Nickel 200 vs 201 vs Monel 400 for Caustic Service

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: Nickel 200, Nickel 201 or Monel 400 - Which Do I Need?

Nickel 200 and Nickel 201 are commercially pure nickel for caustic and chlor-alkali service and differ only in carbon: 201 is the low-carbon grade for service above about 315 C, where 200 graphitises. Monel 400 is a nickel-copper alloy chosen for seawater, aerated chlorides and hydrofluoric acid. Match the grade to the medium and the temperature.

Key Takeaways

• Carbon is the only meaningful difference between 200 and 201. Both are commercially pure nickel with a minimum 99.0 % nickel, but Nickel 200 permits up to 0.15 % carbon while Nickel 201 is capped at 0.02 %; that cap is what makes 201 usable above about 315 C.
• Nickel 200 graphitises above about 315 C. In the 315-540 C band, carbon in Nickel 200 precipitates as graphite at grain boundaries and embrittles the metal, so 200 is not the grade for hot caustic above that ceiling.
• Monel 400 is the chloride and seawater grade. Its nickel-copper matrix resists aerated chlorides and seawater far better than pure nickel, at the cost of some caustic performance and a serious sensitivity to mercury contamination.
• Nickel is the wet-chlorine and chlor-alkali material. Both 200 and 201 are standard choices for wet chlorine gas and chlor-alkali cell components, while Monel 400 is attacked by wet chlorine.
• None of the three is an oxidising-acid alloy. All three perform poorly in nitric acid and in strongly oxidising media, and selection for those duties must move to a different family.
• No post-weld heat treatment is normally required. All three weld with matching fillers and require no PWHT, but carbon control, filler selection and cleanliness of the joint are critical.

What Are Nickel 200, Nickel 201 and Monel 400?

Nickel 200 (UNS N02200) and Nickel 201 (UNS N02201) are commercially pure wrought nickel, both specified with a minimum of 99.0 % nickel, and they are metallurgically the same material with one deliberate difference: the carbon specification. They share a face-centred-cubic nickel matrix, they are ferromagnetic at room temperature, they have high thermal and electrical conductivity for a corrosion-resistant metal, and they are used in caustic evaporators, chlor-alkali plant, electronic components, food-processing equipment and chemical process vessels. Our Nickel 200/201 supplier page documents the bar, plate, tube and wire forms we stock and certify against the ASTM product standards.

Monel 400 (UNS N04400) is a nickel-copper alloy containing roughly 63-70 % nickel and 28-34 % copper, and it is a different material family rather than a variant of pure nickel. It is stronger than Nickel 200 or 201, it is only weakly magnetic, and it combines good resistance to seawater, aerated chlorides and hydrofluoric acid with useful resistance to caustic and to many reducing acids. It is the classic alloy for marine hardware, pump shafts, valve trim in chloride service, and for heat exchangers handling brackish or seawater cooling. Our Monel alloy round bar & tube page covers the forms and standards in which we supply it.

The three grades are frequently specified interchangeably by specification writers who see "nickel alloy, corrosion resistant" and do not distinguish between pure nickel and nickel-copper, and that is the source of a large share of the field failures we are asked to review. They are not interchangeable. Pure nickel and Monel 400 behave differently in caustic, differently in seawater, differently in wet chlorine and differently in mercury-contaminated streams, and the carbon difference between 200 and 201 makes one of the two unsuitable above a hard temperature ceiling. This article sets out the differences in composition, properties, corrosion behaviour and fabrication, and closes with explicit selection rules.

It is worth stating the metallurgical relationship plainly at the outset, because it explains every difference that follows. Nickel 200 and 201 are the same solid-solution metal, and their corrosion and mechanical behaviour at ambient temperature are practically identical; the only thing that changes between them is the temperature at which carbon becomes mobile. Monel 400 is a solid-solution alloyed material in which the copper additions change the electrochemical behaviour of the surface, improving resistance to chlorides and to seawater while reducing resistance to the strongly alkaline and the strongly oxidising extremes. If you hold those two facts, the selection rules at the end of this article follow automatically.

The Decisive Difference: Carbon and the Graphitisation Ceiling

The single most important fact in this comparison is that Nickel 200 is limited to about 315 C (600 F) in sustained service while Nickel 201 can be used well above that temperature. The reason is graphitisation. At temperatures above roughly 315 C, the carbon dissolved in Nickel 200 becomes mobile enough to precipitate as graphite at grain boundaries, and the resulting intergranular film embrittles the metal so that it can crack under load or thermal cycling without warning. Because the failure is intergranular and the metal looks unaffected from the outside, a graphitised Nickel 200 component can fail in a way that a visual inspection will not predict.

Nickel 201 solves the problem by reducing the carbon to a maximum of 0.02 %. At that level there is insufficient carbon in solution to form a continuous grain-boundary graphite network at service temperature, so the alloy retains its ductility through the elevated-temperature range where Nickel 200 would embrittle. The practical consequence is a clean division of duties: Nickel 200 for ambient and low-temperature service in any of its normal media, and Nickel 201 for the same media whenever the metal temperature exceeds about 315 C. This is precisely why caustic evaporators, which run hot, are normally built in Nickel 201 while caustic storage and ambient-temperature handling are built in either grade.

Property / limit	Nickel 200	Nickel 201	Monel 400	Basis
Carbon, max (wt %)	0.15	0.02	0.30	ASTM B160 / ASTM B164
Practical ceiling for sustained service	~315 C	~600 C (dry, non-oxidising)	~480-540 C	typical, published guidance
Graphitisation risk above ceiling	Yes, above ~315 C	Effectively eliminated	Not the controlling mechanism	published metallurgy
Controlling high-temperature limit	Graphitisation	Oxidation and creep	Sulphur and oxidation	typical, published guidance

Table note: The carbon limits are standard composition limits from ASTM B160 for Nickel 200 and Nickel 201 and from ASTM B164 for Monel 400 (latest editions). The temperature ceilings are typical published service guidance and are not standard requirements; the actual permissible temperature depends on the environment, the stress and the required life, and must be confirmed against the design code and the corrosion data for the specific medium.

Two practical warnings follow from this. First, a specification that calls up "Nickel 200" without a temperature qualifier will fail on a hot caustic duty, and the substitution to Nickel 201 is not optional. Second, the carbon difference is not visible in service and is difficult to detect in a finished part, so the grade must be confirmed from the mill certificate and, ideally, by a carbon analysis on the delivered material when the duty is temperature-critical. We report carbon as a separate line item on every Nickel 200 and Nickel 201 certificate for exactly this reason, because the two grades are otherwise almost indistinguishable by field inspection.

The Monel 400 carbon limit of 0.30 % looks high by comparison and occasionally alarms a specification writer, but it is not a graphitisation risk in the same way, because Monel 400 is not used in the same temperature band for the same reason. Where the metal temperature is high enough to matter, the controlling mechanisms for Monel 400 are oxidation and, above all, sulphur attack, not carbon precipitation. This is a good example of why a limit should always be read together with the mechanism and the intended service rather than as an isolated number.

Chemical Composition and Product Standards

The composition limits of the three grades are set out below together with the ASTM product standards that govern them in bar, plate and tube form. The nickel grades are essentially pure nickel with tightly controlled residuals, while Monel 400 is a deliberate nickel-copper alloy with an iron addition that improves its resistance to seawater and aerated chlorides.

Element (wt %)	Nickel 200 (N02200)	Nickel 201 (N02201)	Monel 400 (N04400)	Per standard
Nickel	99.0 min	99.0 min	63.0-70.0	ASTM B160 / ASTM B164
Copper	0.25 max	0.25 max	28.0-34.0	ASTM B160 / ASTM B164
Iron	0.40 max	0.40 max	2.50 max	ASTM B160 / ASTM B164
Carbon	0.15 max	0.02 max	0.30 max	ASTM B160 / ASTM B164
Manganese	0.35 max	0.35 max	2.00 max	ASTM B160 / ASTM B164
Silicon	0.35 max	0.35 max	0.50 max	ASTM B160 / ASTM B164
Sulphur	0.010 max	0.010 max	0.024 max	ASTM B160 / ASTM B164

Table note: Values are standard composition limits from ASTM B160 (nickel rod and bar) for Nickel 200 and Nickel 201, and from ASTM B164 (nickel-copper alloy rod, bar and wire) for Monel 400, latest editions. The carbon line is the only composition difference between Nickel 200 and Nickel 201 and is the reason for the different temperature ceilings discussed above. Do not transfer a limit between standards without checking the governing edition.

The product standards that apply to each form are as important as the composition limits, because a heat that is compliant for bar may not be compliant for tube, and the mechanical property minima differ between product forms. Nickel 200 and Nickel 201 bar and rod are covered by ASTM B160, plate, sheet and strip by ASTM B162, and seamless pipe and tube by ASTM B161, with condenser and heat-exchanger tube requirements falling under ASTM B163. Monel 400 bar and rod are covered by ASTM B164, plate, sheet and strip by ASTM B127, and seamless pipe and tube by ASTM B165, with forgings covered by ASTM B564. The same requirements are adopted for pressure-service use through the ASME SB-series in Section II of the ASME Boiler and Pressure Vessel Code.

Product form	Nickel 200 / 201	Monel 400	ASME equivalent
Rod and bar	ASTM B160	ASTM B164	ASME SB-160 / SB-164
Plate, sheet, strip	ASTM B162	ASTM B127	ASME SB-162 / SB-127
Seamless pipe and tube	ASTM B161	ASTM B165	ASME SB-161 / SB-165
Condenser / heat-exchanger tube	ASTM B163	ASTM B163	ASME SB-163
Forgings	Consult specification	ASTM B564	ASME SB-564

Table note: Standard scopes are summarised from the published documents (latest editions). ASTM B163 covers seamless nickel and nickel-alloy condenser and heat-exchanger tubes and therefore applies to both material families in that specific form. For pressure-retaining components the designer should refer to the ASME Code allowable stress values rather than to the raw ASTM document, because the Code is where the design allowables are published.

The cross-system reference deserves a caution that applies throughout this article. In the Chinese system, commercially pure wrought nickel grades such as N6 correspond closely to Nickel 200 and the lower-carbon grades correspond closely to Nickel 201, but a Chinese grade designation is not an ASTM equivalent. Its composition and property limits are written in its own standard and must never be substituted line for line for the ASTM limits in the tables above. Where a drawing originates in China and will be manufactured to an ASTM specification, or vice versa, the conversion must be made explicitly, with the source system identified in its own column, and the delivered material should be verified against the specification actually named on the purchase order.

Mechanical Properties

The three grades are all relatively soft, high-ductility materials used in the annealed condition, and none of them is a high-strength alloy in the sense that Inconel 718 or Monel K-500 is. Nickel 200 and Nickel 201 have similar strength, with Nickel 201 slightly the softer and more ductile of the two because of its lower carbon. Monel 400 is the strongest of the three, and its nickel-copper solid solution gives it an appreciably higher yield strength than the pure nickel grades, which is one reason it is preferred for structural marine components where a pure nickel part would simply be too soft.

Grade, condition	UTS	0.2 % yield	Elongation	Hardness	Basis
Nickel 200, annealed	~462 MPa (typical)	~148 MPa (typical)	~45 % (typical)	~75 HRB (typical)	typical, not a standard minimum
Nickel 201, annealed	~403 MPa (typical)	~103 MPa (typical)	~50 % (typical)	~72 HRB (typical)	typical, not a standard minimum
Monel 400, annealed	~550 MPa (typical)	~240 MPa (typical)	~40 % (typical)	~120-150 HB (typical)	typical, not a standard minimum

Table note: The values shown are typical published annealed properties and are explicitly not standard minima; property minima for a specific product form are fixed by the governing ASTM product standard named on the purchase order (ASTM B160, B162 and B161 for the nickel grades and ASTM B164, B127 and B165 for Monel 400). Published annealed values for Monel 400 vary between sources, so where the property is design-critical, confirm the acceptance minima from the applicable product specification rather than from a datasheet. Mechanical testing is performed to ASTM E8 at room temperature and to ASTM E21 at elevated temperature, and hardness to ASTM E18 or ASTM E10.

Cold work is a significant variable for all three grades and is worth understanding when a drawing calls out a temper rather than an annealed condition. Nickel 200, Nickel 201 and Monel 400 all work harden readily, so a cold-drawn or cold-rolled product can be supplied at a substantially higher strength and lower ductility than the annealed figures in the table. That is useful when higher strength is needed, but it also means that a strength requirement and a ductility or formability requirement can conflict, and the specification must state which condition is required. For corrosion service, the annealed condition is generally preferred because a cold-worked surface can have different electrochemical behaviour and because residual stress can contribute to stress-corrosion cracking in some environments.

One further property distinguishes the three grades in a way that decides real applications: magnetic behaviour. Nickel 200 and Nickel 201 are ferromagnetic at room temperature, with a Curie temperature well above ambient, while Monel 400 is only weakly magnetic and is essentially non-magnetic at the levels that matter for most instrument work. Where magnetism is acceptable or irrelevant, as in chemical process equipment, this is unimportant; where it is not, for example in certain electronic or instrument components that must not distort a magnetic field, the choice may be driven by permeability rather than by corrosion. Conductivity is the mirror image of the same physics: the pure nickel grades are notably more conductive than Monel 400, which is why Nickel 200 and 201 appear in electrical and electronic applications where Monel 400 does not.

Corrosion in Caustic Soda: Concentration and Temperature

Nickel is the reference material for caustic soda service, and the reason is that it resists both the general corrosion and the caustic stress-corrosion cracking that limit carbon steel and many stainless steels. Caustic soda (sodium hydroxide) is aggressive to carbon steel above roughly 50 % concentration and at elevated temperature, and it causes caustic stress-corrosion cracking in carbon steel and in some stainless steels under stress in the affected concentration and temperature ranges. Nickel 200 and Nickel 201 resist caustic across the full concentration range and are the standard materials for caustic evaporators, where the concentration is high and the temperature is well above ambient.

Medium / condition	Nickel 200	Nickel 201	Monel 400	Practical note
NaOH below 50 %, ambient to ~90 C	Excellent	Excellent	Good	All three acceptable
NaOH 50-75 %, up to ~150 C	Excellent	Excellent	Good	Nickel preferred
NaOH above 75 %, boiling	Excellent	Excellent	Fair	Nickel is the standard choice
NaOH service above ~315 C	Not recommended	Good	Fair	Graphitisation limits Nickel 200
Caustic with chlorides present	Good	Good	Very good	Monel 400 tolerates the chlorides better
Wet chlorine gas	Excellent	Excellent	Not recommended	Monel 400 is attacked
Dry chlorine gas	Good	Good	Good	Dry gas is far less aggressive
Hydrofluoric acid, aqueous and anhydrous	Excellent	Excellent	Good to excellent	Both families used in HF service
Seawater, flowing	Good	Good	Excellent	Monel 400 is the marine grade
Seawater, stagnant and aerated	Fair (pitting)	Fair (pitting)	Very good	Pure nickel is prone to pitting
Aerated chlorides generally	Fair	Fair	Good	Chloride SCC resistance favours Monel
Reducing acids (dilute HCl, deaerated H2SO4)	Good	Good	Fair to good	All three are reducing-acid tolerant
Oxidising acids (nitric acid)	Poor	Poor	Poor	None of the three is suitable
Mercury or mercury-contaminated stream	Use with caution	Use with caution	Not recommended	Mercury embrittles Monel 400
Food processing and potable water	Excellent	Excellent	Good	Nickel grades are widely used

Table note: Ratings are qualitative engineering guidance drawn from published corrosion data and our own field experience; they are not the result of a corrosion test on a specific heat and they do not replace one. Where a corrosion rate must be guaranteed for a specific medium, temperature and concentration, confirm it by immersion or electrochemical testing (for example under ASTM G31 for laboratory immersion corrosion testing, or the relevant ASTM G48 method where pitting resistance is the question), and review the result against the design allowance.

The caustic picture has one important subtlety that a specification based only on concentration will miss. Monel 400 performs well in caustic at moderate concentration and temperature, but its margin narrows as the concentration rises, and for the highest concentrations and the hottest duties pure nickel is clearly superior. Conversely, when chlorides are present alongside the caustic, as they frequently are in chlor-alkali and in some process streams, Monel 400 tolerates the chloride component better than pure nickel does. So the selection for a caustic duty is not simply a matter of picking the material with the best caustic rating; it is a matter of asking whether the stream is chloride-free. A chloride-free hot concentrated caustic duty is a pure nickel duty, and specifically a Nickel 201 duty above 315 C. A caustic duty with meaningful chloride contamination often points to Monel 400 despite the lower nominal caustic rating.

Temperature and concentration also interact with the design of the equipment in ways that affect material choice. Caustic evaporators and concentrators typically run at the highest combination of temperature and concentration in the plant, and they are the components most likely to be built in Nickel 201 for exactly that reason. Storage tanks, transfer lines and ambient-temperature handling run cooler and are frequently built in Nickel 200, which is the more economical grade when the temperature ceiling is respected. Where a single material is wanted across the whole battery limit, Nickel 201 is the safer single choice because it covers the full temperature range, and the incremental cost over Nickel 200 is modest relative to the cost of a mis-specified hot section. This is one of the few cases in materials selection where standardising on the more capable grade is usually the right commercial decision.

Chlor-Alkali, Wet Chlorine and Hydrofluoric Acid

Wet chlorine is the environment that most clearly separates pure nickel from Monel 400, and it is the reason Nickel 200 and 201 dominate chlor-alkali plant. Wet chlorine gas is highly aggressive, and pure nickel resists it well, which is why nickel anodes and cell components, wet-chlorine handling equipment and chlor-alkali piping are commonly specified in Nickel 200 or 201. Monel 400 does not resist wet chlorine well: the copper in the alloy is attacked and the material corrodes rapidly in that service. A specification that reads "nickel alloy for wet chlorine" and selects Monel 400 because it is a nickel alloy will fail quickly and visibly, which is at least a fast failure rather than a slow one.

The chlor-alkali process also brings together the two extremes that drive the choice within this family. The cell environment combines wet chlorine, which demands pure nickel, with caustic liquor and, in many plants, chloride-bearing streams. Where the wet chlorine is the controlling environment, pure nickel wins. Where a chloride-bearing caustic stream at moderate temperature is the controlling environment, Monel 400 can be the better answer. The correct approach is to break the plant down by stream, temperature, concentration and chloride content, and to select per stream rather than per plant; a single-material policy across a chlor-alkali battery limit usually represents a compromise that is worse than the correct material in each stream.

Hydrofluoric acid is the other environment where both families appear, and it is a strong point for Nickel 200 and 201. Nickel is highly resistant to hydrofluoric acid, both aqueous and anhydrous, and it is a standard material for HF alkylation equipment and for HF handling in the refining and chemical industries. Monel 400 also performs well in HF and has been widely used in HF service where its greater strength or its chloride tolerance is wanted. Both families are therefore credible for HF duty, and the selection between them comes down to the other conditions in the stream: if chlorides or seawater are also present, or if greater strength is needed, Monel 400 has the advantage; if the stream is a hot concentrated HF duty free of chlorides, pure nickel is the straightforward choice.

Environment	Nickel 200 / 201	Monel 400	Selection driver
Wet chlorine gas and chlor-alkali cells	Preferred	Not suitable	Pure nickel is the material for wet chlorine
Dry chlorine gas	Acceptable	Acceptable	Both used where dry
Hydrofluoric acid, aqueous or anhydrous	Excellent	Good to excellent	Nickel is the classic HF material
HF with chloride contamination	Good	Better	Monel 400 tolerates chlorides
Caustic with chlorides	Good	Very good	Monel 400 for chloride-bearing caustic
Chloride-free hot caustic	Excellent	Fair	Pure nickel, Nickel 201 above 315 C

Table note: Ratings are qualitative engineering guidance from published data and do not replace testing for a specific stream. The distinguishing factor between the two families in this table is the electrochemical effect of the copper addition in Monel 400, which improves chloride tolerance and degrades performance in wet chlorine and in the most concentrated hot caustic.

Seawater, Aerated Chlorides, Acids and Mercury Contamination

Monel 400 is the marine member of this family and is the reason the alloy exists commercially. It resists flowing seawater well, it resists aerated and stagnant chlorides far better than pure nickel, and, importantly, it is highly resistant to chloride stress-corrosion cracking, which is the failure mode that removes many higher-strength materials from seawater duty. Pure nickel is broadly acceptable in flowing seawater but is prone to pitting in stagnant, aerated conditions, so the distinction between the two is not simply "Monel is better in seawater" but "Monel is better specifically where the seawater is stagnant, aerated or chloride-concentrated."

Hydrofluoric acid and seawater are the two classic Monel 400 duties, and marine hardware is the third. Pump shafts, valve stems, propeller-shaft components, marine fasteners and heat-exchanger tubing in brackish and seawater cooling service are all Monel 400 applications where its combination of strength, ductility and chloride resistance has no cheaper equal in the nickel family. Where higher strength is required for the same chloride duty, Monel K-500, the age-hardenable version of the same alloy, is the next step; it is outside the scope of this comparison but is a routine extension of it, and the same restriction on mercury contamination applies to it even more strongly because of its higher strength.

Mercury is the exception that must never be overlooked with Monel 400. Mercury and mercury-contaminated streams cause rapid embrittlement and cracking of Monel 400, and the alloy must not be used in mercury-contaminated service. This matters in practice because mercury can enter a process from trace impurities, from instrument seals, from contaminated feedstock or from upstream equipment, and because a plant that was designed for Monel 400 on the basis of its chloride resistance can be destroyed by a mercury excursion that no one anticipated. If there is any credible mercury source in the stream, Monel 400 is the wrong choice and pure nickel is markedly safer, though even nickel requires a review of the specific conditions. Any Monel 400 application should include an explicit check for mercury in the process.

Environment / contaminant	Nickel 200	Nickel 201	Monel 400
Flowing seawater	Good	Good	Excellent
Stagnant, aerated seawater	Fair, pitting risk	Fair, pitting risk	Very good
Aerated chlorides	Fair	Fair	Good
Chloride stress-corrosion cracking	Resistant	Resistant	Highly resistant
Seawater plus mercury present	Caution, review required	Caution, review required	Not recommended
Mercury-contaminated stream	Use with caution	Use with caution	Not recommended
Nitric acid, oxidising	Poor	Poor	Poor
Deaerated reducing acids	Good	Good	Fair to good

Table note: Ratings are qualitative engineering guidance from published corrosion data and our field experience, and they do not replace corrosion testing for a specific stream composition. The mercury line is a hard restriction rather than a gradual preference: Monel 400 embrittles in mercury-contaminated service and must not be specified where mercury may be present.

The acid behaviour of the three grades follows the same electrochemical logic and is easy to remember. All three resist reducing acids reasonably well, because their corrosion resistance in the absence of oxygen does not depend on a passive oxide film that requires oxidising conditions to form. All three perform poorly in strongly oxidising acids such as nitric acid, because none of them can form the protective passive film that a chromium-bearing stainless steel or a Hastelloy alloy forms. The practical rule is therefore simple: these three grades are caustic, chloride and reducing-acid materials, and for oxidising acid service the selection must move to a chromium-bearing or molybdenum-bearing alloy from a different family. Our alloy technical knowledge center covers the corrosion-alloy families that take over where the nickel grades reach their limit.

Fabrication, Welding and Dissimilar Joints

All three grades weld readily and none of them normally requires a post-weld heat treatment, which is a significant fabrication advantage over the precipitation-hardening nickel alloys and over the low-alloy steels. Nickel 200 and Nickel 201 are welded with matching nickel filler metals, and Monel 400 is welded with matching nickel-copper filler metals. The welds are ductile, they tolerate thermal cycling well, and they do not require the solution treatment and ageing that an alloy such as Inconel 718 requires after welding. That simplicity is one of the reasons pure nickel and Monel 400 remain so widely used in chemical plant despite the availability of stronger alloys.

Process step	Nickel 200 / 201	Monel 400	Practical note
Annealing	~700-925 C typical	~870-980 C typical	typical commercial cycles
Stress relief	~300-500 C typical	~540-600 C typical	typical commercial practice
Welding filler	Matching nickel filler	Matching nickel-copper filler	select per specification
Post-weld heat treatment	Not normally required	Not normally required	confirm against the procedure
Carbon control in welding	Critical for heat-affected zone	Not the controlling issue	specification requirement
Surface cleanliness	Sulphur and lead free	Sulphur and lead free	in-house requirement, not a standard requirement

Table note: The annealing and stress-relief temperatures are typical commercial cycles and the exact cycle is fixed by the governing specification and the process requirement. Welding consumables are selected to match the base material per the applicable specification. The requirement to exclude sulphur and lead from marking materials, lubricants and shop contamination at the joint is a fabrication requirement rather than a standard limit, but it is not optional for these alloys.

Two fabrication issues deserve emphasis for these specific materials. The first is sulphur and lead contamination. Nickel and nickel-copper alloys are embrittled by sulphur and by lead at elevated temperature, so marking crayons, lubricants, cutting fluids and shop dirt that contain those elements must be kept away from the joint before and during welding and heat treatment. A weld that is metallurgically sound but contaminated with sulphur at the surface can crack during subsequent high-temperature service. This is a housekeeping discipline rather than a material property, and it is one of the most common causes of avoidable failures in nickel equipment.

The second is carbon control during welding of the Nickel 200 and Nickel 201 family, which matters more than it first appears. Because the entire distinction between the two grades is carbon, a welding procedure that introduces carbon into the heat-affected zone of a Nickel 201 component can locally defeat the purpose of specifying the low-carbon grade. This is why matching low-carbon filler and clean, controlled welding procedures matter for Nickel 201 in hot caustic service, and why the filler metal should be selected deliberately rather than as "any nickel filler." For Nickel 200 in ambient-temperature service the same care is less critical, because the graphitisation mechanism is not active at the service temperature.

Dissimilar joints between these materials and other alloys are common and are generally straightforward, with one design consideration that is often overlooked. These nickel grades are frequently used as transition materials between carbon or low-alloy steel and higher alloys, because they weld reliably to steel and provide a ductile, corrosion-resistant interface. The difference in thermal expansion between the nickel grade and the steel must nonetheless be considered in the joint design, because thermal cycling of a constrained dissimilar joint produces stresses at the interface that a simple strength calculation will not reveal. Monel 400 is likewise welded to steel in marine and process applications, and the same expansion consideration applies. Where the joint will see significant thermal cycling, the design should account for the expansion mismatch explicitly rather than relying on the ductility of the weld alone.

Decision Matrix: Which Grade for Which Duty?

The selection rules for these three grades reduce to a short sequence of questions, and the matrix below maps the common duties to a recommendation. The logic is consistent throughout: pure nickel where the medium is caustic, wet chlorine or hydrofluoric acid and the stream is chloride-free, with Nickel 201 taking over above about 315 C; Monel 400 where seawater, aerated chlorides or chloride-bearing caustic dominate and no mercury is present.

Application / duty	Recommended grade	Why
Caustic evaporators, high concentration	Nickel 200 or 201	Best caustic resistance; 201 if hot
Caustic service above ~315 C	Nickel 201	Low carbon prevents graphitisation
Caustic storage and ambient handling	Nickel 200	Adequate below the carbon ceiling, economical
Chlor-alkali cells and wet chlorine handling	Nickel 200 or 201	Pure nickel resists wet chlorine
Hydrofluoric acid alkylation and HF handling	Nickel 200 or 201	Nickel is the classic HF material
HF service with chlorides present	Monel 400	Tolerates chlorides alongside HF
Caustic with chloride contamination	Monel 400	Chloride tolerance beats pure nickel
Marine hardware, pump shafts, seawater trim	Monel 400	Seawater and chloride SCC resistance
Brackish or seawater heat-exchanger tubing	Monel 400	Strength plus chloride resistance
Electronic components and high-purity parts	Nickel 201	Low carbon, suitable for high-temperature processing
Food processing and potable water	Nickel 200	Corrosion resistance and non-toxicity
Mercury or mercury-contaminated streams	Nickel 200 or 201, with review	Monel 400 embrittles in mercury
Oxidising acid service	None of these three	Move to a chromium or molybdenum bearing alloy

Table note: The recommendations reflect the standard composition and the typical published corrosion behaviour of the three grades and are engineering guidance rather than standard requirements. For safety-critical or high-value equipment the final selection must be based on the actual stream composition, temperature and concentration, on the applicable design code, and on the required service life, with corrosion testing where the margin is in doubt.

Two rules deserve to be stated as rules rather than as rows in a table. First, if the metal temperature in caustic or in any other medium exceeds about 315 C, the grade is Nickel 201 and not Nickel 200, without exception, because the graphitisation failure mode is not predictable from inspection and cannot be designed around by reducing stress. Second, if there is any credible mercury source in the stream, Monel 400 is excluded, regardless of how well it performs in the rest of the environment. Everything else in this comparison is a trade between cost and margin; those two are constraints that remove a material from consideration outright.

The remaining decisions are genuinely about matching the medium. A chloride-free caustic or HF duty at moderate temperature is a Nickel 200 duty on cost grounds and a Nickel 201 duty if the temperature margin is uncertain. A seawater or aerated-chloride duty is a Monel 400 duty. A chloride-bearing caustic duty is the one case where the two families overlap and where the chloride content and the concentration both have to be weighed; in that situation the honest answer is that the margin depends on the specific composition, and a corrosion test on the actual stream is a better use of money than another datasheet comparison. That is the guidance we give customers who ask us to choose between these grades, and it is the reason we ask for the stream analysis before we quote.

Price Reference (2026, EXW Shanghai)

The three grades are all solid-solution nickel-based materials, and their prices track the nickel price with a modest fabrication premium. Monel 400 is generally the least expensive of the three per kilogram because it contains roughly 30 % copper, which displaces a substantial part of the more costly nickel; the two pure nickel grades are priced close to the cost of their nickel content plus processing.

Form	Nickel 200	Nickel 201	Monel 400
Round bar	USD 26-40/kg	USD 28-42/kg	USD 20-34/kg
Plate / sheet	USD 28-45/kg	USD 30-47/kg	USD 24-38/kg
Seamless tube	USD 35-55/kg	USD 37-58/kg	USD 30-50/kg
Welded pipe	USD 30-48/kg	USD 32-50/kg	USD 26-42/kg
Wire	Quote by size	Quote by size	Quote by size

Table note: Reference range only - floats with LME nickel price. 2026, EXW Shanghai, USD/kg. These figures are a reference range and not a quotation; Nickel 201 normally carries a small premium over Nickel 200 because of the tighter carbon control and the additional processing discipline it requires, and Monel 400 pricing also responds to the copper market in addition to nickel.

Standard Index

Standard	Title / scope	Covers	Form
ASTM B160	Nickel rod and bar	Composition + mechanical	rod, bar
ASTM B161	Nickel seamless pipe and tube	Composition + mechanical	pipe, tube
ASTM B162	Nickel plate, sheet and strip	Composition + mechanical	plate, sheet, strip
ASTM B163	Seamless nickel and nickel-alloy condenser and heat-exchanger tubes	Composition + mechanical	tube
ASTM B164	Nickel-copper alloy rod, bar and wire	Composition + mechanical	rod, bar, wire
ASTM B127	Nickel-copper alloy plate, sheet and strip	Composition + mechanical	plate, sheet, strip
ASTM B165	Nickel-copper alloy seamless pipe and tube	Composition + mechanical	pipe, tube
ASTM B564	Nickel alloy forgings	Composition + mechanical	forgings
ASME SB-160 / SB-164	ASME Code adoption of the above bar standards	Code allowable basis	bar
ASME SB-162 / SB-127	ASME Code adoption of the plate standards	Code allowable basis	plate, sheet, strip
ASTM E8 / E8M	Tension testing of metallic materials	Test method	—
ASTM E10 / E18	Brinell and Rockwell hardness testing	Test method	—
ASTM G31	Laboratory immersion corrosion testing	Test method	—
ASTM G48	Pitting and crevice corrosion resistance	Test method	—
ASTM E1476	Standard guide for metals identification (PMI)	Test method	—
Chinese designation (N6, N7)	Wrought pure nickel grades	Cross-system designation reference only	—

Table note: Standards are listed by number and scope; where an edition year is not quoted, the latest edition applies and the controlling edition is the one named on the purchase order. The Chinese pure nickel designations such as N6 and N7 correspond closely to Nickel 200 and Nickel 201 respectively but are written in a different standard system; they are shown for cross-system recognition only and their limits must not be substituted for the ASTM limits on an ASTM specification.

FAQ

Q1: What is the difference between Nickel 200 and Nickel 201?

Nickel 200 and Nickel 201 are the same commercially pure wrought nickel, both with a minimum 99.0 % nickel, and they differ in exactly one specification: carbon. Nickel 200 permits up to 0.15 % carbon while Nickel 201 is capped at 0.02 %. That single difference is what makes Nickel 201 usable in sustained service above about 315 C, because the low carbon content prevents the graphite precipitation that embrittles Nickel 200 in that temperature band. At ambient and low temperature the two grades behave almost identically in corrosion and mechanical terms, and either can be used in the same media; Nickel 200 is generally chosen because it is slightly less expensive and easier to obtain, while Nickel 201 is chosen specifically when the service temperature exceeds the graphitisation ceiling. The two are not interchangeable above that ceiling, and a specification that calls up Nickel 200 for a hot caustic duty is a specification error that will eventually produce an intergranular failure.

Q2: Why can Nickel 201 be used above 315 C when Nickel 200 cannot?

Nickel 201 can be used above 315 C because its carbon content is held to a maximum of 0.02 %, which is too low to form the continuous grain-boundary graphite network that causes embrittlement. In Nickel 200, which permits up to 0.15 % carbon, the carbon becomes mobile at service temperatures above roughly 315 C and precipitates as graphite preferentially at grain boundaries. That intergranular graphite film reduces the cohesive strength of the boundaries and makes the metal susceptible to cracking without measurable general corrosion and without any external warning sign. The failure is therefore both sudden and difficult to detect in advance, which is why the ceiling is treated as a hard limit rather than as a gradual performance penalty. Nickel 201 removes the mechanism by removing the carbon. The practical ceiling for Nickel 201 is then set by other factors, principally oxidation and creep, rather than by graphitisation.

Q3: Is Monel 400 the same as Nickel 200?

No, they are different materials from different families. Nickel 200 is commercially pure nickel with a minimum 99.0 % nickel, while Monel 400 is a nickel-copper alloy containing roughly 63-70 % nickel and 28-34 % copper with a small iron addition. That copper content changes the electrochemical behaviour of the surface substantially: Monel 400 is far better than pure nickel in seawater, aerated chlorides and chloride-bearing caustic, but it is attacked by wet chlorine and it embrittles in mercury-contaminated service. Monel 400 is also appreciably stronger than Nickel 200 in the annealed condition and is only weakly magnetic. Using the terms interchangeably is a common and expensive mistake, because a specification that reads "nickel alloy" without naming the UNS number can be satisfied by either material, and the two will not perform the same in the intended service.

Q4: Which grade is best for caustic soda service?

Nickel is the reference material for caustic soda, so Nickel 200 or Nickel 201 is the right answer in most caustic duties, with Nickel 201 mandatory above about 315 C. Nickel resists caustic across the full concentration range, including the concentrated hot caustic that attacks carbon steel above roughly 50 % concentration, and it resists caustic stress-corrosion cracking that limits carbon and stainless steels. Monel 400 also performs acceptably in caustic at moderate concentration and temperature, but its margin narrows as the concentration rises, and pure nickel is clearly superior for the most concentrated and hottest duties. The one case where Monel 400 wins is a caustic stream that also contains chlorides, because Monel 400 tolerates the chloride component better than pure nickel does. For chloride-free caustic, choose Nickel 200 below the temperature ceiling and Nickel 201 above it. Our caustic service material selection guide sets out the concentration and temperature combinations for caustic evaporators and concentrators.

Q5: Can Monel 400 be used in chlor-alkali or wet chlorine service?

No. Monel 400 should not be used in wet chlorine service, and it is not the material for chlor-alkali cells or for wet-chlorine handling equipment. Wet chlorine is highly aggressive toward the copper in the Monel 400 alloy, and the material corrodes rapidly under those conditions. Pure nickel, by contrast, resists wet chlorine well, which is why Nickel 200 and Nickel 201 are standard materials for wet-chlorine handling, for chlor-alkali cell components and for piping in that part of the plant. Dry chlorine gas is far less aggressive than wet chlorine, and both families can be considered for dry gas service, but the distinction between wet and dry is critical and is frequently lost in a specification that simply says "chlorine." A chlor-alkali plant typically contains both wet-chlorine and chloride-bearing caustic streams, and the correct approach is to select the material per stream rather than to apply one material across the whole battery limit, because the two streams demand different answers.

Q6: Which of these alloys is better in seawater?

Monel 400 is the best of the three in seawater, and it is the only one of the three that is regarded as a marine alloy. It resists flowing seawater well, it performs far better than pure nickel in stagnant and aerated seawater where pitting is a risk, and it is highly resistant to chloride stress-corrosion cracking. Pure nickel, Nickel 200 and Nickel 201, is broadly acceptable in flowing seawater but is prone to pitting in stagnant, aerated conditions, so it is not the preferred choice for immersed or intermittently wetted marine service. Marine hardware, pump shafts, valve trim, seawater heat-exchanger tubing and propeller-shaft components are therefore Monel 400 applications. The one hard restriction is mercury: Monel 400 embrittles rapidly in mercury-contaminated service and must not be used where mercury may be present, which is a realistic concern in some process streams and a critical check for any marine or process application of the alloy.

Q7: Is Monel 400 suitable for hydrofluoric acid service?

Yes, Monel 400 performs well in hydrofluoric acid and has a long history in HF service. It resists both aqueous and anhydrous HF well, and it is used in HF alkylation and HF handling equipment alongside the pure nickel grades. Nickel 200 and Nickel 201 are also highly resistant to hydrofluoric acid and are regarded as the classic HF materials in the chemical industry. The selection between the two families for an HF duty therefore usually turns on the other conditions in the stream rather than on the HF itself. If the stream is hot concentrated HF with no chlorides and no mercury present, pure nickel is the straightforward and often more economical choice. If chlorides or seawater are also present, or if greater strength is required for a structural component, Monel 400 has the advantage because of its superior chloride tolerance. As always with Monel 400, the stream must be checked for mercury before the alloy is specified.

Q8: What is the maximum service temperature for each of these grades?

For sustained service, Nickel 200 is limited to about 315 C because of graphitisation, Nickel 201 can be used to roughly 600 C in dry, non-oxidising conditions where the limiting factors become oxidation and creep rather than carbon, and Monel 400 is generally used to about 480-540 C, with its high-temperature limit set by oxidation and, above all, by sulphur attack. These figures are typical published service guidance rather than standard limits, and the actual permissible temperature depends on the environment, the stress and the required life. The most important of the three limits is the Nickel 200 ceiling, because it is a hard metallurgical constraint rather than a gradual degradation: a Nickel 200 part above 315 C is not performing below its potential, it is undergoing a mechanism that will embrittle it. Any specification for these grades should state the maximum metal temperature explicitly so that the grade can be checked against its ceiling.

Q9: Can these alloys be welded, and is post-weld heat treatment required?

All three weld readily and none of them normally requires a post-weld heat treatment, which is a significant fabrication advantage. Nickel 200 and Nickel 201 are welded with matching nickel filler metals and Monel 400 with matching nickel-copper filler metals, and the resulting joints are ductile and tolerant of thermal cycling. No post-weld solution treatment or ageing is required, unlike the precipitation-hardening nickel alloys such as Inconel 718, which is one reason pure nickel and Monel 400 remain widely used in chemical plant. Two disciplines are nevertheless essential. First, sulphur and lead must be excluded from marking materials, lubricants, cutting fluids and shop dirt around the joint, because both elements embrittle these alloys at elevated temperature. Second, for Nickel 201 the welding procedure and filler selection must be controlled so that carbon is not introduced into the heat-affected zone, which would locally defeat the purpose of specifying the low-carbon grade.

Q10: Why is mercury a problem for Monel 400?

Mercury causes rapid embrittlement and cracking of Monel 400, so the alloy must not be used in mercury-contaminated service. The mechanism is the same class of liquid-metal embrittlement that affects several nickel-copper and higher-strength nickel alloys: mercury attacks the grain boundaries and the material cracks under low applied stress that it would otherwise carry easily. The practical danger is that mercury often enters a process from a source that is not obvious, such as a trace impurity in feedstock, a failed instrument seal, contamination in recycled material or upstream equipment, and a plant designed for Monel 400 on the strength of its excellent chloride resistance can then be destroyed by an excursion no one anticipated. Any specification for Monel 400 should therefore include an explicit check for mercury in the process. Where mercury is present or cannot be excluded, pure nickel is substantially safer, although even then the specific conditions should be reviewed. Our nickel alloy corrosion case studies include examples of contamination-driven failures and the inspection steps that detect them.

Q11: How do I verify that I received Nickel 201 and not Nickel 200?

Verify by carbon analysis, because the two grades are otherwise almost indistinguishable in the hand and in the field. Confirm first that the mill certificate reports the carbon content and that it is at or below 0.02 % for a Nickel 201 order, then verify the chemistry independently, because the delivered material is what matters and the certificate alone does not prove it. Optical emission spectrometry or an equivalent analysis will separate the grades on the carbon line, and the nickel and residual concentrations should also be checked against the ASTM B160 limits for the specification named on the purchase order. A hardness check will not distinguish them, because the two grades have essentially the same annealed hardness, and a visual inspection will certainly not. Reconcile the heat number on the certificate with the heat number marked on the material, and confirm that the certificate is traceable to the producing mill. For temperature-critical service we report carbon as a separate line item on every Nickel 200 and Nickel 201 certificate for exactly this reason.

Q12: Which grade should I use for electronic components and for food processing?

For electronic and high-purity components, Nickel 201 is normally the better choice because its low carbon content makes it suitable for the high-temperature processing steps used in component manufacture, where a higher-carbon material could graphitise or outgas in ways that affect performance. Its magnetic and electrical properties are also predictable, and the low carbon content reduces the risk of carbide and graphite formation during any elevated-temperature fabrication step. For food processing, potable water and similar hygienic applications, Nickel 200 is widely used and well established, because it resists corrosion in those media, it is non-toxic and it is easy to clean. The choice between the two in food service usually comes down to the process temperature: below the 315 C ceiling either grade is acceptable and Nickel 200 is usually the economical choice, while any high-temperature process step, including some sterilisation and processing operations, moves the selection to Nickel 201.

Q13: How do the prices of these three grades compare?

In 2026 EXW Shanghai reference terms, Nickel 200 bar typically falls in the range of about USD 26-40/kg, Nickel 201 in the range of about USD 28-42/kg, and Monel 400 in the range of about USD 20-34/kg. Monel 400 is generally the least expensive of the three per kilogram because its roughly 30 % copper content displaces a substantial part of the more costly nickel, and Nickel 201 usually carries a modest premium over Nickel 200 because of the tighter carbon control and the additional processing discipline required. All of these figures are reference ranges only and float with the LME nickel price, and Monel 400 pricing also responds to the copper market, so they must not be treated as a quotation. Where the two nickel grades are within a few percent of each other, as they often are, the sensible commercial decision is to standardise on Nickel 201 for a plant that contains any hot section, rather than to carry two nearly identical grades and risk the wrong one being installed.

Q14: Can Monel 400 be substituted for Nickel 200 or Nickel 201?

Only after a full review, and never as a simple drop-in substitution. Monel 400 is stronger and better in chlorides and seawater than pure nickel, so in those media it may be an upgrade, but it is markedly worse in wet chlorine, where it is attacked, and it must not be used in mercury-contaminated service, where it embrittles. It is also a different alloy with different thermal and electrical properties and a different density, so a substitution can affect fit, weight and heat transfer as well as corrosion performance. Substituting in the other direction, pure nickel in place of Monel 400, is similarly not automatic: pure nickel is more prone to pitting in stagnant aerated chlorides and is much softer, so it may not carry the structural load that the original design assumed. Any substitution should be checked against the actual stream composition, the temperature, the load and any mercury presence, and the applicable specification should be re-confirmed. Where the substitution cannot be justified from the process data, we decline it, because these three grades fail in different ways and the differences matter.

Conclusion and Selection Rules

The selection rules for this family are short, and they resolve almost every real case. Use Nickel 200 for caustic, chlor-alkali, hydrofluoric acid, food-processing and ambient-temperature chemical service where the metal temperature stays below about 315 C. Use Nickel 201 whenever the same service exceeds that ceiling, and consider standardising on Nickel 201 for any plant with a hot section, because the graphitisation limit on Nickel 200 is a hard constraint rather than a gradual penalty. Use Monel 400 where seawater, aerated chlorides or chloride-bearing caustic dominate the duty, or where greater strength is needed in a chloride environment, and never where mercury may be present. In none of these cases is the choice a matter of preference between near-identical materials; each grade is correct for a definable set of conditions and wrong outside it.

Two of those rules are constraints rather than preferences and should be treated as such. Nickel 200 is excluded above about 315 C in every case, and Monel 400 is excluded from mercury-contaminated streams in every case. Every other decision in this comparison is a trade between cost and corrosion margin, and where the margin is genuinely in doubt the correct next step is a corrosion test on the actual stream rather than another datasheet comparison. Shanghai Hangbo Alloy Group Co., Ltd. supplies Nickel 200, Nickel 201 and Monel 400 as bar, plate, tube, pipe and wire with full traceability, EN 10204 3.1 certification and third-party inspection, and we will review a stream analysis, a temperature profile and a drawing before quoting so that the grade and the product standard are matched to the duty. Where the environment is beyond the reach of these three grades, for example in oxidising acid service, we will recommend the appropriate chromium or molybdenum bearing family instead, such as the Hastelloy C-276 plate & bar range. Send your process data through our contact page and we will return a material recommendation with the governing standard citations.

Contact & Complete Product Range

Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

All Grades

Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

Product Forms: Bars(6-500mm) | Tubes(OD 6-610mm) | Plates(0.1-100mm) | Forgings | Welded Pipes | Fittings | Wire(0.05-10mm) | Strip(0.02-4.0mm)

SGS/BV/TUV. www.nickel-alloy.com