Incoloy 825 is a nickel-iron-chromium alloy with additions of molybdenum, copper, and titanium. It is designed for exceptional resistance to reducing and oxidizing acids, chloride stress corrosion cracking, and pitting. The alloy is stabilized with titanium to resist intergranular corrosion after welding.

The nominal chemical composition is: nickel 38.0–46.0 percent, chromium 19.5–23.5 percent, iron balance (minimum 22 percent), molybdenum 2.5–3.5 percent, copper 1.5–3.0 percent, titanium 0.6–1.2 percent, and smaller amounts of carbon, manganese, and silicon.

Key mechanical properties (typical, annealed condition):

● Tensile strength: 550–690 MPa

● Yield strength (0.2% offset): 220–310 MPa

● Elongation: 30–45 percent

● Density: 8.14 g/cm³

Incoloy 825 offers excellent resistance to sulfuric acid (up to moderate concentrations), phosphoric acid, nitric acid, and organic acids. It resists chloride and caustic stress corrosion cracking, as well as pitting and crevice corrosion in seawater. The alloy is readily weldable and maintains good ductility.

Common standards: ASTM B424 (plate, sheet, strip), B423 (seamless pipe and tube), B425 (bar and wire), B564 (forgings). ASME SB-424, SB-423, SB-425. Also NACE MR0175/ISO 15156 for sour service.

Typical applications include:

● Chemical processing: tanks, piping, heat exchangers for sulfuric and phosphoric acid service.

● Oil and gas: downhole tubing, flowlines, and components in sour (H2S) environments.

● Marine: seawater cooling systems and pump shafts.

● Nuclear fuel reprocessing: equipment handling nitric acid.

● Pollution control: flue gas desulfurization (FGD) scrubbers.

Comparison with other alloys: Incoloy 825 has higher nickel and molybdenum than stainless steels, giving better acid resistance. Compared to Inconel 625, it has lower molybdenum and no niobium, so lower pitting resistance and strength. Compared to Alloy 20 (Carpenter 20), Incoloy 825 offers similar or better performance in sulfuric acid at higher temperatures. For severe reducing acids, higher nickel alloys like Hastelloy C-276 are preferred.

Selection guidance: Choose Incoloy 825 for moderate to severe acid service (especially sulfuric and phosphoric), for chloride stress corrosion cracking resistance, and for sour gas applications requiring NACE compliance. For higher strength or more severe pitting environments, consider Inconel 625 or Alloy 276.