Higher chloride levels require more highly alloyed stainless steels. Localized corrosion can be difficult to predict because it is dependent on many factors, including: Chloride ion concentration. Even when chloride solution concentration is known, it is still possible for localized corrosion to occur unexpectedly. Chloride ions can become unevenly concentrated in certain areas, such as in crevices (e.g. under gaskets) or on surfaces in vapor spaces due to evaporation and condensation. Temperature: increasing temperature increases susceptibility. Acidity: increasing acidity increases susceptibility. Stagnation: stagnant conditions increase susceptibility. Oxidizing species: the presence of oxidizing species, such as ferric and cupric ions, increases susceptibility. Pitting corrosion is considered the most common form of localized corrosion. The corrosion resistance of stainless steels to pitting corrosion is often expressed by the PREN, obtained through the formula: , where the terms correspond to the proportion of the contents by mass of chromium, molybdenum, and nitrogen in the steel. For example, if the steel consisted of 15% chromium %Cr would be equal to 15. The higher the PREN, the higher the pitting corrosion resistance. Thus, increasing chromium, molybdenum, and nitrogen contents provide better resistance to pitting corrosion. Though the PREN of certain steel may be theoretically sufficient to resist pitting corrosion, crevice corrosion can still occur when the poor design has created confined areas (overlapping plates, washer-plate interfaces, etc.) or when deposits form on the material. In these select areas, the PREN may not be high enough for the service conditions. Good design, fabrication techniques, alloy selection, proper operating conditions based on the concentration of active compounds present in the solution causing corrosion, pH, etc. can prevent such corrosion. Stress Stress corrosion cracking (SCC) is a sudden cracking and failure of a component without deformation. It may occur when three conditions are met: The part is stressed (by an applied load or by residual stress). The environment is aggressive (high chloride level, temperature above , presence of H2S). The stainless steel is not sufficiently SCC-resistant. The SCC mechanism results from the following sequence of events: Pitting occurs. Cracks start from a pit initiation site. Cracks then propagate through the metal in a transgranular or intergranular mode. Failure occurs. Whereas pitting usually leads to unsightly surfaces and, at worst, to perforation of the stainless sheet, failure by SCC can have severe consequences. It is therefore considered as a special form of corrosion. As SCC requires several conditions to