Nickel Aluminium Bronze Investment Castings: Grades, Properties and Marine Applications

TL;DR — Key Takeaways

• Nickel Aluminium Bronze (NAB) investment castings are specified as C95500 (AB1) and C95800 (AB2) — two distinct grades with different composition, strength, and marine performance profiles.

  
  

• C95800 (AB2) is the marine-optimised grade — lower aluminium content for superior dealuminification resistance — used for seawater pump internals, valve bodies, offshore hardware, and naval components.

  
  

• C95500 (AB1) delivers higher tensile strength and hardness — preferred for bearing surfaces, structural hardware, and high-strength marine applications

  
  

• C95800 offers low magnetic permeability (below 1.05) — a hard requirement for naval mine countermeasure vessels, sonar housings, and submarine hardware that stainless steel cannot meet.

  
  

• Applicable standards: ASTM B148, BS 1400 AB1/AB2, EN 1982 CC332G/CC333G, MIL-B-24480 (US Navy), DEF STAN 02-833 (UK Navy), with EN 10204 Type 3.1 material certification.

Nickel Aluminium Bronze investment casting — produced by the lost wax casting process — is the marine engineer’s default specification when a cast component must survive continuous seawater service while resisting corrosion, cavitation, and biofouling simultaneously.

Among copper alloys, Nickel Aluminium Bronze (NAB) delivers the best combination of strength and seawater corrosion resistance available, and when produced by investment casting, it achieves the complex geometries — pump impellers, valve bodies, propulsion hardware — that marine and naval systems require.

This guide covers the two Nickel Aluminium Bronze (NAB) grades most commonly used in marine castings, including their metallurgy, corrosion resistance, applicable standards, and typical applications. For a detailed overview of the investment casting manufacturing process, read our guide on Investment Casting: Process, Materials, and Industrial Applications. For a broader discussion of copper alloy castings, see Copper, Brass, and Bronze Investment Casting: Metallurgy, Process Control, and Industrial Applications.

Why NAB Is the Marine Engineer’s First Choice?

Marine and offshore environments present a combination of challenges that few materials handle well together: continuous chloride exposure, high-velocity flow that causes cavitation and erosion, biological fouling, and galvanic interaction between dissimilar metals in the same system. A material that resists one of these may fail at another.

Nickel Aluminium Bronze addresses all four simultaneously. It forms a tough, self-healing aluminium-rich oxide film that resists seawater corrosion. Its strength and hardness resist cavitation erosion in pump and propeller service. Its copper content provides natural biofouling resistance. And it offers predictable galvanic behaviour in mixed-metal marine systems. This combination is why NAB is the standard specification for seawater pump internals, valve bodies, propulsion hardware, and naval components across the global marine industry.

Pahwa MetalTech manufactures Nickel Aluminium Bronze (NAB) investment castings in C95500 (AB1) and C95800 (AB2), the two most commonly specified grades for marine, offshore, pump, valve, and seawater-handling applications. While this guide focuses on NAB alloys, aluminium bronze grades without nickel are discussed separately in Aluminium Bronze Investment Casting: Grades, Heat Treatment and Applications.

NAB Grades — AB1, AB2 and the Designation Cross-Reference

The single most common source of confusion in NAB specification is the designation system. The same alloy carries different names across ASTM, British, European, and military standards. Getting this right matters for procurement and certification documentation.

Nickel Aluminium Bronze - C95500 (AB1)

C95500 — designated AB1 under BS 1400 — is the higher-strength, higher-aluminium NAB grade. Composition: copper balance, aluminium 10.0–11.5%, nickel 3.0–5.5%, iron 3.0–5.0%, manganese up to 3.5%.

Standard	Designation
UNS	C95500
ASTM	B148 / B505
BS 1400	AB1
EN 1982	CC332G (verify against EN 1982:2008)
DIN	CuAl10Fe5Ni5

Nickel Aluminium Bronze - C95800 (AB2)

C95800 — designated AB2 under BS 1400 — is the marine-optimised NAB grade. Its lower aluminium content (8.5–9.5%) gives superior resistance to dealuminification in seawater. Composition: copper 79% minimum, aluminium 8.5–9.5%, nickel 4.0–5.0%, iron 3.5–4.5%, manganese 0.8–1.5%.

Standard	Designation
UNS	C95800
ASTM	B148 / B505
BS 1400	AB2 (CuAl10Fe5Ni5)
EN 1982	CC333G (CuAl10Fe5Ni5-C)
Military (US Navy)	MIL-B-24480
DEF STAN (UK Navy)	02-833 (formerly NES833)
JIS	H5121 CAC703C Class 3
DIN	W Nr 2.0975.04

When to Specify AB1 vs AB2

Requirement	Recommended Grade	Reason
Marine propellers, seawater immersion	C95800 (AB2)	Lower aluminium — superior dealuminification resistance
Naval components requiring low magnetic permeability	C95800 (AB2)	Permeability below 1.05
High-strength bearing surfaces and structural hardware	C95500 (AB1)	Higher tensile strength and hardness
Seawater pump internals (general)	C95800 (AB2)	Marine-optimised corrosion profile
High-load, slow-moving wear components	C95500 (AB1)	Higher hardness and load capacity

The Metallurgy Behind NAB’s Marine Performance — the α+κ Microstructure

NAB’s marine performance comes from its microstructure. Unlike simple copper alloys, NAB forms a complex structure of a copper-rich alpha (α) matrix containing several distinct kappa (κ) phases — intermetallic compounds rich in iron, nickel, and aluminium.

The kappa phases — designated κI, κII, κIII, and κIV — form at different stages of cooling and serve different functions. The iron-rich kappa phases provide strength and act as nucleation sites that refine the grain structure. The nickel-rich kappa phases are critical to corrosion resistance: they stabilise the microstructure against the selective phase attack that causes dealuminification in seawater.

This is why nickel content matters so much. Aluminium bronze without sufficient nickel is vulnerable to dealuminification — a corrosion mechanism where aluminium is selectively leached from the alloy in seawater, leaving a weak, porous copper structure.

The nickel additions in C95500 and C95800 form the protective kappa phases that resist this mechanism. The lower aluminium content of C95800 reduces dealuminification susceptibility further, which is why AB2 is the preferred grade for the most demanding seawater applications.

Critically, the kappa phase distribution depends on both alloy chemistry and cooling rate. Correct composition alone is not sufficient — the casting process and any subsequent heat treatment together determine whether the protective microstructure is achieved.

Property	C95500 (AB1)	C95800 (AB2)
Tensile Strength (as-cast)	~655 MPa	~590–650 MPa
Yield Strength (as-cast)	~290 MPa	~240–270 MPa
Elongation	~10%	~15%
Brinell Hardness	190–240 HB	150–180 HB
Aluminium Content	10.0–11.5%	8.5–9.5%
Dealuminification Resistance	Good	Superior
Magnetic Permeability	Low	Below 1.05

C95500 (AB1) delivers higher tensile and yield strength and greater hardness, making it the choice for bearing surfaces, gears, structural hardware, and high-load components. C95800 (AB2) trades a degree of strength for superior corrosion resistance and higher elongation, making it the choice for seawater-immersed components, propellers, and naval applications. Both grades offer excellent fatigue resistance — important for propellers and rotating components subject to cyclic loading.

Corrosion Performance in Marine Service

The Self-Healing Oxide Film

NAB forms a dense, adherent, aluminium-rich oxide film on its surface when exposed to oxygenated seawater. This film is self-healing — if mechanically damaged, it re-forms in the presence of oxygen. It is the primary barrier protecting the alloy from seawater corrosion and gives NAB its very low long-term corrosion rates in marine service.

Cavitation Erosion Resistance

Cavitation occurs when rapid pressure changes in high-velocity fluid flow create and collapse vapour bubbles against a metal surface. The collapse generates intense localised impact that progressively erodes the material — a major failure mode for pump impellers and propellers.

NAB resists cavitation erosion through the combination of its hardness and the toughness of its α+κ microstructure, which absorbs and distributes the impact energy of collapsing bubbles rather than fracturing under it. This is why NAB outperforms many stainless steels in pump impeller and propeller service where cavitation is a design concern.

Biofouling Resistance

The copper content of NAB provides natural resistance to biofouling — the accumulation of marine organisms such as barnacles and algae on submerged surfaces. Copper ions released slowly from the surface inhibit organism attachment. For seawater piping, pump, and valve components, this reduces flow restriction and maintenance frequency compared to materials that foul readily.

Dealuminification and Galvanic Compatibility

As covered in the metallurgy section, the nickel-bearing kappa phases in C95500 and C95800 resist dealuminification — the selective leaching of aluminium that destroys lower-grade aluminium bronzes in seawater.

In mixed-metal marine systems, NAB also offers predictable galvanic behaviour, sitting close to other copper alloys and many stainless steels on the galvanic series, which simplifies corrosion management when properly designed.

Low Magnetic Permeability — Naval and Defence Applications

C95800 offers a magnetic permeability below 1.05 — effectively non-magnetic. This is a hard specification requirement for a specific class of naval applications:

• Mine countermeasure vessels (MCMVs) — ships designed to detect and neutralise naval mines must minimise their magnetic signature to avoid triggering magnetic-influence mines. Every component, including pumps and valves, must be non-magnetic.

  
  

• Sonar housings and equipment — magnetic materials interfere with sensitive acoustic and magnetic sensing equipment.

  
  

• Submarine hardware — magnetic signature management is critical for submarine stealth.

Stainless steel — even austenitic grades that are nominally non-magnetic — can develop magnetic permeability through cold work and machining. NAB C95800 maintains its low permeability through fabrication, which is why it is specified for these applications where stainless steel cannot reliably meet the requirement.

Nickel Aluminium Bronze Investment Castings — Process Considerations

Investment casting is well suited to NAB marine components because it produces the complex geometries — curved impeller blades, internal flow passages, valve body cavities — that these applications require, with minimal machining and excellent surface finish.

The Oxide Film Challenge During Melting

The same aluminium content that gives NAB its corrosion resistance creates a foundry challenge. Molten aluminium bronze readily forms a tenacious aluminium oxide film on the surface of the melt. If this film is drawn into the casting during pouring, it creates oxide inclusions that compromise mechanical properties and pressure-tightness. Controlling pour technique, gating design, and melt handling to prevent oxide entrainment is fundamental to producing sound NAB castings — and is a primary reason many general foundries cannot cast NAB reliably.

Gating, Feeding and Simulation

NAB has a relatively narrow solidification range and requires carefully designed gating and feeding to avoid shrinkage porosity, particularly in the heavy sections typical of pump and valve castings. Simulation-based gating and feeding design — modelling mould filling and solidification before tooling is committed — is used to eliminate internal defects in complex marine components such as impellers and valve bodies.

Heat Treatment of C95800

For demanding seawater applications, C95800 castings may be given a corrosion-inhibiting temper anneal — typically heating to approximately 675°C for a minimum holding period followed by controlled cooling, per ASTM B148.

This heat treatment homogenises the microstructure, relieves casting residual stresses, and optimises the kappa phase distribution for maximum corrosion resistance. Note that propeller castings are commonly exempted from this requirement under the relevant specifications. Heat treatment requirements should be confirmed against the applicable standard and application at the specification stage.

Standards and Specifications for NAB Marine Castings

Standard	Scope
ASTM B148	Aluminium bronze sand and investment castings — C95500, C95800
ASTM B505	Copper alloy continuous castings
BS 1400 AB1 / AB2	British Standard NAB casting grades
EN 1982 CC332G / CC333G	European NAB casting designations
MIL-B-24480	US Navy specification for C95800
DEF STAN 02-833	UK Naval specification (formerly NES833)

Marine Applications of NAB Investment Castings

Seawater Pump Impellers and Casings

NAB is the standard material for seawater pump internals — impellers, casings, wear rings, and sleeves. The combination of cavitation erosion resistance, seawater corrosion resistance, and strength makes it the default choice for cooling water pumps, ballast pumps, and firewater pumps on ships and offshore platforms.

Valve Bodies and Trim

Butterfly valve bodies and discs, gate valve components, and valve trim in seawater service. NAB’s non-galling behaviour between moving parts and its biofouling and corrosion resistance make it superior to stainless steel for many marine valve applications. Common in API 609 and ASME class butterfly valves for marine and offshore fluid systems.

Ship Rudder Stocks, Pintles and Propulsion Hardware

Rudder stocks, pintles, propeller hubs, and propulsion system components. C95800 is specified for marine propellers and propulsion hardware where dealuminification resistance and fatigue strength under cyclic loading are critical.

Offshore Platform Hardware

Seawater lift pump components, riser and caisson hardware, valve bodies, and fittings on offshore oil and gas platforms exposed to splash zone and immersion conditions. NAB’s performance in high-chloride, high-velocity seawater makes it indispensable for offshore seawater handling systems.

Naval and Defence Components

Pump and valve components for mine countermeasure vessels, sonar housings, and submarine hardware requiring low magnetic permeability. C95800 to MIL-B-24480 and DEF STAN 02-833 is the specification for these applications, where the combination of non-magnetic behaviour and seawater performance cannot be matched by stainless steel.

Desalination Plant Components

High-pressure seawater pump components, valve bodies, and fittings in reverse osmosis and thermal desalination plants. NAB’s resistance to high-salinity, high-velocity seawater makes it a standard specification for desalination seawater handling equipment — a growing application as coastal desalination capacity expands.

Fire Suppression and Firefighting Systems

Seawater-fed firefighting pump components, valve bodies, and nozzle hardware on ships, offshore platforms, and coastal installations. NAB’s reliability in intermittent seawater service — where the system must function on demand after long idle periods — makes it the preferred material for marine fire suppression hardware.

Nickel Aluminium Bronze vs Alternative Materials for Marine Applications

NAB vs SS316L: SS316L is widely used in marine service but suffers pitting, crevice corrosion, and stress corrosion cracking in warm, stagnant, or high-chloride seawater. NAB resists these mechanisms and offers superior cavitation resistance and biofouling resistance. SS316L is easier to machine and weld and is appropriate for lower-velocity, intermittently wet applications. For continuously immersed, high-velocity seawater service, NAB is generally the better material.

NAB vs Duplex Stainless Steel 2205: Duplex 2205 offers excellent chloride corrosion and stress corrosion cracking resistance with very high strength. The choice between NAB and duplex 2205 depends on the specific application — duplex 2205 for high-strength, high-pressure pressure-retaining components, NAB for cavitation-critical, biofouling-exposed, or low-magnetic-permeability applications. Duplex 2205 investment casting is covered in detail at Duplex Stainless Steel 2205 Investment Casting: Properties, Process and Applications

Pahwa MetalTech manufactures Nickel Aluminium Bronze investment castings in C95500 (AB1) and C95800 (AB2) to ASTM B148, BS 1400, and EN 1982 from our foundry in Chakan, Pune — with EN 10204 Type 3.1 material certification, simulation-based gating and feeding, heat treatment capability, and full NDT inspection.

Send your component drawings, material specification, annual volume, and inspection requirements to info@pahwametaltech.co.in for a technical review and quotation.