Selecting and Specifying Stainless Steels for Refineries and Upgraders
Stainless steels offer some key advantages over carbon steel and low alloy steels when proper grades or types are selected in refineries and upgraders. Key advantages are:
- Resistance to hydrogen embrittlement
- Resistance to hydrogen/hydrogen sulfide mixtures that occur in hydroprocessing operations
- Resistance to sulfidation that occurs in crude distillation units and sulfur plants
- Resistance to acidic corrosion from sour waters and organic acids
Many stainless steels may be applied as cladding or weld overlays which provides corrosion resistance at a lower cost.
Many advances in stainless steel alloys have occurred with newer alloys that are resistant to the following:
- Sulfuric acid solutions
- Salt solutions: resistant to stress corrosion cracking and pitting corrosion. Some alloys are resistant to seawater.
- High temperature applications
There are five “families” of stainless steels that are based upon their structures:
- Austenitic: types 304, 321, 316, 347, etc
- Ferritic: types 403 and 405
- Martensitic: types 410, 416
- Preciptation hardnening: types 17-4 PH. 15-5, etc
- Duplex (austenitic and ferritic): types 2205, 2101
The more corrosion resistant grades are the alloys from the austenitic and duplex families. The martensitic and precipitation hardening types have chromium contents in the 12% to 17% range and so corrosion resistance between these types is similar. Ferritic stainless steels have limited use in refineries because of their instability to high temperature and difficult weldability.
There are several advanced stainless steels used in refineries:
- Super-austenitic: AL-6XN, Alloy 20, 904L, Sanicro 28, 6 Mo, 7 Mo, Alloy 31 and Alloy 33. These alloys have much higher nickel levels than in 304 or 316, hence, the austenitic structure is reinforced. Typical applications are where the typical 304/347/316 SS fail due to stress cracking or pitting corrosion.
- Super-duplex and hyper-duplex: 2507, 2707HD and 3207HD. With higher chromium and molybdenum contents corrosion resistance is improved over standard 2205 duplex SS.