Aspects Of Biofouling And Corrosion On Ship Hulls Clad With Copper-Nickel
Leslie H. Boulton* & Carol A. Powell
Consultants to the Nickel Development Institute
* Auckland, New Zealand, Birmingham, United Kingdom, And
W. Bruce Hudson
Copper Mariner Ltd
Auckland, New Zealand
ABSTRACT
Copper-nickels are alloys, which possess a combination of excellent corrosion resistance and a high natural
resistance to biofouling in seawater. They have a proven performance record over many years in applications
such as seawater piping, intake screens, water boxes, and for cladding of offshore structures. The property
combination also makes copper-nickel an attractive material for boat hulls, either as the hull material
itself or applied as sheathing. A recent development has been the application of 90-10 copper-nickel
sheathing as an adhesive-backed thin foil to ship hulls.
This paper describes trials and evaluation of 90-10 copper-nickel sheathing on the hulls of two commercial
passenger ferries, in service on the Auckland Harbour, New Zealand. One vessel is a slow ferry (10 knots),
constructed of fibreglass-reinforced polymer, which was retrofitted with copper-nickel sheathing in 1993. The
other vessel is a fast catamaran ferry (22 knots) with an FRP hull, which was sheathed with copper-nickel
foil during construction in 1994, and has been in commercial service for about 30 000 nautical miles since
construction.
The five-year project has shown that copper-nickel sheathing of ship hulls provides an effective antifouling
regime on the commercial vessels used in the trials. Corrosion control measures on the sheathed hulls need
special attention, but the control procedures are in line with marine corrosion mitigation practices.
Erosion- corrosion of the copper-nickel foil on ship rudders is one area that needs to be monitored
regularly.
KEY WORDS
Copper-nickel alloys, biofouling, marine corrosion, hull sheathing of ferries, adhesive-bonding of copper-
nickel foil, antifouling.
1. INTRODUCTION
Biofouling is commonplace on immersed marine structures, including the hulls of vessels such as harbour
ferries, which are in routine commercial service. Regular removal of macrofouling is an operational
requirement on passenger ferries, otherwise the vessels suffer hydrodynamic drag problems during service and
fuel consumption rises proportionately. The cost associated with removal of macrofouling from a ship's hull
can be high, particularly when a fleet of commercial ferries requires regular dry-docking for
maintenance.
The principal commercially available copper-nickel alloys, 90-10 (UNS C70600) and 70-30 (UNS C71500), are
solid solution alloys, which exhibit excellent fabrication characteristics, useful mechanical properties and
a high level of corrosion resistance in seawater (1). Forming and welding of these alloys is relatively
uncomplicated and they have enjoyed many successful applications in the marine industry (2,3).
90-10 copper-nickel is recognised for its excellent resistance to corrosion and additionally is known to have
a high inherent resistance to biofouling. It was first used as a boat hull material in 1971 and a survey in
1994 of the behaviour of the vessels built, since that time, showed good performance. The corrosion rates of
90-10 copper-nickel alloy in seawater (1) have also been studied. The 90-10 alloy has an acceptably low
corrosion rate in seawater, except when the water is heavily polluted. As such, coatings and cathodic
protection are unnecessary.
Long term research at the LaQue Center for Corrosion Technology, USA, and service experience on the legs of
Stage 1 of the Morecambe Field gas platforms, UK, have shown that 90-10 copper-nickel alloy has good
characteristics for the sheathing of marine steel structures in the splash, spray and tidal zones, where
corrosion can otherwise be very severe (4,5). The accumulation of biofouling on insulated 90-10 sheathing was
also shown (5) to be less than 2% of that occurring on corresponding areas of bare steel. Thus, 90-10
copper-nickel alloy has a proven record for use as a sheathing material on marine structures, whether they be
static, such as offshore platform piles, or dynamic, as on seagoing ships (4).
Observations of the biofouling resistance of the 90-10 copper-nickel alloy under normal exposure conditions
on vessels and offshore platforms suggest that microfouling does not build up sufficiently to allow
macrofouling to become established. Under quiescent or stagnant conditions macrofouling can eventually occur,
but larger marine growths will periodically slough off during service as seawater flow conditions are
established. In addition, biofouling, which does accumulate on a hull, can be easily removed by wiping, or
with a gentle scraping action.
For optimum fouling resistance, 90-10 copper-nickel must be freely exposed and not subject to galvanic
contact with less noble metals, such as zinc anodes, or skin fittings. Cathodic protection (CP) can
significantly reduce the biofouling resistance of the 90-10 copper-nickel and the alloy must be electrically
isolated from any on-hull CP system (2).
About ten years ago, Mitchell' developed a proprietary adhesive-backed copper nickel foil in the United
Kingdom for application to wood, glass-fibre and steel boats. This was applied to about forty boats over the
years and, when correctly applied, the performance was very encouraging. In 1992, work began to develop the
process further in New Zealand. Copper-nickel foil has now been applied to medium size pleasure craft and two
commercial fleet ferries, by Copper Mariner Ltd, enabling the observation of more exacting service conditions
and allowing a more detailed evaluation to be documented, than had previously occurred.
In parallel trials, test panels prepared using the copper-nickel cladding have been immersed in seawater at
Singapore Harbour (1 year), Langstone Harbour, UK, (3 years), and in the Auckland Harbour (5 years). The
seawater trials in Langstone and Auckland Harbours are to assess the 90-10 copper-nickel cladding performance
on materials such as steel and aluminium alloys, as well as on FRP and painted wood.
EVALUATION OF 90-10 COPPER-NICKEL SHEATHING
2.1 The Sheathing System and Evaluation Programme
The sheathing of a ship's hull with copper-nickel foil involves the application of adhesive-backed panels
(approximately 210mm x 500mm) to the prepared hull, allowing about 15mm overlap. The copper-nickel foil
thickness chosen is about 0.15mm thick. The panels are easily cut and manipulated even over the most
difficult contours.
The bonding system acts as an insulator and as a barrier to seawater, which protects the hull from the
corrosive actions of seawater on its own. An advantage of the system has been that if impact occurs and some
panels are damaged, it takes only a short time to repair the sheathing. The system can be applied to hulls on
new vessels and as a retrofit.
An evaluation programme on the installation and performance of the foil sheathing commenced in August 1993.
This paper concentrates on the MV Koru and the MV Osprey; both of which are in-service on Auckland Harbour.
In addition, test programmes involving trials on immersed test panels, commenced over the same time
(1993-1999). One of the test panel trials, at Langstone Harbour, UK, is continuing.
2.2 MV Koru
An old 48 tonne ferry, the MV Koru (Figure 1), had the FRP hull sheathed with copper-nickel foil in 1993.
Koru is a slow ferry, which travels at about 10 knots, and is held mostly in reserve for emergencies. The
fouling and corrosion performance of the sheathed hull were monitored by inspection at dry-docking intervals,
over the following years.
Several teething problems linked with understanding the galvanic action situation presented by the new hull
protection system, whilst retaining some existing hull fittings, were addressed and overcome. The nature of
these problems were as follows:
A 70-30 copper-nickel shoe had been installed along the keel. Unexpectedly, this design promoted sufficient
galvanic corrosion of the adjacent 90-10 panels to corrode their surfaces. The solution was to replace the
keel shoe with FRP.
Fasteners holding the shoe were also corroding. These were made of silicon bronze, a less noble material than
copper-nickel. The silicon bronze was replaced with 316 stainless steel, which is more corrosion-
resistant.
The rudder shoe and bearing attachments were made from manganese bronze and showed signs of dezincification,
as did the propeller. Cathodic protection had been removed to achieve the optimum biofouling of the
copper-nickel and thus these components were no longer protected. Zinc anodes were then installed on the
rudder shoe in such a way that they were electrically insulated from the hull.
Fouling, in terms of slime formation, had occurred which was generally slight to medium. This tended to be
heavier on sides exposed to sunlight when moored. Some patches of small barnacles had formed, which seemed to
have occurred preferentially on the adhesive at the overlap of the panels. Barnacles on the copper-nickel
were easily removed by finger pressure.
More recent inspections of the Koru's hull during dry-docking (1995-1999) have revealed that the early
teething problems encountered with hull sheathing have been overcome.
MV Osprey
The 21-metre catamaran MV Osprey built as a fast passenger ferry, was launched in December 1994. The ferry
has since been in passenger ferry service for about 30,000 nautical miles on the Waitemata Harbour in
Auckland. Both FRP hull pontoons were sheathed from the waterline down with 90-10 copper-nickel foil panels
during construction (Figure 2).
During slipway inspections of the Osprey hull over the next two years, it was found that this vessel
experienced the same galvanic corrosion problems with the keel strip encountered earlier with the Koru.
Again, this required a replacement with FRP to solve the problem. The adhesive performed very well on the FRP
hull. The other parts of the hull appeared in good condition, apart from the leading edges of the two rudder
skegs which showed evidence of erosion-corrosion after six months'service. The erosion-corrosion problem on
the foil at the stern can be tolerated and periodically the affected panels are replaced during a maintenance
haulout.
Fouling again consisted of moderate algal growth. The growth was heavier on the exterior sides of the
catamaran pontoons exposed to more sunlight. On the sheathed hull surfaces between the pontoons, where
seawater velocity/turbulence are higher and sunlight exposure is less, the development of surface slime was
minimal.
Another factor, which was found to be consistent on both the Koru and the Osprey, was that mechanical damage
to the copper-nickel panels (sustained through collisions with floating debris) was also minimal. The foil
panels were not only durable, but they exhibited excellent mechanical properties, such as toughness and
impact resistance. Additionally, any significant mechanical sheathing damage sustained on both vessels was
quickly and easily repaired during normal maintenance time on the slipway.
Auckland Harbour Test Panel Trials
Between 1993 and 1999, two wooden frames each consisting of five 90-10 copper-nickel sheathed panels, were
immersed at the National Maritime Museum in the Waitemata Harbour, Auckland. The panels were painted
aluminium and steel. The sheathed steel panels were supported by a painted wood frame, whereas the aluminium
panels and wooden frame were totally clad with copper-nickel foil (5).
The panels and frames were recovered, water-blasted and inspected in early 1999 (Figure 3). There was no
evidence on any test panels of surface corrosion of the copper-nickel foil or detachment of the adhesive from
the substrates or the foil. There was no evidence observed of degradation of the adhesive material in
seawater. When the bare adhesive is exposed to seawater for long periods, it becomes fouled with marine
growth at about the same rate as a non-antifouling paint.
Singapore Harbour Test Panel Trials
In September 1994, a test panel assembly was submerged in the harbour at Singapore, in a 1-2 knot current.
This location was chosen because experience was required with the adhesive-bonded 90-10 copper-nickel foil
panels in a warm, polluted seawater environment. The test panels were withdrawn and inspected monthly for a
period of one year, from 1994 to 1995.
Generally, marine growth and fouling on the wooden support frames was very rapid, with heavy barnacle
encrustation evident within three months' exposure. During the same time interval, there was some marine
growth on the copper-nickel panels, but it underwent little further change after the first three
months.
Langstone Harbour Test Panel Trials
In August 1996, exposure panels were immersed from rafts in the tidal flow of Langstone Harbour in the UK.
Triplicate panels of copper-nickel sheathed steel, FRP and aluminium were exposed. Each panel was
copper-nickel sheathed, on both sides, for the lower two-thirds of the panel length. The upper third of each
test panel was paint coated. The copper-nickel sheathing on each test panel was exposed below seawater level.
After 3.5 years' exposure, there were medium slime levels on the copper-nickel and minimal levels of
macrofouling.
Figure 4 shows a copper-nickel sheathed FRP panel after 2.75 years' exposure in Langstone Harbour. This
illustrates the low level of macrofouling on the copper-nickel for this length of exposure time, and the ease
with which the biofouling can be wiped off with finger pressure.
DISCUSSION
It is perhaps a natural progression of copper-nickel alloy technology that development of an adhesive-backed
sheathing system for the hulls of small and medium size pleasure and commercial craft should occur. A system
that was originally developed in the United Kingdom has now been applied and given rigorous service
experience in New Zealand
The overall assessment of the evaluations to date is that it has proved to be a viable system. The adhesive
used under the foil has shown excellent adherence to the FRP boats examined. The biofouling resistance has
been in line with documented accounts, such that microfouling does occur on the copper-nickel but
colonisation of macrofoulers is restricted. If colonisation does eventually occur, it can readily be removed
by a wipe or finger pressure, so that a light waterblast will quickly remove any growth.
Corrosion of the copper-nickel foil has only occurred on rudder skegs, a traditional problem area on any
vessel. The erosion-corrosion damage is due to impingement by fast flowing seawater on the foil, which breaks
down the passive oxide film. If seawater movement over the foil exceeds a certain breakdown velocity, 90-10
copper-nickel alloy can suffer erosion-corrosion damage. Experience with boat hulls has shown that minimal
corrosion occurs after about one year at 24 knots (12 m/s) and the highest recorded fluid velocity tolerated
is 38 knots (19 m/s). This was on a patrol boat which showed no measurable loss of hull thickness after 200
hours at maximum operating speed.
The project has demonstrated that there are a number of factors that must be taken into account when
considering the response of copper-nickel to biofouling, namely:
- Seawater temperature;
- Access to sunlight;
- Velocity of the seawater;
- Avoidance of galvanic action.
3.1 Temperature
The effect of seawater temperature remains somewhat uncertain. However, a review of the test panel results
obtained in quiet seawaters in Singapore and Auckland Harbours indicated that biofouling of the copper-
nickel alloy foil in warm tropical seawater after one year exceeded the marine growth in temperate climate
waters. Nevertheless, the ease of removal of biofouling was about the same in both tropical and temperate
waters.
3.2 Sunlight
The experience on Koru and the Osprey showed that green algae (slime) formed predominantly on the
copper-nickel foil at, or just below the waterline on both vessels. In addition more algae was observed on
one side of the Koru hull which was facing the sun during out-of-service time. Obviously, sunlight affects
the rate of growth of the green fouling (photosynthesis), but the higher temperature of the surface seawater
on sunny days may also be a factor. The green algae was easily removed using rotary brushing underwater, but
the growth became firmly attached and more difficult to remove if it dried when the vessel was on the
slipway.
3.3 Velocity
The velocity of the seawater had a substantial effect on the degree of fouling resistance of the
copper-nickel foil. Areas of the Koru and Osprey hulls were almost entirely free of biofouling where the
velocity of seawater experienced by the alloy exceeded some undetermined speed. Turbulence was possibly a
factor that also contributed to this observation. Typically, the stern and waterline were more likely to show
signs of fouling initiation than other hull areas.
3.4 Galvanic Corrosion
It has been confirmed that the 90-10 copper-nickel alloy must be freely exposed to seawater and it must not
be subject to cathodic protection, or any other type of galvanic coupling, if it is to achieve maximum
biofouling resistance. Any CP system must be electrically isolated from the copper-nickel cladding on the
ship's hull.
3.5 Presence of Pollutants on Corrosion of Copper-Nickel
Finally, although some degree of pollution may be expected in harbours, no obvious effects on the copper-
nickel foil were observed. Sulphide surface films from polluted waters can lead to pitting corrosion and to
higher corrosion rates. Consequently, the use of copper-nickel as a hull material is normally preferred for
operation and moorings in cleaner waters. This is particularly important during the first weeks of exposure
to seawater, while the protective surface oxide films are forming on the copper-nickel. Once mature these
films provide a better resistance to transient exposure to polluted seawaters. However, persistent exposure
to such waters can have a deleterious effect on copper-nickel.
CONCLUSIONS
A five-year evaluation of the feasibility of using 90-10 copper-nickel adhesive-backed foil sheathing on the
hulls of commercial passenger craft, to minimise biofouling, has shown that the system possesses certain
special features:
4.1 The adhesive-backed copper-nickel foil sheathing, trialled on two boat hulls and on test panels, is
readily applied and has shown excellent adherence in the test programme. The system is suitable for new-
build and retrofit.
4.2 For retrofits, care should be taken to establish the alloy identity of hull fittings. Steps may need to
be taken to replace the fittings, or to ensure that their corrosion resistance is maintained and is
compatible with the new copper-nickel foil system on the hull. Any CP system installed must be located with
care on the vessel's sheathed hull.
4.3 Copper-nickel has a good resistance to macrofouling, although microfoulants will form on the alloy.
4.4 Copper-nickel adhesive-backed foil has been found to effectively form part of an antifouling regime on
the ferry hulls, to provide decreased fouling levels and reduced downtime on the slipway. Light waterblasting
of the hull, once a year, removes any biofouling. Even this procedure becomes unnecessary with periodic
underwater hull cleaning, while the vessel is out-of-service but in seawater.
Factors that may influence the resistance of copper-nickel to biofouling are:
- The ambient seawater temperature;
- The degree of exposure to sunlight in seawater;
- The velocity of seawater along the ship's hull;
- Any galvanic coupling to less noble metals.
4.6 Regular inspection of the hull during dry-docking to check for mechanical damage, erosion-corrosion
and galvanic corrosion, is an advisable precaution for maintenance of the system. The sheathing system is
easily repaired if physical damage occurs during service. The sheathing shows good impact resistance and adds
integral strength to the vessel's hull.
4.7 The test programme experience gained during the five year evaluation period of the system confirms
similar performance results published in the USA and Europe during the past 27 years of trials and testing
using 90-10 copper-nickel alloy for the fabrication of ship hulls in marine service.
ACKNOWLEDGEMENT
The authors thank Fullers Group Limited (Auckland) for permission to publish this work. In particular, thanks
are expressed to Messrs George Hudson and Michael Fitchett of Fullers.
REFERENCES
Powell C. A., Corrosion and Biofouling Protection of Ship Hulls Using Copper-Nickel,
Proc. Int. Conf. on Marine Corrosion Prevention, London, UK, 1994.
Powell C. A., Copper-Nickel Sheathing and its Use for Ship Hulls and Offshore Structures,
Int. Biodeterioration & Biodegradation, 321-331, 1994.
Jordan D. E. and Powell C. A., Fabrication of Copper-Nickel Alloys for Offshore Applications, Welding in
Maritime Engineering, October, 22-24, KrK, Croatia, 1998.
Thirteen-Year Results of Long Term Copper-Nickel Sheathed Piling Studies,
International Copper Association Ltd, Project No. 358, Annual Report, USA, 1998.
Boulton L. H., Powell C. A. and Hudson W. B., Controlling Biofouling on Ferry Hulls with Copper-Nickel
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