Corrosion patterns of metals can generally be divided into two categories: comprehensive corrosion and localized corrosion. And localized corrosion can be divided into: pitting corrosion, crevice corrosion, galvanic coupling corrosion, intergranular corrosion, selective corrosion, stress corrosion, corrosion fatigue and wear corrosion.
Comprehensive corrosion is characterized by corrosion uniformly divided in the surface of the metal, so that the metal overall thinning. Comprehensive corrosion occurs under the condition that the corrosive medium can reach all parts of the metal surface uniformly, and the composition and organization of the metal is relatively uniform.
Pitting corrosion, also known as small hole corrosion, is a kind of corrosion concentrated in a very small range of the metal surface and deep into the metal internal pore-like corrosion pattern.
Pitting corrosion conditions generally meet the material, medium and electrochemical conditions:
1, pitting generally occurs in the easy passivation of the metal surface (such as stainless steel, aluminum) or the surface of the metal with cathodic plating.
2, pitting occurs in the presence of special ions, such as halogen ions in the medium.
3, pitting corrosion occurs in a specific critical potential above, called the pitting potential or rupture potential.
Intergranular corrosion is a metal material in a specific corrosive medium along the material grain boundaries or grain boundaries near the corrosion, so that the loss of bonding between the grains of a corrosion phenomenon.
Selective corrosion refers to the more active components in the multiple alloys preferentially dissolved, this process is caused by the electrochemical differences in the alloy components.
Crevice corrosion is the presence of electrolyte between metal and metal and metal and non-metal constitute a narrow gap, the migration of the medium is blocked when a localized corrosion state.
The formation of crevice corrosion:
1, the connection between different structural components.
2, in the metal surface of the deposits, attachments, coating and other corrosion products exist.
Post time: Mar-15-2024