What makes duplex stainless steel corrosion-resistant?

What makes duplex stainless steel corrosion-resistant?
1. Why does stainless steel not rust? ​
The corrosion resistance of stainless steel generally increases as the chromium content increases. When the chromium content in the steel is ≥12%, a very thin but dense oxide film (commonly known as the passivation film) will be formed on the surface of the steel. The substrate is no longer oxidized or corroded, so stainless steel has excellent corrosion resistance.
2. The composition of duplex stainless steel: refers to stainless steel with ferrite and austenite each accounting for approximately 50%. Generally, the content of the smaller phase needs to be at least 30%. In the case of low C content, the Cr content is 18%-28% and the Ni content is 3%-10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N.
3. Classification of duplex stainless steel:
Category 1: Low alloy type, representative brand UNS S32304 (23Cr-4Ni-0.1N), the steel does not contain molybdenum, the PREN value is 24-25 and can be used instead of AISI304 or 316 in terms of stress corrosion resistance.
Category 2: Medium alloy type. The representative brand is UNS S31803 (22Cr-5Ni-3Mo-0.15N). The PREN value is 32-33. Its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel. between.
Category 3: High alloy type: generally contains 25% Cr, molybdenum, and nitrogen, and some also contain copper and tungsten. The standard grade is UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), and the PREN value is 38-39, the corrosion resistance of this type of steel is higher than that of 22%Cr duplex stainless steel.
Category 4: Super duplex stainless steel type: high in molybdenum and nitrogen, standard grade UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), some also contain tungsten and copper, PREN value is greater than 40, suitable for harsh environments Medium conditions, with good corrosion resistance and mechanical properties.
4. Duplex stainless steel VS austenitic & ferritic stainless steel
Compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:
(1) The yield strength is more than twice that of ordinary austenitic stainless steel, and it has sufficient plasticity and toughness required for molding.
(2) It has excellent resistance to stress corrosion cracking. Even duplex stainless steel with the lowest alloy content has a higher resistance to stress corrosion cracking than austenitic stainless steel, especially in environments containing chloride ions.
(3) The corrosion resistance of 2205 duplex stainless steel, which is the most commonly used in many media, is better than that of ordinary 316L austenitic stainless steel, while super duplex stainless steel has extremely high corrosion resistance.
(4) It has good local corrosion resistance. Compared with austenitic stainless steel with the same alloy content, its wear corrosion resistance and fatigue corrosion resistance are better than austenitic stainless steel.

So, What makes duplex stainless steel corrosion-resistant?
When Duplex Stainless Steel contains low C, the Cr content is 18%~28% and the Ni content is 3%~10%.