Copper-nickel alloys, also known as cupronickel or CUNI, are widely used in marine systems such as seawater cooling systems, desalination plants, and offshore structures. Their unique properties make them ideal for harsh aquatic environments where corrosion resistance is critical.

While copper itself performs well in atmospheric conditions and fresh water, copper-nickel alloys offer superior performance in seawater. The addition of nickel not only enhances the mechanical strength of the material but also improves its resistance to both corrosion and erosion in natural waters, including seawater, brackish water, and treated water. These alloys also exhibit excellent resistance to stress corrosion cracking and corrosion fatigue, making them highly reliable in long-term marine applications.

Many navies and merchant vessels have relied on copper-nickel alloys for decades due to their durability and performance in seawater environments.

Originally developed over 50 years ago for use in condensers and piping systems, copper-nickel alloys have proven their effectiveness in a wide range of marine applications. Today, they are extensively used in the offshore, power generation, and desalination industries. They remain a preferred choice for many naval and commercial ships because of their long service life and low maintenance requirements.

One of the key advantages of these alloys is their natural resistance to macrofouling. This means that marine organisms, such as algae and barnacles, find it difficult to attach to the surface of the alloy. As a result, the need for biocides in condenser and seawater systems is reduced, and the drag on offshore platforms and ship hulls is minimized, leading to lower cleaning costs and improved efficiency.

Copper-nickel alloys provide an optimal balance between resistance to flowing seawater and overall corrosion resistance. There are two main grades commonly used in marine applications: 90-10 (90% copper, 10% nickel) and 70-30 (70% copper, 30% nickel). Both alloys contain small amounts of iron and manganese, which enhance their performance in dynamic seawater environments.

The 70-30 alloy is stronger and better suited for high-flow-rate applications, while the 90-10 alloy offers a more cost-effective solution for most standard uses. In fact, the 90-10 grade is more widely used due to its balance of performance and affordability. Another variant, a modified 30% nickel alloy containing 2% manganese and 2% iron, is specifically designed for use in heat rejection systems of multi-stage flash desalination units. This version offers higher impact resistance and corrosion resistance under demanding conditions.

It's important to note that the maximum allowable levels of certain impurities are strictly controlled, as they can affect the alloy’s hot ductility, workability, and weldability. Contamination during manufacturing and welding must be carefully managed. The 2% Mn and 2% Fe grade is typically produced as seamless tubes, allowing for expansion into tube sheets without the need for welding, which further enhances reliability and reduces potential failure points.

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