What is Monel 400?

Monel 400 (UNS N04400) is a solid-solution strengthened nickel-copper alloy containing approximately 63–70% nickel and 28–34% copper, with small controlled amounts of iron, manganese, carbon, silicon, and sulfur. The alloy was developed in the early 1900s and has since become the most widely used member of the Ni-Cu family. Its strength comes from the solid-solution matrix itself — there is no precipitation hardening mechanism, which is why Monel 400 stays readily weldable and formable in all product forms.

The Ni-Cu combination gives Monel 400 a corrosion resistance profile that is genuinely unique. It resists hydrofluoric acid at all concentrations up to the boiling point — a capability that very few alloys share. In seawater, it shows excellent general corrosion resistance and is immune to chloride-induced stress corrosion cracking, the exact failure mode that causes 316L stainless steel pump shafts to crack within months in marine service. Monel 400 also handles alkalis, neutral salts, and reducing chemical environments well. Where it falls short is oxidizing environments — nitric acid, ferric chloride, and chromic acid will attack it. For those applications, Inconel 625 or Hastelloy C276 are the correct alternatives.

Chemical Composition

Per ASTM B127 / B164 / B165. Weight percent. Monel 400 is a binary Ni-Cu alloy — no Al or Ti, which means no precipitation hardening and excellent weldability.

Mechanical Properties

Monel 400 is supplied in three conditions. Unlike K500, there is no age-hardening option — strength is controlled by cold work and annealing only:

Physical Properties

Heat Treatment & Processing

Monel 400 is a solid-solution alloy — there is no phase transformation and no precipitation hardening. Heat treatment is limited to annealing (for softness and stress relief) and stress-relief annealing (for dimensional stability in machined parts).

Corrosion Resistance

Monel 400's corrosion resistance comes from its Ni-Cu solid-solution matrix. The high nickel content provides nobility in reducing environments, while copper contributes immunity to specific acids. Here is how it performs across the key environments:

Seawater & Marine

Excellent resistance to general corrosion, pitting, and crevice attack in flowing seawater. Immune to chloride stress corrosion cracking — the failure mode that cracks 316L stainless steel pump shafts in marine service within months. Monel 400 has been the standard propeller shaft material on naval vessels for decades. Limitation: in stagnant seawater (below 0.6 m/s flow velocity), pitting and biofouling can occur — maintain flow or add cathodic protection. Also note: Monel 400 can show slight ferromagnetism near room temperature (Curie point 21–49°C), which matters for non-magnetic instrument applications — for those, specify Monel K500 instead.

Hydrofluoric Acid (HF)

Monel 400 is the standard material for handling hydrofluoric acid across all concentrations up to the boiling point. This is its signature capability — very few alloys match it. HF acid is used in alkylation units in refineries, fluorochemical production, and glass etching. Monel 400 heat exchanger tubes, pump casings, and valve trims are standard specifications in these services. Note: aerated or high-velocity HF can cause erosion-corrosion — design for low velocity or use Monel 400 with work-hardened surface.

Caustic (Alkali) Services

Excellent resistance to sodium hydroxide (caustic soda) and potassium hydroxide across a wide concentration and temperature range. Monel 400 is used in caustic evaporation service, mercury cell caustic handling, and alkaline scrubbing systems. For caustic concentrations above 75% at temperatures above 260°C, Inconel 600 is sometimes preferred — but for the majority of caustic services, Monel 400 is technically adequate and more cost-effective.

What Monel 400 Does NOT Resist

Avoid Monel 400 in oxidizing acids (nitric acid, chromic acid), ferric chloride, cupric chloride, and sulfur dioxide in the presence of moisture — these will cause rapid attack. For oxidizing acid services, Inconel 625 or Hastelloy C276 are the correct choices. Also avoid Monel 400 in sulfidation environments above 315°C — the copper forms low-melting eutectics with sulfur.

Monel 400 vs Similar Grades

Monel 400 is the default choice for Ni-Cu service — but when strength, temperature, or weldability drive the spec, here is how the alternatives compare:

→ For a side-by-side technical deep-dive: Monel 400 vs Monel K500 — Full Comparison Guide

Typical Applications by Industry

Applicable Standards & Specifications

Available Product Forms from HT PIPE

We carry Monel 400 in annealed and stress-relieved conditions. EN 10204 3.1 certification on every shipment; 3.2 available on request.

HT PIPE Project Experience

In 2024, we supplied 12.8 tons of Monel 400 seamless pipe (ASTM B165, 2″ SCH 80 and 4″ SCH 40) and matching flanges (ASTM B564) to a Middle East refinery for their HF alkylation unit revamp. The order required 100% PMI verification, hydrostatic test at 1.5× design pressure, and EN 10204 3.1 certification with full traceability. All materials were delivered with flange drilling to ANSI B16.5 #300 RF, pre-inspected by the client's third-party inspector, and shipped in 10 weeks from order confirmation.

We also regularly supply Monel 400 plate and bar to marine equipment manufacturers for propeller shaft and pump casing fabrication. Contact our sales engineers for a reference case matched to your application and project location.

Frequently Asked Questions