Corrosion results in huge losses to industrial countries every year. An SSINA - Battelle study estimated that the combined economic loss to the US in 1995 due to metal and alloy corrosion (excluding non-metallics) was $300 billion. Obviously engineers and designers must have an understanding of corrosion and of methods to minimise the losses.
Corrosion is the deterioration and loss of material due to chemical attack. The conditions that promote corrosion involve both chemical and electronic changes and are always with us.
2. Corrosion by Solution
The simplest corrosion is chemical solution, eg dissolving sugar in water. Most engineering materials are not particularly soluble, but some polymers, such as some types of rubber, can be dissolved by strong solvents such as petrol.
There are some generalisations that can be made about solutions:
3. Electro-chemical Oxidation
The most common type of corrosion involves the electro-chemical process of metal oxidation. Strictly 'oxidation' is the removal of electrons from an artom.
The oxidation of iron to ferrous ions:
Different metals have different 'oxidation potentials' ie the energy required to remove electrons varies from metal to metal. For example electrons are removed from iron when both oxygen and water are present and they are removed from aluminium atoms when chloride ions are present.
4. Electrode Potential
Most corrosion occurs through the interaction of the two processes of 'solution' and 'oxidation'. With slight modifications the mechanism of corrosion of iron described by the two equations above, (i) and (ii) may be applied to all metals and even non-metals.
As iron goes into solution, excess electrons are produced and in static liquids,
equilibrium is usually reached quickly as ions and electrons soon combine at the same
rate at which they form.
The production of ions and electrons builds up an electrical potential called an 'electrode potential', which depends upon the nature of the metal and the nature of the solution. All metal does not oxidise to ions and electrons with equal facility. For example, atoms along a grain boundary are less stably located than those in the crystal lattice, so they ionise more readily. Also the reaction (i) will produce equilibrium with a higher electrode potential if the metal ions enter a solution in which they are relatively stable. Positive iron ions are more stable in a uniformly Cl- concentrated solution than in a uniformly dilute Cl- solution.
To measure the electrode potential of any material (and therefore its tendency to corrode) the voltage difference between the metal and a standard hydrogen electrode need to be found. With hydrogen the equation of equilibrium is given by:
Table - Electrode Potentials of Metals - 25oC, Molar solutions of metal ions:
|Li+ (base)||+2.96 (anodic)|
|Au+ (noble)||-1.50 (cathodic)|
If a piece of copper and a piece of iron are immersed in an conducting liquid and the iron is used as a cathode and the copper as an anode, and electrons are forced to the cathode (by a battery) then the corrosion process is reversed and electro-plating occurs (electro-plating is the opposite of corrosion). Corrosion always occurs at the anode and electro-plating always occurs at the cathode.
7. Types of Galvanic Cell
There are three different groups of galvanic cell:
There is no size limit to galvanic cells so these can be set up in two phase alloys when they are exposed to an electrolyte. Often the potential difference between two similar phases is quite small. Ferrite and iron carbide have electrode potential values sufficiently close together for plain carbon steel to normally have a lower corrosion rate than a steel brass combination.
Heat treatment may affect the corrosion rate by altering the microstructure of the metal. After quenching a thin section carbon steel component the structure is a single phase, martensite. After tempering ferrite and carbide produced provide many galvanic cells and the corrosion rate is increased. Tempering at higher temperatures, gives agglomeration of the carbides and reduces the number of galvanic cells, which gives a significant reduction in corrosion rates.
David J Grieve, 28th October 2003.