Pitting corrosion is also called small hole corrosion, pitting or pitting.
It is a form of corrosion damage in which most of the surface of the metal does not corrode or corrodes very slightly, but corrosion holes appear in local places and develop deeper. Some pits exist in isolation, while others are compactly connected and look like a rough surface. Etch holes can be large or small, but generally smaller, as shown in the following figure different pitting pits cross-section, in terms of size, the depth of the etch hole is generally equal to or greater than the diameter of the etch hole. Pitting is one of the most harmful corrosion patterns of duplex stainless steel. Pitting holes are often the starting site of stress corrosion cracks and corrosion fatigue cracks.
Stainless steel surface passivation film due to the presence of defects in stainless steel, inclusions and solutes, such as inhomogeneity, so that the passivation film is more fragile in these places, in a particular corrosive solution is easily destroyed, the destruction of part of the anode becomes activated, the surrounding area becomes a cathode area, the area ratio of the two is very small, the anodic current density is very large, the activity of the solvation accelerated, and then become a number of needle-like holes.
Stainless steel, as well as other metals that rely on passivation for corrosion resistance, in a solution containing a specific anion (chloride, bromide, hypochlorite, or thiosulfate ions). As long as the corrosion potential (or the potential applied during anodic polarisation) exceeds the pitting potential Eb, pitting can occur. The mechanism of pitting corrosion of duplex stainless steel is the same as other stainless steels.
Metal treated with oxidising media, its corrosion rate than the original untreated before a significant decrease in the phenomenon known as metal passivation. The passivation mechanism can be mainly used to explain the thin film theory, that is, passivation is due to the role of metal and oxidising media, the role of the metal surface to generate a very thin, dense, good coverage performance, can be solidly attached to the metal surface passivation film. This film into a separate phase exists, usually oxygen and metal compounds. It plays the role of the metal and corrosion media completely separated from the role of preventing the metal and corrosion media in direct contact, so that the metal basically stop dissolving to form a passivated state to prevent the effect of corrosion.
Post time: Dec-27-2023