Inconel 600 in 90 Seconds

Inconel 600 is a nickel-chromium-iron solid-solution alloy with 76% Ni, 15.5% Cr, and 8% Fe. No molybdenum, no niobium, no copper. It was developed in the 1930s and remains one of the most widely used high-nickel alloys in the world, with annual consumption exceeding that of 625, 690, and 718 combined.

Three properties define 600’s engineering personality:

1. Exceptional resistance to chloride stress-corrosion cracking — better than 300-series stainless by orders of magnitude. This is what made it the standard for nuclear PWR steam-generator U-tubes.
2. Good high-temperature strength with excellent resistance to oxidation and carburization up to 1,150°C in cyclic service.
3. Resistance to caustic and chloride SCC — strong performer in hot NaOH and KOH service up to 50% concentration.

What 600 does not have is molybdenum — so its pitting and crevice corrosion resistance in chloride-bearing environments is significantly worse than 625, C276, or C22. That single fact is the basis of the “600 vs 625” decision.

Composition & ASTM Specifications

ASTM Specifications

ASME “SB” versions of the same specifications exist for Section VIII pressure-vessel service. There is also a low-carbon “Grade 2” (UNS N06600 with 0.05% C max) that is required for nuclear steam-generator service and for service above 600°C where sensitization must be avoided.

Mechanical Properties vs Temperature

For high-temperature service, the annealed 600 retains useful strength to about 1,150°C in oxidation-resistant applications. For sustained creep duty, the comparable grade is 600T (a controlled-grain variant similar to 800H) — but most modern plants have migrated to 800H/HT for new units.

Annealing: 1,010°C, 1,050°C, 1,150°C — Which?

Annealing temperature determines the carbide distribution and therefore the corrosion behavior. Three common regimes:

Field trap: A re-anneal at 925°C after field bending — a common repair procedure — will sensitize the alloy. If field bending is required, specify a 1,150°C re-anneal followed by rapid quench.

Corrosion Resistance: Where 600 Shines

Aqueous Corrosion

• Hot caustic (NaOH, KOH): outstanding resistance to 50% concentration at temperatures up to 200°C. Used in caustic evaporators, chlor-alkali membrane cells, and soap manufacturing.
• Hot pure water: virtually immune to chloride SCC — the reason 600 became the standard for nuclear PWR steam-generator U-tubes.
• Seawater: resistant in flowing conditions; susceptible to pitting and crevice attack in stagnant, hot, aerated service (use 625 there).
• Hydrofluoric acid (HF): good resistance to anhydrous HF and to 50–70% HF at moderate temperature; 600 is the standard alloy for HF alkylation.
• Sulfuric acid: poor — use Alloy 20 or Hastelloy.
• Nitric acid: marginal — better than 304, worse than 310; use 310 or a high-Cr nickel alloy for HNO₃.

High-Temperature Corrosion

• Oxidation: excellent in air to 1,150°C. Forms a tough, adherent Cr₂O₃ scale.
• Carburization: good resistance in CO, CO₂, and hydrocarbon atmospheres up to 950°C — used in ethylene cracking, reformer catalyst tubes.
• Sulfidation: fair to 800°C; for higher-sulfur atmospheres, upgrade to 625 (better Cr) or to 601 (more Cr + Al).
• Chlorination: poor above 200°C — limit Cl₂ to 0.1% in service gas.

Inconel 600 vs 625 — The Real Decision

Use this 4-question decision tree:

1. Is the fluid hot (60°C+) and chloride-bearing (≥ 1,000 ppm)? → Use 625. 600 will pitting/crevice attack within 1–3 years.
2. Is the fluid hot, concentrated H₂SO₄ (50%+) or HCl? → Use C276 or C22, not 600 or 625.
3. Is the service hot pure water or hot caustic, with no reducing acid? → Use 600. Better than 625, cheaper than 625.
4. Is the service high-temperature, non-corrosive, with creep as the design driver? → Use 600 (lower cost) or 800H/HT (better creep code stress). 625’s Mo is wasted in this case.

Indicative 2026 mill prices (plate, USD/kg):

The price gap between 600 and 625 (~$15/kg) is large enough to justify specifying 600 by default in non-chloride service — even when the project is a 625 plant standard.

Nuclear Steam-Generator Tubes

The largest single application of Inconel 600 is in PWR nuclear power plants. The steam generator (SG) is a U-tube heat exchanger that transfers heat from the primary loop to the secondary loop, and the U-tubes (typically 22 mm OD × 1.27 mm wall × 18–22 m long) operate in primary-side water at ~325°C / 155 bar.

Why 600:

• Immune to chloride SCC in pure primary water.
• Compatible with phosphate and AVT (all-volatile treatment) chemistry.
• Weldable, U-bendable, and inspectable by eddy current.
• Over 30 years of validated ASME Section III design rules.

600 vs 690 vs 800: the nuclear industry progressively moved to Alloy 690 (UNS N06690) for new SGs because of its 30% Cr content, which dramatically reduces primary-water stress-corrosion cracking (PWSCC) — the dominant failure mode in 600 tubes. Alloy 800 has been used in German and Russian designs.

If you are specifying nuclear SG tubes today, the order of preference is:

1. Alloy 690 (30% Cr): new builds, PWSCC-resistant
2. Alloy 800 nuclear grade: alternative for new builds, German KTA rules
3. Alloy 600 thermally treated (TT): replacement tubes for existing 600 SGs

Welding & Fabrication

• Filler metal: ERNiCr-3 (Inconel 82) is the universal choice. For matching strength, ENiCrFe-3 covered electrode or ERNiCrFe-3 for flux-cored / GMAW.
• No preheat required for most thicknesses.
• No PWHT required for 600; in fact, PWHT in the 550–850°C range can sensitize the HAZ and is generally avoided for aqueous service.
• Cold forming at room temperature is standard. Hot forming at 870–1,150°C, followed by a 1,150°C anneal and rapid quench.

Selection Cheat Sheet

Inconel 600 in Caustic (NaOH) Service: The Unique Advantage

Perhaps the single most important industrial application of Inconel 600 — and the one where no other alloy can match it economically — is in caustic soda (NaOH) and caustic potash (KOH) service. The 76% nickel content provides a fundamental advantage: nickel forms a chemically stable, adherent oxide film in high-temperature alkaline environments where chromium, iron, and molybdenum dissolve.

Why 600, not 625, for caustic? 625 contains ~9% Mo which forms soluble molybdates in hot NaOH — the corrosion rate is actually higher than 600 in concentrated, hot caustic. 600’s simple Ni-Cr-Fe chemistry with no Mo is actually an advantage in this environment. Also, 600 costs ~60% of 625, making it the clear economic choice.

Stress-Relief Annealing: A Critical Step Often Skipped

After cold forming, welding, or machining, Inconel 600 components benefit from a stress-relief anneal at 870–925°C for 30–60 minutes, followed by air cooling. This is NOT a full solution anneal (which would be at 1,010–1,150°C) — it is a lower-temperature treatment specifically designed to reduce residual stresses without significant grain growth or reduction in tensile strength.

Why stress-relief matters for 600:

• Caustic SCC prevention: In hot NaOH service, residual tensile stress on the surface is the primary driver of intergranular caustic SCC. Stress-relief reduces the peak surface tension from ~300–400 MPa (as-machined) to ~50–100 MPa (stress-relieved) — well below the SCC threshold.
• Dimensional stability: 600 has a relatively high coefficient of thermal expansion (13.3 μm/m/°C). Residual stresses from fabrication can relax during the first thermal cycle in service, causing unpredictable dimensional changes. Stress-relief “pre-relaxes” the component to its service dimensions.
• Intergranular attack resistance: While 600 is not considered “sensitizable” in the same way as 304 stainless (because of the low carbon content, typically ≤ 0.05% in good commercial heats), stress-relief further homogenizes the grain-boundary chemistry and reduces the risk of intergranular corrosion in aggressive media.

Inconel 600 Thermowells: The Unsung Hero

One of the largest single-component applications for Inconel 600 is in thermowells — the protective tubes that house temperature sensors in chemical reactors, boilers, heat exchangers, and piping. Inconel 600 thermowells dominate high-temperature, corrosive applications because the alloy combines three critical properties that no other material offers at its price point:

1. High fatigue strength: Thermowells vibrate in fluid flow — the vibration frequency is proportional to fluid velocity (Strouhal number ~0.2). 600’s high fatigue endurance limit (260 MPa at 10^7 cycles, room temperature) gives a comfortable margin against vortex-induced vibration (VIV) failure — the #1 cause of thermowell breakage.
2. Corrosion resistance in process fluids: 600 is inert to most organic and inorganic chemicals at moderate temperatures — exactly the “universal” resistance needed for a thermowell that sees a different process fluid with every batch.
3. Weldability to dissimilar vessel materials: 600 filler (ERNiCr-3) is the universal filler for joining Ni-alloy thermowells to carbon steel, stainless steel, or nickel-alloy vessel nozzles. A 600-to-316L or 600-to-carbon-steel weld is standard practice and qualifies easily per ASME Section IX.

If you’re buying thermowells for a chemical plant, refinery, or power boiler, Inconel 600 in bar form (ASTM B166 or B564 for forged flanges) is available in diameters from 6 mm to 50 mm — and most common thermowell sizes (3/4″ NPS, 1″ NPS, and 1-1/2″ NPS) are standard stock items at Huaxiao Alloy.

Full Anneal vs Mill Anneal: The Hidden Specification

When ordering Inconel 600 plate, sheet, or tube, the purchase order often says “ASTM B168, Annealed” — but that single word “Annealed” hides two very different heat-treatment practices that produce different microstructures and properties:

• Mill anneal (hot-rolled + air-cool): The as-hot-rolled product is cooled in air from the hot-working temperature (~1,050–1,150°C). This produces a fine, recrystallized grain structure (ASTM 7–9) with yield strength at the high end of the spec range (310–380 MPa). This is the default for stock material. It’s acceptable for most commercial applications but may contain residual hot-working strain and slightly non-uniform grain size.
• Full solution anneal + water quench: The product is reheated to 1,010–1,050°C, held for 1 hour per 25 mm of thickness (minimum 15 minutes), then water quenched. This produces a coarser, uniform grain (ASTM 4–6), yield strength at the lower end of the spec range (240–280 MPa), but MAXIMUM corrosion resistance and ductility. Required for nuclear, caustic, and any service where IGC or SCC is a risk.

Inconel 600 in Chlorine and Chloride Service — The Limits

While Inconel 600 excels in caustic and high-temperature oxidation, it has well-defined weaknesses that procurement teams must respect:

Wet Chlorine (Cl₂ + H₂O)

Inconel 600 is rapidly attacked by wet chlorine at any concentration above 20°C. The attack mechanism is a combination of oxidation (Cl₂ → Cl⁻ + OCl⁻) and acidification (Cl₂ + H₂O → HCl + HOCl) — the HCl component pits the nickel matrix, and the oxidation component attacks the chromium-rich passive film. For wet chlorine service, use titanium (Grade 2) or Hastelloy C-276 — never Inconel 600.

Dry Chlorine (Cl₂, anhydrous)

In contrast to wet chlorine, 600 has good resistance to dry chlorine gas up to ~300°C. Nickel forms a stable, adherent NiCl₂ surface film in the absence of moisture. This is why 600 is acceptable (though not first choice) for dry-chlorine handling in chlor-alkali plants. However, if there is any chance of moisture ingress (e.g., during a shutdown with humid air), switch to Hastelloy C-276.

Chloride Stress-Corrosion Cracking (Cl-SCC)

This is the most important limitation of 600: it is NOT immune to chloride SCC. Unlike Inconel 625 (immune to Cl-SCC in all practical service conditions), 600 has a lower Ni content (76% vs 625’s 58% — actually 600 has MORE Ni, but 625 has Mo which helps). The actual explanation is more nuanced:

• Inconel 600, with 76% Ni and NO Mo, is resistant to Cl-SCC up to ~200°C in neutral chloride solutions — but becomes susceptible in acidified, aerated chloride above ~120°C.
• Inconel 625, with 58% Ni plus 9% Mo, is immune to Cl-SCC at ALL practical temperatures and chloride concentrations because the Mo content shifts the repassivation potential in the noble direction, preventing the crack-tip acidification that drives Cl-SCC.
• Practical rule: If you need guaranteed Cl-SCC immunity, choose 625 (or C276). If you are in a moderate chloride environment with low stress, 600 is sufficient and costs ~40% less. But don’t specify 600 for a hot, acidic, aerated chloride application.

Nitriding Resistance of Inconel 600 — A Hidden Advantage

In ammonia and nitrogen-bearing atmospheres at high temperature, many alloys suffer from nitriding — the inward diffusion of nitrogen atoms that form hard, brittle nitrides (CrN, Cr₂N, Fe₄N) in the surface layer. The nitrided surface is brittle, prone to spalling, and can initiate fatigue cracks. Inconel 600 has a specific advantage:

The high nickel content (~76%) makes the formation of chromium nitrides kinetically slower than in lower-nickel alloys. In a carburizing-nitriding environment (typical of ammonia synthesis gas at 500–600°C), 600 shows a nitrided case depth of ~0.1 mm after 10,000 hours, compared to ~0.3 mm for 304H and ~0.2 mm for Incoloy 800H. This is one reason 600 is specified for ammonia-plant heat-exchanger tubing and thermowells in ammonia synthesis loops.

Inconel 600 in High-Pressure Hydrogen Service — API 941

In refineries and petrochemical plants, Inconel 600 is frequently used in high-temperature, high-pressure hydrogen environments — hydroprocessing reactor internals, heat-exchanger tubing, and thermowells. API 941 (Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants) provides the Nelson curves for hydrogen attack, and 600 is immune to high-temperature hydrogen attack (HTHA) up to at least 650°C and 20 MPa hydrogen partial pressure — essentially the entire operating range of refinery hydroprocessing units.

• Why 600 for HTHA resistance? HTHA occurs when hydrogen atoms diffuse into steel and react with iron carbide (Fe₃C) to form methane (CH₄), which cannot diffuse out and builds up pressure at grain boundaries until the steel cracks. Inconel 600 has no iron carbide (it’s fully austenitic nickel-chromium solid solution with carbon tied up as stable Cr₂₃C₆ or (Cr,Fe,Ni)₇C₃), so there is no Fe₃C for hydrogen to attack.
• Comparison with 625: 625 is also immune to HTHA, but 600 costs less and is equally effective. The Mo in 625 provides no additional benefit in hydrogen service — it only adds cost. This is another example where 600 wins over 625 on a pure cost-for-performance basis.

Why Choose Huaxiao Alloy for Your Inconel 600 Procurement

Procurement Guide: Ordering Inconel 600 by Product Form

Common Mistakes When Specifying Inconel 600

Over decades of supplying nickel alloys, we’ve seen these errors repeatedly:

1. Specifying 600 when 625 is actually needed: 600 cannot handle hot, stagnant chlorides — 625 can. Don’t save 40% on material cost only to lose 100% on a premature failure.
2. Not specifying the anneal condition: “Annealed” means different things to different mills. Write “Fully solution-annealed at 1,010–1,050°C + water quenched” for critical service.
3. Forgetting the stress-relief: After heavy machining of 600 bar or plate, the residual stress can reach 300–400 MPa — enough to cause caustic SCC. Always stress-relieve at 870°C for 1 hour after final machining for caustic service.
4. Using ERNiCr-3 filler on 600 for caustic service without qualification: ERNiCr-3 has ~67% Ni, 600 has 76% Ni. The weld metal is slightly leaner in Ni than the base metal, and in concentrated hot caustic, the weld may corrode preferentially. Always qualify the complete weld procedure in a simulative environment.
5. Ignoring the nickel price volatility: Inconel 600 is ~76% nickel, so its price tracks the LME nickel price directly. When nickel spikes (as it did in March 2022 — +250% in 2 days), Inconel 600 costs go with it. Lock in pricing with a firm mill quote, not a market-index-based price.

Extended FAQ — Inconel 600 Applications and Selection

Inconel 600 in Action: Real-World Service Examples

Order Inconel 600 from Huaxiao Alloy

We stock Inconel 600 in plate (1.5–50 mm), sheet (0.5–4.8 mm), seamless tube (1/4″–8″ OD), seamless pipe (1/2″–12″ NPS), bar (6–300 mm), and welding wire (ERNiCr-3). All material is fully solution-annealed with original mill MTC to EN 10204 3.1 standard. EN 10204 3.2 available on request.

Frequently Asked Questions