Hastelloy C276 vs Inconel 625: Nickel Alloy Selection for Chemical and Sour Service

Hastelloy C276 (UNS N10276, 15Mo-16Cr-4W) is the workhorse for reducing acids (HCl, dilute H2SO4, H3PO4) and severe sour service, with a Pitting Resistance Equivalent Number (PREN) of approximately 68. Inconel 625 (UNS N06625, 9Mo-21Cr-3.5Nb) is the choice for oxidizing media, seawater, aerospace, and moderate chemical service, with PREN approximately 51 but nearly twice the yield strength (414 vs 283 MPa) and one-third the cost. Selection is driven by acid type and chloride-H2S severity, not by price.

Why Is C276 vs 625 the Most Common Nickel Alloy Selection Question?

C276 and 625 are the two most widely specified nickel alloys in chemical and oil and gas service because they offer the best combination of corrosion resistance, mechanical strength, weldability, and field-proven performance, but they occupy overlapping yet distinct application spaces that require careful evaluation rather than a default choice.

Engineers specifying materials for chemical process equipment, sour hydrocarbon service, flue gas desulfurization (FGD), pharmaceutical plants, and marine applications face a recurring decision: should I select Hastelloy C276 (UNS N10276) or Inconel 625 (UNS N06625)? Both alloys contain more than 50 percent nickel, both offer excellent corrosion resistance in aggressive environments, both are readily available in plate, pipe, bar, and fitting forms, and both are accepted under major pressure vessel and sour service codes (ASME, NACE MR0175, EN, JIS).

However, the cost difference is significant: C276 is typically 2.2-3.0 times the price of 625, making the wrong choice a costly error in either direction. Selecting C276 where 625 would suffice wastes money; selecting 625 where C276 is required causes premature failure. This guide provides a structured, data-driven selection framework based on the four key decision dimensions: chemistry (which determines corrosion mode resistance), mechanical properties (which determine pressure rating and design life), sour service qualification (which determines NACE MR0175 acceptability), and cost (which determines project feasibility).

Source: Haynes International Hastelloy C276 Technical Bulletin; Special Metals Inconel 625 Technical Bulletin; NACE MR0175/ISO 15156; aggregated field experience 1980-2025.

What Are the Chemistry Differences Between C276 and 625?

C276 has much higher molybdenum (15-17 percent vs 8-10 percent) plus 3-4.5 percent tungsten, giving it superior resistance to reducing acids and severe sour service, while 625 has higher chromium (20-23 percent vs 14.5-16.5 percent) plus niobium, giving it better oxidation resistance, higher strength, and better weldability.

The fundamental difference between C276 and 625 lies in the balance of alloying elements, which directly controls the corrosion mode each alloy resists. Molybdenum and tungsten are the primary defenses against chloride pitting, crevice corrosion, and reducing acid attack. C276 with 15-17 percent Mo and 3-4.5 percent W has a PREN (Pitting Resistance Equivalent Number, calculated as Cr percent plus 3.3 times Mo percent plus 16 times N percent) of approximately 68, among the highest of any commercial alloy.

Inconel 625 with 8-10 percent Mo and no tungsten has a PREN of approximately 51, which is still excellent but about 25 percent lower. Conversely, 625 has higher chromium (20-23 percent vs 14.5-16.5 percent), which provides superior resistance to oxidizing media such as nitric acid, and contains niobium (3.15-4.15 percent) which precipitates as gamma-double-prime phase to give nearly twice the yield strength of C276. Iron content is also lower in 625 (5 percent max vs 4-7 percent for C276), contributing to its higher corrosion resistance in some environments.

Table 1. Chemical Composition Comparison (ASTM Standards)

Source: ASTM B575-24 (C276 plate); ASTM B443-21 (625 plate); Haynes International datasheet H-2002; Special Metals datasheet SMC-067.

How Do Mechanical Properties and Temperature Limits Compare?

Inconel 625 has nearly twice the yield strength of C276 (414-517 MPa vs 283-355 MPa), superior creep resistance up to 700 degrees C, and better fatigue performance, while C276 offers higher ductility (40 percent vs 30 percent elongation) and slightly better formability for complex fabrications.

The mechanical property difference between 625 and C276 comes primarily from the niobium addition in 625. When 625 is heated in the 550-750 degrees C range, niobium precipitates as gamma-double-prime (Ni3Nb) phase, providing significant strengthening. Grade 2 (annealed) 625 has minimum yield strength of 414 MPa in sheet form, while C276 solution-annealed has minimum yield strength of 283 MPa in sheet form.

This 45-50 percent strength advantage means 625 can be used in thinner sections for the same pressure rating, saving weight and cost in pressure vessel and piping applications. C276 is not precipitation-hardenable and relies solely on solid solution strengthening from Mo and W, giving lower strength but better ductility. Both alloys are approved for cryogenic service down to -196 degrees C with good impact toughness.

At elevated temperatures, 625 retains strength better due to the stable gamma-double-prime precipitates, with creep-rupture strength at 700 degrees C of approximately 140 MPa for 1000 hours, compared to approximately 80 MPa for C276. For continuous service above 600 degrees C, 625 is generally preferred for structural applications. However, C276 has a higher maximum service temperature (approximately 1040 degrees C in sulfur-free atmospheres) compared to 625 (approximately 980 degrees C in air).

Table 2. Mechanical Properties Comparison (Solution Annealed Condition)

Source: ASTM B575/B443-21/24; Haynes International datasheet H-2002; Special Metals Inconel 625 Technical Bulletin.

Which Alloy Is Better for Sour Service (H2S + Chloride per NACE MR0175)?

Both C276 and 625 are approved under NACE MR0175/ISO 15156 for sour service, but C276 is the safer and more universally specified choice for severe sour environments with high H2S partial pressure and high chloride, while 625 is acceptable for moderate sour service and is often preferred when higher strength is needed.

Sour service refers to environments containing hydrogen sulfide (H2S) dissolved in water, which can cause sulfide stress cracking (SSC), stress-oriented hydrogen-induced cracking (SOHIC), and hydrogen-induced cracking (HIC) in susceptible alloys. NACE MR0175/ISO 15156 is the governing standard for materials selection in sour oil and gas service. Both C276 and 625 are listed in NACE MR0175 as acceptable for use in all regions of the standard, including the most severe environments with high H2S partial pressure, low pH (down to 2.5 or lower in some cases), and high chloride concentration (up to saturation).

However, field experience and laboratory testing under NACE TM0177 and TM0284 have shown that C276 has superior resistance to hydrogen embrittlement and chloride-assisted SCC in the most severe conditions, particularly when the combination of H2S partial pressure exceeds 1 MPa, chloride exceeds 50,000 ppm, and pH is below 3.5. 625 in the annealed condition (Grade 2) is more resistant than Grade 1 in these conditions due to the absence of carbide networks.

For downhole tubing and surface equipment in the most severe sour wells, C276 is typically specified. For moderate sour service where strength is a priority (e.g., high-pressure wellhead components), 625 is often selected. Both alloys require solution-annealed condition and hardness below 35 HRC (typically below 90 HRB for C276 and below 240 HB for 625) to meet NACE requirements.

Table 3. Sour Service Selection (NACE MR0175/ISO 15156)

Source: NACE MR0175/ISO 15156 Part 3 (2020); Haynes C276 Sour Service Bulletin; Special Metals Inconel 625 Sour Service Bulletin; field data 1990-2020.

How Do C276 and 625 Compare in Reducing Acids (HCl, H2SO4, H3PO4)?

C276 is the clear winner in reducing acid environments (HCl, dilute H2SO4, H3PO4, organic acids) because its high Mo and W content resists the active corrosion mechanisms specific to non-oxidizing acids, with corrosion rates typically 3-10 times lower than 625 in the same conditions.

Reducing acids such as hydrochloric acid (HCl), dilute sulfuric acid (H2SO4 below 70 percent), and phosphoric acid (H3PO4) are not passivating and instead attack the passive film on stainless steels. Molybdenum is the primary alloying element that resists reducing acid attack, with tungsten providing additional protection. C276 with 15-17 percent Mo and 3-4.5 percent W offers superior resistance in these environments. In boiling 5 percent HCl, C276 corrodes at 0.5-1.0 mm per year while 625 corrodes at 2.5-5.0 mm per year.

In 10 percent H2SO4 at 80 degrees C, C276 corrodes at 0.05-0.1 mm per year while 625 corrodes at 0.5-1.0 mm per year. In 85 percent H3PO4 at 100 degrees C (typical evaporator conditions), C276 corrodes at less than 0.05 mm per year while 625 corrodes at approximately 0.5 mm per year. For phosphoric acid concentrators, sulfuric acid coolers handling dilute acid, hydrochloric acid scrubbers, and acetic acid anhydride reactors, C276 is the standard specification. 625 can be used in mild reducing acid service (room temperature, less than 5 percent concentration) but is generally not cost-effective compared to lower alloys such as 316L or Alloy 20 in these conditions.

Table 4. Corrosion Rates in Reducing Acids (mm per year)

Source: Haynes International Corrosion Resistance Guide; Special Metals Inconel 625 Corrosion Data; ASM Handbook Vol. 13A Corrosion (2003); aggregated field data.

How Do C276 and 625 Compare in Oxidizing Media and Seawater?

Inconel 625 is the better choice for oxidizing media (HNO3, ferric chloride, wet Cl2 mixed with oxidizers) and is equal to or better than C276 in seawater and marine service, while C276 is preferred only when the environment is a mix of reducing and oxidizing, such as in FGD scrubbers.

Oxidizing media such as nitric acid (HNO3) and ferric chloride (FeCl3) are passivating and require chromium (not molybdenum) for resistance. Inconel 625 with 20-23 percent Cr offers better resistance in these environments than C276 with only 14.5-16.5 percent Cr. In boiling 65 percent HNO3, 625 corrodes at 0.1-0.3 mm per year while C276 corrodes at 0.5-1.0 mm per year.

In ferric chloride (FeCl3) pitting tests (ASTM G48), 625 typically has a critical pitting temperature (CPT) of 70-80 degrees C, compared to 60-70 degrees C for C276. In seawater service, both alloys are excellent at room temperature, but 625 has slightly better performance in hot seawater above 50 degrees C, with a CPT above 80 degrees C compared to 60-70 degrees C for C276. In wet chlorine service, C276 is preferred because of its higher Mo content, which is more effective than Cr in this environment. For FGD scrubbers where the environment alternates between oxidizing and reducing, the choice depends on the specific zone: 625 for the oxidizing/absorber outlet, C276 for the reducing/quench zone.

Table 5. Corrosion Resistance in Oxidizing Media and Seawater

Source: ASTM G48-21 Standard Test Methods for Pitting and Crevice Corrosion; Haynes C276 seawater bulletin; Special Metals Inconel 625 marine guide.

How Should C276 and 625 Be Welded for Process Equipment?

Both alloys are weldable using gas tungsten arc welding (GTAW) with matching filler metals (ERNiCrMo-4 for C276, ERNiCrMo-3 for 625), but C276 is significantly more sensitive to heat input and requires stricter control, while 625 is more forgiving and is often the preferred choice for field welding and complex fabrications.

Welding is a critical step in the fabrication of C276 and 625 process equipment because improper procedure can destroy the corrosion resistance of the base metal in the heat-affected zone (HAZ). C276 is particularly sensitive to heat input and interpass temperature because the high Mo and W content promotes precipitation of mu phase and P phase in the 600-1100 degrees C range, which depletes Mo and W from the surrounding matrix and reduces corrosion resistance.

Recommended heat input for C276 is 0.5-1.5 kJ/mm, interpass below 93 degrees C, and no preheat. Filler metal is ERNiCrMo-4 (AWS A5.14) for matching composition. Inconel 625 is more forgiving because the gamma-double-prime strengthening phase in the weld metal can be controlled by heat input. Recommended heat input for 625 is 0.5-2.0 kJ/mm, interpass below 150 degrees C.

Filler metal is ERNiCrMo-3 (AWS A5.14), which is also widely available. For dissimilar joints (e.g., 625 to carbon steel, 625 to stainless steel), ERNiCrMo-3 is the standard choice and provides excellent results. For joining C276 to lower alloys, ERNiCrMo-3 or ERNiCrMo-4 can be used, but a buttering layer of matching alloy is recommended. Post-weld heat treatment is not required for either alloy in thin sections; for thick sections, solution annealing at the manufacturer's recommended temperature followed by rapid water quenching restores optimal properties.

Table 6. Welding Parameters Comparison

Source: AWS D1.6 Structural Welding Code - Stainless Steel; Haynes International C276 Welding Guide; Special Metals Inconel 625 Welding Handbook; ASME Section IX.

C276 vs 625 vs 825 vs 20: Material Selection for Chemical and Sour Service

C276 is the highest-cost and most corrosion-resistant option for the most severe reducing acid and sour environments, 625 is the workhorse for oxidizing media and moderate sour service, 825 fills the middle ground for sulfuric acid and mild sour service at lower cost, and Alloy 20 is the budget option for sulfuric acid and lower chloride.

Material selection for chemical and sour service requires balancing corrosion resistance, mechanical properties, fabricability, and cost. The four alloys C276, 625, 825, and Alloy 20 represent a cost-corrosion hierarchy that covers most chemical process applications. Alloy 20 (UNS N08020) is the lowest cost option (about 30-40 percent of C276) and is suitable for sulfuric acid service up to 40 percent concentration and moderate chloride.

Alloy 825 (UNS N08825) is the middle option (about 50-60 percent of C276) and provides good performance in sulfuric acid, phosphoric acid, and sour service up to moderate severity. Inconel 625 (UNS N06625) is the higher-strength, oxidizing-resistant option (about 35-45 percent of C276) for seawater, nitric acid, FGD, and aerospace.

Hastelloy C276 (UNS N10276) is the top of the line (most expensive) for the most severe reducing acid and sour service. Selecting the wrong alloy typically results in either overspending (using C276 where 625 would work) or premature failure (using Alloy 20 where C276 is required), so the matrix below should be referenced for any chemical or sour service application.

Table 7. Material Selection Matrix: C276, 625, 825, and Alloy 20

Source: NACE MR0175/ISO 15156 (2020); Haynes International Corrosion Guide; Special Metals Inconel 625 Bulletin; ASME BPE Standards for Pharmaceutical.

Case Study: FGD Absorber Tower Alloy Upgrade from 625L to C276

A US coal-fired power plant upgraded the reducing quench zone of its FGD absorber tower from Inconel 625 to C276 in 2015 after 625 experienced severe accelerated corrosion in the high-temperature reducing zone, eliminating unscheduled outages and reducing annual maintenance costs by 75 percent over the following 8 years.

The 600 MW power plant operated a wet limestone FGD system with an inlet quench zone subjected to flue gas at 180-220 degrees C containing SO2, HCl, HF, and fly ash. The original specification was Inconel 625 for the inlet quencher and absorber lower section, chosen for its chloride pitting resistance. However, after only 4 years of service, ultrasonic inspection revealed corrosion rates of 0.5-0.8 mm per year at the inlet quencher zone, with through-wall pitting at several locations.

Failure analysis showed that the combination of high temperature (200 degrees C), reducing conditions (low pH 1-3, high SO2), chloride from combustion, and HF created an environment where 625 was inadequate. The plant replaced the inlet quencher and lower 3 meters of the absorber in 2015 with C276 clad to carbon steel backing. After 8 years of service (2015-2023), the C276 section shows no measurable wall loss and no pitting. The estimated annual saving from eliminated unplanned outages and reduced inspections is approximately 280,000 dollars per year. Importantly, the upper absorber section (oxidizing zone) and outlet duct remained as 625, where it continues to perform well. This case illustrates the importance of zone-specific material selection within a single vessel.

Source: EPRI FGD Materials Handbook 2018; field case provided by JN Alloy customer (anonymized); Haynes International FGD case study database.

Case Study: Sour Service Downhole Tubing Material Selection in HPHT Well

A Middle East operator selected C276 for downhole production tubing in a high-pressure high-temperature (HPHT) sour gas well after 625 was ruled out due to NACE MR0175 limits at low pH, with C276 tubing performing reliably for 7 years at 180 degrees C bottom-hole temperature, 15 percent H2S, and 120,000 ppm chloride.

The operator planned production from a deep gas well with bottom-hole conditions of 180 degrees C, 100 MPa pressure, 15 percent H2S in the gas phase, 120,000 ppm chloride in the produced water, and pH 3.0. NACE MR0175/ISO 15156 Part 3 was applied for material qualification. The operator first considered Inconel 625 Grade 2 (annealed) because of its higher strength and lower cost.

However, the specified pH of 3.0 was below the typical 3.5 lower limit for 625 in chloride-containing sour environments, and laboratory testing under NACE TM0177 Method A at the project conditions showed 625 specimens failed within 200 hours. C276 was then tested under the same conditions, and no failures occurred in the standard 720-hour test. C276 tubing (UNS N10276, 3-1/2 inch OD, 0.276 inch wall) was selected, supplied in solution-annealed condition with hardness 85-95 HRB and full traceability per EN 10204 3.2.

The tubing has been in service for 7 years (2017-2024) with annual inspection showing no measurable wall loss, no cracking, and no pitting. C276 cost approximately 3.2x the price of 825 and 2.5x the price of 625 Grade 2, but the well economics justified the premium because failure of the tubing in this HPHT sour environment would result in well abandonment costing 50+ million dollars.

Source: NACE MR0175/ISO 15156 Part 3 (2020); SPE-185703-MS Sour Service Tubing Selection Paper; Haynes International HPHT Sour Service Bulletin.

What Standards Govern C276 and 625 in Chemical and Sour Service?

C276 is covered by ASTM B462 (forgings), B574 (bar), B575 (plate), B619 (welded pipe), B622 (seamless pipe), B626 (welded tube), B366 (fittings), and ASME SB- equivalents; 625 is covered by ASTM B443 (plate), B444 (pipe), B446 (bar), B564 (forgings), and ASME SB- equivalents. Both are accepted in ASME BPVC Section VIII Division 1 and NACE MR0175/ISO 15156.

The standards framework for C276 and 625 is well-established and provides engineers with confidence in material properties, testing requirements, and design allowables. For C276 (UNS N10276), the key ASTM product standards are: B462 (forged flanges, fittings, valves), B574 (rod and bar), B575 (plate, sheet, strip), B619 (welded pipe), B622 (seamless pipe), B626 (welded tube), B366 (fittings). ASME SB- equivalents are approved for use in Section VIII Division 1 pressure vessels.

For sour service, NACE MR0175/ISO 15156 Part 3 lists C276 with the note that it is acceptable in all regions of the standard in the solution-annealed condition with hardness less than 35 HRC. For Inconel 625 (UNS N06625), the key ASTM product standards are: B443 (plate, sheet, strip - Grade 1 annealed, Grade 2 solution annealed), B444 (pipe), B446 (bar - Grade 1 and Grade 2), B564 (forgings), and B704/B705 (welded tube). ASME SB-443 and SB-444 are approved for Section VIII use.

For sour service, NACE MR0175 specifies 625 in the annealed condition (Grade 2) with hardness less than 35 HRC. For pressure equipment in the EU, PED 2014/68/EU compliance requires CE marking and EN 10204 3.1 or 3.2 certification. For welded fabrication, ASME Section IX or AWS D1.6 provides welding procedure qualification.

Table 8. Applicable Standards for C276 and 625

Source: ASTM International 2024; ASME BPVC 2023 Edition; NACE MR0175/ISO 15156 2020; EN Standards.

What is the 20-Year Total Cost of Ownership for C276 vs 625?

Although C276 costs 2.2-3.0 times the initial material price of 625, in severe reducing acid and sour service applications, C276's longer service life and reduced maintenance deliver a 15-25 percent lower 20-year total cost of ownership, while in oxidizing media and seawater where both alloys work, 625 is the clear economic choice with 50-60 percent lower TCO.

Total cost of ownership analysis for nickel alloys must consider the initial material cost, fabrication cost, scheduled maintenance, unscheduled downtime, and replacement frequency. The table below compares C276 and 625 for two representative applications: (1) a HCl scrubber where C276 is required for technical reasons, and (2) a seawater piping system where 625 is technically sufficient.

In the HCl scrubber case, selecting 625 to save 30 percent on material would result in 3-5x higher maintenance and likely premature replacement, making 625 more expensive over 20 years. In the seawater case, selecting C276 would be over-engineering because 625 has more than adequate corrosion resistance, and the C276 premium cannot be recovered through longer life.

Table 9. 20-Year TCO Comparison: C276 vs 625 in Two Applications

Source: JN Alloy engineering analysis based on aggregated industry data; representative of typical North American chemical plant maintenance cost benchmarks 2018-2024.

Frequently Asked Questions: C276 vs 625

Q: Can Inconel 625 directly replace Hastelloy C276 in existing equipment?

A: In most cases, no. 625 can replace C276 only in the specific environments where 625 is technically sufficient (oxidizing media, seawater, moderate sour service). In severe reducing acid (HCl, dilute H2SO4, H3PO4) and severe sour service (H2S greater than 1 MPa, Cl greater than 50,000 ppm, pH less than 3.5), 625 will fail prematurely. If you are considering substitution to save cost, a corrosion engineer should review the specific service conditions against the data in Tables 4 and 5. If conditions are uncertain, the conservative choice is to retain C276.

Q: Is 625 weldable with standard stainless steel procedures?

A: 625 requires a nickel-based filler (ERNiCrMo-3, AWS A5.14) and procedures qualified to ASME Section IX or AWS D1.6, not standard stainless steel procedures. Using ER308L or ER316L filler on 625 will result in weld metal with lower corrosion resistance than the base metal and potential premature failure. Heat input should be controlled to 0.5-2.0 kJ/mm and interpass below 150 degrees C. C276 is even more sensitive, requiring ERNiCrMo-4 filler, 0.5-1.5 kJ/mm heat input, and interpass below 93 degrees C.

Q: What is the maximum temperature for C276 vs 625 in air?

A: C276 has a maximum service temperature of approximately 1040 degrees C in sulfur-free atmospheres, while 625 has a maximum of approximately 980 degrees C in air. However, in sulfur-containing atmospheres (e.g., H2S, S02), C276 is limited to approximately 400-450 degrees C due to sulfur attack. 625 has slightly better high-temperature strength up to 700 degrees C due to gamma-double-prime strengthening. For continuous service above 600 degrees C with structural load, 625 is typically preferred. For intermittent high-temperature exposure without sulfur, C276 is acceptable to 1040 degrees C.

Q: Which alloy is better for nuclear applications?

A: Inconel 625 is the more common choice for nuclear applications because of its lower cobalt content (1 percent max vs 2.5 percent for C276), which reduces activation products in the primary loop, and its extensive ASME Section III qualification for nuclear service. C276 is used in some nuclear applications, particularly for spent fuel processing where its resistance to nitric acid and chloride is needed. For new nuclear projects, 625 is the typical default unless specific corrosion concerns require C276.

Q: Why is C276 more expensive than 625?

A: C276 is more expensive than 625 primarily because of its higher molybdenum content (15-17 percent vs 8-10 percent) and tungsten addition (3-4.5 percent). Molybdenum costs approximately 8-10 times the price of nickel per unit, and tungsten is similarly expensive. The higher alloy content also makes C276 more difficult and slower to melt, hot-roll, and anneal at the mill, adding to the conversion cost. The market size for C276 is also smaller than 625, reducing economies of scale. As of 2024-2026, C276 plate typically costs 2.2-3.0 times the price of 625 plate.

Q: Can C276 and 625 be welded to each other?

A: Yes, C276 and 625 can be welded to each other using ERNiCrMo-3 (625 filler) or ERNiCrMo-4 (C276 filler) as the filler metal. ERNiCrMo-3 is the more common choice because it is more widely available and has good compatibility with both base metals. For dissimilar joints to carbon steel or standard stainless steel, ERNiCrMo-3 is the standard recommendation, often with a buttering layer of the higher alloy on the carbon/stainless steel side to reduce dilution. Procedure qualification per ASME Section IX is required for production welds.

JN Alloy supplies certified Hastelloy C276 (UNS N10276) and Inconel 625 (UNS N06625) plate, pipe, bar, flanges, and fittings for chemical and sour service worldwide. All materials are supplied with EN 10204 3.1/3.2 MTC, full traceability, and origin from ISO 9001-certified mills. Contact us for technical datasheets, fabrication guidance, and mill-direct pricing.

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