corrosion property evaluation of copper alloy tubes

280

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 : [email protected]

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.


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



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



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.


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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corrosion property evaluation of copper alloy tubes

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