Effective sum
The active sum is used to estimate the corrosion resistance of a nickel-containing alloy against pitting or crevice corrosion . The pitting resistance equivalent (also known under the abbreviation PRE P itting R esistance E equivalent, see PREN ).
The ability of a stainless steel or a nickel-based alloy to protect itself against these forms of corrosion depends on the different contents of alloy elements. These alloy elements lead to a passivation of the metal and thus to protection. If the passive layer is not sufficient, the metal will be attacked and corroded. To establish a passive layer, a required stainless steel a chromium content of at least 12 percent for the formation of a stable passive surface layer. As a rule, the chromium content must be higher, the more oxidizing agents are present.
After chromium, molybdenum is the most important alloying element . The proportions are between 1.6 and 28 percent. Molybdenum increases the resistance to surface-erosive corrosion attack in reducing media and is therefore of essential importance for acid resistance.
Nitrogen has a similar meaning for the material .
The effective sum for high-alloy stainless steels is calculated as follows:
WS =% Cr + 3.3 *% Mo + 30 *% N
The higher the active sum, the more resistant the material is to crevice or pitting corrosion. Alloys with an active sum of more than 33 are considered to be sea water resistant.
Example:
U-boat steel with the material number 1.3974 and the short name X 2 Cr Ni Mn Mo N Nb 23-17-6-3 consists of 0.02% carbon, 23% chromium, 17% nickel, 6% manganese, 3% molybdenum and about 0.5% nitrogen, so it has an effective sum of 23 + 3.3 * 3 + 30 * 0.5 = 47.9.
Individual evidence
- ↑ Stainless Steel Association (ISER): Leaflet 893 "stainless steel for water management". Retrieved September 7, 2017 .
- ↑ Stainless steel - material 1.3974. Retrieved September 7, 2017 .