|Syllabus reference (October 2002 version)|
4. Iron and steel corrode quickly in a marine environment and must be protected
Students learn to:
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
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:
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
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:
This metal will be the one that is oxidised and will be the metal that corrodes.
Many methods have been developed to prevent or slow the corrosion of iron.
Some methods for the protection of iron from corrosion that you may be able to use in your investigation include:
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
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.
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.
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 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.