Alloy 825, designated UNS N08825, is an austenitic nickel-iron-chromium alloy stabilised with titanium and alloyed with molybdenum and copper. The grade was developed to deliver corrosion resistance in both oxidising and reducing media within a single material, which is why it became a workhorse of chemical processing, hydrometallurgy and offshore oil and gas production. Product forms include plate, sheet, strip, seamless tube and pipe, welded tube, round bar and forged fittings.
Sheet and plate are commonly supplied in thicknesses from about 4.8 mm to 50.8 mm, with heavier plate available for pressure vessel work. Because the alloy is readily formed and welded by the techniques used for other nickel-based grades, it can be fabricated into heat exchangers, reaction vessels, piping and pumps without special shop equipment.
Chemical Composition of Alloy 825
The table gives the standard composition range for UNS N08825 in plate, sheet and strip product forms. Values are weight percent.
| Element | Content, wt % | Function |
|---|---|---|
| Nickel, Ni | 38.0–46.0 | Chloride stress corrosion cracking resistance |
| Chromium, Cr | 19.5–23.5 | Oxidising media and pitting resistance |
| Iron, Fe | 22.0 min (balance) | Matrix element |
| Molybdenum, Mo | 2.5–3.5 | Reducing media and pitting resistance |
| Copper, Cu | 1.5–3.0 | Sulphuric and phosphoric acid resistance |
| Titanium, Ti | 0.6–1.2 | Stabilises carbon, prevents sensitisation |
| Aluminium, Al | 0.20 max | Residual element, controlled |
| Carbon, C | 0.05 max | Prevents carbide precipitation |
| Manganese, Mn | 1.00 max | Deoxidation |
| Silicon, Si | 0.50 max | Deoxidation |
| Phosphorus, P | 0.030 max | Residual element, controlled |
| Sulphur, S | 0.030 max | Residual element, controlled |
The titanium addition of 0.6–1.2% is the key stabilising element. It ties up carbon as titanium carbide so that chromium carbides cannot precipitate on grain boundaries during welding or during exposure to the sensitising temperature range, and the alloy therefore retains its resistance to intergranular attack after fabrication. Molybdenum and copper work together on the corrosion side: molybdenum provides resistance to pitting and crevice attack in chloride media, while copper is what makes the alloy serviceable in sulphuric and phosphoric acid.
Corrosion Resistance in Practice
Alloy 825 resists uniform and localised corrosion in a wide range of process environments, including sulphuric acid, phosphoric acid, nitric acid, sulphur dioxide bearing and other reducing acid streams, organic acids, and alkaline solutions such as sodium hydroxide and potassium hydroxide. It also performs well in acidic chloride solutions and in flowing seawater under most conditions.
Chloride stress corrosion cracking: the high nickel content raises the cracking threshold well above that of conventional austenitic stainless steels, so the alloy is used where chlorides and tensile stress occur together.
Pitting and crevice corrosion: molybdenum addition gives useful tolerance in chloride-bearing and acid chloride environments, although the alloy is not a substitute for a high-molybdenum super-austenitic or nickel-molybdenum grade in severely pitting duties.
Seawater and brines: suitable for flowing seawater and for many polluted or brackish streams. In stagnant seawater, deposits and biological activity can create localised attack on any grade, so design should avoid stagnant zones and crevices.
Hydrofluoric acid: resistance to hydrofluoric acid is poor and the alloy should not be selected for that service. Confirmation of the specific acid concentration and temperature is always required for aggressive mixed-acid duties.
Atmospheric exposure: good general performance outdoors, with superficial staining possible in humid conditions; this does not affect structural performance.
Mechanical and Physical Properties
| Property | Value |
|---|---|
| Density | 8.14 g/cm³ |
| Melting range | 1370–1400 °C |
| Tensile strength, room temperature | 586 MPa (85 ksi) min |
| Yield strength, 0.2% offset | 241 MPa (35 ksi) min |
| Elongation in 50 mm | 30% min |
| Magnetic response | Essentially non-magnetic (austenitic) |
| Useful service temperature | Good mechanical strength up to about 550 °C |
The alloy keeps useful strength at both ambient and moderately elevated temperature. Pressure vessel certification for the grade is normally granted with a manufacturing temperature limit of about 450 °C, and above roughly 550 °C the available data become limited, so a creep assessment is required before the material is used hotter than that. Because the grade is austenitic it remains tough at cryogenic temperature, which is another reason it appears in low-temperature and liquefied gas piping.
Heat Treatment, Forming and Welding
Solution annealing: about 940 °C followed by rapid cooling, which dissolves carbides and restores the stabilised microstructure.
Intermediate annealing: preferably above 1050 °C when heavy cold forming is carried out in several steps.
Hot working: carried out in the 870–1170 °C range; working below about 650 °C should be avoided because the alloy has limited ductility there.
Cold forming: readily performed, with a high work-hardening rate that calls for intermediate anneals on severe deformation.
Welding: all common arc processes are used, with a nickel-chromium filler metal of the ERNiCr-3 (UNS N06082) type for matching corrosion resistance; the titanium-stabilised chemistry keeps the weld heat-affected zone resistant to intergranular corrosion.
Machining: normal for nickel-based austenitic materials, requiring rigid tooling, sharp edges and adequate cooling.
Equivalent Grades and Typical Applications
| System | Designation |
|---|---|
| UNS | N08825 |
| Werkstoff number | 2.4858 |
| EN symbol | NiCr21Mo |
| China (GB) | NS1402 / 0Cr21Ni42Mo3Cu2Ti |
| Japan (JIS) | NCF 825 |
Applications follow directly from the corrosion and mechanical profile: heat exchangers, pipework, valves and pumps handling hot sulphuric acid and chloride-bearing solutions; sulphuric acid pickling and ore processing circuits in hydrometallurgy; wood pulp digesters and liquor handling equipment in the paper industry; pollution control scrubbers; and deep sour gas well tubulars where combined resistance to chlorides and to hydrogen sulphide is needed. In each case the material is chosen because a single grade covers both oxidising and reducing process steps, which simplifies fabrication and reduces the number of welded dissimilar joints in a plant.
Frequently Asked Questions
Q: What is the UNS number of Alloy 825?
UNS N08825. Its Werkstoff number is 2.4858 and the EN designation is NiCr21Mo.
Q: What is the titanium content of Alloy 825 and why is it added?
Titanium is specified at 0.6–1.2%. It stabilises carbon as titanium carbide, which prevents chromium carbide precipitation on grain boundaries and keeps the alloy resistant to intergranular corrosion after welding.
Q: Is Alloy 825 suitable for seawater service?
Yes for flowing seawater, brackish water and most polluted streams. Stagnant seawater, deposits and crevices should be avoided in design because they promote localised attack.
Q: Can Alloy 825 be used in hydrofluoric acid?
No. Resistance to hydrofluoric acid is poor and another material family should be selected for that service.
Q: What is the maximum service temperature of Alloy 825?
Mechanical properties remain good up to about 550 °C, and pressure vessel certification is typically limited to a manufacturing temperature of about 450 °C. Hotter applications need a full creep evaluation.
Q: What filler metal is used to weld Alloy 825?
A nickel-chromium filler metal of the ERNiCr-3 (UNS N06082) type is normally used, with a matching or slightly higher-alloyed chemistry to preserve corrosion resistance in the weld zone.
Q: How is Alloy 825 heat treated?
Solution anneal at about 940 °C followed by rapid cooling. Intermediate anneals during severe cold forming are preferably carried out above 1050 °C.
