Reinforced Concrete Structures
Durability Enhancement for New and Deteriorating Infrastructure
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.
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 added benefit of barrier protection to the general environment provided by the cover concrete.
Two factors which 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 breakdown 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 reduce 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.
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
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.
By making the steel negative, all chlorides which are negatively charged are repelled and migrate away from the steel surface to the anode surface.
The cathodic reaction at the steel produces hydroxide which provides a localised pH increase at the steel surface.
The secondary benefits of cathodic protection of steel in concrete (chloride extraction and realkilisation) 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.
How Does It Work?
The basic corrosion cell below can aid in providing a better understand of how cathodic protection works, from this diagram we can see that there are 4 distinct elements within a corrosion cell, as follows:
Electrolyte - Provide reactants
- Remove waste
Anode - Corrosion - metal loss produce 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 we can see 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, these external energy sources can be based on a material with more stored energy (e.g. zinc has more stored energy than steel) or by providing energy via an external power source (mains AC converted to DC, solar panels, batteries etc)
Figure 6 – Basic Cathodic Protection Cell
On a typical metallic surface undergoing generalised 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
Types of Cathodic Protection Systems
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 to 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 platinised 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 platinised 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 platinised titanium. To overcome this natural effect we provide an external amount of energy to the platinised titanium and force it to become the anode.