Inconel 625 holds up in seawater, in hot sulfuric acid, and at 600 °C - three very different jobs that most alloys can only handle one of. What it does not do is come cheap. So the practical question for a buyer is not "is 625 good?" - it is "where does 625 stop being the right answer?"
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
Why the Chemistry Works the Way It Does
Inconel 625 is a solid-solution strengthened nickel-chromium-molybdenum alloy. Every element in the specification earns its place - and the combination is what makes the grade versatile.
| Element | ASTM B444 Limit (wt %) | What It Actually Does |
|---|---|---|
| Nickel (Ni) | 58.0 min | Austenitic base; keeps the alloy ductile at cryogenic and high temperatures |
| Chromium (Cr) | 20.0 – 23.0 | Forms the passive film that resists oxidizing media and high-temperature oxidation |
| Molybdenum (Mo) | 8.0 – 10.0 | The anti-pitting and anti-crevice element; also lifts resistance to reducing acids |
| Niobium + Tantalum (Nb+Ta) | 3.15 – 4.15 | Stabilizes carbides against sensitization and gives solid-solution strengthening - the reason 625 keeps strength at 600 °C |
| Iron (Fe) | 5.0 max | Balance |
| Carbon (C) | 0.10 max | Kept low to limit carbide precipitation in service |
| Titanium (Ti) | 0.40 max | Fine carbide control during processing |
| Aluminum (Al) | 0.40 max | Deoxidation during melting |
The niobium is the element most people overlook. Without it you are not looking at 625 - you are looking at a 6Mo austenitic grade. Niobium is what gives 625 its combination of high-temperature strength and weldability. If a certificate shows niobium near zero, the chemistry is wrong regardless of what the grade name on the paper says.
Strength: Room Temperature and at 600 °C
At room temperature, annealed 625 pipe delivers roughly double the yield strength of 316L. The more interesting numbers are at temperature, where the solid-solution strengthening from molybdenum and niobium keeps working while 316L loses most of its design stress.
| Property (annealed) | Inconel 625 | 316L (S31603) | 2205 Duplex (S32205) |
|---|---|---|---|
| Tensile strength, min | 827 MPa (120 ksi) | 485 MPa | 620 MPa |
| Yield strength, min | 414 MPa (60 ksi) | 170 MPa | 450 MPa |
| Elongation, min | 30 % | 40 % | 25 % |
| ASME B31.3 allowable stress @ 100 °C | ~ 172 MPa | ~ 115 MPa | ~ 138 MPa |
| ASME B31.3 allowable stress @ 400 °C | ~ 117 MPa | ~ 70 MPa | ~ 60 MPa |
| ASME B31.3 allowable stress @ 600 °C | ~ 55 MPa | ~ 25 MPa | not permitted |
Two takeaways. First, above 400 °C the duplex option disappears and 316L loses over half its design stress - this is the zone where a solid-solution nickel alloy earns its keep. Second, 625 is supplied in two conditions: Grade 1 (annealed at ≥ 871 °C) and Grade 2 (solution annealed at ≥ 1093 °C). Grade 2 is the standard call-out for corrosive service; Grade 1 is often chosen where only moderate corrosion resistance is needed but strength matters.
The Corrosion Numbers That Define Its Limits
Corrosion resistance is where 625 gets marketed as "universal" - it is not, and the numbers below show both the strengths and the boundaries.
| Metric | Value | What It Tells You |
|---|---|---|
| PREN = Cr + 3.3Mo + 16N | ~ 51 | Pitting resistance equivalent; higher than 316L (24) and 2205 (34) |
| Critical pitting temperature (ASTM G48 Method A) | typically > 90 °C | No pitting in standard ferric chloride test below this temperature |
| Critical crevice corrosion temperature (ASTM G48 Method B) | ~ 30 – 35 °C | Crevice attack is the real limit in seawater - plan for it in flanged joints |
| ASTM G28 Method A corrosion rate (annealed) | ~ 0.5 mm/yr class | Intergranular attack check; verifies correct heat treatment |
| Seawater service | Excellent, proven 30+ years | Marine piping, seawater cooling, firewater - the classic 625 job |
| Environment | Inconel 625 Performance |
|---|---|
| Seawater, static and flowing | Excellent; one of the most chloride-tolerant wrought alloys |
| Sulfuric acid, dilute (< 20 %), ambient to moderate temperature | Good to excellent |
| Hydrochloric acid, dilute, no oxidizers | Moderate; C276 (higher Mo) is the better choice |
| Nitric acid and oxidizing media | Good, but 316L or 6Mo grades are usually more economical |
| Wet chlorine / hypochlorite | Good at moderate concentrations |
| Hydrofluoric acid | Not recommended; Monel 400 handles HF better |
| Caustic soda, hot | Good; Nickel 201 is the specialist for this service |
The practical boundary: 625 is a chloride-and-mixed-acid alloy. For pure reducing acid service, the higher molybdenum of C276 wins. For hot caustic, Nickel 201 wins. For oxidizing service, stainless steel wins on price. 625 sits in the middle and that is exactly where it belongs.
Physical Properties That Affect Piping Design
| Property | Value | Design Note |
|---|---|---|
| Density | 8.44 g/cm³ | ~ 8 % heavier than 316L; weight matters in offshore topside design |
| Melting range | 1290 – 1350 °C | Above most service limits; not a constraint in practice |
| Thermal conductivity @ 20 °C | 9.8 W/m·K | Roughly one-third of carbon steel - insulation design must account for it |
| Thermal expansion (20 – 100 °C) | 12.8 µm/m·°C | Close to carbon steel; dissimilar welds to CS spools are manageable |
| Electrical resistivity | 1.29 µΩ·m | Relevant for PMI and induction heating parameters |
| Magnetic | Non-magnetic (annealed) | Preferred in instrument and control line service |
The low thermal conductivity is the one that surprises people: 625 pipe runs hotter on the surface than carbon steel for the same heat flow, which changes insulation thickness and personnel-protection rules.
Sizes, Forms and Standards
| Form | Standard | Typical Range |
|---|---|---|
| Seamless pipe | ASTM B444 / ASME SB444 | NPS 1/8" – 12" (10.3 – 323.9 mm OD), Sch 5S – 160S |
| Welded pipe | ASTM B705 / ASME SB705 | NPS 2" and up, larger diameters on request |
| Seamless heat exchanger tube | ASTM B704 / ASME SB704 | OD 6.35 – 127 mm, wall 0.89 – 12.7 mm |
| General requirements | ASTM B751 (welded tube), B775 (welded pipe) | Hydro, NDE, marking per standard |
Most inquiries we handle are for ASTM B444 seamless pipe in NPS 1/2" to 6", Sch 40S and 80S, Grade 2, solution annealed. For heat exchangers in seawater or FGD service, ASTM B704 tube is the usual call-out. If your drawing references an ASME code, make sure the material spec is the SB version (SB444 / SB705).
Where Inconel 625 Pipe Gets Used
| Industry | Typical 625 Pipe Application |
|---|---|
| Marine / offshore | Seawater cooling lines, firewater systems, ballast piping, subsea manifolds |
| Power / FGD | Flue gas desulfurization scrubber circulation lines, reheater piping |
| Chemical processing | Sulfuric and phosphoric acid plant piping, chlorinated organic reactor lines |
| Oil & gas | Subsea umbilicals, control lines, wellhead instrumentation tubing |
| Aerospace / hydraulic | Small-bore seamless tubing for high-pressure hydraulic and fuel lines |
If the service involves seawater, chlorides plus moderate temperatures, or a mix of acids, 625 pipe is usually the safest single-grade answer. That is why it shows up on Middle East and European project MTOs so often.
Where You Do Not Need Inconel 625
Knowing when not to buy 625 saves more money than any discount. The short version:
High temperature + oxidation, no seawater or strong acids → Inconel 600 or 800H does the job at a lower price.
Strong reducing acids (hydrochloric, wet HCl) → C276 carries more molybdenum (15–17 %) and outperforms 625.
Mild chlorides / neutral brine, no pitting risk above 40 °C → 2205 duplex or 316L covers it for a fraction of the cost.
Hot caustic service → Nickel 201 is the specialist; 625 is overkill.
Hydrofluoric acid → Monel 400, not 625.
If your service sits clearly in one of these lanes, tell your supplier - a good one will quote the cheaper grade and explain why. If it sits in the middle (mixed acids, chlorides, heat), 625 is the honest answer.
Why the Price Moves - and How to Budget
625 pricing tracks two exchange-traded metals: nickel and molybdenum. Nickel is the big weight (58 %+ of the alloy), but molybdenum swings are what create the surprise price jumps, because Mo costs several times more per ton than nickel. Practical rules: ask for a price validity period, and when LME nickel moves more than 5 % between quote and order, expect a revised number. Do not lock a project budget on a six-month-old quote.
FAQ
What is the difference between Inconel 625 and Hastelloy C276 pipe?
Molybdenum and niobium. C276 has 15–17 % Mo, which makes it better in strong reducing acids like hydrochloric acid. 625 has 8–10 % Mo plus 3.15–4.15 % niobium, which gives it higher strength, better weldability, and better high-temperature performance. For seawater and mixed-acid service both work; for hot pure HCl, C276 wins.
Can Inconel 625 pipe be used in seawater?
Yes - it is one of the most chloride-tolerant wrought alloys, with a critical pitting temperature above 90 °C in the standard test. Marine piping, seawater cooling, and firewater systems have used it for decades. The limit to watch is crevice corrosion in flanged joints and under gaskets, where the critical crevice temperature is much lower (~30–35 °C).
What is the maximum service temperature for Inconel 625 pipe?
For continuous service, around 650 °C is the practical oxidation ceiling, but pressure design is governed by the ASME B31.3 allowable stress, which drops steadily above 400 °C. For structural high-temperature strength, 625 keeps more allowable stress than 316L at the same temperature.
Why is Inconel 625 pipe so expensive?
Three alloying elements: nickel (58 % min, LME-traded), molybdenum (8–10 %), and niobium (3.15–4.15 %). Melting and hot-working a high-niobium alloy also costs more than standard stainless. The price is the market telling you this is a specialty product - which is exactly why the "do I really need 625?" check matters.
Seamless or welded Inconel 625 pipe - which one should I order?
Smaller diameters and higher pressures point to seamless ASTM B444. Large diameters and long runs are more economical as welded ASTM B705. Both are solution annealed and corrosion performance is equivalent when the weld is made with matching filler and the pipe passes the same hydro and NDE requirements.
Need Inconel 625 pipe for seawater, chemical, or high-temperature service?
HUITONG supplies ASTM B444/B705 UNS N06625 seamless and welded pipe in Grade 1 and Grade 2, with EN 10204 3.1/3.2 MTC and PMI verification on request. Send your size list and service conditions to market@htpipe.com or WhatsApp +86-19339900201 - and if 316L, 2205, C276 or Nickel 201 fits your service better, we will tell you before you pay.
