Crevice corrosion of AISI 316L tube
Crevice corrosion is a form of corrosion that belongs to the family of localized corrosion and therefore has some very similar aspects to pitting corrosion. This type of corrosion is manifested by a loss of material mainly localized near to the coupling between two mechanical parts in the presence of a wetting environment. In these areas, due to dimensional tolerances, they create the gaps sufficient to establish a self-stimulating mechanism that leads to the formation of craters and caverns.
Crevice corrosion can occur:
- At the mechanical joints between metal parts
Crevice corrosion of metal coupling proximity
- Between metal parts and a non-conductive body (gaskets, spacers, etc …)
Crevice corrosion in the housing close to the plastic seal
- Metal part screened by a external substances (soot, deposits of sand, etc …)
Crevice corrosion of a screw used to block of a transmission shaft
The shielding action of the external body can be:
- Primary, if the body is present for mounting error or accidental cause (cavity by inclusions, foreign bodies, etc…);
- Secondary, if the shielded body is a corrosion product created by a corrosion process triggered previously.
The crevice corrosion proceeds very rapidly when the corrosion products created inside the caves, spread outside. In this situation, they are dispersed in the liquid and they precipitate on the surface of the material in the immediate vicinity, causing the shielding action of secondary type (“Occluded cell” ). In particular, it may happen that, following a reduction of oxygen, there is an increase in alkalinity which causes the precipitation of salts (eg. Salts ferritic) that spread near to the cathode area (outside of the gap). The latter process can be a diagnostic element of the cathodic process which defines the whole process of corrosion.
Mechanism of crevice corrosion
The corrosion mechanism typically develops in a neutral environment with a trigger mechanism caused by differential aeration conditions between the outer surface and inner interstice. In this condition, the interstitial area has a lower concentration of oxygen, the ions in solution (eg. Chlorides), in high percentages, they react with the interstitial surface favoring the dissolution of the metal, while in the outer areas the concentration of oxygen is so high that promotes passivation conditions. Once triggered, the corrosion process is self-stimulating and develops according to the model “occluded cell”.
“Occluded cell” process
This type of process is typically observed on the walls of the boilers and is very often mistaken for pitting. In fact, the crevice corrosion is less perforating and one can observe that, near to the area attacked by corrosion, are formed of the corrosion products which circumscribe the corrosive area, while on the outside they develop covered areas of layers of protective magnetite, also favored by a high pH. The accumulation of corrosion products enclose almost completely the cave, delimiting a zone hard electrochemical exchange in which it determines a high content of ions and a low oxygen content. In this situation, the corrosion is of great intensity and self-stimulating.
The factors that stimulate the crevice corrosion are:
- Conductivity: determines the size of the caves that are formed near to the gap; the higher the conductivity plus the size of the caverns are considerable because the corrosion process can exploit wider cathodic areas. In addition to conductivity, the corrosion process involves the transport phenomena that favor the penetration of the ions insidethe interstice is liable to self-stimulation and dissolution of the metal. Unlike the pitting, in which the size of the pit checking the corrosion process, the mere presence of the gap is necessary and sufficient condition for establishing the self-stimulating mechanism of « occluded cell »;
- Temperature: the increase of temperature favors the kinetics of formation of corrosion products and the difficulty of a heat exchange produces easy trigger points. The temperature is an important parameter in heat exchangers, in which the calcareous deposits that form on the surface, cause local overheating that help to trigger this type of corrosion;
- Agitation: agitation of the solution has a different role depending on the conditions of corrosion and by the geometry of the gap. If the gap has a geometry such that agitation of the solution promotes the diffusion of oxygen within the gap, then the stirring mechanism has a positive side on the crevice corrosion resistance. If, instead, the agitation of the solution promotes the oxygen cathode reaction, then this factor has a negative aspect in the crevice corrosion resistance;
- pH and aggressive ions: as in the case of pitting corrosion, a decrease in pH and an increase in the concentration of the ions favors the development of this form of corrosion.
If the pH becomes very low (acid range) the cathodic process is the development of hydrogen. So at initial stage, the solution acts in a constant way both on the inside and on the outside of the gap. With the passage of time can create a discontinuity between the internal corrosion rate, near to the gap, and the outside. This speed differentiation becomes pronounced when trying to pair two different metals nobility. In this case also it is established a galvanic corrosion process which, thanks to a synergic action, enhances the crevice corrosion.
The prevention methods for this type of corrosion can be:
- Avoid gaps in submerged areas; favor of continuous welding processes, avoiding clean process with abrasive paper. it is advisable to use electrochemical pickling which do not favor the occurrence of porosity or cavity (micro gap) and restore the original conditions of the material;
- Use seals with a geometry such as not to create gap favorable to corrosion (eg. seals protruding with respect to the coupling);
- Choose appropriate materials to the conditions of use (i.e. titanium or superduplex stainles steel or AISI 316L), or use of non-wettable material such as Teflon as gasket;
- Remove the corrosion products with more frequency.