RU2230829C1 - Cathodic protection method against corrosion of metals in liquid media - Google Patents

Abstract

FIELD: corrosion protection.

SUBSTANCE: invention relates to controlled voltage-involving methods for protection of metals against corrosion, which may be used in a variety of areas including oil production. Preliminarily, one determines, under laboratory conditions, ability of sacrificial (consumable) metal to corrode more intensively than metal to be protected from data obtained by measuring non-biological and biological constituents of corrosion process on metal being protected and sacrificial metal. Required protective voltage is found from suppression of both constituents of corrosion process on the metal being protected. Non-biological and biological constituents of corrosion process on both metals as well as suppression of indicated constituents of corrosion process on the metal being protected are found by direct amperometric measurement of values of stationary current of corrosion in electrochemical system "metal being protected - reference electrode".

EFFECT: enhanced efficiency of cathodic protection of metals exposed to process liquid media infected with microorganisms, for instance in oil production system.

2 cl, 5 dwg, 1 tbl, 4 ex

Description

The invention relates to methods for protecting metals from corrosion using superimposed controlled potential difference and can be used in various industries, including oil.

Known methods of protecting metal in liquid corrosive environments (for example, oilfield waters) using chemicals - corrosion inhibitors - by dosing them into the formation and into the annulus of the well bounded by the casing and tubing [1]. The main disadvantage of the known methods is the need for a high real chemical consumption with a relatively low actual effectiveness of corrosion protection, which is associated with the presence of the biological component of corrosion processes in conditions of infection of liquid technological environments with corrosive microflora.

Known methods of protecting oilfield equipment, including downhole equipment, from general and including biological corrosion by the combined use of inhibitors and bactericides, as well as complex reagents (bactericidal inhibitors) by dosing them in aggressive process fluids - “oilfield waters” [ 2].

A disadvantage of the known methods is the low protective effectiveness of most chemicals in long-exploited oil fields with high water cut due to the formation in the oil production systems of a fixed biocenosis of oil field microflora, characterized by increased resistance to bactericides.

Known methods for the protection of carbon steel from general corrosion in fresh and mineralized waters using treads of metals of various nature, including the application of a controlled potential difference [3, 4]. The main disadvantage of the known methods is the inability to control the corrosion process in waters containing microorganisms, and therefore the selection of the required potential difference is not effective enough. In addition, the use of traditional design treads in wells is difficult due to the peculiarities of their descent and attachment, which results in tearing off when removing it and the need for additional labor-intensive and expensive work to drill and raise the spent metal. The high cost of traditional tread protection of downhole equipment is also determined by the use of magnesium-zinc alloy treads that remain during attempts to lift in the well and are practically destroyed by drilling.

Closest to the proposed method is the cathodic protection of carbon steel against biological corrosion by applying a protective potential difference to the protected and sacrificial metals of a similar type, developed for unlimited volumes of sea water with a natural background of microflora [5], which includes determining the electrode potential of the protected metal in the process of corrosion and subsequent determination of the necessary protective current density. In the direct implementation of the cathodic protection of the prototype, an overlay of the potential difference is used, the value of which is determined empirically, by constructing the dependencies “protective current density - the number of microorganisms” at a given potential difference. The main criterion for the effectiveness of protection in this case is the assessment of the intensity of the corrosion process by the loss of mass of the protected metal sample before and after the experiment, which does not allow determining the effectiveness of a number of applied protective potentials in one experiment - each of them requires its own experiment. As for the number of microorganisms on the metal, depending on the magnitude of the current determined by the potential difference, it cannot serve as the main criterion for the corrosion process, which is only the corrosion current. From the preliminary determination of the change in the electrode potential of the sample of the protected metal used in the presence of microorganisms in the liquid test medium (seawater) used in the prototype, only a conclusion on the qualitative difference in its corrosion state follows and the conclusion is drawn on the need for cathodic protection by applying a protective potential difference. The disadvantage of the prototype method is the inability to determine its effective value, because there is no data on the values of the corrosion current in the system of the protected metal sample, and it is not possible to estimate the value of the protective current.

It follows from the foregoing that without measuring the corrosion current, it is difficult to assess the effectiveness of cathodic protection.

A disadvantage of the known method of cathodic protection when using carbon steel as a sacrificial metal, similar in properties to the protected, is also the impossibility of an initial assessment of the readiness of such a metal to be a soluble anode, i.e. correlate instead of the object of protection. This situation does not allow you to choose the optimal protective potentials and if the soluble anode is not selected correctly, it requires significant energy consumption to implement the prototype cathodic protection method. The noted disadvantages show the lack of effectiveness of the known method of cathodic protection.

The disadvantages of the prototype also directly follows, in the case of using a metal identical to the protected metal, the limited ability to select a reasonable optimal value of the protective potential difference in the presence of active microflora. This follows both from the non-use in the prototype of available electrochemical means and methods for controlling corrosion processes involving microorganisms to determine the biological component of total corrosion, and the lack of a sound approach to the choice of sacrificial metal, since its ability to corrode more intensively than that protected in the prototype method has not been previously studied.

The problem and the expected technical result of the proposed method are to increase the efficiency of the cathodic protection of metals from corrosion, including biological, in particular, in conditions of microorganism contamination of technological liquid media, for example, oil production systems.

The problem is solved in that in the method of cathodic protection of metals from corrosion in liquid media, including the application of a protective potential difference between the protected and sacrificial metals of a similar type, the ability of the sacrificial metal to corrode more intensively protected in laboratory conditions is preliminarily determined by determining the non-biological and biological components of the corrosion process on protected and sacrificial metals and the necessary protective potential difference to suppress both states vlyayuschih corrosion process on the protected metal.

The non-biological and biological components of the corrosion process on the protected and sacrificial metals, as well as the suppression of both components of the corrosion process on the protected metal, are determined by direct amperometric measurement of the values of the steady-state corrosion current in the protected metal-reference electrode electrochemical system.

A comparative analysis of the essential features of the proposed technical solution and prototype allows us to conclude that the claimed invention meets the criterion of "novelty."

A preliminary determination of the ability of a sacrificial metal to correlate more intensely than that of a protected metal by determining the non-biological and biological components of the corrosion process on the protected and sacrificial metals with cathodic protection of the metal has not previously been proposed.

Unlike the prototype, the proposed technical solution involves a direct amperometric determination of the effectiveness of the cathodic protection directly during the experiment, in the kinetic mode.

The possibility of direct amperometric determination of the intensity of corrosion processes follows from RF patent No. 2095786 (1997) [6], the technical solution of which is based on the established experimental proof of the properties of real corrosion systems in which only a stationary state characterized by the total charge transfer recorded as effective current. In the case of the predominance of the process of loss of mass of the metal, the algebraic current completely characterizes the corrosion process, and its registration under various conditions allows us to give a direct quantitative assessment of the non-biological and biological components of the general corrosion process.

The method is carried out by the following sequence of operations.

1. A preliminary determination in laboratory conditions of the ability of a sacrificial metal to correlate more intensely than that of a protected metal by determining the non-biological and biological components of the corrosion process on the protected and sacrificial metals, including the sequence of actions:

1) preparation of samples of protected and sacrificial metals;

2) sampling of a liquid corrosive medium;

3) bringing metal samples into contact with samples of a liquid corrosive medium;

4) amperometric determination of the intensity of the process of the non-biological component of general corrosion of the samples of protected and sacrificial metals with the establishment of stationary values of the current strength of the non-biological component of corrosion;

5) amperometric determination of the intensity of the process of general corrosion of samples of protected and sacrificial metals with the establishment of a stationary value of the current strength of general corrosion;

6) the imposition of the potential difference between the samples of the protected and sacrificial metals;

7) amperometric determination of the suppression of the process of general corrosion with the establishment of a stationary value of the current strength of the general corrosion of the sample of the protected metal to establish the necessary protective potential difference;

8) determining the appropriateness of using a sample of sacrificial metal in a pair of “protected and sacrificial metal” on the basis of the actions in paragraphs 4-7.

2. The imposition of the protective potential difference established by claim 1, subparagraph 7 between the protected and sacrificial metals.

The implementation of the method involves the use of a flowing electrochemical system, shown in figure 1, consisting of three similar cells - for the protected metal sample 1, the reference electrode 2, and the sample of the sacrificial metal 3 connected by means of an electrolytic key-tee 4.

In contrast to the prototype, the proposed method is very simple to automate using available equipment, which is reflected in figure 2, which shows a block diagram of the measuring and control circuits of the flowing electrochemical system used and the implementation of the method. In this case, the electrochemical cells - 1, 2, 3 are connected to the inputs of the highly sensitive galvanometer-microammeter 4 and the recorder 5 through the operational amplifier 6, which has feedback from the peristaltic pump 7. The electrochemical systems are connected through the contact of the time relay 8, the additional contact of which controls the operation of the peristaltic pump. The following is a description of the sequence of the method in an automated embodiment.

A two-channel version was used using two identical containers with a capacity of 200 cm and cells with a capacity of 10 cm. The flow through the system was set by a multichannel peristaltic pump of type 304. The system is thermostated in a dry-air thermostat. The determination of the total, background, and biological components was carried out using low carbon steel electrodes with an external diameter of 2 mm and linear dimensions of 40 mm.

The current was measured using a M2005 high-precision microammeter, accuracy class 0.2, connected with the transistor converter of the F195 device in the operational amplifier mode. The output signal was recorded using a KSP-4 type self-recording potentiometer.

Samples of the protected and sacrificial metals (electrodes) were cleaned, degreased, and then the electrochemical cell systems were assembled by connecting them to the flow system. The systems were filled with the investigated liquid medium, followed by removal of air bubbles and sealing. We used the stochastic regime of changing the fluid supply rate by the pump using a 2РВМ type electromechanical time relay, interlocked with the control circuit, whose software comparator is oriented for generating random sequences of time intervals.

The measuring and control circuits were connected to the sample electrodes and the reference electrode, manually measuring the current value during the first hour with an interval of 2 minutes, while simultaneously recording its value with a recorder. Upon reaching stationary values of the current strength of the non-biological component, the system was transferred to automatic mode, the control of which was carried out by the control circuit.

The method is illustrated by examples.

Example 1. Determining the feasibility of using a carbon steel sample as a sacrificial metal (soluble anode) before the influence of the biological component of the corrosion process begins.

In oilfield practice, in the implementation of cathodic protection of downhole equipment made of carbon steel, various versions of soluble anodes of the same metal can be used as sacrificial, which requires determining the appropriateness of their use.

The feasibility of using two carbon steel samples as the “sacrificial” metal — elements of replaceable well construction structures — tubing couplings — was carried out by direct amperometric measurements of corrosion currents in the “protected metal - reference electrode” and “sacrificial metal – reference electrode” systems. At the same time, the electrode potentials of the protected and sacrificial metal samples were measured relative to the silver-silver reference electrode used. Similar measurements (for contrast) were carried out for a sample of high-carbon tool steel (sample 3). The table shows the results obtained for the oilfield water of the Sergeevskoye field of the Ufaneft NGDU, from which it clearly follows that the choice of metal for the soluble anode in the absence of the influence of the biological component of the corrosion process corresponds to intensive corrosion of the soluble anode. The total (algebraic) current characterizes the rapid establishment of a stationary value of the dissolution rate of the anode, which “blocks” the cathode corrosion process. For a sample of high-carbon tool steel, the measured values of currents and potentials indicate the inappropriateness of using it as a sacrificial metal in tandem with the protected metal.

However, for oilfield environments containing the biocenosis of active corrosion-hazardous microflora, “point” (short-term) measurements are insufficient due to the presence of the biological component of the corrosion process, the determination of which requires many hours of experiment, which is reflected in example 2.

Example 2. Determination of the biological component of the corrosion process of carbon steel and evaluation of the feasibility of using a carbon steel sample as a sacrificial metal (soluble anode).

The biological component of the corrosion process on the protected and sacrificial metals was determined under the influence of the microbiological factor of oilfield waters - the biocenosis of microflora, which forms fixed forms. Used wastewater from the Sergeevskoye field of the Uganeft NGDU with a total microorganism content of up to 10 cells per 1 cm. The kinetics of the development of corrosion of the two aforementioned carbon steel samples in the presence of microorganisms is presented in Fig. 3. The results indicate that the presence of microorganisms determines the rapid increase in the corrosion current in all three studied systems (the protected metal is the reference electrode; sample 1 of the sacrificial metal is the reference electrode and sample 2 of the sacrificial metal is the reference electrode). For the protected metal, the corrosion current increases 2.23 times, for one of the sacrificial samples - 3.54, which indicates the possibility of using the latter as a soluble anode paired with the protected sample. At the same time, the second sample of the sacrificial metal in the presence of the biological component of the corrosion process exhibits less ability to corrode in comparison with the protected metal sample. Despite the higher initial value of the current before the development of microorganisms, the final corrosion current on this sample is lower than the current on the protected one, since it increases 1.31 times from the initial one. From the results obtained, it follows that it is inappropriate to use the metal of the second sample as a sacrificial one. For comparison, the change in the corrosion current of sample 3 of high-carbon tool steel, obtained under the same conditions, is presented. The inappropriateness of using such a metal together with the protected one was unequivocally established, which corresponds to the results of Example 1. Experience allows us to identify the high electrochemical (corrosive) activity of the biocenosis of oilfield microflora and suggests the need to apply a larger controlled potential difference when performing cathodic protection when the sacrificial metal is unsuccessful, which is reflected in examples 3 and 4.

Example 3. Determination of effective values of the controlled potential difference before the development of the biological component of the corrosion process of carbon steel.

Prior to the development of the biological component of the corrosion process in the water of the Sergeevskoye deposit, the protective potential difference superimposed on the pair “protected metal - sacrificial metal (sample 1)”, which allows “dumping” the corrosion current to almost residual exchange currents close to zero, is 2.1 In what is presented in figure 4 in the form of an amperometric curve. A similar potential difference is also effective when using sample 2 of the sacrificial metal. The use of high-carbon tool steel (sample 3) as a sacrificial metal is initially impractical, since the effective protective potential difference in this case is 2.4 V, which corresponds to a more positive value of the electrode potential of tool steel compared to the protected one (example 1).

The further development of microorganisms determines a steady increase in current, which is given below, and requires an increasing potential difference, which “closes” the corrosion process.

Example 4. Determination of the effective values of the controlled potential difference after the development of the biological component of the corrosion process of carbon steel.

After the development of corrosion in the presence of microorganisms in the water of the Sergeevskoye deposit is completed, in order to inhibit the dissolution of the protected carbon steel sample, initially higher potential differences are required than before the establishment of stationary currents determined by the formation of a biocenosis attached to the metal. The results of determining the effective values of the protective superimposed potentials for the pair “protected metal - sacrificial metal (sample 1)” are shown in figure 5. Full suppression of the biological component current is possible only in the range of 4.6 V. After successive application of the potential difference to 7.6 V, the corrosion process can be “locked up”, dropping the current to residual values close to zero.

For the “protected metal – sacrificial metal (sample 2)” pair, since as a result of the influence of the biological component, the protected carbon steel sample acquires the properties of a sacrificial one (has a more negative potential and higher values of the corrosion current), a much larger protective potential difference is required, and complete “locking ”Of the corrosion process is possible only with a potential difference of 9.2 V, which is almost equal to the protective potential difference (9.4 V) for the pair“ protected metal is a high-carbon tool Ordering steel (sample 3) ", obviously unsuitable for effective cathodic protection, according to the results of Example 1.

From the above examples it follows that the biological component of the corrosion process can cause a modification of the electrochemical properties of metals, which is expressed in the shift of the stationary electrode potential to the negative side and an increase in the corrosion current, and the primary values of these parameters, determined before the influence of microorganisms on the corrosion process, do not reflect the further electrochemical the behavior of a particular metal sample.

The conclusion following from the unexpectedness of the technical solution of the claimed method is the need to determine the appropriateness of using one or another sample of the sacrificial metal in the presence of the biological component of the corrosion process. The presence of the modification does not follow unambiguously from the current level of knowledge about the electrochemical properties of metals and has been established experimentally.

The claimed method is more efficient than the prototype and is industrially applicable, since affordable laboratory tools and field equipment are used.

Sources of information

1. Ibragimov G.Z., Khisamutdinov I.I. Reference manual on the use of chemicals in oil production. - M: Nedra, 1977 .-- 271 p.

2. Khisamutdinov N.I. Ibragimov G.Z. Oilfield development. Volume 1V. - M: VNIIOENG, 1994 .-- 262 p.

3. Protection of metal structures from underground corrosion: Reference / Strizhevsky I.V. et al. - M: Nedra, 1981. - 293 p.

4. Corrosion resistance of chemical equipment: Methods for protecting equipment from corrosion / Ed. B.V. Strokana, A.M. Sukhotina -L .: Chemistry, 1987 .-- 280 p.

5. Microbiological corrosion in sea water. Some protection methods. - M .: Nauka, 1983. - S. 50-53, 76-79, 81-84.

6. RF patent 2095786 (1997). A method for determining the biological component of the corrosiveness of liquid media / V.V. Leonov.

Claims (2)

1. A method for the cathodic protection of metals from corrosion in liquid media, comprising applying a protective potential difference between the protected and sacrificial metals of a similar type, characterized in that they preliminarily determine the ability of the sacrificial metal to corrode more intensively than the protected metal in laboratory conditions by determining the non-biological and biological components of the corrosion process on protected and sacrificial metals and the necessary protective potential difference in suppressing both components of corrosion process on the protected metal. 2. The method according to claim 1, characterized in that the non-biological and biological components of the corrosion process on the protected and sacrificial metals, as well as the suppression of both components of the corrosion process on the protected metal, are determined by direct amperometric measurement of the values of the stationary corrosion current in the protected metal - electrode electrochemical system comparisons. ”

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