Common forms of localised offshore corrosion

Pitting corrosion 

Pitting is an extremely localised form of attack where the wall loss is confined to a very small
area of the surface. The conditions within the pit can quickly become increasingly aggressive
causing corrosion pits to rapidly advance through the wall thickness whilst the vast majority
of the pipe or vessel wall remains unaffected. This can lead to very rapid failures as the pit
quickly penetrates the wall. This form of attack is one of the main forms of corrosion
observed in corrosion resistant alloys, however it is also found with corrosion of carbon
steels.

Crevice corrosion 

Crevice corrosion is similar to pitting corrosion, in that it is likely to be observed under the
same environmental conditions that have given rise to pitting. In crevice corrosion the area of
localised attack is found within crevices which typically form around and under washers, bolts
and seals. The solution within the trapped pocket can become increasingly aggressive and
significant localised attack can occur around the crevice.

Galvanic corrosion 

Galvanic corrosion occurs at the junction of two dissimilar metals which are in electrical
contact with each other. According to their relative positions within the galvanic series one
metal will be protected from corrosion at the expense of the other. Depending on the relative
surface areas of each metal this form of corrosion can proceed extremely quickly. If the
cathodic metal is much larger than the anodic metal surface then the observed corrosion
rates can be extremely high as a large cathodic area is driving corrosion at a relatively small
anodic point.

Source:http://www.hse.gov.uk/offshore/ageing/ageing-plant-summary-guide.pdf