APPLICATION OF COPPER-NICKEL
ALLOY UNS C70600 FOR SEAWATER SERVICE
ABSTRACT
Over
several decades, many thousands of tons of the copper-nickel alloy UNS C70600
have been used as a seawater piping material in marine engineering. In order to
provide reliable service performance of the material, this paper discusses its
metallurgical properties, the relevant mechanisms of general, localized and
erosion corrosion, the unique befouling resistance and the performance in
polluted water. The guidelines for the production of UNS C70600 products, the
design of pipe work, the limits of the alloy and useful service recommendations
are outlined.
Keywords: UNS C70600, corrosion resistant alloys, seawater,
piping systems, service recommendations
INTRODUCTION
The
choice of an appropriate material for seawater service is a difficult decision
that has to be made by a designer prior to specification of the system. Since a
broad range of conditions will usually be imposed on the piping material, the
impact of seawater on material performance is determined by numerous variables
such as condition of the material, system design, fabrication procedure,
various seawater temperatures and flow regimes, biological activity, and
presence of oxidizing compounds. Further factors that are relevant in choosing
a material for a seawater piping system are: physical and mechanical
properties, availability, material costs, ease of fabrication and maintenance,
anticipated design-life and previous design experience.
Over
several decades, many thousands of tons of the copper-nickel alloys UNS C71500
and UNS C70600 have been installed in different marine engineering structures
for the shipbuilding, offshore, power and desalination industries. These
alloys, which have been applied for seawater piping and heat exchangers, are
adopted by various standards. UNS C71500 is predominantly used for military
submarine service due to its higher strength and maximum allowable flow rate,
as well as low magnetic permeability. However, the wider commercial application
of this alloy is limited to a certain extent because of its higher material
cost. The work-horse, therefore, is the UNS C70600 (CuNi 90/10, cupronickel).
This alloy reveals a well-balanced combination of characteristics allowing its
widespread and economical use. To ensure the further reliable application of
the material, there is a need for a detailed discussion on its properties. In
particular, attention should be drawn to the quality of CuNi 90/10 products,
the performance in waters containing hydrogen sulfide and the prevention of
erosion as well as galvanic corrosion.
This
paper is based on a literature review and experience of KME as a manufacturer
of copper-nickel piping systems. It describes the relevant corrosion mechanisms
and provides useful service recommendations particularly for UNS C70600.
CORROSION AND BIOFOULING RESISTANCE
Metallurgical
considerations
Copper
and nickel have similar atomic radii and lattice parameters and so the phase
diagram is relatively simple. At all temperatures, Cu-Ni alloys are represented
by a single phase face centered cubic structure. The absence of phase
transformation during thermal cycles reduces the effect of welding on
mechanical characteristics and the corrosion resistance of the material. This
crystallographic structure reveals very good ductility and impact strength even
at temperatures well below freezing point. The slow diffusion rate for nickel
in copper leads to concentration gradients in the melt and consequently
increases a segregation tendency in the cast structure at normal cooling rates
(Figure 2). Thus, to provide a uniform establishment of protective oxide
layers, the homogenization of the segregated structure is required by hot
forging or cold working with a subsequent recrystallisation anneal.
Table
1 compares the common international compositional standards for cupronickel.
Some specifications monitor more strictly the iron content which is an
essential alloying element and responsible for the improvement in corrosion
resistance of the alloy. The alloying range between 1.5 and 2.0 wt.% Fe
provides optimum resistance in flowing seawater (Figure 3)3. To provide the
appropriate quality of cupro-nickel products, the cooling rate from the
solution annealing temperature must keep the precipitation of iron containing
particles to a minimum. The quality of the piping can be easily accessed by
measurement of the relative magnetic permeability, which should be lower than
1.5.4 As for all metallic materials, if the CuNi 90/10 has to be welded, the
maximum limits for some impurities such as lead, sulfur, carbon, and phosphorus
should be carefully controlled.
As
a confirmation of above considerations, Figure 4 illustrates one of many weld
necks installed on an offshore unit operating off the coast of South America. A
great number of weld necks downstream valves and pumps suffered corrosion
attack. The system was operated at ambient temperature and at a flow rate below
3,5 m/s. The cross-section of the weld neck shows a cast structure. The
chemical analysis of the weld neck revealed the presence of 0.952 % Fe, 0.012 %
Mn, and 0.087 % S.
General
Corrosion
The
corrosion behavior of copper-nickel depends on the presence of oxygen and other
oxidizers because it is cathodic to the hydrogen electrode. During the primary
corrosion reaction, a cuprous oxide film is produced that is predominately
responsible for the corrosion protection. The products of corrosion reactions
can react with compounds in seawater e.g. to CuCl23Cu.(OH)2
or Cu2(OH)3Cl and in so doing build a multi-layered oxide
structure. The corrosion rate quickly decreases significantly over a few days,
with one study indicating that the associated discharge of copper ions was
reduced tenfold during 10 min and 100-fold in the first hour. The long-term
general corrosion rates of UNS C70600 have been found to continuously decrease
with time of exposure to below 2.5 µm/yr.6 However, establishment of
a mature film takes from 27 to 35 months at temperatures
of 15-17°C. At 27°C, common inlet temperature for the Middle East, the
establishment of the protective film within a few hours was reported.8 In spite
of a wide utilization of cupronickel, the positive effect of the seawater
temperature9 is still not well understood and more research is required on this
area.
It
is important to ensure appropriate formation of the protecting film and, thus,
to avoid premature failures. The following recommendations work well during
hydrotesting and commissioning:
·
The system should
be cleaned from dirt, lubricants and debris. Introduction of solid matter
should be avoided by installation of strainers or screens.
·
The usage of clean
seawater or fresh water for hydrostatic testing is advisable. If polluted water
has been used, it should be disposed and the pipe work should be rinsed
properly with clean seawater or fresh water. If subsequent stagnant conditions
are expected, blow-drying of the system is advisable.
·
During
commissioning of a new or retubed system, continuous exposure to clean seawater
for up to 3 months, depending on water temperature, is needed to establish a
mature protective film.
·
A system with
continuous pumping activity, such as a cooling system, can be commissioned
under normal operating conditions. In seawater and water containing high levels
of suspended matter, the minimum flow rate should not be below 0.5-1 m/s in
order to prevent the formation of deposits.
·
The commissioning
of a system with intermittent flow, such as a fire fighting system, should be
conducted in seawater free from suspended matter, e.g. in water from the open
sea. However, the sea-water has to be replaced by oxygenated seawater within
4-5 days to avoid putrefaction.
Biofouling
The
bacteriostatic properties and resistance of cupronickel to macrofouling are well
recognized and widely used. There are two different aspects to which these
characteristics can be attributed:
On
the one hand, there is a gradual release of cuprous ions that is not tolerated
by many marine organisms. It has been shown, that the establishment of a
bacterial layer on the surface of cupronickel is significantly retarded. The
natural corrosion rate of cupronickel is sufficient to decelerate the initial
development of the biofouling layer.
On
the other hand, the cupronickel generates a passive film, which consists of
several different layers. A thin cuprous oxide layer, which is formed on the
bulk metal, provides sufficient corrosion resistance. The top layers, which are
formed during secondary corrosion reactions, have a weakly adherent porous
structure which reduces the release rate of cuprous ions. This may allow some
organism settlement to take place on the surface in quieter conditions
although, due to weak the adherence of the top layers, biofouling sloughs off
periodically revealing the protective inner cuprous oxide layer again.
Figure
5 compares the appearance of cupronickel and steel plates exposed under similar
conditions. The significant difference in the accumulation of biological mass
on steel samples is evident. Costly cleaning procedures and chlorine treatment
of seawater become unnecessary. However, it has been widely recognized that, if
cathodically protected, the biofouling resistance of cupronickel is decreased.
Localised
Corrosion
In
clean natural seawater or in seawater chlorinated to levels sufficient to
control the biological metabolism, UNS C70600 is resistant to localized attack.
Due to its biofouling resistance, the number of potential sites for pitting
attack is limited, even in slowly moving seawater. However, in polluted water
containing hydrogen sulfide, pitting usually takes place in the form of wide
and shallow pits. The undercut type of pitting attack normally associated with
stainless steel is not common for CuNi 90/10.
Only
a limited amount of information is available on failures of copper-nickel due
to crevice corrosion. Theoretically, the crevice corrosion behavior of the
alloy is generally controlled by an ion concentration cell mechanism where the
accumulation of copper ions in the crevice leads to ennoblement. Thus, if
encountered, the corrosion attack would tend to take place in the region
adjacent to the crevice, which is exposed to the bulk water.
Under-deposit
corrosion is really crevice corrosion beneath deposits. There is a tendency for
deposition of suspended matter at flow rates below 0.5-1 m/s. Cupronickel
performs well as a material for firefighting systems with predominantly quiet
conditions. However, if the region under deposits become anaerobic, contributing
to the establishment of sulfate reducing bacteria, the situation can become
critical.
Since,
stagnating conditions should be avoided in seawater containing high levels of
suspended particles or biological matter; it is advisable to use the water from
the open sea in stand-by systems. In addition, the frequent refreshment of
seawater is preferable at least once every 4-5 days. The chlorination of
seawater could improve the water quality by control of microbiological
activity. During prolonged shut-down periods of the plant, it is recommended to
rinse the system with clean seawater and keep it dry.
Effect of
Polluted Water
Most
accelerated corrosion problems and premature failures of UNS C70600 have been
related to the activity of sulfate reducing bacteria (SRB) and presence of
hydrogen sulfide originating from biochemical reactions. It was demonstrated14
that in seawater containing no sulfide, the free corrosion potential of
cupronickel lies on the noble side of hydrogen evolution. In the presence of
sulfide, however, the corrosion potentials are shifted to negative values.
Therefore, hydrogen evolution becomes possible as a part of the cathodic
reaction. Sulfur can be reduced to sulfide at the cathodic site. The elemental
or colloidal sulfur contained in seawater may be reduced to sulfide. HS- reacts
with Cu+ and produces a non-protective black cuprous sulfide which is poorly
adherent and results in enhanced probability of erosion attack.
Eiselstein
et al demonstrated that samples pre-exposed to aerated water corroded much
slower even than fresh material in aerated polluted water. This indicated that
corrosion films formed in sulfide-free environments offered some protection
against accelerated attack although not longer than for three days. Finally,
samples pre-exposed to aerated polluted and then exposed to aerated unpolluted
water were re-passivated in less than five days. In contrast, Kirk and
co-worker reported that protective films formed on cupronickel during four
months exposure to clean seawater provided nearly total corrosion immunity in
seawater containing up to 0.5 ppm H2S.
From
general experience, UNS C70600 installed on offshore systems or seagoing
vessels provides sufficient resistance to sulfide induced corrosion. However, it
is desirable to replace the water in systems with stagnating conditions every
4-5 days with oxygenated clean seawater. For ships, it is recommended to fill
the system with clean seawater prior to entering, while within and when leaving
the harbor. Care should be taken if there is a risk of prolonged exposure to
sulfide polluted water as is commonly the case in harbors and brackish water.
It
has been recognized that the presence of Fe2+ in seawater, which
originates from additions of ferrous sulfate (FeSO4) or installation
of stimulated iron anodes, can reduce the extent of sulfide induced corrosion.
It
was indicated that protection was due to establishment of a protective film
consisting primarily of lepidocrocite (g-FeO.OH).
Two possibilities for film formation were suggested: lepidocrocite was
electrophoretically deposited on the surface from g-FeO.OH colloid formed in the solution, or, through an
intermediate step, ferrous ions are transported to cathodic sites and
ultimately oxidized to g-FeO.OH. The substance acted as a cathodic inhibitor
increasing the cathodic polarization. In another study18, considering the zeta
potentials and colloidal chemistry of FeSO4, it was postulated that
the film formation takes place due to direct attraction of lepidocrocite from
the colloid by Cu2O.
However,
it has been recognized that extensive application of ferrous ions can result in
formation of a bulky scale on the tube surface leading to deterioration of heat
exchanger performance. Therefore, the control of treatment efficiency and
periodical mechanical cleaning of the heat exchanger might be required. Sato19
recommended a gradual decrease in dosing levels after the initial film
formation. The concentration and type of dosage capable of preventing sulfide
induced corrosion are given in Table 2. It is advisable to stop the treatment
with ferrous ions one hour before chlorination since the simultaneous
treatments were reported to be ineffective.
It
should be emphasized that during normal service on offshore units or seagoing
vessels, additional ferrous sulfate dosing is seldom required. However, if
exposure to polluted water is going to occur (e.g. when entering port), a
reasonable additional precaution might be to apply dosing prior to entering,
while in and after leaving port.20 In addition it has been reported that
application of ferrous sulfate treatment is beneficial in combating the erosion
of condenser tubing.
Erosion-corrosion
Erosion-corrosion
is a combined process, which is partly the mechanical impact of a moving medium
over a metal surface, and partly electrochemical processes. In the case of
copper alloys, it has been generally recognised that increasing flow velocities
have no significant effect on the corrosion rate until a critical velocity –
the so called breakaway velocity – is reached. Depending of the inner pipe
diameter, the maximum flow rates in CuNi 90/10 systems should to be conservatively
limited to 3.5 m/s. It has to be emphasized, that no erosion failures are known
in clean seawater within the above velocity range. Moreover, the reported
erosion failures are usually associated with the basic design mistakes of the
piping system.
The
mechanism of erosion-corrosion is affected by hydrodynamic characteristics of
the flow depending on the thickness of both the velocity boundary layer and the
diffusion boundary layer at similar average flow velocities. As a result, the
increasing boundary layer causes a decreased concentration gradient and thus
reduces the mass transport. Consequently, if the corrosion process is
determined by the mass transport from or to the surface, it can be expected
that an increasing pipe diameter results in lowering erosion-corrosion rates
and an increased breakdown velocity. Efird estimated that the critical velocity
was 4.4 m/s for a tube 0.03 m in diameter and 6 m/s for a tube 3 m in diameter
at 27°C. The calculated critical shear stress was 43.1 N/m² for UNS C70600 in
seawater indicating the conditions, under which the passive film is formed,
must be considered.
Severe
conditions might be expected if the pipe opening has a foreign body lodged in
it causing throttling of the flow and leading to an abnormal increase in local
flow velocity. Therefore, entrance of debris has to be prevented by
installation of strainers. Also growth of macrofouling accumulations must be
prevented by appropriate biocide treatment.
Campbell
evaluated the effect of controlled bubble size (1.0 and 2.3 mm) on
erosion-corrosion resistance and compared these results with bubble free water
using a jet impingement apparatus. The deleterious effect decreases with
smaller bubble size. To minimize this effect of gas bubbles, consideration
should be given to the position of inlet boxes and the construction of venting
systems.
It
appears reasonable to suppose that an occasional reduction in velocity may
contribute to repassivation and thickening of the surface film. Knutsson
presented results of an examination of flow regimes over 12 months using a
copper alloy. From 1 to 25 % flow duration, no attack was observed at 11.9 m/s.
Continuous flow produced results of <15 µm/yr. at 6.1 m/s and 76 µm/yr. at
11.9 m/s. Therefore, for emergency situations, as in fire fighting systems
which do not experience frequent pumping activity, the flow velocities up to
10-15 m/s are reasonable for UNS C70600. During two years of testing with sand
loaded natural seawater by means of a once-through loop including pipes from 4”
to 7” operated intermittently, no appreciable corrosion attack was detected at
velocities up to 7 m/s.
Negative
effects of erosion are preventable by good design which is also necessary to
improve the efficiency of the piping system. The general guidelines for
reduction of friction loss depend on the flow velocity, pressure drop in the
system due to the geometry of bends and valves, the required pumping power, and
the probability of erosion-corrosion:
·
The layout of the
system should be as direct as possible.
·
Control the flow
with the least number of valves.
·
Ask the valve
manufacturer for data related to the effect of valve geometry on the pressure
drop in the system. In most instances, there are considerable variations for
nominally similar valves.
·
Consider the
effect of r/d-ratio of elbows and the effect of sudden enlargement and
contractions on the pressure drop.
·
When fittings are
used, specify long radius and full-form types.
·
Cut the gasket
flush with the inner surface of the pipe.
·
Provide a minimum
distance of 5 x I.D. between a pump or a valve and a bend.
The
exact adjustment of pipe ends and outlet is important to avoid protrusions that
can lead to erosion damage in service. In all cases, 100 % weld penetration
without excessive penetration of the root into the tube cavity is required to
avoid causing any turbulence of the fluid in service. Mismatching of pipe-ends
should not exceed half of the wall thickness; however, it should be less than 2
mm.
Effect of
Chlorination
Chlorination
of seawater is the most common method to control biofouling and it has been
reported that continuous chlorination to a residual free chlorine level of 0.25
ppm can be 100 % effective against fouling.
It
has also been reported that, in the presence of 0.25 ppm free chlorine, the
corrosion of CuNi 90/10 increased during 30 days of exposure but the effect of
the chlorine weakened subsequently. Kirk and co-worker stated that according to
general experience no negative effect of chlorine concentrations 0.2-0.5 ppm
was indicated on the corrosion behavior of copper-nickel alloy during many
years in coastal power and process industries. In spite of the biofouling
resistance of copper-nickel, the chlorine treatment extended the intervals
between mechanical cleanings to restore the performance of heat exchangers from
2 months without chlorination to up to 1 year under chlorinated conditions in
coastal plants.
Francis
published the results of tests related to the effect of chlorine additions in
the range between 0.3 and 4.0 ppm on corrosion and jet impingement tests (jet
velocity 9 m/s for 2 months) of UNS C70600 exposed to natural seawater. The
products formed on cupronickel during chlorination led to appreciable anodic
and cathodic polarization, and, thus, somehow to an improvement in corrosion
resistance. Nevertheless, these products impaired the mechanical resistance of
the copper-nickel surface leading to an increase in impingement susceptibility.
For continuous and intermittent chlorine additions, the concentrations of 0.3
and 0.5 ppm respectively were recommended. Finally, the author pointed out that
his results require more additional research.
Another
study proposed a possible mechanism for the effect of free chlorine on
corrosion performance of CuNi 90/10 in highly polluted brackish water
containing appreciable amounts of planktonic and sessile sulfate reducing
bacteria. It was assumed that the corrosion process in the presence of chlorine
was controlled by transformation of Cu2O layer into secondary
compounds such Cu2(OH)3Cl due to the high oxidizing power
of the medium. These secondary products are not well adherent to the surface
and easily removed allowing further Cu2O formation. Unfortunately,
no mass loss or depths of corrosion attack were presented in this paper.
Obviously,
more research is needed on the effect of chlorination on CuNi90/10. However, it
can be concluded that an over-chlorination of seawater should be avoided.
Galvanic
Corrosion
The
avoidance of galvanic corrosion is a principal design consideration. Stainless
steels undergo significant potential variations depending of chlorine and
oxygen content, temperature, as well as presence of a biofilm, e.g. up to +800
mV SCE in seawater containing 0.5-1.0 ppm free chlorine to less than –400 mV
SCE in deaerated seawater. In contrast, UNS C70600 reveals only small changes
(Figure8). It has been reported that the corrosion potential of this alloy
remains in the range between 0 and –300 mV SCE in natural aerated seawater at
10 and 40°C, in seawater flowing at 3 to 15 m/s at same temperatures, and in seawater containing 0.5
ppm free chlorine at 15 °C. Figure 9 demonstrates the galvanic series of
different commercial alloys in flowing seawater. UNS C70600 has a central
position in the series. It is nobler than aluminum alloys, carbon and
galvanized steel and less noble than stainless steels and titanium alloys. It
can be coupled to tin and aluminum bronze.
Bardal
et al studied coupling of CuNi 90/10 with high-alloy stainless steel in natural
and chlorinated seawater. During connection of the metals in chlorinated water,
no significant effect of galvanic corrosion was found. The difference was
attributed to the establishment of biofilm on high-alloy stainless steel in natural
seawater providing a much higher cathodic efficiency. However, precautionary
measures should be taken if the chlorination might be turned off. For
prevention of galvanic corrosion, it is recommended to use compatible materials
wherever possible. However, in multi-material systems, the combination of
different metals is often unavoidable. Thus the electrical contact of UNS
C70600 with aluminum, nickel and titanium alloys, carbon and stainless steels
should be avoided by application of commonly applied protection measures.
Stress-Corrosion Cracking
It
has been reported36 that cupronickel failures due to stress-corrosion cracking
are unknown in seawater polluted with ammonia. The alloys are also immune to
chloride and sulfide stress corrosion cracking. Thus, no stress relief
treatment is required for cold worked or welded material.
CONCLUSIONS
The
merits of the UNS C70600 as an appropriate alloy for seawater pipework can be
attributed to various aspects. First of all, it is a simple alloying system
with a single phase face centered cubic structure providing excellent hot and
cold workability. The absence of phase transformations during welding
contributes to its easy weldability with no need for extensive post-weld
treatments. However, the chemical composition and the manufacturing of
cupronickel products must comply with international standards.
The
low uniform corrosion rates of the alloy allow the specification of thinner
walled piping and, therefore, provide weight saving. Cathodic protection is not
usual for UNS C70600 piping. The alloy is resistant to biofouling and does not
reveal sensitive corrosion potential variations under different seawater
conditions. Such a combination of features leads to the improved resistance to
localized corrosion and the elimination of extensive monitoring procedures
associated with chlorination and higher seawater temperatures which other alloy
systems might require. The alloy has high resistance to crevice corrosion and
is resistant to stress corrosion cracking under marine conditions.
For
lower seawater temperatures, protective surface films may take up to 3 months
to fully mature. For this reason, hydrotesting and commissioning
recommendations have been given. In addition, to avoid premature failures in
the presence of hydrogen sulfide, precautionary measures are needed. The given
practical recommendations should be followed to avoid corrosion problems. The
susceptibility to erosion- corrosion and galvanic corrosion can be eliminated
by recommended design considerations.