# Pitting corrosion

Pitting corrosion , also known as pitting corrosion or pitting corrosion , refers to the appearance of small areas of corrosion or punctiform holes in the surfaces of passivated metals, some of which expand considerably in the depth of a trough. Pitting corrosion often goes unnoticed for a long time because of its small extent on the surface.

## Emergence

While in active metallic materials, such as structural steels , the metallic erosion takes place uniformly through corrosion, in passivated metals that are in a chloride or bromide-containing electrolyte , punctiform corrosion often occurs at defects in the passive layer consisting of oxides. At these flaws, for example due to embedded foreign metal particles, the oxygen is displaced from the oxide layer of the passivated metal by chloride or bromide ions. The addition of more chloride and bromide ions creates an area that is no longer protected by an oxide layer. This point now offers a point of attack for corrosion. Under favorable circumstances, what is known as repassivation can occur: The chloride ion is displaced again by oxygen, the protective oxide layer is thus repaired.

Initiation of pitting corrosion

Otherwise, the pitting corrosion will proceed. The following mechanisms promote pitting corrosion:

• Due to the small hole diameter, little oxygen gets into the hole, which hinders repassivation. Since the oxygen content outside the hole is much greater than in the hole, a concentration element is also formed .
• The small hole forms the anode, the rest of the surface the cathode. Since the rate of corrosion is determined by the area ratio of the cathode to the anode, the reaction proceeds at great speed.
• The dissolved metal in the hole forms e.g. B. with chloride ions salts. Hydrolysis creates oxonium ions that acidify the electrolyte in the hole . The lower pH value increases the free corrosion potential and thus the rate of corrosion.${\ displaystyle {\ rm {FeCl_ {2} \, + \, H_ {2} O \, \ rightarrow \, FeClOH \, + \, H ^ {+} \, + \, Cl ^ {-}}} }$

## conditions

Critical conditions for the occurrence of pitting corrosion are:

• high chloride content in the corrosive medium
• high sulphate content (for copper materials)
• high temperatures
• low electrode potential of the material
• low electrolyte pH
• low oxygen concentration in the electrolyte (therefore no repassivation)
• low flow velocity of the medium (e.g. in water cycles)

## Avoidance

### material

Materials on which pitting corrosion can occur include: a. stainless steels (e.g. 1.4301, V2A ), copper alloys in water circuits or aluminum or magnesium alloys.

Chromium-nickel steels are made resistant to pitting corrosion by alloying them with molybdenum , for example . The addition of molybdenum stabilizes the passive layer on the steel surface and the steel can repassivate more quickly. Local chromium depletion, such as the glow brought chromium carbide formation, this protection can, however, destroy and leads to intergranular corrosion . In order to avoid this “sensitization”, so-called ELC steels with only a very low carbon content were developed, which therefore cannot form any chromium carbides.

Particularly resistant materials are 1.4362, 1.4404, 1.4462 and 1.4571.