The naturally-formed passive layer is only a few nanometers thick (approx. 2-7nm) and always has a few weak points. Furthermore certain ions (e.g. chloride ions) are able to destroy the passive. This localized process leads to pinholes and is called pitting corrosion. The preferred areas of attack are imperfections, cracks or flaws, which are always present, e.g.: manganese sulphide, non-metallic enclosures, carbides, scaling residues, rings and scratches, Fe – impurities etc. Added to these are imperfections which are the result of fabricating or installing a component, e.g. fats or oils, non-alloyed Fe and stainless steel shavings, heat tint, dusts or strong internal stresses (welds, scratches).