Steel Corrosion In Concrete Costal Structures ERT

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Steel Corrosion inside Reinforced Concrete Costal Structures

Chemistry

2012


Contents

1.0 Introduction        

2.0 Background Information        

3.0 Why does Steel corrode in a costal environment?        

4.0 Method of Corrosion Prevention        

4.1 Epoxy Coating        

4.2 Cathodic Protection System        

5.0 Conclusion        

6.0 Appendix        

7.0 Bibliography        


1.0 Introduction

Steel has been around since BC times however it has only been with the relatively recent invention of the Bessmer process in the mid-19th century which has turned it into the mass produced, inexpensive material it is today. More than 1.3 billion tons of steel is produced annually (Kolb, 2004 ), it is one of the most common materials in the world being used in everything from buildings and infrastructure to weapons and machines (The Steel Reinforcement Institute of Australia , 2008). Corrosion is a chemical or electrochemical reaction between a material, usually a metal, and its environment which results in deterioration of the material and its properties. For steel embedded in concrete, corrosion occurs mainly in the form of rust which is the oxidization of iron(III). The formation of rust is two to four times the original volume of the steel that has corroded and it maintains none of the good mechanical properties that make it useful as reinforcement (Cadman Inc., 2012). Because of the high pH and relatively low permeability of concrete it provides a passive film of protection from corrosion in most places (The Steel Reinforcement Institute of Australia , 2008). However steel reinforcing that is constantly exposed to aggressive corroding chemicals such as seawater (Chlorides) often requires an extra level of protection beyond what the concrete alone can provide. The most common method used to protect against corrosion is an epoxy coating on the steel. Other methods used include switching to Galvanised and stainless steel reinforcement, either Impressed Current or Galvanic cathodic protection systems, and chemical and mineral corrosion protection systems. This report will be investigating two corrosion protection systems, epoxy coating and impressed current cathodic protection, analysing their advantages and disadvantages in order to determine which method is the most effective to protect against corrosion in steel reinforcement in concrete coastal structures.


2.0 Background Information

Steel is an alloy of iron containing about 1-2% carbon and occasionally other elements such as manganese chromium, vanadium, nickel and tungsten (Kolb, 2004 ). Steel is a mixture of elements not a compound, therefore it does not have a chemical formula. When writing chemical equations with steel, Fe is used to represent the steel, as Iron (Fe) is the main ingredient in steel, comprising of about 98-99% Iron. The melting point of steel varies between 1540°C and 1400°C, depending on the exact chemical composition of the steel, as carbon content increases there is a decline in the melting point (Kolb, 2004 ). The density of steel also varies based on its composition however it usually ranges between 7,755 and 8,050 kg/m3. Steel is best for reinforcing concrete because they both hold very similar properties of expansion and contraction under normal conditions (Kolb, 2004 ).

Carbon and other elements added to the steel act as hardening agents, aiding in preventing dislocations in the iron atom crystal lattice. Different amounts of alloying elements added to steel can control qualities including hardness, ductility and tensile strength. The addition of elements to steel’s composition will help improve certain properties of the alloy. For example steel with chromium added is harder and has a higher melting point (Kolb, 2004 ). The two main advantages of steel over iron is that it has better weld-ability and increased corrosion resistance. Pure iron alone is relatively soft and is very prone to corrosion. Steel can be hard, tough, and more corrosion resistant. Steel is used to build almost everything and is the least expensive and most widely used of all metals because of its many favourable properties. Steel is a shiny, lustres metal with an attractive finish, it is very malleable and can be rolled into thin sheets, rods, bars or beams or forged into different shapes. It is very strong and resistant to fracture. It conducts both heat and electricity, is ductile enough to be stretched into thin wires or pressed into different shapes and is very durable and resistant against wear (Kolb, 2004 ). Steel is a very strong and economically wise choice to use in construction, there are hundreds of varieties of steel, so there will always be a certain variety suited for a specific application.

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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, ...

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