Atmospheric Corrosion of Copper in the Presence of Acid
Ammonium Sulfate Particles
R. E. Lobnig
and C. A. Jankoski
Lucent Technologies, Bell Labs, Murray Hill, New Jersey 07974, USA
ABSTRACT
Basic copper sulfates, CuSO4(OH)4 and cuprite, Cu20, are the major corrosion products found during field exposure
of copper. The active species of the atmosphere which lead to formation of these corrosion products are, however, still
uncertain. In this study, we investigated whether submicron acid ammonium sulfate particles, NH4HSO4, can lead to formation of a patina with the above composition. The corrosion process was followed by various analytical techniques,
including in situ analysis by X-ray diffraction, pH, and scanning Kelvin probe measurements. The main corrosion products found at 300 K are mixed ammonium copper sulfates, (NH4)2Cu(SO4)(OH)2xH2O, and small amounts of cuprite, Cu20.
Those formed at 373 IC are antlerite, Cu3(S04)(OH)4, and cuprite Cu20. From the room temperature results, NH4HSO4 does
not appear to be responsible for patina formation. The corrosion mechanism is discussed and compared to that of copper
with (NHJ2SO4 particles, which has earlier been shown'2 to lead to the corrosion products found in field exposed copper.
Iniroduction
During atmospheric corrosion of copper, a patina is
formed over time. Initially, a layer of Cu20, CuO, and Cu0
xH,O forms. Cuprite, Cu20, is the main component. Later, a
patina with several corrosion products is formed. The basic
copper sulfates, posnjakite, Cu4SO4(OH)6'H20, brochantite,
Cu4SO4(OH)6, and antlerite, Cu3SO4(OH)4, are primary con-
stituents. Basic copper chloride and carbonate are also
found.33 The mechanism of basic copper sulfate formation
has been the subject of several studies which have been
reviewed.4'5 There is still uncertainty about which sulfur
species is the active species leading to the formation of the
basic copper sulfate. SQ was thought to be the most likely species, but copper sulfate formation could only be simulated in laboratory studies with SO2 concentrations much
higher than in normal outdoor conditions. Another possible sulfur species is ammonium sulfate, (NHJ2SO4, one of
the main components of fine dust particles in the atmosphere. Vernon demonstrated that ammonium sulfate exposures in the laboratory produced patinas with at least some
of the characteristics of natural patina.6 In metropolitan
areas, most of the sulfur acquired by surfaces is not in
gaseous form by the reaction with SO2 but as dry deposition]'8 Outdoor exposures of copper show that the main
factors influencing the weight gain rates are relative
humidity and concentration of aerosol particles.9 The most
abundant ions found in fine particles are SO and NH:,
with the ratio typically being between that of NH4HSO4
and (NHJ2SO4.
The effect of submicron (NH4)2504 particles on the cor-
rosion of copper at varying relative humidities (RHs) and
temperatures was studied earlier.1'2 Laboratory simulations have shown that (NH4)2S04 particles lead to the corrosion products found in natural patinas. It was unclear
whether NH4HSO4 or (NH4)3H(SOJ, are also possible reac-
tion partners.
In this study, the effect of NH4HSO4 and (NHa3H(50J2
on the atmospheric corrosion of copper was investigated
at 300 and 373 K at various RHs.
Experimental
Copper of 99.999% purity (Fe: 4 ppm, Mg: 0.4 ppm) was
obtained from Aldrich Chemical Co. Coupons (10 X 20 ><
0.1 mm) were successively polished with diamond paste of
15, 3, and 0.1 p.m particle size. They were then ultrasoni-
cally cleaned in methyl ethyl ether, deionized water, and
acetone.
The generation and deposition of dry, submicron sized
(NH4)2S04 particles has already been described earlier.' A
diluted solution of (NH4)5S04 is atomized to a fine spray
using high pressure nitrogen. The fine aerosol is then dried
* Electrochemical Society Active Member.
946
using hot nitrogen (350 K). Residual moisture is removed
using diffusion dryers filled with silica gel. This method
did not work, however to generate NH4HSO4 particles.
When NH4HSO4 was dissolved in water, atomized, and
dried, the generated particles had a different chemical
composition as shown by X-ray diffraction. In addition to
NH4HSO4, there was always a considerable amount of
letovicite, (NHJ3H(504)2.
The drying process of aqueous solutions of NH: and
SO ions in varying concentrations has been studied by
Tang et al. and Kim et al."'3 In (NH4)2S04 aerosols, the dry
(NH4)2S04 particle is the only stable compound at RHs
below 47.5%. In NH4HSO4 aerosols, however, no solid com-
pounds were observed above 25% RH. Other authors'4
report even lower threshold values (10% RH). That RH is
considerably lower than the critical relative humidity (CRH)
of NH4HSO4 which is 39.5% RH."2 For aerosols with a
[NHj/[SOI molar ratio of 4/3, drying leads to a particlegas mixture. Below 50% RH letovicite, (NH4I13H(S04)5, is
formed. At RHs below 39.5%, a mixture of letovicite and
NH4HSO4 is found.
In order to avoid the chemical changes during the dry-
ing process a different method was used to generate
NH4HSO4 particles with a defined size range. To obtain a
sufficient yield of particles in the submicron range with
this new method, NH4HSO4 (AlfaAesar, high purity) was
first recrystallized from aqueous solution at 308 K, then
pressed between filter paper, and dried above concentrated sulfuric acid. NH4HSO4 was then fed into the high pressure nitrogen stream of a jet mill (Garlock Laboratory Jet
Mill, TX, see Fig. 1) and pulverized by particle to particle
collision. Coarser particles were separated by an air clas-
sification system and recirculated into the jet mill.
Separation of different size fractions took place in an external cyclone in the jet mill discharge line, with the coarse
particles settling gravitationally into a container, and the
fine particles being carried away by the gas stream. This
mixture of fine particles and nitrogen was then directed
through a cascade impactor (California Measurements, Incorporated, Model MPS-4G1), which separates the particles into four size fractions and deposits them on the copper samples.
The same method was used for the pulverization, size
classification, and deposition of (NH4)3H(S04)2 particles.
The material used was taken from an old bottle with highpurity NH4HSO4 (Johnson Mathey). The salt was decomposed after years of storage. X-ray diffraction showed the
lines of NH4HSO4 and (NH4)3H(S04)2. The molar ratio of
[NH:l/[Sol was determined by ion chromatography as
1.44:1. This is very close to the ratio for pure (NH4)3H(S04)2
of 1.5:1. Therefore, only a small portion of the material is
NH4HSO4. Since the molar ratio of lNHI/[SO found in
atmospheric dust particles is between that of NH4HSO4 and
J. Electrochern. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.
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(NH4)2S04, the effect of this material on corrosion of copper was also investigated.
In this study, 19 to 350 p.g NH4HSO4 or (NH4)3H(S04)2
particles with diameters of 0.5 to 1.5 p.m were deposited
on copper coupons. The samples were then exposed to
humid air at 300 K with the RH of 32, 40, 75, or 93%, and
also at 373 K with the RH of 40 or 88%. The CRH for
NH4HSO4 is 40%. The exposures at 300 K were performed
in closed glass vessels with the samples hanging above a
saturated salt solution for humidity control. The exposures at 373 K were performed in a temperature-humidity
chamber from Tenney. Exposure times varied from 1 h to
25 days.
After exposure, the samples were analyzed by optical
microscopy, scanning electron microscopy (SEM) combined
with energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and metallography. In another set of experiments, the corrosion process was monitored by various
in situ measurements.
For the in situ XRD measurements, a high temperature
diffractometer (Philips X-ray generator XRG 3100, 45 kV,
35 mA; diffraction control unit PW171O/00) was used. The
sample was mounted on a 1 mm Pt heating filament fitted
made at 300 and 373 K. For measurement at 373 K, the
chamber was immersed in a heated glycerin bath. The
reaction was started by covering the bottom of the chamber with a saturated salt solution for RH control, or by
introducing flowing air H20.
The apparatus and technique for the in situ scanning
Kelvin probe measurements are described in Ref. 15. A
NiCr rod with a tip diameter of 250 p.m was used for
The sinusoidal
measuring the corrosion potential
change of distance between the probe tip and the corroding surface produces an alternating current due to the
electric field across the gaseous gap between tip and specimen. When this electric field is exactly opposed by an
applied potential VM, so that the alternating current vanishes, the electrochemical open-circuit potential, E01,. =
V, + constant, can be evaluated. The constant was determined by calibrating the tip over a saturated solution of
CuSO4 on copper. All corrosion potentials are reported relative to the standard hydrogen electrode (SHE).
Results
Copper with NH4HSO4 particles, RH c CRH.—Below the
critical relative humidity (CRH) of NH4HSO4, that is at
with a Pt 10% RhPt thermocouple to control the sample
300 K and 32% RH, no reaction between copper and
temperature. The RH was controlled by bubbling air
NH4HSO4 particles was observed. No change in chemical
through water held at a temperature corresponding to the
desired dew point. The chamber was heated by a circulating mixture of glycerol and water to a temperature slightly above the dew point of the gas, to avoid condensation of
water vapor. All tubing between the water bubbler and the
reaction chamber was heated to a temperature between
the dew point of the gas and the reaction temperature. The
sample was heated to the reaction temperature in dry aix;
to avoid moisture reaction with the particles before the
humid air was introduced.
Measurements of pH were made using a glass micro-pH
electrode (Microelectrodes Inc., MI-406 flat membrane pH
composition or morphology of the particles took place
within the 25-day exposure time of this study. Figure 2
shows an SEM picture of copper with NH4HSO4 particles
after 25 days of oxidation at 300 K and 32% RH. The inhomogeneous pattern of the particle deposits is caused by the
design of the cascade impactor used for particle deposition
(see Experimental).
Copper with NH4HSO4 particles, RH = CRH.—At the
critical relative humidity, that is 40% RH, the NH4HSO4
electrode) and a Ag/AgC1 microreference electrode
(Microelectrodes Inc., MI-403 double junction micro-reference electrode). The Ag-AgC1 reference electrode, filled
with a 3 M KC1 solution saturated with AgCl, was sur-
rounded by a reference chamber filled with saturated
(NH4)2S04 solution to avoid contamination of the sample
surface with AgC1 or KC1. With these electrodes, only surface contact with the absorbed solution is necessary. The
copper specimen holding the deposited particles was
placed in the middle of a glass chamber. The electrodes
were positioned above the particles. Measurements were
LARGE
RflCLE
PT
DISCHARGE
MflS Szes)
particles reacted with copper at 300 and 373 K. The
amount of adsorbed water was too small to be visible. At
300 K, the reaction was confined to the area of original
particle deposits where the original white color of the
NH4HSO4 became darker with time. On other areas, only
interference colors appeared, indicating formation of a
thin copper oxide layer. Figure 3a and b shows SEM photographs of copper with 100 p.g NH4HSO4 after 1 day, and
after 7 days of exposure at 300 K and 40% RH for areas of
heavy particle deposition. After 1 day of oxidation, coarse
grained particles can be seen on top of a layer of fine
grained corrosion products. Only those fine grained particles showed the presence of Cu by EDX analysis. After 7
days of oxidation, the morphology is more homogeneous,
with grains of —10 p.m diameter. The chemical composition
was identified by XRD as (NH4)2Cu(SO4)26H2O. After 25
days of oxidation, the same XRD lines were observed and
two additional lines which can probably be attributed to
IM,CT
(Variable tzea, CHAMBER
Fig. 1. Scheme of the Garlock jet pulverizer.
Fig. 2. SEM of Cu with 100 pig NH4HSO4 after 25 days at 300 K
and 32% RH.
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948
J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.
(NH4)2Cu(S04)2 0.5H20 indicating a loss of water of crystallization with time.
At 373 K and 40% RH, the corrosion products were
spread over large areas of the sample. Figure 4a and b
shows SEM photographs of copper with 100 g NH4HSO4
after 25 days of oxidation. Corrosion products showing
Cu-, 0-, S-, and possibly N-peaks with EDX, have formed
with a needle-like morphology. Additionally, small circles
of corrosion product deposits are observed with only Cuand O-EDX-peaks. The exact chemical compositions,
identified by XRD, are cuprite, Cu20, and a mixed ammonium-copper-sulfate (NH4)2Cu(S04)2.xH2O. The exact
value of x is uncertain. The best matching ASTM standards were those with 0.5 <x < 2, that is, the amount of
water of crystallization is smaller than at 300 K.
The development of corrosion products with time for the
various temperatures and RHs is summarized in Table I.
Copper with NH4HSO4 particles, RH> CRH.—Above the
CRH (93 and 75% RH at 300 K, 88% RH at 373 K), macro-
scopically visible droplets formed within 2 mm of expo-
sure to humid air. The droplets stayed colorless at all
exposure conditions used in this study and dried with
(5 X 5 mm) with time. After 2 h at 373 K, the areas of
heavy deposits turned green.
Table I shows the corrosion products identified by XRD
after oxidation to different temperatures, RHs, and exposure times.
Depending on oxidation temperature, different corrosion
products were formed above the CRH. At 373 K and 88%
RH, the first corrosion product was cuprite, Cu20, followed
by the basic copper sulfate antlerite, Cu3(SOj(OH)4. At
300 K and 93 or 75% RH, mixed ammonium-copper-sulfates were formed first, followed by cuprite, in later stages
of the corrosion process. The amount of Cu20 formed at
373 K was much higher than at 300 K.
The chemical composition of the corrosion products did
not change with the amount of deposited NH4HSO4 particles within the investigated range. However, the time after
which they appeared was affected. At 300 K and 93% RH
the reaction was followed in situ by XRD. The amount of
deposited NH4HSO4 was 150 or 310 g. The sequence of
reaction product formation was the same as found in ex situ
experiments. On both samples, the mixed (NH4)2Cu(S04)2
6H20 was already present after 1 h of exposure. Cu20
appeared after 4 to 5 hon copper with 310 g, but after only
time. The dry-out took —30 mm at 300 K, less than 10 mm
18 h on copper with 150 g. Additionally, the amount of
corrosion product increased with increasing amount of
posits, while areas with very heavy deposits (only on sam-
deposited NH4HSO4, as indicated by increasing intensities
of the XRD line intensities.
The morphology of the corrosion products depended on
at 373 K, and was accompanied by a color change. At
300 K, a light pink color appeared on areas of light deples with 350 g total particle deposits) appeared green.
At 373 K, the surface became dark violet immediately
after drying, the color spreading to the whole sample area
temperature, RH, and the amount of deposited particles.
At 300 K and 75% RH, reaction mainly took place on
Fig. 3. SEM of Cu with 100 pg NH4HSO, (a, top) after 1 day at
300 K and 40% RH, (b, bottom) after 7 days at 300 K and 40% RH.
Fig. 4. SEM of Cu with 100 pg NH4HSO4 after 25 days at 373 K
and 40% RH. (a top; b, bottom).
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,J. tI8crrocn8m.
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Table I. Corrosion products during corrosion of Cu with NH4HSO4 particles at varying exposure conditions (T, RH, exposure time).
Exposure conditions
300 K 40% RH 1 d
300 K 40% RH 5 d
300 K 40% RH 25 d
373 K 40% RH 1 d
373 K 40% RH 5 d
373 K 40% RH 25 d
300 K 75% RH 5 d
300 K 93% RH 1 h
300 K 93% RH 1 h
300 K 93% RH 1 d
300 K 93% RH 1 d
300 K 93% RH 5 d
300 K 93% RH 5 d
300 K 93% RH 5 d
300 K 93% RH 5 d
300 K 93% RH 5 d
300 K 93% RH 25 d
100
100
100
373K88%RH1h
100
50
100
19
50
100
100
373 K 88% RH 2.5 h
373 K 88% RH 1 d
373 K 88% RH 5 d
373 K 88% RH 5 d
373 K 88% RH 5 d
373 K 88% RH 25 d
Phases
Amount of NH4HSO4 in jig
100
100
100
100
100
350
50
100
19
33
50
100
350
100
Cu + (NH4)2Cu(SO4)26H2O
Cu + (NH4)2Cu(SO4)26H2O
Cu + (NHJ2Cu(S0j2.6H20 + possibly (NH4)2Cu(SO4)20.5H2O
Cu + Cu20 + (NH4)2Cu(SOj2.xH2O, x 2
Cu + Cu20 + (NH4)2Cu(SO4)2xH2O, x 2
Cu Cu20 + (NH4)2Cu(S04)2xH2O,
Cu + (NH4)2Cu(S04)26H20 + Cu20
Cu + (NH4)2Cu(SO4)26H2O
Cu + (NHJ2Cu(S04)2'6H20
Cu + (NH4)2Cu(S04)26H,O + Cu20
Cu + (NTH4)2Cu(SO4)26H2O + Cu20
Cu + (NH4)2Cu(SO4)26H2O + Cu20
Cu + (NH4)2Cu(S04)26H20 + Cu20
x2
Cu ÷ (NBJ2Cu(S04)2.6H20 +
Cu + (NH4)2Cu(S0j2.6H20 +
Cu + (N114)2Cu(S04)26H20 +
Cu + (NH4)2Cu(S04)26H20 +
Cu20
Cu20
Cu20
Cu20
Cu+Cu20
Cu +
Cu +
Cu +
Cu +
Cu +
Cu +
Cu20 + Cu3(S04)(OH)4
Cu20 + Cu3(S04)(OH)4
Cu2O
+ Cu3(S04)(OH)4 (only trace amount)
Cu20 + Cu3(S04)(OH)4
Cu20 + Cu3(S04)(OH)4
Cu20 + Cu3(S04)(OH)4
areas where particles were originally deposited, as
observed at the CRH of 40%, with additional smaller
original solid particles to a needile-like morphology. A
sample. Figure 5 shows a SEM photograph of Cu with 100
jig NH4HSO4 after 5 days of exposure to 300 K and 75%
RH. At higher RH, 300 K and 93% RH, the corrosion spread
The Cu matrix below the needle-like sulfur-containing
corrosion products was unevenly dissolved, but no Dxide
could be identified by EDX. Copper oxide has, instead,
formed on other surface areas, for example, the light col-
grains of corrosion products spread over the rest of the
further over the surface. The copper surface between the
ammonium-copper-sulfate corrosion products (diameter —10
jim) was very rough as shown in Fig. 6, a SEM photograph of
Cu with 100 p.g NH4HSO4 particles after 1 h of exposure.
EDX-analysis only showed a Cu-peak in those areas indicating the dissolution of Cu. The observed morphology is consistent with a dissolution-precipitation mechanism.
For Cu with 350 jig NH4HSO4 particles, the corrosion
products have precipitated to form a ring around the original area of deposition, leaving the original area almost
free of corrosion products (Fig. 7a). This can be explained
by a differential aeration cell in the droplet formed by
water vapor adsorption from the humid air. The center of
the droplet is the local anode where Cu dissolution takes
place. The local cathode is located at the outer edges of the
droplet where oxygen reduction takes place. After 1 day of
exposure, EDX-analysis only showed the element Cu in
the center area. After 5 days (Fig. 7b and c), an additional
0-peak was observed, indicating the formation of Cu20, as
identified by XRD. After longer exposures, the morphology of the product (NHJ2Cu(S04)2.6H20 changed from the
Fig. 5. SEM of Cu with 100 jig NH4HSO4 after 5 days at 300 K
and 75% RH.
cross section of Cu with 350 p.g NH4HSO4 after 5 days of
exposure to 300 K and 93% RH is shown in Fig. 8a and b.
ored layer on the left side in Fig. 8a.
At 373 K and 88% RH, much thicker Cu20-layers were
formed than at 300 K. The antlerite that was formed after
—2 h exposure also had a needle-like morphology, as
shown in the SEM photograph in Fig. 9. Cross sections of
samples after 2.5 h exposure showed a —3 p.m thick oxide
layer with a flat oxide-gas interface, but a rough Cu-Cu20
interface. This observation is consistent with a dissolution-precipitation mechanism. After 2.5 h, the needle-like
antlerite was not undergrown by Cu20, but some undergrown antlerite structures could be found after longer exposure times. Figure 10 shows examples of antlerite without and with an underlying Cu20 layer on the same sample
after 5 days of exposure. On the left side of Fig. lOa the
—3 p.m thick oxide layer is visible. The antlerite in Fig. lob
has a layer structure, which is partly detached from the
surface. Below that, still attached to the Cu matrix, a Cu20
layer can be seen.
The change in pH with time on Cu with varying amounts
of NH4HSO4 at 300 K and 93% RH or at 373 K and 88%
Fig. 6. SEM of Cu with 100 jig NH4HSO4 after 1 hat 300 K and
93% RH.
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950
J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.
BMRAY
*ijU2O
Fig. 8. Cross section of Cu with 350 pig NH4HSO4 after 5 days at
300 K and 93% RH. (a, top; b, bottom).
For Cu with 50 jig NH4HSO4 this took 25 mm while for Cu
with 243 jig it took about 1 h. The following pH decrease
is smaller than at 373 K, and the pH reached an almost
constant value of pH 5.5 after about 10 h. A small second
maximum may be present after 15 h for Cu with 50 jig
NH4HSO4 and after 40 h for Cu with 243 jig NH4HSO4.
The variation in the corrosion potential with time on
copper with 0.1 p.L saturated solution of NH4HSO4 at
Fig. 7. SEM of Cu with 350 pg NH4HSO4 (a, top) after 1 h at
300 K and 93% RH; (b, middle) after 5 days at 300 K and 93% RH;
and (c, bottom) after 5 days at 300 K and 93% RH.
RH is shown in Fig. 11. The surface pH was —1 for both
temperatures at the beginning of exposure to humid air,
and then it increased during the first minutes. At 373 K
(Fig. lic) a first maximum of pH 4 was reached after
3 mm, the pH then dropped during the next 2 h to a minimum of pH 2. A second maximum of --pH 5.5 was reached
after --15 h, after which the pH dropped only slightly. At
300 K (Fig. ha and b), the first maximum of pH 6 was
reached after longer exposure times than it had at 373 K.
Fig. 9. SEM of Cu with 100 jig NH4HSO4 after 7 days at 373 K
and 88% RH.
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tio I
Fig. 10. Cross sections of Cu
with 100 pig NH4HSO4 after 5
days at 373 K and 88% RH. (a,
top; b, bottom).
300 K and 93% RH is shown in Fig. 12. The corrosion
potential in the area of the droplet was about 300 mV
lower than that of clean copper in that environment.
above 40% RH as observed with NH4HSO4 particles. This
may be explained by the small amounts of NH4HSO4 in the
Copper with (NH4,}3H(S04)2.—The same exposure condi-
originally deposited particles. At lower RH, no reaction
was observed. As with NH4HSO4, macroscopic droplets
tions were chosen for Cu with letovicite, (NH4)3H(S04)2,
containing small amounts of NH4HSO4 (see Experimental)
as for Cu with NH4HSO4, that is 32, 40, or 93% RH at 300 K
and 40 or 88% RH at 373 K. 100 p.g of particles were deposited in each case. The corrosion of Cu with (NH4)3H(S04)2
was very similar to that of Cu with NH4HSO4. Therefore,
only the differences between the two corrosion mechanisms are addressed.
Reaction between Cu and particles took place at and
formed on the (NH4)3H(S04)2 deposits at 373 K and 88%
RH, and at 300 K and 93% RH. The dry-out, however, took
much longer than with pure NH4HSO4. At 300 K, the
droplets stayed colorless for 1.5 h, were milky white after
2.5 h, and became dry and white after 4 h. After 6 h, the
surface on areas of heavy particle deposition became darker, and turned green after 10 h. Areas with smaller amounts
of particle deposits turned violet. At 373 K, the colorless
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952
J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.
Table II. Corrosion products during corrosion of Cu with
(NH4)3H(504)2 particles at varying exposure conditions
CL RH, exposure time).
pH 4
Exposure
conditions
(days, hours)
4
It 20 24 34
II
time (boon)
time (boon)
'4
I'
(4,)
12
10
pH 4
.2
o
10
2*
to
a
to
to
Phases
I 02 14 0.0 tO 1 12 14 1.0 12
20
2
02 0.4 o.o to i 13 1.4 1.4 1.4
time (boon)
time (hoots)
300 K 32% RH 5 d Cu + NH,HSO4 + (NHJ3H(S04)2
300 K 32% RH 28 d Cu + NH4HSO4 + (NH4)3H(SO,)2
300 K 40% RH 5 d Cu + (NHJ2Cu(S0j2H20 + Cu20
300 K 40% RH 28 d Cu + (NH4)2Cu(S04)26H20
300 K 93% RH 1 h Cu + NH4HSO4 + (NHJ2H(S0J2
300 K 93% RH 1 h Cu + (NHJ2Cu(S0j2-6H20
300 K 93% RH 1 d Cu + (NHJ2Cu(504)26H20
300 K 93% RH 5 d Cu + (NH4)2Cu(S04)26H20 + Cu20
300 K 93% RH 28 d Cu + (NH4)2Cu(S04)26H20 + Cu20
373 K 40% RH 28 d Cu + Cu20 + (NH4)2Cu(SO4)2xH2O, x S 2
373 K 88% RH 1 h Cu + Cu20
373 K 88% RH 1 d Cu + Cu20 + Cu3(S04)(OH)4
373 K 88% RH 5 d Cu + Cu20 + Cu3(S04)(OH)4
373 K 88% RH 28 d Cu + Cu20 + Cu3(SO(OH)4(onlytrace amount)
(C)
pH
0
0
4
ing plates. After 5 days, the smooth particles had comn
ii
to
, ii
time (hours)
'3 03 04 0.0 5.0 01 0.0 tO
time (hours)
Fig. 11. (a) pH-time curve for Cu with 50 pg NH4HSO4 at 300 K
and 93% RH, (b) pH-time curve for Cu with 250 pg NH4HSO4 at
300 K and 93% RH, (c) pH-time curve for Cu with 500 pg NH4HSO4
at 373 Kand 88% RH.
droplets dried in less than 20 mm, and the dry corrosion
product first had a white grayish color that became darker
after 1 h. After 6 h, a blue-green ring appeared around
areas of main deposits with a black color inside. The
greenish color became more intense with time. The rest of
the sample was dark violet.
At 40% RH, no macroscopic droplets were observed at
either exposure temperature. The original white color of
the particles became a dark pink. The corrosion products
identified with XRD at the varying conditions are shown
in Table II.
The corrosion products that have formed for Cu with
(NH4)3H(S04)2, and the sequence and time of their appearance, are almost the same as for Cu with NH4HSO4 particles. Also, the morphology was similar.
At 300 K and 93% RH, corrosion products changed from
solid particles to a needle-like morphology as on Cu with
NH4HSO4 particles. Figure 13 shows an SEM photograph
of areas with high particle deposition after 1- and 5-day
exposures. The originally formed smooth particles, similar
to those in Fig. 7a, changed their morphology to upstand-
E (my)
pletely changed to the new morphology (Fig. 13c). The amount
of Cu20 formed was much larger than with NH4HSO4. Figure
14 shows a SEM picture of a cross section after 28 days of
exposure. Below the sulfur-containing porous needle-like
structure, a 10 p.m thick oxide layer can be seen.
Figure 15 shows the morphology of products formed at
373 K after 28 days at 88% RH, that is, a needle-like
antlerite surrounded and under grown by coarse crystalline Cu20. The oxide layer was still much thinner than
for Cu with (NH4)2504 as shown by comparison of cross
sections in Fig. 16 and 17.
Discussion
Reaction of copper with NH4HSO4 particles was only
observed at and above the CR11 of the salt, that is 40% RH.
The same has been observed previously for copper with
(NH4)2S04 particles, where reaction only occurred at or
above the CR11 of (NH4)2S04, that is, 81% at 300 K, and
75% at 373 K. For atmospheric corrosion of copper, the
presence of NH4HSO4 can be considered more dangerous
than that of (NH4)2S04, since 40% RH is easily exceeded
outdoors and indoors. The amount of corroded copper is,
however, much lower in the case of NH4HSO4. This can be
demonstrated by comparing cross sections after corrosion
exposure (Fig. 8, 10, 17). On copper with (NH4)2S04 particles, pits 20 p.m deep were formed at 300 K, and 100 p.m
deep pits were formed at 373 K.'2 Pits of that size did not
develop on copper with NH4HSO4 particles. Even at the
higher temperature of 373 K, the maximum pit depth was
only -5 p.m. The laterally nonuniform dissolution of copper may be explained by formation of small differential
aeration cells around the deposited particles with anodic
copper dissolution in the center area.
The corrosion of Cu with NH4HSO4 particles proceeded
similar to that of Cu with (NH4)2SO4 particles. With both
salt deposits, macroscopic droplets were formed on the
surface as soon as the sample was exposed to high humidity. With NH4HSO4, the droplets stayed colorless at 300
and 373 K. With (NHà2SO4, they stayed colorless at 373 K,
but almost immediately turned blue at 300 K. The blue
color indicates the presence of Cu2t The difference in
x(pm)
Fig. 12. Corrasian potential-time curve for Cu at 300 K and 93%
RH (a) before, (b) 2 mm after, (c) 30 mm after, and (d) Ii after
addition of a droplet of 1 pA. saturated NH4HSO4 solution to the
surface.
droplet color for the two salts corresponds to the different
corrosion potentials measured with the scanning Kelvin
probe. On Cu with NH4HSO4, the potential in the area of
the droplet was about 100 mV vs SHE (see Fig. 12), and,
therefore, not high enough for the formation of Cu ions.
Figure 18 is a Pourbaix diagram showing the stable phases in the Cu-H20-SO3 system.16 In the presence of (NHJ2SO4,
the corrosion potential in the droplet area was 400 mV,
which is sufficiently high for Cu2t formation.
At 373 K and 88% RH, the corrosion products and their
sequence of formation were the same in the presence of
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j. tiecrrocnem. .,oc., vol. i ', i'io. , Maren I ö I ne Liectrocnemlcal society, Inc.
NH4HSO4 as observed earlier with (NH4)2S04. In both cases,
Cu20 was formed first followed by antlerite, Cu3(S04)(OH)4.
For all other exposure conditions, the corrosion products
were different for the two salts. At 300 K and 93% RH,
(NH4)2S04 particles lead to formation of posnjakite,
Cu4SO4(OH)6H2O, while in the presence of NH4HSO4, the
mixed (NHJ2Cu(S04)2•6H20 was formed. At the CRH of
each salt, (NH4)2Cu(SO4),xH2O in the presence of NH4HSO4,
and the basic copper sulfates, antlerite Cu3(S04)(OH)4, or
posnjakite Cu4(S04)(OH)6H20 were formed in the presence
Fig. 14. Cross section of Cu with 100 p.g (NH4)3H(S04)2 after 28
days at 300 K and 93% RH.
Fig. 15. SEM of Cu with 100 pg (NH4)3H(S0J2 after 1 day at
373 K and 88% RH.
of (N}14)2S04. The amount of Cu20 was larger on copper
with (NH4)2S04 deposits than with NH4HSO4 deposits.
The corrosion products that are found in the natural
patina of copper during field exposures, mainly basic copper sulfates and cuprite, Cu20, can, therefore, only be
simulated using (NH4)2S04 deposits not NH4HSO4.
Fig. 13. SEM of Cu with 100 p.g (NHJ3H(SOJ2: (a, top) after I
day at 300 K and 93% RH, (b, middle) after 1 day at 300 K and
93% RH, and (c, bottom) after 5 days at 300 K and 93% RH.
Fig. 16. Cross section of Cu with 100 g (NH4)3H(SOJ2 after 5
days at 373 K and 88% RH.
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954
J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.
phase. Therefore the amount of deposited NH4HSO4 parti-
cles per area determines the thickness of the electrolyte
layers or droplets which make electrochemical reactions
possible. All detected corrosion products contained Cu. In
bulk solution of (NH4)2504, Cu is mainly dissolved as
Cu(NH3), and possibly as Cu(NH3). 17,18 The necessary
NH3 is supplied by dissociation of NH: ions. Which Cu
complex is stable in the presence of NH: strongly depends
on pH since the reactions of complex formation involve
protons
4Cu + 02 + 4W = 4
Fig. 17. Cross section of Cu with 3.4 jig (NH4}2S04 after 5 days at
373 K and 88% RH.
Cu +
2H20
[Oa]
Cu + NH: = Cu(NH3) + W
fOb]
Cu(NH3) + NH: = Cu(NH3) + H
[Oc]
Additionally, Cu or Cu2 ions or their complexes can be
formed depending on potential. Since the droplets formed
on NH4HSO4 particle deposits were colorless, it can be
assumed that only Cu ions were present in the first stage.
Figure 19 shows a Pourbaix diagram for the homogeneous system Cu-NH3-H20 with 0.05 M Cu and 1 M (NH3
As already shown above, the morphology of the corrosion products can be explained by a dissolution-precipitation mechanism. When Cu is exposed to humid air at RH>
plus NH:).'9 According to the diagram, formation of CuNH3 complexes is only possible above pH 3.3 and not in
the acid droplet that initially forms by water vapor ad-
CRH of NH4HSO4 (40%), water vapor is adsorbed until the
solution that forms is in chemical equilibrium with the gas
species under those conditions. The first reaction taking
sorption on the NH4HSO4 particles. Cu is the stable
Fig. 18. Pourbaix diagram for
the homogeneous system CuS03-H20 with a concentration of
1O' mol 503/L at 298 K.
'sty"
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.L Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.
a0
The dissolution rate of copper in aqueous ammonia solution has been investigated by Halpern.2° According to his
results, the dissolution rate is independent of pH, independent of Po2 for sufficiently high Po2, and independent of the
concentration of Cu-NH3 complexes. At high Po2' the dissolution rate can be quantified with the equation
0.4
a 0.3
RT = kNH: [NHfl + kms [NH3]
I..
zi.J
o
0.
955
The constant kNH3 is smaller by a factor of 18.5 than kwH:.2°
0
C,,.
Since in solutions of NH4HSO4 or (NH4)3504 the concentration of NH3(aq) is orders of magnitude lower than that of
C.45H322'
z
NH:, the second part of the equation can be neglected.
The reaction rate is then approximately proportional to
the NH: concentration, that is 8.15 mol/L for saturated
U
.0.2
4
6
5
7
5
9
LI
tO
It
NH4HSO4 solution, and 8.2 mol/L for saturated (NH4)3504
solutions. The dissolution rate of copper in the presence of
(NH4)3S04 particles or NH4HSO4 particles should, therefore, be comparable. In our experiments, however, copper
pH
Fig. 19. Pourbaix diagram for the homogeneous system CuNH3-
1430 with concentrations of 0.05 M Cu and I M (NH3 +
298 K.
place
NH) at
was much more severely corroded in the presence of
(NH4)3S04. The reason is probably that the surface of Cu
with NH4HSO4 dries out much faster than with (NHJ,S04.
during corrosion of copper in the presence of
Dry-out in the presence of NH4HSO4 takes less than
NH4HSO4 particles, therefore, is
4Cu + 02 + 4W = 4Cu + 2H30
30 mm at 300 K and less than 10 mm at 373 K. In the presence of (NH4)3504, those times were 2 days at 300 K and 78 h at 373 K. The longer period for electrochemical reactions together with the comparable dissolution rate explain
why Cu is more severely corroded in presence of (NH4)3504.
[1]
This reaction increases the pH of the solution, as observed
during in situ pH measurements. In contrast, in droplets
formed on (NH4)2S04 deposits, the initial pH of 4-5 allows
for formation of ammonia complexes at the beginning of
the corrosion process.
Since the NH3 concentration is strongly influenced by
pH, the NH3 concentration is much lower in NH4HSO4
solution than in (NHj3504 solution. The concentration can
be calculated using the mass action law for the reaction
Dry-out occurs as soon as the water vapor pressure of
the solution is higher than that of the gas phase. The water
vapor pressure of the solution can increase by precipitation of insoluble corrosion products, which decreases the
concentration of ions in solution. The ion concentrations
of saturated solutions of NH4HSO4 or (NH4)3504, calculated from solubility and density data from Ref. 21, are summarized in Table III.
The initial concentration of NH: is similar in saturated
NH3 + H30 = NH + OW
[21
The value of the equilibrium constant K = [NH][0H]/
[NH38)] is 1.8 x i0 at 300 K.2° A saturated solution of
solutions of both salts, but decreases much faster in
(NH4)3504 solution, as discussed previously. Additionally,
the sulfate concentration is a factor of two lower in
NH4HSO4 contains 66.67 wt % NH4HSO4.2° This value
together with the solution density of 1.43 leads to 8.15 mol
(NHJ2SO4 solution. Since the concentrations of S0 and
of NH: with time are higher in NH4HSO4 solution, solubility products of components that contain these ions are
more easily reached than in droplets formed on (NHj3504
deposits. This leads to earlier drying of the surface.
At 300 K, the first solid corrosion product observed is
NH4HSO4/L. A saturated NH4HSO4 solution of pH 1, therefore, contains 4.5 X 10-8 mol NH3(aq). An analogous calcu-
lation for saturated (NH4)3504 solution (solubility of
43.3 wt %, density of 1.25) at pH 5 leads to a NH38, concentration of 4.5 x 10 mol/L, which is about four orders
of magnitude higher.
During corrosion of copper with (NH4)3S04 deposits,
most of the NH: disappeared from the surface within 1 h
by evaporation of NH3. If the kinetics of NH3 evaporation
is determined by diffusion through a diffusion boundary
layer in the gas phase, then the evaporation will increase
linearly with the concentration of NH3 in solution. The
evaporation rate of NH3, and the accompanying loss of
NH: from the surface from the NH4HSO4 solution, is then
slower by a factor of i04 compared to that of (NH4)2S04.
Therefore, NH3 and NH: remain much longer on the surface of Cu with NH4HSO4. This difference explains the ten-
(NH4).,Cu(S04)3xH3O containing Cu2-ions. The Cu which
is present in the originally formed droplets must, therefore, be oxidized
4Cu + 03 + 4W = 4Cu2 + 2H30
Since no blue droplets were observed, the oxidation to
Cu2 probably takes place after or during dry-out of the
macroscopic droplets. The reaction increases pH, as ob-
served in the first stage of the corrosion process. The overall reaction of the ammonium-copper-sulfate precipitation
(reaction 4) should not change pH
Cu3 + 2NH + 2S0 + xH3O = (NH4)3Cu(504)3.xH3O [4]
dency to form mixed ammonium-copper-sulfates
(NH4)3Cu(S04)3.xH3O in the case of NH4HSO4 deposits,
while corrosion products without ammonium ions, that is
basic copper sulfates, are formed on Cu with (NH4)3S04.
After -1 h, a pH of 6 is reached at 300 K, and the macroscopic droplets have dried. At pH 6, the concentration of
NH3(aq) has strongly increased due to reaction 2 leading to
an increase in evaporation rate of NH3
Table Ill. Calculated ion concentrations in saturated solutions of
NH: = NH3I + W
NH4HSO4 and (NH4)3S04.
Density in kg/L
Solubility in wt % at 300 K
Concentration of NH in mol/L
Concentration of SO in mol/L
Solubility in wt % at 373 K
Concentration of NH: in mol/L
Concentration of SO in mol/L
NH4HSO4
(NH4)3S04
1.43
1.25
43.4 %
8.2
4.1
50 %
9.46
4.73
66.6 %
8.15
8.15
93.9 %
11.66
11.66
[3]
.
[5]
At this high pH, copper-ansmonium complexes are stable
according to Fig. 19 and can therefore be formed
Cu + XNH = Cu(NH3) + xH
Cu2 + xNH = Cu(NH3) + xW
[6a]
[6b]
Reactions 5 and 6a and b decrease pH, as observed in the
second stage of reaction at 300 K.
The Pourbaix diagram in Fig. 18 shows that Cu30 is only
stable at pH> 3, which is not the case in the initial stages of
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956
J. Electrochem. Soc., VoL 145, No. 3, March 1998 The Electrochemical Society, Inc.
the corrosion process. At 300 K, formation of Cu20 is, there-
fore, only observed after 4 h to 1 d of exposure
2Cu(NH3) + 1120 = Cu30 + 2W + 4NH3t
2Cu + H30 = Cu20 4- 2W
['lal
[7b1
Reaction 7 decreases pH. The slight pH decrease after 15 h
in Fig. ha and at 40 h in Fig. hib may be attributed to this
reaction. However, the amount of Cu30 formed at 300 K is
very low, so that no drastic pH change results.
At 373 K, the sequence of reaction product formation is
different from that 300 K. First Cu30 forms, followed by
sulfate containing products. One reason for this is the
faster pH increase compared to that at 300 K. After only
6 mm, the pH reaches a value of 4, which is within the stability range of Cu20 (see Fig. lic and 18), and also facili-
tates the evaporation of NH3 compared to 300 K.
Additionally, the stahility of Cu compared to Cu2t increases with temperature,22'23 so that the solubility of Cu(I)
salts as Cu2O is more easily exceeded. The first observed
pH decrease must, therefore, be attributed to Cu2O formation (reaction 6), in contrast to the change at 300 K. Since
at 373 K larger amounts of Cu2O are formed, the related
pH decrease of 2 is more pronounced than at 300 K.
The second increase of pH can be attributed to the oxidation of Cu(I) to Cu(II) ions (reaction 3). This reaction is
ammonium copper sulfates may react further after longer
exposure times to form basic copper sulfates needs to be
investigated. At 373 K, the naturally occuring corrosion
products, antlerite and cuprite, were found. 373 K is, howevei far above the temperature copper would be exposed
to during normal atmospheric exposure. Acid ammonium
sulfate, NH4HSO4, is, therefore, less likely than ammonium
sulfate, (NH4)2SO4, the sulfur containing species leading to
formation of basic copper sulfates in copper patinas.
Acknowledgments
Manuscript submitted July 29, 1997; revised manuscript
received October 16, 1997.
Lucent Technologies assisted in meeting the publication
costs of this article.
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Conclusion
In this study, we investigated whether acid ammonium
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