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Desalination 205 (2007) 140–146

0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved

Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by theEuropean Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.

*Corresponding author.

Corrosion investigation of Cu–Ni tube desalination plant

K. Abouswa*, F. Elshawesh, O. Elragei, A. ElhoodPetroleum Research Center, P. O. Box 6431, Tripoli,Libya

Tel.+214 830022; Fax +214 830031; email: [email protected]

Received 23 February 2006; accepted 1 May 2006

Abstract

Copper-based alloy tubes (i.e. Cu–Ni) are extensively used in desalination plants. This is in order to completethe heating and evaporating process for the seawater and to obtain distillate water that can be used in the steamgeneration plant and as drinking water. A number of these tubes were found to suffer from severe localized corrosionat 6 o’clock position (corrosion throughout the pipe wall thickness). Several samples from the failed tubes weresubjected to metallographic examination and electrochemical test in the simulated working environment (i.e. chloridecontent, temp. non evacuated system) in order to establish the main cause of corrosion and failure of the tubes.

Keywords: Condenser tube failure; Pitting corrosion; Multi-stage flash; Desalination plant; Dissolved gases

1. Background

Copper–nickel alloys have a remarkable com-bination of good resistance to both corrosion andbiofouling in seawater. As they are also readilywelded and fabricated, they are an obvious choicefor pipe systems, heat exchangers, boat hulls andother structures engineered for marine use.

Copper–nickels have been specified for sea-water use for over 50 years; they are the materialsof first choice for seawater pipe work and con-

denser service for many of marine applications.They are used in desalination, power plants andoffshore fire water systems, and for the sheathedprotection of oil and gas platform legs and risers.

Piping of 90–10 copper–nickel is used for bothnatural seawater and hot de-aerated brine service.Large pipes up to 1.37 m OD are fabricated fromplate; seamless pipe is used for sizes up to about400 mm.

The multi-stage flash (MSF) process of desali-nation involves large heat exchangers producingup to 57,000 m3/d of water. Copper–nickel alloys

K. Abouswa et al. / Desalination 205 (2007) 140–146 141

are widely used to fabricate piping, water boxes,evaporator shells, tube plates, etc. The 90–10 nickelalloy is usually used in such fabrications, althougha 70–30 copper–nickel with 2% iron and 2% man-ganese (C71640, CW353H) is also widely usedfor heat exchanger tubing.

2. Copper–nickel seawater corrosion resistance

The resistance to seawater corrosion of cop-per–nickel alloys results from the formation of athin, adherent, protective surface film which formsnaturally and quickly on exposure to clean seawa-ter. The film is complex and predominantly cup-rous oxide with the protective value enhanced bythe presence of nickel and iron. The initial filmforms fairly quickly over the first couple of daysbut takes up to three months to fully mature. Thisinitial exposure is crucial to the long-term per-formance of copper–nickel [1–4].

Once a good surface film forms, the corrosionrate will continue to decrease over a period ofyears. For this reason, it has always been difficultto predict the life of copper–nickel alloys basedon short-term exposures. Normally, corrosion ratesof 0.02–0.002 mm/y are anticipated.

3. Role of flow rates

With increasing seawater flow rate, corrosionremains low due to the resilience of the protectivesurface film [2]. But when the velocity for a givengeometry is such that the shear stress action ofthe seawater on the film is sufficient to damage it,impingement attack can result. General experiencehas shown that 90–10 copper–nickel can success-fully be used in condensers and heat exchangerswith velocities up to 2.5 m/s; the 70–30 alloy canbe used up to 3 m/s. For pipeline systems, higherseawater velocities can safely be used in largerdiameter pipes as indicated by BS MA18. Saltwater piping systems in ships which suggested amaximum design velocity of 3.5 m/s in pipes of100 mm and larger for 90–10 copper–nickel, and

4 m/s for the 70–30 alloy. Although these guide-line values are now considered to be conservative,they work well because they take into account eff-ects from things like bends which cause areas ofhigh local flow rate. Nevertheless, extreme tur-bulence has to be avoided from elements like tightradius bends, partial blockages and areas down-stream of partially throttled valves.

Minimum flow rates of more than 1 m/s areusually preferred to avoid sediment build up.

4. Localized corrosion

Copper–nickels have good inherent resistanceto chloride pitting and crevice corrosion. Crevicecorrosion is seldom found. The mechanism is ametal ion concentration cell type totally differentto that of stainless steels. Copper–nickels are notsusceptible to chloride or sulphide stress corrosioncracking or hydrogen embrittlement and unlikebrasses do not suffer cracking due to ammonia inseawater service. But ammonia can cause highercorrosion rates, although copper–nickels are moreresistant than many other copper-based alloys.Copper–nickel tubing is resistant to chlorinationat the dosing levels used to control biofouling.Excessive chlorination can be detrimental, as itreduces erosion-corrosion resistance.

Dealloying is not common with copper–nickelalloys. De-nickelification of the 70–30 alloy hasbeen encountered occasionally in refinery over-head condenser service, where hydrocarbonstreams condense at temperatures above 150°C.This appears to be due to thermo-galvanic effectsresulting from local hot spots. The solution hasbeen to remove the deposits which lead to the hotspots either by more frequent cleaning or by in-creasing flow rates. Ammonia in seawater canproduce a type of de-alloying which looks similarto hot spot corrosion. This happens at aroundambient temperature, but only under heat transferconditions. It can be controlled by adding ferroussulphate to the seawater.

142 K. Abouswa et al. / Desalination 205 (2007) 140–146

5. Corrosion failure investigation

5.1. Visual investigation

Visual investigation of the corroded tuberevealed the presence of several corrosion pitswith different morphology and size as shown inFig. 1a–b, few external pits throughout the wallthickness were visible. Corrosion hole on theexternal surface of the tube is clearly shown inFig. 1a. Cluster of small corrosion pits on thelower part of tube bundle can be observed clearlyin Fig. 1b.

Fig. 1. Large hole (a) and cluster of small corrosion pits (b).

a b

Fig. 2. Localized corrosion pits.

a b

5.2. Low magnification microscopy

External and internal examination on the failedtube was carried out using low magnification mic-roscope. Fig. 2a–b shows the extent of the corro-sion pits observed on the external tube surface.Pits were isolated and some were of cluster typeencountered Cu–Ni tubes at the lower part of tubebundle.

A sign of some corrosion products and scalelike were visible around the pits. Some pits werefound to be covered with the while corrosion pro-ducts.


5.3. Chemical analysis

Chemical analysis of Cu–Ni tubes was con-ducted using SEM-EDAX and the results confirmthat the tubes were made of Cu–Ni (90–10 alloy).The added iron was found to be less than 2.0 wt%.This was confirmed by the ferrite-scope meter.These results exclude the role of iron in thecorrosion mechanism and tubes failure.

5.4. Investigation using SEM

Detailed examination of the failed Cu–Ni tubesusing SEM confirms the observation obtainedfrom the optical microscopic examination. Themorphology of corrosion pits on the external tubessurface was found to be different as shown inFig. 3a–b. Some pits were found to be shallow,others were narrow and deep while most werewide and shallow. Many of pits were penetratedthroughout the pipe wall thickness. Few pits regu-lar in shape and large in size penetrated throughoutthe wall thickness were visible.

The analysis of the corrosion products andscales precipitated on the tube revealed the pre-sence of a complex of compounds composed ofcuprous oxide, CuCl and cupric oxide, as shownin Table 1. This was confirmed by the results of

Fig. 3. Wide and shallow pits penetrated throughout the wall thickness.

a b

XRD analysis. A sign of chloride, Mg, high oxy-gen and carbon were detected in the analyzed thinfilm scale, this indicates that the tubes were ex-posed directly to seawater vapor. The heavy scalewas observed on the stainless steel demisters andsurrounding areas.

5.5. Electrochemical tests

Electrochemical study using potentiodynamictest technique was conducted on samples takenfrom the failed Cu–Ni tubes in the environmentcontaining a wide range of chloride ions, air andsome of CO2 gas. The latter was made to simulate

Table 1Results of XRD analysis

Elements Concentration (wt. %) C 2.22 O 27.00 Cu 51.32 Mg 0.077 Si 0.138 Cl 9.365 Fe 1.949 Ni 7.459


the real condition when the vacuum system wasnot properly working.

Samples of 1 cm2 exposure area were preparedfor this study and tested in the simulated test en-vironment. Cu–Ni samples were tested in solu-tions with different chloride concentration (100,1000, 22,000 and 35,000 ppm) with addition ofblown air in one case and some CO2 in anothercase. The latter were added in order to simulatethe real environment when the vacuum systemdoes not working properly. The tests were carriedout at room temperature (RT) and 70°C.

Fig. 4 shows that the corrosion potential of Cu–Ni alloys is markedly affected by the chloride con-centration. This was evident when tests were con-ducted at a chloride concentration of 1000 ppm at70°C under static condition.

Fig. 5 shows the role of CO2 gas present withinseawater on the Cu–Ni tubes at RT and 70°C. Theresults showed detrimental effect of CO2 on thecorrosion potential particularly at 70°C. Lowpotential of (–250 mV) was recorded.

Fig. 4. Polarization curves of Cu–Ni alloy in distilledwater with CO2 at (RT) and at 70°C at different concen-tration of Cl–.

Fig. 6. Polarization curves of Cu–Ni alloy in seawateropen to air and blowing air at 70°C.

Fig. 5. Polarization curves of Cu–Ni alloy in seawaterwith and without CO2 at (RT) and at 70°C.

Fig. 6 shows the role of seawater or when22,000 ppm of chloride ions are present on theCu–Ni tubes alloy when tests were carried out inopen air and when the air was blown into the testcell.


6. Discussion

The results of examination and tests conductedon Cu–Ni tubes for the desalination revealed thefact that the corrosion was localized and initiatedfrom the external surface.

The mechanism of failure can be given as fol-lows: the results of visual and microscopic exa-mination revealed the fact that the corrosion wasinitiated externally on the Cu–Ni tubes. Severallocalized corrosion pits (isolated and clustered)were observed on the examined tubes at 6 o’clockposition. SEM-EDAX analysis of the corrosionproducts and deposit was found to compose ofcuprous oxide, high oxygen and carbon content.The high carbon content may be from build-up ofscale (carbonate-HCO3), as a result of CO2 gaspresent within the system (dissolved within thewater droplets), particularly when the vacuumsystem does not working properly.

The conducted examinations on the receivedtubes excluded the suggestion that the Cu–Nimaterial composition was the main cause of Cu–Ni tubes failure. The results of chemical analysisconfirm that the tubes were made of Cu–Ni alloy(90–10).

The mechanism of Cu-Ni tubes corrosion andfailure could be attributed to the process controlproblem which can be given as per these twoscenarios.

6.1. Scenario I

i Deposition of seawater droplets or water vapordroplets carrying some chloride ions on Cu–Ni tubes. This may occur when the demisterunit was not in the right position.

ii Leakage of one or more tubes as a result ofinternal corrosion. This may lead to depositionof seawater salts on Cu–Ni tubes.

One of the above scenarios has led to build-upof non continuous film-like scale on Cu–Ni tubes.This was evident from microscopic observationand conducted examination using SEM-EDAXanalysis, as shown in Figs. 2 and 3. Carbon, oxy-

gen, some chloride ions, K and Na, were all detect-ed at areas around the localized corrosion sites(pits).

The present thin film of scale may have createdcorrosion micro-cells leading to autocatalyticprogressive in the corrosion process [1]. The cor-rosion process is aggravated by surrounding temp-erature in addition to the presence of chloride ionswithin the build-up thin scale (salts, carbonatesresult of CO2 and water). The presence of this filmin wet condition in addition to the dissolved gasesCO2 and O2 is expected to aggravate the corrosionprocess via formation of carbonic acid and reduc-tion of pH.

The conducted electrochemical tests confirmedthat CO2 gas alone seem to be not enough to causesevere or localized corrosion on Cu–Ni tubes/material and the chloride ions in concentrationmore than 100 ppm need to be present, as shownin Fig. 4. The corrosion was found to be pro-nounced when the chloride ions concentration wasfound to be more than 1000 ppm in presence ofCO2 as shown in Fig. 5. Highest corrosion ratewas expected when high concentration of chlorideions (22,000 ppm Cl ions H– seawater) was presentalong with CO2 gas as shown in Figs. 4–6. Therequirements to chloride ions may be due to thefact that pH was not low enough to cause corro-sion.

6.2. Scenario II

The other suggested mechanism can be givenas follows:

The effect of vapor side environment is ex-pected to play some role in Cu-Ni tube corrosion.The surrounded environment is steam containsCO2 and oxygen gases [1]. The non condensableCO2 gas is expected to form low pH carbonic acid,in the vapor zone leading to increased vapor sidecorrosion (VSC).

As mentioned above, most desalination unitshave inadequate venting system (small, not effi-cient etc.) and some gas like CO2 and oxygen isexpected within the system. The condensed vapor


droplets on tubes are expected to contain CO2 gasin addition to oxygen. As per tube bundle designthe water droplets with incondensable bubbles ofCO2 are expected to persist in the lower part oftube bundle at cold end tube sheet. This allowsenough sub-cooling for water droplet and reduc-tion in pH sufficiently for corrosion to occur par-ticularly when enough air is present.

7. Conclusions

1. The severe corrosion of Cu–Ni tubes afterthey have been in service for a long period of timecan be attributed to the process control problem.It seems that quite a large amount of seawater wasevaporated and precipitated as film-like scale onthe external surface of Cu–Ni tubes. This maygenerate corrosion micro-cells which in turn actas a potential site for localized corrosion to pro-ceed particularly when gases such as CO2 and O2are present within the condensed water on Cu–Nitubes. The later reduce pH and enhance the corro-sion attack in a short period of time.

2. Alternative conclusion of Cu–Ni tubes cor-rosion is that the condensation of water dropletson Cu–Ni tubes contains non condensable CO2gas and oxygen can be the main cause of tubecorrosion and failure. The marked reduction inpH of condensed water droplets condensed on Cu–Ni will enhance and accelerate the corrosion pro-cess.

8. Recommendations

1. Identify the source of seawater salts foundon Cu–Ni tubes as a result of internal corrosionleakage or as a result of demister/vacuum problem.The presence of chloride ions was confirmed byEDAX analysis.

2. Check the performance of the vacuum sys-tem of the desalination unit(s). This is an importantstep and need to be considered. Presence of gasesshall assist in initiation and acceleration in the cor-rosion process.

References[1] G.J. Danek and R.B. Niederberger, Accelerated

corrosion of copper–nickel alloys in polluted waters,Corrosion/76, Paper No.76, NACE, 1976.

[2] A. Syrett, Accelerated corrosion of copper in flowingpure water contaminated with oxygen and sulfide,Corrosion, 33 (1977) 257–262.

[3] C. Giulani and G. Bombard, Influence of pollutionon the corrosion of copper alloys in flowing saltwater, Br. Corrosion J., 8 (1973) 20–24.

[4] R. Dooly and J. Glater, Alkaline scale formation inboiling sea water brines, Desalination, 11 (1972) 1–16.

[5] M.A. Finan and M.N. Elliot, A theory of the forma-tion of magnesium scales in sea water distillationplants, and means for their prevention, Desalination,14 (1974) 325–340.

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