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9.6 Shipwrecks, Corrosion and Conservation: 4. Marine environment protection
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002). Board of Studies, NSW.
[Edit: 26 Jun 08]
Prior learning: Preliminary module 8.3.1
Background: Throughout history many different materials have been used to construct ocean-going vessels. As vessels became larger the materials used in their construction had to become stronger and be longer lasting. Wooden ships are subjected to attack by marine organisms and many methods have been tried to slow this attack. Vessels made from iron and steel are stronger than wooden vessels and are not attacked by marine organisms but can corrode rapidly in the saline environment.
identify data, gather and process information from first-hand or secondary sources to trace historical developments in the design and construction of ocean-going vessels with a focus on the metals used
- To identify data sources, consider what is the most appropriate technology or strategy.
- When this has been done gather information from these sources such as encyclopaedias, CDs and the Internet.
- Process the information by organising it in a chronological order. Note that the most important part of this investigation is to find the different metals used and the reasons for their uses.
Some information that may assist in this investigation
As vessels have changed through history, people have endeavoured to find materials that are strong, will resist attack by marine organisms and will not corrode in the electrolyte solution that is salt water.
The earliest boats were constructed from animal skins, bamboo, logs or reeds. The material used depended on what was available to the people.
The early Egyptians constructed boats using small pieces of wood.
Wooden planked boats were constructed by the:
- Phoenicians using copper alloy spikes, nails and fasteners
- Greeks and Romans using iron to replace the weaker copper
- Vikings who nailed on overlapping shingles to cover oak planks
- Europeans who added decking, forecastles and aftercastles, as well as two or three masts.
Lead and then copper sheeting was fixed to the hull of ships to prevent biofouling by marine organisms. The sheeting was fixed initially using iron nails but these were found to rust. Copper nails were used in their place.
A bronze alloy (60% copper and 40% zinc) was developed to replace the copper sheeting. This alloy was found to be stronger and could be used to make thinner sheets, a cheaper option.
Sheet iron ships began to replace wooden boats from 1855.
Steel was used as it was less likely to be corroded than iron or bronze.
Keels have attachment plates made of magnesium or zinc so they will corrode instead of the steel.
identify data , choose equipment , plan and perform a first-hand investigation to compare the corrosion rate, in a suitable electrolyte, of a variety of metals, including named modern alloys to identify those best suited for use in marine vessels
- Identify the variety of available metals and alloys that should be used for this investigation. Include those that are used, or have been used, for the construction of ocean going vessels, such as iron, copper, stainless steel or bronze. The electrolyte solution should have a similar composition to sea water.
- Choose equipment that will be suitable for the investigation. Glassware will be suitable to use as vessels as it won’t corrode and interfere with the results.
- Plan the investigation by thinking carefully about the type of data that you will be able to collect. How will you record this data so that you can readily compare the rate of corrosion of the metals and alloys? Your data could be qualitative, such as changes in surface appearance, or quantitative, such as changes in mass or thickness or strength of the metal and alloy samples.
- Perform the investigation. It may take many weeks before you can observe significant differences between the metals and alloys.
predict the metal which corrodes when two metals form an electrochemical cell using a list of standard potentials
The HSC data sheet lists the standard potentials for a number of half reactions. Reading the equations from left to right as reductions they start at the top with the positive ions that are the least likely to be reduced. These half reactions have the lowest reduction potentials, that is, they are given the most negative Eo values.
To predict which metal will corrode:
- select the reduction half equations for the two metals
used in the electrochemical cell.
-
reverse the half equation that has the most negative
Eo value (the half equation of lowest
reduction potential) . This equation will now represent
an oxidation half reaction and will have a positive
Eo value.
This metal will be the one that is oxidised and will be the metal that corrodes.
plan and perform a first-hand investigation to compare the effectiveness of different protections used to coat a metal such as iron and prevent corrosion
Many methods have been developed to prevent or slow the corrosion of iron.
- When planning your investigation you should include as many of these methods as possible.
- In performing the investigation identify how each method protects the iron from corrosion and compare their advantages and disadvantages.
Some methods for the protection of iron from corrosion that you may be able to use in your investigation include:
- iron covered in regular paint
- iron covered in rust-preventing paints
- iron covered in polymer-based paints
- iron covered in vitreous enamel
- tin plate (iron covered with a thin coating of tin)
- galvanised iron (iron covered with a layer of zinc).
The control for this investigation should be uncoated iron.
identify the ways in which a metal hull may be protected including:
- corrosion resistant metals
- development of surface alloys
- new paints
- Identify examples of each of the methods for protecting the hulls of vessels listed in the syllabus. For each example, explain how the method offers protection to the hull.
Stainless steels are favoured corrosion resistant metals because of a passive film of chromium (III) oxide on the surface that resists corrosion.
Surface alloys can be created by various methods to give ordinary steel a surface similar to stainless steel. The hull would have the corrosion protection of an outer layer of stainless steel without the expense of this material.
Polymer paints protect against rust by forming a film over the surface of the steel that is impervious to oxygen and water. These paints also form a layer of a very insoluble ionic substance called pyroaurite. This ionic layer bonds strongly to the surface of the steel and well into the polymer layer. It prevents the movement of ions across the surface of the steel.
outline the process of cathodic protection, describing examples of its use in both marine and wet terrestrial environments
gather and process information to identify applications of cathodic protection, and use available evidence to identify the reasons for their use and the chemistry involved
The two syllabus points referring to cathodic protection are very similar. In order to cover them, you need to identify examples of cathodic protection used in a marine environment and a wet terrestrial environment.
- Identify why cathodic protection is used in these environments.
- Outline the process of cathodic protection.
-
Describe the oxidation/reduction
chemistry of examples of cathodic protection
To gather information on applications of cathodic protection go to the Internet. You could use a search engine and type ‘cathodic protection’. If you get too many hits you could add ‘chemistry’ and ‘redox’.
When you have gathered enough information from various sources, process it by highlighting the useful information and compiling it into a suitable format.
Use this information to formulate cause and effect relationships.
describe the process of cathodic protection in selected examples in terms of the oxidation/reduction chemistry involved
- An example of cathodic protection, described in terms of the oxidation/reduction chemistry:
An iron tank buried in the ground may be protected from corrosion by attaching a block of zinc to it. When someone is available to monitor the tank and manage the situation, this method of cathodic protection is cheaper than using a galvanised tank. The zinc block is easily replaced when it has nearly completely corroded. The zinc is more active than the iron and corrodes preferentially. The zinc block and iron from the tank form an electrochemical cell and the zinc is oxidised as the anode.
Zn --> Zn2+ + 2e–
The electrons flow into the iron preventing the formation of Fe2+ ions. The electrons produced by the oxidation of the zinc reduce any Fe2+ ions that form, back to Fe atoms. The site where reduction occurs is called the cathode and the method is called cathodic protection.
Fe2+ + 2e– --> Fe
The rate of corrosion of the zinc block is slow because it forms a coating of dull white zinc hydroxide carbonate which protects the underlying zinc.
In marine environments sacrificial anodes are usually used. Zinc sacrificial anodes attached to outboard motors gradually dissolve as they are “sacrificed” instead of the metal parts of the outboard motor.
In wet terrestrial environments, such as the surroundings of a metal pipeline, the impressed current method is usually used. Insertion of a Direct Current power supply between a metal or graphite anode and the pipeline provides a flow of electrons to protect the pipeline metal. The surroundings must be wet so that electrolytes can flow through the ground to complete the electric circuit between the negative pipeline and the positive anode. The anode should last a long time as it is not sacrificed when an impressed current is used.
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