Volume 7 Preprint 3
Cu-Ni-Zn-Mn Alloys for Sulphide Polluted Seawater Applications
A.P.Patil and R.H. Tupkary
Keywords: Copper alloys, Corrosion, Seawater, Sulphide and Film
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Volume 7 Paper 3
Cu-Ni-Zn-Mn Alloys for Sulphide Polluted
A.P.Patil* and Dr. R.H. Tupkary
Department of Metallurgical and Materials Engineering, Visvesvaraya
National Institute of Technology, Nagpur – 440 011 (India), Email:
Cu-10Ni alloy suffer from accelerated corrosion in sulphide polluted
seawater. New copper base alloy containing 10% Ni, 29% Zn and, 3% and
5% Mn have been developed and tested vis-a vis Cu-10Ni alloy in
synthetic seawater both clean and polluted with sulphide ions. It is found
that Cu-10Ni-29Zn-5Mn and Cu-10Ni-29Zn-3Mn alloys have better
corrosion resistance in both the test solutions. Observed behaviour in
synthetic seawater is attributed to modification of defective structure of
Cu2O by trivalent cations of Mn. Observed behaviour in sulphide polluted
synthetic seawater is attributed to formation of ZnS containing multiphased film and incorporation of Mn3+ in Cu2S lattice
Key words: Copper alloys, Corrosion, Seawater, Sulphide and Film
As a result of excellent corrosion resistance in seawater, 90-10 Cu-Ni has
become standard condenser / heat exchanger tube material for seawater
applications. However, this alloy suffers from accelerated corrosion in
seawater polluted with sulphide. Its poor corrosion resistance in sulphide
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and
Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it should
not normally be referenced in published work. © UMIST 2004.
polluted seawater is a cause of concern. Like other copper alloys the 9010 Cu-Ni develops a film of Cu2O, which accords the alloy excellent
corrosion resistance in seawater . If seawater is polluted with sulphide,
Cu2S forms in the film . Like the Cu2O the Cu2S is a p-type
semiconductor but possesses more defective structure than the Cu2O and
thereby causes accelerated corrosion. The accelerated corrosion in
sulphide polluted seawater can be mitigated in two ways viz. (i) by
modifying defective structure of the Cu2S and the Cu2O and (ii) by
modifying composition of the film altogether. The defective structure of
the Cu2O is modified when multi-valent cations originating from the alloy
get incorporated in its lattice . This improves ionic and electronic
resistance of the film and consequently improves corrosion resistance of
the alloy. This is achieved by alloying Cu-Ni alloys with Fe, Mn, Al and Cr.
These alloying additions produce bi-valent or tri-valent cations like Ni2+,
Fe2+, Fe3+, Mn2+, Mn3+, Al3+ and Cr3+. Examples of these are, (i) additions
of 1-1.8% Fe and 0.5-1% Mn in alloy C70600, (ii) additions of 2% each of
Fe and Mn in alloy C71640, (iii) additions of 0.5% Fe, 0.2-1% Mn and 2-3%
Cr in alloy C71900, (iv) additions of 0.7-1.2% Fe, 3.5-5.5% Mn, 1-2% Al
and 0.5% Cr in alloy C72420 and, (v) additions of 1-2% Fe, 4.5% Mn, 0.5%
Cr and 1.9% Al in MARINEL. However structure of Cu2S seems to be too
defective to be modified effectively with these alloying additions.
Certain alloying additions modify the composition of the film and instead
of a single-phased film of Cu2O, either a multi-phased film or a single-
phased film of some other compound is formed. Examples of such
additions are (i) Al addition which produces a film of Al2O3 (as in
Aluminium brass-C86700 and Aluminium bronze-C95800) and (ii) Sn
addition which produces a film of SnO2 (as in AP bronze). Films of Al2O3
and SnO2 accord better corrosion resistance than film of Cu2O and
thereby improve corrosion resistance of the alloys. It is known that
addition of Zn improves resistance to sulphide attack as in case of singlephase Cu-Zn alloy but data is scanty. The formation of ZnS, which is a
bad conductor, probably accords better resistance to sulphide attack.
However, role of Zn addition to Cu-Ni alloy remains to be investigated.
Role of Mn in Cu-Ni system is found to be secondary to that of Fe  but
its role in Cu-Ni-Zn system remains to be investigated. The present work
is an attempt in that direction. Accordingly, it is aimed at developing a
series of single-phased Cu-Ni-Zn-Mn alloys by modifying 90-10 Cu10Ni with additions of 29% zinc and, 3% and 5% manganese and, testing
these alloys vis-à-vis Cu-10Ni alloys for corrosion resistance in synthetic
seawater, both clean and polluted with sulphide ions.
The test alloys were prepared in the laboratory. These were melted in
graphite crucible, then cast in steel mould (10 x 20 x 300 mm), then
annealed (for solutionising) at 900°C for three hours, then cold rolled
from 10 mm to 1.2 mm thickness, then annealed at 800°C for 2 hours
and finally quenched in water. Actual chemical composition of the test
alloys and other relevant information are presented in Table 1.
Table 1: Actual chemical composition of the test alloys
The alloys possess single-phase microstructure as presented in Fig. 1.
Testing for corrosion involved weight loss, cathodic and anodic
polarisation methods. Test solutions used in these studies were clean
synthetic seawater (SSW) and sulphide polluted SSW. Nominal
composition of clean SSW used in this investigation is as per ASTM
standard D114-75 (reproved 1980) for synthetic seawater but without
heavy metal ions. For preparing sulphide-polluted seawater LR grade
Na2S was added to SSW. In general 0.1 g Na2S was added to 1 litre SSW.
The nominal pH of SSW was 8.2, but changed to 9.2 on addition of Na2S.
The pH was then adjusted to 8.2 by addition of 0.05N H2SO4. It was
found, that almost whole of sulphide ions were oxidised to sulphate in
one day. This required the test solution to be replenished daily with
sulphide ions. Hence, a fresh solution of Na2S in SSW was added to the
test solution daily and resultant rise in pH was neutralised by addition of
Fig.1: Microstructure of test alloys; showing well developed single-phased
grains (A: Cu-10Ni, B: Cu-10Ni-29Zn, C: Cu-10Ni-29Zn-3Mn and D: Cu10Ni-29Zn-5Mn).
Specimens (10 x 25 x 1.2 mm) were cut and prepared by abrading on a
series of emery paper (1/0, 2/0, 3/0 and 4/0) for the weight loss tests.
Specimens were degreased in 5% NaOH solution and then washed with
water just before testing. The pH of SSW was 8.2 and temperature was
27°C. Test duration was 15 days, but to study effect of exposure duration
the specimens were taken out at planned intervals of 0-3, 0-6, 0-9, 0-12
and 0-15 days. The corroded specimens were then subjected to cleaning
by scrubbing with bristle brush using scourer powder and distilled water.
The samples were then washed in distilled water, then rinsed in methanol
and then air dried before weighing for weight loss. One set of Cu-10Ni29Zn-5Mn specimens was prepared in the same way, then exposed to
respective test solutions for 15 days and then subjected to scanning
electron microscopy. The films were analysed using EDX attached to SEM
(JEOL 840A) and also by XRD (Philips PW1017 based).
The specimens for polarisation studies were prepared by soldering a
piece of insulated copper wire at one flat surface of specimen (10 x10
x1.8 mm). These were then mounted in cold setting resin in such a way
that the soldered joint was completely embedded and the other flat
surface was open. The open surface was prepared by abrading on series
of emery papers (1/0, 2/0, 3/0, 4/0 and 5/0). Polished specimens were
then washed with soap and distilled water just before setting up cell.
These specimens were allowed to reach a stable open circuit potential
(OCP) for almost 30-35 minutes, before carrying out the polarisation test.
The OCP of the test alloys was found to be in the range of –50 mV (SHE)
to +30 mV (SHE) except that of Cu-10Ni in sulphide polluted SSW being –
600 mV (SHE).
Anodic and cathodic polarisation studies were carried out with a threeelectrode system using a computer-controlled potentiostat (Sycopel
AUTOSTAT 253). Platinum electrode (cylindrical mesh) was used as
counter electrode and saturated calomel electrode was used as reference
electrode however, all potentials are referenced to Saturated Hydrogen
Electrode (SHE). The cathodic polarisation was started from -560 mV
(SHE), increased towards OCP and stopped when current became positive.
The solution was stirred with a magnetic stirrer to minimise effects of
concentration polarisation. The anodic polarisation was started at –60 mV
(SHE) and stopped after +500 mV (SHE). The solution was stagnant in
anodic polarisation studies. The scan rate in cathodic polarisation was
0.166 mV/s and that in anodic polarisation was 0.08 mV/s.
Results and Analysis
Figures 2 and 3 present results of weight loss studies in clean and
sulphide polluted SSW, respectively and shows the effect of exposure
duration on weight loss and corrosion rate of the test alloys. It is evident
that manganese-containing alloys have better corrosion resistance than
other two alloys. It is evident that rate of corrosion is high initially and
then decreases with increasing exposure duration. It is also evident that
rate of change of weight loss is high initially and then tapers down.
However, the plots have not reached to a plateau, it means that the rate
of corrosion will fall further if the exposure duration is extended beyond
Figure 4 presents SEM photomicrographs of the film formed on Cu-10Ni29Zn-5Mn exposed to clean SSW. It shows that the film covers entire
surface and is compact. It also shows that the film is made up of two
layers. The inner layer is crystalline and forms the bulk of the film and,
the outer layer is powdery. Figure 5 presents SEM photomicrographs of
the film formed on Cu-10Ni-29Zn-5Mn in sulphide polluted SSW. It
shows that the film covers entire surface but is defective and has many
clusters of globular substance.
Fig. 4: SEM photomicrograph of the
Fig. 5: SEM photomicrograph of the
alloy in clean synthetic seawater.
alloy in sulphide polluted synthetic
film formed on Cu-10Ni-29Zn-5Mn film formed on Cu-10Ni-29Zn-5Mn
Figure 6 presents EDX spectrums of polished Cu-10Ni-29Zn-5Mn alloy.
Various peaks in this figure in general represent composition of the Cu10Ni-29Zn-5Mn alloy. Whereas, Fig. 7 presents EDX spectrums of the
film formed on Cu-10Ni-29Zn-5Mn in clean SSW and Fig. 8 shows EDX
spectrums of the base film formed on Cu-10Ni-29Zn-5Mn in sulphide
polluted SSW. Comparison of Figs. 6 and 7 indicates that the film formed
in clean SSW has alloying elements almost in the same ratio as that in
substrate and, has substantial oxygen, sulphur and chlorine. Comparison
of Figs. 7 and 8 indicates that the film has large quantity of Zn and small
quantities of Cu and Ni and, large quantity of O, S and Cl.
Figure 9 presents XRD spectrum of the polished Cu-10Ni-29Zn-5Mn
alloy. The positions of all the peaks match with those of copper,
indicating thereby that the alloy is single-phased. These peaks are
slightly shifted to left of copper positions, suggesting thereby some strain
in the lattice. This lattice strain is attributed to formation of solid solution
of alloying elements in the copper. Figure 10 shows XRD spectrum of the
alloy corroded in SSW for 15 days. Like Fig. 9, in this figure also the
positions of all the peaks match with the copper position. This indicates
that the film is coherent with the substrate matrix and is single-phased.
The Cu2O is formed epitaxially and is coherent with the substrate matrix
. It therefore suggests that the film formed on Cu-10Ni-29Zn-5Mn in
SSW is made of Cu2O. Figure 11 presents XRD spectrum of the film
formed on Cu-10Ni-29Zn-5Mn alloy in sulphide polluted SSW. There are
many peaks in this figure and therefore it indicates that the film is multiphased. Out of these peaks, four peaks match with the copper positions
and indicate presence of Cu2O. The remaining peaks indicate presence of
oxides and sulphides, although exact identification is not possible owing
to non-stoichiometric composition of these oxides and sulfides . It is
likely that oxides like Cu2O, CuO, ZnO, NiO and, sulfides like Cu2S, CuS,
ZnS and NiS are formed in this film
Fig. 9: XRD spectrum of polished specimen of Cu-10Ni-29Zn-5Mn alloy
Fig. 10: XRD analysis of film developed on Cu-10Ni-29Zn -5Mn alloy
exposed to SSW for 15 days
Fig. 11: XRD spectrum of film developed on Cu-10Ni-29Zn -5Mn alloy
exposed to sulfide polluted SSW for 15 days
Figure 12 presents cathodic polarisation plots of the test alloys in clean
SSW. It is seen that Ecorr of all the test alloys is almost in the range of 0
to 50 mV (SHE) although Ecorr of Cu-10Ni alloy is relatively nobler. It is
evident that at potential lower than –400 mV (SHE) the cathodic c.d.
reaches a limiting value. This indicates that the cathodic reaction is under
diffusion control. At potentials higher than –400 mV (SHE) the cathodic
plot is near linear. This indicates that the cathodic reaction in this
potential range is under activation control. This linear region is
extrapolated to obtain icorr. The icorr of Cu-10Ni alloy is higher than
other test alloys. The data obtained from these plots is presented in Table
Table 2: Data obtained from cathodic polarisation vis-à-vis immersion
Where, CRT = Corrosion rate from Tafel extrapolation and,
CRW = Corrosion rate from weight loss in immersion test.
Figure 13 presents cathodic polarisation plots of the test alloys in
sulphide polluted SSW. From this figure it is evident that the Ecorr of Cu-
10Ni is –600 mV (SHE) and those of other alloys are in the range of –80 to
–120 mV (SHE). It is evident that the Cu-Ni-Zn-Mn alloys have relatively
much nobler Ecorr than Cu-10Ni in sulphide polluted SSW. It was noticed
that specimens of Cu-10Ni had developed a violet tinge in 30 minutes
exposure before the cathodic polarisation test and that the specimen had
lost the tinge completely at the end of cathodic polarisation tests. It
means that the film that was developed in the initial exposure was
reduced in cathodic polarisation. Whereas, the specimens of Cu-Ni-ZnMn alloys developed a light golden tinge during initial exposure of 30
minutes and that the specimens retained the tinge even after cathodic
polarisation tests. It means that the components of the film were stable in
the range of potential in which the polarisation test was conducted. This
suggests that the film formed on Cu-10Ni in sulphide polluted SSW was
different from that formed on Cu-10Ni-29Zn-5Mn alloy. The data
obtained from these plots is also presented in Table 2. In general there
seems to be reasonably good agreement between corrosion rate obtained
in weight loss and that from cathodic polarisation.
Figure 14 presents anodic polarisation plots of the test alloys in clean
SSW. It is evident that the test alloys undergo rapid active dissolution up
to 180-200 mV (SHE) and give a peak c.d. of 10-20 mA/cm2 at this
potential. The active dissolution region does not show a well- defined
Tafel region and consists of two or more linear regions with different
slopes. Milosev and Metikos-Hukovic  obtained almost similar active
dissolution region in anodic polarisation plots of 90-10 Cu-Ni in borate
buffer containing different NaCl concentrations and termed it as ‘the
apparent Tafel region’. On increasing the potential further the anodic c.d.
decreases to approximately 1 mA/cm2. In this region the c.d. is rather
limited and increases slowly with increasing potential up to 450 mV (SHE),
after which film breaks down. Anodic polarisation plots of commercial
Cu-9.4Ni-1.7Fe alloy in air-saturated 3.4 wt% NaCl solution obtained by
Kato and coworkers  had similar features. Their plots had limiting
current region with c.d. of 1-2 mA/cm2 following a peak c.d. of 2
mA/cm2. They termed the observed limiting current region as
‘brightening region’. In view of relatively higher c.d. of 1-2 mA/cm2 and
almost equal peak c.d. (2 mA/cm2) it was appropriate for them to
mention this region as ‘brightening region’. But in present study, the c.d.
drops noticeably from a peak value of 10-20 mA/cm2 to 0.8-1 mA/cm2
for Cu-10Ni-29Zn-5Mn in SSW. The drop in c.d. is too substantial to be
termed as ‘brightening’ region. Secondly metal is supposed to exhibit
passivity when passivation c.d. (ip) is lower than corrosion c.d. (icorr)
obtained from cathodic polarisation (Tafel extrapolation). The icorr for
Cu-10Ni-29Zn-5Mn in SSW is 9 µA/cm2. Considering this icorr value, c.d.
of 1 mA/cm2 is too high for this region to be termed as passive region. In
a way this situation lies in between ‘passivity’ and ‘brightening’.
Therefore, this passivity is termed as ‘pseudo-passivity’.
Figure 15 presents anodic polarisation plots of the test alloys in sulphide
polluted SSW. Anodic polarisation plots of test alloys in SSW+Na2S exhibit
two pseudo-passive regions. For example in plot of Cu-10Ni-29-5Mn
first region starts at –37 mV (SHE) and second region starts at 181 mV
(SHE). The c.d. in first region is 70 µA/cm2 and that in second region is 16
mA/cm2 whereas, icorr is 15 µA/cm2. Considering the icorr of 15 µA/cm2,
the c.d. in first region is too high for this feature to be termed as
passivity. Therefore, this feature is termed as primary pseudo-passivity.
Accordingly, these anodic polarisation plots can be divided in three main
regions: (i) the apparent Tafel region or free corrosion region, (ii) the
primary pseudo-passive region and (iii) the pseudo-passive region. The
slope of apparent Tafel region is denoted by dE/dlog(i). Anodic
polarisation plots obtained in present investigation for Cu-10Ni is similar
to the plots obtained by Alhaji and Reda  for 90-10 Cu-Ni alloy in
Table 3: Data obtained from anodic polarisation plots
Alloy and solution
Cu-10Ni in clean SSW
Cu-10Ni-29Zn in clean SSW
Cu-10Ni-29Zn-3Mn in clean SSW
CU-10Ni-29Zn-5Mn in clean SSW
Cu-10Ni in sulphide polluted SSW
Cu-10Ni-29Zn in sulphide polluted SSW
Cu-10Ni-29Zn-3Mn in sulphide polluted
Cu-10Ni-29Zn-5Mn in sulphide polluted
The data (Ecorr and Tafel slope) obtained from anodic polarisation plots is
presented in Table 3. Comparison of cathodic and anodic Tafel slopes
indicate that corrosion of the test alloys in clean SSW is under cathodic
control. It also indicates that corrosion of zinc containing alloys in
sulphide polluted seawater is also under cathodic control but that of Cu10Ni alloy in sulphide polluted seawater is under anodic control.
The pH of the test solutions was 8.2 therefore hydrogen reduction
reaction would require operating potential lower than –484 mV (SHE).
While, oxygen reduction reaction would require operating potential lower
than +0.743 mV (SHE). The open circuit potentials (OCP) of the test alloys
were in the range of –50 to +32 mV (SHE). Therefore oxygen reduction
reaction will certainly occur, but hydrogen reduction reaction will not
occur during free corrosion of these alloys. In SSW+Na2S solution, OCP of
Cu-10Ni was –500 mV (SHE), i.e. lower than –484 mV (SHE). But even in
this case hydrogen reduction should not occur at OCP because of lack of
overvoltage required for this reaction to occur. Hence, in general oxygen
reduction reaction was the only possible cathodic reaction in the system
Formation of Cu2O by direct anodic reaction and cations by oxidation
reactions is possible
2Cu + 2OH¯ Æ Cu2O + H2O + 2e
Cu Æ Cu+ + e
Out of the alloying elements added to the test alloys, Ni and Zn produce
bivalent cations and, Fe and Mn produce bivalent and trivalent cations.
Table 4 shows the redox potentials for the formation of various cations
and the ionisation potentials of these cations.
Table 4: Redox potentials and ionisation potentials of ions of alloying
Ni2++ 2e- → Ni
Zn2++ 2e- → Zn
Mn2++ 2e- → Mn
Mn3++ 2e- → Mn
Fe2++ 2e- → Fe
Fe3++ 2e- → Fe
# Estimated value assuming cation concentration to be 10-6 g-ion/litre.
Open circuit potentials of the test alloys were found to be ranging from 50 to 32 mV (SHE) in SSW. Therefore, Mn3+, Fe3+, Ni2+, Zn2+, Fe2+ and
Mn2+ cations are stable species. It is reported that Ni is present as Ni2+
and Fe is present as Fe3+ ion in the film of Cu2O formed on 90-10 Cu-Ni
alloy . The stability of Mn3+ ion is more than that of Fe3+ ion, owing to
ionisation potential (51.2eV) and redox potential (-0.401 V (SHE)) of Mn3+
being lower than those of Fe3+ (54.8eV and -0.1545 V (SHE), respectively).
Therefore Mn3+ will certainly form.
North and Pryor  found the Cu2O and Cu2(OH)3Cl to be forming on Cu,
Cu-10Ni-1Fe-0.5Mn and Cu-30Ni-0.4Fe in boiling NaCl solution. Kato
and coworkers  studied the mechanism of corrosion of Cu-9.4Ni-1.7Fe
alloy in air saturated NaCl solution. They found by XRD analysis that the
corrosion product developed on the surface of Cu-Ni alloy had Cu2O and
Cu2(OH)3Cl. In both the cases the inner layer formed by direct anodic
reaction and outer layer by precipitation. The inner layer was Cu2O and
outer layer was Cu2(OH)3Cl. The film formed on Cu-10Ni-29Zn-5Mn
shown in Fig. 4 too has two layers, crystalline inner layer and powdery
outer layer. XRD analysis has indicated that this film is made of singlephased Cu2O. The ionic and electronic resistivities of such a composite
film would depend upon resistivities of these individual layers. The Cu2O
is a metal-deficient p-type semiconductor . Various anions and cations
entering the film affect ionic and electronic resistivities of Cu2O.
Incorporation of cations having valency more than one increases ionic and
electronic resistivities of Cu2O. Whereas, incorporation of anions like Cl¯
and OH¯ decreases the ionic and electronic resistivities of Cu2O. The
cations enlisted in Table 4 originating from substrate can modify the
defect structure and therefore improve corrosion resistance accorded by
the Cu2O film.
The incorporation of Ni2+ ions in the Cu2O film has been reported to
increase ionic and electronic resistivities of the Cu2O film forming on 90-
10 and 70-30 Cu-Ni alloys . In the similar manner incorporation of
Ni2+ ions in the Cu2O film forming on test alloys should improve ionic and
electronic resistivities of the film. Since, nickel is added to all the test
alloys in equal quantities hence, no specific relative effect of nickel can be
highlighted for these alloys. But, it is natural that nickel would contribute
to ionic and the electronic resistance of the film. Trivalent cations of Fe
are going to be more effective than bivalent ions in increasing ionic and
electronic resistivities. This is so because, when one Fe3+ ion replaces Cu+
ion in the Cu2O lattice, it increases two positive charges. This rise in
positive charges requires incorporation of two electrons to maintain
charge neutrality. Incorporation of two electrons in the Cu2O lattice
annihilates two positive holes and consequently shall increase electronic
resistivities. If trivalent cation of Fe occupies a vacant position in the
lattice then it would be even more effective. This is as per findings of
North and Pryor  that the rise in the electronic resistivity for Ni2+ ions
entering the Cu+ vacancy is 2.2x108 ohm.cm and that for Ni2+ ions
replacing Cu+ is 1.1x108. Considering the fact that diffusion of Cu+ ions
in the Cu2O lattice takes place via vacancy-assisted mechanism, in which
vacancies move inward and Cu+ ions move outward. Drop in number of
vacant sites due to placement of Fe3+ ion in place of Cu+ ion in the lattice
makes cationic movement that more difficult and consequently increases
The test alloys have varying quantity of Mn therefore it is expected that
proportional quantity of Mn3+ be incorporated in the film. It is expected
that incorporation of Mn3+ in Cu2O lattice is as effective as the
incorporation of Fe3+ is. In that event it should increase ionic and the
electronic resistance of the film with increasing manganese content and
this effect should manifest itself in polarisation plots. It has been found
to be so in cathodic and anodic polarisation studies of test alloys.
Cathodic polarisation plots of the filmed specimens have been found to
shift to lower current density with increasing manganese content.
Similarly, icrit have been found to decrease with increasing manganese
content in anodic polarisation plots of polished specimens. The effect of
manganese is more pronounced in cathodic polarisation than in anodic
polarisation. This is consistent with the findings of North and Pryor ,
who found the ionic resistivity of the Cu2O film to increase by 4.5 times
and the electronic resistivity by 6.5 times on three fold increase in nickel
content of Cu-Ni alloys (from 10wt% to 30wt%). Therefore, corrosion
resistance of Cu-10Ni-29Zn-3Mn and Cu-10Ni-29Zn-5Mn alloys being
better than Cu-10Ni is attributed to single-phased Cu2O film and
incorporation of bi-valent and trivalent ions of Mn.
Traverso et al. found MxSy type sulfide of alloying elements when Cu30Ni-2Fe-2Mn allot was exposed to natural seawater containing 8 ppm
sulphide. In the similar way the film formed in SSW+Na2S solution is likely
to contain sulphides of alloying elements. The estimated redox potential
for the formation of Cu2S, ZnS and MnS are –0.88 V, -1.46 V and –1.52 V,
respectively (considering 1 gl¯ Na2S = 41 ppm S2¯). The OCP of zinc
containing alloys was –50 mV (SHE) or higher thus, it is natural that these
sulphides would form. Secondly solubility product of Cu2S (1.9x10-48) is
relatively lower than those of ZnS (3.2x10-25) and MnS (5x10-14).
Therefore, Cu2S should always be the first compound to form. However,
considering the fact that Zn is surface-active element and that ZnS has
more negative redox potential (-1.46 V) than Cu2S (-0.88 V),
simultaneous formation of ZnS and Cu2S is possible. The ionic and
electronic resistivities of the overall film depend upon its structure and
phases. The ZnS is a bad conductor and when it forms as a separate
phase in any part of film, it will not allow electronic and ionic conduction.
If few grains of ZnS are formed then it will reduce the area of the film
through which ionic and cationic conduction can take place. In that event
the corrosion rate of the test alloys should be lower. This is what has
been found out in the present studies in case of zinc-containing alloys.
Besides, incorporation of various ions in the lattices of different phases
may also affect resistivity of the film. The Cu2S is p-type semiconductor
and its lattice is more metal-deficient than that of Cu2O . Therefore,
the resistivity of the Cu2S shall always be less than that of Cu2O if similar
cations are incorporated in similar quantities. In this way the corrosion
resistance of Cu-10Ni alloy should be relatively lower in sulphide polluted
SSW than other alloys. This has been found in the present investigation.
However, incorporation of Mn3+ ions in the Cu2S lattice would improve its
ionic and electronic resistance. Therefore, Mn containing alloys should
show better corrosion resistance than Cu-10Ni. This has been found to
be so in the present investigation.
The Cu-10Ni-29Zn-5Mn and Cu-10Ni-29Zn-3Mn alloys exhibit better
corrosion resistance than Cu-10Ni alloy in clean synthetic seawater. Their
relatively better corrosion resistance is attributed to formation of single-
phased Cu2O film and incorporation of bi-valent and tri-valent cations of
Mn. These alloys are more corrosion resistant than Cu-10Ni alloy in
sulphide polluted synthetic seawater. Their relatively better corrosion
resistance is attributed to the formation of multi-phased film containing
ZnS, which is a bad conductor. This reduces the area of the film through
which ionic and electronic conduction can take place. Incorporation of
Mn3+ cations in Cu2O and Cu2S lattice too has some role to play in
improving corrosion resistance of manganese containing test alloys in
sulphide polluted synthetic seawater.
‘The corrosion of copper-nickel alloys 706 and 715 in flowing
weawater I-effect of oxygen’, D.D. Macdonald, B.C. Syrett and S.S.
wing, Corrosion, 34, 9, pp289-301, 1978.
‘A study of de-alloying of 70Cu-30Ni commercial alloy in sulfide
polluted and unpolluted seawater’, A.M. Beccara et al., Corrosion
Science, 32, 11, pp1263-1275, 1991.
‘The influence of corrosion product structure on the corrosion rate of
Cu-Ni alloys’, R.F. North and M.J. Pryor, Corrosion Science, 10,
‘Copper-nickle-iron alloys resistant to seawater corrosion’, G.L.
Baily, J. Institution of Metals, 79, pp243-292, 1951.
‘Behaviour of copper in artificial seawater containing sulfides’, E.D.
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