The corrosive attack in metal expansion joints
Causes of corrosion in metal expansion joints and metallic compensators
The service life of a metal expansion joint or metallic compensator can be drastically reduced due to corrosion issues. The design and operation conditions can influence meaningfully, in such a way that the bellows may be affected by corrosion issues whilst the surrounding pipes and supports may not, even though they are made of similar materials.
These are the most common sources of corrosion in the metallic compensators or metal expansion joints:
- Stress corrosion cracking, which is shown by a mechanical failure of the material as a result of the combined effect of stresses and a corrosive environment.
- Intergranular corrosion, marked by a preferential attack to the grain boundaries of the material.
- Pitting corrosion, that is one of the main localized corrosion mechanisms.
- Uniform corrosion, which occurs gradually and evenly.
- Erosion under corrosion attack, tied with the impact of a liquid or gaseous media on the surface of the material.
The oxidation at high temperatures is also a very common corrosion mechanism that occurs very frequently in exhaust pipes or pipings processing hot air.
The impact of the corrosion will depend on its type and on the conditions of each material, as well as on the original surface state.
The material selection should be such that there is no possibility of corrosion or that the corrosion rate is not higher than 0.002 inches per year. The resistance against corrosion of the stainless steel rests on naturally occurring chromium rich oxide film formed slowly on the surface of the steel. Some stainless steel impurities as well as welding spatters can prevent or hinder the formation of this chromium oxide film, therefore, the remaining impurities must be removed by pickling, in order to get the maximum resistance against corrosion.
Regarding the presence of welding splashes, this is an issue that must be prevented in the workshop as well as during normal operation by protecting the bellows or by applying some anti-splash compound on its surface.
When designing a piping system integrating metal expansion joints or metallic compensators, particular care is required not only to the internal conditions but also to the external ones. In practice, numerous corrosion attacks can be significantly minimized with a well-planned study, which would help to get a proper design. The designer or engineer must foresee the situations in which the corrosive attack may occur, due to a poor design as well as to a wrong material selection. Given that corrosion can be a complex issue, we recommend you to seek advice from engineers that are highly-skilled in materials, as those making up the Quality Department in SACOME.