Pitting Corrosion

What is Pitting Corrosion?

Pitting corrosion refers to a highly localised form of corrosion, where small but deep cavities or pits form on the surface of the material. Even when most of the surface remains passive, a tiny anodic site starts dissolving rapidly, creating pit corrosion that can penetrate deeply with limited overall material loss. Pitting corrosion is particularly insidious because it’s often hidden and hard to detect until significant damage occurs beneath the surface.

What Causes Pitting Corrosion?

Pitting corrosion is typically triggered by chloride ions (Cl⁻) or other aggressive species like thiosulfates. These ions can locally disrupt the passive chromium oxide layer, especially in stagnant or low-oxygen environments, initiating localised pitting attacks.

Other causes include mechanical damage, surface inclusions (e.g. sulfides), and improper fabrication or creviced joints, all of which compromise the passive layer and start pit corrosion.

How to Measure and Repair Pitting Corrosion

How to repair pitting corrosion:

• Surface defects are removed by grinding, polishing, or abrasive blasting, eliminating pit corrosion sites.
  
• Deeper pits may be welded or filled with corrosion-resistant alloys or epoxy repair compounds.
  
• Protective or barrier coatings, and re-passivation treatments, can restore the passive layer to prevent further corrosion.