Corrosion resistance of new nickel free stainless steel inalkaline solutions containing chloride ions

Localized corrosion is described as corrosion that occurs at discrete locations on an otherwise passive material. Many engineering alloys, such as stainless steels and aluminum alloys, are useful only because of passive films, which are thin (nanometer-scale) oxide layers that form naturally on the metal surface and greatly reduce the rate of corrosion of the alloys.
Such passive films, however, are often susceptible to localized breakdown, resulting in accelerated dissolution of the underlying metal. If the attack initiates on an open surface, it is called pitting corrosion; at an occluded site, it is called crevice corrosion.
It should be noted that, whereas localized dissolution following breakdown of an otherwise protective passive film is the most common and technologically important type of pitting corrosion, pits can form under other conditions as well.
For instance, pitting can occur during active dissolution if certain regions of the sample are more susceptible and dissolve faster than the rest of the surface.
Classical pitting corrosion caused by passive film breakdown will only occur in the presence of aggressive anionic species, and chloride ions are usually, although not always, the cause. The severity of pitting tends to increase with the logarithm of the bulk chloride concentration. Chloride is an anion of a strong acid, and many metal cations exhibit considerable solubility in chloride solutions. Pitting is considered to be autocatalytic in nature; once a pit starts to grow, the local conditions are altered such that further pit growth is promoted. The anodic and cathodic electrochemical reactions that comprise corrosion separate spatially during pitting.
The concentration of an anionic species such as chloride must also increase within the pit in order to balance the charge associated with the cation concentration and to maintain charge neutrality. This enrichment of anions occurs by electromigration from the bulk solution in response to the potential gradient that develops as a result of the ohmic potential drop along the current path between the inside of the pit and the cathodic sites on the passive surface