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Home > Chemistry > Options > Shipwrecks, Corrosion and Conservation > Shipwrecks, Corrosion and Conservation: 5. Rate of decay and corrosion

9.6 Shipwrecks, Corrosion and Conservation: 5. Rate of decay and corrosion

Syllabus reference (October 2002 version)
5. When a ship sinks, the rate of decay and corrosion may be dependent on the final depth of the wreck	Students learn to: outline the effect of: temperature pressure on the solubility of gases and salts identify that gases are normally dissolved in the oceans and compare their concentrations in the oceans to their concentrations in the atmosphere compare and explain the solubility of selected gases at increasing depths in the oceans predict the effect of low temperatures at great depths on the rate of corrosion of a metal	Students: perform a first-hand investigation to compare and describe the rate of corrosion of materials in different: oxygen concentrations temperatures salt concentrations use available evidence to predict the rate of corrosion of a metal wreck at great depths in the oceans and give reasons for the prediction made

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

Prior learning:  Preliminary module 8.4.3, 8.4.4

Background: The rate of decay of a vessel may depend on the depth of the wreck. The conditions that affect the rate of corrosion change as the depth of the seawater increases. This section asks students to investigate the factors that affect the rate of corrosion and to use this knowledge to predict the rate of corrosion of ships that have sunk to great depths in the oceans.

perform a first-hand investigation to compare and describe the rate of corrosion of materials in different:

• oxygen concentrations
• temperatures
• salt concentrations.

• Care needs to be taken with the design of this investigation to ensure that valid results are obtained. Oxygen concentrations, temperature and salt concentration may all influence the rate of corrosion of a material. Each variable should be changed separately while the other two are controlled (kept constant). The investigation should be trialed so a suitable set of conditions can be determined and used as a control for the experiment The results from all other trials can be compared with this standard.  Check that the rate of corrosion increases with:
  • increasing oxygen concentration
  • increasing temperature
  • increasing salt concentration.
• Note that increasing the temperature and increasing salt concentration each have the effect of decreasing the amount of oxygen that dissolves.
• When you are happy with the design of your investigation, perform it and record the experimental results.
• From your experimental results draw a conclusion or conclusions.
• Discuss your conclusion(s) with the other students.

Galvanic corrosion Key to metals, Zurich, Switzerland

outline the effect of:

• temperature
• pressure

on the solubility of gases and salts

• Solubility of a gas increases as the pressure of that gas in contact with the solution increases
• Solubility of gases decreases as the temperature increases.
• The solubility of salts generally increases as the temperature rises. (There are a number of exceptions to this generalisation. The solubility of some salts remains the same as the temperature increases, while the solubility of others decreases.)
• The solubility of salts is largely unaffected by increasing the applied pressure.

Gas solubility information , Elmhurst College, USA

identify that gases are normally dissolved in the oceans and compare their concentrations in the oceans to their concentrations in the atmosphere.

• The atmospheric gases most dissolved in the oceans are carbon dioxide, nitrogen and oxygen, in that order.

  Gas	Formula	air % (v/v)	sea water % (v/v)
  nitrogen	N2	78.1	0.8 – 1.5
  oxygen	O2	20.9	0 – 0.9
  carbon dioxide	CO2	0.03	4.5 – 5.4

  
  
  
• The concentration of most gases in the oceans is lower than their concentration in the atmosphere.  Acidic carbon dioxide is an exception.
• The proportion of carbon dioxide in seawater when compared with oxygen and nitrogen is much higher than its proportion in the atmosphere when compared with the same two gases.  CO₂ molecules can react with water molecules to form ions which are more soluble in water.  The hydrogen ions produced lower the pH of seawater.

  CO₂(g)    +   H₂O(l)   -->    HCO₃^–(aq)    +    H⁺(aq)

• The rate of gas movement from the atmosphere, the use and production of gases by living organisms, and chemical reactions can all affect the amount of gas present at lower depths.
• The concentration of oxygen in seawater is greatest near the surface because wave action increases contact between water and air.  The concentration then decreases to a minimum at depths between 500m to 1000m because of the respiration of marine organisms. Below 1000m the oxygen concentration steadily increases as the depth increases.

compare and explain the solubility of selected gases at increasing depths in the oceans

• The solubilities of all gases tend to increase with depth because of the increasing pressure.
• Carbon dioxide reacts with water to form hydrogen carbonate ions and carbonate ions leading to increased solubility when compared with oxygen and nitrogen.

  CO₂(g) + H₂O(l)    H₂CO₃     H⁺ + HCO₃  2H⁺ + CO₃^2–

• As ocean depth increases the pH of the water changes from basic (pH 8) to neutral to acid.  The increased solubility of acidic CO₂ with depth is the main cause of the drop in pH.

use available evidence to predict the rate of corrosion of a metal wreck at great depths in the oceans and give reasons for the prediction made.

• Using knowledge of the behaviour of electrochemical cells it can be predicted that the rate of corrosion of a metal wreck will be slow at great depths because of:
  • the low temperature of the water
  • the low concentration of oxygen compared with the atmosphere.

predict the effect of low temperatures at great depths on the rate of corrosion of a metal

• Water at the bottom of oceans will be about 4⁰C because salt water has its maximum density at this temperature. Water colder than this is of lower density and will rise upwards.
• The rate of corrosion of metals will decrease as the temperature decreases. (You will need to draw on your knowledge of the effect of temperature on the rate of reactions to make this prediction).