Learn About...Crevice Corrosion
20 March 2020
Crevice Corrosion refers to a form of localised corrosion attack. Crevice corrosion occurs because part of a metal surface is in a shielded or restricted environment, compared to the rest of the metal which is exposed to an electrolyte. A crevice is typically a gap that is 25-100µm wide. Crevices can arise during fabrication where there are overlaying surfaces such as welds, seams, joints, threads, etc. Crevices can also arise due to surface contamination such as dirt and corrosion products
Crevice corrosion occurs in cracks or crevices formed between mating surfaces of metal assemblies. Both surfaces may be of the same metal or of dissimilar metals or one surface may be a non-metal. It can occur also under scale and surface deposits and under loose fitting washers and gaskets allowing water ingress.
Initially, the electrolyte is assumed to have uniform composition. Corrosion occurs slowly over the whole of the exposed metal surface, both inside and outside the crevice. The normal anode and cathode processes occur, the generation of positive metal ions is counterbalanced electrostatically by the creation of negatively ions.
Followed by oxygen concentration reduction or depletion within the crevice. A concentration cell likely establishes whereby electrons flow through the metal and ions through the electrolyte.
The crevice now becomes a focal anode and the net charge in the crevice becomes positive. This sets up a potential difference and negative ions from the solution flow into the crevice to try and balance the charge. This can result in an acidic environment in close proximity to the anode
The negatively charged ions diffuse towards the anode and further encourage corrosion forming complexes with the metal ions. This combination of events and processes can result in an autocatalytic process.