High Strength Copper Nickels

High strength Cu-Ni alloys, with their unique set of properties, enjoy a wide variety of engineering applications. These can be summarised by market sector in the following way:

• Oil and Gas Industry Bolting for risers, subsea manifolds, heat exchangers, caissons and splash zone, as well as for down-hole equipment. Stab plate connectors, actuator components, shafts for pumps & valves, valve stems and subsea clamps.
• Defence Applications Bolting for ships and submarines. Towing and winch components. Components in sonar equipment. Propeller shafts, drive bushes and bearings.
• Aerospace Components in head-up displays. Electrical connectors and bearings.
• Automotive Valve components, connector rods and cylinder liners.

From the above tables, it can be seen that copper-nickel alloys are available with high strengths and, in some cases, equivalent strengths to carbon steel. Additionally, it should be noted that, unlike many materials, impact properties of high strength copper-nickels are fully retained at sub-zero temperatures down to -196°C.

Results for the fatigue limit in air for NES 835, DTD 900/4805 and Cu-Ni-Mn-Al-Nb are given below:

Cu-Ni-Mn-Al-Nb possesses a rotating bend fatigue S-N curve as shown in the graph below. No noticeable difference is found between testing in air and testing in artificial seawater, producing the following results:

Fatigue limit in air (10⁸ cycles): 310 N/mm²

Fatigue limit in artificial seawater (10⁸ cycles): 310 N/mm²

Figure 1

Resistance to Hydrogen Embrittlement

Most high strength engineering alloys suffer a reduction in mechanical properties when they interact with hydrogen. This phenomenon is known as hydrogen embrittlement (HE) and it is a particular concern when cathodic protection is applied to protect an immersed steel structure or when dissimilar metals are coupled in seawater. These situations will stimulate the release of nascent hydrogen which can penetrate the metal surface and act to reduce ductility. Hydrogen embrittlement can occur by a number of different mechanisms, depending on the alloy type concerned. Zirconium and titanium alloys undergo embrittlement due to the formation of internal hydrides within the material. Steel and nickel alloys lose ductility through the retention of hydrogen at internal discontinuities within the microstructure. Thus, if bolts or other components of these materials are under tension under cathodic protection conditions in seawater, material failure through HE can occur relatively rapidly.

During the 1980’s, hydrogen embrittlement was experienced as a particular problem in offshore nickel alloy bolting and the oil & gas industry sought a solution by sponsoring extensive testing programmes on a number of high strength materials. Most high strength alloys have been tested including nickel alloys, stainless steels, carbon steel and the high strength Cu-Ni alloys NES 835 and Cu-Ni-Mn-Al-Nb. Of all the materials tested, Cu-Ni-Mn-Al-Nb and NES 835 have been the only materials to consistently show no effects of hydrogen embrittlement, as shown by the table below, which is taken from work carried out at the BNF Laboratories, UK in 1990.

The reasons why high strength Cu-Ni alloys are resistant to HE have been proposed to be due to the observation that hydrogen is less able to enter the material and to be retained by internal discontinuities than alloys which are known to be susceptible to HE. Thus, unlike steel and nickel alloys, where hydrogen entry is rapid and accumulation of hydrogen at particles and precipitates is considered to promote crack growth, high strength Cu-Ni alloys do not suffer hydrogen embrittlement.

Abuse Tolerance and Resistance to Stress Corrosion Cracking

Cu-Ni alloys are noted in the literature for being much more resistant to stress corrosion cracking (SCC) than brasses and aluminium bronzes. Work which has been carried out on the stress corrosion cracking behaviour of 90-10 Cu-Ni’s with additions of iron in ammonia and this has shown that resistance to SCC is produced by restricting the iron content to below 1% and retaining the iron in solution. If iron is above a 1% level and out of solution, intergranular stress corrosion has been observed to occur. MARINEL 220, which represents the latest version of the alloy (with an iron content controlled to be less than 0.85%) shows resistance to environmental effects and significant abuse tolerance.

C-ring stress corrosion testing has been carried out after the application of adhesives, coatings and locking media which are often used on bolting materials in service. The overall result is that MARINEL 220 is suitable to use both in a coated or uncoated condition without the risk of experiencing environment-assisted degradation of properties.

Resistance to Corrosion and Stress Corrosion Cracking in Hydrogen Sulphide Environments

Scanning electron microscopy of Cu-Ni-Mn-Al-Nb, carried out after exposure in acidified hydrogen sulphide solution at room temperature, reveals that the aluminium content of the material produces an oxide layer which gives the alloy a considerable degree of corrosion protection. This would explain its superior behaviour over standard Cu-Ni alloys in sulphide environments.

It has been demonstrated that Cu-Ni-Mn-Al-Nb successfully resists sulphide stress corrosion and is well able to satisfy the requirements of NACE MR-01-77.

Anti-Galling Properties

The resistance to pickup by high strength Cu-Ni alloys is very good, the alloys generally running against nickel aluminium bronze and stainless steels. DEF STAN 02 835 (formerly NES 835), DOD-C-24676 and Cu-Ni-Mn-Al-Nb have demonstrated superior anti-galling properties when used in bearing and actuator applications.

General Corrosion Resistance

The general corrosion rates of the high strength Cu-Ni alloys have been determined by immersion in seawater off the south coast of Britain and can be compared to other corrosion resistant alloys which have been investigated under similar conditions. A chart showing the comparative seawater corrosion rates for different materials is given below and demonstrates the low corrosion rates of the two high strength Cu-Ni alloys DEF STAN 02 835 (formerly NES 835) and Cu-Ni-Mn-Al-Nb. The corrosion rates of Cu-Ni-Al alloys are observed to reduce with time, as initially a dark copper oxide-rich layer is formed which acts to stifle further corrosion of the material.

Figure 2

Biofouling Resistance

High strength Cu-Ni alloys are able to effectively resist the deposition and build up of marine organisms, making them ideal in terms of inspection and maintenance when employed in submerged seawater service.

Galvanic Compatibility

Laboratory tests and application experience has demonstrated that the aluminium-hardened Cu-Ni alloys are galvanically compatible with most copper based alloys, including 70-30 Cu-Ni and nickel aluminium bronze. However, where cathodic protection is not applied, it is good design practice to minimise the danger of galvanic corrosion by ensuring the more noble components (cathodic) are of smaller surface area than the less noble material (anodic).

The table below shows the results of galvanic coupling tests carried out at the LaQue Center for Corrosion Technology, USA and Langstone Harbour, UK on common marine alloys coupled with S32550 superduplex stainless steel. Visually, no enhanced attack on the Cu-Ni alloys under the applied crevice conditions was observed. Generally, when dissimilar metals are present, Cu-Ni-Al alloys are more noble than carbon steel thus, if these alloys are used for bolting carbon steel, any hydrogen liberated by the couple will not adversely affect the Cu-Ni bolts.

High Resistance to Erosion Corrosion

Jet impingement tests have been carried out on high strength Cu-Ni-Al alloys in aerated seawater flowing at 9.3 m/s for 28 days and show that these materials have resistance to impingement attack. Their corrosion properties in this case are similar to cast nickel aluminium bronze and superior to wrought iron-containing 70-30 and 90-10 Cu-Ni alloys. Results are given in the table below.

Cavitation erosion tests in sodium chloride also show the superiority of Cu-Ni-Mn-Al-Nb over other marine corrosion resistant materials, as shown below.

Welding

Welding can generally be carried out, either by the use of matched welding consumables (where available) or nickel aluminium bronze welding consumables. In the latter case, the strength properties of the weld region will be probably lower than the parent material.

Brazing & Soldering

High strength Cu-Ni-Al can be brazed by the same procedures as for the aluminium bronzes using special fluoride containing fluxes and silver brazing alloys with low melting point (about 650°C). Soft soldering can also be carried out by similar procedures to those employed for the aluminium bronzes using a flux of phosphoric acid solution with a minimum specific gravity of 1.75.

Machining

High strength Cu-Ni alloys can be easily machined to give an extremely good finish, using methods normally employed with high strength aluminium bronzes. Thread rolling in the production of fasteners may be employed and a relatively small increase in hardness in the thread form is encountered.

Dimensional stability is good, and although normally no stress relief is required, in special cases where dimensional tolerances are critical it may be desirable to heat treat for I to 2 hours in an air circulating furnace at 300°C followed by air cool.

Tooling should be carbide tipped with water soluble lubricant applied.