2001 HSC Question.

[Fortify]

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For ONE specific metal, evaluate the steps that can be used to clean, stabilise and preserve artefacts recovered from shipwrecks. Let's use Iron . Can anyone help me out ?

[sam5]

Originally Posted by Fortify

For ONE specific metal, evaluate the steps that can be used to clean, stabilise and preserve artefacts recovered from shipwrecks. Let's use Iron . Can anyone help me out ?

i will do this cos im a nice person

Im not gonna evaluate - cant be bothered - ur smart enough to do that - i will describe.

Get the iron thing and wash it repeatedly in a 5 - 10% weak base solution (e.g. NaHCO3). Do this over a period to wash the silt off, stop corrosion occuring in its paces and remove lightly embedded salts. To check this process is working each time, you can get a probe to measure concentrations of chlorides and other anions to check that they are leeching out of the iron. Then you would get the iron thing and use a hammer to lightly chip off any concretions.

Then comes electrolysis. Get the iron thing and put it as the cathode in an electrolytic cell. Connect the negative terminal of a DC power source to the iron thing and connect the positive terminal to a piece of stainless steel (inert)wrapped in stainless steel mesh. Use a 2 - 5% NaOH electrolyte. Use a small charge at first to slowly remove the outer salt layers in the iron thing, then increase the charge to get the rest out. This is done because the hydrogen produced at the cathode (iron thing) can cause rust flakes to be removed prematurely which can tear metal away with it, causing damage. The electrolye and steel mesh should be replaced every few days to prevent anion saturation.

Iron salts reduced: Fe(OH)2 + 2e- --> Fe + 2OH-
Water is reduced at cathode: 2H2O + 2e- --> H2 + 2OH-
Water is oxidised at anode: H2O --> 1/2O2 + 2H+ + 2e-
Hydroxides oxidised at anode: 2OH- --> 1/2O2 + 2e- + H2O

Finally to protect the iron thing forever, coat it in a wax, lacquer or PEG (polyethylene glycol). This stops water and oxygen reaching the object, preventing corrosion.

(does that deserve rep? LOL)

[harry4]

soaking the iron in acid would increase the rate of corrosion which is undesirable. you use acid for lead and copper

[sam5]

Originally Posted by harry4

soaking the iron in acid would increase the rate of corrosion which is undesirable. you use acid for lead and copper

im not doubting what ur saying - but why is that?

Is it cos iron is more reactive than the other 2?

[harry4]

ah i think the H+ ions in acids act as a catalyst in the conversion from Fe(OH)2 into Fe2O3.xH20 (RUST)