.
3.0 Why does Steel corrode in a costal environment?
Although steel is a very versatile strong cheap metal it has one major disadvantage, corrosion. Steel corrosion in concrete is a worldwide problem, causing billions of dollars in repairs and maintenance (Kolb, 2004 ). Corrosion can cause a number of structural problems within concrete including spalling (refer to Appendix 3), cracking, decreased strength and even complete failure/collapse of structure (NRMCA, 1995). And when applied in coastal regions the problem gets worse. In a coastal region a number of factors accelerate corrosion. These include salinity, temperature, moisture, constant wetting and drying and pH levels (corrosion-doctors, 2006). The two main causes of steel corrosion in reinforced concrete is chloride penetration and carbonation. Due to the porosity of concrete, oxygen, carbon dioxide and salts can easily penetrate and diffuse into concrete, this is the cause of corrosion within concrete (corrosion-doctors, 2006).
In high quality reinforced concrete the steel is protected by the high alkalinity caused by the reaction of water and cement which deposits a passive film on the surface of the steel which prevents corrosion (The Steel Reinforcement Institute of Australia , 2008). Chlorides are deposited onto the concrete through saltwater spray or coastal winds. The chloride will permeate into the porous concrete especially in areas of constant wetting and drying. Eventually the Chloride will reach the surface of the steel and begin to build up beyond a certain concentration level at which the protective film is destroyed and corrosion will begin (corrosion-doctors, 2006). Steel in the presence of an electrolyte such as sodium chloride can cause rapid corrosion.
Steel when in contact will water and oxygen will corrode in the form of rust. There are two types of Iron present in every steel Iron(II) and Iron(III), Iron(III) being the most abundant. When Iron(II) corrodes it forms Ferrous Oxide and when Iron(III) corrodes it forms Ferric Oxide. Ferrous Oxide is however not rust, it is a black powder and it is fairly uncommon as most Iron in steel is Iron(III). Ferric oxide is the typical red coloured rust seen on most aged steel structures. The equations for both reactions are bellow. (Kolb, 2004 )
Iron(II) = Fe++ + 2(OH)- →. Fe(OH)2 (Ferrous Oxide)
Iron (III) = 4Fe(OH)2+ 2H2O + O2 → Fe(OH)3 (Ferric Oxide)
(Cathodic Protection Co. Limited, 2011)
When electrolytic corrosion begins electrochemical cells are formed on the surface of the metal and the electrolyte or solution surrounding the metal. Each cell consists of an anode and cathode (somewhere on the steel bars), a return circuit (steel bars) and an electrolyte (surrounding concrete) (corrosion-doctors, 2006). In a relatively anodic spot on the metal the steel undergoes oxidation (corrosion) which results in the production of electrons which then travel through the steel bars to a relatively catholic spot on the metal where they are consumed. The concrete must be wet enough or have enough salt content (salt bridge) to provide an electrolytic path to transport ions. Therefore this form of corrosion mostly happens in wet coastal areas. Cathodes do not corrode. This reaction is known as a redox or reduction and oxidization reaction (corrosion-doctors, 2006).
Reaction at anode, Iron losing its valance electrons becoming oxidized
Fe → Fe++ + 2e-
Reaction at cathode, electrons being absorbed into water and oxygen to make hydroxide
O2 + 2H2O + 4e- → 4OH-
(Ltd, 2004)
Carbon dioxide from the air can neutralise the alkalinity of concrete by reacting with the lime component of the concrete which gives concretes high alkalinity of about pH 12 (The Steel Reinforcement Institute of Australia , 2008). When the alkalinity drops below pH 10 corrosion can occur but only in the presence of oxygen and moisture, which for costal environments is inevitable.
Carbonation in concrete is represented as the following equation.
Ca(OH)2 + CO2 → CaCO3 + H2
(Ltd, 2004)
This is known as carbonation, it is a slow and predictable process. It is not the main cause of corrosion in steel and does not create nearly as many problems as chloride induced corrosion.
4.0 Method of Corrosion Prevention
4.1 Epoxy Coating
Although steel reinforcement represents only a fraction of the cost of a concrete structure (about 3-7%), in most cases it is the corrosion and failure of the steel that leads to completely replacing the structure or costly repairs. One method that has been widely used by the construction industry is to epoxy coat the steel to give it a physical barrier between aggressive chemicals which commence the formation of corrosion (National Reasearch Council , 2001). The epoxy coating can last and protect steel reinforcement for sometimes up to 50 years depending on the quality of the epoxy and the environment in which it is placed (American Galvanizers Association, 2012). Epoxy resins have desirable properties including high ductility and good heat resistance. A high quality epoxy applied in a thick coat has very low permeability when it comes to chloride ions and oxygen so it is great at preventing corrosion however the epoxy resin is permeable to moisture which greatly reduces the epoxy’s adhesion to the steel (American Galvanizers Association, 2012). This can lead to the peeling of and flaking of the epoxy resin, exposing the steel, (refer to Appendix 5) making epoxy coating not suitable for costal environments.
The transportation and handling of steel coated in epoxy is very important to its life span. The steel bars have to be handled very carefully during transportation and installation in order not to damage the epoxy coating creating holes in the seal which open the floodgates to corrosion (American Galvanizers Association, 2012). Because the coating provides no cathodic protection any breach in the seal will allow corrosive elements to make contact with the steel and initiate corrosion. All it takes is one outbreak of corrosion and the entire bar is compromised as the corrosion will spread through the steel underneath the epoxy (American Galvanizers Association, 2012). Eventually after years of use (depending on the quality of the epoxy and the handling of the bars) the epoxy will disintegrate and begin cracking and flaking of the steel to expose it to corrosion. If no other corrosion prevention system is in place then the corroded steel will have to be removed and replaced. In concrete applications such as bridges the cost and difficulty of replacing the reinforcing bars can be phenomenal and in most cases impossible (American Galvanizers Association, 2012). The structure will eventually be rendered unsafe for use and will have to be knocked down and rebuilt. In conclusion epoxy coated steel reinforcing bars cannot provide sufficient long-term protection from corrosion in structures exposed to costal environments.
4.2 Cathodic Protection System
As discussed above in 2.0, when corrosion begins electrochemical cells are formed between the metal and the concrete. In the cell there is a cathode and an anode, the anode corrodes and the cathode absorbs the electrons (NDT Resource Centre, 2002). Cathodic protection prevents the anode forming on the steel reinforcing bars and instead a foreign metal is used as the anode. The anode system that provides the current can be made of a variety of materials including silicon, cast iron, graphite, mixed metal oxides, niobium, platinum and titanium. However the most effective anode systems are made of Titanium (refer to Appendix 4) or mixed metal oxides (Cathodic Protection Co. Limited, 2011). Cathodic Protection systems are the most effective method against corrosion prevention in costal environments as it is able to entirely halt corrosion and make the steel completely unreactive as long as the current is applied. The cost of installation, electricity and maintenance is minute compared to the cost of repairs and possible replacement of the entire structure if epoxy coated steel was used, as it would eventually corrode.
A major benefit of cathodic protection is that it can be applied in both brand new and already corroded structures to stop any corrosion from starting and to stop any further corrosion occurring. In many cases, owners of deteriorating concrete structures have been told to demolish their entire structure (Oko, 2008). This is where impressed current cathodic protection should be considered if the structure is still in reasonable condition. In many cases the price of cathodic protection installation can be insignificant compared to the price of a building a new structure (refer to appendix 1). Installation of cathodic protection to existing structures can be as simple as encapsulating the exterior in an anode mesh (refer to Appendix 2) or cutting slots in the concrete to insert anodes (Cathodic Protection Co. Limited, 2011). The anode is applied usually to non traffic surfaces like walls and ceilings. The Anode can then be covered in a second layer of concrete of just directly painted over.
Rectifier is used to power the anode which release electrons that travel through the concrete to the cathode which is the steel bars (Oko, 2008). The electrons are then consumed on the surface of the steel in the reduction reaction. The steel does not corrode because it is now the cathode in what appears to be a redox reaction however no actual oxidation occurs as the electron flowing to the anode does not come from the oxidization of the anode they are instead coming from electrons flowing through the anode provided by the rectifier, this is known as an ‘inert’ electrode as it is not consumed or destroyed by the reactions (NDT Resource Centre, 2002). Because of this impressed current cathodic systems can last beyond 100 years without need to replace the anode.
Iron(II) has 2 valance electrons, meaning it wants to lose two electrons and become Fe2+ its natural oxide state. But because Iron is the cathode it only receives electrons, which it does not want, it wants the opposite, to loose electrons, but it can’t because it is the cathode. This is why cathodic protection systems work so well, because as long as the steel remains the cathode it cannot oxidize. The electrons that were released are consumed at the cathode by electrolysing water and oxygen to make hydroxide ions (Cathodic Protection Co. Limited, 2011). As hydroxide ions are formed the pH surrounding the steel increases which prevents Carbonation (Discussed in 2.0). These are the half equations for the reactions occurring.
Fe → Fe2+ + 2e- (EMF=+0.41V)
O2 + 2H2O +4e- → 4OH- (EMF= +0.40V)
(Cathodic Protection Co. Limited, 2011)
5.0 Conclusion
Steel is an alloy which has a vast variety of applications in construction all over the world and particularly in the concrete industry. It has many favourable properties which makes it perfect for reinforcing concrete however corrosion remains a constant problem in every application of steel. It is an inevitable process in which the Iron try’s to return to its natural oxide state through a redox reaction. Corrosion can be controlled through various prevention methods including epoxy coating and cathodic protection. Through the points discussed in this report it has been concluded that a cathodic protection system is a better choice for costal structures as it can provide continued corrosion protection for as long as desirable. Whereas on the other hand epoxy coatings deteriorate after several years of use (depending on the quality) and then the structure is completely prone to corrosion and will eventually become unusable and beyond economically feasible repair meaning it will have to be knocked down and re-built or if the problem is caught before it is too late a cathodic protection system can be applied in most cases to prevent further corrosion inside the concrete. Hopefully the implementation of more cathodic protection systems to save and protect corroding costal structures will secure the use of steel in the costal construction industry.
6.0 Appendix
To save money, the owner applied cathodic protection
to only the bottom floor. The bottom floor ceilings
are in excellent condition, but costly patching
had to be applied to the upper three floors.
Appendix 1
Appendix 2
Appendix 3
Appendix 4
Appendix 5
7.0 Bibliography
Books
{SOURCE 8} National Reasearch Council . (2001). Performance of Epoxy Coated Reinforcing Steel In Highway Bridges . Washington D.C.: Transportation Reasearch Board.
Reports
{SOURCE 12} NRMCA. (1995). Corrosion of Steel in Concrete. Silve Spring: NRMCA.
{SOURCE 9} Cathodic Protection Co. Limited. (2011). CATHODIC PROTECTION OF CONCRETE STRUCTURES . Lincolnshire: Cathodic Protection Co. Limited.
{SOURCE 7} Administration, F. H. (2002). Electrochemical Chloride Extraction: Influence of Concrete Surface on Treatment. United States: Federal Highway Administration.
{SOURCE 3} Ltd, C. C. (2004). CORROSION OF STEEL REINFORCEMENT IN CONCRETE. Sydney : CTI CONSULTANTS PTY LTD.
Websites
{SOURCE 2} corrosion-doctors. (2006). Nature of the Problem. Retrieved 9 26, 2012, from corrosion-doctors:
{SOURCE 5} American Galvanizers Association. (2012). Alternative Rebar Materials. Retrieved 9 26, 2012 , from Galvanized Steel Transportation Solutions:
{SOURCE 11} Cadman Inc. (2012). Corrosion of Steel In Concrete. Retrieved October 23, 2012, from CADMAN:
{SOURCE 6} Oko, D. U. (2008). Extending the Life of Old Concrete Structures. Retrieved 10 20, 2012, from Corrosion Services :
{SOURCE 1} The Steel Reinforcement Institute of Australia . (2008, March 9). Durability. Retrieved 9 25, 2012, from SRIA:
{SOURCE 4} NDT Resource Centre. (2002). Corrosion. Retrieved 9 29, 2012, from NDT Resource Centre:
Encyclopaedia
{SOURCE 10} Kolb, D. K. (2004 ). Steel. Retrieved 10 20, 2012, from Encyclopedia : http://www.encyclopedia.com/topic/steel.aspx