Definition - What does Ringworm Corrosion mean?
This is a type of bimetallic corrosion that has the shape of a ring and is located near an upset pipe. One of the characteristics of the ring is that it is either very smooth or has pits in it.
The cause of ringworm corrosion is the process of upsetting, where heat results in the creation of two different structural grains. One of the grain structures goes in the direction of upset, while the other goes the direction on the pipe. To avoid this condition, the pipe should be normalized after upsetting. This is achieved by heating the grains to achieve a uniform structure.
Corrosionpedia explains Ringworm Corrosion
This can be described in terms of heat affected zones (HAZ) corrosion. In wet CO2 service, there are frequent failures of the upset of J-55 tubing which has not been fully normalized after being upset. This corrosion occurs about six inches below the upset part found on the heat affected zones. The heat affected zone has a completely different micro-structure from the other sections of the tubing.
Deformation of the upset part of the tubing should be done through heating of the parts. Welded joints may also result in the same problem, but this can be solved. Full-length welding, where the welded joints are heated to the austenitic temperatures and then cooled normally, is the only solution. This is to enable the joint to attain the same micro-structure and resistance to corrosion as the rest of the tubing.
There are numerous methods that one can use to prevent ringworm corrosion, especially on steel tubing. This is done by upsetting the tubing material at a specified temperature. The procedure of upsetting the material is done by upsetting at 1100-W1 and 200°C in order to curb ringworm corrosion. This occurs at the interface of the upset part and the base part of the material. A little structural difference in the same temperature range is seen at the boundary of the upset part and the base part. This is where elimination of a large grain size difference occurs.