Study of galvanic corrosion
comparing DOX-Steel®
and other kinds of steel using three
electrochemical techniques
Universidad Nacional Autónoma de México (UNAM)
Faculty of Chemistry
Department of Metallurgical Engineering
Corrosion Laboratory
Design, development and supervision of study: José Hernández Espinoza (*), MSc
Project consultant: Dr. Joan Genescá Llongueras (*)
Text: Jose Hernandez Espinoza (*) and Juan Alberto Ruiz de Velasco
(*) Unam-Faculty of Chemistry-Departament of Metallurgical Engineering-Corrosion laboratory
UNAM - Faculty of Chemistry-Departament of Metallurgical Engineering-Corrosion laboratory
Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
1.- Study objectives
To evaluate the characteristics of the galvanic pair that forms between DOX-Steel® and other kinds of steel
in acid and neutral media.
2.- Hypothesis
The electrochemical characteristics of the Nickel-Cobalt alloy used in the manufacture of DOX-Steel® confer
it a high polarization to cathodic reaction (reduction of hydrogen or water) and a small exchange current
density (corrosion density), meaning that in order for these reactions to occur as the result of a cell formed
by a galvanic pair, a high potential change is required.
If enough energy is present in a galvanic pair to set off an electrochemical reaction between DOX-Steel® and
another metal it is attached to, the reaction will occur very slowly, with a galvanic pair current of such values
as will not contribute to the corrosion of the most active metal of the pair.
3.- Summary
Galvanic corrosion is one of the most common forms of corrosion found in oil industry facilities and in the
transportation of liquids generally. This is due to the fact that the transportation equipment comprises a
wide variety of fittings, which are made from a range of materials that create galvanic corrosion cells when
placed in contact with each other.
Galvanic series have been considered a source of information for predicting the behavior of galvanic pairs
between different kinds of noble or active metals. However, there are metals or metal alloys that, despite
having a noble electrochemical potential, present very low levels of corrosion currents when paired with
another metal, which makes for a galvanic pair that is non-aggressive to the most active metal.
The objective of the study was to analyze the galvanic pairs formed with the Nickel-Cobalt alloy when used as
a coating for steel (also known by the brand DOX-Steel®) in contact with conventional steels, such as stainless and B7 steel, and steels with protective coatings such as cadmium-plated and galvanized steel.
The evaluation was carried out using three well-established techniques: potential sweeping, polarization
curves and monitoring of galvanic potential and galvanic current using a zero-resistance ammeter.
The results obtained show that DOX-Steel® does not create an aggressive galvanic pair since the current
output that develops is very low when compared with other metals, allowing it to be successfully used
as a long-term anti-corrosion coating. This behavior is attributed to the Nickel-Cobalt alloy that is deposited on the DOX-Steel® forming a stable passivation film which is polarized by cathodic reaction only at very
negative values. The alloy presents a very low current output which reduces the speed of galvanic corrosion when compared with the speed of corrosion observed in the metals in the pair independently.
UNAM - Faculty of Chemistry-Departament of Metallurgical Engineering-Corrosion laboratory
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
4.- Design of the Experiment
4.1.- Techniques of investigation
To accomplish the objectives set out in this investigation the following design of the experiment was made:
a) Potential sweep of the metals to be evaluated
b) Realization of the polarization curves
c) Measurement of the galvanic potential and galvanic current
Three different systems were set up, one for each technique to be used.
4.1.1.- Technique 1: Potential sweep
For the potential sweep a cell with electrodes was set up:
a) Electrodo de trabajo: Acero a evaluar
b) Electrodo de referencia: Electrodo de calomel saturado.
The system that was set up is illustrated in the following figure and was used for all the tested media.
Figure 2.1 System used for the potential sweep. Three
media. The reference electrode is saturated calomel.
The potential sweep lasted 60 minutes.
4.1.2.- Technique 2: Polarization curves
To obtain the polarization curves a system of three electrodes was set up, with the following functions
and compositions:
•• Working electrode: steel under evaluation
•• Reference electrode: saturated calomel.
•• Auxiliary electrode: high purity and high density graphite.
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UNAM - Faculty of Chemistry-Departament of Metallurgical Engineering-Corrosion laboratory
Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 2.2. Cell for electrochemical evaluation using
polarization curves. Gill AC Potentiostat.
Figure 2.2. shows the Gill AC potentiostat used to carry out the corresponding polarizations.
The conditions for each test were:
•• Cathodic polarization at 600 mV more negative than the rest potential.
•• Anodic polarization at 600 mV more positive than the rest potential.
The sweep rate was 1 mV per second.
4.1.3.- Technique 3: Measurement of the galvanic potential
and galvanic current
To carry out this test the following system was set up:
A
B
C
Figure 2.3. In figures A, B and C, the equipment used to generate the galvanic pairs can be seen. A saturated
calomel reference electrode was used for the potential sweep in the pair. The distance between the metals
forming the pair was 10 cm. A Gill AC potentiostat was used as a zero-resistance ammeter.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
As can be seen in figure 2.3., the ratio of the surface areas of the galvanic pair is of 1:1. Though
international norms for this kind of tests indicate that real geometries should be used, this test seeks
to compare the aggression of the pairs, and as such it was decided to use areas in a ratio of 1:1.
A saturated calomel reference electrode was used in support of the potential sweep of the galvanic
pair and a Gill AC potentiostat was used as a zero-resistance ammeter, such that both the galvanic
potential and the galvanic current were monitored. Monitoring of the galvanic potential and current
was carried out for 7 days.
4.2.- Conditions of the techniques employed:
For the three techniques used the following conditions were fulfilled:
4.2.1.- Steel under evaluation
••
••
••
••
••
Grade B7 alloy steel
B7 Steel treated with DOX® process, 20 micron coating
316 stainless steel
Galvanized B7 steel with 20 micron coating
Cadmium-plated B7 steel with 20 micron coating
4.2.2.-Steel surface treatments
Coated steel
Uncoated steel
DOX®
Cadmium-plated
Galvanized
Surface treatment
Washed with detergent to eliminate greasiness and
any organic residues present. Rinsed with doubledistilled water and dried with acetone.
Stainless
B7
Sanded with 240-grit up to 600-grit to obtain a
homogenous surface over the whole test area.
Degreased and washed.
The surface treatment is carried out to achieve the above and to avoid effects resulting from
differences in the geometry.
4.2.3.- Test solutions
The steels were evaluated using three different solutions: acids, saline (neutral) and sweet. The
solutions were prepared with double-distilled water.
•• A
cid medium: the acid media were prepared with concentrated sulfuric acid which was taken to
a solution of 0.1 molar by concentration in order to obtain a pH equal to 1.
•• Saline medium: the saline media were a solution of 3% sodium chloride by weight.
•• Sweet medium: the sweet media were prepared on the basis of a 3% sodium chloride by weight
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
through which gaseous carbon dioxide was bubbled for an hour. Bubbling for this length of
time ensures that the saturation pH is achieved, which for these media is around 3.4 (see section
on results). The CO2 purity of the gas used was 99.9% by composition.
All the reactants used were of analysis grade and all equipment was calibrated before use.
All the tests were made at a room temperature of around 20ºC and at the atmospheric pressure in Mexico
City, which is approximately 0.7 Bar.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
5.- Analysis of results
5.1.- Reproducibility
All the results shown in this work were taken solely from tests that are reproducible. Figure 3.1 shows an
example of this..
Figure 3.1 Polarization Curve for evaluating B7 steel in saline media.
5.2.- Saturation of CO2 in saline media
Figure 3.2. shows the saturation pH value in the solution, which is approximately 3.9, and the time
necessary to achieve this saturation is around 20 seconds.
Figure 3.2 the saturation pH value is approximately 3.9.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Equation 1 shows the relation of equilibrium constants required to demonstrate that the value
obtained in the experiment does in fact correspond to the saturation value.
Equatión 1
The values of the constants used in equation 1 are shown in the following table.
kdis
= 0.03386 mol dm-3 Bar-1
k hid
= 0.00258
= 1 Bar
= 1.74 x 10 –4moldm-3
p(co2)
k1
Hence:
[H+] =1.2329x 10-4 mol dm-3
pH = 3.9090
5.3 Corrosion potential
Given that the corrosion potential of a corrosion cell will define how much energy is involved in the
process, and hence whether a reaction will occur spontaneously or not, it is necessary to identify
which element behaves as the cathode and which as the anode.
This behavior can be detected by studying each metal independently in the presence of an electrode
with a known potential. The potential generated by the cell (test and reference metals) will indicate
the potential value of the metal under evaluation. Later, by comparing the results of the metals
evaluated in the different media, it will be possible to know which metal will be the cathode and
which the anode when the two form a galvanic pair.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.3 Potential sweep in an acid solution, H2SO4 0.1 molar.
Figure 3.4 Potential sweep in an saline solution, 3% wt NaCl.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.5 Potential sweep in a saline solution, 3% wt NaCl.
Figures 3.3, 3.4 and 3.5 show the behavior of the various metals under evaluation in the different media. Both
in acid and neutral media, the DOX-Steel® presents the most positive potentials. In all the metals evaluated
a stabilization of potential was reached after approximately 15 minutes.
Figure 3.6 Comparison among the different metals in the different media..
Figure 3.6 clearly shows that regardless of the media used for the potential test, the DOX-Steel®
presents the most positive values, followed by stainless steel, cadmium-plated, B7 and galvanized.
It may also be observed that for all steels, the medium generating the most active potentials is that
saturated with CO2. Even with regard to the most corrosive medium, the DOX-Steel® presents a more
noble character (with more positive potentials) than any other metal. In this medium the readings
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
were:
•• DOX-Steel® - 326 mV
•• Stainless steel - 576mV
•• Cadmium-plated steel - 751mV
•• B7 steel - 807mV
•• Galvanized steel -1266mV.
Figure 3.7 shows the behavior of DOX-Steel® in a medium saturated with O2. It may be observed that
the potential drops rapidly towards more negative values, but after a short time recovers its noble
character, though in this case stabilization is not achieved quickly. In a highly-corrosive medium such
as this, the DOX-Steel® still displays a more noble profile than the other metals evaluated.
Investigations carried out with DOX-Steel® in acid and basic media reveal that the material produces
a film formed principally of nickel and cobalt oxides. Given that nickel and cobalt are the components
of the DOX-Steel® alloy and that these display very similar characteristics, it has been surmised that
the film formed is a very stable solid solution of these oxides.
The fact that the potential quickly stabilizes suggests that the formation of the film occurs rapidly and
that it is very stable. This indicates that the oxygen content in the media plays a key role in controlling
the thickness of the oxide film formed and in maintaining its protective qualities in DOX-Steel®.
Figure 3.7. Potential sweep on oxygen-saturated media.
Comparison with a medium with no oxygen saturation.
In addition, the fact that DOX-Steel® displays a more positive potential value implies that when
coupled with other kinds of steel it will behave as a cathode, forcing the other metal to take the role
of anode.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
The difference in potential between DOX® and other steels can be observed in the following table..
Pair formed
Acid medium
DOX®–Stainless steel
DOX® - Cadmium-plated
DOX® – B7
DOX® – Galvanized
207
377
578
763
Saline medium
58
355
490
765
Saline medium with CO2
250
425
481
940
Table 1. Difference in potential between metals forming a theoretical galvanic pair. The values are obtained on the basis of the
potential values found in the individual potential monitoring of each steel. Values are in millivolts (mV).
As can be observed in Table 1, DOX-Steel® shows the greatest difference with galvanized steel in a
medium with CO2. The smallest differences are with stainless steel and the lowest of these is with the
saline medium.
For example, a pair formed by B7 and galvanized steel in an acid medium has a potential of 175 mV
compared with the DOX®-galvanized pair, which is 763 mV. This might lead us to think that if DOXSteel® is placed in contact with these metals a highly-damaging galvanic pair will be generated, and
that the speed at which the metal it is in contact with corrodes will augment considerably. However,
measurements of potential are not sufficient to evaluate the behavior of metals in galvanic pairs,
since they do not account for the polarization of the metal in different media and faced with different
kinds of reaction.
As such, the potential values only confirm that a galvanic pair is formed, without answering two key
questions relating to the kinetics of the pair:
•• How long it takes for the galvanic pair to form.
•• Once formed, whether the galvanic pair will behave aggressively or not.
5.4.- Steel polarization
The results of the polarization curves are presented below. The treatment of this data is focused on
describing the kinetic behavior of DOX-Steel® in different media in comparison with the other metals
under evaluation. This allows its distinguishing characteristics to be identified and the way these
determine the kinetic profile of DOX-Steel®.
To facilitate readability of the text the following abbreviations will be used:
••
••
••
••
Corrosion current density (CCD)
Cathodic slope (CS)
Anodic slope (AS)
Speed of corrosion (SC)
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.8. Polarization curves for different steels in acid medium,
solution of sulfuric acid, H2SO4 0.1 molar, temperature 20ºC.
••
Figure 3.9. Polarization curves for different steels in saline medium, solution of
Sodium Chloride, 3% wt NaCl, temperature 20ºC.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.10. Polarization curves for the different steels in saline medium, solution of Sodium Chloride, 3% wt
NaCl, saturated with Carbon Dioxide, CO2, temperature 20ºC.
5.5.- Kinetic data:
Figure 3.11. Comparison of the current density values (exchange current density) among the
different steels, immersed in the three test media.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.11 shows that the most aggressive media was the acid. In the case of the galvanized steel
the value was an order of magnitude higher when compared to the CO2-saturated saline medium.
The CCD is a parameter that indicates the speed at which the reduction and oxidization reactions
occur, once they achieve equilibrium.
Since the DOX-Steel® displays the lowest CCD this allows us to note that the speed of the cathodic
reaction occurring on the surface of the DOX-Steel® is also very slow when compared to the other
metals under evaluation.
In the saline medium the CCD displayed by DOX-Steel® is 2.51 x 10-2 A/m2; cadmium-plated steel in
the same medium gives 7.5 x 10-2 A/m2 and galvanized steel gives 24.86 x 10-2 A/m2, an order of
magnitude higher than DOX-Steel®.
Stainless steel displays a higher CCD than DOX® and cadmium-plated steel, especially in the presence
of a saline medium.
As has been observed, DOX-Steel® is the material that presents the lowest CCD value, while the other
materials vary depending on the medium they are exposed to.
Figure 3.12 Relationship of cathodic slopes among the different metals and media.
The CS is a measure of the degree of polarization undergone by metals, that is, the magnitude of
the change of an electrochemical reaction when, from a position of equilibrium, it is disturbed by
a flow of current. Figure 3.12 shows how the cathodic reaction on DOX-Steel® undergoes a greater
polarization than in other metals in all media; this tells us that once the corrosion process in a system
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
involving DOX-Steel® has begun, and current (movement of electrons) begins to flow, the DOX-Steel®
will tend to take the cathodic reaction to more negative values.
As such, the cathodic reaction will require a more negative potential value for it to take place; if the
potential of the cell generated does not reach this value, the process will halt, as it lacks sufficient
energy to function.
The cathodic polarization value of the cadmium-plated steel is not as high. For example, in saline
solution, which is the medium in which it gives the highest reading, the following polarization values
are obtained: DOX-Steel® 205 mV, and cadmium-plated steel 160 mV.
The CS values presented by DOX-Steel® are higher than those of any other metal, and among these
there are variations according to the medium to which they are exposed, with cadmium-plated steel
giving high values, though lower than DOX®, and B7 steel the lowest.
5.6.- Anodic slopes
In figures 3.8 to 3.10 it may be observed that the metals display different behaviors at polarizations
higher than 300 mV. In an acid medium stainless steel rapidly displays passivation, very probably due
to the chrome oxides that form on its surface. This film is non-conductive.
The other metals may form a film in this medium, but in the case of DOX-Steel®, this film polarizes in
an unusual manner. For media with dissolved CO2 it displays a behavior similar to that in media with
NaCl and H2SO4.
The galvanized steel displays passivation when it is subject to high current outputs. In the case of the
saline solution, all the metals show passivation to a greater or lesser degree, forming a film which will
be non-conductive for a narrow range of potentials; however at higher potential ranges it will lose
stability.
In the case of DOX-Steel®, the film will conduct the current in such a way that an anodic reaction takes
place on it.
The undefined linear relationship shown by the curves obliges us to be very cautious when evaluating
the anodic curve (polarization) undergone by the metals under evaluation.
Figure 3.13 shows the AC values which indicate how the anodic process occurs for the metals under
evaluation. Given that these curves are found very close to the equilibrium potential, the anodic
process may lead to the metal dissolving in the medium, though later it may or may not achieve
passivation, as has been stated.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.13 Relationship of anodic slopes among the different metals and media
Figure 3.13 shows that the anodic behavior of the steels tested is very similar. In the CO2-saturated
saline solution, the curve values correspond to those given theoretically for the oxidization of iron, at
40 mV. The curve values change for the other two solutions, but in general the steels all behave in a
similar manner to each other.
The values of the slopes in the acid medium are:
•• DOX-Steel® 70 mV
•• Stainless steel 85 mV
•• Cadmium-plated steel 80 mV
•• B7 steel 87 mV
•• Galvanized steel 110 mV
It may be pointed out that both cadmium-plated and DOX-Steel® give very similar curve values in the
acid medium, just as they do for the CCD and CS values.
The anodic slope values of all the steels tested are very similar:
•• Stainless steel 78mV
•• Galvanized steel 10 mV
•• Cadmium-plated steel 103 mV
•• DOX-Steel® 98 mV
•• B7 steel 83 mV
Once again we observe similar values for DOX® and cadmium-plated steel, though the latter gives
higher values, indicating that it undergoes a more marked change in potential to the current flow
generated by the system.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Comparison among the metals under evaluation tells us that the polarization value of the anodic
reaction of DOX-Steel® does not show significant change.
Taking into account the three kinetic values presented above (corrosion current density, cathodic
slopes and anodic slopes), and if the DOX-Steel® is compared with the results of the rest of the metals
under evaluation, it may be stated that:
•• DOX-Steel® presents the lowest CCD in all media.
•• DOX-Steel® presents a very similar anodic polarization to the rest of the metals under evaluation.
•• The polarization of the cathodic reaction of DOX-Steel® is very high.
This leads us to think that the behavior of DOX-Steel® is governed by the cathodic process that occurs
on its surface. It must develop a film composed of nickel and cobalt oxides that slow down the speed
of the cathodic reaction, and in general, the speed of corrosion of DOX-Steel®.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
5.7.- Galvanic potential and galvanic current over time
In order to be able to evaluate the behavior of DOX-Steel® in a galvanic pair, different pairs were
simulated using the metals under evaluation. All the possible combinations were generated with the
aim of evaluating the pairs that all the metals under evaluation develop among themselves.
DOX®-stainless steel, galvanized-cadmium-plated steel, B7-DOX-Steel®, etc. All such pairs were
evaluated in the three media under study (saline -neutral-, acid and carbonic).
In order to evaluate the galvanic corrosion and the galvanic potential an Gill AC potentiostat was
used, functioning as a zero-resistance ammeter (ZRA). This apparatus has the ability to measure both
the potential and the current of a galvanic pair, allowing us to know with precision the current output
from the pair over time, and as such the speed of corrosion generated as an effect of the pair formed,
that is, the galvanic corrosion current (GCC).
The moment at which the behavior of the potential and current in the galvanic pairs stabilizes was
sought in order to determine the length of the experiment.
It was determined that the length of time for the test would be 7 days. Over this time the majority of
the galvanic pairs formed showed signs of a stable current and potential, allowing us to assume that
the pairs will not show changes in polarity in the long term.
The first set of results shown are for pairs formed with DOX-Steel® and other metals and later for
the rest. A comparison is also made between the galvanic current values that indicate the speed of
corrosion generated by the galvanic pairs.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.14. Pairs formed with DOX-Steel® in acid medium, H2SO4 pH=1, temperature 20ºC,
pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
Figure 3.15. Pairs formed with DOX-Steel® in saline medium, 3% wt NaCl, pH=6.4,
temperature 20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.16. Pairs formed with DOX-Steel® in CO2-saturated saline medium, 3% wt NaCl, pH=3.6,
temperature 20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
The values shown in the figures 3.14 to 3.16 indicate that:
A) The range of GCC values in acid medium is between 23 and 82 μA:
•• Pair with stainless: 24 μA
•• Parr with B7: 79 μA
•• Pair with cadmium-plated: 82 μA
•• Pair with galvanized: 79 μA
According to these values we can say that the pair DOX®-stainless steel presents the lowest GCC value
and the potential value is in the order of -350 mV. It is interesting to note that the GCC value of the
other pairs is very similar, at around 80 μA, with a potential value of around -650 mV. This indicates
that when a galvanic pair is formed between DOX-Steel® and cadmium-plated, galvanized or B7 steel,
the DOX-Steel® induces in these metals a comparable galvanic pair behavior.
The GCC output from the DOX®-Stainless pair is very low.
At zero hours the pair DOX®-stainless steel gives almost the same GCC values as after 100 hours,
a behavior that does not occur in the other steels under evaluation. For example, for the DOX®galvanized steel pair at zero hours the GCC value is of an order of 900 μA and for the DOX®-B7 pair it is
around 100 μA. This indicates that the surface of the plated steels (cadmium-plated and galvanized)
changes but stabilizes over time and also, as will be seen below, that deterioration does not increase
as a result of being in contact with DOX®.
In the saline medium similarities are found with the acid medium. For example, the DOX®-stainless
steel pair is the one that stabilizes most quickly and displays the most noble potential (-480 mV),
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
though in this medium the current output is not the lowest, giving a value of 30 μA compared to the
11 μA and 7.9 μA readings for the cadmium-plated and galvanized steels respectively. Just as in the
acid medium these two latter pairs start out at very high values (336 μA for cadmium-plated and 275
μA for galvanized steel).
As for the potentials in the saline medium, the DOX®-stainless steel pair tends towards more negative
values, as do the pairings with galvanized and B7 steel, with only the cadmium-plated pairing
tending towards more positive values. This implies that the process with the pairs, except in the case
of cadmium-plated steel, tends to favor corrosion, which is logical considering the thermodynamic
character of the metals taken individually. In the case of the cadmium-plated steel it appears that a
film forms that provides a more protective character in the saline medium than in the acid medium.
In saline media the lowest values are registered:
•• Stainless steel 0.56 μA
•• Cadmium-plated 8 μA
•• Galvanized 16 μA
•• B7 42 μA
With regard to B7 steel the change is not so marked given that at zero hours it the starting value is 97
μA. This is not the case with the cadmium-plated steel which starts at 139 μA and least of all for the
galvanized steel at 850 μA. This indicates that these latter two pairs display a rapid corrosion process,
which over time decreases in speed and stabilizes. The potential values fit very well with the above,
given that all the pairs show a more positive character over time and stabilize.
The potential value of the DOX®-B7 steel pair changes very little over the course of the test, starting
out at -692 mV and finishing at -680 mV. This is a very slight change, like that registered in the GCC
test. The stainless steel continues to present very positive potential values, more positive than any
other. All of this may be explained by the fact that the carbonic media tend to form carbonate films
over the surface of the steel. It appears that these films remain very stable, and it seems that the
presence of the pairing with DOX-Steel® foes not lead to instability in the films and as such they may
be avoiding the advance of the corrosion process.
Another important point to highlight is the fact that after the 100 hours the systems stabilize both
in terms of the potential and the current value. Moreover (given the characteristics of DOX-Steel®
that have been mentioned), the possibility that over very long lengths of time a change in behavior
occurs due to the presence of the activation or passivation of the metals in the pair is very small.
Figure 3.17 shows the galvanic corrosion speed, GCS, registered by the DOX® in galvanic pair.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.17. Evaluation of the speed of corrosion caused by the galvanic pair to the anodic metal.
Comparison among different media.
The GCS values shown in the DOX® pairs are very small in comparison with the independent metals;
this can be seen in figure 3.18
Figure 3.18 Values for corrosion speed, VC, in mm/year, for the different metals under evaluation.
The values are obtained on the basis of the corrosion densities shown in figure 3.11. These values are
presented in mA/cm2 and using Faraday’s laws are converted to mm/year.
Figure 3.18 indicates that the VC values of the metals evaluated are very high in comparison to those
registered by pairing with DOX®. For example, galvanized steel gives a VC of 2.1 mm/year in acid
medium by itself, while paired with DOX® this is 0.185 mm/year, that is to say, a reduction by two
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UNAM - Faculty of Chemistry-Departament of Metallurgical Engineering-Corrosion laboratory
Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
orders of magnitude. Hence the contribution of the DOX-Steel® is of less than 10 %. One way of
better evaluating these results is by comparing the pairs established among the different metals and
observing which generate higher current outputs and hence faster speeds of corrosion.
The following figures reveal the behavior of the pairs formed in the remaining metals in the three
media.
Figure 3.19. Pairs formed with stainless steel in acid medium, H2SO4 pH=1, temperature 20ºC,
pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1. Pair not formed with DOX-Steel®.
Figure 3.21 Pairs formed with stainless steel in CO2-saturated saline medium, 3% wt NaCl
pH=3.6, temperature 20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
Pair not formed with DOX-Steel®.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.22 Pairs formed with B7 steel in acid medium, H2SO4 pH=1, temperature 20ºC, pressure 0.7
Bar approx. Surface area ratio between electrodes 1:1. Pair not formed with DOX® or stainless steel.
Figure 3.23 Pairs formed with B7 steel in saline medium, 3% wt NaCl. pH=6.4, temperature
20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1. Pair not
formed with DOX® or stainless steel.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.24. Pairs formed with B7 steel in CO2-saturated saline medium, 3% wt NaCl pH=3.6,
temperature 20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
Pair not formed with DOX-Steel®.
Figure 3.25 Pair formed with cadmium-plated steel in acid medium, H2SO4 pH=1, temperature
20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1. Pair not formed with
DOX®, B7 or stainless steel.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.26 Pair formed with cadmium-plated steel in saline medium, 3% wt NaCl pH=6.4,
temperature 20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
Pair not formed with DOX®, B7 or stainless steel.
Figure 3.27 Pair formed with cadmium-plated steel in CO2-saturated saline medium, 3% wt NaCl pH=3.6,
temperature 20ºC, pressure 0.7 Bar approx. Surface area ratio between electrodes 1:1.
Pair not formed with DOX®, B7 or stainless steel.
The behaviors shown in figures 3.19 to 3.27 may be summarized as follows.
Pairs with stainless steel in acid medium.
The potential and current values stabilize over time.
The potential values for all pairs are practically identical, with no change between B7, cadmiumplated and galvanized steel. The galvanic current value appears in the range 112 μA to 350 μA, much
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
higher values than those shown by DOX-Steel®. Further, the initial GCC values are very high: 4633 μA
for cadmium-plated, 11847 μA for galvanized, and 888 μA for B7steel. These values are higher than
when paired with DOX®. Though the GCC values are higher than when paired with DOX-Steel®, in this
medium the rest of the materials evaluated display a very similar behavior in terms of stabilization,
since all steels reach similar potential values (around -645 mV.).
Pairs with stainless steel in saline medium.
In this medium, a number of key moments occur for the galvanic pair:
•• The first occurs between zero hours and approximately 6 hours, when the GCC drops rapidly
from 321 μA to 136 μA.
•• It then remains stable for almost 50 hours.
•• From that point on the GCC falls more gradually to 38 μA, a value reached after 126 hours, and
it then stabilizes.
The behavior of the other metals shows similar characteristics, with an initial abrupt change in
the GCC values which then stabilizes until reaching the reported value, which in the case of the
stainless-B7 pair is 6.6 μA and 26 μA for cadmium-plated steel. The potential values remain very
stable throughout the test.
Pairs in saline medium with CO2
In this medium very interesting behavior is displayed.
The GCC value readings for the pairs formed by stainless-galvanized, B7-galvanized and cadmiumplated-galvanized display a cyclical nature (see figures 3.21, 3.24 and 3.27), while the potential values
remain stable.
The other pairs, B7 and cadmium-plated with galvanized in acid register high GCC outputs at the
start of the test, of around 500 μA, and subsequent stabilization after about 50 hours.
This cyclical conduct may indicate the presence of films that cause passivation, but which are unstable
and over time dissolve in the medium, allowing further corrosion before they reform. In the case of
the DOX-Steel®, this film appears to be more stable as it does not demonstrate this cyclical behavior.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
5.8.- Comparison of the activity of different galvanic pairs:
In order to be able to carry out a better comparative evaluation of the different galvanic pairs formed
let us consider figures. 3.28 to 3.31:
Figure 3.28. Comparison of DOX® paired with stainless steel and the other metals under evaluation.
Measurement of the speed of corrosion caused by the galvanic pair to the anodic metal.
Different media.
In all media DOX-Steel® generates the least aggressive pair, since it displays the lowest GCC. The
smallest current output is seen when paired with stainless steel in carbonated media.
Figure 3.28 shows the lowest GCC values of the cycles.
The highest values are observed in both sulfuric and carbonic acid, with the pair formed with
galvanized steel displaying the highest value in carbonic acid and the pair with B7 steel the highest
in sulfuric acid.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
Figure 3.29. Comparison of DOX® paired with B7 and the other metals under evaluation.
Measurement of the speed of corrosion caused by the galvanic pair to the anodic metal.
Different media.
Figure 3.29 shows galvanic corrosion values for DOX® and cadmium-platedsteel. In acidic solutions
the reading for cadmium-plated steel is higher than that for the DOX-Steel®: 0.39 mm/year for the
cadmium-plated and 0.1752 mm/year for DOX®. In both cases the pairing is with B7 steel. The most
corrosive pair is that formed between B7 and stainless steel in acid medium, which gives 0.8mm/year.
Figure 3.30. Comparison of DOX® paired with cadmium-plated steel and the other metals under evaluation.
Measurement of the speed of corrosion caused by the galvanic pair to the anodic metal. Different media.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
In general the pairs formed by cadmium-plated steel and the rest of the metals under evaluation are
not aggressive in any of the media. The most aggressive pair formed is with stainless steel in acid
medium, but in this case it is the stainless steel which causes this behavior in the pair. As was seen
in figure 3.28, the pairings with stainless steel tend to be the most aggressive, above all in acidic and
carbonic media.
Figure 3.31. Comparison of DOX® paired with galvanized steel and the other metals under evaluation.
Measurement of the speed of corrosion caused by the galvanic pair to the
anodic metal. Different media.
Using kinetic values and the measurement of the current in galvanic pairs, it has been seen that
DOX-Steel®, as well as being a metal that is very resistant to corrosion, generate galvanic pairs that
are less corrosive than stainless or galvanized steel, and even cadmium-plated steel, which as can be
observed in figure 3.31, in comparison with DOX-Steel® in saline and carbonic media shows very high
galvanic corrosion speed values when paired with galvanized steel.
6.- Mixed potential theory applied to the behavior of
DOX-Steel® in a galvanic pair
Using the mixed potential theory, that is, the way in which a process or reaction is polarized by the action
of another reaction, an argument may be presented to explain the behavior displayed by DOX-Steel® up to
this point.
Figure 3.32 shows the cathodic behavior of DOX® in the system and the consequent polarization of the
anode towards more positive potential values, meaning that the speed of anodic reaction will be slower in
the paired steel than if it were not paired.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
This can be explained using the Evans Tafel curves, which show the cathodic control of DOX® over the
galvanic pair.
Figure 3.32 Behavior of DOX-Steel® in a galvanic pair. The current output diminishes, meaning the galvanic pair
formed will not increase the current output of the anodic metal.
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Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
7.- Conclusions
•• DOX-Steel® displays a more noble potential than the other metals for all media tested, meaning
it will always behave as a cathode in any galvanic pair formed with these metals.
•• The corrosion current density produced by DOX-Steel® is very low, meaning that the speed of
reactions will be very slow.
•• The cathodic polarization on DOX-Steel® is very high, meaning that a very large energy input is
required for this to occur.
•• The potential of the galvanic pair formed with DOX-Steel® is more positive than the anodic
potential of the rest of the metals under evaluation, meaning that these will assume more noble,
and less corrosive, behavior.
•• The current output generated by DOX-Steel® in a galvanic pair is very low and is not considered
aggressive.
•• In comparison with metals such as stainlesssteel, cadmium-plated steel and galvanizedsteel,
DOX-Steel® generates a non-aggressive galvanic pair, meaning that the speed of corrosion of
the paired metal will not increase.
8.- Future work
Based on the different variables involved in the evaluation of galvanicpairs, it will be necessary to undertake
further research covering:
••
••
••
••
••
••
••
••
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Tests in which the surface area of DOX-Steel® is larger than that of the paired metal
Temperatures simulating those present in the field
Concentration cells
Handling of systems with dynamic flows
Analysis of the films formed on the different pairs
The use of inhibitors
Increasing the resistance of the electrolyte
Extending the length of the experiment
UNAM - Faculty of Chemistry-Departament of Metallurgical Engineering-Corrosion laboratory
Study of galvanic corrosion comparing DOX steel
and other kinds of steel using three electrochemical techniques
9.- Bibliography
•• Genescá Ll. J. and Ávila J., Más allá de la herrumbre, Vol. 1 and 2, Fondo de Cultura Económica,
Mexico 1986.
•• Bard A. J. and Faulkner L. R., Electrochemical methods, Fundamentals and Applications, John
Wiley and sons, Second edition, USA 2001.
•• Standard guide for conducting and evaluating galvanic corrosion tests in electrolytes,
designation g 71-81 (reapproved 1998, ASTM).
•• Standard guide for development use of galvanic series for predicting galvanic corrosion
performance, designation g 82 98 (reapproved 1998, ASTM).
•• http://www.doxsteel.com.
•• Ajit K. Roy Dennos and l. Fleming, Galvanic corrosion study of container materials using zero
resistance ammeter, Paper no 156, Corrosion conference, NACE 1998.
•• Baldera N., Evaluación de la corrosión de la aleación DOX en medio ácido, thesis paper, UNAM,
Mexico, 2003.
•• Ramírez Rodríguez R., Caracterización electrochemical de una aleación Ni-B-Co depositada
electrolíticamente, thesis paper, UNAM, Mexico 1998.
•• Rodríguez C, Campillo B, Albarrán, J. Genescá, Caballero L. X. , Corrosion behavior of electrolytic
NICOB coatings, Corrosion Review, vol. 17 no 2 1999.
•• Oldfield J., Electrochemical theory of galvanic corrosion, Galvanic corrosion ASTM Stp 978, USA
1988.
•• Randle T.H., Galvanic corrosion–a kinetic study, Journal of chemical education, Vol. 71 number
3, March 1994.
•• Cramer S.D. and Jones P., Planning and design of tests, Corrosion tests and standards, Robert
Baboian, editor.
•• Kim J-G., Lee H-D. and Chung S., Effects of flow, velocity, ph, and temperature on galvanic
corrosion in alkaline chloride solutions, Corrosion NACE, vol 59, 2, February 2003.
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Promicom
Miguel Angel Rodriguez
E. mar@promicom.com
P. (832)-349-4589