INCOLOY Alloy 25-6HN (UNS N08367),AL-6XN: The Optimal Choice for Extreme Chloride Environments

INCOLOY Alloy 25-6HN, AL-6XN (UNS N08367, W.Nr. 1.4501)—commonly known as a 6-Moly super austenitic stainless steel—fills the performance and cost gap between standard austenitic grades and high-end nickel-based superalloys. Engineered to deliver outstanding resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking (SCC), this nitrogen-strengthened alloy achieves a Pitting Resistance Equivalent Number (PREN) above 46, making it an ideal material for seawater, desalination, flue gas desulfurization, and petrochemical systems. Below, we break down its chemistry, properties, and application advantages, along with the complete product range we supply.

These three materials share the same UNS designation and composition standards and are fully interchangeable; they differ only in their brand names and manufacturer designations.

UNS N08367: United States National Standard (UNS) number (statutory material standard number, unique)
AL‑6XN®: Registered trademark and trade name of ATI (formerly Allegheny Ludlum); the original designation under which this alloy was developed
INCOLOY Alloy 25‑6HN: Registered trademark designation with the same composition from Special Metals (Incoloy brand)

1. Material Overview: What Is INCOLOY Alloy 25-6HN?

INCOLOY Alloy 25-6HN is a low-carbon, iron-nickel-chromium-molybdenum alloy fully compliant with ASTM and ASME pressure equipment standards. Its carefully balanced high-alloy formulation provides a unique combination of superior local corrosion resistance, higher mechanical strength, and excellent fabricability—outperforming 316L, 904L, and Alloy 926.

Key Differentiators vs. Conventional Alloys:

• Self-healing passive film: The high chromium, molybdenum, and nitrogen content creates a dense oxide layer that repels chloride attack in stagnant seawater, acidic brines, and scrubbing media.
• Full austenitic microstructure: Remains completely non-magnetic and free from detrimental phase transformations after welding and thermal cycling, avoiding risks common to duplex stainless steels.
• Cost-effective upgrade: Delivers corrosion performance approaching that of pure nickel-base alloys while maintaining a significantly lower material cost.

We supply a comprehensive inventory of AL-6XN finished and semi-finished products—including seamless pipes, forged fittings, anchor flanges, plates, bars, and custom pressure forgings—for global marine, environmental, and industrial projects.

2. UNS N08367 Standard Chemical Composition (wt%)

The chemical makeup strictly adheres to UNS N08367 registration and original ATI material specifications, ensuring stable passive film formation, solid-solution strengthening, and immunity to intergranular corrosion.

The ultra-low carbon design combined with high molybdenum and nickel eliminates weld sensitization and guarantees permanent intergranular corrosion resistance.

3. Heat Treatment & Mechanical Properties

3.1 Solution Annealing Process

All UNS N08367 products are delivered in the fully solution-annealed condition. Standard treatment involves heating to 1120°C–1170°C followed by rapid water quenching. This process dissolves secondary phases, homogenizes the grain structure, and removes residual stresses—maximizing ductility, weldability, and uniform corrosion resistance. Every batch is accompanied by complete heat treatment records, PMI spectral inspection, and certified mechanical test reports.

3.2 Room-Temperature Mechanical Properties (Solution-Annealed)

• Tensile Strength: ≥740 MPa (107 ksi)
• Yield Strength (0.2% offset): ≥330 MPa (48 ksi)
• Elongation: ≥40%
• Brinell Hardness: ≤225 HB

3.3 Physical Characteristics & Service Limits

• Density: 8.24 g/cm³
• Average Thermal Expansion (20–400°C): 14.9 μm/m·°C (14.9 × 10⁻⁶ /°C)
• Service Range: Stable from cryogenic temperatures (-196°C) up to 425°C. The alloy exhibits excellent toughness at low temperatures without a ductile-to-brittle transition.
• Design Strength Advantage: Under ASME codes, its allowable design stress is approximately 40% higher than that of 316L, enabling thinner-wall piping, reduced weight, and cost savings.

4. AL-6XN Core Performance Advantages

4.1 Superior Resistance to Pitting & Crevice Corrosion

With a PREN >46, AL-6XN significantly outperforms 316L, 904L, and even duplex 2205 in chloride-rich waters. Its Critical Crevice Corrosion Temperature (CCCT) reaches 43°C, effectively preventing localized perforation in marine splash zones, dead legs, and stagnant brine service.

4.2 Superior Resistance to Chloride Stress Corrosion Cracking (SCC)

The high-nickel austenitic matrix provides exceptional resistance to chloride-induced SCC—a common failure mechanism for standard stainless steels under elevated temperatures and tensile stress. This makes UNS N08367 a trusted choice for buried marine pipelines and high-pressure desalination equipment.

4.3 Excellent General Corrosion Resistance

AL-6XN maintains stable passivation in dilute sulfuric acid, phosphoric acid, organic acids, and aggressive flue gas desulfurization (FGD) scrubbing liquids. It withstands both uniform corrosion and oxidative acid attack far better than conventional corrosion-resistant grades.

4.4 High Strength & Permanent Non-Magnetic Structure

Nitrogen solid-solution strengthening boosts yield and tensile properties substantially. The fully austenitic structure remains permanently non-magnetic after welding, cold forming, and long-term thermal cycling—suitable for precision instrumentation and marine electromagnetic systems.

4.5 Outstanding Fabricability & Weldability

In the solution-annealed condition, AL-6XN offers excellent hot forging, cold bending, and machining characteristics. It can be readily welded using GTAW, GMAW, and SAW processes with matched super-austenitic filler metals. There is no post-weld sensitization risk, making it ideal for on-site assembly of large-diameter piping and pressure vessels.

5. Product Range & Specifications

We provide a full spectrum of ASTM/ASME-compliant AL-6XN products:

• Forged Pipe Fittings & Anchor Flanges: Seamless elbows, equal/reducing tees, concentric/eccentric reducers, caps, and marine anchor flanges. Sizes from NPS 1/2 to NPS 60, wall thickness SCH10–SCH160. Custom heavy-wall anti-corrosion forgings available.
• Seamless Alloy Pipes & Tubes: Hot-finished and cold-drawn. Outer diameters: 16 mm to 720 mm; wall thickness: 3 mm to 90 mm. Custom cut lengths and non-standard dimensions supported.
• Plates, Sheets & Round Bars: Thick plates, thin sheets, round bars, and flat bars for marine, desalination, and equipment components. Custom forging blanks and precision CNC machining available.
• Pressure Flanges & Custom Forgings: WN, SO, blind flanges, and special-shaped structural forgings for offshore and environmental projects.

Factory Quality Assurance: Every batch undergoes PMI spectral recheck, mechanical testing, hardness verification, and 100% UT/MT non-destructive testing. All deliveries include EN 10204 3.1 mill test certificates.

6. Key Application Fields

• Marine & Offshore Engineering: Seawater intake/discharge lines, subsea process piping, riser anchor flanges, and marine cooling systems.
• Seawater Desalination: RO and multi-stage flash piping, brine circulation heat exchangers, and high-chloride pump & valve components.
• Power Plant FGD Environmental Systems: Scrubber tower piping, acid mist internals, flue gas heat exchangers, and high-chloride circulating water lines.
• Petrochemical & Chemical Processing: Dilute acid reactor internals, salt-solution heat exchanger tubes, high-chloride wastewater piping, and crude oil treatment equipment.
• Pulp & Paper: Chlorine-containing bleach plant piping, high-corrosion washing tank structures, and process anti-rust components.
• Cryogenic & Precision Equipment: Low-temperature corrosive media lines, non-magnetic instrument piping, and seawater storage vessels.

7. Quality Control & Product Marking

Our AL-6XN products are manufactured under a strict ISO 9001 certified quality management system. The unified inspection workflow includes:

1. Raw material spectral recheck
2. Hot forming & professional solution annealing
3. Mechanical property testing
4. Non-destructive flaw detection (UT/MT)
5. Dimensional tolerance inspection
6. Laser permanent marking & vacuum moisture-proof packaging

Finished products are free of surface cracks, inclusions, laps, and oxidation defects. Standard laser marking includes brand, INCOLOY Alloy 25-6HN / AL-6XN / UNS N08367 grade, industry standards, heat batch number, dimensions, and wall thickness—fully traceable and ready for third-party inspection.

8. Industry Development Trends

• Desalination & Offshore Wind Expansion: As global freshwater demand grows and offshore renewable energy investments rise, AL-6XN is replacing duplex stainless steel in critical coastal projects, becoming the mainstream cost-effective super austenitic alloy.
• Lightweight Pipeline Design: High allowable design stress supports FEA-driven thin-wall design, reducing pipe weight and project CAPEX without compromising safety.
• Full Component Standardization: Standardized production of matching anchor flanges, special tees, and pressure forgings will streamline one-stop procurement for EPC contractors.
• Green Manufacturing: Precision energy-saving forging and environmentally friendly coatings align with global carbon neutrality goals.
• Wastewater & CCUS Projects: Industrial high-chloride wastewater treatment and carbon capture systems represent new growth markets where UNS N08367 is becoming the premier material for harsh flue gas and brine circulation piping.

Contact Us for INCOLOY Alloy 25-6HN,AL-6XN,UNS N08367 Solutions

Complete mill test reports, PMI, and NDE certificates available. Request a quote today.