Indoor atmospheric corrosion of copper and
steel under heat trap conditions in Cuban
tropical climate
Y. Martı́n-Regueira*1, O. Ledea1, F. Corvo2 and C. Lariot2
Published by Maney Publishing (c) IOM Communications Ltd
Corrosion behaviour of copper and steel under heat trap conditions in Cuban tropical climate is
reported. Temperature and humidity reach higher values than those reported for traditional
outdoor and indoor conditions. Annual calculated time of wetness is in the range corresponding to
outdoor or ventilated sheds. This behaviour is not reported for other indoor conditions. Sulphur
compounds deposition rate is higher than chloride deposition rate at all corrosion stations. Main
corrosion products formed on steel and copper are goethite and brochantite respectively. No
significant differences in the statistical influence of exposure time and time of wetness on
atmospheric corrosion process of copper and steel under heat trap conditions are determined.
Keywords: Atmospheric corrosion, Steel, Copper, Electronic materials
Introduction
Environmental aggressiveness under indoor conditions
is lower than outdoors; however, the performance of
electro-electronic equipment very often depends on this
factor. Corrosion of electric contacts is very important
for a good performance and high reliability of electroelectronic devices. Copper is used as a main material for
contacts, cables, wires, coatings for printed circuits and
so on. Steel is employed as material for metallic boxes,
usually coated by a metallic or organic coating. In
general, these two metals are widely used in manufacturing of electro and electronic devices.
Indoor corrosion aggressiveness depends upon the
types of climate. It is reported that tropical humid
climate is more aggressive than temperate climate
regarding indoor corrosion.1,2
Usually, electro and electronic devices are designed
taking into account the influence of temperate climate,
which is very different from tropical humid climate.1
Many tests recommended to evaluate the performance
of electronic equipment are carried out under typical
conditions of temperate climate. Materials performance
may significantly change under tropical conditions as,
for example, Cuban tropical conditions.
The corrosion rate of domestic telephone apparatus
exposed at different locations in Mexico City was
evaluated.3 It was found that correct environmental
control, particularly of temperature and relative humidity, significantly reduced damage produced on telephone
1
Centro Nacional de Investigaciones Cientı́ficas, Apartado 6412, Ciudad
de la Habana, Cuba
2
Instituto de Ciencia y Tecnologı́a de Materiales, Universidad de la
Habana, Vedado, Plaza, Ciudad de la Habana, Cuba
*Corresponding author, yarelys.martin@cnic.edu.cu
printed circuits. The importance to take preventive
actions in designing to avoid future failures is clear.
Electric and electronic devices and their parts exposed
under heat trap conditions are subject to significant
changes of temperature and humidity regarding outdoors and other different indoor exposure conditions.
Humidity condensation takes place due to the decrease
in temperature, for example, during the night or because
of the increase in the relative humidity, which induces an
increase in time of wetness (TOW).4
Time of wetness is calculated supposing that an
aqueous electrolyte layer is present on the metallic
surface at air temperature .0uC and relative humidity
.80%. It is defined as the time during which corrosion
takes place.5 However, recent reports have proposed a
modification, because an upper limit for temperature has
not been considered which takes into account the
conditions of tropical climate.6
Under indoor conditions, temperature and humidity
depend mainly on ventilation, air conditioning and
exchange conditions with external air. Rainfall and solar
radiation have not a direct influence. The presence of
atmospheric contaminants inside the devices is lower
than outdoors in some cases studied in our country. The
deposition of chloride and sulphur compounds is not
predominant as it occurs outdoors, but there are also
present gases like ozone, hydrogen sulphide and nitrogen dioxide. These gases can penetrate inside the
equipment and reach a relatively high and significant
concentration.6
Corrosion aggressivity of a place in a given climate
and exposure conditions can be classified according to
the level of deposited contaminants and weight loss of
the metals under test.3,4 Under outdoor conditions,
extreme corrosion aggressiveness has been determined at
a few metres from the north shoreline in Cuba after one
year of exposure.7,8
ß 2011 Institute of Materials, Minerals and Mining
624
Corrosion Engineering, Science and Technology
2011
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Published by Maney on behalf of the Institute
Received 4 April 2009; accepted 19 November 2009
DOI 10.1179/147842209X12579401586762
Martı́n-Regueira et al.
Indoor atmospheric corrosion of copper and steel
Published by Maney Publishing (c) IOM Communications Ltd
1 a metallic box located at Cojimar coastal station and b samples inside the metallic box
The aim of the present work was to determine the
behaviour of carbon steel and copper exposed to heat
trap condition under different environmental conditions
(rural, urban and coastal) during two years of exposure
in Cuban tropical climate.
Experimental
Test samples
Copper and carbon steel samples of 1065060,5 mm
were exposed at three atmospheric corrosion stations for
periods of 1, 3, 6, 12 and 24 months. Samples were
organised in groups of three specimens, corresponding
series no. 1 to one month of exposure, no. 2 to three
months of exposure and so on.
New samples were exposed after retiring samples at
six and 12 months of exposure each year. Three samples
of each metal were exposed for every exposure period (1,
3, 6, 12, 24 and repetition of them at six and 12 months).
These samples were used for gravimetric evaluation of
the corrosion rate. Two additional samples 206506
0?5 mm for X-ray diffraction (XRD) and SEM analyses
were also exposed for two years.
The total number of samples of each metal used and
their size is presented in Table 1. Table 2 shows the
chemical composition of the two metals tested. Before
exposure, samples were prepared according to ISO
11844-2.9
Test sites
Three metallic boxes designed and manufactured using
galvanised steel were used as containers for indoor
exposure to represent heat trap conditions (class 3?4
according to International Standard ETS 300-019-10).10 Every metallic box presented a front door and two
small windows on each side for ventilation. The boxes
were located in a position where the small windows are
Table 1 Size and total number of copper and steel
samples
Material
Samples for weight loss
Samples for XRD
and SEM analyses
Size, mm
Number
Size, mm
24
24
2065060.5 10
2065060.5 10
Carbon steel 1065060.5
Copper
1065060.5
Number
placed in the dominant wind direction. Boxes were
placed on a structural base to reach ,1?5 m height from
the ground level (Fig. 1a) and the samples were placed
upright (Fig. 1b).
The exposition sites presented the following characteristics:
(i) Cojimar corrosion station: located at ,150 m
from the north shoreline and classified as
coastal. Samples exposure began on 26
November 2003
(ii) Quivican corrosion station: rural corrosion station located at ,30 km away from the north
shoreline and 18 km of the south shoreline.
Samples exposure began on 10 December 2003
(iii) CNIC corrosion station: classified as urban
station, located at ,2 km away from the north
shoreline. The exposure period began on 7
October 2003.
Environmental conditions
Temperature and relative humidity were determined
continuously (every hour) inside the metallic boxes and
outdoor using automatic data loggers. Time of wetness
was calculated according to ISO-9223.5
Sulphur compounds deposition was determined using
alkaline plates placed inside the metallic box (according to
ISO 9225).11 Chloride and particulate material deposition
were also determined, but having some modifications
according to previous tests. 1,12 Contaminants deposition
rate was determined every six months taking into
account the commonly low levels found at indoor
environments.
The corrosion rate of samples was determined by
weight loss using an analytical balance (accuracy 1025 g)
according to ISO/11844-1:2006.13 The classification of
indoor corrosion aggressiveness was determined based
on ISO/11844-2:2005 (Table 3).10
Table 2 Chemical composition of metals tested*
Chemical elements, %
Metal
Fe
Cu
C
Mn
P
S
Carbon steel
Copper
99.51
…
…
99.96
0.1
…
0.3
…
0.04
…
0.05
…
*Fe: iron; Cu: cooper; C: carbon; Mn: manganese; P: phosphorous; S: sulphur.
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Indoor atmospheric corrosion of copper and steel
Published by Maney Publishing (c) IOM Communications Ltd
2 a behaviour of temperature and b relative humidity under heat trap effect (inside metallic boxes) during two years at
three atmospheric corrosion stations
X-ray diffraction
Results and discussion
X-ray diffraction analysis of copper and steel corrosion
products formed at the Cojimar station and two-year
exposure were obtained using a polycrystalline HZG4
difractometer with Co Ka radiation filtered by nickel. An
angular sweep 2h of 10–70u was used at a rate of 0?05u
and a counting time of 5 s. The samples were prepared
in situ and phase identification was carried out by
comparison with International Centre for Diffraction
Data using the software PCPDFWIN version 2.01.
Environmental conditions
Scanning electron microscopy
The samples of cooper and steel were stuck to supports
using conducting double face adhesive tape and subject
previously to high vacuum during 24 h. The surface of
the samples was analysed without metal coating in a
TESCAM TS 5130 SB scanning electron microscope
working at accelerated voltage between 10 and 20 KV,
and an INCA 350 energy dispersive X-ray spectrometer
(EDX). The samples were analysed from Cojimar
station at two-year exposure.
Influence of environmental parameters on
atmospheric corrosion process
A statistical analysis was conducted using F Fisher in
order to determine the correlation between the exposure
time and calculated TOW. If a significant correlation is
obtained, it could be considered that there is no need to
calculate the TOW, because the two variables can be
equally representative for the corrosion process. A
simple linear regression analysis was carried out between
corrosion rate, exposure time and TOW.
Table 3 Corrosivity categories of indoor
(IC) according to ISO 11844
atmospheres
Indoor corrosivity categories
IC1
IC2
IC3
IC4
IC5
626
Very low
Low
Medium
High
Very high
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Atmospheric corrosion stations differ in the types of
atmosphere. Cojimar corrosion station could be classified as coastal, Quivican as rural and CNIC as urban;
however, the three stations show a similar behaviour on
the temperature humidity regime (Fig. 2). This is
because the three stations are placed in the same climate
and in the same region of the country (western).
Concerning metallic boxes, it is important to note that
the maximum recorded inside temperatures reach
,40uC and sometimes, higher values were reported.
Quivican corrosion station presents a slightly higher
relative humidity, because it is placed in the inner part of
the Cuban Island.
The Cuban Island is long and narrow and almost
parallel to Equator, it causes that changes in meteorological variables are not very high. Under these
conditions, the higher influence on the corrosion rate
is given by atmospheric contaminants.14
Concerning the temperature humidity regime, Cojimar (Fig. 3) and Quivican (Fig. 4) stations showed
similar behaviour when comparing outdoor and heat
trap conditions. Temperature and humidity under heat
trap conditions showed higher values than outdoors,
which means higher differences between maximum and
minimum average temperature, as well as between
maximum and minimum average humidity. These
conditions do promote the corrosion process; however,
it is well known that, under outdoor conditions, higher
corrosion rate is observed. This is explained based on
the influence of temperature humidity regime. It creates
conditions for the development of corrosion process, but
there are other factors influencing the rate of the
process. Under heat trap conditions, in respect of
outdoors, the effect of pluvial precipitation does not
exist; there is a lower deposition of contaminants and a
lower exchange with the external environment.7 The
absence of direct water precipitation implies the lack of a
washing effect on the contaminants deposited.
Negligible differences in the temperature and relative
humidity regime were found between the three atmospheric corrosion stations, as was the case for the TOW
NO
5
Martı́n-Regueira et al.
Indoor atmospheric corrosion of copper and steel
Published by Maney Publishing (c) IOM Communications Ltd
3 a behaviour of temperature and b relative humidity regime under heat trap effect and outdoor during two years at
Cojimar coastal corrosion station
4 a behaviour of temperature and b relative humidity regime under heat trap effect and outdoor during two years at
Quivican rural corrosion station
calculated. The TOW increases as a function of relative
humidity (Table 4). It is important to remark that
Quivican rural station presented the highest TOW values
at relative humidity in the range from 90 to 100% and
even some peak values higher than 100% were recorded.
At all test stations, under heat trap and outdoor
conditions, the highest TOW values were determined at
temperatures ,30uC, with maximum values recorded at
temperatures from 20 to 25uC (Table 5). It means that
under both conditions, outdoor and heat trap, the
corrosion process should take place more frequently at
the temperature range mentioned above.
A classification of TOW inside the metallic boxes was
made according to ISO 9223.5 The three corrosion
stations can be classified in the category t4
(2500,t,5500). This category corresponds to ventilated
shed under humid conditions; however, under storehouses and other conditions the values of TOW reported
are higher. It is important to note that metallic boxes do
not fulfil the conditions of ventilated sheds because
normally the last are wooden boxes having double
Venetian blinds, allowing the free entrance of air from
outside and does not reach considerably high temperatures (wood has a lower capacity to absorb heat than
metals). In this case, the metallic boxes have only two
small windows (Fig. 1); and the air entrance should be
lower than in a ventilated shed. The influence of sun
radiation should be higher on metal boxes due to the
great capacity of metals to increase their temperature by
Table 4 Estimated TOW (h/year) as function of RH under
heat trap conditions at three stations*
Relative humidity, %
CNIC
Cojimar
Quivican
RH
RH
RH
RH
RH
RH
RH
RH
RH
431
1677
1607
1210
1456
2145
3197
4597
0
58
592
1587
1985
1692
2032
3174
3140
432
143
1201
1821
1374
1292
1334
1836
5237
2023
20–30
30–40
40–50
50–60
60–70
70–80
80–90
90–100
100–110
*TOW: time of wetness; RH: relative humidity.
Table 5 Estimated
TOW
(h/year)
as
function
of
temperature under heat trap conditions at three
stations*
Temperature, uC
CNIC
Cojimar
Quivican
10–15
15–20
20–25
25–30
30–35
192
1671
4474
1970
27
33
753
3557
2348
47
644
1965
4480
1650
240
*TOW means time of wetness at relative humidity higher than
80%.
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Indoor atmospheric corrosion of copper and steel
5 Behaviour of contaminants deposition at three test
stations
Published by Maney Publishing (c) IOM Communications Ltd
direct solar radiation. Perhaps the small size of the
metallic boxes creates conditions similar to ventilated
sheds, because the exchange with the environment could
be relatively easy.
The higher deposition rate of chloride ions inside the
metallic boxes was found to take place at the corrosion
stations located at Cojimar (coastal) which presented
sulphur compounds and deposited dust (Fig. 5). This is
in agreement with the type of atmosphere. This station is
located at ,150 m from the Havana north shoreline. In
this region, trade winds come from the northeast
direction. The influence of marine breeze increases the
presence of contaminants inside the metallic boxes in
relation with other corrosion stations.
It is worth mentioning that in all exposure stations,
deposition of dust presented high values. Dust is
composed by multiple particles, very heterogeneous in
nature, having different sizes and properties. Dust can
also include the presence of chloride and sulphate.
Another important result was that sulphur compounds deposition was higher than chloride deposition
in the three test stations, including Cojimar (coastal
station). This behaviour is characteristic of indoor
environments, because SO2, SO3 are gases and sulphate
ions are small particles (,2 mm diameter), which can
more easily penetrate through the windows of metallic
boxes.4,6
A classification was made using ISO 9223 for chloride
and sulphur dioxide deposition (Table 6). All corrosion
stations were classified in the lower levels (So and Po),
excepting Cojimar that showed a medium level of
chloride deposition S1. The deposition level at indoor
(heat trap) condition is lower than that for outdoor;
however, if the classification is made using the ISO
Standard for indoor conditions,9 the exposure stations
CNIC and Quivican can be classified as level II whereas
Cojimar as level III.
628
finally by the station at the CNIC (urban station). This
behaviour is in agreement with the level of contaminant
deposition. It is important to note that only copper
showed a decrease in corrosion rate vs. time on Quivican
and CNIC stations. Copper at Cojimar and steel in all
stations show a constant increase in corrosion rate
versus time. It indicates the formation of non-protecting
corrosion products.
A classification of corrosivity was made using ISO
9223 and ISO 11844 standards (Table 7). A low corrosivity classification for copper and steel was obtained
using ISO 9223 (C2), whereas using ISO 11844, a high
indoor corrosivity classification is obtained. It is important to note that at Cojimar, the indoor corrosivity for
copper is over the maximum level established by ISO
11844 standard and steel is in the top. The corrosivity
classification for the station at CNIC is high and at
Quivican is in the top for the two metals.
Steel presented the highest weight loss at the three
atmospheric corrosion stations. As well known, steel is
an active metal and very sensitive to the influence of all
environmental parameters. Copper is a semi noble metal
having an initial corrosion product: cuprite, which acts
as a protective layer on the surface of the metal. The
presence of this corrosion product on the surface
diminishes corrosion rate of copper.15 It has been
recently reported that under indoor conditions in coastal
regions, corrosion rate of copper increases significantly
with time.16 In the present work, the same behaviour is
observed for copper for three different environmental
conditions, perhaps due to an insufficient coverage
degree of copper corrosion products caused by a
relatively lower corrosion rate, but also a possible nonprotective corrosion products layer formed under
particular conditions.
Copper corrosion products: XRD and SEM
analyses
X-ray diffraction spectra of copper corrosion products
obtained during exposure at three corrosion stations
during two years are presented in Fig. 7. Atmospheric
corrosion process is an electrochemical process. For the
oxidation of a metal, it is necessary the presence of water
on the surface. It has been reported that under
conditions of high relative humidity, a protective layer
of cuprite is formed (Cu2O), but without the formation
of a patina.17 This phase is reported in copper
difractogramms together with copper oxide II (CuO)
(Table 8).
Table 6 Sulphur compounds and chloride deposition rate
during test period: classification according to
ISO 9223:1992*
Contaminants depositions
Stations
Cl2, mg m22/day
SO2, mg m22/day
Weight loss
CNIC
Quivican
Cojimar
The corrosion rate of steel and copper as a function of
time for the three stations can be observed in Fig. 6. The
highest weight loss for these two metals inside metallic
boxes was found to take place at Cojimar coastal
atmospheric station, due to a higher contaminant
deposition, followed by the station at Quivican and
1.76 (S0)
1.41 (S0)
3.47 (S1)
3.67 (P0) (II)
3.96 (P0) (II)
4.48 (P0) (III)
*10 mg m22/day is equivalent to 12 mg m23 of SO2. Inside the
first parenthesis it is placed ISO classification for chlorides (S)
and sulphur compounds (P). Inside the second parenthesis it is
placed the classification according to ISO 11844-1.It is
important to know that the Cl2 is chloride deposition rate and
SO2 is sulphur compounds deposition rate.
Corrosion Engineering, Science and Technology
2011
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Indoor atmospheric corrosion of copper and steel
Published by Maney Publishing (c) IOM Communications Ltd
6 Corrosion versus time behaviour for a steel and b copper under heat trap conditions at three corrosion stations during
two years
It is also reported that when SO2 deposition reaches a
level that allow to compete with chlorides presented in
airborne salinity, the formed patina is a complex mix of
basic cupric chlorides [paratacamite Cu2(OH)3Cl] and a
structural modification (atacamite), basic cupric sulphates [antlerite Cu3(SO4)(OH)4] and brochantite
[Cu4(SO4)(OH)6].18
As has been shown above, sulphur compounds
deposition rate is higher than chloride deposition rate
at all test stations. It does not mean that SO2
concentration should be higher, because sulphur content
is not only due to SO2, but also to sulphate contained in
marine aerosol and in soil and other possible sulphur
compounds. The presence of a significant amount of
deposited dust is also important, because it may contain
hygroscopic products. These conditions favoured that
wet copper surface could have conditions for absorption
and oxidation of gaseous pollutants. Brochantite was
also identified as a phase present in the copper corrosion
products (Table 8), indicating the presence of SO2 or
other sulphur compounds having oxidising properties
like SO3. The presence of sulphate coming from marine
aerosol, dust or biogenic origin could include sulphate
ions in the electrolyte and probably could favour the
formation of Brochantite. On the copper humid surface
covered by corrosion products, Cuz is oxidised to Cu2z
that finally forms a patina mainly composed of basic
cupric sulphates in rural and urban sites.
It is important to note that XRD spectra for samples
exposed at Cojimar show very few signals. The observed
picks corresponding to cuprite and brochantite. It could
indicate that copper under these conditions presents a
lower corrosion rate; however, it is not in agreement
with the results obtained by weight loss, because a high
corrosion rate is determined for copper, particularly at
Cojimar station. It is reported that corrosion rate for
copper can be very dependent on the morphology of the
corrosion products on the copper surface.16 It has been
confirmed that corrosion products of the same composition could give higher corrosion rates when they do not
dry out than when they do. These high corrosion rates
have been associated with corrosion products in the
form of a gel rather than an oxide. An explanation to
this behaviour could be that atmospheric corrosion
products formed on this station are mostly non-crystalline and they are not detected by XRD. The use of
another analytical technique could confirm this hypothesis. An explanation for the existence of a non-crystalline structure of copper corrosion products at Cojimar is
the presence of a relatively significant deposition of
contaminants, sulphur compounds, chloride and dust,
under conditions where no rain or dew influence is
possible. These contaminants could produce a layer of
hygroscopic products that create conditions for the
presence of many small active sites for the occurrence of
the corrosion process; however, the conditions for
solubilisation and recrystalisation of corrosion products
forming crystalline structures are more difficult, because
there is no presence of water precipitations on the
surface as occurs outdoors.
The results from SEM analysis confirmed the
presence of amorphous corrosion products. The above
Table 7 Annual corrosion rate for samples exposed
inside metallic boxes at three corrosion stations:
classification according to ISO 9223 (C) and ISO
11844-1 (IC)
Weight loss
Stations
Carbon steel, g m22
Copper, g m22
CNIC
Quivican
Cojimar
66.17 (C2) (IC4)
75.68 (C2) (IC5)
100.00 (C2) (IC5)
1.87 (C2) (IC4)
4.54 (C2) (IC5)
5.86 (C3) (.IC5)
7 X-ray diffraction spectra of copper corroded samples
exposed at three different corrosion stations during
two years of exposure
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a,b dark green zone of sample was selected; c EDX
8 Micrograph of copper samples exposed at Cojimar coastal station during two years
9 Micrograph of copper samples exposed at Cojimar coastal station under a heat trap conditions and b outdoors conditions or salt spray cabinet
explanation could be considered as possible. Micrographs obtained from the surface of copper samples
exposed at Cojimar corrosion station are shown in
Fig. 8. These micrographs were made in a zone of the
sample characterised by green colour. Copper corrosion
products show not a clearly defined crystalline structure.
A comparison with micrographs reported for outdoor18
and salt spray cabinet19 show the presence of cubic
crystals of cuprite and basic sulphates of significant size
(Fig. 9). It seems that the corrosion products of copper
formed inside the metallic boxes exposed at Cojimar,
due to its small size and not defined crystalline structure
Table 8 Interplanar distances (d) and relative intensities (I/Io) of copper and its corrosion products determined on
samples, exposed at Cojimar coastal station for two years
Experimental
630
Reported
No.
2h
d, Å
I/Io
2h
d, Å
I/Io
Identification/PDF no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
13.96
16.72
23.00
27.92
29.73
32.90
35.08
36.64
39.12
41.36
42.46
43.52
45.42
48.60
50.72
61.56
66.08
6.34
5.30
3.87
3.20
3.00
2.72
2.56
2.45
2.303
2.183
2.129
2.001
1.996
1.873
1.800
1.506
1.414
95
75
50
40
10
35
30
75
45
30
40
100
30
25
50
30
15
13.89
16.56
22.81
27.98
29.67
32.25/33.48
34.51/35.27
36.38/36.55
38.50
41.28
42.27/42.45
43.33
46.20
48.63
50.47
61.39/61.60
65.95
6.37
5.35
3.89
3.18
3.01
2.77/2.67
2.59/2.54
2.46/2.45
2.33
2.18
2.13/2.129
2.08
1.96
1.87
1.80
1.51/1.50
1.41
80
65
100
50
55
49/25
15/90
20/100
100
12
35/10
100
20
40
45
25/35
30
Brochantite/43-1458
Brochantite
Brochantite
Brochantite
Cu2O/77-0199
BrochantitezCuO
BrochantitezCuO
BrochantitezCu2O
CuO/44-0706
Brochantite
Cu2Ozbrochantite
Cu/4-0836
CuO
CuO
Cu
Cu2OzCuO
CuO
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10 X-ray diffraction spectra of carbon steel corroded
samples exposed at three different corrosion stations
during two years of exposure
do not present significant protective properties. It
explains why the corrosion aggressiveness for copper
classification at Cojimar is over the maximum of ISO
indoor standard. On the other hand, its analysis shows
that the corrosion process occurs in a uniform manner,
similar to other studies, but in different periods of
exposure and different stations.20
It is important to note that any compounds containing chlorides ions were detected on XRD spectra;
however, EDX picks of oxygen, sulphur and chlorine
(small) were detected on the sample. It means that
detection limit of XRD does not allow to show the
presence of basic cupric chlorides.
Steel corrosion products: XRD and SEM
analyses
X-ray diffraction spectra obtained for samples exposed
at the three corrosion stations are presented in Fig. 10.
Indoor atmospheric corrosion of copper and steel
The level of indoor corrosivity at the three corrosion
stations is classified as high and very high, and the visual
aspect is very similar for samples of the three stations.
It was already established that lepidocrocite
(c-FeOOH) is the first crystalline phase formed in the
corrosion process. As time increases, this phase is
transformed into others, mainly goethite (a-FeOOH) in
urban and rural atmospheres or magnetite (Fe3O4) and
akagenite in coastal atmospheres. A given amount of the
corrosion products remains amorphous. Lepidocrocite
and goethite are the main crystalline phases found in
corrosion products of carbon steel under heat trap
conditions (Table 9). They are very often found in
atmospheric corrosion products of steel.21 Main phase is
goethite, corresponding with a relatively high level of
sulphur compounds inside the metallic boxes. It is
important to note that at Cojimar coastal stations no
detection of magnetite and akagenite was reported, as
occurs for outdoor exposure.
The morphology of steel corrosion products could be
observed in Fig. 11. There were products showing
different morphologies. A few zones showed the
presence of an oxide layer having a compact and regular
structure, presenting conditions that could act as a
barrier to corrosive agents, although in Fig. 11 a crack
across the surface was observed, it could be related to
the corrosion process under exposure time, indicating a
higher probability of penetration of aggressive agents or
by the dehydratation of the corrosion products due to
evacuation of the SEM chamber. Energy dispersive Xray spectrometer confirmed that there is no sulphur on
this zone (Fig. 11).
The majority of the sample showed irregularities, first
with the presence of a lamellar structure that should
correspond to lepidocrocite morphology and needles in
the upper part, very possible due to the transformation
of lepidocrocite into goethite. As it is an irregular layer,
it should not act as a barrier for corrosive agents.
Although under not highly corrosive conditions, the
structure of steel corrosion products formed showed
Table 9 Interplanar distances and relative intensities of carbon steel and its corrosion products determined on samples,
exposed at Cojimar coastal station for two years
Experimental
Reported
No.
2h
d, Å
I/Io
2h
d, Å
I/Io
Identification/PDF no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
14.20
18.08
21.20
27.08
29.88
33.32
34.84
36.60
38.12
41.44
43.36
44.84
47.20
50.76
53.28
59.12
60.68
61.36
63.12
64.04
6.24
4.91
4.19
3.29
2.99
2.69
2.58
2.45
2.36
2.18
2.087
2.021
1.925
1.798
1.719
1.563
1.526
1.511
1.473
1.454
55
50
85
50
30
55
60
100
35
35
20
30
35
30
50
40
30
40
35
35
14.14
17.81
21.24
26.34/27.10
…
33.26
34.73
36.37/36.68
38.13
41.22
43.29/43.31
45.08
46.90/47.34
50.65
52.86/53.28
59.07
60.77
61.44
63.38
64.03
6.26
4.98
4.18
3.38/3.29
…
2.69
2.58
2.47/2.45
100
10
100
10/90
…
35
12
50/80
20
20
5/20
5
5/20
5
20/40
10
40
10
10
10
Lepidocrocite/8-0098
Goethite/29-0713
Goethite
Goethitezlepidocrocite
…
Goethite
Goethite
Goethitezlepidocrocite
Lepidocrocite
Goethite
Goethitezlepidocrocite
Goethite
Goethitezlepidocrocite
Goethite
Goethitezlepidocrocite
Goethite
Lepidocrocite
Goethite
Goethite
Goethite
2.19
2.09/2.08
2.01
1.93/1.92
1.8
1.73/1.71
1.56
1.524
1.509
1.467
1.453
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Martı́n-Regueira et al.
Indoor atmospheric corrosion of copper and steel
11 a micrograph of carbon steel samples exposed at Cojimar coastal station, b EDX for irregular zone and c EDX for
regular zone
non-protecting properties. The results coincide with
XRD analysis showing that the main phase formed is
goethite.
Influence of environmental parameters
Meteorological conditions such as humidity, temperature,
rain, dew and wind rate along with contaminants (gases,
particles, aerosols, wet deposition) determine the intensity
and nature of the corrosion process in the atmosphere.
Corrosion process is a function of time of exposure.
Time of wetness is considered as the time during the
corrosion process takes place because it is the time
during which an electrolyte is present on the metallic
surface. Dependence between time of exposure and
TOW could be possible for specific climatic conditions.
Taking into account that TOW or exposure time is not
the only variable influencing the atmospheric corrosion
process it is proposed to make a comparison between the
model proposed by Pourbaix22 and widely used by
different authors in the last 20 years23–26 and other
models including TOW by fitting to a simple regression
statistical analysis, see equations (1) and (2)
K~atb
(1)
K~atb
(2)
22
where K is corrosion rate (g m ); t is time of exposure
(months);t is TOW (h). Constant a is proposed to
represent corrosion rate for the first month of exposure.
Constant b is related to the possible protective
behaviour of corrosion products formed, it depends on
metal composition, physicochemical characteristics of
the atmosphere and exposure conditions.
The results obtained after fitting experimental data
using a simple regression analysis to models 2 and 3 for
every metal and station are presented in Table 10.
A comparison of the values of r2 obtained for time and
TOW showed P50?07 and a50?05. It means that there
are no significant differences between time and TOW.
It is important to remark that coefficient b, instead of
the different natures of the metals and the different
characteristics of the test sites, in all cases, is over one,
except for copper at Quivican corrosion station. It
indicates that there is no retard on time of corrosion
process, because corrosion products do not act as
protective within the time frame of the test. These
results confirmed that indoor corrosivity is classified as
high in the atmospheric test stations.
Table 10 Values of constants a and b obtained by statistical regression of copper and steel data for stations for two
models proposed*
K5atb
K5atb
Station
Metal
a
b
R2
a
b
R2
CNIC (urban)
Carbon steel
Copper
Carbon steel
Copper
Carbon steel
Copper
2.53
0.06
5.98
0.13
1.89
0.83
1.24
1.08
1.10
1.33
1.44
0.56
93.66
51.8
92.51
32.11
93.29
23.99
1.69
0.761025
0.012
0.461024
0.2961023
0.029
1.26
1.13
1.10
1.4
1.46
0.56
91.40
53.23
90.65
34.86
92.77
22.39
Cojimar (coastal)
Quivican (rural)
*t is exposure time (months); t is time of wetness (h); K is weight lost (g m22); a and b constant values.
632
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Conclusions
1. Under indoor heat trap conditions, temperature
and humidity reach higher values than those reported
for traditional outdoor and indoor conditions in Cuba.
2. Annual calculated TOW for heat trap conditions
in Cuban tropical climate is in the range corresponding
to outdoor or ventilated shed according to ISO 9223.
This behaviour is not reported for other indoor
conditions.
3. 20–25uC is the most frequent interval of temperature for TOW under heat trap conditions, as it occurs
outdoors. Relative humidity is frequently over 90%.
Sometimes the saturation of the atmosphere is reached.
4. Weight losses of copper under heat trap conditions
at Cojimar coastal corrosion station is over the higher
classification of indoor corrosivity established on ISO
11844-1:2006. It could be explained based on the
structure of corrosion products formed.
5. Sulphur compound deposition rate is higher than
chloride deposition rate at the three corrosion test
stations. It could causes that main corrosion products
formed on steel is goethite and on copper brochantite
for Cojimar station.
6. There are no significant differences in the statistical
influence of time and TOW on atmospheric corrosion
process of copper and steel under heat trap conditions in
Cuban tropical climate.
Acknowledgements
These results have been obtained based on an
International CYTED Project named ‘TROPICORR’
headed by Brazil and with the participation of eight
countries including Cuba. Thanks Dr A. Dago from
Cuban Oil Research Centre to help during the X-ray
diffraction characterisation.
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