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.
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 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
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 (H2
S) 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