Understanding Corrosion Inhibition of Iron Carbonate Films Through Molecular Modeling

By S. Ramachandran, V. Jovancicevic and M.B. Ward

Introduction

Various test methods have been used to develop new technologies for corrosion inhibition, including flow loops, rotoclave and high-speed autoclave. Coupled with computerized models for fluid dynamics, these tests allow corrosion inhibitors to be matched with field conditions with confidence.

Without the use of computer modeling, the development of new inhibitors has been an iterative process that starts with an educated guess. Many possible molecules must be synthesized and screened to find ones that pass the bench tests, and these must be tested in the field to prove their worth.

With computer modeling, however, one has a way to predict the effectiveness of inhibitor models based on their molecular structure, opening the door to the discovery of new inhibitor chemistries and shortening the time and cost of development, to the benefit of both customer and suppliers.

Baker Petrolite has been working the past several years on developing such models. Last year, we reported on a molecular model that described how imidazolines work with Fe₃O₄ (magnetite) to prevent corrosion, and this year we expand the work to ferrous carbonate, FeCO₃, or siderite.

Corrosion products of mild steel

During the corrosion of mild steel in a carbon dioxide environment, FeCO₃, Fe₃O₄, and Fe₂O₃ can form as corrosion product films.¹ Several studies of the corrosion of mild steel in flowing carbon dioxide saturated solutions have identified iron carbonate as a major constituent.^2-4 Iron carbonate film is porous,² and films up to 80 mm thick have been obtained in conditions where uninhibited corrosion rates varied from 390 to 1580 mils per year.³ Iron carbonate is not conductive to electricity⁵, whereas both Fe₂O₃ and Fe₃O₄ are: Fe₂O₃ has an electrical resistance around 10⁶ ohm-cm,⁶ while Fe₃O₄ has an electrical resistance of only 0.005 ohm cm.⁷

Inhibitors work at active sites, either by blocking active cathodic or anodic reaction sites on the metal itself, or by incorporating in the corrosion product and preventing further access of the corrosive fluid.

Developing the model - surface binding energies

The basis of molecular modeling is the replication of molecular structures (obtained through x-ray and neutron diffraction studies and microwave spectroscopy), of vibrational modes (obtained through infrared and Raman spectroscopy) and bond energies (obtained by calorimetric studies and quantum mechanics).

The general structures of saturated straight chain imidazolines, amides and amines studied in this work are shown in Figure 1. The use of molecular modeling to study the adsorption of these molecules on Fe₃O₄ and Fe₂O₃ has been previously described,^8,9 and the current work extends those procedures to FeCO₃.

A model for Fe₃O₄

The first step was to determine the surface of the crystal structure that would be used as a model for inhibitor attachment. The most stable plane is the one that involves the breaking of the weakest (normally the longest) bonds. In Fe₃O₄, this is the 111 plane of the crystal.⁸

This 111 surface of Fe₃O₄ was used to model the binding energy of three simple small molecules that represent the functional binding groups of important corrosion inhibitors: methylamine, formamide, and 1,2-dimethyl-2-imidazoline, shown in Figure 1. The binding energies, shown in Table 1, indicate that all three of these molecules should prevent corrosion because all have higher binding energies than water.

A model for FeCO₃

The most stable plane for the FeCO₃ crystal is the 001 miller plane shown in Figure 2. The area of each iron site on this plane is quite a bit smaller than the 111 plane of Fe₃O₄ or the 001 plane of Fe₂O₃, as shown in Table 2. Would the same model for imidazoline surface bonding work?

FIGURE 2 - The (001) miller plane of FeCO₃ . The surface Fe ²⁺ sites are in white. The oxygen atom in CO₃^2- anions are in dark gray while the carbon atom in CO₃^2- is in a lighter shade of gray. The hexagonal spacing of surface Fe²⁺ sites is highlighted in the diagram.

TABLE 2: Area per iron site on different surfaces.

In the binding of a molecule to the surface there are two important considerations: (1) the area that a molecule occupies on the surface, and (2) the binding energy of the molecule to an iron site.

Nitrogen-containing compounds such as methylamine, formamide, 1,2-dimethyl-2-imidazoline and 1-aminoethyl-2-methyl-2-imidazoline are the simplest small molecules that represent binding functional groups of important corrosion inhibitors in oil field applications (e.g. imidazolines). These molecules are shown together with water in Figure 3 to illustrate the different sizes of the molecules. The van der Waals surface area shown in Figure 3 is the contact surface area between the inhibitor and the iron carbonate surface.

FIGURE 3 - The size and van der Waals surface area of atoms that point towards the surface of the FeCO₃ scale for water (A), methyl amine (B), formamide (C), 1,2-dimethyl-2-imidazoline (D) and 1-aminoethyl-2-methyl-2-imidazoline (E).

As stated earlier, binding energy is another important consideration in the ability of an inhibitor to bind to the surface of a FeCO₃ crystal. For this work, the binding energy of the four molecules shown in Table 3 also includes the effect of displacing a molecule of water from the surface, so the values shown in this table are not exactly analogous to those shown in Table 1. The main point, however, remains unchanged: These are strongly exothermic reactions, which means that all of these molecules can displace water from the surface, establish an anchoring point, and prevent corrosive species from transporting to the surface.

TABLE 3: Binding energy for four amine-based inhibitor molecules.

It is interesting to note that the size of the siderite iron unit and the inhibitor's van der Waals contact area do not match exactly, yet binding energy is substantial. This is due to the way the inhibitor packs on the surface of the siderite.

Packing of Imidazolines on Siderite

The top view of a packed layer of 1-aminoethyl-2-methyl-2-imidazoline on the (001) surface of FeCO₃ is shown in Figure 4. The model shows nine 1-aminoethyl-2-methyl-2-imidazoline molecules, of which eight are shown with cylinders while one is shown with a Corey-Pauling-Koltun (CPK) model that represents the space occupied by the van der Waals bulk of the molecule.

FIGURE 4 - Surface structure of 1-aminoethyl-2-methyl-2-imidazoline on siderite. The surface Fe ²⁺ sites are in black. A Corey-Pauling-Koltun (CPK) model is used to illustrate the space occupied by the van der Waals volume of one of the nine inhibitor molecules, while the others are shown with cylinders. The solid lines show the spacing of surface Fe²⁺ sites, while the dotted lines illustrate the spacing of imidazoline molecules. The distances between the sites are given in the figure.

When the inhibitor packs on the surface of the siderite, the once symmetric hexagonal pattern of Fe²⁺ sites shown in Figure 3 changes somewhat. The distances between the iron sites vary from 3.92 Å to 5.36 Å, and the spacing between imidazoline molecules varies from 7.99 Å to 8.22 Å.

Bilayer Structure of Imidazolines

Inhibitors can retard corrosion by strengthening the corrosion product layer, reducing its porosity and preventing its removal in conditions of flow; they can retard the transport of reactants to the surface; and they can interfere with anodic or cathodic corrosion reactions. Since FeCO₃ has a low electrical conductivity and is unlikely to be involved in anodic or cathodic reactions, the current work focuses on the first two mechanisms.

Hydrophilic polar groups are most likely to interact with water and with the hydrophilic surface of the substrate.⁸ We believe bilayer films act as the starting structure by which a multilayer film precipitates; hence the stability of such a bilayer structure on the siderite surface is important.

Table 4 shows the relationship of binding energy to the length of the imidazoline molecule's tail.

TABLE 4: Binding energy of a bilayer of imidazoline molecules on a 001 surface of FeCO₃. The longer the length of the molecule's tail, the stronger the binding force.

It is clear that the energy of bilayer film formation becomes more exothermic with alkyl chain length, meaning that longer chain inhibitor molecules form tougher films. Similar patterns were found in studies of the alkyl chain dependence of imidazolines on hematite and magnetite surfaces.^8,9

The most probable physical structure of the inhibitor film is shown schematically in Figure 5.

Correlation with Experimental Studies

To test the predictions of the computer model, experimental studies were conducted on the relationship between the alkyl chain-length of imidazoline inhibitors and their effectiveness in inhibiting CO₂ corrosion using rotating cylinder electrodes and linear polarization techniques.¹⁰ In these experiments, the minimum effective concentration to inhibit CO₂ corrosion was measured as a function of chain length. Figure 6 shows the minimum effective concentration to inhibit corrosion plotted against the number of carbon atoms in the tail chain.

FIGURE 6:

It is clear that experiments validate the hypothesis of the model.

Conclusions

Inhibitor bilayer film formation on FeCO₃ can increase the stability of the FeCO₃ film and reduce its porosity, thereby blocking access to the reacting surface. Our computer modeling studies indicates that imidazolines strongly adsorb to FeCO₃ and hence contribute to this mechanism, even though FeCO₃ is not electrically conductive and cannot be a source of anodic or cathodic reaction sites.

Previous work showed that imidazolines also adsorb strongly to Fe₃O₄, which results in direct blockage of anodic and cathodic reaction sites. The combination of both studies indicates that imidazoline inhibitors work both by reducing the porosity of electrically nonconductive FeCO₃ and blocking reaction sites by adsorbing to electrically conductive Fe₃O₄.

While molecular modeling work is still in the early stages of what can be done, it gives us clues about how inhibitors work and it helps relate laboratory performance to field performance. Ultimately, that should help us develop new inhibitor chemistries more scientifically, more quickly, and at lower cost.

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