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9.6 Shipwrecks, Corrosion and Conservation: 2. Corrosion of iron and its alloys
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 7 Jul 09]
Prior learning: Preliminary module 8.3.2
Background: The water of the oceans is a massive electrolyte that enables electrochemical reactions, including corrosion, to occur. Galvanic cells that lead to corrosion can form between different metals connected by sea water electrolyte.
Ocean going ships are constructed from iron or alloys of iron. The term steel is used to identify alloys of iron. Different steels have different properties because of their different composition. Rusting is the term applied to the corrosion of iron and steel and is an important chemical property of these substances.
identify data, select equipment, plan and perform a first-hand investigation to compare the rate of corrosion of iron and an identified form of steel
- When planning your investigation, you need to identify the type of data that you collect. Will you collect qualitative data or quantitative data? How will you record this data?
- When you have decided on what type of data to collect you will need to select the most appropriate equipment for the investigation.
- Plan to carry out some trials for this procedure in order to identify those conditions and procedures that allow you to make a valid comparison of the rate of corrosion of iron and your identified alloy of iron.
- The investigation should be a fair test in which all variables apart from the independent variable (type of iron or steel alloy) are controlled. The rate of corrosion is the dependent variable.
use available evidence to analyse and explain the conditions under which rusting occurs
- Evidence about rusting is readily available from standard chemistry textbooks, encyclopaedias and from the World Wide Web. You need to identify the factors that affect the rusting of iron and be able to explain why they act in this way.
Some information you might find useful
Both oxygen and water are necessary for rusting because together they can be reduced to form hydroxide ions.
Salt water accelerates rusting because it is an electrolyte solution and its ions are able to move freely between the sites of the oxidation and reduction half reactions.
Impure iron rusts more quickly than pure iron because the impurities may act as the cathode. The O2 is reduced on these impurities.
The rusting of iron is accelerated when it is in contact with a less active metal such as copper or tin because less active metals serve as the cathode.
Iron under mechanical stress rusts more quickly because individual Fe atoms are less strongly bonded together. This makes it easier for atoms to break away from the crystal lattice as Fe2+ ions.
describe the conditions under which rusting of iron occurs and explain the process of rusting
Background information
Rusting is the corrosion of iron that forms the hydrated iron(III) oxide, Fe2O3. x H2O. This compound is commonly known as rust.
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The conditions under which iron will rust are
described here:
- Iron will rust when it is in contact with both oxygen and water
- Rusting is faster if the water is acidic and if it contains dissolved salts (an electrolyte solution)
- The rusting of iron is accelerated if the iron is impure, is in contact with a less active metal or is under mechanical stress.
-
An explanation of the electrochemical
process of rusting involves the following:
- - The iron is the anode for this reaction and
is oxidised to the Fe2+ ion.
Fe --> Fe2+ + 2e– - - Reduction of oxygen occurs at the cathode.
O2 + 2H2O + 4e– --> 4OH– - Electrons move from the anode to the cathode through the iron. The anode and cathode may be separated from each other by some distance.
- Insoluble iron(II) hydroxide Fe(OH)2 is formed and the Fe2+ ions are further oxidised to Fe3+ to form rust Fe2O3.xH2O.
- - The iron is the anode for this reaction and
is oxidised to the Fe2+ ion.
Read about
Rust formation 
, Fact-index.com, Wikipedia
gather and process information from secondary sources to compare the composition, properties and uses of a range of steels
- The information about the composition, properties and uses of a range of steels can be gathered from a variety of secondary sources.
- This information will need to be processed so that a clear comparison of the composition of the steels can be made. The properties of the steels should be shown to relate to their compositions. The uses of the steels will depend on their properties.
Steel for many purposes
- Relate uses of the object to properties and composition.
identify the composition of steel and explain how the percentage composition of steel can determine its properties
- Steel is an alloy of iron with no more than 2% carbon and with varying amounts of other metals and silicon.
- The carbon may form cathodic sites where reduction reactions take place.
- The deposits of carbon impurities may lead to mechanical stress of the iron
- Mild steel contains < 0.2% carbon, is soft, malleable and corrodes readily.
- Structural steel has between 0.2% and 0.5% carbon, is hard but malleable with high tensile strength and corrodes readily.
- Stainless steel, like mild steel contains < 0.2% carbon, but also typically contains 10 to 20% chromium and 5 to 10% nickel. It is hard and very resistant to corrosion due to formation of a protective layer of chromium oxide.
identify iron and steel as the main metals used in ships
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Iron or steel have been used to make ships because they:
- are relatively hard
- are mechanically strong
- can be worked into different shapes and structures
- can be welded.
account for the differences in corrosion of active and passivating metals
- Corrosion is usually the oxidation of metal in the presence of oxygen and water.
- Only a few metals naturally occur as the element (e.g. gold and silver).
- Most metals react with other elements and occur in nature as compounds.
- Iron continues to corrode because rust is a porous compound that allows oxygen and water to reach the fresh layers of iron below.
- Passivating metals such as aluminium and chromium react to form non-porous layers of oxides that prevent further corrosion.
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