Volume 17 Preprint 37
Galvanic Corrosion between Carbon Steel and CuAlBe: Experimental Design with Anodic Dissolution Current as the Dependent Variable
M. C. P. Cruz , M. S. Leite,.C. M. F. Soares, R. T. Figueiredo; E. B. Cavalcanti, R. E. Souzac, P. M. M. AraÃºjo
Keywords: galvanic corrosion, anodic dissolution, carbon steel, CuAlBe
Galvanic corrosion between carbon steel and the shape-memory alloy CuAlBe was investigated. A fractional factorial design of 26-2 including 6 central points was used with the following variables: Cl-, SO42â€“, S2â€“, HCO3â€“, ratio of anodic to cathodic area (1:1, 1:0.5, 1:0.25) and temperature (25, 35 and 45 Â°C). The experimental design was used to investigate the main factors influencing the anodic dissolution current (dependent variable) as calculated from the galvanic current density. The dependent variable was directly proportional to the cathodic area, even in complex electrolyte solutions such as the water produced in mature oil wells. The experimental design provided information that could be used to minimize the dissolution of carbon steel. An analysis of the response surfaces showed that the area, [SO42â€“] and [HCO3â€“], and the SO42â€“ - HCO3â€“ interaction significantly (p<0.1) influenced the anodic dissolution current. The relationship between the cathodic and anodic areas had the largest influence on the anodic dissolution current.
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Galvanic Corrosion between Carbon Steel and CuAlBe: Experimental
Design with Anodic Dissolution Current as the Dependent Variable
M. C. P. Cruza1, M. S. Leiteb,.C. M. F. Soaresb, R. T. Figueiredob; E. B. Cavalcanti b, R.
E. Souzac, P. M. M. Araújod
Faculty Pio Décimo, Av. Tancredo Neves, 5655, Sergipe-Brasil
Institute of Technology and Research, Av. Murilo Dantas, 300, Sergipe-Brasil
Petrobras, Rua Acre, 2504, Sergipe-Brasil.
Department of Mechanical Engineering, Federal University of Sergipe, Sergipe-Brasil
Galvanic corrosion between carbon steel and the shape-memory alloy CuAlBe
was investigated. A fractional factorial design of 26-2 including 6 central points was
used with the following variables: Cl-, SO42–, S2–, HCO3–, ratio of anodic to cathodic
area (1:1, 1:0.5, 1:0.25) and temperature (25, 35 and 45 °C). The experimental design
was used to investigate the main factors influencing the anodic dissolution current
(dependent variable) as calculated from the galvanic current density. The dependent
variable was directly proportional to the cathodic area, even in complex electrolyte
solutions such as the water produced in mature oil wells. The experimental design
provided information that could be used to minimize the dissolution of carbon steel. An
analysis of the response surfaces showed that the area, [SO42–] and [HCO3–], and the
HCO3– interaction significantly (p<0.1) influenced the anodic dissolution
Corresponding author : Faculty Pio Décimo, Av. Tancredo Neves, 5655, Sergipe-Brasil.
Tel: +5579 8866 45 87 E-mail: firstname.lastname@example.org
current. The relationship between the cathodic and anodic areas had the largest
influence on the anodic dissolution current.
Keywords: galvanic corrosion, anodic dissolution, carbon steel, CuAlBe
The petroleum industry faces a wide variety of conditions in its activities, which
range from exploration to refining. Corrosion is one condition that often occurs in this
industry, it is possible to state that it is also included among those conditions and this
requires in-depth analysis. The electrolyte in corrosion in the petroleum industry is
complex and highly saline. It contains CO2 and H2S and is an aggressive corroding
medium. The interaction between dissolved salts, corrosive gases and suspended solids,
together with hydrocarbons and water in petroleum fields, results in a synergistic
phenomenon of degradation of metallic materials that is still not fully understood [1,2].
Carbon steel is commonly used in engineering, and because it is relatively
inexpensive, it is widely applied in oil production and refining (3, 4). In the petroleum
industry, materials such as zinc (5), austenitic stainless steel (6), aluminum (7) are also
commonly used in combination with carbon steel. Galvanic corrosion can occur where
there is contact between carbon steel and other metallic materials.
Shape-memory alloys are metallic materials that demonstrate the capacity to
return to a previously defined shape or size when subjected to an appropriate thermal
cycle. Generally, these materials can be plastically molded at a relatively low
temperature and, when exposed to a higher temperature, return to the form they had
before molding (original form) . One possible application of shape-memory alloys is
in connection of steel pipes without the use of welding.
Because of the use of shape-memory alloys in the petroleum industry, galvanic
corrosion between these alloys and carbon steel should be studied. Conventional
isolation of each of them factors arduous and potentially inconclusive, as synergistic
effects can occur that either retard or accelerate corrosion. Experimental design offers a
practical way to study the factors that contribute to the corrosiveness of the aggressive
medium encountered in the petroleum industry. It can be used to determine the main
variables that significantly influence the corrosion process. Because of the number
variables involved in this process, a fractional factorial design (lk-p, where l=number of
levels, k=number of factors, and p=size of the fraction of full factorial) can be used. A
fractional factorial design can be used to assess the effects of the main variables with
the same accuracy as a full factorial design .
The objective of this study was to develop an experimental design to study the
main variables affecting galvanic corrosion between carbon steel and the shape-memory
alloy CuAlBe in a complex fluid similar to that found in the oil and gas industry. In the
present study, a two-level fractional factorial design of 26-2 + 6 (central points) was
selected for studying galvanic corrosion between carbon steel and the CuAlBe alloy in
just 22 assays. This included 16 assays of a 24 design with six repetitions of the central
point. By comparison, the corresponding full factorial design (26) would require 64
assays. Because central point repetitions are essentially the same experiment, they can
be used to estimate the experimental error rather than repeating each experimental
condition . The anodic dissolution density was selected as the response variable,
because weight loss of the anode (i.e. its dissolution) would affect industrial application
of the galvanic couple by compromising its physical integrity. The main variables
affecting maximization/minimization of the anodic dissolution current were analyzed,
and used to understand the corrosion process of the galvanic couple.
API Grade D 11B carbon steel (Table 1) was obtained from Tenaris Confab
Table 1. Manufacture supplied chemical composition of API Grade D 11B carbon steel.
0.40 to 0.45
0.75 to 1.00
0.15 to 0.25
0.15 to 0.35
0.80 to 1.10
2.1 Shape Memory Alloy
The Laboratory of Rapid Solidification (LRS) at the Federal University of
Paraiba (Brazil) provided samples of the shape-memory alloy CuAlBe (patent pending).
2.2 Experimental Design
The physical and chemical properties of water produced in the petroleum and
gas industry was used to develop a 26-2 fractional design with six replicates at the central
points. This experimental design was used to evaluate the effect of the following six
independent variables: [Cl–], [SO42–], [S2–], [HCO3–], the ratio of cathodic to anodic
area, and temperature on the anodic dissolution current dependent variable .
Statistical analysis was performed using Statistics software, version 8.0.
Corrosion in the oil industry is complex (Table 2), and this is difficult to
simulate in the laboratory because it requires working with H2S and CO2 at high
pressure and temperature.
Table 2. Physicochemical characteristics of the water
total iron (g/L)
Flow velocity (m.s-1 )
38.9 ± 3.4
22.67 ± 1.13
18.40 ± 3.96
7.25 ± 2.48
6.84 ± 2.35
7.31 ± 0.45
42.5 ± 3.3
12.8 ± 2.6
22.1 ± 3.3
12.75 ± 1.21
4.46 ± 1,56
11.66 ± 2.09
10.95 ± 1.92
7.33 ± 0.38
252.58 ± 40.81
8.8 ± 0.6
118.3 ± 8.3
8.8 ± 0.6
8.4 ± 2.3
5.09 ± 1.39
2.43 ± 2.59
25.38 ± 11.82
23.92 ± 11.18
7.00 ± 0.54
275.39 ± 238.86
0.6 ± 0.4
74.1 ± 23.4
12.7 ± 2.3
10.6 ± 3.3
5.47 ± 2.18
0.54 ± 0.41
13,0 ± 11,0
14.29 ± 2.97
13.45 ± 2.80
7.68 ± 0.12
226.56 ± 85.27
9.5 ± 1.2
132.5 ± 3.5
4.0 ± 0.0
Total suspended solids
26.38 ± 4.84
19.27 ± 0.09
21.72 ± 5.63
Total Solidsperformed; ** Values changed by the content of total iron *** Not measured
* Only solids
Corrosion of the system (Table 3) was studied to determine how the variables
influenced degradation of the material at 1 atm (1.033 kgf/cm2). The ranges of the
factors were determined in accordance with the results shown in Table 2, while trying to
adjust the experimental design to the minimum, maximum and medium values found in
the wells to the extent possible within the laboratory conditions.
The concentration range of Cl– (mg/L) was regarded as the total salinity.
Consequently, to obtain a more aggressive corroding medium, the chloride
concentration in the medium was increased. The S2– concentration was defined, without
considering its supersaturation by the effects of temperature and pressure. Sodium
sulfide (Na2S) was used to substitute for H2S. The minimum concentration of S2– used
in the experiments was 10 mg/L because it is rapidly oxidized by oxygen dissolved in
the solution. In simulated aerated seawater, the concentration of S 2– fell from 10 mg/L
to 1.2 0.2 mg/L after 1 day . Bicarbonate (HCO3–) was used to simulate CO2,
because when it is contained in a slightly alkaline solution, it is present as H2CO3,
HCO3– and CO32–.
Temperature analysis was restricted to values below 100 °C because of
experimental limitations in the laboratory. To evaluate the effect of the CuAlBe alloy
area on the dissolution of carbon steel alloy the following ratios of anodic to cathodic
area (in cm2) exposed to the electrolyte were tested: 1:1; 1:0.5 and 1:0.25.
The values of the variables in the 26-2+ 6 factorial design are detailed in Table 3.
Table 3 Values of the variables studied using the fractional factorial design 26-2 + 6
central points with the system in equilibrium with the atmosphere.
Area of CuAlBe/Steel
2.3 Galvanic Corrosion
Galvanic contact between the carbon steel alloy and the CuAlBe alloy was
established by forming a small circuit with the metals using a potentiostat (Model G300, Gamry, Warminster, PA) controlled by Gamry Framework software. The
potentiostat acted as a zero resistance ammeter. The tests were carried out in three
electrode cells with carbon steel alloy as the primary working electrode, CuAlBe alloy
as the secondary working electrode and using a coiled platinum electrode as a reference.
The experiment took approximately 12 h to complete. Tests were performed
immediately after preparation of the samples, which were sanded with metal sanders
(400, 800, 1200, and 2000 grit) and then washed with distilled water and dried.
Results and discussion
For galvanic corrosion of Al alloy coupled with dissimilar metals, the effect of
the ratio of cathodic to anodic areas (Ac/Aa) was studied by Mansfeld et al. . The
rate of dissolution of an anode is directly proportional to the area of the cathode and can
be calculated for any Ac/Aa ratio using Eq. (1):
Id = Ig (1 + Ac/Aa)
According to Schiefler (2002)  this equation is only applicable to a
proportion of substrates with coatings in a 1/1 ratio (coating/substrate). When there are
two different alloys coupled together, the galvanic current density can be used to
estimate the anodic current density. In this way the anodic dissolution current density
(Id) can be calculated using the galvanic current density (Ig).
Table 4 shows the matrix of the 26-2 design with the results for the anodic
dissolution current density (Id). The experimental runs were randomly conducted.
Depending on the experimental conditions, the anodic dissolution current density varied
from 0.00 to 29.06 μA/cm2. The solutions were slightly alkaline, with pH values
ranging from 7.63 to 8.23.
Table 4 Fractional factorial design 26-2 used to study galvanic corrosion density
between carbon steel and CuAlBe alloy with the anodic dissolution current density as
the dependent variable (Id).
In the galvanic current density results (Table 4), the largest increase in the
response variable was observed for when the area of cathodic to anodic. This indicates
the area ratio is the main factor affecting corrosion of the galvanic couple. The
magnitude of increase because of this effect was even greater when the anodic
dissolution current density of the carbon steel (Id) was taken into consideration, because
it increased the number of electrons per second produced in the anode. Because of this,
the estimated values of Id moved from higher values than the current statistical model
proposed. This confirms the effect of area on the middle and validates the results, as
was observed in the response surfaces for the area.
To verify the experimental results, variables that affected the response of the
dependent variable were estimated. This variation is called experimental error and it is a
statistical error arising from uncontrollable test conditions. In this study, the level of
significance level was defined as p=0.10. Among the variables studied, only the Ac/Aa
ratio and the concentrations of [SO42–] and [HCO3–] were statistically significant (Table
Table 5 Effects of the independent variables on the dependent variable.
Data obtained for a statistical study with a 90% rate of accuracy.
Figure 1 shows a Pareto chart for the results. All the effects that go beyond the 0.10 line
are significant. This graph confirms that [S2–], [Cl–], and the temperature did not
significantly affect dissolution of the anode. The fractional factorial design can be used
to define the variables of that influence the response variable, and to study their effects
on the current anodic dissolution. When the effect of one variable depends on the level
of the other variables, first-order interaction occurs between the variables. First-order
interactions (e.g. area and HCO3–, area and SO42–) can be confused between themselves,
difficult to distinguish.
Figure 1 Pareto diagram for analyzing the effects of independent variables on
the anodic dissolution current.
Temperature is an important variable because it increases electrolyte
conductivity and transport of ions, acceleration of electrochemical reactions at the
electrode/electrolyte interface, decreases polarization, and increases solubilization of
protective films . However, in the studied range, the effect of this variable was not
significant (Figure 1).
Cruz et al. studied the effect of the temperature of the electrolytic medium on the
galvanic current density intensity (response variable) in chloride ions . A complex
situation of passivation and despassivation was observed, in which the film was both
protective and destroyed. Therefore, a more complex electrolyte is necessary to use a
statistical approach as interaction among different variables may increase or reduce the
effects of the other variables.
Dissolution of iron present in the carbon steel occurs in accordance with Eq. (2):
Fe → Fe2 + + 2e–
Iron oxidation in an alkaline solution occurs in two main stages, the first of
which follows Eq.(3):
Fe + 2OH– → Fe (OH)2 + 2e–
The reaction occurs in two partial steps (Eqs. 4 and 5), along with the adsorption
of OH– ions .
Fe + OH–→ Fe (OH)ads + e–
Fe (OH)ads + OH–→ Fe (OH)2 + e–
Formation of Fe(OH)2 is subject to the formation of an intermediate product
When the anodic dissolution density is analyzed, it is important to consider that
passivation of the anodic surface can diminish or even completely stop dissolution of
the anode. Depending on the solution, the passive film on the surface of carbon steel can
composed of an oxide, hydroxides, or basic salts such as carbonates and sulfates. In this
way, the negative effect in Table 5 can indicate formation of a protective film. It should
be noted that negative values for the effects of the independent variables on the
dependent variable indicate negative effects on the anodic dissolution current density,
that is increases in the ion concentration decrease the values of the response variable.
This is particularly true when, as a result of highly concentrated solutions, the solubility
limit is exceeded and a film forms on the surface of the anode through precipitation.
However, the type of precipitated film was not determined in the present study.
The electrons on the surface of the CuAlBe alloy interact with a complex system
composed of Cl–, SO42–, S2–, HCO3–, HO–, O2, and H2O. Therefore, the possible
cathodic reactions (Eqs. 6 and 7) are as follows:
HCO3– + 2e– → 2CO32– + H2
H2O + O2 + 4e–→ 4OH–
These equations describe the types of electrochemical activity, but there is a
possibility of reaction between copper metal and oxygen molecules:
2Cu0 + ½O2 → Cu2O (cuprite)
Cu0 + ½O2 → CuO (tenorite)
These hypotheses were raised because of a visual inspection, in which formation
of a dark film was observed on the surface of the cathode. This film was probably
cuprite and/or tenorite. It is known that, depending on the nature of the corrosive
medium, the cathodic reaction may involve absorption of oxygen, but deposition of
films has also been observed.
Ig is associated with the concepts of thermodynamics and chemical kinetics. The
thermodynamic process of galvanic corrosion is spontaneous, and ΔG is negative. This
implies that corrosion will occur naturally without an external potential.
Electroneutrality dictates that, for a galvanic couple, the dissolution rate of the anode
must be equal to the rate of the reduction reaction on the cathode. However, in the
present case there would be passive film formation, chemical reactions and ionic
interference in the medium. In this manner, Ig was analyzed also taking into account
film formation, which tends to minimize the response variable. Physically, when
studying Ig one must understand that it is associated with both anodic and cathodic
current densities. The generated response surfaces with Id as the response variable can
be contrasted with the response surface generated from Ig . Trends observed are
caused by the cathodic current density (i.e. changes in the cathode).
Cruz et al. analyzed the response surface for Ig/T/[Cl–] dependence .
Comparison with that obtained in this study for Id/T/[Cl–] (Figure 2), indicates that some
event occurs in the cathodic region that contributes to the quasi-suppression of the
galvanic current density. It is hypothesized that chloride ions accelerate precipitation of
a protective film on the cathode. This implies that the anodic dissolution can be affected
by formation of this film on the cathode, hindering galvanic corrosion.
Figure 2 Response surfaces as a function of temperature and [Cl–].
The use of experimental design is important for understanding of the corrosion
process. With this set of experiments it is possible to assess several response variables
for an electrochemically complex medium. In this way, equations that are representative
of the process can be proposed, demystifying the concept that experimental design only
proposes equations without physical meaning.
Figure 3 shows the response surfaces for the anodic to cathodic area and the
other variables. The anodic to cathodic area showed a positive effect on the anodic
dissolution current density, indicating that increasing cathodic area increased
dissolution of the anode (Figure 3 a–e). The ratio between the areas is unfavorable when
the surface of the more noble metal is larger than that of the more active member, in
which case accelerated dissolution of the anode is predicted. This result has been
discussed extensively in the literature [16-18]. However, the use of a complex
electrolyte is what sets this study apart.
Figure 3 Response surfaces as a function of a) area and temperature; b) area and
[Cl–]; c) area and [S2–]; and d) area and concentration of [HCO3–] e) area and [SO42–].
Increases in [HCO3–] (Figure 2 d) and [SO42–] showed a negative effect on the
anode dissolution density.
Figure 4 shows that within the range studied, [SO42–] and [HCO3–] have
synergistic effects on anode dissolution.
Figure 4 Response surfaces according to [SO42–] and [HCO3–].
Carbonate and sulfate ions present potentiating effects, because they enhance the
anodic dissolution current when their concentrations in the electrolyte solution decrease.
This indicates that the protective films are not formed under these conditions and that
dissolution of the anode is maximized.
The best conditions for minimizing the dissolution current of the anode were
determined. A lower anodic current density was found when the cathode area as
minimized compared to the anodic area. Furthermore, an analysis of the response
surfaces showed that [SO42–] and [HCO3–], and the SO42–
significantly influenced the anodic dissolution current. An interesting fact, was the
analysis of the response surface for Ig/T/[Cl–] comparing with that obtained in this study
for Id/T/[Cl–], indicates that some event occurs in the cathodic region that contributes to
the quasi-suppression of the galvanic current density. Moreover, anodic and/or cathodic
passivation could suppress dissolution of the metals and an experimental design can be
used to understand the physical process of its dissolution. Anodic dissolution is a
complex process that can result in overlapping of a number of phenomena.
We thank Petrobras (Rio de Janeiro, Brazil) for financial support and Rosane Fernandes
de Brito Petrobras (Rio de Janeiro, Brazil) for tireless support in the early stages of this
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