Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly what gases that are in contact with the metal, determine the form and rate of deterioration. Low pH acid waters accelerate corrosion by supplying hydrogen ions to the corrosion process. Although even absolutely pure water contains some free hydrogen ions, dissolved carbon dioxide (CO2) in the water can increase the hydrogen ion concentration. From the less noble metal, anode, metal atoms dissolve leaving electrons as positive ions. At the same time, on the other electrode, electrons are driven to the positive hydrogen ions. A special feature of this corrosion is the building up of hydrogen on the cathode. This type of corrosion is also known as hydrogen corrosion. The dissolution of metals and thus of the material always occurs in the electrode whose capacity is lower that the capacity of hydrogen. This electrode becomes anode. The cathode having a higher potential remains protected from corrosion.
In this procedure, there were 2 electrodes of different metals. Acid corrosion can also occur if there is only one metal in a solution. In such a case, destruction can be attributed to local elements. The creation of elements is due to certain non-homogeneity of metal surfaces that are also the cause of potential differences.
The outside appearance of an acid corrosion is a homogenous erosion of the material on the surface. In case of materials with an inhomogeneous composition, a selective corrosion might occur. This corrosion attacks only one component of the alloy, and does not affect the others.
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