Use of copper-nickel in marine systems

Copper-nickel (CuNi or cupronickel) alloys are used for seawater systems, desalination units and to protect offshore structures

Copper has excellent resistance to corrosion in the atmosphere and in fresh water. In seawater, the copper-nickel alloy has superior corrosion resistance

Copper cladding of wooden-hulled warships, introduced by the Royal Navy in the 18th century to prevent damage by wood-boring insects and worms such as the teredo, and reduce coverage of weed and molluscs. This meant that ships could stay at sea for long periods without cleaning. Nelson’s successful blockade tactics, and subsequent victory at Trafalgar was partly due to the superior speed and manoeuvrability of his clean-hulled ships

A brass replacement for copper was developed in the 19th century, composed of around 60% copper and 40% zinc. Called Muntz Metal, it was cheaper than copper and could be hot rolled into thin sheets. An early application was cladding of the wooden hull on the Cutty Sark

The addition of nickel to copper improves its strength and durability, and also the resistance to corrosion and erosion in natural waters, including seawater, brackish and treated water. These copper alloys also show excellent mechanical properties, including resistance to stress-corrosion cracking and corrosion fatigue

“…used by many Navies and in merchant shipping”

Copper-nickel alloys were developed specifically for seawater service over five decades ago, initially for condensers and piping systems. Their overall suitability has since been confirmed by continuing use in these and other marine applications. The alloys are currently used in sizeable quantities for offshore, power, and desalination industries and also continue to be used by many Navies and in merchant shipping

“…inherent resistance to macrofouling”

Although developed for seawater corrosion resistance, it was soon recognised that the alloys also had an inherent resistance to macrofouling. This property has been advantageous in avoiding or reducing the necessity for biocide dosing in condensers and seawater systems, and in reducing drag forces and cleaning regimes on offshore platforms and boat hulls

“…provide the best combination of resistance to flowing seawater and overall corrosion resistance”

There are two main copper-nickel alloy grades used in marine service which are generally available in most product forms. These are copper base alloys with either 10% or 30% of nickel and are described as 90-10 (90/10) and 70-30 (70/30) copper-nickel respectively. Both alloys contain small but important additions of iron and manganese which have been chosen to provide the best combination of resistance to flowing seawater and overall corrosion resistance. The 30% nickel alloy is stronger and can withstand higher seawater velocities but, for most applications, the 90-10 alloy provides good service at a lower cost and of the two alloys tends to be the one that is more widely used. Also worthy of mention is a modified 30%Ni alloy containing 2%Mn and 2% Fe, which is only commercially available as condenser tubing and is being used particularly in the heat rejection section of multistage flash desalination units where higher resistance to impingement corrosion is required. Maximum levels are defined for some specific impurities because of their effects on hot ductility, hot workability, and weldability. These elements can also arise from external contamination and therefore precautions are necessary when the alloys are handled during forming and welding. The 2%Mn and 2%Fe grade is produced as a seamless tube for expanding into tube sheets so that welding is not necessary

Ocean Footprint are distributors for a range of 90-10 cupronickel press-fit piping system called MarinePRES manufactured by Raccorderie Metalliche (RACMET) which when installed by a trained installer can achieve many certifications without the requirement for hot works or costly x-ray inspections