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Microalloyed steel corrosion inhibition efficiency by a IMP ICCA 9710 imidazoline-type inhibitor under sour environment conditions

E. Valenzuela1, S. Serna 2, J. G. González-Rodríguez2, B. Campillo3, A. Torres2, J. Juárez-

Islas4

1Universidad Politécnica de Chiapas, Tuxtla Gutierrez, México; 2CIICAp-UAEM, Cuernavaca, México; 3Instituto de Ciencias Físicas-Facultad de Quimica-UNAM, México;

4Instituto de Investigaciones de Materiales-UNAM, D.F., México

Abstract Microalloyed steels have a relevant importance for sour gas transport in the oil and gas industry, new challenges for this industry are being pointed out due to the discovery of new wells with diverse corrosion environmental conditions and facilities that require improved mechanical and welding properties. As a result, an important demand is the continuously development of new grades of microalloyed pipeline steels that can resist the new or diverse corrosion environment conditions; here the attention is focused on the development of the X80 grade microalloyed steel. Variations of grade X80 microalloyed steel were obtained in the laboratory under thermomechanical and controlled cooled treatments. On the other hand the most economically way to prevent corrosion is by the application of corrosion inhibitors. However, the uncertainty is to know how often and what concentration the inhibitor could be effective to control corrosion, and to evaluate new kinds of inhibitors. A local laboratory developed a new kind of imidazoline (IMP ICCA 9710). The efficiency of the IMP ICCA 9710 inhibitor has been evaluated by using electrochemical impedance spectroscopy (EIS) technique under sour environment with different inhibitor concentrations at room temperature. The corrosion resistance of the samples was calculated as a function of the inhibitor concentration and also related to the different steels microstructures. These results indicate that the correlation of the microstructure and the inhibitor concentration determine the corrosion resistance of the steel, the application of the imidazoline type inhibitor and an adequate thermomechanical and cooling treatment can reduce the corrosion rate of the microalloyed steels in sour media. The results presented could impact in the application of new inhibition batch schedules in combination with the development of the best corrosion resistant microstructure for the same grade of microalloyed steel. Keywords: sour corrosion, organic inhibitor, steels microstructure, protective films 1. Introduction Moving wet gas from offshore production facilities for onshore treatment often is an economically attractive alternative instead of offshore drying and allows more flexibility in field development. Furthermore, in several situations offshore drying could not be considered as a feasible option thus making unrefined fluids transportation unavoidable. Produced fluids include CO2, H2S and acids that in combination with water make these media highly corrosive [1, 2]. For the corrosion control of offshore wet gas pipelines several options are available. The use of corrosion resistant alloys (CRAs) is a relatively new sound solution, although the costs of materials and the added laying time make this alternative unattractive for long distance, large diameter pipelines. The use of low carbon microalloyed steels for transportation of wet, corrosive gas offers potential savings over the more expensive alternatives, such as use of corrosion resistant alloys or gas drying, but with a higher risk.

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However, the use of microalloyed steels could involve high operating costs due to inhibition practices, inspection, and monitoring. The economic incentive for the use of a low carbon microalloyed pipeline steel needs to be evaluated for each specific project, as the advantages of the use CRAs and microalloyed steels can vary widely depending on external factors and project conditions. The general trend is that life cycle costs of microalloyed steels will be more attractive for long, large pipelines of relatively short life. The risk of failure needs to be added to any cost comparison between these two alternatives. Extensive investigation and on field experience with the operation of gas pipelines in corrosive service exists in literature of recent pipeline projects involving transportation of corrosive gas [3]. From these investigations it has been demonstrated that plain carbon steel pipelines can be safely operated in very corrosive service if the corrosion control system is properly designed and implemented. The corrosivity of the environment and the temperature of the fluids, can limit carbon steel use also as the flow velocity. These criteria could be extended to low carbon microalloyed alloys steels. The effect of temperature on corrosion has been extensively investigated. It is generally agreed that a 'critical temperature' exists that limits the applicability of corrosion inhibitors. Most investigations coincide that sour corrosion can be inhibited at temperatures below about 120 °C. Inhibition is also possible at higher temperatures, but then the inhibitor selection process and the design and operation of the inhibitor injection and the corrosion monitoring systems become extremely critical. However there is less agreement on how steels microstructures developed by different process relates to other factors such as inhibitor concentration, temperature, etc. Thus increasing or decreasing organic inhibitors efficiency. With actual technology corrosion control systems with 100% corrosion control efficiency can be designed and constructed. The cost of this approach, however, can be cost prohibited and it needs to be balanced against a reduction of corrosion risks. In most cases, pipeline design is based on a value of 95% corrosion control as the maximum achievable following normal equipment and operating procedures. The use of organic inhibitors could raise this efficiency. So, most of the discussion in this paper will focus on corrosion inhibition, which is one of the most common and versatile methods of corrosion control systems, by a new developed organic imidazoline inhibitor using Electrochemical Impedance Spectrum (EIS) technique for corrosion monitoring. An attempt was made to discuss the corrosion results in conjunction with different microstructures developed by distinct routes of fabrication that converges in an API X80 grade microalloyed pipeline steel. 2. Experimental Procedure 2.1 Development and characterization of the X80 steels X80 microalloyed steel chemical composition in weight percent was 0.044-C, 0.271-Si, 1.69-Mn, 0.0091-P, 0.0103-Cr, 0.240-Ni, 0.0010-V, 0.014-Ti, 0.2170-Cu, 0.25-Mo, 0.0310-Al, 0.0001-B, 0.0554-Nb, 0.0070-Sn, 0.055-As, 0.0018-Pb, 0.0037-Sb, 0.0016-Ca, 0.0040-N2 and balance Fe. Steel slab samples were heated at 1250ºC, soaked for 90 minutes and immediately hot rolled in a Fenn reversible mill. Rough rolling was followed by a cooling period until an experimental initial temperature for the final rolling procedure of 1051ºC was reached. A finishing total deformation of 37% was achieved in five passes ending at 867ºC. After the last final rolling pass, steel plates were cooled in forced nitrogen gas, accelerated cooling or water quench.

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Microstructure of resulting plates was observed under a scanning electron microscope Stereoscan 440 equipped with EDAX microanalys is. Thin foil specimens were prepared by a standard jet-polisher, and then examined with a JEOL-2000 EX transmission electron microscope (TEM). Flat tensile (ASTM E-8) tests were conducted on an Instron 1125 (10 ton) test machine. 2.2 Sour environment for electrochemical testing Sour environment consists of 5% NaCl on 1,700 mg/l of acetic acid (CH3COOH) and 3,530 mg/l of Na2S-9H2O within 1L of water to simulate a satured 500 mg/l of H2S solution reaching a pH between 3.5 and 4. The test solution was prepared with analytical reagents and deionised water. This test solution is in accordance with that reported in NACE Standard 24007 for inhibitor evaluation in sour environments for pipelines used in hydrocarbon transportation. The sour solution was also deareated with high purity nitrogen gas for 1 hour before the introduction of the steel working electrodes. 2.2 Electrochemical technique used for inhibitor evaluation and monitoring The EIS electrochemical technique was carried out with and without the presence of a IMP ICCA 9710 inhibitor. In the tests with inhibitor this was injected to the test solution immediately after the deareation process and introduction of the steels working electrodes. For each steels the following inhibitor concentrations were used: 0, 5, 10, 20, 50 and 100 ppm at a controlled temperature of 23 oC. For these tests a Satured Calomel reference electrode (SCE) and a graphite auxiliary electrode were used. The EIS tests were done with a Solartron SI-1287 potentiostat at the free corrosion potential (Ecorr) applying a 10 mV amplitude sine signal with a frequency interval between 10 kHz to 0.01 Hz. For determining the protective film inhibitor behaviour as a function of time for each inhibitor concentration, the measure of the Impedance was carried out within 60 minutes intervals during 6 hours. The charge transfer resistance was taken as a measure of the steels corrosion resistance with and without the inhibitor. The RCT values were obtained by adjusting the experimental data to equivalent electric circuits using the Z-VIEW software. 3. Results and Discussion 3.1 Steels microstructural and mechanical characterization The hot rolled plates which were water quenched (steel 1), showed a martensite microstructure as that presented in Fig. 1a. In martensite laths, it was observed the presence of precipitates which were identified as Nb(C, N). These precipitates showed an average size of 10 nm. The microstructure observed in specimens who were hot rolled plus cooled in forced nitrogen gas (steel 2) is shown in Fig. 1b and consisted mainly of ferrite grains. Apart from ferrite grains; it was possible to observe some patches of bainite growing from ferrite grain and subgrain boundaries. During SEM observations, it was detected the presence of TiN precipitates. TEM observation performed in these specimens showed the presence of small precipitates with a size of ∼20 nm, which were identified mainly as Nb(C,N). The microstructure observed in specimens hot rolled plus accelerated cooled (steel 3), consisted of ferrite plus bainite grains (Fig. 1c). Bainite developed form ferrite grain or subgrain boundaries. During TEM observations performed in these specimens, it was detected the presence of particles which were identified as TiN, Nb(C,N) and Mo2C.

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Figure 1, Steels microstructures obtained by different processing routes: a) Steel 1, b) Steel

2, and c) Steel 3 The steels resulting mechanical properties reached after cooling the plates in different media, achieved the specified goal strength of 80 kpsi of yield strength to accomplish the API specification for microalloyed steels for sour service. However, all the steels are within the limits required, the martensitic steel shows the lesser strength with 77 Kpsi of yield strength. On the other hand the controlled hot rolling procedures accompanied by the cooling of plates in forced nitrogen gas or by means of an accelerated cooling procedure, are above and equal to the reached target properties with 87 and 80 kpsi of yield strength respectively. 3.2 Steels sour corrosion Corrosion of low carbon microalloyed steels in presence of H2S dissolved in water depends on the molecular H2S dissociation, and the iron oxidation in the form of Fe++ [2]. H2S corrosion is strongly dependent on the surface films formed during the corrosion processes.

b

c

a

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The protectiveness, rate of formation/precipitation, and the stability of the film controls the corrosion rate and its nature. On Figures 2, 3 and 4 the steels Nyquist diagrams under sour environment without inhibitor are presented. These diagrams illustrate the corrosion behaviour of the steels from the EIS experimental data. Is common the observation of one or two semi-circles overlapped in the mid frequency zone. The intersection of these semicircles with the impedance real part axis, tell us about the magnitude of the charge transfer resistance (RCT). In this section the RCT serve to indicate the relative corrosion resistance of the steels microstructures without inhibitor. From these diagrams typical corrosion behaviour could be observed for all steels. Advancing from left to right of the diagram curves a small semicircle is formed at high frequencies that corresponds to the initial corrosion response of the steel to the sour environment. The high frequency semicircle is overlapped in the mid frequency zone with another curve that corresponds to the possible formation of a film in the steel surface that could be protective meanwhile the steel is being corroded, following the overall governing corrosion equation between iron and aqueous (aq) H2S:

Fe + H2S(aq) ? FeS + 2H

Figure 2 Impedance diagrams corresponding to steel 1 at different testing times without inhibitor

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Figure 3 Impedance diagrams corresponding to steel 2 at different testing times without inhibitor

Figure 4, Impedance diagrams corresponding to steel 3 at different testing periods of time without inhibitor

The possible formation of a protective layer is due to the almost uniform formation of a FeS corrosion product type. It is well known that under sour environments irreversible corrosion FeS products are formed, and could be present in different crystalline forms [4.] such as pirite (FeS2), pyrrhotite (Fe7S8), and kansite (Fe9S8). Corrosion products formed inside pipelines depends on operation conditions, for instance at low H2S concentrations, corrosion products can be a combination of pirite, troilite and small amounts of kansite. When pH values are between 3 and 4 or higher than 9, corrosion products will be composed by pirite and troilite. For pH values between 6.3 and 8.8 the corrosion products could be a

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combination of Kansite (major component), pirite and troilite. The presence of chlorides and carbon dioxide can change the characteristics of these corrosion products. Some researchers [5] establish that for pH ranges from 3 to 5, species like FeSH+ exists and this can form Mackinawite corrosion products according to the following reaction: FeSH+ k1 ? Fe(1-x) + xSH- + (1 - x)H+. For the environments studied, the pH was between 3.5 and 4, this suggests that corrosion products like, pirite, troilite and Mackinawite could be present. The formation of these corrosion products has the tendency to form protective films. Inhibitor film could be formed and anchored on top of these corrosion protective films, which, in turn, will depend, among other factors, upon the microstructure of the steel. Depending of the FeS crystalline form this corrosion product could be precipitated into the test solution, or protect the steel surface. Further to the corrosion products hydrogen could be evolved as a by-product of the corrosion reaction. At zero testing time where the steels immediately get in contact with the corrosion environment the steels curves present only one semicircle. These semicircle has a marked lower radii compared with the curves evaluated at grater periods of time. The reason of this behaviour is that the steels are not passivated at the beginning of the test, and only is observed the response of an interface formed by the metal surface and the test solution. The RCT for the three steels calculated from these semicircles is very similar falling between 15 and 17 O, thus indicating little difference on steels corrosion behaviour due to their different microstructures. Subsequently, conforming the period of time is increased, as already stated, a protective film is developed over the steels surfaces. So, the charge transfer between steel and sour environment is progressively restricted and the corrosion rate is being decreased. From the Nyquist diagrams at the corresponding steels elapsed times, a second semicircle is developed, this semicircle represent the passive film formation. Also, it can be observed that for steels 2 and 3 (Figs. 3 and 4 respectively), the second semicircle grows until reaches a RCT value of 20 O. For steel 1 this change is more dramatic reaching a radii equivalent to 60 O (Figure 2) for the RCT value. These results suggest that steel 1 form the most protective film as the time is running. Table 1 shows the RCT values obtained by simulating an equivalent electric circuit (Figure 8) from the in laboratory steels-media systems. These results are in good accordance to the steels experimental data. From these results we can say that steels microstructures has a decisive role on their corrosion and passivation behavior once the corrosion processes begin to actuate over them. As can be observed from Table 1 almost no difference exists between RCT for all steels during the first minutes of testing indicating null passivation. However, the corrosion resistant values begin to grow as time is elapsed, indicating a passive film formation. For steels 2 and 3 stabilization was achieved at relative low RCT values between 17 and 33 O. On the other hand steel 1 shows a dramatically improvement of their RCT value much more higher than steels 2 and 3 reaching 120 O after 360 minutes of immersion. Table 1 RCT values comparison between steels without inhibitor at different periods of time

Steel RCT (O)

0 min

30 min

120 min

240 min

360 min

1 9 25 85 103 120 2 8 9 10 19 33 3 8 9 15 17 17

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3.3 Effect of inhibitor addition over steels corrosion behavior In Figures 5, 6 and 7 the steels behavior could be observed when inhibitor ICCA 9710 is added in different concentrations to the test solution. Dealing with steel 2 with an inhibitor concentration of 2 ppm (Figure 5a), the increment of their corrosion resistance represented by the RCT at the very beginning period of time (time=0) is considerably better than its counterpart test without inhibitor, with an RCT value of 500 O, at this inhibitor concentration the increase was of two orders of magnitude. When monitoring and registering the Nyquist curves at longer periods of time for the same steel, it could be observed that RCT is incremented as a function of time elapsed, reaching a maximum value of 6,000 O after 360 minutes of testing. Increasing the inhibitor concentration to the test solution resulted in higher corrosion resistant of steel 2 until 50 ppm is reached (Figure 5b) obtaining RCT values at time zero of 4800 O that is threefold grater than steel RCT value without inhibitor. In the Nyquist diagram at 50 ppm for steel 2 two semicircles overlapped are present in the middle frequency zone at 2 Khz. The passivation behaviour of the steel for preventing corrosion is indicated by the presence of a semicircle at high frequencies between (10 khz and 4 Khz). However, the gross protection against corrosion is due to the formation of an inhibitor film, presenting a RCT value as high as 15,000 O at the end of the monitoring period.

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Figure 5, Impedance Nyquist diagrams corresponding to steel 2 at different inhibitor concentrations: a) 5 ppm y b) 50 ppm.

On Figure 6 the results corresponding to steel 1 in the presence of the ICCA 9710 inhibitor are illustrated. As in steel 2 the addition of 5 until 50 ppm of inhibitor reached considerable their corresponding RCT values as compared with the tests without inhibitor. For 5 ppm at cero time the RCT value was of 6,500 O, increasing monitoring time the inhibitor protective film becomes more resistive. Meanwhile at 50 ppm the corrosion resistant of steel 1 is notable improved by inhibitor addition with a corrosion resistance value of 18,000 O in 360 minutes. In both cases of inhibitor concentration for steels 1 and 2, a clear tendency of corrosion rate diminishing in function of time is clearly observed. This behaviour apparently could be attributed at a consequence of the reinforcement of the inhibitor protective film created on

a

b

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steels surface conforming testing time is advancing. However beyond 50 ppm of inhibitor these effect disappears and the inhibitor is no longer efficient to reduce steels corrosion rates.

Figure 6, Impedance diagrams corresponding to steel 1 at different inhibitor concentrations: a) 5 ppm y b) 50 ppm.

a

b

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Figure 7, Impedance diagrams corresponding to steel 3 at different inhibitor concentrations: a) 5 ppm y b) 50 ppm.

Figure 7a shows the performance of steel 3 at 5 ppm of inhibitor concentration. From this figure typical corrosion behaviour is observed in this steel, with semicircles incrementing their radii meanwhile immersion time is also incremented, any second semicircle is formed. When inhibitor concentration was incremented to 50 ppm, (Figure 7b) the results differ in the formation of a second semicircle, located in the high frequency zone. These semicircle corresponds to the anticorrosive steel response (passivation), that when added to the protective inhibitor film effect, increment the corrosion resistance of the steel.

a

b

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Table 2 shows the maximum RCT values for steels assessed in the presence of different ICCA 9710 inhibitor and selected times of immersion. Table 2. RCT values comparison between steels with different inhibitor concentrations and

assed periods of time. 0 min 360 min Steel

RCT (O) 5 ppm 50 ppm 5 ppm 50 ppm 1 9,000 9,000 25,000 18,000 2 500 4,800 6,000 15,000 3 500 8,000 5,000 14,000

From this table, steel 3 presents the major corrosion susceptibility obtaining a maximum RCT value of 14,000 O at 50 ppm of inhibitor and 360 minutes of immersion. For steel 2 again the maximum value of corrosion resistance was achieved with 50 ppm of inhibitor concentration at the end of the period of test time. In both cases corrosion resistance depends directly from inhibitor concentration and immersion time. A different history applies to steel 1 for which only 5 ppm of inhibitor was needed for get a maximum corrosion resistance that was also the highest for all steels tested (25,000 O). Also, this result show that is unnecessary to increment inhibitor concentration, apparently the major efficiency is achieved at 5 ppm. 3.4 Electrical equivalent circuits for steels corrosion simulation For steels in conditions where is not applied any inhibitor concentration the impedance experimental data was simulated using the equivalent circuit f Figure 8. The electric equivalent of an interface metal-solution in which there not exist any electron transference and can occur over a wide potential range is symbolized only by a capacitor. However, electrochemical reactions with charge transfer happen through the interface, known as charge transfer currents, or faradic currents. The interface acts as an electron transfer barrier from or into surface metal. This can be represented by a resistance, called charge transfer resistance (RCT). This resistance is not only an ohmic resistance, because only is constant in a limited range of potential. Thus the representative electrical circuit of this interface could be elaborated combining in parallel a double layer capacitance (CDL) and a RCT like is shown in Figure 4.10. Rs is the ohmic resistance of the solution, it has to be noted that this equivalent circuit represents and interface that indicate the electrode (steel) corrosion. The electrical circuit equivalent to an interface possible covered by a protective film is quite more complicated, as discussed below.

Figure 8 Equivalent electric circuit to simulate metal solution interface of steels without the presence of inhibitor

The Figure 9a equivalent electrical circuit was the best fitted to steels 2 and 3 corrosion behaviour alter inhibitor additions. Rf and CPEf represents the resistance and the capacitance

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of an inhibitor film formed over a surface, CPEd is the interface capacitance. This suggests that a film is formed in these steels after inhibitor addition.

Figure 9 Equivalent electric circuits used to simulate experimental impedance data, under

the presence of inhibitor: a) For steels 2 and 3, b) For steel 1. For steel 1 the electrical equivalent circuit used to adjust the experimental impedance data is shown on Figura 4.9b. In this case, is important to note that a Warburg (W) type impedance is present on the circuit. This is indicative that the corrosion process in this steel is under diffusion control. However after 5 ppm of inhibitor concentration apparently the protective film is progressively destroyed, due that experimental data after this concentration is being adjusted to the equivalent electrical circuit of Figure 8 again after some elapsed time. 3.5 Effect of steels microstructure on inhibitor protective film formation. Inhibition is the most cost effective and flexible means of corrosion control in the oil and gas production industry. In the mid-1940s, long-chain polar compounds were shown to have inhibitive properties. This discovery dramatically altered the inhibitor practices on primary production oil wells and gas wells. It permitted operation of wells that, because of the corrosivity and volume of water produced along with the hydrocarbons, would not have been used due to economics. Perhaps entire reservoirs would have been abandoned because of the high cost of corrosion. Inhibitors also allowed the injection and production of high volumes of corrosive water resulting from the secondary-recovery concept of waterflooding. Organic inhibitors based on Nitrogen, like imidazolines has been utilized successfully even though the protective mechanisms over metal surface is still uncertain and under debate [6]. The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solution interface. Three possible types of adsorption are associated with organic inhibitors: p-bond orbital adsorption, electrostatic adsorption, and chemisorption [7]. It has been reported that inhibitor adsorption depends on molecular physico-chemical properties related to their functional group. Also it have been suggested that adsorption depends on possible steric effects and to the electronic density of donate atoms. More recently, adsorption is attributed to the inhibitor p-orbital interaction with d-orbital of superficial atoms which can induce a grate affinity to inhibitor molecules at metals surface, thus inducing the formation of a protective film against corrosion [8]. There are different approaches to inhibitor application such as continuous treatment and batch treatment. Continuous treatment is a technique which simply injects inhibitor into oil or gas well fluid and keeps a certain inhibitor concentration in the fluid for preventing corrosion. However, in deep wells and in three phase gas pipelines batch treatment techniques are often used because of the difficulty of injecting an inhibitor continuously downhole and for the inhibitor to reach the top wall of the gas flowlines. Here a problem arises as to how often the

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batch treatment should be repeated. This is a question which is frequently asked everywhere batch treatment inhibitors are used. Inhibitor film persistency evaluation is essential for determining how often the batch treatment must be repeated and for optimising the batch treatment procedures. However, the current technology for assessing inhibitor film persistency is a subject of much discussion and, in many cases, disagreement [9, 10]. EIS has been shown to be a useful technique for studying inhibitor film formation and destruction processes and for evaluating inhibitor film persistency [11, 12]. Nevertheless, the interaction of inhibitor molecular structure and the influence of microstructure over film formation are rarely studied. The tendency of the studies over inhibitor film formation has been focused on the dependence this film to the corrosion products layer previously formed, for instance, Rosenfield et al. [13], has been observed that inhibitors incorporate to corrosion product layer and forms a protective barrier between the base metal and the corrosive environment. French et al [14] present results showing that the structure of the corrosion products is modified by the inhibitor presence. They have been suggested that inhibitor structure has to be appropriated for their interaction with the corrosion products. Furthermore they report that imidazolines can be effective over iron sulphides and carbonates, but not over their oxides. Within the systems under study an inhibitor film will form and get attached to a FeS corrosion layer. In carbon steels after application of different heat treatments showed that either corrosion rates as well the ability of protective film formation decreases as the temper temperature was increased. The results of these experiments indicate that the microstructures generated play an important role in the development of protective films against corrosion [15]. The microstructure attachment effect it’s believed to become from the size and distribution of carbides dispersed on steels microstructures as a principal factor that influences film adhesion over steel surface. Thus the inhibitor film stability would depend of the adhesion ability of the formed corrosion products. Dugstad [16], working with low carbon steels with different heat treatments, found that the corrosion film adhesion was related to the carbides anchorage effect present on steel surface. Also, from this study the quenched steel, with no carbides, and the steel with any heat treatment, with carbides of major size, presented the more elevated corrosion rates. As described on the microstructural characterization part, steels under study have different microstructures. Steels also presents fine carbides in the order of nanometers like those showed on Figure 10.

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Figure 10 Nano-carbides distribution observed on the different steels microstructures: a) Steel 1, b) Steel 2 c) Steel 3

In good agreement with the electrochemical results, Figure 10a shows the greater number of fine carbides homogenously distributed within martensite laths and in some retained austenite zones that corresponds to steel 1. This steel presents the bigger RCT values when inhibitor is added. On the other hand Steel 3 (Figure 10c) has the bigger carbides poorly distributed along grain boundaries, being the steel that present the lowest corrosion resistant although inhibitor addition. For steel 2 fine carbides are principally homogeneously distributed within grain boundaries and a few of them in grain boundaries (Fig. 10b) and present a relatively same electrochemical behaviour as steel 3. Until know exist uncertainty why corrosion rates tend to be higher when big carbide particles are present on the steel. However a good explanation can be that when small carbides, they distribute more uniformly and a major number of particles could be present. As a consequence the number of anchorage sites is increased to form a protective film. 4. Conclusions 1. The preliminary results on the steels without inhibitor additions shows that the corrosion resistance represented by RCT values is being incremented as a function of immersion time. These suggest that the resistive corrosion products layer developed on steels surfaces, progressively increment their protective capacity while time is running. 2. In general all steels under study incremented considerable their corrosion resistance in the presence of the ICCA 9710 inhibitor by three orders of magnitude. Additionally was observed that inhibitor concentration influences directly their protective capacity. This conclusion is

a b

c

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strictly valid for steels 2 and 3, where an augment of the RCT was proportional to inhibitor concentration raise in the test solution. However, for steel 1 the maximum protective efficiency was reached at 5 ppm after 360 minutes of test. 3. Steel 1 microstructure plays an important role in the corrosion resistance of this steel with or without the presence of the inhibitor. Either with or without inhibitor for steels 2 and 3 their corrosion behaviour shows few differences (maintain the same tendency). The major corrosion resistance exhibited by the martensitic microstructure is principally due at the homogeneously distribution of very fine carbides between martensite laths as well as in retained austenite. 4. For a batch inhibitor treatment with the ICCA 9710 organic inhibitor among all steels, the martensitic steel (steel 1) under sour environments proved to be the more efficient and economic. It could be stated that this type of microstructure on conjunction with fine carbides suitably distributed, it is not necessary to use more than 5 ppm of inhibitor concentration to reach the optimum corrosion protection, being too much bigger as compared with the other two steels under study. Acknowledgements The authors thank the technical support of I. Puente, O. Flores and R. Guardian for the realization of this work. References

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2. A. Fragiel, S. Serna, R. Pérez International Journal of Hydrogen Energy 30 (2005) p. 1303

3. S. D. Kapusta and B. F. M. Pots, “The Materials and Corrosion View of Wet Gas Transportation”, in: Advances in Corrosion Control and Materials in Oil and Gas Production Edited by P. S. Jackman and L. M. Smith European Federation of Corrosion Publications 26, 1998, p. 5

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Persistent Inhibitors for Oilfield applications’, NACE Publication 1D182, item No. 54238, 1982

10. Y. J. Tan, S. Bailey and B. Kinsella, An investigation of the formation and destruction of corrosion inhibitor films using electrochemical impedance spectroscopy (EIS) Corrosion Science, 38, 1996, pp. 1545-1561

11. Huey J. Chen, ‘Evaluation of Oilfield Corrosion Inhibitors by EIS’, Corrosion 94, paper 92, NACE (1994)

12. J. L. Dawson, A. N. Rothwell et al, ‘Electrocherrical measurements for inhibitor assessments’ Corrosion 93, paper 108, NACE (1993)

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13. I. L. Rosenfield, D.B. Bogomolov, A.E. Gorodetskii, L.P. Kazanszkii, L.V. Frolova, L.I. Shamova, Z. Metallakd 18, 1982, p. 163.

14. E. C. French, R. L. Martin, J.A. Dougherty, Materials Performance 28, 1989, p. 46. 15. G.B. Chitwood, W.R. Coyle, R.L.Hilts, A Case-Hystory Analysis of Using Plain

Carbon and Low Alloy Steels for Completion Equipment in CO2 Service CORROSIION/94, Paper 90, (Houston, TX, NACE, 1994).

16. A. Dugstad, H. Hemmer, M.Sefesten, Effect of Steel Microstructure upon Corrosion Rate and Protective Iron Carbonate Film Formation CORROSIION/2000, Paper 23, (Houston, TX, NACE, 2000).