Rates of corrosion
Iron and iron alloys such as steel are the major structural metals that have been used to construct ships since the late eighteenth century, when the first sheet iron ships were built. Pure sheet iron is very malleable and readily welded. Steel is harder than iron and has greater mechanical strength. Eventually, large steel-hulled ships allowed world trade to develop to high levels in the twentieth century.
The rates of corrosion of iron and the various types of steel differ considerably. It is very difficult to obtain pure iron as it usually contains traces of other elements including carbon. It has been shown that extremely pure iron oxidises nearly as readily as aluminium but will not easily corrode as it also forms a passivating oxide layer on its surface. If, however, impurities such as carbon are present (as in mild steel), then a different and incoherent, porous oxide layer forms which leads to extensive rusting of the iron.
• Iron (wrought or cast)
• Mild steel (e.g. steel bolts)
• Stainless steel
Place the samples in test tubes under the following condition:
1. Dry air only (use drying crystals)
2. Water and air present
3. Water only (degassed by boiling – then stoppered)
4. Salt water.
Examine the samples each lesson and rate their degree of rusting using a five-point scale. The stainless steel should show no corrosion or the slowest rate of corrosion as it has alloyed elements such as chromium and tungsten that form passivating layers. Cast iron has a higher carbon content than mild steel. Both corrode fairly rapidly although you may find that the cast iron rusts more rapidly than the mild steel. Wrought iron may be found to rust the least as it has a lower carbon content than mild steel. These results depend on the samples and their size/shape.
Rusting is a galvanic process. Some regions of the iron behave as anodes and others behave as cathodes. Electrons are conducted between the two sites through the metallic iron lattice. The electrolyte may be a film of water on the surface of the iron. Any ions in the water film help to conduct the charge in the galvanic cell.
The conditions that promote rusting include:
• the presence of electrolytes. Iron rusts more in salty water than in fresh water. The presence of ions such as sodium ions and chloride ions in the water film provides a conducting path between the anodic and cathodic sites on the surface of the iron.
• impurities in the iron. The presence of impurities such as carbon and silicon in the iron lattice act as cathodic sites for reduction of oxygen in the water film. The regions not containing these impurities are anodic and the iron oxidises at these sites.
• regions of stress in the iron lattice. Cracked or stressed regions of the iron or steel lattice contain iron atoms that are not strongly bound. These atoms form ions that more readily dissolve in the water film. Stressed regions include the tips and sharp edges of iron nails or regions where steel wires are bent such as steel fencing.
• regions of differential aeration. Crevices between two riveted iron plates are sites of low oxygen concentration. These sites are anodic whereas exposed areas of the iron plate that have a higher oxygen concentration are cathodic. This is an example of differential aeration. Rusting occurs in sites where there is a relatively lower oxygen concentration. Concretions or encrustations on the steel also create areas of differential aeration.
• contact with metals less active than iron. Wherever iron is joined to less active metals such as tin or copper, the rate of rusting of the iron is accelerated. The less active metal acts as the cathodic site and the iron as the anodic site in the galvanic cell. These less active metals provide large surface areas on which reduction of oxygen can occur.
• acidic environments promote rusting as the reduction potential is more positive than in neutral or alkaline solutions. The reduction half-equation in acidic environments becomes.