Crevice Corrosion and Pitting Corrosion

Crevice corrosion and pitting corrosion are related because they both require stagnant water, chloride, and oxygen or carbon dioxide. The mechanism of corrosion is very similar for both.

Crevice corrosion tends to occur in crevices (stagnant, shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.

Pitting Corrosion

Pitting Corrosion is "self nucleating" crevice corrosion, starting at occluded cells. Corrosion products often cover the pits, and may form "chimneys". Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and prevent. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system.

Schematic of an actively growing pit in iron

Once initiated, both crevice and pitting corrosion can be explained by differential concentration cells, Cathodic reactions, i. e. oxygen reduction or hydrogen evolution may start in the crevice or the pits. Large surface areas will become cathodic and pits or crevices will become anodic and corrode. Metal dissolution will thus be concentrated in small areas and will proceed at much higher rates than with uniform corrosion. Large crevices are less likely to corrode because water movement causes mixing and replenishes oxygen, hydrogen ions, bicarbonate or hydrogen sulfide.

The chloride ion acts as a catalyst in pitting and crevice corrosion. In other words, increases the corrosion rate but is not used up in the reaction. It has the ability to absorb on the metal surface or the passive films and polarize the metal, initializing localized corrosion. (e.g., pitting corrosion of austentic stainless steels (304) in salt water). This photo is an example of crevice corrosion on a tubing end.

Pitting corrosion is frequently observed in CO₂ and H₂S environments in the oilfield. Pits will generally initiate due to local breakdown of corrosion product films on the surface and corrosion will proceed at an accelerated rate. In sweet (CO₂) systems, the pits are generally small with sharp edges and smooth rounded bottoms. Pits may become connected as the corrosion damage increases. Corrosion products are dark brown to grayish black and loosely adhering. In sour (H₂S) systems, the pits are usually shallow round depressions with etched bottoms and sloping sides. Generally, the pits are not connected, and corrosion products are black and tightly adhering to the metal surface.

The first image is an example of CO₂ pitting, and the second is an example of H₂S pitting.