In the 1930s, the Royal Navy used 70/30 cupro-nickel to replace 70/30 copper zinc brass tubes for their condenser tubes. The latter suffered corrosion problems when in service, particularly when there were sand particles present in the seawater it was exposed to. To increase the strength and corrosion resistance of 70/30 cupro-nickel for marine applications, up to 1% manganese and 0.6-1.0% iron was added to the alloy.
During the 1970s, both the US and UK backed research to find an alternative to 70/30 cupro-nickel and nickel-aluminium-bronze (NAB) that could resist the corrosion effects of flowing seawater. In 2000, UK Chief of Defence Procurement, Sir Robert Walmsley, revealed that over £6m had been spent on CNC castings research. With no definitive results, research was abandoned.
In 2012, Copper Alloys (in liaison with the UK Ministry of Defence) picked up the project to trial wrought CNC production. The work carried out proved that it was possible to overcome the problems that had blighted earlier trials (including catastrophic forging failure, linear oxide inclusions and production unpredictability) and Copper Alloys successfully produced several forgings of CNC-1®.
CNC-1® is a wrought copper nickel chrome alloy. It was developed with the intent of replacing cast CNC and NAB in submarine applications where wear resistance and corrosion resistance is paramount.
Developed over seven years by Copper Alloys, the research involved developing the manufacturing process technology followed by a long-term corrosion program: ‘The British Corrosion Project’. This program involved exhaustive support from the UK Ministry of Defence with mechanical and physical testing.
In total, over 400 samples were placed into three racks across three different locations – two racks in sea water and a third in a neat sewerage channel where high levels of hydrogen sulphide were present.
Within the first seven months of testing, it was clear to see the superior anti-fouling properties of copper-based alloys – with none of the eight copper alloys tested showing any signs of marine growth. This was the case even after four years of being submerged, showcasing how resistant CNC-1® was to marine biofouling.
After four years of being submerged in seawater, the alloy samples were also tested to measure crevice corrosion and pitting. Pitting corrosion was observed on all the copper alloys except for the wrought CNC-1® sample. Only super duplex, titanium samples and CNC-1® remained free of both crevice corrosion and pitting.
Read more about the British Corrosion Project
The composition requirements of CNC-1® are:
| Cu |
Ni |
Cr |
Fe |
Mn |
Si |
Zr |
Ti |
| Remainder |
29-32% |
1.6-2% |
0.5-1% |
0.5-1% |
0.2-0.4% |
0.05-0.15% |
0.03-0.15% |
The maximum permitted impurities for CNC-1® are:
| Pb |
P |
Bi |
S |
C |
Co |
B |
| 0.005% |
0.005% |
0.001% |
0.005% |
0.02% |
0.05% |
0.001% |
Wrought alloys have many advantages compared to cast alloys – a much finer grain size, uniform properties, and extremely high internal integrity. This creates components that have higher mechanical strength and greater toughness than a cast product. Wrought alloys can also be ultrasonically tested at various stages of the manufacturing process to prevent issues with final machining.
After extensive testing, copper nickel chrome (CNC-1®) was found to have several clear advantages over NAB, especially in a marine environment:
| Feature |
CNC-1® |
NAB |
| Non-sparking? |
Yes |
Yes |
| 0.2% proof stress |
350-390 MPa |
280-380 MPa |
| Can be ultrasound tested? |
Yes |
No, requires radiograph test |
| Selective phase corrosion |
None |
0.5 – 1mm per year |
| Impact strength |
120 – 150J |
30 – 40J |
CNC-1®’s incredible resistance to seawater corrosion and biofouling meant that Royal Navy submarines could simply clean the surface of components with glass bead shot blasting during their 10-year refit. Minimal corrosion wear (with only a slight etching showing) meant CNC components could be returned to service.
Discover the full benefits of CNC-1®
