Corrosion
The right selection of material makes the difference
Stainless steel is needed for numerous applications and industries. This material is used for a huge variety of purposes in essential sectors such as raw material extraction, 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 ultraclean 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, for which in general sea water-resistant material is needed. 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.
Used corrosion-resistant materials
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
Stainless steels, i. e. corrosion-resistant steels, become resistant to corrosion because a so-called passive layer forms on the surface. Such layer 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 (1) or crevice corrosion (2) 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.
Used corrosion-resistant materials
| Material | Nr. | Norm | Major alloying element - % | Pitting index (PREN) |
|||
| DIN EN | ASTM | Cr | Ni | Mo | |||
| Stainless steel | 1.4404 | X2CrNiMo17-12-2 | 316L | 16,5 - 18,5 | 10,0 - 13,0 | 2,0 - 2,5 | 23,0 - 28,0 |
| 1.4571 | X6CrNiMoTi17-12-2 | 316Ti | 16,5 - 18,5 | 10,5 - 13,5 | 2,0 - 2,5 | 25,0 | |
| Duplex | 1.4462 | X2CrNiMoN22-5-3 | A182F51 | 21,0 - 23,0 | 4,5 - 6,5 | 2,5 - 3,5 | 30,0 - 38,0 |
| 1.4539 | X2NiCrMoCu25-20-5 | N08904 | 19,0 - 21,0 | 24,0 - 26,0 | 4,0 - 5,0 | 34,0 - 40,0 | |
| Super Duplex | 1.4410 | X2CrNiMo25-7-4 | S32750 | 24,0 - 26,0 | 6,0 - 8,0 | 3,0 - 4,5 | 35,0 - 42,0 |
| 1.4501 | X2CrNiMoCuWN25-7-4 | S32760 | 24,0 - 26,0 | 6,0 - 8,0 | 3,0 - 4,0 | 37,0 - 44,0 | |
| Cronifer 1925hMo | 1.4529 | X1NiCrMoCu25-20-7 | N08926 | 19,0 - 21,0 | 24,0 - 26,0 | 6,0 - 7,0 | 41,0 - 48,0 |
| 245 SMO® | 1.4547 | X1CrNiMoCuN20-18-7 | S31254 | 19,5 - 20,5 | 17,5 - 18,5 | 6,0 - 7,0 | 42,0 - 48,0 |
| Hastelloy® C-4 | 2.4610 | NiMo16Cr15Fe6W4 | N06455 | 14,5 - 17,5 | 66,0 | 14,0 - 17,0 | |
| Titanium | 3.703 | R50400 | |||||
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
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