Incoloy 825 vs Inconel 625 - Cost, Strength & Acid Service
Date: 2026年9月18日 Categories: News Views: 329
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: Is Incoloy 825 Good Enough, or Do I Need Inconel 625?
Incoloy 825 (UNS N08825) is a nickel-iron-chromium alloy with molybdenum and copper that handles sulphuric and phosphoric acid, seawater-cooled exchangers and mildly sour oilfield service at roughly 40-50% of the cost of Inconel 625. Inconel 625 (UNS N06625) has twice the molybdenum, no deliberate copper, far higher strength and a much wider corrosion envelope. Specify 825 where the process is proven compatible; step up to 625 where chlorides, higher temperature or higher H2S partial pressure exceed 825's limits.
Key Takeaways
- 825 is the cost-effective acid alloy. Copper plus molybdenum make it a workhorse for sulphuric and phosphoric acid plants and for acid-cooling exchangers.
- 625 is roughly twice as strong at room temperature and much stronger at temperature, which sometimes lets you use a thinner wall.
- 825 is not an HF alloy and 625 is not a copper alloy. The chemistries solve different corrosion problems; swapping them without review is a common and expensive mistake.
- Sour service is a limit question, not a brand question. Both grades appear under NACE MR0175 / ISO 15156, but the acceptable H2S partial pressure and temperature window differs.
- Calculate total cost, not unit price. 825's lower price per kilogram is real, but 625's strength can reduce wall thickness on pressure parts.
What Are Incoloy 825 and Inconel 625?
Incoloy 825 is a nickel-iron-chromium alloy with additions of molybdenum, copper and titanium. The nickel content of 38-46% gives it resistance to chloride stress-corrosion cracking while remaining substantially cheaper than high-nickel alloys; the copper addition is what makes it effective in sulphuric acid; the titanium addition stabilises it against intergranular corrosion after welding.
Inconel 625 is a nickel-chromium-molybdenum-niobium solid-solution alloy with no intentional copper and a molybdenum content roughly three times higher than 825. That chemistry buys corrosion resistance in chlorides and oxidizing/reducing mixed acids that 825 cannot match, plus substantially higher mechanical strength.
Product pages: Incoloy 825 / 800H supplier and Inconel alloy supplier.
Composition Comparison (wt.%)
| Element | Incoloy 825 (N08825) | Inconel 625 (N06625) | Per standard |
|---|---|---|---|
| Nickel | 38.0-46.0 | 58.0 min | ASTM B424 / ASTM B446 |
| Chromium | 19.5-23.5 | 20.0-23.0 | ASTM B424 / ASTM B446 |
| Iron | 22.0 min (balance) | 5.0 max | ASTM B424 / ASTM B446 |
| Molybdenum | 2.5-3.5 | 8.0-10.0 | ASTM B424 / ASTM B446 |
| Copper | 1.5-3.0 | — | ASTM B424 |
| Niobium + Tantalum | — | 3.15-4.15 | ASTM B446 |
| Titanium | 0.6-1.2 | 0.40 max | ASTM B424 / ASTM B446 |
| Carbon | 0.05 max | 0.10 max | ASTM B424 / ASTM B446 |
| Manganese | 1.0 max | 0.50 max | ASTM B424 / ASTM B446 |
Table note: Values are standard composition limits from the listed ASTM specifications. The three practical discriminators in a warehouse PMI check are molybdenum (2.5-3.5% vs 8-10%), copper (present in 825, absent in 625) and iron (825 is iron-rich, 625 is not).
Mechanical Properties Comparison
| Property | Incoloy 825, annealed | Inconel 625, annealed | Per standard |
|---|---|---|---|
| UTS at 20°C | 586 MPa min (85 ksi) | 827 MPa min (120 ksi) | ASTM B424 / ASTM B446 |
| 0.2% yield at 20°C | 241 MPa min (35 ksi) | 414 MPa min (60 ksi) | ASTM B424 / ASTM B446 |
| Elongation at 20°C | 30% min | 30% min | ASTM B424 / ASTM B446 |
| Typical hardness | 75-90 HRB (typical) | 95-100 HRB (typical) | ASTM E18 |
| UTS at 650°C | ~330 MPa (typical) | ~560 MPa (typical) | ASTM E21 (typical, not a standard minimum) |
| Useful load-bearing ceiling | ~550-600°C (in-house guidance) | ~650°C (in-house guidance) | — |
Table note: Room-temperature figures are standard minima. Elevated-temperature figures are typical values shown for orientation only and are not standard minima; for design use project-specific ASTM E21 data.
The strength gap matters commercially. On a pressure vessel or a heat-exchanger tube bundle, higher allowable stress in 625 can reduce required wall thickness by 20-30%, which recovers part of the price difference.
Which Medium Does Each Alloy Actually Handle?
| Environment | Incoloy 825 | Inconel 625 | Practical note |
|---|---|---|---|
| Sulphuric acid, moderate concentration, 60-80°C | Excellent | Good | Copper in 825 is the advantage here |
| Phosphoric acid (wet process, with chlorides and fluorides) | Very good | Good | Both widely used; 825 is the traditional choice |
| Hydrochloric acid | Poor | Good to very good | Do not specify 825 for HCl service |
| Hydrofluoric acid / alkylation units | Unsuitable | Not the first choice either | Monel 400 is the classic HF material |
| Seawater, ambient to 40°C | Good (flow-dependent) | Excellent | 825 pits in stagnant, polluted seawater |
| Chloride stress-corrosion cracking | Very good | Excellent | Ni content drives this behaviour |
| Sour gas with H2S and CO2 (NACE MR0175 / ISO 15156) | Good within defined limits | Better for higher H2S partial pressure | Always verify against the standard's limits |
| Caustic (NaOH) service | Fair | Good | Caustic SCC is a known risk for 825 |
Table note: Ratings are qualitative engineering guidance based on published corrosion data and our field experience. They are not ASTM G48/G28 results for a specific heat — request corrosion testing when the service is critical.
Where Does 825 Stop and 625 Become Mandatory?
The decision is usually driven by three variables acting together, not by one of them alone:
- Chloride level. A stream with a few hundred ppm chloride at 60°C is comfortable for 825. The same stream at 150°C with 50,000 ppm chloride is not, and 625 (or a higher-molybdenum alloy) becomes the safe specification.
- Acid type. Reducing acids push you toward molybdenum and tungsten. 825's strength in sulphuric and phosphoric acid comes from copper, which does not help in hydrochloric acid.
- H2S partial pressure. Under NACE MR0175 / ISO 15156 both alloys have defined sour-service windows; operations above 825's window require an upgrade.
A second, more commercial factor is wall thickness. Because 825's yield strength is roughly 60% of 625's, an 825 exchanger tube or vessel wall may be thicker for the same design pressure — and thickness, not alloy price, is what drives fabrication hours, welding consumable consumption and weight-related freight.
Which Alloy for Which Application?
| Application | Best choice | Why |
|---|---|---|
| Sulphuric acid cooling and pickling lines | 825 | Copper-bearing chemistry, well-proven, lower cost |
| Wet-process phosphoric acid evaporators | 825 | Resists the chloride and fluoride contamination in the process |
| Seawater-cooled heat exchanger tubing | 825 or 625 depending on velocity/pollution | 625 where chlorides and temperature are aggressive |
| Oilfield tubulars and wellhead components | 825 first, 625 for harder duty | Selected against the project's sour-service envelope |
| Flue-gas desulphurisation dampers and ducting | 625 or a clad option | Hot, chloride-bearing, oxidizing conditions |
| Nuclear and chemical process vessel internals | 625 | Broader envelope plus higher strength |
| Large fabricated tanks with proven chemistry | 825 | Lowest capital cost for the duty |
| Cryogenic piping | 825 | Good toughness, economical |
Fabrication and Welding Notes
Incoloy 825 welds well and, because it is titanium-stabilised, resists sensitization in the heat-affected zone without post-weld heat treatment. It is commonly welded with matching filler metal; dilution from a dissimilar steel substrate must be managed with an appropriate buffer layer when cladding. 825 is softer than 625 and machines more easily, with a tendency to work-harden, so positive feeds and sharp tooling are used.
Inconel 625 is one of the most weldable high-nickel alloys, which is why it is used as a cladding and overlay material on carbon steel. Heat input is less critical than for C-276, but interpass temperature control and cleanliness are still required, and consumables must be low-carbon.
A practical detail from our own shop: when a client specifies 825 tubesheet-and-shell construction but upgrades only the tubes to 625, the difference in thermal expansion and yield strength concentrates stress at the rolled joint. The correct fix is usually to upgrade the tubesheet and tube-to-tubesheet connection together, not the tube alone. This is exactly the kind of partial upgrade that should be reviewed before the drawing is released — see our alloy technical knowledge center and related technical guides.
Price Reference (2026, EXW Shanghai)
| Form | Incoloy 825 | Inconel 625 | Comment |
|---|---|---|---|
| Round bar, annealed | \$28-42/kg | \$48-75/kg | 825 typically 40-50% cheaper |
| Plate / sheet | \$30-45/kg | \$50-78/kg | Width and thickness add spread |
| Seamless tube | \$35-55/kg | \$60-95/kg | Cold-work and testing dominate |
| Welded tube / pipe | \$30-48/kg | \$52-80/kg | Lower cost where the duty allows |
| Forging / flange | Quote by drawing | Quote by drawing | Yield and NDT drive cost |
Table note: Reference range only — floats with LME nickel price. 2026, EXW Shanghai, USD/kg. Confirm against current raw-material levels and the required certification package.
Standard Index
| Standard | Title / scope | Covers | Form |
|---|---|---|---|
| ASTM B424 | Ni-Fe-Cr-Mo-Cu alloy plate, sheet, strip | Composition + mechanical | plate, sheet, strip |
| ASTM B423 | Ni-Fe-Cr-Mo-Cu alloy seamless pipe and tube | Composition + mechanical | seamless tube |
| ASTM B425 | Ni-Fe-Cr-Mo-Cu alloy bar and forging billet | Composition + mechanical | bar, billet |
| ASTM B446 | Ni-Cr-Mo-Nb alloy bar, forging | Composition + mechanical | bar, forging |
| ASTM B444 | Ni-Cr-Mo-Nb alloy pipe and tube | Composition + mechanical | tube |
| ISO 15156 / NACE MR0175 | Materials for use in H2S-containing environments | Selection limits | — |
| ASTM E1476 | Standard guide for metals identification (PMI) | Test method | — |
| ASTM A262 / G28 | Intergranular corrosion testing | Test method | — |
How We Verify Both Grades Before Shipment
Both alloys arrive in the warehouse as bright, non-magnetic material and cannot be separated by eye, so we run an enforced three-step release:
- OES heat verification against the specification on the purchase order, with molybdenum, copper and iron read explicitly — that trio separates 825 from 625 and from 316L in a single pass.
- Certificate reconciliation. The EN 10204 3.1 certificate is matched line by line against the stencilled heat number on the bar end or plate corner, and the reconciliation is photographed into the batch record.
- Mechanical confirmation where the application is pressure-critical. Room-temperature tensile per ASTM E8/E8M plus hardness, and where relevant an intergranular corrosion check per ASTM A262/G28 for welded assemblies.
A substitution case we caught in 2026: a customer's drawing called for 825 for a phosphoric acid line, and the delivered tube analysed as Cr 21%, Ni 18%, Mo 3%, Cu 0.1% — an 800-series chemistry with no copper at all. It would have passed a visual inspection and a hardness check. It was rejected on the copper reading before shipment. Ask for that reading; it takes seconds and it is the difference between a working plant and a shutdown.
FAQ
Q1: Is Incoloy 825 or Inconel 625 better for sulphuric acid?
Incoloy 825 is usually the better economic choice for moderate-concentration sulphuric acid service up to roughly 60-80°C because its copper addition specifically improves resistance. Inconel 625 is the safer specification when the acid is more aggressive, the temperature is higher, or chlorides are present alongside it.
Q2: How much more expensive is Inconel 625 than Incoloy 825?
Typically 40-50% more per kilogram for bar, plate and tube in 2026 EXW Shanghai terms, because 625 carries 8-10% molybdenum with niobium and only 5% maximum iron, while 825 is iron-rich with 2.5-3.5% molybdenum. On fabricated equipment the gap can narrow because 625's higher strength can reduce wall thickness.
Q3: Can Inconel 625 be substituted for Incoloy 825?
In most cases yes — 625 has the broader corrosion envelope and higher strength. The exceptions are where the process specifically relies on copper-bearing chemistry (certain sulphuric acid duties) and where the material must be non-magnetic with very tight magnetic permeability limits, a constraint that usually favours other alloy families entirely.
Q4: Is Incoloy 825 suitable for sour gas service?
Yes, within defined limits. Both 825 and 625 are addressed by NACE MR0175 / ISO 15156, and 825 is widely used in oilfield tubulars and wellhead equipment. The acceptable window depends on H2S partial pressure, chloride content, temperature and pH. Because those limits are project-specific, the material selection must be confirmed against the standard's tables rather than by grade reputation.
Q5: Which alloy is better in seawater?
Inconel 625, particularly in stagnant or polluted seawater where 825 is susceptible to pitting and crevice attack. 825 performs acceptably in flowing, clean, aerated seawater and is common in seawater-cooled exchangers, but 625 and the super-austenitic stainless grades carry a larger margin.
Q6: Can Incoloy 825 be used for hydrofluoric acid?
No. Incoloy 825 is not recommended for hydrofluoric acid service. Monel 400 is the classic nickel-copper material for HF and alkylation service, and that is a different alloy family entirely. Our Monel alloy round bar & tube page covers that grade.
Q7: Does Incoloy 825 need post-weld heat treatment?
Generally no. The grade is titanium-stabilised, which resists sensitization in the heat-affected zone, so post-weld heat treatment is normally unnecessary — an important fabrication-cost advantage over unstabilised austenitic grades.
Q8: What are the UNS numbers?
Incoloy 825 is UNS N08825. Inconel 625 is UNS N06625. Your purchase order and certificate should both carry the UNS number rather than only the trade name.
Q9: Which alloy is stronger at elevated temperature?
Inconel 625 by a wide margin. At 650°C, 825 is typically well below 400 MPa ultimate tensile strength while 625 is typically in the 550-600 MPa range. Where the design is strength-limited at temperature, 825 is usually not competitive.
Q10: What forms and certifications do you supply?
Bar (6-500 mm), plate and sheet (0.1-100 mm), seamless and welded tube (OD 6-610 mm), forgings, flanges and wire — with EN 10204 3.1 or 3.2 certification, PMI and UT on request, and third-party witness inspection by SGS, BV or TUV.
Contact & Complete Product Range
Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier
Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com
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