Corrosion is defined as an undesirable chemical or electrochemical destruction of the material.
Safety-related corrosion is an important factor. Types as stress corrosion cracking, intergranular corrosion, crevice corrosion and corrosion fatigue occur fast and destroy an entire construction.
The corrosion resistance of stainless steel depends on the alloy and the oxide film that covers the surface of the material. Different media can ruin this film and cause different kinds of corrosive attacks.
Typically corrosive attacks seen on compensators are: |
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Corrosion fatigue
Fatigues are frail and the cracks are often opposite grained as at stress corrosion cracking, but not branched. The picture illustrates a primary crack of corrosion fatigue, which at one end is wider because of a secondary corrosive reaction.
Corrosion fatigue arises because of simultaneous influence of dynamical influences and corrosion, which lead to faster extensive damage than the two influences one by one.
Most corrosion damages that involve corrosion flakes have an element of corrosion fatigue in it. Generally the compensator is designed to avoid corrosion fatigue. |
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Intergranular corrosion
Intergranular corrosion is, as the name indicates, a kind of corrosion that follows the grain boundaries with relatively low chromium content.
Intergranular corrosion occur if the steel has been heated to long at degrees between 550ºC and 850ºC. For that reason welding must be done at low heat input and preheating should be avoided as preheating is a technical increasing of the heat input.
Intergranular corrosion occurs if the steel is sensitized and the steel has been exposed to heat treatment that has used the chromium content in the grain boundaries. For that reason this chromium is no longer available for protection against corrosion. Sensitiveness occurs if the steel contains to much carbon (C) that should be less than 0,03%. At these temperatures the carbon will settle in the grain boundaries. C-atoms will diffuse in the grain boundaries where they react with Cr under forming chromium carbide (CrC). The result is a quite narrow zone alongside the grain boundaries where the content of chromium has become to low (less than 12 % Cr) for preservation against corrosion.
If the necessary precautions against intergranular corrosion are taken the risk is minimized. The easiest way to avoid intergranular corrosion is by specifying a C-content that is lower than 0,03% C and likewise via the material certificate demand a special test for intergranular corrosion and an eventual stabilization of the steel with titanium (or niobium). |
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Stress corrosion cracking
Stress corrosion crackingis characterised by the crack where branching on the surface as well as in depth occurs.
Stress corrosion cracking occurs if austenitic stainless steel is exposed to a combination of tensile stress and increased temperatures in an environment that contains chlorides or other chemicals.
Stress corrosion cracking and crevice corrosion are often seen on compensators (in cases where corrosion is seen on expansion joints).
Stress corrosion cracking occurs on the compensator because stress in the bellow cause larger movements than the compensator is designed for. Likewise a high operating pressure could cause stress corrosion cracking.
Stress corrosion cracking is avoided by removing one or several of these factors, but that is seldom possible in practice. For that reason there is only one option which is use of higher alloyed material, and especially the content of nickel is important. |
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Pitting corrosion
Pitting corrosion is located corrosion and is characterized by small attacks on small discreet spots on the surface of the steel.
Pitting corrosion occurs if the oxide film is not strong enough to resist the environment the steel is exposed to.
That results a local destruction of the oxide film which is not capable of repairing itself. For that reason the corrosive attack continues. Attacks from pitting corrosion normally looks rather innocent on the surface but underneath the corrosion extends to a significant size. Pitting corrosion is either caused by steel that is not good enough (not sufficiently alloyed) or the oxide film is ruined from the outside by for example grinding dust.
Pitting corrosion and crevice corrosion is avoided by:
- Using a steel type that is sufficiently high alloyed and therefore resists the requested environment.
- Avoiding cracks.
- Avoiding pollution of the stainless steel. Minor damages are repaired by pickling.
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Crevice corrosion
Crevice corrosion occurs in narrow liquid-charged cracks where the oxygen is quickly used up and for that reason the passivity is not sustained.
Crevice corrosion is closely related to pitting corrosion. In narrow cracks like overlap a concentration of aggressive mediums is possible and will break down the passive film. This is caused by lack of oxidization of the oxide film and this means that the crevice corrosion can continue.
A type of crevice corrosion occurs under incrustation in maritime environments.
This kind of corrosion is often seen on compensators in assemblies of connection ends (for example flanges) and the bellow. These assemblies result cracks and in these cracks crevice corrosion occurs.
Crevice corrosion and pitting corrosion are avoided by:
- Using a steel type that is sufficiently high alloyed and for that reason resists the requested environment.
- Avoiding cracks.
- Avoiding pollution of the stainless steel. Minor damages are repaired by pickling.
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Corrosion caused by turbulence
This kind of corrosion is very special and not often seen. Even so attention to this kind should be given. A high velocity of flow constitutes a risk. For that reason it is a good idea to contact Belman if the velocity of flow exceeds 20m/s. |
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