** Crevice corrosion **, or crack corrosion, better known by the name given in English to crevice corrosion, is a corrosive phenomenon that occurs in spaces in which the access of the fluid being worked with to the medium sees its diffusion limited. These spaces are commonly called “slits”, they are also known by the name given to them in English, that is, “crevices”.
Examples of gaps are the spaces and areas of contact between pieces, in joints or seals, within cavities, cracks and seams.
Mechanism
The corrosion resistance of metals depends on the presence of the protective oxide layer on the surface, but it is possible that under some circumstances this layer breaks down, for example in reducing acids, or in some environments where the atmosphere itself serves as a reducing medium. In either case, the areas where this oxide layer breaks down serve as cavities that promote crevice corrosion.
The most important factor in crevice corrosion is the width of the gap, as it must be wide enough to allow the solution to enter, but must also be narrow enough for the solution to remain stagnant within the gap. Therefore, this type of corrosion normally occurs in cracks or spaces a few micrometers wide, and is not found in places where these spaces have good circulation of the solution. An easy way to avoid this corrosion method is to design correctly. If the design can avoid cracks or small places with little or no flow of the working solution, this should be done or, failing that, reduce these types of places to the minimum possible. Crevice corrosion has a very similar mechanism to pitting corrosion"), so alloys resistant to one of these types of corrosion are generally resistant to both; however, the difference between these two types of corrosion is that crevice corrosion is a more aggressive corrosion than pitting corrosion and occurs at much lower temperatures than this.
The immediate surrounding medium of the slits can develop chemical conditions (both concentration and composition) very different from those of the rest of the medium or fluid solution.
There are two determining factors in the initiation of this type of corrosion:
The chemical composition of the electrolyte inside the slit.
The drop in potential within the slit.
Chloride attack
Introduction
** Crevice corrosion **, or crack corrosion, better known by the name given in English to crevice corrosion, is a corrosive phenomenon that occurs in spaces in which the access of the fluid being worked with to the medium sees its diffusion limited. These spaces are commonly called “slits”, they are also known by the name given to them in English, that is, “crevices”.
Examples of gaps are the spaces and areas of contact between pieces, in joints or seals, within cavities, cracks and seams.
Mechanism
The corrosion resistance of metals depends on the presence of the protective oxide layer on the surface, but it is possible that under some circumstances this layer breaks down, for example in reducing acids, or in some environments where the atmosphere itself serves as a reducing medium. In either case, the areas where this oxide layer breaks down serve as cavities that promote crevice corrosion.
The most important factor in crevice corrosion is the width of the gap, as it must be wide enough to allow the solution to enter, but must also be narrow enough for the solution to remain stagnant within the gap. Therefore, this type of corrosion normally occurs in cracks or spaces a few micrometers wide, and is not found in places where these spaces have good circulation of the solution. An easy way to avoid this corrosion method is to design correctly. If the design can avoid cracks or small places with little or no flow of the working solution, this should be done or, failing that, reduce these types of places to the minimum possible. Crevice corrosion has a very similar mechanism to pitting corrosion"), so alloys resistant to one of these types of corrosion are generally resistant to both; however, the difference between these two types of corrosion is that crevice corrosion is a more aggressive corrosion than pitting corrosion and occurs at much lower temperatures than this.
The immediate surrounding medium of the slits can develop chemical conditions (both concentration and composition) very different from those of the rest of the medium or fluid solution.
There are two determining factors in the initiation of this type of corrosion:
Researchers had previously stated that one or the other of the two factors was responsible for initiating crevice corrosion, but it has recently been shown that it is a combination of the two that causes the beginning of corrosion.[1] Both factors are of great importance in the beginning of the corrosive phenomenon, both the drop in potential and the change in the chemical composition of the electrolyte in the crevice are caused by a deoxygenation of the crevice and a separation of electroactive areas, this allows a series of cathodic reactions occur on the outside of the slit and anodic reactions occur inside it.
The difference between the areas of the cathodic and anodic regions is also important, as is the difference in concentration between the solution inside and outside the slit, especially the concentration of oxygen, since part of this phenomenon is due to the fact that a concentration cell is created between the metal in the slit and the rest of it, where as we know, the part where the concentration is lower works as an anode and the rest acts cathodically, which is clearly seen in this type of corrosion, since within the slit there is a shortage of oxygen (or low concentration of solution) and it (the slit) will act as an anode. Crevice corrosion is also increased by the presence of chloride ions.
Some of the corrosive phenomena that occur within the slit are reminiscent of galvanic corrosion and a concentration cell:
The mechanism of crevice corrosion can (but not always) be similar to pitting corrosion. However, there are enough differences to justify differentiated treatment. For example, in crevice corrosion, one must consider the geometry of the gap and the nature of the concentration process that leads to the development of local differentiation of chemical composition.
Attack mode
• - Chopped.
• - Filiform corrosion (a type of crevice corrosion that can occur on the surface of aluminum beneath an organic coating).
• - Intergranular corrosion.
• - Fatigue corrosion.
fatigue corrosion
A common form of “crevice failure” occurs due to stress corrosion, where a crack or cracks develop from the base of the crack, where the stress concentration is highest. This was the cause of the 1967 Silver Bridge collapse in West Virginia, where a single crack only about 3 mm long grew rapidly and caused a retaining bar joint to fracture. The rest of the bridge collapsed in less than a minute.
Importance
The susceptibility of each material-environment system varies greatly from one system to another. In general, crevice corrosion is of greatest concern for materials that are normally passive, such as stainless steel or aluminum. It is also important to take this corrosive phenomenon into account for components made of superalloys with high corrosion resistance and that must work with water as pure as possible, such as in steam generators in nuclear power plants, where their main corrosive problem is crevice corrosion.
This type of corrosion is very dangerous due to its localized attacks, which can cause the failure of a component, while the loss of metal in the rest of the structure is minimal, and the initiation and progress of crevice corrosion can be difficult to detect.
The chemical composition of the electrolyte inside the slit.
The drop in potential within the slit.
Researchers had previously stated that one or the other of the two factors was responsible for initiating crevice corrosion, but it has recently been shown that it is a combination of the two that causes the beginning of corrosion.[1] Both factors are of great importance in the beginning of the corrosive phenomenon, both the drop in potential and the change in the chemical composition of the electrolyte in the crevice are caused by a deoxygenation of the crevice and a separation of electroactive areas, this allows a series of cathodic reactions occur on the outside of the slit and anodic reactions occur inside it.
The difference between the areas of the cathodic and anodic regions is also important, as is the difference in concentration between the solution inside and outside the slit, especially the concentration of oxygen, since part of this phenomenon is due to the fact that a concentration cell is created between the metal in the slit and the rest of it, where as we know, the part where the concentration is lower works as an anode and the rest acts cathodically, which is clearly seen in this type of corrosion, since within the slit there is a shortage of oxygen (or low concentration of solution) and it (the slit) will act as an anode. Crevice corrosion is also increased by the presence of chloride ions.
Some of the corrosive phenomena that occur within the slit are reminiscent of galvanic corrosion and a concentration cell:
The mechanism of crevice corrosion can (but not always) be similar to pitting corrosion. However, there are enough differences to justify differentiated treatment. For example, in crevice corrosion, one must consider the geometry of the gap and the nature of the concentration process that leads to the development of local differentiation of chemical composition.
Attack mode
• - Chopped.
• - Filiform corrosion (a type of crevice corrosion that can occur on the surface of aluminum beneath an organic coating).
• - Intergranular corrosion.
• - Fatigue corrosion.
fatigue corrosion
A common form of “crevice failure” occurs due to stress corrosion, where a crack or cracks develop from the base of the crack, where the stress concentration is highest. This was the cause of the 1967 Silver Bridge collapse in West Virginia, where a single crack only about 3 mm long grew rapidly and caused a retaining bar joint to fracture. The rest of the bridge collapsed in less than a minute.
Importance
The susceptibility of each material-environment system varies greatly from one system to another. In general, crevice corrosion is of greatest concern for materials that are normally passive, such as stainless steel or aluminum. It is also important to take this corrosive phenomenon into account for components made of superalloys with high corrosion resistance and that must work with water as pure as possible, such as in steam generators in nuclear power plants, where their main corrosive problem is crevice corrosion.
This type of corrosion is very dangerous due to its localized attacks, which can cause the failure of a component, while the loss of metal in the rest of the structure is minimal, and the initiation and progress of crevice corrosion can be difficult to detect.