1. Shipwrecks, Corrosion and Conservation
Summary Slides PART 2 – Jack Dengate

2. Rusting is the corrosion of iron that forms the hydrated iron(III) oxide, Fe2O3.xH2O
This compound is commonly known as rust.
Rusting

3. 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.
Rusting

4. 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–
Rusting Process

6. Electrons move from the anode to the cathode through the iron
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.
Rusting Process

7. 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
Composition of Steel

8. Mild steel contains < 0
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.
Composition of Steel

9. Ships 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.
Ships

10. 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.
Corrosion of Metals

11. 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. .
Corrosion of Metals