Stainless steels rely on a thin protective film composed of chromium oxides to provide corrosion resistance. This film, known as a passive layer, exhibits high resistance to corrosion. When stainless steels are exposed to moist air or oxygenated water, this film begins to form within milliseconds on damaged, freshly cut, or clean, abraded surfaces.
Unlike carbon steels, which corrode uniformly across the entire exposed surface, stainless steels fail at weak points or defects in the passive film when exposed to aggressive environments. This results in the formation of pits, i.e. attack much deeper than its diameter.
There are many hundreds of alloys with varying properties and levels of corrosion resistance. The Pitting Resistance Equivalent (PRE) is a practical index that quantifies the resistance to pitting corrosion based on composition. It ranges from a minimum value of 10.5 (the threshold for recognition as stainless steel) up to approximately 60. The PRE is calculated as %[chromium] + 3.3 [%molybdenum] + 0.5[%tungsten] + 16[% nitrogen]. This index has been calibrated against the Critical Pitting Temperature (CPT), which measures the temperature required to induce pitting in an oxygenated chloride test solution. It is important to note that the CPT is not indicative of service temperature but serves as a reference point for alloy comparison. Generally, stainless steels with a PRE value greater than 40 exhibit resistance to ambient temperature seawater.
In the presence of crevices, such as under fasteners or at lap joints, the temperature required to cause corrosion in the test solution will be lower. This temperature is known as the Critical Crevice Temperature (CCT) and is always lower than the CPT.