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The Effect of Corrosion Inhibitor Films on Deposition and Adhesion of
Paraffin Wax to Metal Surfaces
Final Report
1
Summary
Molecular dynamics simulations have been used to study the deposition of alkane waxes onto
representative oil pipeline surfaces, and to determine how the deposition process is modified
in the presence of a corrosion inhibitor coating. It is shown that the oxide films found on
pipeline surfaces do not act as nucleation sites for wax growth. On the other hand, the
additives often used to form a protective film against corrosion of the pipeline were shown to
promote deposition of long-chain alkanes in a manner likely to seed wax formation. The use
of molecular simulation has been shown to be a viable and powerful tool for gathering
mechanistic information about the interplay between corrosion prevention and wax
formation. Such mechanistic information is vital for developing multipurpose additive blends
to control the plethora of undesirable deposition processes that can occur from oil.
2
Background/Context
The industrial cost involved in preventing problems such as corrosion, scale, wax and hydrate
deposition during oil and gas transport is high, and so there is a considerable drive from the oil
and gas industry to find cheap and effective additives to control these problems. Protecting
pipelines from all these effects simultaneously, however, is an extremely complex problem.1
Most research has therefore been targeted at studying these processes independently, with
separate research programmes to develop scale inhibitors, wax inhibitors, corrosion inhibitors
or hydrate inhibitors. In each case there has been a focus on developing surface-active
materials, usually in the form of polymeric or self-assembled monolayer coatings. The target
for these coatings may be the pipe itself, as in the case of corrosion and scale inhibition; or the
surface of the growing crystals, as in the case of hydrate and wax deposition.
There has been considerable success in developing the separate inhibitors, and well
tested lead compounds exist for corrosion,2 scale,3 wax4 and hydrate inhibition.5 However,
there is little or no understanding of the interplay between the different inhibition and
deposition processes. It is clear that such interplay does occur. It has been found, for example,
that threshold inhibitors for hydrate formation do not work effectively unless a corrosion
inhibitor is also present.6 The interaction is not always synergistic and the balance between
competitive and synergistic effects can be difficult to predict. A good example of this is the
interdependence of corrosion and wax inhibition. Wax formation has been found to enhance the
effectiveness of corrosion inhibitors (CIs), with indications that the wax can form an additional
protective coating.6 Thus the use of wax inhibitors might be expected to reduce the efficacy of
Langmuir film corrosion inhibitors. On the other hand, the paraffinic nature of common CIs7
may actively promote deposition of hydrocarbons onto the surface of the CI film. Without an
understanding of the molecular mechanisms underlying this interplay, it will not be possible to
design wax or corrosion inhibitors that will not exacerbate the complementary problem.
Prior to this project there was little information about the phenomena involved in this
competition between corrosion inhibition and wax deposition, and in particular there was no
molecular-level information that could be used to guide further inhibitor development. The
purpose of this project was to show that molecular simulation techniques could be used to
redress this situation, providing essential mechanistic information about the growth of long
chain alkane films on pipeline surfaces that had been coated with a corrosion inhibitor.
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Key Advances and Supporting Methodology
AIMS AND OBJECTIVES
As outlined in the original proposal, the central aim of the project was a proof of concept: to
verify that there is a significant interplay between corrosion inhibitors and wax deposition
(two distinct problems in oil and gas transportation), and thereafter to prove that molecular
simulation is an effective means of obtaining valuable information about this interaction.
This aim has been achieved in full. Realistic simulations of a system as complex as a
metal oxide surface, coated with oleic imidazoline and under a heptane or octacosane liquid
have been shown to be viable, and the results clearly show that an ordered alkane layer forms
around the CI layer, presenting a surface similar to the (001) surface of alkane crystals. The
simulations have been analysed to produce considerable information about the molecular
behaviour associated with the wax / CI interplay; much of this information would not be
accessible to other techniques. The success of this “proof of concept” can be seen from the
interest shown by industry, as detailed in section 5.
In addition to this central aim, the proposal identified several specific goals. These
have also been achieved in full, as detailed hereafter.
• Molecular simulation of wax formation on iron oxide surfaces, and on oxide surfaces
protected by a film of corrosion inhibitor, have been shown to be viable.
• Simulations have confirmed that the presence of a layer of oleic imidazoline inhibitors
(taken as a prototype for many other corrosion inhibitors) does promote the formation of
an ordered layer of long-chain alkanes at the pipe surface, and with a structure that is
compatible with paraffin waxes.
• The influence of the CI film on paraffin deposition was found to be influenced by factors
such as the relative chain length of the alkane and the CI tail, the surface coverage of the
CI layer, and the tilt angle of the CI tail.
• Most importantly, the proof of concept — that molecular simulations can be used
advantageously to study the interplay between the different deposition / inhibition
processes associated with oil and gas transport — has been established.
DETAILED RESULTS
Surface Models
One of the concerns in the proposal was to identify appropriate models for pipeline surfaces.
Pipelines are typically made of mild steel, and it is known that under common operating
conditions and oxide film forms on the iron surface; this oxide film mostly presents a
hematite (α-Fe2O3) structure. It has been observed that the cationic termination of the (0001)
surface and anionic termination of the ( 1012 ) surface dominate the morphology of hematite,
and so these have been used as the primary model surfaces for this project.
Force Fields
Force fields for this project were adapted from existing potentials, as no suitable combination
of potentials was available in the literature. The oxide surface was described using Paulingtype potentials, with the parameters adapted to reproduce the relaxation of the (0001) surface
reported in the literature;8 no experimental measurements are available for these surfaces.
A united atom model (UA) was used to describe the paraffin interactions. Parameters
were based on the OPLS CHARMm potentials, but refined to reproduce the major features of
the radial distribution functions from C20 and C36 crystals (taken from the Cambridge
crystallographic database). The main effect of this refinement was to increase the C–C–C
bond angle to about 112°.
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Oleic Imidazoline was used throughout the project as a suitable model corrosion
inhibitor (CI).7 This was modelled in a hybrid manner. The alkane tail was modelled using
the same UA potential used for the alkanes, while the headgroup was described using a fully
atomic potential based on CHARMm parameters, but modified to reproduce the bonding
distances of Ramachandran et al.9 This gave both a suitably detailed description of the metal
surface – CI interactions and a consistent description of the paraffin interactions.
Wax Deposition on Metal Oxide Surfaces
Waxes are microcrystalline aggregates of alkane molecules, consisting predominantly of nalkanes with chain lengths typically in excess of 20. Accurate crystal data has only recently
become available for paraffin waxes,10 but this data indicates that the structure of the waxes
contains a very similar lamella structure to that found in pure paraffin crystals.11 We have
therefore modelled wax deposition using a pure component system based on octacosane
(C28). Further modelling has also been done with both dodecane (C12) and heptane (C7).
Single alkane adsorption sites on the two oxide surfaces were determined: the alkane
was found to lie flat to the surface, fitting between the rows of protruding Fe (0001) or O
( 1012 ) atoms. The adsorption energy was more favourable on the (0001) surface, but both
were less favourable than corresponding deposition onto wax. This suggests that these
surfaces will not readily act as nucleation sites for wax formation.
Various protocols were developed for simulating wax growth. At one extreme these
involved an iterative cycle in which individual alkane molecules were added to the surface,
and then annealed using molecular dynamics (MD) simulations at a range of temperatures. At
the other extreme, idealised layers of wax molecules were constructed about 10 Å above the
surface and allowed to drop onto the surface using finite temperature MD. The first method
should overstate the number of defects in the resulting crystal, while the second should
understate the defects; in practice they were found to give consistent results. A similar
approach was found to give a good model of wax growth in the absence of the oxide
surface.12
The results of these MD simulations showed that the alkanes tended to cluster in
lamellar sheets with structures similar to those seen on the (100) and (110) surfaces of C28
crystals (which are the main growth surfaces for long chain alkane crystals), but differing in
the orientation of the alkane about its long axis. On the (0001) oxide surface the alkanes tilted
in a very similar manner to that found in the wax crystals, and this was reproduced in
subsequent layers grown in the simulations (up to 8 layers were grown). On the other hand,
the alkanes lay flat to the ( 1012 ) surface, with consequent addition of more layers resulted in
curvature and reconstruction of the surface.
An energetic analysis of these systems confirmed that wax deposition was not
activated by either Fe2O3 surface, but that, if it does occur, deposition is more likely to occur
on the (0001) oxide surface where it generates a structure nearly commensurate with the bulk
wax crystal structure, and with a surface similar to the main growth surface of wax crystals.
Formation of Corrosion Inhibitor Films
Oleic imidazoline (OI) molecules were used as model CIs for our simulations. They were
found to adsorb onto the surface by means of two strong interactions: (i) the aliphatic
nitrogen in the ring bound to a surface iron atom, and (ii) the nitrogen in the amine group
bound to a neighbouring iron. For an isolated OI the alkane tail was then found to lie along
the surface, but for surface coverages more akin to those used in real corrosion inhibition
applications (ca. 66 Å2 / molecule13) the tails were found to lift off the oxide surface. In
vacuum studies, the stable tilt angle of the tail from the surface normal was found to depend
on the initial configuration used, but was typically 65° — i.e. nearly flat to the surface. When
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an alkane fluid was also present, however, (see below) this changed to be consistently in the
range 20–30° — i.e. almost perpendicular to the surface.
Simulations were performed on a range of different surface coverages, on both
surfaces, and for temperatures in the range 2–300 K. In every case, once the monolayer
formed, the structure of the headgroup on the oxide surface was found to be very stable. The
main variations were found in the structure of the OI tails, as just described.
Wax Deposition on Corrosion Inhibitors
The major part of this project has involved an extensive set of MD simulations to study the
behaviour of an oxide surface, coated with OI as a corrosion inhibitor, and covered by liquid
hydrocarbon. Both heptane and octacosane were used for the liquid hydrocarbon. New
protocols and analysis tools were developed to perform and study these systems.
Simulations of 300–1000 ps were performed to ensure that a steady state / equilibrium
structure was achieved. Several different starting configurations were also used in this study,
varying in factors such as the initial average angle of the OI tail, and the extent of order /
disorder in the alkane fluid phase. In particular, some simulations were started with the fluid
pre-aligned to fit with the OI tails, while other simulations used a completely random
arrangement taken from a simulation of the neat liquid. For a corrosion inhibitor coverage of
66 Å2/molecule there was a remarkable consistency in the steady-state achieved in all this
simulations, with the long axis of the OI tails tending to adopt an angle of ca. 20–30° to the
surface normal.
When heptane was used as the solvent, the solvent was found to fill the space between
the tails and stabilise the resulting OI film, but no significant order was found in the heptane
above the OI film. When octacosane was present, however, it was found to form an ordered
layer that interpenetrated with the OI film, but that extended considerably above the film (by
ca. 20 Å). The orientation of the C28 was about 20° to the surface normal, which is slightly
smaller than for the OI tail. This arrangement was found both with the ordered and the
disordered C28 liquid starting configurations. The resulting structure is akin to that found in
wax crystals, but showing the (001) wax surface rather than the (100) surface found for
deposition on the bare oxide surface. We note that one of the more efficient growth
mechanisms for paraffin crystals is spiral growth from a screw dislocation on the (001)
surface.14
Typical “snapshot” of the interfacial region for oxide + OI under C28 fluid
(left) and C7 fluid (right). The tails of the OI have been coloured black.
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Analysis of the interaction energies confirmed that the minimum energy arrangement
for these oxide/OI/C28 interfaces did occur when the OI tails were aligned perpendicular to
the surface, with the C28 oriented in the same direction and penetrated into the OI layer.
Conclusions
This project has been successful in showing that realistic MD simulations designed to study
the interaction between wax formation and corrosion inhibitors are now viable, and that such
simulations produce important mechanistic information that is not available from other
techniques.
The project has also confirmed that the presence of a protective corrosion inhibitor
film, such as that formed from oleic imidazoline, does generate an ordered layer of long chain
alkane molecules with a structure akin to that found in the bulk alkane crystal. While such an
alkane layer will increase the efficiency of corrosion inhibition, it is also likely to act as a
nucleation site for wax deposition.
4
Project Plan Review
The project has proceeded substantially as outlined in the original application. Simulations of
hydrocarbon molecules and films on several different metal oxide surfaces have been
performed at different temperatures. Similarly, studies of films of oleic imidazoline corrosion
inhibitor (CI) on the same metal oxide surfaces have been performed. Finally, extensive
simulations of inhibited surfaces under several different alkane surfaces have been
performed.
The main variation from the original plan has been that balance of the project shifted
to stage iii (i.e. simulations of the metal/CI + alkane fluid) and away from stage ii (metal/CI +
wax). This change was necessary because the structure of the CI film was found to be
affected strongly by the presence of a hydrocarbon fluid, and so realistic simulations were not
possible without the fluid phase. The inclusion of the fluid made the calculations substantially
more CPU-intensive, and so by expanding stage iii it became necessary to omit the shear
simulations originally proposed as part of stage ii.
5
Research Impact and Benefits to Society
The ability to control both wax deposition and corrosion in oil pipelines is important for oil
and gas transport, and oil companies currently have large capital and operating costs
associated with trying to alleviate the problems they cause. Given the interplay between the
two phenomena, this is a very complex problem, and a complete solution will only result
from the synthesis of a large number of different research studies. The aim of this project was
not to solve this problem, but to validate and develop a major tool — molecular simulations
of very complex systems — that will help to solve this and similar problems.
The success of this project in achieving this aim, and the impact of the results on the
oil and gas industry, can best be seen from the fact that the Instituto Mexicano del Petroleo
(IMP) invited the PI to Mexico City to consult on the wax / corrosion problem, and that both
the IMP and Lubrizol have begun discussions with the PI with a view to placing contracts for
further modelling work on oil-related problems. This will supplement the long standing
collaboration the PI already has with BP-Amoco (which was extended with a new research
contract during the course of this grant), and a contract with another oil-related company
(Cabot Specialty Fluids, U.S.A.) which the PI won partly due to the expertise demonstrated
on this ROPA project.
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6
Dissemination Activities and Further Research
The results of this work have been disseminated in a number of different ways. In part this
has been through standard scientific channels such as publication in primary journals
(including RSC and ACS journals) and presentation at conferences (including annual CCP5
conferences — the natural forum for condensed phase modelling in the UK — and the 5th
World Congress of Theoretically Oriented Chemists).
In addition, there has been considerable effort to disseminate the results of this project
directly to the oil industry. This has been achieved by personal communication, offers of
presentations, and mailing publications and reports to appropriate contacts. Specific activities
have included discussions with Lubrizol (both US and UK research groups), a presentation to
seven different oil companies involved in the Wax Attack joint industry venture, extended
discussions with BP-Amoco (Sunbury-on-Thames), and a presentation and discussions with
the IMP.
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