Copper Nickel Alloys – Austral Wright Metals

Unfouled copper nickel alloy mesh (right) with adjacent heavily fouled galvanised steel mesh after exposure in sea water.

Photo: International Copper Research Association

Boat Hulls

Copper-nickel is one of the few engineering materials with good inherent resistance to both corrosion and biofouling making the alloy a suitable material for boat hulls without recourse to cathodic protection or antifouling and anticorrosion paints.

Hydraulic Lines

With adequate strength to withstand pressures in most marine hydraulic and instrumentation systems, copper-nickel provides good service, combined with ease of manipulation at installation.

Desalination Units

Considerable quantities of 90-10 copper-nickel are used in Multi-Stage Flash Desalination Units predominately located in the Middle East. The alloy is a prime condenser tubing material for the Heat Recovery Section and is also used for water boxes, tube plates and other fabrications as solid material or as clad steel plate.

Alloy Properties

Mechanism of Corrosion Resistance

The corrosion resistance of the alloys is due to the protective surface film formed when in contact with water. On initial immersion cuprous oxide is formed but complex changes occur in sea water which research work is only now beginning to elucidate. At a flow rate of 0.6 m/s the equilibrium corrosion rate is an almost negligible 0.002 mm/year. Normally, design flow rates of up to 3.5 m/s give a satisfactory safety factor for use in pipework systems. This figure makes allowance for the fact that local speeds may be higher at changes of direction, points of divergence, etc. If water velocity is excessive, it can cause vortices leading to impingement attack which can cause premature failure. Where surfaces in contact with water allow smooth flow, as in ships hulls, different design criteria apply.

As mentioned, the fouling resistance is due to the copper ions at the surface, making it inhospitable to most marine organisms in slowly moving water. In static conditions there may be some deposition of chemical salts and biological slimes, possibly leading to some weakly adherent fouling but such residues are easily detached from the metal’s corrosion resistant surface, exposing a fresh, biocidally active surface.

Easy to Fabricate

Hot and cold working techniques can be used but because of the good ductility of the alloy, cold working is normally preferred.

Sea Water Piping Systems can last a Ship’s Lifetime

Experience over the last 40 years has confirmed the durability of copper-nickel.

Inherent Resistance to Biofouling

The protective oxide surface film which forms naturally on CuNi in sea water also provides an inhospitable surface to deter marine growth.

Corrosion in Sea Water

The 90/10 and 70/30 cupronickel alloys both have excellent resistance to bio-fouling and corrosion in sea-water with some variations in the performance of the alloys under different conditions as shown in Tables 10 and 11. For example, the 90/10 alloy has better bio-fouling resistance. The corrosion resistance of the 90/10 and 70/30 alloys in heat exchangers and condensers is compared with a number of other alloys in Figure 1. Table 7 gives the relative resistance of various alloys to fouling in quiet sea-water. If water velocity is accelerated above 1 m/sec, any slight bio-fouling on metal with good fouling resistance will be easily detached and swept away. On a material that does not have this good fouling resistance, strongly adherent, marine organisms would continue to thrive and multiply.

Design Conditions for Sea Water Service

The effect of water velocity on fouling and corrosion rates of various metals is shown in Figure 1, which also shows the typical service design speeds for some items of common equipment in contact with sea-water. The excellent corrosion resistance of 70/30 and 90/10 copper nickel alloys and their suitability for many applications can be seen. Some materials with apparently better corrosion resistance may have disadvantages such as lack of resistance to bio-fouling, lack of availability in the forms required, or susceptibility to crevice corrosion. They may also be more expensive and therefore less cost-effective over the required service lifetime.

Crevice Corrosion and Biofouling

Crevice corrosion can occur in components in sea-water when they are locally starved of oxygen at a joint or under attached bio-fouling. Table 8 shows the good tolerance of the copper-nickel alloys to this type of attack, giving these alloys advantages over other materials of equal corrosion resistance.

The copper-nickel alloys have good corrosion resistance in the quiescent or stagnant conditions which may occur during the commissioning or overhaul of plant. Where plant is not being used at design speeds some other materials may fail.

Development of a Corrosion Resistant Surface

When first brought into use, care must be taken to allow copper-nickel alloys to form their protective corrosion resistant surface freely. Normally, this protective film will develop in six to eight weeks. Contact with other less noble metals or with cathodic protection systems must be avoided to ensure development of the corrosion resistant surface film and the non-fouling properties.

Stress Corrosion Cracking

Copper-nickel alloys do not suffer the stress-corrosion problems associated with some other materials, such as copper zinc alloys (brass) with more than 15% zinc.

Titanium

Hastelloy C

Inconel 625

Titanium will pit at temperatures above 120°C.

Inconel 625 after 2-3 years shows signs of incipient pitting in some tests in quiet seawater

• carbon steel
• copper-nickel cladding on steel
• copper-nickel panels with aluminium anodes
• freely exposed copper-nickel (far right)

90-10

Copper

Nickel

70-30

Copper

Nickel

90-10

Copper

Nickel

70-30

Copper

Nickel