Crevice corrosion is regarded as localized corrosion. Problems with crevice corrosion mainly occur in chloride-bearing solutions in combination with a crevice that is wide enough to allow penetration of solutions, but narrow enough to create stagnant conditions. Crevice corrosion is most likely to occur in seawater applications.
How to reduce the risk of crevice corrosion
The risk of crevice corrosion can be reduced, or problems eliminated, by proper engineering design. Typical examples of crevices are confined regions created by and associated with the design of flanges, tube-to-tube sheet joints, bolt heads to washers, washers to base plates and threaded connections. Deposits can also be crevice formers. The tighter the crevice, the greater the risk of problems with crevice corrosion.
Problems with crevice corrosion are greatest in stagnant solutions. At flow rates over 1.5 m/s the risk decreases since there will be no deposit formation and build-up of a corrosive environment. Whenever a structure requires a crevice, an open design is recommended, where the surrounding solution is allowed to flow as freely as possible.
To ensure freedom from crevice corrosion, design temperatures should in general be at least 15–25°C (59–77°F) below the temperatures where pitting corrosion is a risk.
Stainless steel should normally not be painted, because crevice corrosion will result if the paint is damaged.
The mechanisms for crevice corrosion and the associated problems are in many respects the same as for pitting corrosion. Thus, good resistance to crevice corrosion most often goes hand in hand with good resistance to pitting corrosion.
Effect of alloying elements
These alloying elements have the following influence on a material's resistance to crevice corrosion:
- Chromium (Cr) +
- Molybdenum (Mo) +++
- Nitrogen (N) +++
Crevice corrosion testing
Crevice corrosion testing is performed using a sample piece with a crevice. It is preferred to use a sample with a standardized crevice to achieve reproducible results. For example, the spring load crevice former.
Sandvik grades with very good or excellent resistance to crevice corrosion
- Sandvik SAF 2707 HD
- Sandvik SAF 3207
- Sandvik SAF 2507
- Sandvik 254 SMO*
- Sanicro 28
Facts in brief about crevice corrosion
- Crevice corrosion occurs in crevices and other narrow spaces, mainly in chloride-bearing solutions
- Stagnant solutions should be avoided; flow rates over 1.5 m/s are recommended
- The risk of crevice corrosion can be reduced, or problems eliminated, by proper engineering design.
* 254 SMO is a trademark owned by Outokumpu OY