Volume 6 Preprint 37
The Influence of Hexamethylenetetramine on the Corrosion and Hydrogen Permeation of Type API 5L-X52 Steel
R. RÃƒÂ©quiz, A. Delgado, A. Rivas and A. Ruiz
Keywords: corrosion inhibitor, hexamethylenetetramine, API 5L-X52 steel, hydrogen permeation, hydrogen induced cracking, blistering, ultrasonic testing, attenuation
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Volume 6 Paper C086
The Influence of Hexamethylenetetramine on the Corrosion and
Hydrogen Permeation of Type API 5L-X52 Steel
R. Réquiz, A. Delgado; A. Rivas and A. Ruiz
Departamento de Ciencia de Materiales, Universidad Simón Bolívar,
Apartado 89000, Caracas 1080, Venezuela. E-mail: firstname.lastname@example.org
The inhibitive effect of hexamethylenetetramine on the corrosion and
hydrogen permeation behaviour was evaluated on a type API 5L-X52 steel
in 1N H2SO4 solution. Potentiodynamic polarization curves were employed
to determine the electrochemical behaviour of the steel, while the
Devanathan-Stachurski technique was used to estimate the hydrogen
permeation rate. Optical and scanning electron microscopies and energy
dispersive X-ray analysis were used for surface analysis. Additionally, to
detect the hydrogen damage in the steel, an ultrasonic technique based on
the sound wave attenuation was employed. The obtained results clearly
indicate that Hexamethylenetetramine inhibits efficiently the corrosion of
the steel but increase the quantity of absorbed hydrogen in the bulk metal.
The organic compound seems to have a specific influence on the hydrogen
evolution reaction, reducing the active site numbers were hydrogen could
be reduced and partially blocking the recombination of atomic hydrogen to
molecular hydrogen on the steel surface. Apparently, this effect leads to a
larger hydrogen absorption into the metal lattice. In the samples that were
tested in solutions containing inhibitor, considerable superficial damages
such as blistering and hydrogen induced cracking were observed.
Moreover, the higher inhibitor concentration, the larger is the mechanical
damage observed in steel.
Keywords: corrosion inhibitor, hexamethylenetetramine, API 5L-X52 steel,
hydrogen permeation, hydrogen induced cracking, blistering, ultrasonic
Metallic materials may absorb considerable amounts of hydrogen in
practical conditions such as electroplating, cathodic protection and
corrosion, but especially in industrial processes that involve production,
transport and crude oil refining. Atomic hydrogen may diffuse through a
metal lattice because of its smaller size, and then it can recombine to form
molecular hydrogen within structural defects such as inclusions, voids and
microcracks among other places. Hydrogen embrittlement occurs as a
result of hydrogen adsorption on the metal surface, with its consequent
absorption within the metal lattice. The irreversible hydrogen trapping into
metals lead to a progressive degradation of their mechanical properties
and increases the probability of cracking. The above described phenomena
could occur in a higher extent when the hydrogen sulphide concentration is
high in the aqueous environments where the metal is immersed [#ref1-5].
Corrosion inhibitors have been widely used to decrease hydrogen
demonstrated the effectiveness of several organic compounds to reduce
the corrosion rate and hydrogen absorption in metals exposed to sour
environments, inhibiting the hydrogen evolution reaction by means of
physical isolation of metal surface and/or modifying the electrical double
layer. Amines are well know by their inhibiting effect on the corrosion in
carbon and low alloy steels, specially in systems containing H2S. The
presence of nitrogen in the organic compound with unsaturated bonds
allows the adsorption of the organic compound on the metal surface,
therefore hindering the metal dissolution [#ref8]. However, it also have
been found that some organic compounds can effectively inhibit the
corrosion of metals but stimulate the hydrogen absorption within the metal
lattice, because the recombination reaction of atomic adsorbed hydrogen
on the metal surface is partially blocked [#ref9].
Ultrasonic inspection of metallic materials is an adequate technique
that could help to reduce and prevent catastrophic failures in pipelines
systems and industrial equipments. Conventional techniques have not
exhibited good detectability for hydrogen damage in steels, but very good
attenuation, backscattering signal, spectral analysis and ultrasonic velocity
change [#ref10,11]. The damage caused by hydrogen changes the way in
which ultrasonic waves are propagated. The presence of dissolved
hydrogen into the metal bulk may affect the elasticity modulus reducing
the longitudinal and shear ultrasonic waves.
The objective of this study was to investigate the inhibitive action of
hexamethylenetetramine on the corrosion and hydrogen permeation
behaviour of type API 5L-X52 steel, which is commonly used in the oil
industry for production and transportation of natural gas and crude oil.
The metallic material used in the present work was an API 5L grade
X52 steel. The chemical composition of the steel is presented in table 1.
Table 1. Chemical composition of the API 5L-X52 steel.
Concentration (wt %)
0.30 ± 0.01
0.004 ± 0.0001
The specimens were made from rectangular pieces of steel cut
lengthwise from an 11 mm thick pipeline. Some stripes were machined to 8
mm thickness and subsequently cold rolled to 1mm thickness in several
steps with two intermediate annealings in pure nitrogen atmosphere at
600ºC for 60 min. The reason for the heat treatments was to keep, as far
as possible, the original microstructure. For polarization studies, samples
measurements specimens of about 2cm x 5cm were employed. Another
group of samples was machined to 5mm in thickness. This group was used
for the ultrasonic evaluation performed after the hydrogen permeation
Polarization studies were performed according to ASTM G5-87
standard. A five electrodes corrosion cell was used. Steel specimens, with
an exposed area of 1cm2, were employed as working electrodes while a
saturated calomel electrode (SCE) and two platinum electrodes were used
as the reference electrode and counter electrodes respectively. The working
electrodes were mechanically polished with different grades of emery paper
(180, 240, 320, 400 and 600 grid), rinsed with double distilled water and
acetone and then dried. Prior to the experiment, the samples were
prepared by polishing with 1µm high purity alumina powder, degreased in
an ultrasonic bath with xylene, rinsed with acetone and dried quickly by
warm air blowing.
All tests were carried out in 1N H2SO4 solution prepared from
hexamethylenetetramine (HMT) was used at 3 different concentrations, that
is, 10-2, 10-3 and 10-4M. This was achieved by dissolving the appropriate
amount of solid HMT into the sulphuric acid solution. High purity nitrogen
was bubbled through the electrolyte prior and during the experiment in
order to keep the system free of dissolved oxygen. The Potentiodynamic
polarization curves were obtained using a potential scan rate of 10 mV/min
starting from 500mV below Ecorr and moving in the anodic direction to
200mV above the open circuit potential. A PC4 750 Gamry potentiostat was
used for polarization measurements. All experiments were performed at
Hydrogen Permeation Studies
In order to investigate the hydrogen permeation rate through type
API 5L-X52 steel, the electrochemical technique developed by DevanathanStachurski was employed. The experiments were performed in a specially
designed and built two compartment cell separated by the hydrogen
permeation test specimen. The electrolytes on the cathodic and anodic side
of the cell were 1N H2SO4 and 0.1M NaOH respectively. On the cathodic
side, the inhibitor (HMT) was added to the electrolytic solution prior to
filling the compartment, at the same concentrations stated before. Both
solutions were prepared from analytical grade reagents and double distilled
water. In order to reduce any possible electrolyte impurities, the solutions
were pre-electrolyzed in separate electrolytic cells by imposing a current
density of 3mA/cm2 during 3 hours. Additionally, the electrolytes were
deaerated with pure hydrogen gas before placing it into the permeation
cell. The transfer was made without any air contact.
The steel membranes were prepared, as described above, by
conventional metallographic grinding and polished using 1µm high purity
alumina powder. Specimen surfaces were rinsed with double-distilled water
and ultrasonically cleaned in xylene during 15 minutes, rinsed with acetone
and then dried by warm air blowing. Later on, just before the experiments,
the anodic side of the membrane was coated with electroless palladium
using a commercial solution (Pallamerse). An area of 0.9 cm2 of the steel
membrane was exposed to the solution.
The anodic side of the cell was potentiostatically maintained at
0.150mV vs. SCE using an EG&G PARC model 363 potentiostat/galvanostat
to ensure that the steel was in its passivation zone and to oxidize the
atomic hydrogen that diffused through the membrane. Prior to the
permeation experiment, the anodic side was left polarized at that potential
for 12 h. After that time has elapsed a typical passivation current of 300
nA/cm2 was obtained. Then, the cathodic compartment was filled with the
acid solution with or without the inhibitor and the potential was held
vs. SCE employing a Microstat model 1503
potenciostat. The anodic current was recorded as a function of time using a
DakBook k/216 data acquisition system. All the experiments were carried
out at room temperature.
Ultrasonic Evaluation and Surface Analysis
In order to detect the hydrogen damage into the steel, an ultrasonic
analysis was conducted on steel samples of 5mm thickness before and
after the hydrogen permeation tests. Thick specimens were selected for
this study since evaluation on thin specimens (1mm thick) produced noisy
signals. All the measurements were performed using a Krautkramer USM
22 ultrasonic equipment and a 10Mz frequency transducer model KBA525.
The attenuation coefficient α was calculated based on the first and second
backwall echoes and the test specimen thickness. On the other hand,
optical and scanning electron microscopy techniques were employed to
study the steel surface morphology after the hydrogen permeation tests,
and energy dispersive X-ray analysis was used to characterize some
inclusions observed in the metallic material.
Results and Discussion
The steel microstructure in the as-received condition, observed by
optical microscopy, was a typical banded ferrite/perlite structure (figure
1a). Elongated and rounded inclusions, mainly MnS, were observed; some
aluminates, silicates and calcium inclusions were also found within the
material. The cold rolled and annealed samples showed a microstructure
mainly comprised by ferrite with fine pearlite and fragmented spherodized
cementite (figure 1b).
Figure 1. Microstructure of the API 5L-X52 steel in: a) as-received condition, b) cold rolled
Figure 2 shows the potentiodynamic polarization curves of the
samples at different hexamethylenetetramine concentrations. As it can be
seen, the corrosion potential was only slightly affected as the concentration
of HMT in the electrolyte increased from 10-4M to 10-2M. Additionally, the
form of the curves is very similar either in the cathodic or in the anodic
side, which indicates that the mechanisms of iron dissolution and hydrogen
The main difference in the above mentioned curves is observed in
the current density values. It is clear from figure 2 that for the same
potential, the samples tested in presence of the organic compound showed
current density values, both anodic and cathodic, lower than those samples
tested without inhibitor. Moreover, for the same electrode potential, the
current densities decreased with increasing inhibitor concentration. This
indicates a reduction in the corrosion rate of the steel with increasing
inhibitor concentration. Apparently, adsorbed molecules of the organic
compound decrease the active surface area where both electrochemical
reactions take place, delaying the corrosion of the steel. Based on these
results, if the samples are polarized at a specific cathodic potential in
presence of inhibitor, the hydrogen atoms will be produced at a lower rate.
The corrosion inhibitor efficiency and cathodic inhibitor efficiency at –
900mV (SCE) were calculated from figure 2 and the results are presented in
table 2. As it can be noticed, the highest inhibition effect is attained at
concentrations of 10-3M of HMT. If the inhibitor is added to the electrolyte
at concentrations greater than this, the inhibition efficiency diminished. As
a result, the corrosion rate increases again. On the other hand, when the
potential value is above -350mV (SCE), the curves tend to be overlapped.
This is a clear indication that the organic compound is not efficient when
the steel potential is ennobled. This could be due to the repulsion between
the protonated organic compound and the increasingly positive metallic
surface. This result suggests that the organic molecules are adsorbed on
the steel surface mainly by electrostatic forces.
Figure 2. Potentiodynamic polarization curves for the steel in deareated
1N H2SO4 + HMT at 25ºC.
Table 2. Corrosion inhibitor efficiency.
E = Ecorr
Hydrogen Permeation Studies
The hydrogen permeation transients of the steel for the various
concentrations of hexamethylenetetramine were measured as a function of
time and the results are displayed in figure 3. As it can be seen from this
figure the curves followed a similar pattern, that is, an increase in the
permeation current density until a steady state is attained after several
hours of exposure. It can also be observed that the permeation current
density increases with the increase on the inhibitor concentration.
hexamethylenetetramine, showed permeation currents four times greater
than those of the samples tested without inhibitor. Opposite to what was
expected, the organic compound stimulated the hydrogen permeation,
even though less atomic hydrogen was produced on the steel surface. As a
result, more hydrogen damage such as blistering and cracking would be
expected in the samples tested in the inhibitor presence.
Figure 3. Hydrogen permeation curves for 1mm thick membranes
in deareated 1N H2SO4 + HMT at 25ºC.
Figure 4 shows the possible mechanism of hydrogen evolution and
Essentially, in absence of inhibitor, the hydrogen evolution reaction (HER)
involve the hydrogen ions discharge on the steel surface, followed by two
electrochemical recombination) and hydrogen absorption within the metal
lattice (figure 4a) [#ref12]. Nevertheless, if there are adsorbed molecules of
the organic compound in the steel surface, the recombination reaction of
electrochemical reduced hydrogen could be inhibited. Therefore, adsorbed
atomic hydrogen activity may increase and as a result more hydrogen
penetration is promoted into the steel (figure 4b) [#ref12].
The hydrogen diffusion coefficients were determined for the steel
membranes by four different methods [#ref6] and the results are presented
in table 3. The values obtained in this work are similar to those previously
obtained for similar steels by other researchers [#ref2,7]. Nevertheless,
diffusion coefficients values around one order of magnitude greater than
those have been reported for the same steel too [#ref14]. It could be
possible that the cold rolled and the annealing treatments have slightly
modified the hydrogen diffusivity on the steel.
Figure 4. Schematic representation of the HER: a) in the absence of HMT,
b) in the presence of HMT.
Table 3. Hydrogen diffusion coefficients attained by different analytical techniques for the
API 5L-X52 steel in deareated 1N H2SO4.
Laplace method: slope
Laplace method: intercept
The microstructural analysis made after the samples were exposed to
the 1N H2SO4 solution did not show any evidence of hydrogen damage,
neither at the surface nor into the metal. These results seem to indicate
that, under these conditions, the hydrogen concentration within the metal
was insufficient to promote the formation of blisters and cracks. However,
the plates exposed to the acid solution in the presence of inhibitor were
concentrations. Moreover, the higher the inhibitor concentration, the larger
the mechanical damage observed in the steel. Figure 5 shows several
micrographs obtained by scanning electron microscopy for the sample
tested with the highest inhibitor concentration. As it can be seen from
these figures, large blisters as long as 300µm, were seen on t he steel
surface. Most of them, as shown in figure 5a, are aligned in the cold rolled
direction. Figures 5b and 5c are another micrographs showing cracking
propagation around the blisters. This result is a clear indication of the high
hydrogen pressures generated inside the blisters. Figure 5d is a detail of a
crack growing on the steel surface. Cracks propagation seems to be
intergranular. Nevertheless, transgranular propagation of cracks was also
Figure 5. SEM micrographs of a sample exposed to the 1N H2SO4 +10-2M HMT solution.
a) general view. b,c) blisters details. d) crack details.
Micrographs of the sample exposed to the 1N H2SO4 + 10-4M HMT
solution are displayed in figure 6. As it can be noticed, long and
continuous cracks are located on the steel surface parallel to the rolling
plane. Moreover, a stepwise cracking pattern was observed (figure 6a). As
pointed out by Domizzi et al. [#ref15], this result could be related with a
more uniform distribution of MnS particles throughout the material. On the
other hand many cracks, like those presented in figure 6b, were also
associated to MnS inclusions. This finding is consistent with previous
investigations made on similar steels [#ref15].
Figure 6. SEM micrographs of a sample exposed to 1N H2SO4 +10-4M HMT.
a) microcracks developing near the largest crack. b) cracks detail.
Figure 7 shows the backwall echoes spectra for 5mm thickness
samples exposed to the 1N H2SO4 and 1N H2SO4 +10-2M HMT solutions for
over 24 hours. In addition, the backwall echoes spectrum for the samples
before the hydrogen permeation test is also presented in the same figure
as reference. All measurements were carried out using a 10MHz transducer
and a 46dB gain. As it can be noticed, the ultrasonic spectrum remains
unaltered after the samples were cathodically charged in 1N H2SO4 solution
(figure 7b). This could means that the backwall echoes have the same
amplitude than those echoes registered before the hydrogen permeation
test. This result suggests that the material under the experimental
conditions did not show any susceptibility to hydrogen induced cracking or
blistering, which is consistent with the results previously discussed for
similar specimens in the microstructural analysis. However, the ultrasonic
hexamethylenetetramine showed higher attenuation levels than that of the
samples exposed to the acid solution without inhibitor (figure 7c). Since
the samples evaluated with the highest concentration of inhibitor exhibited
the larger mechanical damage such as blisters and cracks, it could be
possible to relate these changes in the attenuation levels with the hydrogen
damage produced in the steel. Some researchers [#ref10,11] have
previously demonstrated that the backwall echoes spectrum changes with
hydrogen attack is due to the presence of extra of scatterers. Higher
attenuation levels are observed in attacked samples because hydrogen
damage can be considered itself as an additional scatterer. This is
especially certain at high ultrasonic frequencies.
Figure 7.Pulse-echo signal for steel samples: a) before hydrogen permeation test, b) after
testing in H2SO4 solution and c) after testing in H2SO4 + 10-2M HMT solution.
Attenuation measurements were taken on the samples at an
ultrasonic frequency of 10MHz and the results are presented in table 4. As
it can be seen, there are no appreciable changes in the sound wave
attenuation of those samples exposed to the 1N H2SO4 solution. This
means that the hydrogen effect within the metal is not large enough to
Nevertheless, for those samples exposed to the solution containing
significant increase in
attenuation coefficient was
registered. Once again, the obtained results allow drawing the conclusion
that the organic compound increases the steel susceptibility to hydrogen
Table 4. Attenuation coefficients measured with an ultrasonic frequency of 10MHz.
Before hydrogen permeation test
Tested in H2SO4 solution
Tested in H2SO4 + 10-2M HMT
coefficient α [dB/cm]
The hexamethylenetetramine inhibits efficiently the corrosion of the
steel but increases the quantity of absorbed hydrogen into the metal
lattice. The organic compound seems to have a specific influence on the
hydrogen evolution reaction, apparently diminishing the active sites were
recombination of atomic hydrogen to molecular hydrogen on the steel
In the absence of inhibitor, the hydrogen that diffused through the
metal did not induce considerable damages such as blisters or hydrogen
The susceptibility to hydrogen damage of the API 5L-X52 steel in
sulphuric acid increases in the presence of hexamethylenetetramine.
Moreover, the higher inhibitor concentration, the larger the mechanical
damage observed in the steel.
Ultrasonic analysis of the samples showed that the height of the
backwall echoes decrease and the sound wave attenuation coefficient is
greater when the quantity of hydrogen in the sample increases.
The authors wish to thank the Fondo Nacional de Ciencia, Tecnología
e Innovación, FONACYT, for the financial support to the USB-Proyecto
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