Monel Pipe

Quick Answer

Monel is a nickel-copper alloy (approximately 67% Ni, 30% Cu) with exceptional resistance to hydrofluoric acid (HF), seawater, and reducing acids. Monel 400 (UNS N04400) pipe per ASTM B165 is the standard material for HF alkylation units in refineries and seawater service in marine/offshore applications.

Monel alloys were among the first nickel-based alloys developed commercially. They maintain a single solid-solution phase that resists attack from a range of corrosive media that would damage both carbon steel and stainless steel.

Monel Alloy Grades

Property	Monel 400	Monel K-500
UNS	N04400	N05500
Ni (%)	63-70	63-70
Cu (%)	28-34	27-33
Fe (max %)	2.5	2.0
Al (%)	-	2.30-3.15
Ti (%)	-	0.35-0.85
Yield (min, annealed)	172 MPa (25 ksi)	690 MPa (100 ksi, aged)
Tensile (min, annealed)	483 MPa (70 ksi)	1000 MPa (145 ksi, aged)
Hardness	~70 HRB	~30 HRC (aged)
Pipe spec (seamless)	ASTM B165	ASTM B865 (bar/rod)
Pipe spec (welded)	ASTM B725	-
Max. service temp	~480°C	~480°C

Why Monel 400 Is Used

Monel 400’s unique resistance profile targets specific corrosive environments:

• Hydrofluoric acid (HF): Monel 400 is the standard material for HF alkylation unit piping, vessels, and equipment. No other commercially available alloy offers the same combination of HF resistance and mechanical strength at reasonable cost.
• Seawater: Excellent resistance to flowing and stagnant seawater, with corrosion rates below 0.025 mm/year. Used for seawater cooling systems, pump shafts, and propeller shafts.
• Reducing acids: Sulfuric acid (concentrations above 80%), deaerated hydrochloric acid (low concentrations)
• Alkalis: Caustic soda and potash at all concentrations and temperatures up to boiling

Applications

• HF alkylation unit piping and equipment (refineries)
• Seawater intake and cooling systems (offshore platforms)
• Marine hardware and propulsion components
• Salt production and brine handling
• Chemical processing with reducing acid environments
• Valves, pumps, and instrumentation components

Monel K-500

Monel K-500 adds aluminum and titanium to the base Monel composition, enabling precipitation hardening (aging) to achieve yield strengths above 690 MPa. K-500 is used for:

• Pump and impeller shafts
• Doctor blades and scrapers
• Oil and gas wellhead components
• Springs and fasteners in marine service

K-500 is rarely used for pipe; it is primarily a bar, forging, and wire product.

Common Mistake

Monel 400 is susceptible to stress corrosion cracking in aerated, high-temperature fluoride solutions and mercury environments. It is also attacked by oxidizing acids (nitric acid, ferric chloride). Always verify the specific corrosive medium before specifying Monel; it excels in reducing environments but can fail in oxidizing ones.

Monel pipe costs approximately 8-12x more than carbon steel and 2-4x more than 316L stainless. Procurement lead times are 16-24 weeks for seamless pipe.

Read the full guide to pipe types