Corrosion Science 51 (2009) 1263–1272
Contents lists available at ScienceDirect
Corrosion Science
journal homepage: www.elsevier.com/locate/corsci
Corrosion behaviour of several aluminium alloys in contact with a thermal
storage phase change material based on Glauber’s salt
A. García-Romero a,*, A. Delgado a, A. Urresti b, K. Martín b, J.M. Sala b
a
Universidad del País Vasco-Euskal Herriko Unibertsitatea, Dpto. Ing. Minera, Metalúrgica y Ciencias de los Materiales, Escuela Universitaria de Ingeniería Técnica de Minas y de
Obras Públicas, Beurko s/n, Barakaldo 48902, Spain
b
Universidad del País Vasco-Euskal Herriko Unibertsitatea, Dpto. de Máquinas y Motores Térmicos, Escuela Superior de Ingeniería Industrial, Alameda Urkijo s/n, Bilbao 48013, Spain
a r t i c l e
i n f o
Article history:
Received 19 December 2008
Accepted 5 March 2009
Available online 19 March 2009
Keywords:
A. Aluminium
B. XRD
B. SEM
C. Alkaline corrosion
C. Pitting corrosion
a b s t r a c t
This article presents the results of a study about the corrosion behaviour of four aluminium alloys (EN
AW 2024, 3003, 6063, and 1050) in contact with a commercial thermal storage material based in the Glaubeŕs salt (Na2SO410 H2O). Results indicate that the Al 2024 alloy is not compatible with this material
due to the extense formation of NaAlCO3(OH)2 in contact with air. The aluminium alloys 3003, 6063
and 1050 showed to be fully compatible with the material.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction and state of the art
Since early 70s, much research has been done for the development of efficient thermal storage devices based in latent heat in
a phase change. The materials employed for this application are
widely known as PCMs, or phase change materials, and the results
attained so far have rendered some interesting industrial applications. Presently, the global warming and the energy crisis have renewed the interest in the topic. In spite of the extense bibliography
and the large social and commercial interest in this subject, only
some few works have been devoted to the study of the degradation
of the systems containing PCMs, and no more than a dozen international references have been found regarding the corrosion of
metals with potentially corrosive hydrated salts used as PCMs. Table 1 gathers these references, together with the PCMs and alloys
evaluated in them.
This article presents the results of a study on the corrosion
behaviour of several aluminium alloys with a commercial material
developed for the storage of thermal energy in the form of latent
heat by means of a solid-liquid phase change. The materials employed for this purpose are commonly known as PCMs, phase
change materials. The work has been carried out in the frame of
a project where the objective is to develop modular components
for thermal storage at temperatures between 30 and 40 °C. The
main requirements of these modules are that they have to be com* Corresponding author. Tel.: +34 94601 4982; fax: +34 94601 7800.
E-mail address: anemiren.garcia@ehu.es (A. García-Romero).
0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2009.03.006
pact, light, with high thermal conductivity, and must be produced
of comercially available materials and procedures, at a marketable
price. According to these requirements, three commercial aluminium products were selected, capable of containing the PCM while
maximizing the thermal conductivity, at a reasonable price and
with good availability. These products were made of four different
aluminium alloys, specifically Al alloys 3003-H19, 6063, 2024-T3
and 1050-H24. And two of them required a sealing stage with
epoxy adhesives. It has to be remarked that the alloy selection
for the PCM containers was made according to market availability
in shapes and prices to produce the designed modules. As a result,
the materials selection has not been based in the corrosion resistance of the alloys.
The selection of the PCM was an important part of the study.
Inorganic commercial PCMs were preferred, and a commercial
product with phase change at 32 °C was selected, called PLUS ICE
E32. This product is produced and supplied by EPS LTD, Cambridgeshire (UK). The active PCM in this product was not disclosed
by the supplier, but its analysis showed that Glaubeŕs salt was the
main component.
The first studies about the corrosion behaviour of the PCM hydrated salts in contact with metallic containers were reported by
Abhat, Heine, and other researchers at the end of the 70s and
beginning of the 80s. Abhat made a summary of those results in
a review [1]. His main conclusions were that stainless steel was
fully compatible with all the evaluated hydrated salts and (literally) the evaluated aluminium alloys were incompatibility with
all the evaluated hydrated salts, with the exception of the hydrated
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A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
Table 1
Bibliography about corrosion of alloys by molten hydrated salts.
Author
Year
PCM
Metals/alloys
Ref.
Aboul-Enein ref. by A. Abhat
1977
Na2SO41/2 NaCl10H2O
[1]
Heine et al.
1978
Na2HPO412H2O
Na2S2O35H2O
Heine D.
1981
Na2HPO412H2O
CaCl26H2O
Zn(NO3)26H2O
Na2S2O35H2O
Schöder et al. ref. by A. Abhat
1980
Porosini et al.
1988
LiClO33H2O
NaOAc3H2O
Mg(NO3)26H2O
Ca(NO3)24H2O + Mg(NO3)26H2O
(67/33)
CaCl26H2O
Na2SO410H2O
Na2SO41/2 NaCl10H2O
NaOH3,5 H2O
SS 1.4301
Al AG3 (5000 series)
Cu
SS 1.4301
Mild steel 1.0330
Al 99.5 (1000 series)
Al AG3 (5000 series)
Cu
SS 1.4301
Mild steel 1.0330
Al 99.5 (1000 series)
Al AG3 (5000 series)
Cu
Tin plated Stainless Steel
[2]
Groll et al.
1990
SS304L
Steel C20
Al AG3 (5000 series)
Al Dural (2007)
Al A net (1000 series)
Cu
Several steels
Cabeza et al.
Cabeza et al.
Cabeza et al.
2001
2001
2002
Nagano et al.
2004
Cabeza et al.
2005
Farrell et al.
2006
High temperature (160–420 °C)
anhydrous salts with water impurites
Zn(NO3)26H2O
Na2HPO412H2O
CaCl26H2O
NaOAc3H2O
Na2S2O35H2O
Mg(NO3)26H2O + MgCl26H2O
TH29 (Commercial PCM based in CaCl26H2O)
TH29 + MgCl26H2O
Plus ice E17 (commercial PCM based in
Na2SO410H2O + NaCl) ClimSel C18
(commercial PCM based in NaOAc + additives)
sodium thiosulphate. Nevertheless, a detailed observation of the
results included in this article show that hydrated sodium sulphate
was compatible with the tested aluminium alloy (1000 series).
Porosini [2] made corrosion tests lasting for more than 2 years.
The main conclusions remarked by the author are that all the
tested aluminium alloys suffered from corrosion, severe pitting in
most cases. However, observing the data included in the tables of
this article, it is remarkable that ‘‘pure aluminium” and stainless
steel perform similarly when tested immersed in Glaubeŕs salt,
and the results are reported as ‘‘slight corrosion”. The highly alloyed aluminium alloys suffer from pitting corrosion in all tested
conditions.
Cabeza et al. [4–6,8] measured the corrosion rate of five alloys
in contact with seven PCMs. The corrosion behaviour was evalu-
[1]
[1]
[1]
[3]
Al (2007 alloy)
Brass Ms58
Cu
C Steel 345
SS 301
Pure Cu
Brass (70–30)
C Steel
SS 304
SS 316
Pure Al
As in previous articles
[4]
[5]
[6]
Al (2024 alloy) Cu pure (C38600)
[9]
[7]
[8]
ated in many conditions and the main conclusions were given as
a recommendation for use/no use of the each pair of alloy/PCM. Tables 2 and 3 show the materials as well as the main conclusions
and the recommendations for each alloy-PCM.
Nagano [7] evaluated the corrosion of six alloys in contact with
one eutectic mixture of hydrated nitrate and chloride. The aluminium alloy tested (reported as pure aluminium) did not show any
sign of corrosion. The main conclusion reported is that aluminium
and stainless steel 316 can be used as storage tanks for the tested
PCM. No further explanation is given for the choice of stainless
steel as container in spite of the clearly visible pits on the surface
of the stainless steel.
Farrell [9] evaluated the potential galvanic coupling of Cu and Al
in contact with several PCMs. This choice of these materials was
Table 2
Tests and recommendations by Cabeza et al. [4–6,8].
Test temperature
56–60 °C
56–60 °C
56–60 °C
80 °C
80 °C
80 °C
80 °C
Open air testing
PCM
Recommendation
Recommendation
Recommendation
Recommendation
Recommendation
Yes
Zn(NO3)26H2O
No
No
No
No
Yes
Yes
Na2HPO412H2O
No
Yes
Caution
Caution
Yes
Yes
CaCl26H2O
Caution
Yes
Yes
Caution
Yes
Yes
TH29
No
Yes
Yes
No
Yes
Yes
TH29–MgCl26H2O
No
Yes
Yes
No
Yes
Not specified
NaOAc3H2O
Yes
Caution
Caution
YES
YES
Not specified
Na2S2O35H2O
Yes
No
No
Yes
Yes
to
to
to
to
to
use
use
use
use
use
Al (2007 alloy)
Brass Ms58
Cu
C Steel 345
SS 301
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A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
Table 3
Observations and conclusions by Cabeza et al. in aluminium alloy 2007.
Zn(NO3)26H2O
Corrosion takes place, but the corrosion rate slows down as time advances. Extensive formation of white precipitates, regarded as Al(OH)3. Severe
corrosion in all conditions, independent of the air tightness. The coupling to graphite enlarger the corrosion.
The formation of a cotton-like white precipitate, regarded as Al(OH)3 is reported. Dissolution of the aluminium in this PCM is reported, with a high mass
Na2HPO412H2O
loss, and a pH change from 11 to 14 after the dissolution. This reported observation is in agreement with the usual behaviour of aluminium alloys in
alcaline ambients, and in alkaline phosphates specifically.
Production of bubbles is reported. Low corrosion rate, increment of the pH from 6 to 7–8, and the formation of a non-continuous white layer on the
CaCl26H2O
surface are reported and regarded as indicative of a process of pitting corrosion taking place, included the appearance of the alloy samples
TH29
Production of bubbles is reported. White precipitates supposed to be Al(OH)3. Graphite enhanced corrrosion. Mass loss is not a problem, but the severe
TH29MgCl26H2O damage can be visually appreciated.
No corrosion
NaOAc3H2O
No corrosion
Na2S2O35H2O
due to the usual combination of both in heat exchangers. The main
data reported show that aluminium alloy 2024 corrodes in the
form of pitting with all the evaluated PCMs, and that the largest
corrosion occurs when aluminium is coupled with copper in the
acetate PCM. The result is based in visual examination and no
chemical analysis or discussion is included about the reason why
the aluminium alloy is corroded more dramatically by acetate than
by an eutectic mixture of sulphate and chloride salts.
As a summary of the bibliographic review, it can be stated that
there is a lack of information about the mechanisms leading to the
corrosion behaviour of the alloys in contact with the hydrated salts
employed as PCMs. Contradictory observations are reported by different researchers (Abhat [1] and Porosini [2]), and rare results are
found in some other cases (Farrell [9] and Nagano [7]). In nearly all
the studies, the results show a superior resistance of stainless steel
in comparison with any other material, included the evaluated aluminium alloys, and the conclusions reported by the authors in
most cases generalize that aluminium alloys are incompatible with
the molten hydrated salts.
However, there are at least as many types of aluminium alloys
as types of steels. According to the international unified alloy system (Aluminium Association, UNS, EN, ISO, etc.), there are eight
series of wrought aluminium alloys (1000–8000), gathering a total
of 290 different alloys, and all the studies herein reported (except
Nagano [7]) have included Al alloys of the 2000 series, with a high
copper content. Only three of the studies have included one aluminium alloy with low copper content (Abhat [1], Porosini [2]
and Nagano [7]). Therefore, if the data reported in the literature
are not carefully evaluated they mislead to the general conclusion
that aluminium alloys in contact with molten hydrated salts corrode by pitting. This could turn down industrial products where
aluminium alloys offer excellent advantages when compared to
stainless steel, such as higher thermal conductivity, lightness,
higher conformability, and much reduced price.
The copper containing aluminium alloys of the 2000 series are
known as Duraluminium due to their high strength. They are commonly employed for mechanical components, but their use is limited due to corrosion problems. During dissolution of Al–Cu–Mg
particles, regions of metallic copper are formed on the alloy, thereby promoting accelerated galvanic attack and increasing the pitting severity [10]. A copper-ion concentration of 0.02–0.05 ppm
in neutral or acidic solutions is generally considered to be the
threshold value for initiation of pitting on aluminium [11]. A recent
article by Bakos et al. [12] show that a thick anodizing layer onto
aluminium can prevent the galvanic coupling between aluminium
and copper to a large extent. However, this is a preliminary laboratory study and no working procedure has been attained so far.
Therefore, when corrosive environments are foreseen, non-copper
containing aluminium alloys are presently employed.
From a corrosion resistance point of view, Al alloys from 1000
and especially from 3000 and 5000 series are usually employed
for weather resistance applications where mechanical demands
are limited [13]. Other alloys are employed when mechanical
requirements are added to the resistance in corrosive environments, and specifically the alloys of the 6000 series were developed for this purpose. Nevertheless, it has to be specified that
almost all kinds of Al alloys will suffer from pitting corrosion in
contact with chlorides [14]. Extense studies have been developed
so far to study the pitting mechanisms in order to help in the
development of a ‘‘stainless” aluminium. An interesting review
was published by Szklarska et al. [15], while the main conclusion
attained is that systematic studies are yet to be carried out before
a marketable stainless aluminium, or an efficient corrosion inhibitor, be attained.
2. Experimental
2.1. Materials
In order to evaluate the possible corrosion four different aluminium alloys were exposed to a commercial inorganic PCM,
named PLUS ICE E32 (PCM LTD Cambridgeshire). The data sheet
description of the material declares that it is a blend of inorganic
salts and thickening agents, with melting point at 32 °C, density = 1.7 g cm 3, and latent heat of 186 kJ kg 1. The chemical composition of the aluminium alloys is shown in Table 4, and the shape
and size of the specimens is shown in Table 5. The specimens produced with the different alloys could not be the same in all cases
because the material selected was already preshaped for the required containment of the PCMs in the intended application. In
addition, two different preshaped products made of alloy EN AW
6063 were required, and therefore specimens of both types were
prepared.
Table 4
Composition of the alloys evaluated.
Alloy EN AW
%Si
%Fe
%Cu
%Mn
%Mg
%Ti
%Zn
%Cr
%Al
1050-H24
2024-T3
3003-H19
6063
<0.25
0.5
0.6
0.3–0.6
<0.4
0.5
0.7
0.5
<0.05
3.8–4.9
0.05–0.2
0.1
<0.05
0.3–0.9
1–1.5
0.15
<0.05
1.2–1.8
–
0.6–0.9
<0.05
0.15
0.1
0.25
<0.05
0.25
0.1
–
–
–
–
0.05
Min 99.5
Balance
Balance
Balance
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A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
Table 5
Characteristics of the tested specimens.
Alloy
Supplier
Specimens dimensions and shape
EN AW 1050-H24
EN AW 2024-T3
EN AW 6063
Alustock, S.L., Vitoria
Alustock, S.L., Vitoria
Alustock, S.L., Vitoria
Rectangular Test Specimens: 100 25 mm, 0.6 mm thickness
Rectangular Test Specimens: 100 25 mm, 1.5 mm thickness
Rectangular test specimens: 40 20 mm, 3 mm thickness
EN AW 3003-H19
Ò
Euro-composites ,
Luxemburg
Shaped extruded hollow
specimens, 70 mm length
Rectangular test specimens: 100 25 mm, honeycomb of 5.7 mm thickness
2.2. Instruments
In this work X-ray fluorescence spectroscopy (XRF, PHILIPS
PW1480, Sc/Mo dual tube) was used to analyse the chemical composition of the PCM. Scanning electron microscope (JEOL, model
JSM-6400, Japan) aided with EDS microprobe for chemical analysis,
was used to carry out microstructural observation and composition
analysis of specific corrosion features of carbon coated specimens.
Additional low voltage (5KV) EDS microanalysis on non-coated
specimens was carried out in a JEOL-Field Emission Scanning Electron Microscope JSM-7000F, for the determination of carbonated
products. X-ray diffractometer (XRD, PW1719 Philips, X-ray
source: Cu-Ka) was used to identify the main crystalline compounds formed due to the corrosion process. Stereoscopic microscopy (NIKON smz1500) was used to visualize specimen’s corrosion
and to aid in the determination of the compounds by colour
identification.
For the preparation of the metallographic specimens Buehler
procedures were employed in a Buehler automatic polishing machine: Samples were cut from the corroded specimens with a hand
saw without refrigeration, mounted in epoxy resin, grinded with
SiC paper number P240 and polished with diamond suspension
of 9 and 3 lm. A final finishing stage with suspension of colloidal
alumina of 0.05 lm was also carried out.
Mass changes were measured in a Sartorius ED224S weighing
scale with 0.1 mg accuracy.
2.3. Procedure
The main compound of the commercial PCM was determined by
XRD. In addition XRF was used to determine if chlorides or other
type of elements, potentially corrosive for aluminium alloys, were
present in this material.
The corrosion study consisted in submerging alloy specimens in
molten PCM at 45 °C for a fixed period of time. The containers consisted of 250 ml laboratory glass beakers where 175 ml of molten
PCM was added in each one. Prior to the immersion, the specimens
were cleaned with distilled water and acetone, and were weighed.
The testing schedule was defined according to the potential conditions to be encountered in the real application: It can initially be
foreseen that the metallic containers will never be fully filled in
with PCM, and some corrosion processes can take place when an
alloy is subjected to different oxidative conditions in different
zones. The presence of oxygen in the uppermost zone of the specimens, in comparison with the low oxygen content available in the
molten PCM can act as a (galvanic) concentration cell. As a result,
the main testing schedule is performed with specimens partially
immersed in the PCM, but also tests with specimens fully immersed in PCM were carried out for comparison reasons.
The schedule of the tests is shown in Table 6. Specimens were
carefully placed in each glass beaker in order to avoid direct contact between each other and to ensure that each specimen was
fully or partially immersed in the PCM, as required. The beakers
were covered with a plastic film to avoid water loss by evaporation
to the ambient, and the subsequent PCM degradation. This closure
cannot be considered airtight, although it has undoubtly limited
the available air for the corrosion processes to proceed. The temperature selected is the maximum temperature calculated to be
reached in peak periods in the designed system. No higher temperature was employed in order to avoid degradation of the PCM.
When Cu containing alloys are immersed together with other
alloys in the molten PCM, corrosion of the latter can occur due to
the deposition of Cu particles (leached from the former) on its surface. Therefore, tests were carried out with and without the presence of the copper containing alloy.
The pH of the molten solution was measured before the specimens were introduced, and after each testing period. Measurement
was made by pH paper.
After each testing period, visual evaluation of the specimens
was done in order to identify any sign of corrosion (pitting, general,
etc.). When corrosion signs were visible, the specimens were
rinsed with distilled water and acetone for a visual inspection
and analysis when required. No removal of the corrosion products
on the surface was performed for the examination.
Table 6
Tests schedule.
Beaker
Testing period
(days)
Three specimens of each alloy
Condition
Inspection
1
30
1050
2024
–
Mass loss measurements. Visual. Microstructural analysis
2
60
1050
2024
–
3
90
1050
2024
–
4
30
60
90
30
60
90
30
60
90
6063
3003
1050
Partially
immersed
Partially
immersed
Partially
immersed
Partially
immersed
6063
3003
1050
5
6
2024
Mass loss measurements. Visual. Microstructural analysis
Mass loss measurements. Visual. Microstructural analysis
Visual Inspection each 30 days. Microstructural analysis after 90 days
when required
Fully immersed
Visual inspection each 30 days. Microstructural analysis after 90 days
when required
Fully immersed
Visual inspection each 30 days. Microstructural analysis after 90 days
when required
A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
Mass loss measurements were performed in the specimens
clearly corroded. Prior to the measurement, these specimens were
cleaned by immersion in a commercial product specific to eliminate the deposits onto aluminium alloys avoiding damage, called
Turco Alcalino (Turco Española, Spain) of pH 9, aided with a polymer brush to eliminate the well adhered deposits.
Some of the corroded specimens (90 days) were selected for
inspection of the corroded areas in the SEM prior to cleaning for
mass loss measurement. Cut-through samples of the corroded
specimens were also metallographically prepared from the width
of the specimens. Due to the non-conductive nature of the corrosion deposit, carbon coating of the specimens was required in
the preliminary observations and analysis, where high voltage
(20 KV) was used. Low voltage (5 KV) EDS analysis was performed
subsequently directly onto the corroded surface to analyse the nature of the corrosion deposits and the different particles observed in
both types of specimens (direct surface and cut-through samples).
The observation of the cut-though samples yielded an indication of the depth of the corrosion deposit formed onto the specimens surface.
XRD of the corrosion product was made directly to the corroded
deposit formed onto the test specimen.
3. Results and discussion
3.1. Identification of the Inorganic PCM
The results of the X-ray diffraction analysis of the PCM show
that it is a mixture of two salts: anhydrous disodium sulphate
(Thenardita, Na2SO4) and disodium sulphate decahydrated, (Mirabilita, Na2SO410H2O), this last one known as Glauber’s Salt. The
XRF chemical analysis highlights a high silica Si02 content (around
4%), which could not be detected with XRD analysis due to its
amorphous structure, and the lack (no trace) of chlorides. The pH
measured to the fresh molten PCM is 10.
3.2. Corrosion analysis of the 2024 Al alloy partially immersed in
Glaubeŕs salt
The alloy 2024 was highly corroded even in 30 days. However,
the data of the mass loss are negligible. Taking into account that
three specimens of each material were tested each permanencetime, the scattering of the results showed to be larger than any valid measurement (i.e. for a specimen with initial weight of
1267
10.4023 g a net weight loss of 0.4 mg was measured after 90 days
exposure; a specimen with an initial weight of 10.4278 g a net
weight loss of 0.2 mg, etc.). The measured values cannot be considered reliable because they are in the range of the scattering due to
the cleaning and manipulation procedures.
The first remarkable feature is that the alloy zone submerged in
the PCM did not show corrosion, while the zone immediately
around the PCM surface, and 5–10 mm above it, is a highly corroded zone with white and grey deposits even after only 30 days.
The pH of the molten PCM after 90 days of immersion did not
change, being around 10. Fig. 1 shows a macrograph of the specimens tested for 90 days. This corresponds to a corrosion mechanism due to differential oxygen concentration, which makes the
low oxygen content zone become cathodic with respect to the high
oxygen content zone. Some dark green spots are visible in the aerated area, and brownish zones can also be observed.
The SEM observation of the corroded area showed some small
zones of compact deposits of around 1 lm depth, with a characteristic cracked feature (see Fig. 2). The analysis corresponds mainly
to Alumina, with silicon containing particles of non-crystalline
appearance, deposited onto it. However, most of the surface was
covered by large porous deposits, more that 30 lm depth (see
Fig. 3), covering most of the corroded area. Included in this porous
deposit bright particles could be seen scarcely (see Fig. 4). The
analysis of the bright particles corresponds to Copper. The analysis
of the porous deposit revealed to contain a ‘‘soup” of elements
either leached from the alloy and contained in the PCM, and specially a high Si amount (between 10–20%, depending on the zones),
which could either mean that colloidal silica was acting as a binder
of the corrosion product, or that a silicate was reacting with the alloy (no information about the nature of the silica product contained in the PCM material was available).
The observation of copper particles in the corrosion product is
in agreement with the corrosion mechanism of alloy 2024 in
watery systems, where the Al a-phase surrounding Al2Cu precipitates become anodic to these precipitates, dissolving into the solution. The copper contained in the a-phase goes into solution, from
where it reduces onto the specimeńs surface, forming small precipitates that become strong cathodic sites, and severe pitting corrosion takes place. This mechanism is well known [13] and in the
present case it is the responsible for the large corrosion seen in this
alloy. However, the oxide that usually forms in the corrosion of this
alloy is a dense thin alumina scale, while the large porous deposits
formed onto the surface of the tested specimens are not a usual
corrosion product. In addition, the Si content in these deposits is
Fig. 1. Macrograph of aluminium alloy 2024 specimens tested for 90 days, at 45 °C, partially immersed in PCM E32.
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Fig. 2. Scanning electron micropgraph of the cracked Al2O3 surface onto a specimen of aluminium alloy 2024 tested for 90 days, at 45 °C, partially immersed in PCM E32.
Fig. 3. Cross-sectional scanning electron micrograph of an aluminium alloy 2024 specimen tested for 90 days, at 45 °C, partially immersed in PCM E32 showing the high
porous deposit formed in extense areas. EDX spectrum of the deposit showing the composition.
A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
very high and no pits could be observed onto the surface due to the
large-thick deposits formed.
Since the porous zone was deep and covered a large zone of the
specimen, it was feasible to perform XRD to analyse its crystallographic nature. The results showed that a basic carbonate of Al
and Na had formed as corrosion product, specifically Dawsonite,
orthorrombic NaAICO3(OH)2 (Fig. 5). No silicate was detected by
XRD. The presence of the high Si content in this porous deposit
can only be explained as colloidal silica entrapped and acting as
a binder of the carbonate.
1269
The formation mechanism of this carbonate has not been
investigated in this work, but formation of Dawsonite has been
researched along the last years as a mean to capture CO2 from
the atmosphere. It has been concluded [16] that the dawsonite
formation occurs in presence of Al+3 dissolved hydroxides, at
pH >8. The optimum dawsonite crystallization takes place at
pH 10.3, and at pH >10.8 the crystallization decreases again,
and high temperatures (100–300 °C) also promotes its formation.
On the other hand, recent studies reported by Bengtsson et al.
[17] about the corrosion behaviour of pure aluminium in the
Fig. 4. Scanning electron micrograph of the surface of an aluminium alloy 2024 specimen tested for 90 days, at 45 °C, partially immersed in PCM E32. The micrograph has
been taken in an area of the high porous deposit and show the bright Cu particles precipitated in it. The EDX spectrum included in the figure corresponds to the analysis of the
particles and part of the sorrounding area due to the voltage (20 kV) employed for the analysis.
Fig. 5. XRD spectrum of the corroded surface of an aluminium alloy 2024 specimen tested for 90 days, at 45 °C, partially immersed in PCM E32. The main peaks corresponding
to Dawsonite are directly identified in the spectrum.
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A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
presence of atmospheric salt pollutants and CO2 conclude that
the presence of atmospheric CO2 drastically reduces the pitting
corrosion of pure aluminium due to chloride or sulphate pollutants. Since in our study no carbonate formation could be seen
in the alloys that do not contain Cu, it can be concluded that
a copper promoted mechanism is taking place. This mechanism
leads to large dissolution of aluminium preceding to the reaction
with the Glaubeŕs salt for the formation of the carbonate. Glaubeŕs salt will not transform to carbonate unless a large amount
of aluminium is previously dissolved into it.
In addition, it can also be foreseen that Cu might have played an
important role in the formation of the carbonates. Visual inspection of the deposit in the stereoscopic microscope showed that
although most of the carbonate is white, some green coloured
and pink coloured crystals had formed on some zones of the alloy
surface (Fig. 6). They are regarded as Cu and Mn carbonates,
respectively (no other carbonates/oxides show these colours). Both
metals are well known for their large affinity to formation of carbonates when in contact with atmospheric CO2. EDS analysis in
the SEM confirmed the presence of Carbon and Cu in the green
products.
The surface of the specimens was inspected again after removing the corrosion deposit. Large pits were easily found. In their
interior of the pits crystals could be seen, white and green coloured, confirming the idea that Cu plays an important role in the
formation of the carbonates.
3.3. Corrosion of Al alloys 1050, 3003 and 6063 partially immersed in
Glaubeŕs salt
The specimens of Al alloy 1050 immersed together with the
2024 alloy showed a slight corrosion in the zone immediately
above the surface of the molten salt (see Fig. 7). Most of the specimens showed a thin white scale onto the surface area adjacent to
the molten PCM surface. The white scale is dense, non-porous, and
can be regarded as alumina. However, in one of the specimens a
green deposit could bee seen in the uppermost part of the specimen. The specimens of Al alloy 1050 immersed together with the
Fig. 6. On the upper left of the figure, the optical Macrograph of the corroded surface of an aluminium alloy 2024 specimen tested for 90 days, at 45 °C, partially immersed in
PCM E32, is shown. The scanning electron micrograph of the area marked with a white dotted line is shown in the right hand of the picture, showing a corrosion pit
surrounded by the corrosion deposit. The deposit is green coloured under natural light, and the EDX spectrum on the lowest left of the picture, taken under 5 KV with no
metallization, shows that the corrosion product corresponds to a copper carbonate.
Fig. 7. Picture of the Specimens of Aluminium alloy 1050 tested together with specimens of Al alloy 2024 for 90 days, at 45 °C, partially immersed in PCM E32. Spots of
corrosion can be clearly seen.
A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
3003 and 6063 showed no sign of corrosion. Not even a stain or a
loss of brightness. Therefore, it was concluded that the light corrosion determined in the 1050 specimens tested together with Al
2024 specimens can be regarded as Cu deposits onto the Al 1050
surface, initiating therefore localized galvanic corrosion also in this
alloy. The large pit and the green deposit observed in one of these
specimens are due to the condensation of the vapours on the polymer lid, from which they slipped down the pitted specimen. Vapours had copper dissolved in them, which precipitated onto this
site of the surface.
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The specimens of Al alloys 3003, immersed together with 1050
and 6063 showed no sign of corrosion, like in the case of the 1050
alloy immersed together with it. Not even a stain or a loss of
brightness was seen in these cases. Both Al alloys 1050 and 3003
are fully compatible with the PCM and the testing conditions herein evaluated.
Alloy 6063 showed a large resistance to corrosion. No sign of
corrosion or loss of brightness could be seen in the fully immersed
specimens. The specimens partially immersed, showed a stained
surface with a loss of brightness in some zone, being always the
Fig. 8. Picture of two specimens of Alloy 6063 tested for 90 days in PCM E32 at 45 °C. On the left fully immersed specimen. On the right partially immersed specimen with the
surface exposed to air shown in the picture.
Fig. 9. Picture of one representative specimen of each evaluated alloy after having been fully immersed in PCM E32 at 45 °C for 90 days.
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A. García-Romero et al. / Corrosion Science 51 (2009) 1263–1272
zone outside the PCM. Fig. 8 corresponds to two specimens of this
alloy, one fully immersed and one partially immersed. No pits,
extensive formation of oxide, or other sign of corrosion could be
seen or measured. Longer time testing of this alloy should be required before taking a decision about its compatibility with the
PCM E32, and in case corrosion proceeds, the corrosion mechanism
must be determined because in the case that the corrosion is due to
the low Cu content of this alloy, another alloy of the same series
with no copper content could be a good choice.
3.4. Additional observations and discussion
All the specimens that were fully immersed in the PCM, without
contact to air, remained non-corroded. Fig. 9 gathers several of
these specimens. As it can be seen, even the specimens of Alloy
2024 remain not corroded. This confirms the corrosion mechanism
observed in the aerated specimens, due to different oxygen concentration in different zones, and opens a doubt about whether
the tests reported in the literature were performed in specimens
fully immersed in PCMs or partially in contact with air.
Among the studies in the bibliographic review included in this
article, only two studies have reported corrosion tests with Glaubeŕs salt [1,2]. The results reported by Abhat [1] show a good compatibility between Glaubeŕs salt and Al alloy 1000 series (the only
Al alloy tested by this author), Porosini [2] reports slight corrosion
in a similar alloy, and pitting in the case of the 5000 and 2000 series alloy. Results obtained in this work are in good agreement with
those reported by Abhat [1] regarding the 1000 series alloys and
with those reported by Porosini [2] in the 2000 series alloy. In
the present study 5000 series alloys have not been evaluated, but
this serie is formed by high corrosion resistant alloys developed
for outdoor products in the building industry. An explanation for
the bad results attained by Porosini et al. can be found in the high
magnesium content of the selected alloy (3.5%), which has been
previously reported to have detrimental effect on the corrosion
resistance [13]: ‘‘Alloys in which magnesium is present in amounts
that remain in solid solution, or is partially precipitated as Al8Mg5
particles dispersed uniformly throughout the matrix, are generally
as resistant to corrosion as commercially pure aluminium and are
more resistant to saltwater and some alkaline solutions, such as
those of sodium carbonate and amines. Wrought alloys containing
approximately 3% or more magnesium under conditions that lead
to an almost continuous intergranular Al8Mg5 precipitate, with
very little precipitate within the grains, may be susceptible to exfoliation or SCC”. Taking this information into consideration and the
results attained in our tests with the 3000 and 6000 series alloys, it
can be foreseen that by selecting a low Mg content 5000 serie Al
alloy, a good compatibility with Glaubeŕs salt could be attained.
4. Conclusions
Aluminium alloys are a large family of materials with different
composition that behave in distinct ways when in contact with different PCMs. Aluminium alloys of the 2000 series are not generally
resistant to hydrated molten salts due to a well known mechanism
of galvanic couples originated between the Al2Cu particles and the
a-phase solid solution inside the alloy, starting a corrosion process
leading to severe pitting.
Compatibility of aluminium alloys with Glaubeŕs salt is good
except in the case of Cu containing alloys, specially the 2000 series
alloys. Some other specific high alloyed compositions of other series could also present corrosion problems mainly due to the formation of differential electric potential between the intermetallic
particles and the a-phase.
Aluminium alloy 2024, when partially in contact with Glaubeŕs
salt and partially with air (usual situation in containers) suffers
from severe pitting corrosion, and also degradation of the PCM occurs due to the formation of large porous Na–Al alkaline carbonates. A similar behaviour can be expected in all alloys of the
2000 series when similar conditions be found.
Aluminium Alloys 1050 and 3003 show an excellent resistance
to corrosion by Glaubeŕs salt.
Aluminium Alloy 6063 also presents a good corrosion behaviour, although some reaction has to be foreseen if open air is available. It is initially foreseen that this corrosion is due to the small Cu
content in this alloy, although longer testing of this alloy is required before reaching a conclusion about its corrosion mechanism
and the extent and risk of its use in contact with the PCM E32.
Acknowledgements
The authors thank to the University of the Basque Country and
the company AH Asociados, and also to the Spanish Ministry of Science and Tecnology for the funding of the projects UE06/13 and
BIA2007-65896, respectively, where this study has been performed. We also want to thank to Dr. S. Fernández, Dr. J. Sangüesa
and Mrs. V. Madina for their assistance in the analysis and evaluation of the results, and to Mr. G. Vargas and D. Serna for their helpful collaboration and the assessment in the selection of the
materials from an industrial point of view.
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