18 Onstream   considered to be small. Stresses which are induced during service are either cyclic or static, or a combination of both. Residual stresses occur in nearly all welds and can cause an increased rate of corrosion; however they can be reduced by Post Weld Heat Treatment (‘PWHT’). This is particularly important as these stresses act as a prerequisite for Stress Corrosion Cracking (‘SCC’). Primary types of corrosion Corrosion typically requires moisture and oxygen, and is accelerated by the presence of salts, particularly chlorides. These are of course all in abundant supply in the marine environment but can be equally present in other industrial environments. Corrosion can result in either: • a general slow material loss (‘General Corrosion’); or • accelerated localised pitting • the latter, of these two possibilities is often more dangerous because of greater difficulties in detection, perhaps not being visible until after catastrophic failure. General corrosion This occurs over extended periods of time and typically results in the general loss of material thickness through the formation of an oxide layer on the exposed surface of a metal such as a ship’s shell plates (pictured above). It can be controlled through a number of methods, the most popular being coatings (such as paint). General corrosion should always be monitored through regular surveys to ensure that sufficient thickness of metal remains. Pitting corrosion This is a form of localised corrosion in which small pits or holes form within the affected material (pictured left). Pitting is a particularly dangerous form of corrosion as it can go undetected, due to the localised nature of the damage. It typically starts at small defects on the exposed surface and propagates downwards through the material. Further pitting corrosion then occurs within such pits, leaving a very porous structure that will be susceptible to failure. General corrosion on shell plates.   Localised pitting corrosion. © 2013 Xchanging