Engineering Materals - Essay Example

Crevice corrosion is a localized and geometrically controlled form of corrosion. They are the products of situations containing a stagnant solution and corrosion friendly microenvironments. This type of corrosion can occur between rivet heads, lap joints, in threads and anywhere a small crevice is formed in which at least one side is a metal. As in all types of corrosion, an anodic reaction: exists which propagates over the entire surface. However, the geometry of the crevice restricts cathodic reaction such that the dissolved oxygen cannot be replaced.

Thus the region within the crevice becomes exclusively anodic while the region outside becomes cathodic. An electric imbalance result attracting the negative ions into the crevice. The small size of the crevice screens the incoming ions such that the concentration of the chloride and the hydrogen ions increases within the crevice. The pH of the solution within the crevice decreases from 6 to as much as 2 or 3. This increased acidity causes further corrosion within the crevice resulting in massive material erosion from within the crevice region.

In most cases, crevice corrosion is not visible from the outside and this hidden nature makes inspection as well as rectification difficult. This is schematically represented in the figure shown: Pitting corrosion is a form of localized corrosion as it does not spread laterally across an exposed surface rapidly but penetrates into the metal very quickly, usually at an angle of 90o to the surface. Stagnant solution conditions favor pitting corrosion. The presence of halide ions, chloride, fluoride bromide and iodide, can all pit metals.

The most aggressive ion depends on the metal and ion combination. The effect of pitting is to cause holes on the metal surface which can corrode a large amount of material from the surface. The mechanism of pitting can be divided into two processes – the initiation