The right selection of material
makes a difference
Stainless steel is required for numerous applications and industries. This material is used for a huge variety of purposes in essential sectors such as raw material extraction, the pharmaceutical and chemical industries, plant construction, oil & gas, offshore applications etc. Mankenberg’s product range of flexible standard valves or project-related special valves
is correspondingly broad. The operating conditions at the customer’s site sometimes require ultra-clean surfaces of the valves whilst other valves must be capable of sustaining the flow of dirty or highly corrosive media. Hence, the optimum solution is selected in close consultation with our engineers, technicians and sales staff. A particular challenge is to select the suitable material for applications in chemical-technical processes, in which caustic and / or corrosive fluids are used.
The same applies to the maritime domain or saline liquids, which is generally referred to as sea water resistance. It requires special diligence and clarification of all the technical and chemical details in order to properly assess the loading conditions of the material and the interaction between the medium and the environmental conditions. Stainless steels, i.e. corrosion-resistant steels, become resistant to corrosion because a so-called passive layer forms on the surface. Such layers consists of chromium-rich metallic oxide or metallic oxide hydrate preventing the direct contact of the metal with the corroding medium. Even in the event of small lesions, a new layer builds up independently at the relevant area. If this is not the case, for example due to a lack of oxygen, either pitting corrosion or crevice corrosion may occur.
Stainless steels have a percentage by mass of the element chromium of not less than 12 % and of the element carbon that should not exceed 0,12 %. Hence, the percentage of the alloying element chromium is decisive for the corrosion resistance of stainless steel. In case the steel contains further alloying elements such as molybdenum or the like, the material becomes more resistant also to highly aggressive operating conditions.
The suitable valve from the right material for your application:
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Stainless steel and its resistance to corrosion
General corrosion
- The passive layer is completely destroyed
- This type of corrosion depends on the consistency of the fluid
- Can be avoided by optimally selecting the material with respect to the medium
Pitting corrosion
- Can be avoided by selecting molybdenum-containing steel types
Crevice corrosion
- Occurs in a design-related enclosed hollow space of the equipment and in a chloride-containing environment
- Can be prevented through avoiding enclosed hollow spaces already during the design phase
Corrosion by contamination
- Is caused by ferrous deposits and brings about a contamination
Intergranular corrosion
- Diffuses from the grain boundaries in heat-sensitized areas
- The chromium degrades and impairs the passivating effects
- Can be avoided by selecting low-carbon steel types
Stress corrosion cracking
- Occurs in a chloride-containing environment, when the equipment is exposed to high stress, for example
- Can be avoided by selecting a suitable stainless steel
- Transcrystalline or intergranular crack path at sensitized microstructures
Behaviour of corrosion-resistant stainless steel 1.4529 / 1.4547
Excellent corrosion resistance owing to an increased percentage of chromium and molybdenum
Pitting corrosion is a particular type of corrosion in media containing chloride ions. In the event that the protective passive layer of the stainless steel is interrupted owing to small lesions,
a local corrosion attack occurs. Pits or holes that are often as small as pinholes, form more readily. As long as the exposure
persists, the pits or holes will enlarge.
Crevice corrosion can be found in already existing gaps or fissures which are often generated by the overall design. The passive layer of the stainless steel cannot form there at all and aggressive media such as salt water accelerate the corrosion process. If, in addition, the oxygen, which is necessary to form the passive layer, is not available, heavy crevice corrosion may occur.
Used corrosion-resistant metals
The higher the PREN (pitting resistance equivalent), the more resistant to pitting and crevice corrosion | alloys with a PREN of > 33 are classified as sea water resistant | Hastelloy® C-4 and Titanium are classified as being highly resistant to sea water | PREN of stainless steels = % Cr + 3.3*% Mo + 16*% N | a higher PREN is required for an increasing salt content and/or rising temperature
Used seal materials
