Shipwrecks, Corrosion and ConservationSummary Slides PART 2 – Jack Dengate

Rusting

• Rusting is the corrosion of iron that forms the hydrated iron(III) oxide, Fe2O3.xH2O

• This compound is commonly known as rust.

Rusting

• 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 Process

• 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

• 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.

Composition of Steel

• 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

• 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.

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.

Corrosion of 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.

Corrosion of Metals

• 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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