-
Chinese Journal of Aeronautics, (2015), 28(3): 954–960
Chinese Society of Aeronautics and Astronautics& Beihang
University
Chinese Journal of Aeronautics
[email protected]
REVIEW ARTICLE
Fouling corrosion in aluminum heat exchangers
* Corresponding author. Tel.: +86 22 24092074.
E-mail addresses: [email protected] (J. Su),
erzascrlet@163.
com (M. Ma), [email protected] (T. Wang),
guoxiaomei@
ameco.com.cn (X. Guo), [email protected] (L. Hou),
[email protected] (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