Corrosion and Rusting
Some people may be annoyed by their car ?wearing out?. Kids may have trouble with rust forming on their bicycles. One may think how to prevent rusting, but do one knows what is happening when a metal corrode?
?Corrosion is defined as the involuntary destruction of substances such as metals and mineral building material by surrounding media, which are usually liquid (i.e. corrosive agents).?Most metals corrode. During corrosion, they change into metallic ions. In some cases, the product of corrosion itself forms a protective coating. ?For example, aluminium forms a thin protective oxide layer which is impervious to air and water. In other cases (e.g. iron), however, the coating either flakes off or is pervious to both air and water. So the whole piece of metal can corrode right through.?
The most common forms of metallic corrosion are caused by electrochemical reactions, wherein two metallic phases (e.g., iron oxide and iron) react in the presence of electrolytic solution. Another mechanism of metallic corrosion is caused by chemical reaction, which explains how the protective layer of the metal is formed.
Rusting is the corrosion of iron which is the most widely used structural metal. Most of it is used in making steel. The wide range of products made from steel includes all types of vehicles, machinery, pipelines, bridges, and reinforcing rods and girders for construction purposes. Therefore, rusting causes enormous economic problem and is the reason why extensive measures of corrosion protection have had to be developed.
The economic importance of corrosion and corrosion protection can be shown by the following example: ?It is estimated that roughly 3% of the annual production of steel is lost by corrosion. In 1974, 140 millions tons of steel were produced in the United States at a cost of approximately $400 per tons. So this gives a monetary loss of about 1.7 billion dollars.?It is clearly of the utmost importance to reduce as far as possible the financial loss by corrosion, which not only affects steel but to the extent all other building metal as well.
It is obvious that corrosion and rusting affect significantly the life of the society, so it is worthy to investigate this topic. In this essay, the cause of the corrosion and rusting and consequently the protection of the corrosion will be explored.
Electrochemical corrosion reactions
This type of corrosion takes place when two metallic phases with different electrochemical potentials are connected to each other by means of an electric conductor.
Electrolytes such as acids, alkalis, salt solutions, or even milder media (e.g., rainwater, river water, groundwater, or tap water) also need to be present. ?Metallic phases with different electrochemical potentials exhibit electric potential differences. Potential differences may also arise because of impurities, internal stresses, corrosion products, damaged protective coatings, etc. They also occur when different metals are used. The larger the potential difference, the faster the rate of corrosion.?
The electrochemical EMF series (Table 1) gives the electrochemical potential of metals under normal conditions with respect to hydrogen (hydrogen is 0). The farther two metals in electrochemical series are apart, the larger the potential difference between them. A metal is said to be less noble than those which stand to its right in the electrochemical series. In the case of electrochemical corrosion it is always the less noble metal which is removed.
Table 1. Electrochemical Potential Series, Volts.
not noble —————————————————————–> noble
Likelihood of passing into solution decreases from left to right.
The potential difference does not, however, always fully correspond with the corrosion phenomena experienced in practice. The reason is that oxide and other metal compounds have differing electrochemical potentials.
Chemical corrosion reactions
Metals have a tendency to combine with oxygen to form oxides and this is one of the chemical reactions. This tendency is the stronger the less noble the metal. The layers of oxide on the metal surface which are formed even in dry air may be insoluble and stable against an aqueous medium in contact with them. Therefore, if the oxide layers are dense and adhere well to the metal, they prevent further attack and act as a corrosion prevention layer. An example of this is aluminum oxide. However, iron differs in that, although it does form a surface oxide layer, this layer is loose and enables oxidation to proceed into the depth of the metal.
Chemical corrosion also takes place by the action of acids and alkalis on metals. Hydrochloric acid, for example, reacts with iron, and sodium hydroxide with aluminum (Figure 1). If soluble reaction products are formed, the reaction only ends when either the aggressive medium, or the metal are used up; if salts are formed which are sparingly soluble they can form protective layers.
Figure 1. Chemical corrosion as shown by acid attack..