Chinese Journal of Aeronautics, (2015), 28(3): 954–960

Chinese Society of Aeronautics and Astronautics& Beihang University

Chinese Journal of Aeronautics

cja@buaa.edu.cnwww.sciencedirect.com

REVIEW ARTICLE

Fouling corrosion in aluminum heat exchangers

* Corresponding author. Tel.: +86 22 24092074.

E-mail addresses: 13920616639@139.com (J. Su), erzascrlet@163.

com (M. Ma), wangtianjing@ameco.com.cn (T. Wang), guoxiaomei@

ameco.com.cn (X. Guo), houliguo@ameco.com.cn (L. Hou),

zpwang@cauc.edu.cn (Z. Wang).

Peer review under responsibility of Editorial Committee of CJA.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.cja.2015.02.0151000-9361 ª 2015 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Su Jingxin a,*, Ma Minyu a, Wang Tianjing b, Guo Xiaomei b, Hou Liguo b,Wang Zhiping a

a Tianjin Key Laboratory for Civil Aircraft Airworthiness and Maintenance, Civil Aviation University of China, Tianjin

300300, Chinab Aircraft Maintenance and Engineering Corporation, Beijing Capital International Airport, Beijing 100621, China

Received 15 September 2014; revised 30 October 2014; accepted 15 December 2014

Available online 20 March 2015

KEYWORDS

Aluminum;

EIS;

Heat exchanger;

Pitting corrosion;

Tafel plots

Abstract Fouling deposits on aluminum heat exchanger reduce the heat transfer efficiency and

cause corrosion to the apparatus. This study focuses on the corrosive behavior of aluminum cou-

pons covered with a layer of artificial fouling in a humid atmosphere by their weight loss, Tafel

plots, electrochemical impedance spectroscopy (EIS), and scanning electron microscope (SEM)

observations. The results reveal that chloride is one of the major elements found in the fouling

which damages the passive film and initiates corrosion. The galvanic corrosion between the metal

and the adjacent carbon particles accelerates the corrosive process. Furthermore, the black carbon

favors the moisture uptake, and gives the dissolved oxygen greater chance to migrate through the

fouling layer and form a continuous diffusive path. The corrosion rate decreasing over time is con-

formed to electrochemistry measurements and can be verified by Faraday’s law. The EIS results

indicate that the mechanism of corrosion can be interpreted by the pitting corrosion evolution

mechanism, and that pitting was observed on the coupons by SEM after corrosive exposure.ª 2015 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA. This is an open access article

under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

When heat transfer surfaces become fouled, the coefficiency ofheat transfer will be dramatically reduced with even a

minimum of scales, causing major issues to the design andoperation of the heat exchangers. New Zealand scientist,Steinhagen, investigated more than 3000 heat exchangers from

thousands of different companies and found that over 90% ofthem had varying degrees of scaling.1 The economic losscaused by this fouling accounted for about 0.2% of the GDPper year in developed countries.2 Thackery estimated that

the loss caused by fouling was nearly 500 million pounds eachyear.3 The inefficiency of heat exchangers in civil aircraft’ airconditioning (AC) systems induced by fouling may shut down

the system, and jeopardize the pressure and temperature con-trols in the passengers’ cabin.

Studies of the fouling problems beginning in the 1980s have

been developing along three directions: theoretical analysis ofthe formation of the fouling in order to supply universal and

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Table 1 Results of elements’ contents in fouling samples.

Element Content (wt%)

EDS EDX

O 19.93

Na 2.24 7.283

Mg 2.98 2.288

Al 42.61 11.96

Si 3.87 27.17

P 2.31 3.87

S 1.22

K 1.28 4.385

C 24.8

Ti 1.36

Cl 0.665

Ni 0.403

Co 0.0226

Ca 1.37 2.863

Fe 20.19 12.2

Cu 1.82 0.122

Fouling corrosion in aluminum heat exchangers 955

accurate predictive models;4–6 fouling development monitoringdevices;7–9 and the technology to control and remove the foul-ing.10 There is little actual published literature on the corrosive

fouling found on heat exchangers. The heat exchangers on theaircrafts’ AC modules are made of an aluminum alloy withinferior corrosion resistance than the other two types of com-

monly applied materials: the copper alloy and the stainlesssteel. The specific active ingredients in the fouling, chloridefor example, can damage the passivated surface of the alu-

minum alloy, reducing the service life of the heat exchangers.The serious corrosion may even cause a leak in the pressurizedair, which may de-pressurize the passengers’ cabin. Some otherstudies on the atmospheric corrosion of aluminum alloy have

certain reference value for this study.11–13

In this study, the fouling was sampled from actual aircrafts’heat exchangers and analyzed for its composition. Then, the

aluminum coupons covered with a coating of homogeneousartificial fouling were put into an environment chamber, whichwas set as close as possible to the actual service environment of

the heat exchangers. The corrosive behavior were character-ized by measuring the corrosion rate, comparing the effect ofthe corrosion on different components, and studying the

mechanism of corrosion at different stages.

2. Experiment

2.1. Fouling analysis and experiment design

Samples of the fouling were collected from the heat exchangersby scratching the heat transferring surface or by centrifugallyseparating a sample from the cleaning solution used to removethe fouling. Each sample was then tested with an energy dis-

persive spectrometer (EDS) and an energy dispersive X-rayspectroscopy (EDX) respectively. The results are shown inTable 1. The results of these two tests deviate from each other

on elements contents of O, S, and C because the fundamentalcharacteristics of these two analysis methods and thecorresponding samples’ nature, with one being deposit sedi-

ment while the other, the liquid extract of the sediment, are dif-ferent. The artificial fouling design therefore cannot be merelydetermined by these tests, the operation conditions of the heatexchangers must be taken into consideration additionally.

The fouling on the gas-side of aircraft heat exchangerscomes from atmospheric aerosols and the engine exhaust.Atmospheric aerosols are complicated dispersions of liquid

and solid particles into the atmosphere, which are mainly com-posed of soluble components, organic carbon, element carbon,carbon black, and inorganic alumino-silicate, among

others.14,15 Water-soluble ions, such as SO42�, NO3

�, Cl�, K+,Na+, Ca2+, etc., account for a large proportion of the particlesin the aerosols. The organic carbon and the black carbon

mainly come from the combustion of fossil fuels in aircrafts’engines. At working temperature (100–200 �C), the fouling onthe heat exchangers transforms into porous scales, whichabsorb humidity from the air to form a water-soaked coating

over the aluminum surface in which water-soluble componentstravel through the solution phase to form an electrolytic cell.The Cl� has been well accepted as one of the dominant factors

of aluminum alloys’ corrosion for it damages the passivation ofthese alloys. The black carbon favors the moisture uptakethrough porous covering layers down to the surface of the base

metal, promoting the onset of micro-cell in the voids of the cov-ering layers, and increases the corrosion rate due to thegalvanizing effect. The other fouling components are not as

important under the electrochemical corrosion mechanism ofaluminum alloys. So the contents of Cl� and carbon must betaken into account as the key variables in the study.

Based on the above analysis, this paper presents a test forcorrosion caused by the artificial fouling, which is very similarto the widely applied ASTM corrodokote test,16 in order to

simulate the actual corrosive factors.

2.2. Preparation of fouling and samples

NaCl, Fe2(SO4)3, CuSO4, Na3PO4, Na2CO3, Na2SiO3,Ca(NO3)2, Na3PO4, Na2CO3, and carbon black were addedto 100 mL distilled water as per the amounts listed inTable 2, then continuously stirred for 24 h. After that, 35 g

of kaolin clay was added and the mixture was stirred foranother 30 min. The mixture was left standing for about 1 huntil a paste formed. The paste had a gray-green to dark-green

appearance.2024 aluminum alloy (nominal composition by weight per-

centage: 0.50 Si, 0.50 Fe, 3.8–4.9 Cu, 0.30–1.0 Mn, 1.2–1.8 Mg,

0.10 Cr, 0.25 Zn, the balance is Al) sheets were machined intocoupons of 50 mm · 5 mm · 2 mm with a 1.5 mm diameterhoisting hole near one of the short edges. Each coupon was

sanded up to 1200 # abrasive paper. The fouling paste wasapplied on one side of the coupon using a KW-4A spinningcoater. The thickness of the fouling layer was approximately0.2 mm after drying out. The typical appearances of the testing

coupons are shown in Fig. 1.

2.3. Corrosion rate measurements

The fouling covered samples were hung in a temperature-hu-midity test chamber, which was set at 38 �C and 80% RH.After every 24 h during the experiment, 10 samples from each

group were withdrawn from the chamber for measurements.The bare aluminum coupons were weighed (m1) before the

fouling preparation. After being exposed to the corrosive

Fig. 1 Typical appearances of the testing coupons with artificial

fouling layer.

Fig. 2 Coating evaluation cell used in electrochemical

measurements.

956 J. Su et al.

environment, the fouling layer and the corrosion product werechemically removed by rinsing the coupons the solution, whichwas compounded of 20 g CrO3, 50 mL H3PO4 and 950 mL

deionized water, at 90 �C for 5 min,17 and then the couponswere weighed for the second time (m2).

The Tafel plot was also measured, and the corrosion cur-

rent density (icorr) was calculated for the same batch of samplesas a means of validating the weight loss data. The measure-ments were carried out on a PARSTAT 4000 potentiostat

(Princeton Applied Research), which connected to a computerwith one copy of VersaStudio software installed. One three-electrode system was used in the experiments: a saturated calo-mel electrode (SCE) as the reference electrode, a carbon rod as

the counter electrode, and a corroded coupon as the workingelectrode. The specimens were mounted to a coating evalua-tion cell after being taken out of the test chamber (see

Fig. 2(a)), so as to leave a 1.77 cm2 window (Fig. 2(b)) inthe test solution. The test solution was 3.5wt% NaCl solution,except for samples from group A where 3.5wt% Na2SO4 solu-

tion was used in order to exclude the effect of the chloride.Tafel curves were obtained by using the linear scanning voltagemethod, where the scanning range was between �300 and300 mV versus the open circuit potential, and the scanning ratewas 2 mV/s. The icorr was obtained from the Tafel curve byextrapolating the line of the region of the strong polarization.18

2.4. EIS measurements

EIS measurements were taken at the open circuit potential in afrequency range from 100 kHz to 100 mHz, with a 10 mV

amplitude signal on one PARSTAT 4000 potentiostat. Thesame cell setup was used for EIS measurements as was usedfor the Tafel tests.

2.5. Fouling layer porosity measurements

The pores in the fouling layer play a critical role in moisture

absorption and oxygen delivery, which could influence the cor-rosion rate. The porosity property of the fouling was tested byQuantachrome Autosorb-1 (Quantachrome, US) according toISO 9277:1995 (the determination of the specific surface area

of solids by gas adsorption using the BET method) for betterknowledge of the corrosion mechanism.

3. Results and discussion

3.1. Weight loss

The weight loss from the fouling sample groups, each with dif-ferent amounts of NaCl, are shown in Fig. 3. The results

Table 2 Components of artificial fouling.

Group Components in one batch of artificial fouling (g)

Fe2(SO4)3 CuSO4 Ca(NO3)2 Na3PO4

A 5 5 10 15

B 5 5 10 15

C 5 5 10 15

D 5 5 10 15

E 5 5 10 15

indicate that almost no corrosion occurs for group A, andthe weight loss data of groups B and C are almost the same.

This illustrates that very little corrosion happens when thechloride is not present, but adding more chloride does notincrease the weight loss necessarily. In the corrosion system

used in this research, the base metal of the fouling layer is atype of aluminum alloy that keeps passivated by itself until itcomes in contact with halogen ions in neutral solutions, show-ing a very low corrosion rate. However, the corrosion rate of

the activated aluminum is determined by other factors thanthe halide ion concentration.

The values shown in Fig. 4, by contrast, obviously indicate

that the weight loss increases with the increasing proportion ofcarbon, indicating that the very existence of carbon black par-ticles indeed accelerate the corrosion. This behavior can be

Na2SiO3 Na2CO3 Kaolin clay NaCl Carbon

15 15 35 0 10

15 15 35 5 10

15 15 35 10 10

15 15 35 10 5

15 15 35 10 0

Fig. 4 Accumulated weight loss of groups with different carbon

proportion at different time.

Fouling corrosion in aluminum heat exchangers 957

partially interpreted under the galvanic corrosion effectbetween the metal and the adjacent carbon particles.Furthermore, the porosity characteristics of the fouling layers

(Table 3) indicate that the fouling layers from Groups Cthrough E have smaller specific surface areas and total porevolumes as the carbon quantity decreases. The cathodic

depolarizer in this experiment was the dissolved oxygen inthe solution in the micro pores of the fouling layer. A foulinglayer with a greater porosity is favorable for the dissolved oxy-

gen to migrate through it because the micro pores have greaterchance to connect with one another to form a continuous dif-fusive path. The weight loss increases during an initial phase,but then the growth rate decreases over time for each group,

which manifests as a decrease in the real time corrosion rate.

3.2. Electrochemical behavior

The icorr values obtained from the Tafel plots are presented inFig. 5. It is clear that the icorr decreases by the time, revealingthe corrosion rate slowdowns. This result further validates the

weight loss measurements.According to the Faraday’s Law, the corrosion rate (CR,

mA/cm2) can be calculated using the weight loss data by the

following equation:

CR ¼ 3F3:6MA

� dðm1 �m2Þdt

ð1Þ

Fig. 3 Accumulated weight loss of groups with different NaCl

proportion at different time.

Fig. 5 The icorr data of the fouling covere

where m1 � m2 is the accumulated weight loss (g) due to corro-sion over exposure time t (h) until the sample was withdrawn,

A is the coupons’ surface area (cm2), F is Faraday’s constant(96500 C/mol), and M is the atomic weight of Al (27 g/mol).The C.R. values calculated from Eq. (1) compared with icorrvalues obtained from the Tafel plots of Group B are shownin Fig. 6. The two sets of data match each other when takingboth tendency and magnitude into consideration, which vali-dates both measurements.

The EIS diagrams of group A in a Na2SO4 (3.5wt%) solu-tion and the other four groups in a NaCl (3.5wt%) solution areshown as a function of the exposure time in Figs. 7 and 8, in

Table 3 Porosity characteristics of the fouling layers.

Group Carbon

(g)*Specific

surface area

(m2/g)

Total pore

volume (cm3/

g)

Average

pore size

(nm)

A 10 48.923 7.374 · 10�2 6.029B 10 63.635 1.037 · 10�1 6.517C 10 41.907 6.235 · 10�2 5.968D 5 26.637 5.668 · 10�2 8.511E 0 3.877 2.208 · 10�2 3.827

Note: * Carbon content in artificial fouling as described in Table 2.

d samples from Group A to Group E.

Fig. 6 CR and icorr data of fouling covered samples of Group B.

Fig. 7 EIS diagrams for Group A in 3.5% Na2SO4 solution at

different exposure time.

958 J. Su et al.

which the points represent measured values and the solid linesare the results of Nonlinear least squares fitting using equiva-lent electrical circuit models. The fitting lines coincide well with

the experimental data. The models applied in the fittings aregiven in Fig. 9.

The EIS diagrams of group A at different measurementsresemble one another and are all characterized by a capacitive

loop at high and low frequency ranges. All of the diagramscoincide with the equivalent electrical circuit in Fig. 9(a), inwhich Rs reflects the solution resistance between the working

Fig. 8 EIS diagrams for four groups in 3.5%

electrode surface and the tip of the reference electrode, Q isthe capacity of the fouling layer, Rpo represents the solution

resistance in the micro pores of the fouling layer, Cdl is theelectric double layer capacity at the sites of the metal surfacewhich is in contact with the pore solution, and Rf is the equiva-lent resistance of the passivating film growth on the surface of

the aluminum. In the theory of Cao and Zhang19 the faradaicimpedance of the passivated metal can be reduced to Rf if

NaCl solution at different exposure time.

Fig. 9 Equivalent circuit models proposed to carry out curve fitting for EIS data.

Fig. 10 SEM observations for pitting corrosion holes.

Fouling corrosion in aluminum heat exchangers 959

maintained at a steadily passivated state, which occurred in theexperiment system, because there was no halide present during

the corrosion exposure. Moreover, Rf is usually on the order of105–106 XÆcm2 and tremendously larger than Rpo, so that thelow frequency capacity loop has a maximal curvature radius.

The EIS diagrams of the samples with halide in the foulinglayer, i.e. Groups B–E, manifest an evolution quite dissimilarfrom that of Group A. Group E has the slowest corrosion

development (Fig. 4) and therefore, gives the distinct EIS pat-tern of the early stage of the corrosion evolution, the ‘‘pittinginitiation’’, which is characterized by two capacitive loops fol-lowed by one inductive loop after 24 h of moisture exposure.

The inductive loop at low frequencies reflects the pitting corro-sion initiation on the metal surface under the micro pores ofthe fouling layer.20,21 It can be explained by the equivalent

electrical circuit in Fig. 9(b), in which Rt, RL and L are equiva-lent elements in relation to the corrosion pitting sites’ repas-sivation.22 The EIS results of the other three groups in Fig. 8

present no such traits, because of the faster corrosiondevelopment.

From 48 to 240 h, the inductive loop is no longer present inthe diagrams for Group E, which evolve gradually into a two-

loop pattern with each capacitive loop at high and low fre-quency ranges in the Nyquist plot and coincide with theequivalent electrical circuit shown in Fig. 9(c). Quite similar

patterns can also be identified in the diagrams of Group B(from 24 h through 240 h of exposure), Group C (24 h and48 h), and Group D (from 24 h through 144 h).

This pattern indicates that the pitting corrosion is under-going stable development and/or propagation at the interfaceof the fouling layer and the metal.23,24 One of the samples with

such pattern was implanted with an epoxy resin, cut in half,and then observed under the SEM (1530VP, LEO, Germany)to verify the existence of pitting. The section surface image isshown in Fig. 10, in which numerous micro pores can be seen

at the bottom of the larger corrosion pits. Therefore, the two-time-constant model in Fig. 9(c) is a satisfying simulation forthe interfacial procedures, while Cdl and Rp are, respectively,

the solution resistance and the charge transfer resistance inthe corrosion pits.

The EIS diagram of Group C and D at 240 h represent

another characteristic with a capacitive loop at high frequen-cies and radial at low frequencies, which coincide with the

equivalent electrical circuit shown in Fig. 9(d), where W is

the diffusion resistance. It reflects that the corrosion processis controlled by the diffusion of the dissolved oxygen throughthe fouling layer and the micro pitting caused by corrosion onthe metal surface.25–27

4. Conclusions

A research method including spin coating an artificial foulinglayer and recreating exposure in humid air has been designedand validated to study the fouling corrosion of aluminum heatexchangers.

The results show that weight loss rate data coincide with thecorrosion current density, as the weight loss measurement isequivalent to the Tafel method. The accumulated weight loss

curves and the EIS diagrams demonstrate that two kinds ofcorrosion promoting elements, inorganic carbon and chloride,express themselves during the experiments.

Pitting was observed on the section of the testing couponsafter certain exposure times. The EIS data can also be inter-preted by the pitting corrosion evolution mechanism of passi-vated metals. In the halide-free systems, the EIS plots are

composed of two capacitive loops, one of which is related tothe passivated state of the aluminum surface. In the systemswith chloride added to the fouling layer the EIS diagrams

demonstrate different traits during the stages of pitting initia-tion, pitting development, and diffusion control.

960 J. Su et al.

Acknowledgements

This study was supported by the National Natural Science

Foundation of China (No. 21303261), and the Major Science& Technology Research Project of Civil Aviation of China(No. MHRD20140110).

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Su Jingxin is an associate professor at School of Science, Civil

Aviation University of China. The current research interest is local

corrosion of aircrafts’ structures.

Ma Minyu is a master degree candidate at School of Science, Civil

Aviation University of China, graduated from the same university with

bachelor’s degree in 2008.

Wang Tianjing is a assistant engineer at Aircraft Maintenance

Engineering Co. Ltd., Beijing, majoring in aircraft accessories trou-

bleshooting, graduated from Beijing University of Aeronautics and

Astronautics in 2004.

Guo Xiaomei is a engineer at Aircraft Maintenance Engineering Co.

Ltd., Beijing, majoring in aircraft air-conditioning system.

Hou Liguo is a senior engineer at Aircraft Maintenance Engineering

Co. Ltd., Beijing, majoring in aircraft air-conditioning system.

Wang Zhiping is a professor at School of Science, Civil Aviation

University of China. The current research interest is Failure Analysis

of Engineering Materials.

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Fouling corrosion in aluminum heat exchangers1 Introduction2 Experiment2.1 Fouling analysis and experiment design2.2 Preparation of fouling and samples2.3 Corrosion rate measurements2.4 EIS measurements2.5 Fouling layer porosity measurements

3 Results and discussion3.1 Weight loss3.2 Electrochemical behavior

4 ConclusionsAcknowledgementsReferences