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Monday, January 2, 2012

Corrosion/Oxidation

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For centuries, it has been known that elements are separated into two groups- metals and non-metals. Not surprisingly, the larger of the two is the metals. Metals in general can be characterised as hard, shiny solids that can be shaped, and are a relatively good conductor of heat and electricity. The discovery and utilisation of metals is the primary reason civilisations have moved beyond the Stone Age.


Today many of the known metal elements are used extensively by society for almost everything (particularly iron and steel). This is why corrosion is such a destructive problem for our global community.


When metal corrodes (oxidises) it loses its structural purpose � it weakens significantly. This generates a great economic bearing on industries and the utilisers of the metals. Australia alone spends over three billion dollars a year on preventative measures and replacement of corroded metals.


Corrosion is defined as the state of deterioration in metals, caused by oxidation or chemical action. The process of corrosion is often electrochemical. It involves an anode (a metal that readily gives away electrons), a cathode (a metal that readily accepts electrons) and an electrolyte (a liquid that helps electrons move).


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Corrosion if based on redox reactions. Redox reactions occur in two half-cells, where more reactive metals always give its electrons to the ions of less reactive metals.


The electrode where oxidation takes place is called the anode, and the electrode where reduction occurs is called the cathode. The electrons move from the more reactive electrode, through the circuit, to the less reactive electrode.





There are two types of corrosions, which can occur - dry corrosion and Wet corrosion.


Dry corrosion occurs when a metal reacts directly with oxygen. An example of dry corrosion is the burning of magnesium ribbon to produce magnesium oxide. Also such metals as potassium and sodium are good examples of dry corrosion as they react dangerously with oxygen; thus why they must be stored in oils.


Wet corrosion occurs when a metal reacts directly with oxygen, but it in the presence of water. Water speeds up corrosion reactions. For example iron corrodes much more readily in a moist environment.


The corrosion of iron is commonly referred to as rusting. When water drops onto the iron it often isn’t pure- salts for example are a common impurity. When this occurs, a simple electrochemical cell is set up on different sites of the metal. As mentioned above an electrochemical process must have an anode, cathode and electrolyte present. The anode in this process is usually a point of stress on the metal (dent, scratch etc.). At this point the iron is oxidised and it releases electrons.


The electrons then travel through the metal- being accepted by the oxygen at the cathodic site. This cathode is usually situated where the edge of the water drop meets the air and iron surface. This is because the cathodic area requires a plentiful amount of oxygen. The oxygen accepts the electrons and is reduced.


The hydroxide ions that are released at the cathode move towards the Fe+ ions produced at the anode. These two react to produce iron (II) hydroxide, a green insoluble precipitate.


This precipitate is very unstable in air, so it reacts with oxygen to produce brown iron (III) hydroxide.


The iron (III) hydroxide now partially dehydrates, producing iron (III) oxide (FeO)-rust. The amount of water lost affects the colour of the rust, which varies from black, yellow, or the common reddish brown.


Anything that is made of iron and steel will undergo this process if it isn’t protected in any way. Cars, tools, bridges, building, and machinery are all examples of what corrosion can effect and damage severely. Because iron (III) oxide is soft and powdery, the rust is dislodged easily, and the corrosion process is able begin again. This is why if rust isn’t dealt with immediately it can continue to eat away at the metal.


Water is very important in the above process because it acts as a salt bridge, and with out a salt bridge iron will not rust. Therefore cars last much longer in dryer areas. On the other hand in coastal areas because the air has a high dissolved salt content, it accelerates the rate of corrosion. This is because dissolved ions increase the conductivity of moisture.


There are many other factors that increase the rate of corrosion, some of which include the presence of acids and pollutants (sulphur dioxide, and nitrogen dioxide), and flaws such as bumps, dents and scratches on the metals surface.


Corrosion has a damaging effect on the properties of metals; every day, people take precautions to try and prevent/delay it from happening. For example, the gutters around the house must be painted regularly which help to delay the effects of oxidation. When the car gets scratched, it must be repaired immediately or else further corrosion occurs and the costs become very expensive. Machinery must be constantly coated with rust preventers so that breakdowns are avoided.


There are many adverse effects that corrosion has on metals, some of which are as follows (taken from the chemical connections rd edition textbook (p416)).


• Metal oxides, or rust, have less tensile strength and less elasticity than un-corroded metal. Corroded buildings, bridges and machinery may develop weaknesses, which result in malfunction or breakage.


• A severely corroded metal cannot conduct electricity. Products of corrosion formed in a car battery, between the terminals and the leads, may cause the electrical system of the car to malfunction as the conductivity of the terminals is diminished.


• Corroded copper pipes and how water tanks leak as the copper compounds dissolve in the water flowing through them


• Products of corrosion are brittle and flake off, resulting in holes in car mufflers or iron roofing. In car radiators iron oxide flakes may cause blockages.


• A corroded metal occupies a larger volume than the original metal. Corroded nuts and bolts may jam machinery.


Prevention is an extremely important and vital aspect especially if the steel is to be used for the hull of a ship, or the support beams of a bridge. There are three methods by which corrosion is prevented- Surface protection, alloying and electrochemical protection are the three main techniques.


Surface protection prevents the air and water from coming into contact with the metal. There are numerous methods of surface protection that can be used on iron and steel


Type How it is used


Plastic This is the most common surface protection; it is used to protect electrical cables and wires, or even the everyday coat hanger.


Paint This is used to protect large objects such as ships and bridges. Paint containing phosphoric acid is a very effective preventative as it formes a layer of insoluble iron (III) phosphate with any rust present on the surface. Also zinc dust can also be added to some paints acting as an anti-corrosion reagent. However with paint it is important that they are carefully selected as environments differ in their corrosive nature.


Grease/Oil This is used to coat the moving parts of machinery. It allows mobility also providing protection from corrosion.


Metal coatings This is used on steel. There are two types of these coatings. To coat a less reactive metal than steel, a noble coating would be used; this purely serves as a covering. If this covering is scratched or bumped rapid corrosion occurs to the exposed steel.


The noble coating is used where scratching is not likely and where zinc coating would not be safe, as zinc salts are poisonous (pots and pans are an example).


A coating of a more reactive metal than steel is sacrificial coating. If the sacrificial coating is broken, it results in the formation of an electrochemical cell; the coating becoming the anode. Thus the coating corrodes and the steel cathode is protected. Zinc is the most common sacrificial coating. When metal is coated in this way it is called a galvanised metal. When this process is used on an iron roof, the zinc coating reacts with the carbon dioxide in the area to form a layer of zinc carbonate over its surface. This protects the iron and slows the rate of corrosion. Galvanised steel is used for roofing, guttering, pipes, rubbish bins, fencing wire, and nails.


Alloying is a very effective technique of corrosion prevention. Alloying is the process of mixing different metals together to produce a more resistant, and stronger metal. Iron can be alloyed with small quantities of chromium, nickel, manganese and molybdenum. This produces stainless steel. The formation of the film layer called chromium (III) oxide (CrO) provides a very strong surface protection of the alloy. But despite this, the alloy is still vulnerable to chloride ions. Therefore it is important to choose the right type of alloy to suite the purposes of the metal.


Electrochemical protection works on the metaphor “to sacrifice one for another”. This process involves placing a more reactive metal in electrical contact with the metal that needs protection. There are a few ways in which this process can be achieved.


 If a metal that is lower in the electrochemical series is put together with iron, then that metal will then form the anode; being corroded while the iron, which acts as the cathode, remains un-corroded. Zinc and magnesium are commonly used for this. One example of its application is its use for the protection of underground pipes. Many underground pipes have attached bags of magnesium scraps placed along the pipe in intervals. When the magnesium corrodes, the bags are replaced (these bags of zinc are ‘sacrificed’ to protect the pipe. This process is often labelled sacrificial protection or cathodic protection. An appropriate sacrificial anode must be carefully selected for this process to be executed effectively.


 Another method of cathodic protection is connection iron to a negative terminal of a battery and a metal such as graphite to the positive terminal. The iron is then acting as the cathode and the graphite the anode. Fe+ ions are then formed and are inhibited by the negative potential on the iron.


Today with what scientists know about redox reactions, and electrochemical processes, corrosion will soon become a thing of the past. There is already an American scientist who Sais that they have developed a paint that will possibly permanently prevent corrosion. Who knows in the next 100 years, a new scientific breakthrough could see us depart from the age of the metals and corrosion would no longer even be relevant to our society.





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