AL-6XN (UNS N08367): Super Austenitic Alloy for Chloride & Acid Service
ASTM B688 / ASTM A240 · Published: 2026-06-01 · Updated: 2026-06-02
AL-6XN (UNS N08367) is a super austenitic alloy with 6.2% molybdenum and 0.22% nitrogen, designed for exceptional chloride pitting and crevice corrosion resistance. While commercially positioned as a stainless steel, its iron content of ~48%...
AL-6XN (UNS N08367) is a super austenitic alloy with 6.2% molybdenum and 0.22% nitrogen, designed for exceptional chloride pitting and crevice corrosion resistance. While commercially positioned as a stainless steel, its iron content of ~48% technically classifies it as a nickel-base alloy under some standards — the designation reflects market positioning rather than strict metallurgical classification. Its PREN of ~46 places it above 254 SMO (PREN 43) and directly competitive with super duplex 2507 (PREN 42+).
AL-6XN is the material of choice for reverse osmosis (RO) desalination high-pressure piping, where the combination of high chloride (up to 60,000 ppm in RO concentrate) and moderate temperatures (25-40°C) creates aggressive crevice corrosion conditions at flange gasket surfaces and threaded connections. The alloy's high nitrogen content (0.18-0.25%) provides solid-solution strengthening and delays sigma phase precipitation during welding — a significant practical advantage over 254 SMO. AL-6XN is also specified for chemical process equipment handling mixed acids (sulfuric + hydrochloric), where its nickel content (~24%) provides sulfuric acid resistance that 254 SMO cannot match.
Quick Facts
| Category | Stainless Steel |
| Standard | ASTM B688 / ASTM A240 |
| Density | 8060 kg/m³ |
| Yield Strength | 310 MPa |
| Tensile Strength | 690-760 MPa |
Global Equivalents & Cross-Reference
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Frequently Asked Questions
What is the difference between AL-6XN and 254 SMO?
Both are 6% Mo super austenitics. AL-6XN has higher nickel (24% vs 18%), higher molybdenum (6.2% vs 6.1%), and is produced by Allegheny Technologies (ATI) as a proprietary alloy. 254 SMO is produced by Outokumpu. In practice, the two alloys are functionally interchangeable for most applications, though AL-6XN's higher nickel content gives a slight advantage in hot sulfuric acid environments.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu