corrosion property evaluation of copper alloy tubes
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한국표면공학회지J. Kor. Inst. Surf. Eng.Vol. 42, No. 6, 2009.
<연구논문>
Corrosion Property Evaluation of Copper Alloy Tubes
against Sea Water
Pang Beilli, Ong Sang-kil and Lee Hong-Ro*
Department of Applied Materials Engineering, Chungnam National University,
Daejeon 305-764, Korea
(Received December 10, 2009 ; revised December 29, 2009 ; accepted December 30, 2009)
Abstract
In this study, the corrosion property of copper alloy tubes in seawater has been investigated. Three copperalloys of nominal composition Cu-20Zn-2Al(Al-Brass), Cu-30Ni(CN70/30) and Cu-10Ni(CN90/10) were con-sidered. The samples were immersed in 3%NaCl flowing solution at 90
o
C for 30, 50 and 80 days. Corrosionrate of copper alloy tubes in 3%NaCl flowing solution was investigated by weight-loss measurements andelectrochemical test. The CN70/30 showed lowest corrosion rate among three copper alloy tubes. Becauseof passive films formation, corrosion rates of three types of copper tubes were decrease with time. Surfacecharacteristics of copper alloy tubes were analyzed by optical micrograph(OM), scanning electronic microscopy(SEM), energy dispersive X-ray analysis(EDAX) and X-ray diffraction patterns(XRD). CN70/30 showed partlypitting problem on the surface owing to high Fe content, even though having high resistant against corrosion.Cracks appeared on the surface of CN90/10 and CN70/30 after more than 50 days immersion, which couldbe derived from high nickel contents.
Keywords: Copper alloy tube, Sea water, Heat exchanger, Immersion test, Corrosion rate measurement
1. Introduction
Copper alloy tubes used for heat exchange systems
should be contacted with seawater for cooling.
According to cooling seawater flowing rate and
temperature increase during passing through the heat
exchanging system, usually corrosion problems take
place. Also, copper alloy tubes are usually included
with other impurity component such as lead, silver,
and aluminum etc. These impurities promote corrosion
rates which lead to general problem of copper alloy
tubes used for heat exchanger. Corrosion problems
such as dezincification, stress corrosion cracking and
pitting are most concern to copper alloy tubes used
for heat exchange systems1-4). Corrosion behaviors of
copper alloys about above properties have been
studied by many researchers5-12). Especially concerning
to pit problem, Hukovic examined the breakdown of
passive films on 90Cu-10Ni in borate buffer solutions
containing chloride (pH 9.25). Their results showed
that film breakdown occurred in the form of pitting
corrosion, and the increase in NaCl concentration
resulted in a shift of the breakdown potential Eb to
less positive value13). But still corrosion properties of
copper alloy tubes according to type of alloy and
comparison between these alloys have not been
defined clearly yet. For various application of copper
alloy tubes to evaporation unit or brine heater unit in
seawater, corrosion properties such as corrosion rate
according to immersion time and solution temperature,
pitting and crack formation, should be analyzed. In
this paper, for corrosion property comparison of
copper alloys in 3%NaCl solution, Al-Brass, Cu-
30Ni and Cu-10Ni copper alloy tubes were selected
for immersion test in hot and 3%NaCl flowing
solution. To compare corrosion rate among them,
immersion test with electrochemical measurement
were investigated with dipping time variation at a
fixed conventional operation temperature of 90oC.
Also, for examination of corrosion mechanism and*Corresponding author. E-mail : leehr@cnu.ac.kr
Pang Beilli 외/한국표면공학회 42 (2009) 280-286 281
protection properties, XRD, EDX analyze and SEMobservation were investigated.
2. Experimental
2.1 Weight loss measurement
In this experiment, three copper alloys of nominalcomposition Cu-20Zn-2Al (Al-Brass), Cu-30Ni (CN70/30) and Cu-10Ni (CN90/10) were selected. Weight-loss measurements were conducted by suspendingsample tubes in a 3000 ml vessel (Fig. 1). Sampletubes used for weight-loss measurements were copperalloy tubes (44.5 mm × 2.0 mm × 20 mm). Sampletubes were washed with acetone, ethyl alcohol anddistilled water, respectively. Sample tubes with freshlyprepared surfaces were then fully immersed in 3%NaClflowing solution at 90oC for 30, 50 and 80 days,respectively. The flowing velocity was about 60 cm/s.
2.2 Electrochemical test
Corrosion tests were performed in 3%NaClsolution by using PARSTAT2263 potentiostat. Three-electrode cell method was used for electrochemicalmeasurements with 1 cm2 of working electrode area.Carbon auxiliary electrode and SCE reference electrodewere used. All experiments were executed in aFaraday cage in order to minimize the externalelectronic interference with the system at the roomtemperature. For measuring corrosion rate, Tafel andlinear polarization method were used. Also, foranalyzing general corrosion property of copper alloytubes, dynamic anodic polarization curve was evaluated.To obtain a steady state condition, after maintain30 minute dipping every test was executed.
2.3 Sample tubes surface characterization
Microstructures were observed in digital opticalmicroscope and SEM (JEOL 840A). Elemental analyseswere obtained by using EDAX (GENESIS). Also,the corrosion products were analyzed by using XRD(DE/D-5000) measured with Cuk
α monochromatic
radiation (λ=1.5405Å). Measurements were conductedin a step scanning mode (2o/min) from 2θ=15o to2θ=75o. XRD data were taken directly from surfaceof the copper alloy tubes which were before and afterimmersed in 3%NaCl solution at 90oC. The XRDpatterns were analyzed by using MDI Jade 5.0Software and JCPDF database14). With the help ofMDI Jade 5.0 Software, it was much effective forremoving some noise and texturing effects.
3. Results and Discussion
3.1 Weight-loss test
As shown in Fig. 2 corrosion rates of the sampleswhich were immersed in 3% NaCl solution for 30,50 and 80 days were lower than 5mpy and fell invery good range bordering. CN70/30 copper alloytubes showed the lowest corrosion rate as average0.071mpy,which was much lower than those of Al-Brass (average 0.349mpy) and CN90/10 (average0.308mpy). Corrosion rates of Al-Brass, CN70/30and CN90/10 decreased with time owing to passivityfilm formation on surfaces. After 50days immersion,CN70/30 copper alloy tubes showed about 60%decreased corrosion rate which was twice than othertwo types of about 30% decrease. But after 80daysimmersion, CN70/30 copper alloy tubes showedabout 70% decreased corrosion rate in case of othertwo types about 45% decrease, which means passivityfilm formation on CN70/30 copper alloy tubes weremore speedy than other two types of copper alloys.
3.2 Electrochemical test
Fig. 3 shows Tafel curves for three types of copper
Fig. 1. The sketch map of immersion corrosion testing. Fig. 2. Corrosion rates of copper alloy tubes.
282 Pang Beilli 외/한국표면공학회 42 (2009) 280-286
alloy tubes in 3%NaCl test solution. The Tafel
polarization curves were measured between 250 mV
at the open circuit potential at the rate of 0.5 mV/s
and started after 30 minute immersion of samples in
the solution. The corresponding corrosion potentials
(Ecorr), corrosion currents (icorr), anodic Tafel slopes
(βa), cathodic Tafel slopes (βc) polarization resistance
(Rp) and are listed in Table 1. CN70/30 showed the
lowest corrosion current among the three types of
copper alloy tubes, and corrosion currents of CN90/
10 is lower than Al-Brass.
The polarization resistance Rp was calculated
according to the Stern-Geary equation:
Rp = βaβc / 2.3icorr (βa + βc)
The anodic dynamic polarization curves (Fig. 4)
show that three types of copper alloy tubes had
passive region when immersed in 3%NaCl test
solution. The corrosion rates of samples showed
decreased value because of passive film formed on
the surfaces. The passive current (ip) of CN70/30 was
lower than other two copper alloy tubes. This reason
may be derived from high content ratio of Ni amount
in copper alloy.
Ni-rich micro cathodic surface usually has two
effects. Firstly, these areas behave as barrier of low
electronic and ionic conductivity that inhibit migration
of ions and electrons between the matrix and seawater.
Secondly, as transfer to cathodic electrode potential,
prevents the matrix from being attacked in the sea
water. This result could be confirmed by Zhu’s report15).
3.3 Surface analysis
3.3.1 Optical photographs
Fig. 5 shows optical photographs of Al-Brass,
CN70/30 and CN90/10 alloy tubes after immersion
in 3% NaCl solution at 90oC for 80 days. Al-Brass
alloy tubes after 80 days of immersion in 3%NaCl
solution showed thin gray-green tarnished film covered
by corrosion. But CN70/30 and CN90/10 copper
Fig. 3. Tafel curves of copper alloy tubes in 3%NaCl test
solution (a) Al-Brass, (b) CN70/30, (c) CN90/10.
Table 1. Corrosion parameters obtained for copper alloy
tubes in 3%NaCl solution
Specimen Al-Brass CN70/30 CN90/10
icorr (µA/cm2) 21.11 6.20 13.19
Ecorr (mV vs SCE) −255.54 −239.95 −281.82
βa (mV) 0.63 0.79 0.90
βc (mV) 1.56 1.79 1.45
Rp (Ω) 9.24 38.47 18.31
Fig. 4. Dynamic anodic polarization curves of copper
alloy samples in 3%NaCl test solution (a) Al-
Brass, (b) CN70/30, (c) CN90/10.
Fig. 5. Optical photos of (a) Al-Brass, (b) CN70/30, (c) CN90/10 alloy tubes after immersion in 3%NaCl solution at
90oC for 80 days.
Pang Beilli 외/한국표면공학회 42 (2009) 280-286 283
alloy tubes after 80 days immersion in 3%NaCl
solution showed reddish-black film formation on
surfaces. These films could be confirmed as CuO or
Cu2O passivity films by XRD analysis. But CN70/30
copper alloy had a little crack problem in spite of
high resistant to sea water, which seemed to have
relation of high Ni content in copper alloy result in
high internal residual stress.
Fig. 6. SEM & EDAX images of Al-Brass before immersed (a) and after immersed in 3%NaCl solution at 90oC for 30
days (b), 50 days (c), 80 days (d).
Fig. 7. SEM & EDAX images of CN70/30 before immersed (a) and after immersed in 3%NaCl solution at 90oC for 30
days (b), 50 days (c), 80 days (d).
284 Pang Beilli 외/한국표면공학회 42 (2009) 280-286
3.3.2. SEM and EDAX analysis
SEM and EDX analyses were used to define the
morphology of surface attack and the chemical
composition of corrosion products on specimens after
immersion in 3%NaCl flowing solution at 90oC for
30, 50 and 80 days. A typical photo for the localized
attack is shown in Fig. 7 for Cu70-Ni30 alloy with
EDX spectra of corrosion products. Note localized
attack in Cu70-Ni30 was more intense than that in
the case of Al-Brass. The localized attack spread and
covered the whole exposed surface after 30 days
immersion. EDX analyses of these corrosion products
revealed the presence of chloride along with small
amounts of the major alloying elements. It is likely
that this corrosion product consists mainly of CuCl.
In addition, some Ni(OH)2 or NiO corrosion product
could be formed in the Cu-Ni alloy. This conclusion
was well consistent with EDX result. Fig. 6 of Al-
Brass shows this alloy is soft more than other two
Cu-Ni alloys and corrosion proceeded with total
corrosion attack against matrix instead of localized
attack. So this alloy didn’t show pitting. Also, EDX
analysis shows this attack related with Cl− and O+2
ions. Fig. 6(d) shows surface matrix shape of more
decreased corrosive attack. In Fig. 8 EDX analysis
shows corrosion products were consisted by oxide or
chloride of Ni or Cu after 30 days but thereafter
passivity film formed on surfaces. In case of Cu-Ni
alloy, usually after 30 days immersion, cracks were
appeared on the surface but from Fig. 2 of weight-
loss test these phenomena have nothing to do with
corrosion rate owing to well development of passivity
film formation.
3.3.3. XRD analysis
Fig. 9 shows that XRD results of Al-Brass alloy
tubes before and after immersion in 3%NaCl flowing
solution at 90oC, respectively. After 30 days, copper
corrosion product revealed the presence of crystalline
oxide film on the surface as Cu(OH,Cl)2·2H2O. This
film may be formed as following,
Cu++ + OH− + Cl− + 2H2O→Cu(OH, Cl)2 · 2H2O
Aluminum and zinc element solved in the solution
which resulted in general corrosion instead of localized
pit corrosion. Morales et al. used cyclic voltammetry
technique to study the passivation behavior of α and
β brasses in NaCl solution buffered with a solution
of sodium borate and boric acid16). They related the
passivation of brass to a complex layer of ZnO and
Cu2O. The dealloying rate of Al from the alloy’s
surface increased in proportion to the Al contents of
the alloy and was found to be dependent on the
kinetics rate of solid state diffusion of Al atoms to
Fig. 8. SEM & EDAX images of CN90/10 before immersed (a) and after immersed in 3%NaCl solution at 90oC for 30
days (b), 50 days (c), 80 days (d).
Pang Beilli 외/한국표면공학회 42 (2009) 280-286 285
the outer surface layers as Al2O3·H2O. But in this
experiment, such ZnO and Cu2O layer or Al2O3·H2O
layer could not be detected. Fig. 10 shows XRD
results of CN90/10 before and after immersion in
3%NaCl flowing solution at 90oC. There were two
oxide layer peak of Cu2(OH)3Cl and CuO. Cu2(OH)3Cl
could be derived as following,
Cu2O + 1/2O2 + Cl−+ 2H2O→Cu2(OH)3Cl + OH−
But after 80 days immersion, CuO layer could be
detected by XRD analysis, which means well developed
passivity film on the surface. Fig. 11 shows XRD
results of CN70/30 before and after immersion in
3%NaCl flowing solution at 90oC for 30, 50 and
80days. In this case, there were three type of oxide
layers such as Cu(OH)2, CuO and Cu2O. This means
CN70/30 alloy has relatively good resistant property
against corrosion. The anodic reaction of copper
alloys is slow, i.e. Cu = Cu+ + e−, and in meanwhile,
Cu+ reacts with the absorbed oxygen on the surface
to for the oxide, 2Cu+ + 2OH−
→Cu2O + H2O, i.e.
passivation. The oxide film formed on the surface by
this way is uniform and adherent to the underlying
substrate. About CN70/30 and CN90/10 the alloy
element Ni is also oxidized to be ions Ni2+ and/or
Ni3+ and enters the oxide Cu2O. Furthermore, the
thickening of the corrosion product film is by the
diffusion of oxygen through and the oxidation of Cu
and Ni into the film. The incorporation of nickel ions
in the film Cu2O enhances the ionic and electronic
resistance of the film whether the nickel ions occupy
the cation vacancies of Cu+ or substitute for Cu+
ions. Blundy and Pryor demonstrated that the Cu2O
structure did not change until it contained more than
40% of nickel17). In this experiment also show same
result, namely Ni(OH)2 or NiO peaks did not appeared.
But in this case, owing to high Fe content in CN70/
30 copper alloy, this alloy shows a little localized
attack resulted in pitting problem.
4. Conclusions
1. CN70/30 copper alloy tubes showed the lowest
corrosion rate as average 0.071mpy, which was much
lower than those of Al-Brass and CN90/10. After
80days immersion, CN70/30 copper alloy tubes
showed about 70% decreased corrosion rate in case
of other two types about 45% decrease, which means
Fig. 9. XRD results of Al-Brass (a) before immersion (b)
after immersion in 3%NaCl flowing solution at
90oC for 30 days (c) same for 50 days (d) same
for 80 days.
Fig. 10. XRD results of CN90/10 (a) before immersion
(b) after immersion in 3%NaCl flowing solution
at 90oC for 30 days (c) same for 50 days (d)
same for 80 days.
Fig. 11. XRD results of CN70/30 (a) before immersion
(b) after immersion in 3%NaCl flowing solution
at 90oC for 30 days (c) same for 50 days (d)
same for 80 days.
286 Pang Beilli 외/한국표면공학회 42 (2009) 280-286
passivity film formation on CN70/30 copper alloy
tubes were more speedy than other two types of
copper alloys, but owing to high ratio of nickel
content resulted in crack appearing after 50days
immersion test.
2. From Tafel polarization curves for three types of
copper alloy tubes in 3%NaCl solution, CN70/30
showed the lowest corrosion current among the three
types of copper alloy tubes, and corrosion currents of
CN90/10 is lower than Al-Brass. Anodic dynamic
polarization curves of three types of copper alloy
tubes show well developed passive region when
immersed in 3%NaCl solution.
3. After immersion in 3%NaCl flowing solution at
90oC for 80 days, Al-Brass alloy tubes showed thin
gray-green tarnished passive film of Cu(OH,Cl)2·
2H2O covered. But at the same condition, CN70/30
and CN90/10 tubes showed reddish-black film of
Cu2O or CuO on surfaces. As a result, in spite of
CN70/30 copper alloy had a little crack problem
showed high resistant to sea water.
4. EDX analyses of corrosion products revealed the
presence of chloride along with small amounts of the
major alloying elements. This corrosion product
consists mainly of CuCl. In addition, some Ni(OH)2or NiO corrosion product in the Cu-Ni alloy. From
SEM observation, Al-Brass alloy showed more total
corrosion attack against matrix instead of localized
attack rather than other two Cu-Ni alloys. From EDX
analysis this attack was related with Cl− and O+2 ions.
5. From XRD results of Al-Brass alloy tubes
before and after immersion in 3%NaCl flowing
solution at 90oC, corrosion product revealed the
presence of crystalline oxide film on the surface as
Cu(OH,Cl)2·2H2O. In case of CN90/10, two oxide
peaks of Cu2(OH)3Cl and CuO appeared. Also, in
case of CN70/30, Cu(OH)2,CuO and Cu2O oxide
layers were detected rather than Ni(OH)2 or NiO
layers.
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