• Question: why does metal rust, what does the water actually do to it?

    Asked by jodlington to Kate, Kieren, Nicola, Rowena, Roy on 14 Mar 2014.
    • Photo: Rowena Fletcher-Wood

      Rowena Fletcher-Wood answered on 14 Mar 2014:

      It oxidises it to form metal oxides. Fe2O3, iron +3 oxide is our common rust. It forms when the metal reacts with oxygen in the air or carried by water. Water facilitates this reaction though it can react woth the metals itself releasing hydrogen gas. If the metal is in a compound not as a pure metal it can make an oxide by swapping bits of its compound with water.

    • Photo: Kate Nicholson

      Kate Nicholson answered on 14 Mar 2014:

      The water makes it easier for the iron to form iron oxide (rust if its Fe2O3 as Rowena says). It will also help expose new un-rusted parts of the metal as it freezes and thaws. Mostly it just keeps the bits that are oxidising the metal in close contact with it, rather than waiting for gas molecules to collide with it and do the same job.

    • Photo: Nicola Rogers

      Nicola Rogers answered on 17 Mar 2014:

      water helps the formation of rust by acting as a electrolyte, i.e. a medium that allows the transport of ions. Water and oxygen combine with iron to form a sort of electrochemical cell; at the ‘cathode’: oxygen can gain electrons from the iron surface, and react with H+ ions in the water to form more H2O, and as the iron gives up its electrons at the ‘anode’ it forms Fe3+, which dissolve in the water and can react with oxygen dissolved in the water form hydrated Fe2O3, which is the rust.

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