1. Chemical Composition
Nickel (Ni): ~57% (balance)
Chromium (Cr): 14-16%
Molybdenum (Mo): 15-17%
Tungsten (W): 3-4%
Iron (Fe): 4-7%
Small amounts of carbon, silicon, manganese, and cobalt.
Nickel (Ni): 63-67% (balance)
Copper (Cu): 28-34%
Small amounts of iron (1.25% max), manganese (1% max), and carbon (0.3% max).
2. Corrosion Resistance
Excels in severe reducing environments, such as concentrated hydrochloric acid (HCl), sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), and hydrofluoric acid (HF) (when properly diluted). It resists pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride-rich solutions (e.g., seawater, brines) even at high temperatures.
Performs well in mixed acid systems (e.g., sulfuric + nitric acid) and oxidizing-reducing cycles.
Maintains stability in high-temperature corrosive environments (up to ~1,093°C/2,000°F) due to its chromium content.
Thrives in neutral to slightly acidic/alkaline environments, particularly those containing saltwater, seawater, and dilute non-oxidizing acids (e.g., acetic acid, sulfurous acid).
Resists corrosion by hydrofluoric acid (HF) and fluorides, even at high concentrations-a key advantage over many other alloys.
Performs poorly in strong oxidizing acids (e.g., nitric acid, concentrated sulfuric acid) and reducing acids like hydrochloric acid, where its copper content accelerates corrosion.
Is susceptible to SCC in aerated, high-temperature (>60°C/140°F) chloride solutions, limiting its use in such conditions.
3. Mechanical Properties
Hastelloy C276 has higher tensile and yield strength, making it more suitable for high-stress applications at elevated temperatures. Its strength remains stable at temperatures up to ~815°C (1,500°F), with good creep resistance.
Alloy 400 has moderate strength but excellent ductility, making it easy to form (e.g., bending, machining). However, its strength drops significantly above 315°C (600°F), limiting high-temperature structural use.
4. High-Temperature Performance
Resists oxidation and sulfidation up to ~1,093°C (2,000°F).
Maintains mechanical integrity in high-temperature corrosive environments (e.g., flue gases, chemical reactors).
Oxidizes rapidly above ~538°C (1,000°F) due to its low chromium content.
Loses strength significantly at temperatures exceeding 315°C (600°F), making it unsuitable for high-heat structural roles.
5. Applications
Chemical processing: Reactors, valves, and piping for handling strong acids (HCl, H₂SO₄) and chloride-rich streams.
Aerospace: Combustion chambers, exhaust systems, and heat exchangers.
Environmental engineering: Wastewater treatment equipment for acidic or chlorinated effluents.
Oil and gas: Downhole components and offshore equipment exposed to corrosive brines.
Marine engineering: Seawater valves, pumps, and hull fittings (resists seawater corrosion but not high-temperature chloride SCC).
Chemical processing: Equipment for HF handling, caustic solutions, and food-grade acids (e.g., acetic acid).
Automotive: Fuel tanks and lines (resists gasoline and alcohol blends).
Electronics: Electrical components (good conductivity and corrosion resistance in mild environments).
6. Cost and Machinability
Cost: Hastelloy C276 is significantly more expensive than Alloy 400 due to its high molybdenum and tungsten content (molybdenum is ~5x more costly than copper).
Machinability: Both alloys are considered "difficult" to machine due to work hardening, but Alloy 400 is slightly easier than C276. C276's high molybdenum and tungsten increase tool wear, requiring specialized machining techniques.
C276 dominates in severe reducing/oxidizing corrosive conditions and high temperatures, making it ideal for extreme industrial and aerospace applications.
Alloy 400 excels in milder, neutral-to-slightly-corrosive environments (e.g., seawater, HF) and offers cost and formability advantages for less demanding uses.
