While all the information in this module is believed to be technically correct, it is prepared for general reading and should not be relied on for specific application without first obtaining COMPETENT advice.
The grades of copper-nickel alloys chosen for marine environments have a wide range of applications. They are most often used as heat exchangers and condensers of various sizes , and piping systems. Applications include desalination, offshore oil/gas production, naval and commercial shipping, and the power industry.
Copper-nickels are recognized for their excellent resistance to sea water corrosion and are also one of the few alloy systems with natural antifouling properties. Their mechanical properties allow them to be produced in all product forms and readily fabricated even into the most complicated designs.
The ease of fabrication is explained by the single phase, face centred cubic metallurgical structure of copper- nickels at all temperatures. This provides good ductility and toughness through to cryogenic temperatures.
Copper-nickels are used to meet modern challenges with a wealth of service experience to draw on.
The actual compositions of the alloys vary slightly among International standards. All aim at controlled levels of iron to provide the best balance of resistance to flowing sea water and localized corrosion. Trace impurity elements such as sulphur, carbon, phosphorus, lead and zinc are also carefully controlled by manufacturers to avoid weld cracking and to ensure hot workability.
Whereas the 90-10 and 70-30 alloys are available in all product forms, such as plate, sheet, tube, rod and wire, the 30% nickel alloy with 2% each of iron and manganese, seen here as 66-30-2-2 and the alloy with 16%Ni and 0.5% Cr presented as 82-16-Cr are usually only available as condenser and heat exchanger tubing. As they are normally expanded into tube plates, weld consumables have yet to be developed.
In standards, the iron content varies from 1-2% for the 90-10 copper-nickel alloy, to give good balance between resistance to localised corrosion and resistance to flow. This graph shows how impingement resistance varies with iron content. Iron levels between 1.5 and 2.0% are seen to have the best resistance to flowing sea water. For optimum resistance the alloy should be processed to keep precipitation of iron within the copper-nickel matrix to a minimum.
In terms of mechanical properties the 2% iron and manganese alloy is stronger than the 70-30 alloy which, in turn, is stronger than the 90-10. The alloy with 16% nickel and chromium lies between the 10 and 30% nickel alloys. All the alloys are very ductile at 35% or more elongation in the annealed temper. They have moderate strengths in the annealed condition which can be increased by cold working but not by heat treatment.
The physical properties of the 90-10 and 70-30 alloys are similar apart from the 90-10 alloy having significantly higher thermal conductivity than the 70-30 alloy and a lower electrical resistivity. The higher thermal conductivity makes the 90-10 alloy more efficient as a heat exchanger material.
Both 90-10 and 70-30 alloys are readily welded by the majority of welding techniques. Autogenous welding, which means welding without using a weld consumable, is not recommended due to the risk of weld porosity nor is the oxy-acetylene process practicable.
A 70-30 type welding consumable is preferred for both 90-10 and 70-30 alloys. This is because the weld metal is stronger than the 90-10, has better deposition characteristics and is at least as corrosion resistant as the base metal.
No post weld heat treatment is required for the 90-10 and 70-30 alloys.
When welding to steel, 65% nickel-copper consumables should be used as they are required to absorb the high levels of weld iron dilution in the weld.
The tendency for weld cracking when welding copper-nickel to steel is illustrated in this diagram which maps the cracking / no cracking areas for selecting a suitable filler metal.
Copper-nickel alloys can also be joined using appropriate silver-base brazing alloys. Phosphorus-bearing brazing alloys are unsuitable. All processes can be used except vacuum brazing due to the high vapour pressure of some brazing alloy constituents. Torch brazing is the most common.
Cold worked parts containing significant internal stresses can be susceptible to intergranular penetration by molten filler material during the brazing operation and must be stress-relieved before hand. This can be simply done by an oxyfuel torch taking care the part is heated uniformly.
The seawater corrosion resistance offered by copper-nickel alloys results from the formation of a thin, adherent and protective surface film which forms naturally and quickly upon exposure to clean seawater. The film is complex and multi-layered. It is predominantly comprised of cuprous oxide, and often contains nickel and iron oxide, cuprous hydroxychloride and cupric oxide as well. The film can be brown, greenish brown or brownish black.
Initial exposure to clean seawater is crucial to the long-term performance of copper-nickel. The initial film forms fairly quickly over the first couple of days but, depending on the sea water temperature, takes 2-3 months to fully mature. Here, the rate of film formation on 90-10 copper-nickel in seawater at 16°C is shown indirectly by copper measurements in the effluent of a condenser over a 3-month period after start up. Copper content was found to decrease to one tenth in ten minutes and one hundredth in an hour. After three months, the copper in the effluent was virtually the same level as that of the intake water. The maturity of the protective film reduced the corrosion rate of the condenser surface.
At higher temperatures, the film forms and matures faster. At 27°C, a common inlet temperature for Middle East desalination plants, rapid film formation and good protection can be expected in a few hours. At lower temperatures the process is slower but the film does form even in Arctic and Antarctic waters.
Once a good surface film forms, the corrosion rate will continue to decrease over a period of years. For this reason, it has always been difficult to predict the life of copper-nickel base alloys based upon short-term exposures.
Specific corrosion rate measurements taken over fourteen years in quiet, tidal and flowing (0.6m/s) sea water on the coast of North Carolina show the corrosion rate decreases over a period of 5-6 years, stabilizing out at about 1.3µm/yr.
Low general corrosion rates are maintained with copper-nickels with increasing seawater flow rate due to the resilience of the protective surface film, and the corrosion rates continue as long as the flow velocity for a given geometry does not exceed a critical value called the" breakaway velocity". Above this, shear stresses are sufficiently high to strip off the protective corrosion film causing damage in the form of impingement attack otherwise known as erosion corrosion. At very high continuous velocities, the corrosion rate for copper-nickel can be in the order of 1mm/yr. Maximum design velocities, therefore, are specified to stop this from occurring.
It should be noted, however, that copper-nickels have a higher relative resistance to the onset of erosion than copper and aluminum brass. The 70-30 alloy is better than 90-10. The 2% iron and manganese alloy is even better again with the 82-16-Cr the best of all.
The behavior of copper-nickel in flowing sea water described so far has been for continuous flow situations in piping and tubes. Fire-mains can encounter intermittent very high velocities of 7-15m/s during test practices as well as during actual fires. Experience has shown that these much higher flow rates are acceptable for the short term practices used in such applications.
The maximum flow velocity increases with diameter upon going from condenser to piping systems and are greater still for ship hulls. Experience to date from boat hull trials has shown minimal corrosion after 14 months at 24 knots, which equates to 12m/s, for the 90-10 alloy whereas the highest recorded velocity is 38 knots, or 19m/s, for a patrol boat which showed no measurable thickness loss after 200 hours at maximum operating speed. The upper service velocity for hulls is still to be established.
Copper-nickel alloys have good inherent resistance to localized corrosion in the form of chloride pitting and crevice corrosion which can plague other alloy systems such as stainless steels. This resistance is maintained even at higher temperatures where more highly alloyed stainless steels may become susceptible.
Copper-nickel alloys are not susceptible to chloride or sulfide stress corrosion cracking or hydrogen embrittlement. Unlike brasses, they have not been found to suffer cracking due to ammonia in seawater service although higher general corrosion rates may be experienced.
Sulfides are present in polluted water either as industrial effluent or when the water conditions support the growth of sulfate reducing bacteria. They can also occur in stagnant conditions as a result of decomposition of organic matter. Copper-nickels can be subject to higher corrosion rates and pitting in the presence of sulfides. If exposed to polluted water, especially if this is the first service water to come into contact with the alloy surface, any sulfides present can interfere with surface film formation, producing a black film containing cuprous oxide and sulfide.
This is not as protective as films formed in clean water and exposure to sulfides should be restricted wherever possible, particularly during the first few months of contact with seawater while the oxide film is maturing.
The sulfide film can be replaced by an oxide film over several days during subsequent exposure to aerated conditions, although high corrosion rates can be expected in the interim. However, it is known that, if an established cuprous oxide film is already present, periodic exposure to polluted water can be tolerated without damage to the film.
There are various ways to avoid sulfide corrosion .
It is important to build up a good surface oxide film as this will provide added resistance to subsequent exposure to polluted waters.
Commissioning requires the use of clean sea water or potable water.
For prolonged standby during service, the water should be drained away to avoid decomposition and putrefaction of the water.
If polluted conditions are expected, ferrous sulfate dosing has been shown to accelerate surface film formation and provide additional protection against waters containing sulfides.
Copper-nickel alloys lie mid-way in the galvanic series and are compatible with other copper alloys. The 70-30 alloy is slightly more noble than the 90-10 alloy.
Both alloys are more noble than zinc, aluminum, steel and manganese bronze, and large areas of copper-nickel may cause those alloys to corrode preferentially. Copper-nickels are less noble than passivated stainless steels, nickel alloys and titanium and may suffer higher corrosion rates than normally anticipated if they come into electrical contact with those metals. As with all bimetallic couples, careful attention should be given to avoiding unfavorable galvanic area ratios, or suitable insulation to should be provided to prevent galvanic current flow.
Copper-nickel alloys have a high natural resistance to marine macrofouling which the majority of metals experience.
This property can reduce the frequency of cleaning of piping systems and condensers, and it can decrease wave loading and fouling removal costs for platform structures. Also, the down-time and expense of applying antifouling coatings to boat hulls is avoided, and fuel consumption is improved. The biofouling resistance of copper-nickel tubing allows shipboard condensers to maintain good heat transfer capability for several months between mechanical cleanings without the need for onboard chlorine generators.
This picture shows the level of fouling found on 70-30 copper-nickel plate after 14 years exposure on the coast of North Carolina.
The discovery of such low levels of fouling led to a further 5 year trial to examine more closely the performance of 70-30 and 90-10 in quiet sea water.
These are the results for 90-10 alloy and although not shown here, the exposures for copper and 70-30 copper-nickel looked very similar. A slime layer of microfouling formed over the first 18 months and slowly thickened. Fouling colonized on the bolts, which were not copper-nickel, and gradually spread over the surface of the coupons.
In more open waters, slimes would still form, but macrofoulers in the form of grasses and shells would find it very difficult to colonise, and, if they did, they would tend to slough away at intervals.
From such observations, it was considered that it was the cuprous oxide surface film that provides biofouling resistance. To explain the gradual, restricted colonization of fouling when under quiet conditions in the 5 year trials, it was thought possible that the cuprous oxide film is oxidized to green cupric hydroxychloride, which is less biofouling resistant and adherent, allowing any macrofouling attachment to periodically slough away. This re-exposes a cuprous oxide film.
However the most important requirement for optimum biofouling resistance is that the alloy should be freely exposed or electrically insulated from less noble alloys such as steel, less noble copper alloys and aluminum or be free of cathodic protection.
Here, exposure panels are ready for immersion on the south coast of England. From left to right is a plain steel panel, a steel panel with the upper area sheathed with copper-nickel and a copper-nickel panel. All these three have sacrificial anodes attached to give cathodic protection. The fourth panel is copper-nickel without any cathodic protection.
After 12 months immersion, the unprotected copper-nickel panel has only a light slime covering whereas, for the panels with cathodic protection, it is difficult to distinguish copper-nickel from steel.
In applications requiring the full biofouling resistance of copper-nickel, such as boat hulls, the use of anodes and impressed current cathodic protection systems should therefore be avoided.
Current thinking is that the most likely explanation for the biofouling resistance is a combination of copper ion release and the nature of the protective film. It is most probable that the high resistance to macrofouling is due to the unoxidized copper ions normally present in the protective film. However, more detailed research is required to confirm this.
To summarise, copper-nickels will form microfouling slimes but have a high resistance to macrofouling. Any macrofouling which does adhere over a period of time is loosely attached and easily removed. To achieve optimum biofouling resistance, it is important to electrically insulate the copper-nickel from less noble alloys or cathodic protection systems.
A practical example is the Copper Mariner, a shrimp fishing vessel operating off the coast of Nicaragua, which had a solid 6mm 90-10 copper-nickel hull welded to a steel frame.This is the hull (right) after 4 years without cleaning.
In several inspections the majority of any biofouling attachments were dislodged apparently during its 3-8 knot travel to and from shrimping grounds; any remaining could be flicked off with a thumbnail. Attachment when it did occur was very light compared to the manner in which barnacles embed themselves onto steel, titanium and other materials without significant biofouling resistance.
To obtain good service from copper-nickels and take into account the properties they exhibit, several important guidelines emerge. First of all, it is important to use a material that has a carefully controlled composition; so, alloys should be ordered to internationally recognised standards.
Second, it is important to keep flow velocities within recognized design limits to avoid areas of turbulence.
Securing a good surface film formation and avoiding exposure to polluted or extended stagnant condition is also necessary particularly during commissioning or early stages of operation while the film is maturing.
Where this is not possible, consideration should be given to including ferrous sulfate dosing systems or simulated iron anodes as a source of ferrous ions.
When using copper-nickel for its antifouling properties, care needs to be taken to ensure the material is insulated from cathodic protection or galvanically less noble material, so that the best resistance is achieved.
The following section provides more detailed guidelines for copper-nickel systems.
For condenser and piping systems, it is often the fitting-out and commissioning period when problems are most likely to occur from sulfides . The ideal situation whether in a ship or power plant is to recirculate aerated, clean seawater at initial start up for sufficient time to form a good protective film. When formed, this provides a high degree of corrosion protection to subsequent exposure to sulfides.
For shutdown and standby conditions, it is important to avoid deposit build up and long term stagnant conditions which can cause the sea water to putrify. Continuous or intermittent flow or draining down, cleaning and drying are required for long term shut down durations of more than 4 to 5 days.
In situations where it is not possible to use clean seawater, circulating the system initially with fresh water containing ferrous sulfate additive will encourage effective film formation.
Ferrous sulfate treatment has been found to reduce corrosion rates of copper-nickel in seawater in both polluted and unpolluted conditions. Ferrous sulfate treatments are not absolutely essential to the successful performance of copper-nickel, but they can be viewed as remedies when trouble has occurred or as a precaution if trouble is likely. Many ships in service have operated successfully without any ferrous sulfate dosing.
An alternative method of releasing ferrous ions into the system is by fitting iron anodes. This however, is more suited to maintaining a protective layer than forming an initial oxide film. It will also reduce the biofouling resistance, if directly connected to the piping.
In mixed metal systems chlorination may be necessary to control fouling. Copper-nickel tubing is resistant to chlorination at normal dosing levels. Chlorination treatment and ferrous sulfate treatment should not be carried out simultaneously, because it will lead to flocking of the ferrous sulphate making it ineffective. To avoid this, the treatment with ferrous ions should be discontinued one hour before chlorination.
Copper-nickel is regularly used for tubing in condensers and heat exchangers of all sizes.
General experience has shown that 90-10 and 70-30 copper-nickel alloys can successfully be used in condensers and heat exchangers with flow velocities up to 2.5m/s and 3m/s respectively; with the 66-30-2-2 alloy up to 4.5m/s. Limited data is available for the 82-16-Cr alloy but velocities up to about 9m/s* are thought to be acceptable.
Copper-nickel as solid or steel backed clad plate can also be used for the tube plates. Here the tube is being welded to the tube plate using automatic tungsten inert gas welding heads.
Copper-nickel water boxes have been made; some with external steel strengtheners. There are also examples of copper-nickel being attached as a welded sheet lining to a steel box or clad plate.
An important application is that of seawater piping systems.
Generally, larger diameters tend to be used for piping systems than for heat exchangers, and guidelines and standards allow higher maximum seawater velocities. Typical maximum flow rates are given here.
Such guidelines have worked well, because they take into account normal velocity raisers within piping systems, such as bends, which can cause areas of high local flow rates. Nevertheless, extreme turbulence should be avoided. Instances where this may occur include tight radius bends, partial blockages and areas downstream of partially throttled valves. Low flow conditions of less than 0.5m/s in systems are not normally a problem in clean seawater. However, if the seawater contains entrained sand or silt and, particularly, if the water is polluted as in some harbours, minimum flow rates of more than 1m/s are usually preferred to avoid sediment build up.
The desalination industry is a major user of copper-nickel for Multistage Flash units, which are common in the Middle East. The alloys are used for what is really a series of very large heat exchangers. A typical plant can use several hundred tons of copper-nickel.
In the process, evaporation and condensation is split into many stages, thereby increasing efficiency. Incoming sea water is passed through heat exchanger tubing on the exterior of which water vapor at progressively higher temperatures is condensing. Finally, the seawater is passed to a heater where steam from an external source applies the energy for the process and heats it to the maximum process temperature (up to 120°C). The temperature depends on the type of anti-scalant used.
The seawater then passes to the evaporator vessel, when pressure is released, causing it to boil or flash. This process is repeated in many stages; the pressure being reduced so that flashing occurs at progressively lower temperatures. The condensed vapor is collected and passed down the plant, flashing at each stage. It is removed at the lowest temperature and is the product from the plant.
The final stages in the plant are known as the heat rejection stages and are the only ones exposed to aerated sea water.
66-30-2-2 copper-nickel is often used in large plants for the heat rejection section, while 90-10 is more often used in smaller plants.
70-30 is normally used for the brine heater. 70-30 is also often selected for the top temperature recovery stages, as it has better resistance to gases that may be found in the vapor in these stages, such as ammonia and carbon dioxide. 90-10 is preferred for the lower stages.
Some plants use 66-30-2-2 in the brine heater and high temperature recovery stages.
90-10 is used for evaporator shells in solid or clad to steel.
It is also used for tube plates and flash chamber linings.
The generation of electricity in both nuclear and fossil fired power plants involves important heat transfer processes; steam generators, heat exchangers, coolers, condensers, tanks, pipework, valves and fittings.
Steam produced in the steam generator of a nuclear power plant, or in the steam boiler when fossil-fuel fired, loses part of its heat energy in the turbine when converted into electricity. In the condenser, the balance of heat energy is transferred to the coolant, usually fresh water from lakes and rivers, sea water or estuarine water, or the natural draft in power stations using wet cooling towers.
For main steam turbine condensers seamless and welded copper-nickel tubes have been used. 90-10 copper-nickel is used in many power plants for the main body, air removal, and steam inlet areas of the tube bundle, but many designers still specify 70/30 for the steam inlet and air removal sections for an extra margin of safety. To avoid erosion corrosion due to abrasive particles 70-30 is better than 90-10 with 66-30-2-2 being the best choice where for example sand is carried in the cooling water.
For oil cooler and the auxiliary cooling systems the same alloys will be considered as for the main steam condensers. For high pressure pre-heaters called feed water heaters, 70-30 copper-nickel or 65% nickel-copper alloy 400 are preferred. They are normally used in the cold worked and stress relieved condition.
There are many different systems, within power generating plants where copper-nickel components can be used.
Copper-nickel applications on offshore oil/ gas platforms and floating structures include seawater, cooling water and firefighting systems and splash zone sheathing.
An example is the Thunderhorse platform where 700 tonnes of copper-nickel piping is for use as sea water carrying systems.
Metal sheathing can also be attached to the legs and risers of offshore platforms in the splash zone to protect against very high corrosion rates experienced by steel in those areas. 65% nickel-copper alloy 400 sheathing directly welded to the steel has been used for many years in this type of application; copper-nickel can also be used as exemplified by the Morecambe Field in the Irish Sea where it has enjoyed successful service since 1985.
In this instance, the 4mm thick sheathing covered legs on production and accommodation platforms, three drill platforms and a flare stack. The sheathing was welded directly to the steel and spanned from 13m above the lowest astronomical tide level to 2m below. This was applied for corrosion rather than biofouling resistance, although low fouling levels which are very easy to remove have been observed. If maximum biofouling resistance is required the sheathing has to be insulated from the steel and the platform cathodic protection system.
A major application for copper-nickel is for naval and commercial shipping. Seawater is used for cooling, tank cleaning and heating, ballasting, waste disposal, firefighting and, by distillation as a source of fresh water for boiler feed water and sanitary hot and cold water. All require piping systems which essentially consist of pumps valves, pipes and fittings. In cooling systems, heat exchangers are also required.
Applications for copper-nickel alloys covering seawater and other systems in shipping include those shown here.
Since 1968, several dozen boats have been made with copper-nickel hulls to capitalize on the combination of its high resistances to biofouling and corrosion, and, thus avoid application of anti-corrosion and anti-fouling paints. The hulls have been fabricated out of solid plate or copper-nickel roll clad onto steel. In addition, adhesive -backed foil has been examined too. The results have been encouraging, and the time is still awaited when the concept will be applied to larger ships.
The oldest solid 70-30 copper-nickel hulled vessel is the Asperida, built in 1967 in Holland. In 2004, it was being refurbished in New Jersey, when an inspection was made.
According to the records, the Asperida hull had been biofouling resistant . When biofouling did occur, it was loosely attached and was easily removed. Overall, after thirty-six years of exposure to seawater, the 4mm thick hull appeared in excellent condition, even though it had several owners, suffered damage during a few mishaps, and had sailed great distances.
The Pretty Penny is a 10m yacht built in 1979. It is made of 3mm 90-10 copper-nickel plate and has been moored for the greater part of its life in the Thames Estuary on the east coast of the UK. The hull remained clean for the first 3 years, but did exhibit some fouling from mainly grasses and some barnacles in later years, particularly 150-300mm below the water line. This fouling was easily removed manually by a light scraping action while still in the water once or twice a year.
Here, we see the condition of the hull when it was beached for the first time in 16 years. Unfortunately, this slide shows the vessel after the hull had been cleaned by hand with a wooden scraper to remove the growth. It had only taken half an hour and the owner was then 80, so must have been very light work!
An evaluation program on the performance of foil sheathing with an adhesive backing commenced in August 1994 and included a commercial passenger ferry, the MV Osprey.
The MV Osprey is a fast catamaran ferry (22 knots) with a FRP hull, which was sheathed during construction. After 4 years and 40,000 nautical miles, the hull was examined for the first time in 12 months, as shown here.
Some grasses were apparent, particularly where the vessel was exposed to the sun during mooring. Between the twin hulls, where velocities were high, hardly any fouling had occurred.
The biofouling resistance is in line with documented accounts such that slime or microfouling does occur on copper-nickel but colonization of macrofoulers is restricted. If colonization does eventually occur, it can readily be removed by a wipe or with finger pressure, such that a light water-blast or an underwater rotary brushing will quickly remove any growth.
Here is the same foil product applied to a boat in Vancouver. This photo was taken in 2005 after the boat had been used for 20 years. Although the foil is thinning near the water line, the lower panels are still in excellent condition. The hull had been given a light spray wash to clean the hull of any fouling before the photos were taken.
Concurrent with the Osprey trials, the same product was exposed on a rig at Langstone Harbour, UK, to examine fouling progression. The fouling was monitored over 7 years, as seen here.
This module has provided an overview of the properties and applications of copper-nickels in seawater service. These are established alloys which continue toenjoy an extremely active role in marine engineering