Hastelloy C-4 is a nickel-chromium-molybdenum alloy (approximately 65 percent nickel, 16 percent chromium, 16 percent molybdenum) with controlled low carbon and silicon. It is designed for excellent thermal stability and resistance to sensitization, making it highly resistant to pitting, crevice corrosion, and stress corrosion cracking in a wide range of aggressive environments, including oxidizing and reducing acids.

The nominal chemical composition is: nickel 65.0 percent (balance), chromium 14.0–18.0 percent, molybdenum 14.0–17.0 percent, iron up to 3.0 percent, titanium up to 0.7 percent, cobalt up to 2.0 percent, and smaller amounts of carbon (max 0.015 percent), manganese, and silicon.

Key mechanical properties (typical, annealed condition):

● Tensile strength: 690–790 MPa

● Yield strength (0.2% offset): 310–380 MPa

● Elongation: 40–50 percent

● Density: 8.64 g/cm³

Hastelloy C-4 offers excellent resistance to pitting and crevice corrosion in chloride-containing environments, and to stress corrosion cracking. It resists oxidizing acids (e.g., nitric, chromic) as well as reducing acids (e.g., dilute sulfuric, hydrochloric) up to moderate temperatures. The alloy’s high thermal stability (due to the absence of tungsten and controlled low iron) makes it resistant to the formation of embrittling intermetallic phases after long-term exposure in the 600–800°C range, which is a key advantage over earlier C-series alloys.

Common standards: ASTM B575 (plate, sheet, strip), B574 (bar, wire), B622 (seamless pipe and tube), B619 (welded pipe), B626 (welded tube), ASME SB-575, SB-574, SB-622, and NACE MR0175/ISO 15156 for sour service.

Typical applications include:

● Chemical processing: reactors, heat exchangers, distillation columns, and piping for mixed acids and chloride-contaminated media.

● Flue gas desulfurization (FGD) systems.

● Pharmaceutical and fine chemical equipment.

● Nuclear fuel reprocessing and waste treatment.

● Oil and gas: downhole components and flowlines in sour service (H₂S, CO₂, chlorides).

Comparison with other Hastelloy alloys:

● Hastelloy C-4 has higher thermal stability than C-276 and C-22, making it more resistant to embrittlement during long-term high-temperature exposure. However, C-276 and C-22 offer better overall corrosion resistance in strongly reducing or mixed acids due to higher molybdenum and tungsten content.

● Compared to C-22, C-4 has lower chromium and no tungsten, so it is less resistant to oxidizing acids and pitting.

● For applications requiring the broadest corrosion resistance (both oxidizing and reducing), C-22 or C-276 are preferred. For high-temperature service where thermal stability is critical (e.g., long-term exposure above 600°C), C-4 is an excellent choice.

Selection guidance: Choose Hastelloy C-4 for applications involving long-term exposure at intermediate to high temperatures (600–800°C) where resistance to sensitization and embrittlement is essential, combined with good general corrosion resistance. For lower temperature, highly aggressive mixed acid environments, C-22 or C-276 are more suitable. For purely reducing acid service, B-3 is preferred.