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9.6 Shipwrecks, Corrosion and Conservation:  6. Corrosion at great depths

Extract from Chemistry Stage 6 Syllabus (Amended October 2002) ;© Board of Studies, NSW.
[Edit: 26 Jun 08]

Background: Predictions that wrecks should corrode slowly at great depth have proved to be incorrect. Scientist have had to look for other explanations. The action of anaerobic bacteria appears to be an important factor in the corrosion of vessels that have sunk to depths below 1000 metres.

explain that ship wrecks at great depths are corroded by electrochemical reactions and by anaerobic bacteria.

• In contrast to the predictions already made based on your knowledge of electrochemical reactions, shipwrecks at great depth are extensively corroded. This corrosion is the result of anaerobic bacteria as well as electrochemical reactions.
  Some evidence of this is found at The Swedish Ship Vasa's Revival , University of Miami,Florida, USA.
  Saving Shipwrecks, Electrochemical and spectroscopic methods help conserve historic metal artifacts Chemical and Engineering News, American Chemical Society, USA
  

describe the action of sulfate reducing bacteria around deep wrecks

• The wreck of the Titanic was found in 1985 in nearly 4000 metre deep waters.  Some parts had long red rusticles ('rust-like, icicle-like') hanging while other parts had black iron(II) sulfide.  Other parts of the Titanic had little corrosion.  Experiments on samples collected from the wreck showed that sulfate reducing bacteria (SRB) were important in forming the red rusticles and black FeS.

  Bacteria associated with the rusticles are sometimes called "iron-eating bacteria".  Two types of bacteria are found with the rusticles.  Anaerobic SRB which do not need oxygen are found on the inside.  Oxygen-dependent aerobic bacteria are found on the outside of the rusticles.  Chemical reactions carried out by this combination of bacteria increase the rate of corrosion of the iron Titanic. One microbiologist has suggested that iron corrosion driven by bacterial action will completely reduce the Titanic to a huge iron-ore deposit within 100 years.

• SRB produce the compound hydrogen sulfide (H₂S) from the sulfate ions that are plentiful in sea water:

  SO₄^2–   +   10H⁺   +  8e^–    -->   H₂S    +   4H₂O

  Note that the oxidation state of sulfur has been reduced from +6 in SO₄^2– to –2 in H₂S.

  This is why the anaerobic bacteria that cause this change are called sulfate reducing bacteria.

• Sea water normally has a pH of about 8.  The increased solubility of CO₂ with depth makes deep ocean water slightly acidic.  As the pH drops with depth in ocean water the presence of more hydrogen ions favours corrosion of metals.
  
  
• Corrosion of metals produces metal ions,

  e.g. Fe + 2H⁺  -->  Fe²⁺  +  H₂

  Some metal ions produced can undergo hydrolysis (reaction with water) to produce more hydrogen ions. The SRB are able to change the H₂ to 2H⁺ , which they then use to reduce sulfate ions to hydrogen sulfide.

• The release of hydrogen ions can produce small acidic environments as low as pH 4 in some locations around a shipwreck.

  H₂S, produced by the action of SRB is a weak acid that releases hydrogen ions and sulfide ions.;

  H₂S      2H⁺  +   S^2–

  The sulfide ions from the H₂S can precipitate Fe²⁺ ions to form insoluble, black iron (II) sulfide FeS:    Fe²⁺  +  S^2–  -->  FeS(s) 

  The presence of black FeS indicates that SRB are present.

  The precipitation of FeS removes sulfide ions and encourages further ionisation of H₂S releasing more H⁺.

• Metal near wood on the Titanic was badly corroded.  As the wood cellulose, (C₆H₁₀O₅)_n, decayed, it released oxygen which stimulated the growth of aerobic bacteria.  Waste from these aerobic bacteria provided nourishment to the anaerobic SRB.  The SRB flourished and increased corrosion of the metal near wood.

  The Grave of the Titanic Gulf of Maine Aquarium, USA.

perform a first-hand investigation to compare and describe the rate of corrosion of metals in different acidic and neutral solutions.

• To perform the investigation, compare the rate of corrosion in at least two different acidic solutions and one neutral solution. Consider which acidic solutions to use.  The investigation should involve a fair test and all variables must be controlled to ensure valid results. The results could be recorded in a table and repetition could be used to increase validity.

explain that acidic environments accelerate corrosion in non-passivating metals.

• Acidic conditions lead to an acceleration of the corrosion process compared with basic or neutral conditions.
• Hydrogen ions can react with non-passivating metals, such as iron.
  Fe(s)   +   2H⁺   -->  Fe²⁺   +   H₂(g)

  Non-passivating metals are metals that do not have a protective oxide layer that would prevent hydrogen ions reacting with metal atoms. Metals with protective passivation layers include aluminium, chromium, titanium and tin.

• Lead and copper objects taken from shipwrecks have been found to be corroded. Hydrogen sulfide produced by sulfate reducing bacteria can react with just about any metal except gold.

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