ASTM B688/B688M is the standard specification by ASTM International covering UNS N08367 super-austenitic stainless steel plate, sheet, and strip — commercially known as AL-6XN (Alloy 6XN). With 6.0–7.0% molybdenum and 0.18–0.25% nitrogen, AL-6XN achieves a PREN ≥ 43, delivering chloride pitting and crevice corrosion resistance approaching Ni-Cr-Mo alloys at a significantly lower cost. It is the standard material for offshore seawater piping, FGD scrubbers, and high-chloride chemical processing. The companion specification ASME SB688 is adopted for ASME Code-stamped equipment.

Zhengzhou Huitong (HT Pipe) supplies ASTM B688 / ASME SB688 compliant plate, sheet & strip with full Mill Test Certificates (MTC) per EN 10204 3.1. 

1. Standard Scope & Designation

ASTM B688/B688M covers UNS N08367 chromium-nickel-molybdenum-iron plate, sheet, and strip — commonly known as AL-6XN (or Alloy 6XN). This super-austenitic stainless steel, with 6% molybdenum and high nitrogen, provides outstanding resistance to chloride pitting and crevice corrosion with PREN ≥ 43, bridging the cost-performance gap between standard austenitics (316L) and Ni-Cr-Mo alloys (C-276).

2. Grades Covered by ASTM B688

ASTM B688 covers the following grades. Each grade is defined by its UNS number and specific chemistry targeted at different service environments.

3. Chemical Composition

Values are weight percent maximum unless shown as a range. Heat analysis. Per ASTM B688 Table 1.

4. Mechanical Property Requirements

All values are minimum requirements. Tensile testing per ASTM E8/E8M.

5. Heat Treatment Requirements

6. Testing & Inspection Requirements

7. Dimensions & Dimensional Tolerances

ASTM B688 covers plate, sheet, and strip from 0.010 in (0.25 mm) to 4 in (100 mm). AL-6XN is typically stocked in plate gauges 0.187–1.500 in (4.75–38 mm) and sheet 0.036–0.187 in (0.91–4.75 mm). Standard widths 48–96 in (1220–2440 mm). Light-gauge strip available for gaskets and lining applications.

8. Industries & Applications

9. ASTM B688 vs ASTM B625 (UNS N08904 / 904L Super-Austenitic Plate) — Comparison

10. Related Standards

11. How to Order — Required Information

12. Frequently Asked Questions

Q: What makes AL-6XN better than 316L for chloride service?

AL-6XN (UNS N08367) contains 6.0–7.0% Mo and 0.18–0.25% N, giving a PREN of 43–48 vs 24–27 for 316L — approximately double the pitting resistance. The critical pitting temperature (CPT) in 6% FeCl₃ (ASTM G48 Method C) is ≥ 72°C for AL-6XN vs ~10–15°C for 316L. AL-6XN can handle seawater and high-chloride brines that would rapidly pit 316L. The high Ni content (23.5–25.5%) also provides superior resistance to chloride stress corrosion cracking (SCC) — 316L fails SCC in hot chlorides above ~60°C, while AL-6XN resists SCC in boiling MgCl₂. AL-6XN is roughly 2–3× the cost of 316L plate but 3–5× lower cost than C-276.

Q: Can AL-6XN be welded without loss of corrosion resistance?

Yes, with proper filler metal and procedure control. AL-6XN is highly weldable using GTAW (TIG), GMAW (MIG), SMAW, and SAW processes. The recommended filler metal is ERNiCrMo-3 (AWS A5.14 / ENiCrMo-3, e.g., Inconel 625) — this over-alloyed filler ensures the weld deposit has Mo content ≥ 6% and maintains PREN ≥ 43. Autogenous welding (no filler) is not recommended because Mo segregation during solidification can reduce local PREN. Heat input should be moderate (≤ 2.0 kJ/mm), interpass temperature ≤ 150°C (300°F). Post-weld heat treatment is not required; solution annealing improves corrosion resistance if fabrication allows.

Q: What is PREN and what does it mean for AL-6XN?

PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3 × %Mo + 16 × %N. For AL-6XN (Cr 20–22%, Mo 6–7%, N 0.18–0.25%): PREN = 20 + 3.3×6.5 + 16×0.22 ≈ 43–48. Higher PREN = greater resistance to localized chloride attack (pitting and crevice corrosion). For reference: 304L PREN ~19, 316L ~24–27, 2205 duplex ~34, 904L ~34–39, AL-6XN ~43–48, C-276 ~63–67. AL-6XN bridges the gap between standard stainless steels and Ni-Cr-Mo alloys — offering near-alloy performance at stainless steel cost.

Q: Is ASTM B688 AL-6XN suitable for sour service?

Yes. AL-6XN (UNS N08367) is listed in NACE MR0175/ISO 15156 as acceptable for sour service (H₂S-containing environments) up to certain temperature and chloride limits. In the solution-annealed condition, it resists sulfide stress cracking (SSC) and stress corrosion cracking (SCC) in produced water with H₂S partial pressure up to 0.1 bar and moderate chlorides. For more severe sour service conditions (higher H₂S, higher temperature), Ni-Cr-Mo alloys such as C-276 (B575) or Alloy 625 (B443) should be considered. Always verify against the current NACE MR0175/ISO 15156 tables for your specific service conditions.

Q: What are the main offshore applications for AL-6XN plate?

AL-6XN plate and sheet are extensively used in offshore oil and gas platforms for: (1) seawater lift pump caissons and risers, (2) fire water (deluge) system piping, (3) produced water treatment and injection piping, (4) heat exchanger tube sheets and shells in seawater cooling service, (5) subsea umbilical tubing, and (6) FPSO topside seawater systems. AL-6XN's combination of high strength (YS ≥ 310 MPa), excellent weldability, and proven 20+ year track record in North Sea and Gulf of Mexico offshore installations makes it the default specification for super-austenitic seawater service.

Request a Quote — ASTM B688 AL-6XN (UNS N08367) Super-Austenitic Plate, Sheet & Strip

Zhengzhou Huitong (HT Pipe) supplies ASTM B688 / SB688 UNS N08367 AL-6XN super-austenitic plate, sheet, and strip for seawater, offshore, and high-chloride chemical service. Full EN 10204 3.1 CMTRs with optional ASTM G48 corrosion testing. Competitive lead times from qualified mills.