What is Crevice Corrosion?
Crevice corrosion is a type of corrosion that occurs in confined spaces, often in the presence of an electrolyte.
It can occur when two different metals are present or on the same metal. It is one of the most common forms of localized corrosion, and it has been estimated to be responsible for almost 50% of all premature failures due to corrosion.
Crevice corrosion forms between two parts or in gaps or cracks in the metal in which an electrolyte is held in place by a capillary action. Due to large potential differences between the different metals, an electrochemical voltage series is created.
The term ‘crevice’ refers to an enclosed space with limited access for air and water. The size and shape of crevices will vary greatly depending on the application, but they are typically found between joints, gaskets, fasteners or flanges.
When these locations contain electrolytes such as saltwater or other liquids that conduct electricity, they provide ideal conditions for crevice corrosion to occur.
Difference between pitting and crevice corrosion
Pitting and crevice corrosion are two forms of corrosion caused by the presence of oxygen and water in metal surfaces. While these two types of corrosion can cause similar damage, there are key differences between them that make it important to properly identify which one is present.
Pitting corrosion is a form of localized attack that results from an electrochemical process in which an area on the surface becomes anodic and corrodes away, creating a cavity or pit in the metal surface.
The driving force behind pitting corrosion is the lack of oxygen in a small area. This zone becomes anode, while the zone of excess oxygen becomes cathode, causing narrowly localized galvanic corrosion. Corrosion of this type has the property of penetrating deep into the metal.
Crevice corrosion, on the other hand, occurs when oxygen-depleted conditions exist in confined spaces such as cracks, seams or crevices. This type of corrosion generally advances slower than pitting but can be more severe due to its location within tight spaces where it can go unnoticed until major material losses occur.
Crevice Corrosion Mechanism
Crevice corrosion is caused by a combination of factors including environmental conditions, chemical reactions and mechanical stresses.
In crevices (damage to surfaces) and dead spaces (e.g., in pressed joints) in metallic materials (pipelines, connections) which form oxidic protective layers, the liquid reacts with the material surface and reduces the oxygen content.
Here, the resulting corrosion products hydrolyze, the pH value drops and anions such as chloride ions can migrate. As a result, the oxidic protective layer can no longer be maintained and severe corrosion occurs in the gap. Due to the very small volume, an aggressive liquid quickly forms in this gap.
The crevice itself acts like a miniature system with its own unique environment. It traps oxygen-rich water vapor around the material which creates an electrolyte solution, allowing ions to move between the two sides of the metal surface.
This increases the activity level at the interface and makes it more prone to corroding at an accelerated rate than the surrounding material. In addition, if there are any impurities present in this confined space, they can accelerate the process further.
How to Prevent Crevice Corrosion?
To protect against crevice corrosion, preventive measures must be taken to ensure the long-term integrity of the material. Here are some ways to prevent crevice corrosion:
- Use welding joint instead of riveted joint.
- Selection of a suitable material that is not susceptible to corrosion.
- Use of non-absorbent solid gaskets.
- Design changes that avoid or enlarge gaps (larger gaps prevent oxygen exclusion and thus crevice corrosion)
- Sealing the gaps, for example with plastic or certain adhesives.
- Use materials that are resistant to crevice corrosion
In summary, it is important to choose materials with good resistance to crevice corrosion. Stainless steel alloys such as 304L and 316L are commonly used for their excellent resistance in many applications.
It is also important that the surface is cleaned properly before use and installation – this includes removing any dirt or debris from the surface, as well as ensuring that no oxidation has occurred on the surface.
Additionally, areas prone to moisture accumulation should be sealed off with protective coatings or sealants. This helps keep oxygen away from metal surfaces and reducing the risk of oxidation occurring in those areas.