Pitting - what is it?


Some metals protect their surface from corrosion by forming an oxide layer. This is the case with all non-ferrous metals, but also with passivated steels, whose alloy also forms a passive layer of oxides. In these cases, there can be no continuous corrosion more - it creates pitting. Read on here.

Conditions for pitting

Pitting corrosion occurs only in aqueous solutions in which chloride or bromide ions are present. In the case of copper and copper alloys, on the other hand, above all, a high sulphate content is decisive for the formation of pitting corrosion.

In addition, some other factors favor:

  • low pH of the electrolyte
  • high temperature
  • low oxygen content in the electrolyte
  • low flow rates

Above all, materials with a low normal potential are more severely affected. With them, the formation of pitting is favored significantly.

repassivation

The defects in the material, which form the starting points for corrosion, usually arise from the fact that in the oxide layer, an oxygen atom is displaced by a chlorine or boron atom. As a result, more and more chlorine or boron atoms accumulate at the defect, which accumulate. As a result, the corrosion proceeds.

In the case of a high oxygen content in the electrolyte, it is occasionally possible for an oxygen atom to displace the chlorine atom from its position. The passive layer is thus repaired again. This "self-healing" of the passive layer is particularly pronounced in individual metals. It is called repassivation in the chemical-technical terminology.

Affected materials

Not all materials are affected by the risk of pitting corrosion. Most at risk are:

  • aluminum alloys
  • magnesium alloys
  • Copper alloys under certain conditions (water cycle)
  • some steel alloys such as V2A

Pitting corrosion in steel

Not all steels are susceptible to pitting corrosion. Susceptibility affects only a few types of steel, almost all stainless steels.

The susceptible to pitting corrosion steels can be protected very effectively by alloying with molybdenum against the occurrence of pitting. In addition, the repassivation is promoted.

Tips & Tricks

For the use of metals in chlorine-containing solutions, you should always resort to very suitable materials. With unsuitable materials, massive damage can very quickly threaten, even strength losses are to be feared after a short time. This can be avoided by selecting the right material.


Video Board: What does pitting look like?