Being an important nature reserve this had to be powered by an environmentally friendly source — wave power or sunlight. Rough Scientist Mikey B was given the challenge of stopping the wire rusting over time. He decided to use a lump of magnesium as a sacrificial electrode.
To find out more about how magnesium stops iron rusting and how it is used to protect oil drilling platforms and boats, read this extract from the second level OU course Our Chemical Environment ST According to this table aluminium should react with oxygen more readily than iron, which is easily oxidized in the presence of water and oxygen.
How is it then that aluminium can be used for aeroplanes which are in constant contact with water and oxygen? In reality, aluminium does react with oxygen very rapidly and forms aluminium oxide, Al 2 O 3. The oxide has the ability to adhere to the surface of the underlying metal with great tenacity. If the oxide becomes scratched and aluminium metal exposed, more oxide is formed and the protected layer renewed.
The situation with iron is very similar in the sense that iron oxidizes and forms the familiar red—orange rust. However, rust takes up a greater volume than the iron from which it is formed and consequently splits and cracks. It is also porous to water and oxygen so that more rust can be formed from the underlying metal and eventually the oxidation process is complete. Moisture, in addition to oxygen, is also necessary for the rusting of iron. Rust is not the simple oxide Fe 2 O 3 but is a compound that actually contains water molecules.
It can be represented by Fe 2 O 3. The water molecules are incorporated into spaces within the crystal structure of iron oxide. Iron and steel is not weather-resistant yet it does find widespread use because of its relative cheapness and its mechanical properties.
Perhaps the most obvious way in which steel can be prevented from rusting is to protect the metal surface with a coating of something that does not rust. The most widely used coating is paint and it is very effective provided that the paint does not become damaged. If this happens, or the metal surface is not completely free of rust when it is painted, then further rusting can occur. The situation is made worse because rusting can spread under the paintwork and is often at an advanced stage when signs of blistered paint become apparent.
Where a hard paint surface is not required such as the inside of car body panels , waxes and paints that do not set completely are useful. If such a surface suffers a small scratch, the paint will flow a little and effectively self-heal the scratch.
Another coating that protects steel is to use a metal that is further down the activity series, a metal that does not oxidize in the air. The ideal metals are gold, silver and copper, which are towards the bottom of the activity series.
Galvanized buckets and dustbins that used to be commonplace did not rust even though they were made from steel. The metal surface of these containers appears to have irregular shiny crystals. Galvanizing is the process by which a metal surface is given a coating of zinc.
The process of galvanizing does seem to work; it is more expensive than conventional paint protection but is finding increasing use in the motor industry. Where does zinc come in our activity series?
We have argued that to coat steel with a metal that does not oxidize in air makes sense, so it seems odd to use a metal that oxidizes even more readily than does iron. Zinc, like aluminium, oxidizes easily. A common form is galvanizing, in which the iron surface is coated with a layer of zinc. Even if the zinc layer is scratched, the iron does not rust because zinc ions are formed in solution in preference to iron ions. Pieces of magnesium alloy are similarly used in protecting pipelines, etc.
From: sacrificial protection in A Dictionary of Physics ». Subjects: Science and technology — Chemistry. View all related items in Oxford Reference ». Search for: 'sacrificial protection' in Oxford Reference ». All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single entry from a reference work in OR for personal use for details see Privacy Policy and Legal Notice.
Oxford Reference. According to the table of Standard Reduction Potentials, the standard reduction potential of zinc is about The standard reduction potential of iron is about This difference in reduction potential means that Zinc would oxidize much faster than iron would.
In fact, zinc would oxidize completely before iron would begin to react. The materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminum alloys that have been specifically developed for use as sacrificial anodes.
In applications where the anodes are buried, a special backfill material surrounds the anode in order to insure that the anode will produce the desired output. Since the sacrificial anode works by introducing another metal surface with a more negative electronegative and much more anodic surface.
The current will flow from the newly introduced anode and the protected metal becomes cathodic creating a galvanic cell. The oxidation reactions are transferred from the metal surface to the galvanic anode and will be sacrificed in favor of the protected metal structure. Figure 1. Partially corroded sacrificial anode on the hull of a ship. Figures courtesy of Wikipedia.
Sacrificial anodes are normally supplied with either lead wires or cast-m straps to facilitate their connection to the structure being protected. The lead wires may be attached to the structure by welding or mechanical connections. These should have a low resistance and should be insulated to prevent increased resistance or damage due to corrosion.
When anodes with cast-in straps are used, the straps can either be welded directly to the structure or the straps can be used as locations for attachment.
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