Performance and application of nickel-based alloy Incoloy 825 fasteners

Incoloy 825 fasteners — bolts, studs and nuts rolled from a nickel-iron-chromium alloy stabilized with titanium — are specified for flanged joints in sour gas, seawater and acidic chemical service. The nickel-chromium-molybdenum-copper balance resists reducing acids and chloride pitting far better than 316 stainless hardware, and it works with corrosion-resistant flange gaskets when a joint must stay sealed to the end of its design life.

Composition, Physical and Mechanical Data

Incoloy 825 keeps carbon at a very low level and adds titanium, so welded joints are not easily sensitized and show little tendency to intergranular corrosion. Chromium, molybdenum and copper deliver the acid resistance, while the high nickel content stabilizes the alloy against stress corrosion cracking.

Composition limits for alloy 825, % by mass. Iron (Fe) forms the balance of the alloy.

• Density: 8.14 g/cm³.
• Melting range: 1370–1400 °C.
• Tensile strength Rm: 550 N/mm².
• Yield strength Rp0.2: 220 N/mm².
• Elongation A5: 30%.
• Brinell hardness: ≤200 HB in the solution-annealed condition.

Corrosion Resistance in Acids and Chlorides

Alloy 825 is a general engineering alloy that resists acid and alkali corrosion in both oxidizing and reducing media. It performs well in sulfuric, phosphoric and nitric acids, in organic acids, in caustic solutions such as sodium hydroxide and potassium hydroxide, and in hydrochloric acid solutions.

• Sulfuric acid service — pickling lines, evaporators, and wash and dip tubes, where the copper addition supports resistance to the reducing acid.
• Chloride service — seawater cooling and marine piping, where the chromium, molybdenum and nickel content limits pitting and crevice corrosion.
• Welded assemblies — very low carbon plus titanium stabilization reduce sensitization and intergranular attack in the heat-affected zone.
• Mixed-media plants — nuclear fuel dissolvers, where sulfuric acid, nitric acid and sodium hydroxide are handled in the same equipment.

The alloy keeps useful mechanical properties at room temperature and up to 550 °C, which is why it is specified for hot acidic service rather than for ordinary ambient duty.

Where Incoloy 825 Fasteners Are Used

In pipeline and refinery projects, Incoloy 825 bolts, studs and nuts are fitted on pipe flanges, pumps, valves and vessels that carry acidic or sulfide-bearing fluids, because the alloy resists both corrosion and harsh service conditions.

• Oil and gas production — fasteners in sulfide-bearing environments, where resistance to sulfide stress cracking matters as much as general corrosion resistance.
• Offshore platforms — seawater piping and fasteners exposed to chloride ions, high salinity and high pressure, where the alloy keeps stable mechanical behaviour.
• Sour gas transmission — lines carrying gases with sulfides, where stress corrosion cracking attacks standard stainless hardware.
• Refineries — piping, pumps and valves handling hot acidic and oxidizing liquids at temperatures up to 550 °C.
• Acid plants — heating tubes, containers, baskets and chains in sulfuric acid pickling, plus evaporators and wash tubes in phosphoric acid production.

For fastener selection the decisive points are the medium, the chloride level and the maximum design temperature: within those limits alloy 825 keeps its strength and keeps the joint sealed.

Frequently Asked Questions

How does Incoloy 825 compare with 316 stainless steel fasteners?

316 stainless works in mildly chloride-bearing service, while Incoloy 825 adds molybdenum and copper on a nickel-rich base. It therefore holds up better in reducing acids, sulfide-bearing media and hot chloride environments.

What is the maximum service temperature for Incoloy 825 fasteners?

The alloy is widely used where the operating temperature stays at or below 550 °C. Above that range, strength and oxidation behaviour have to be reviewed case by case.

Are Incoloy 825 bolts suitable for sour service?

The alloy shows good resistance to sulfide stress cracking, which is why it is used in equipment handling corrosive gases and liquids. Check hardness and heat-treatment condition against the project specification before ordering.

What mechanical data should be used for design?

In the solution-annealed condition the alloy shows 550 N/mm² tensile strength, 220 N/mm² yield strength, 30% elongation and hardness at or below 200 HB.