Hangbo Alloy Group
Shanghai · Est. 2012
Caustic soda is the opposite problem from acid — the enemy is caustic stress corrosion cracking, not general corrosion. This guide shows why pure nickel (Nickel 200 / Nickel 201) is the only material immune to it, and how to choose the right alloy by concentration, temperature and contamination.
Caustic soda (sodium hydroxide, NaOH) is one of the highest-volume commodity chemicals in the world, produced almost entirely by chlor-alkali electrolysis and then concentrated by evaporation. As a strong alkali, its material-selection logic is fundamentally different from that of acids: the dominant failure mode is caustic stress corrosion cracking (CSCC), not general dissolution. Pick a material by its acid resistance and you can still watch a caustic evaporator crack apart.
This selection guide gives plant engineers and procurement teams a practical framework for specifying nickel alloys for caustic soda service. As a direct nickel alloy manufacturer and supplier, Hangbo Alloy Group produces and stocks Nickel 200, Nickel 201, Monel 400 and Inconel 600 in all product forms, with mill test certificates and third-party inspection.
Sodium hydroxide is a strong base, so the alloys that win in acid often lose in caustic — and vice-versa. The decisive points:
Golden rule: For caustic soda, specify the material by its immunity to caustic stress corrosion cracking and by the worst-case metal temperature — not by its acid-resistance reputation. Pure nickel (Nickel 200 below 315°C, Nickel 201 above) is the default for hot concentrated service.
Caustic resistance follows from chemistry. The caustic winners are high-nickel, low-iron compositions; stainless and carbon steel are only partial solutions limited to mild conditions.
| Alloy (UNS) | Family | Ni % | Cu % | Cr % | Fe % | C max |
|---|---|---|---|---|---|---|
| Nickel 200 (N02200) | Pure Nickel | ≥99.0 | ≤0.25 | — | ≤0.40 | 0.15 |
| Nickel 201 (N02201) | Pure Nickel (low C) | ≥99.0 | ≤0.25 | — | ≤0.40 | 0.02 |
| Monel 400 (N04400) | Ni-Cu | 63–70 | 28–34 | — | ≤2.5 | 0.30 |
| Inconel 600 (N06600) | Ni-Cr-Fe | ≥72 | ≤0.50 | 14–17 | 6–10 | 0.15 |
| Alloy 800H (N08810) | Fe-Ni-Cr | 30–35 | — | 19–23 | 39–46 | 0.10 |
The contrast tells the story: the caustic-standard alloys are essentially pure nickel, whose near-total lack of iron is exactly what makes them immune to caustic stress corrosion cracking. Chromium and copper (Inconel 600, Monel 400) add strength or impurity tolerance but are secondary to the pure-nickel base.
Typical room-temperature, annealed values help with pressure-part and structural sizing:
| Alloy | Density (g/cm³) | Tensile (MPa) | Yield (MPa) | Elongation % | Caustic Temp Limit* |
|---|---|---|---|---|---|
| Nickel 200 | 8.89 | ≥462 | ≥138 | ≥40 | ~315°C (graphitization) |
| Nickel 201 | 8.89 | ≥414 | ≥103 | ≥40 | ~427°C+ (no graphitization) |
| Monel 400 | 8.80 | ≥483 | ≥172 | ≥35 | High (caustic-tolerant) |
| Inconel 600 | 8.47 | ≥550 | ≥240 | ≥30 | High (strength-driven) |
| Alloy 800H | 8.0 | ≥450 | ≥180 | ≥30 | High-temp struct. duty |
*“Caustic temp limit” reflects the metallurgical ceiling for pure nickel in NaOH (graphitization of Nickel 200 above ~315°C; Nickel 201 avoids it). These are not corrosion limits — nickel's corrosion rate in caustic stays very low; the limit is embrittlement, governed by the actual metal/wall temperature.
The summary below reflects well-established chlor-alkali and manufacturer guidance for reasonably clean caustic. Rates illustrate typical selection bands; confirm against your exact concentration, metal temperature and impurity profile.
| NaOH Condition | Recommended Alloy | Notes |
|---|---|---|
| Dilute (≤~30–50%), low T (<~60–80°C) | 304L / 316L SS, Carbon steel | Economical; avoid if stressed + hot |
| ≤~50% NaOH, <~315°C | Nickel 200 (N02200) | Standard; immune to CSCC |
| 50–73%, 120–175°C (evaporators) | Nickel 201 (N02201) | Chlor-alkali standard |
| >73%, 175–315°C (concentrators) | Nickel 201 (N02201) | Avoid Ni200 (graphitization) |
| Need higher strength / toughness | Monel 400 / Inconel 600 | Good caustic resistance, stronger |
| Chlorides / oxidizers present | Hastelloy C-276 / C-22 | Evaluate by coupon test |
| Molten NaOH (≥~320°C) | Nickel 201 (special) | Ni200 will embrittle |
Understanding caustic SCC is the single most important step in specifying caustic-soda equipment:
Engineering practice: select the alloy by the hottest local metal temperature (tube walls and heating surfaces run hotter than the bulk liquor), and control welding residual stress on all caustic-wetted joints.
Commercially pure wrought nickel — the standard material for caustic soda service below about 315°C (600°F). Its stable NiO film gives very low corrosion rates from dilute to 100% NaOH, and it is inherently immune to caustic stress corrosion cracking. Used for storage tanks, ambient/medium-temperature piping, and low-temperature caustic handling. Limit Ni200 to below ~315°C to avoid graphitization.
The low-carbon (≤0.02% C) grade of pure nickel and the default for hot concentrated caustic. By removing carbon it eliminates graphitization, so it is used for the hottest duty: chlor-alkali evaporators (50–73% NaOH, ~120–175°C), concentrators (73–98% NaOH, up to ~315°C and beyond), heating coils and spargers. Most chlor-alkali plants specify Nickel 201 by default because of the risk of temperature excursions that would embrittle Ni200.
The nickel-copper alloy offers good caustic resistance across a broad concentration/temperature range and is often a lower-cost alternative to pure nickel for valves, pumps and many caustic-handling components. It tolerates some impurities that can affect pure nickel and resists caustic SCC far better than steel or stainless. For the hottest, most concentrated evaporator service, pure nickel (Nickel 201) remains first choice.
Nickel-chromium-iron alloy chosen when the design needs greater strength, heating-coil performance or improved tolerance of certain sulfur-bearing contaminants. It resists caustic SCC much better than steel/stainless but, like all nickel alloys, can be susceptible to cracking in severe hot concentrated caustic if stress and metallurgical condition are not controlled — so it is qualified on a case-by-case basis.
Alloy 800H is used where caustic service coincides with high structural temperature. When the NaOH stream is contaminated with chlorides, chlorates, hypochlorites or heavy-metal ions, the protective nickel-oxide film can break down; in those cases Hastelloy C-276 (UNS N10276) or C-22 (UNS N06022) is evaluated, ideally using representative coupon corrosion testing.
| Your Caustic Service Condition | First Choice | Upgrade When… |
|---|---|---|
| Dilute, low T (<~60–80°C) | 304L / 316L, Carbon steel | Hot/stressed → Nickel 200/201 |
| ≤~50% NaOH, <~315°C | Nickel 200 (N02200) | Hotter → Nickel 201 |
| 50–73% NaOH, evaporators | Nickel 201 (N02201) | Molten → Ni201 special |
| >73%, concentrators | Nickel 201 (N02201) | Ni200 forbidden (embrittle) |
| Need higher strength / toughness | Monel 400 / Inconel 600 | Severe hot conc. → Ni201 |
| Chlorides / oxidizers present | Hastelloy C-276 / C-22 | Coupon-test first |
| Pure acid instead (HCl/HNO₃/H₂SO₄) | Inconel 625 / C-276 / 690 | Only if NOT caustic |
In chlor-alkali plants, membrane-cell liquor is produced at roughly 33% NaOH and evaporated to ~50% commercial product; diaphragm-cell and concentrator service push concentrations to 73% and beyond, with heating surfaces running well above the bulk temperature. This is exactly the regime where material selection must be zone-by-zone and where pure nickel wins:
Hangbo Alloy Group manufactures and supplies caustic-service alloys in a full range of forms, with mill test certificates and third-party inspection (SGS, TÜV) available:
| Alloy (UNS / W.Nr) | Plate | Rod/Bar | Pipe/Tube | Forgings |
|---|---|---|---|---|
| Nickel 200 (N02200 / 2.4060) | ASTM B162 | ASTM B160 | ASTM B161/B163 | ASTM B564 |
| Nickel 201 (N02201 / 2.4068) | ASTM B162 | ASTM B160 | ASTM B161/B163 | ASTM B564 |
| Monel 400 (N04400 / 2.4360) | ASTM B127 | ASTM B164 | ASTM B165 | ASTM B564 |
| Inconel 600 (N06600 / 2.4816) | ASTM B168 | ASTM B166 | ASTM B167/B163 | ASTM B564 |
| Alloy 800H (N08810 / 1.4958) | ASTM B409 | ASTM B408 | ASTM B407 | ASTM B564 |
All grades are also referenced by EN/DIN (W.Nr.), ISO, AMS and GB standards, and are covered by ASME pressure-vessel codes. For export projects, always specify the UNS number plus the applicable ASTM/EN/JIS execution standard (e.g. ASME SB160–SB163 for Nickel 200/201). Welding of Nickel 200/201 is qualified with ERNi-1 (Nickel 61) filler per AWS A5.14 / A5.11.
Not sure whether your caustic service calls for Nickel 200, Nickel 201, Monel 400 or Inconel 600 — or whether stainless/carbon steel is sufficient for your dilution? Send us your concentration, metal temperature and impurity profile and our metallurgists will recommend the most cost-effective certified alloy. As a direct manufacturer and supplier, we offer competitive price quotes, full material certification and worldwide delivery.
Email: [email protected] | Phone / WhatsApp: +86-136-1165-6360
For caustic soda, commercially pure nickel is the benchmark. Nickel 200 (UNS N02200) is specified for caustic service below about 315°C (600°F); Nickel 201 (UNS N02201), the low-carbon grade, is used wherever the metal temperature can reach or exceed 315°C because its low carbon content eliminates graphitization (carbon embrittlement). Pure nickel is uniquely immune to caustic stress corrosion cracking (CSCC), which destroys carbon steel and attacks austenitic stainless steels in hot concentrated NaOH. Monel 400, Inconel 600 and Hastelloy grades are used for specific conditions (strength, impurities, or chloride/oxidizer contamination).
Pure nickel forms a stable, self-healing, tenacious nickel-oxide (NiO) film in caustic environments that protects it across the full concentration range, from dilute to 100% molten NaOH. Critically, its face-centered-cubic structure and near-total lack of iron make it inherently immune to caustic stress corrosion cracking — the failure mode that embrittles carbon steel and cracks stainless steel in hot NaOH. Nickel also resists caustic embrittlement, so corrosion rates stay very low (typically well under 0.025 mm/year / 1 mpy) even in 50–73% caustic at elevated temperature.
Caustic stress corrosion cracking (also called caustic embrittlement) is a cracking failure that occurs when a susceptible metal is exposed to hot, concentrated sodium hydroxide under tensile stress. Carbon steel and many stainless steels crack in this combination, often with little warning. Pure nickel (Nickel 200/201) is immune because it does not undergo the metallurgical mechanism that drives CSCC. Nickel-copper (Monel 400) and nickel-chromium (Inconel 600) alloys also resist caustic SCC far better than steel or stainless, though hot concentrated caustic can still cause cracking in nickel alloys if residual stress and metallurgical condition are not controlled.
The only meaningful difference is carbon content: Nickel 200 has up to 0.15% C; Nickel 201 has a maximum of 0.02% C. Above about 315°C (600°F), the carbon in Nickel 200 can precipitate as graphite at grain boundaries — a phenomenon called graphitization that embrittles the metal and can cause sudden failure. Nickel 201 removes this risk. Rule of thumb: use Nickel 200 below ~315°C (storage tanks, ambient/medium-temperature piping) and Nickel 201 for ≥315°C or for any chlor-alkali evaporator/concentrator where temperature excursions are possible. In practice most chlor-alkali plants specify Nickel 201 by default.
Yes, but only within limits. Austenitic stainless steels such as 304L and 316L are acceptable for dilute caustic (typically below about 30–50% NaOH) at moderate temperatures (below roughly 60–80°C) and where stress is low. As concentration and temperature rise, stainless becomes susceptible to caustic stress corrosion cracking. Stainless is common for mild process-water and dilute caustic lines in chlor-alkali plants, but it must NOT be relied upon for hot concentrated caustic — that is pure-nickel territory.
Carbon steel is widely used for caustic service, but only in a limited band: relatively low concentration and temperature, and especially for sodium hydroxide below about 50% concentration at ambient-to-moderate temperature. Carbon steel suffers caustic embrittlement (a form of stress corrosion cracking) and accelerated general corrosion as concentration and temperature increase. For hot concentrated caustic — the heart of chlor-alkali evaporation — carbon steel is unsuitable and pure nickel (Nickel 200/201) is specified.
Yes. Monel 400 (UNS N04400), the nickel-copper alloy, has good resistance to caustic soda across a wide range of concentrations and temperatures and is often a lower-cost alternative to pure nickel for many caustic services. It resists caustic SCC far better than steel or stainless. In chlor-alkali and chemical plants Monel 400 is used for valves, pumps and components handling caustic, and it tolerates some impurities that can affect pure nickel. For the hottest, most concentrated evaporator service, pure nickel (Nickel 201) remains the first choice.
As a practical guideline: Nickel 200 is used for caustic up to about 315°C (600°F); Nickel 201 extends well beyond that (often cited to ~427°C / 800°F and higher depending on pressure and concentration) because of its low carbon content. For membrane-cell liquor evaporated from ~33% to 50% NaOH, and diaphragm-cell/concentrator service to 73% and above, Nickel 201 is the standard. The limiting factor for pure nickel is not corrosion rate (which stays very low) but, for Ni200, graphitization above ~315°C. Always size to the worst-case metal temperature, not the nominal bulk temperature, because tube walls and heating surfaces run hotter than the liquor.
Graphitization is the precipitation of carbon (as graphite films) at grain boundaries in Nickel 200 when it is held for prolonged periods above roughly 315°C (600°F). The graphite films weaken the grain boundaries, embrittle the material and can lead to sudden cracking under stress. This is why Nickel 201 — with its maximum 0.02% carbon — is specified for all hot concentrated caustic service. The ~315°C boundary is a graphitization limit, not a universal corrosion or design-temperature rating; below it, Nickel 200 is perfectly sound.
Pure nickel's caustic resistance assumes a reasonably clean caustic stream. If the NaOH is contaminated with strong oxidizers (chlorates, hypochlorites) or heavy-metal ions, or with significant chlorides, the protective nickel-oxide film can break down and localized corrosion accelerates. In those cases, a high-chromium-molybdenum nickel alloy such as Hastelloy C-276 (UNS N10276) or Hastelloy C-22 (UNS N06022) is evaluated, and representative coupon corrosion testing or documented service evidence should drive the final choice rather than a generic 'clean-caustic' chart.
Nickel 200 = UNS N02200 (W.Nr 2.4060); Nickel 201 = UNS N02201 (W.Nr 2.4068); Monel 400 = UNS N04400 (W.Nr 2.4360); Inconel 600 = UNS N06600 (W.Nr 2.4816); Alloy 800H = UNS N08810 (W.Nr 1.4958); Hastelloy C-276 = UNS N10276 (W.Nr 2.4819); Hastelloy C-22 = UNS N06022 (W.Nr 2.4602). These grades are cross-referenced in ASTM/ASME, EN/DIN (W.Nr.), ISO, GB (China) and AMS standards. For export projects always specify the UNS number plus the applicable ASTM/EN/JIS execution standard.
Nickel 200 and 201 are welded with ERNi-1 (AWS A5.14) / ENi-1 (AWS A5.11) filler metal — the consumable known as Nickel 61 — which matches the base metal, gives matching corrosion resistance and stays ductile in the as-welded condition. GTAW/TIG is preferred for thin sections, GMAW/MIG and SMAW for heavier plate and field joints. Pure nickel is sensitive to sulfur, lead and phosphorus contamination, so degrease thoroughly, use dedicated nickel tools, control heat input (stringer beads, low interpass temperature) and use pure argon shielding. Monel 400 is welded with ENiCu-2/ERNiCu-7 and Inconel 600 with ERNiCr-3.
Send your caustic concentration, metal temperature, any impurities (chlorides, oxidizers, metal ions), pressure and required form to Hangbo Alloy Group at [email protected] or call/WhatsApp +86-136-1165-6360. As a direct manufacturer and supplier we provide competitive price quotes, mill test certificates, expert grade selection (Nickel 200 vs 201, Monel 400, etc.) and worldwide delivery of bars, plates, tubes, forgings and the matching welding consumables.
For help choosing between Nickel 200, Nickel 201, Monel 400 and Inconel 600 for your caustic soda service — or for a quotation on bars, plates, tubes, forgings or welding consumables — contact Hangbo Alloy Group. We typically respond within 10 minutes and ship worldwide from our Shanghai facility.
Email: [email protected]
Phone: +86-136-1165-6360
WhatsApp: +86 13611656360
Send us your concentration, metal temperature and impurity profile and we will recommend the most cost-effective certified nickel alloy — with a competitive quote and full material certification.