection of steel in concrete becomes a very cost-effective solution for life extension, preservation, or maintenance-free design.
Figure 8 – Effects of Cathodic Protection on Chloride
Induced Corrosion of Steel in Concrete
Figure 9 – Effects of Cathodic Protection on Carbonation Induced Corrosion of Steel in Concrete
Types of Cathodic Protection on Steel in Concrete
Galvanic cathodic protection relies on the natural energy difference between metal types also known as the galvanic or electro-chemical series to provide the energy to the structure been protected.
Basically, some materials have more stored energy than others, and if we connect two different materials together the one with the higher stored energy will corrode and provide energy to the material with less stored energy.
By connecting zinc which has more available energy than steel, the zinc will corrode (becoming the anode) and provide energy to the steel which will not corrode (becoming the cathode).
The zinc will be used up or ‘Sacrificed’ to provide energy to the steel, which will suffer no corrosion as it uses up the free energy in the non-corrosion (cathodic) reaction.
Impressed current cathodic protection use materials with a lower amount of stored energy than steel, typically platinized titanium or Mixed Metal Oxide (MMO) coated titanium.
These materials are used because they are smaller, lighter, and require less bulk material when compared with galvanic materials.
When connecting steel to a titanium anode, the steel will automatically start to corrode (act as the anode) this is due to the higher stored energy of the steel when compared with the titanium.
To overcome this natural galvanic effect, an external voltage is applied to the titanium which forces it to become the anode and forces the steel to be com the cathode (impressed current cathodic protection). This is achieved by using an external power source such as AC power which is converted to DC using a transformer rectifier and requires a continuous supply of power to keep working.
As engineers, owners, and operators of sustainable structures we have a responsibility to ensure the best use of our current natural resources, using less material and ensuring that the materials used now will last as long as possible and can be completely recycled and reused in the future.
This can be achieved by using cathodic protection on your assets.