RU2171463C1 - Method evaluating efficiency of inhibitory protection of metal in corrosion-active media ( versions ) - Google Patents

Abstract

FIELD: evaluation of protective effect of inhibitors of corrosion of metals and welds in sorption-active media, oil, gas and chemical industries. SUBSTANCE: method evaluating efficiency of inhibitory protection of metal in corrosive media consists in application of sample of corrosive medium on specimens of metal in whose capacity specimens of basic metal, specimens of metal covered by products of corrosion and specimens of metal simulating sections of zone of thermal effect of welds are used, in measurement of corresponding electrode potentials, of edge angle of wetting, surface tension of tested medium and in determination of adhesive effect by them. Value of adhesive effect and corresponding difference of electrode potentials are used to make judgment on reliability of protection of metal in corrosive medium. EFFECT: increased authenticity of evaluation of efficiency of inhibitory protection of metal under conditions of corrosive media, simplification of method and accelerated determination of properties of tested specimens of metal. 3 cl, 1 dwg, 8 tbl

Description

 The group of inventions relates to the field of evaluation of the protective effect of metal corrosion inhibitors and welded joints in sorption-active environments and can be used in the oil, gas and chemical industries.

 Known gravimetric method for assessing the effectiveness of inhibitory protection of metal in corrosive environments, which consists in measuring the loss of mass of the metal over a certain period of time and calculating the corrosion rate. In this case, a metal sample of a certain surface area, processed on the grinding wheel to the required degree of purity, is degreased with acetone, adjusted to constant weight and placed in a corrosive environment. The exposure time depends on the degree of aggressiveness of the environment and on the objectives of the study. Then the sample is removed, the surface is treated with a soft rubber to remove corrosion products, degreased and after the indicated operations are again brought to constant weight. The corrosion rate is determined by the change in the mass of the sample according to the corresponding formula.

K = Δ m / t • S,
where K is the corrosion rate, (g / m ² • h),
Δ m is the change in mass of the sample during the exposure, g,
t is the exposure time, h,
S - sample area, m ²
(see GOST 9.506-87. Unified system of protection against corrosion and aging. Corrosion inhibitors of metals in water-oil media. Methods for determining the protective ability.)
One of the main disadvantages of the method is its low reliability: it is difficult to completely remove all corrosion products without affecting the base metal, it is impossible to evaluate uneven corrosion. Gravimetric studies provide averaged data on the development of the corrosion process, while it is often necessary to carry out corrosion control at a given point in time.

 Another disadvantage of the gravimetric research method is the difficulty of installing witness specimens and the difficulties associated with their removal. These operations are usually tied to the shutdown of technological installations for scheduled repairs, or a shutdown of the process is required. These tests do not allow to intensify the corrosion processes and carry out accelerated tests with little time and money when conducting full-scale tests. The method does not make it possible to evaluate the effect of the medium on the welded joints.

 There is also a method of evaluating the effectiveness of inhibitory protection of a metal in corrosive environments, based on electrochemical test methods, that is, on measuring the magnitude of the current density of corrosion or the values of the electrode potential. Among them, a linear polarization resistance method has now been developed and is widely used to assess the instantaneous corrosion rate, which allows one to determine active dissolution, pitting formation, or inhibition of the corrosion process in technological media in the case of the formation of protective films as a result of adsorption on the metal surface. The essence of the method is to measure the corrosion rate using a three-electrode probe with simultaneous determination of the electrode potential. Instrumentation using domestic and foreign measuring instruments allows these studies to be carried out in the field.

(see, Balaban-Irmenin Yu.V., Kurashev V.D., Lezezin N.E. et al. Methods for studying local corrosion of carbon steels used in the construction and operation of fuel and energy complex facilities. - M.: IRZ GAZPROM, 1994. 42 pp., Obz. Inform., Ser. Corrosion and protection of structures in the gas industry.)
There are a number of factors limiting the use of this method: the unreliability of the results obtained when films of corrosion products with very high resistance are present on the metal surface in cases where concentration changes in the solution affect the course of polarization curves. The semi-logarithmic relationship between the potential and the current density in this case is violated and the method becomes unusable. In addition, it is not possible to install the probe in existing equipment.

There is also a method of evaluating the effectiveness of inhibitory protection of metal in corrosive environments, based on chemical-analytical analysis of the environment, including the determination of iron ions as an indicator of the rate of corrosion and inhibitory protection. In this case, to assess the corrosiveness of the medium, it is necessary to take samples of the medium with subsequent determination of the concentration of iron ions in it. (see Romanov V.V. Methods for the study of metal corrosion. - M.: Metallurgy, 1965 - 280 p.)
The disadvantage of this method is its low reliability in the case of the formation of insoluble corrosion products or complex compounds with corrosion inhibitors in the medium.

 Closest to the claimed one is a method that implements an integrated approach to assessing the effectiveness of inhibitory protection of a metal, based on measuring electrochemical parameters: corrosion currents and electrode potential, as well as ion-salt composition as factors determining the degree of corrosion aggressiveness of a medium (see A.P. Mitina, LV Frolova and others. Results of the study of the compatibility of broad-spectrum chemicals with natural and industrial environments at GAZPROM facilities. - M .: IRC GAZPROM, 1999 - 56 pages).

 The method includes sampling the test fluid, applying the test medium to samples of the base metal in the absence of corrosion products, conducting rapid analyzes of the ion-salt composition of the medium and operational monitoring of the metal surface by measuring the electrode potential or corrosion currents.

 Determination of the ion-salt composition of the medium is carried out using wet chemistry methods (volumetric analysis and physico-chemical methods) in order to assess the effect of the ions present on the corrosiveness of the medium.

 The disadvantage of this method is the impossibility of a complete analysis of the environment in order to take into account the influence of all components of the medium, including surfactants on the corrosive environment of the studied environments and its associated low reliability and high complexity. It is inherent in all the disadvantages of the electro-chemical methods of measurement indicated above.

 In addition, the method does not allow to evaluate the effectiveness of the inhibitory protection of welded joints and metal samples coated with corrosion products.

 The objective of the proposed method is to increase the reliability of the assessment of the inhibitory protection of metal in corrosive environments and in a simple and quick determination of the necessary properties on an existing facility by measuring the wetting angle, the surface tension of the test medium at the liquid-gas interface, which determine adhesion work and metal electrode potential.

In the first embodiment, the task is achieved by a method of evaluating the effectiveness of the inhibitory protection of a metal in a corrosive environment, including applying a sample of a corrosive environment to samples of the base metal and measuring the electrode potential in which according to the invention, metal coated corrosion products, additionally measure the wetting angle and surface tension of the test medium at the liquid-gas interface, by which th determined job adhesion, the work of adhesion value of not more than 100 mJ / m ² and the electrode potential difference between the base metal and the metal coated with corrosion products, no more than 10 mV indicates the reliable protection of the metal in a corrosion-aggressive environment.

In the second embodiment, the task is achieved by a method of evaluating the effectiveness of inhibitory protection of a metal in a corrosive environment, including applying a sample of the investigated corrosive environment to the samples of the base metal and then determining the electrode potential, in which according to the invention, metal samples are used along with the base metal and metal samples simulating areas of the heat affected zone of welded joints additionally measure the contact angle of contact and surface the remaining tension of the medium under study at the liquid-gas interface, by which the adhesion work is determined, and by the highest value of the adhesion work and the maximum value of the negative electrode potential between the base metal sample and metal samples simulating portions of the heat affected zone of welded joints, the least stable the corrosion section of the welded joint, after which the corrosion inhibitor is introduced and re-measurement of the contact angle, surface tension I investigated medium at the liquid-gas phase, which determine the work of adhesion, and the difference of electrode potential between samples of the base metal and the least corrosion-resistant portion of the welded joint, and the value of the work of adhesion of not more than 100 mJ / m ^2, and the difference of electrode potential not more than 10 mV indicates the reliability of the inhibitory protection of the metal in a corrosive environment.

 In a preferred embodiment of the second method, to increase the reliability of the assessment of the inhibitory protection of the metal, the electrode potential is additionally measured on metal samples simulating different sections of the welded joint, the potential difference between the sections of the welded joint of not more than 10 mV indicates the reliability of the inhibitor protection in all sections of the welded joint.

The method according to the first embodiment is as follows:
- as a metal sample, base metal and metal coated with corrosion products are used;
- take a sample of the environment from the existing equipment without stopping it;
- a sample of the medium (with a syringe or pipette) is applied to the surface of the base metal and the metal coated with corrosion products;
- determine the contact angle by the known method (see Zimon A.D., Leshchenko N.F. Colloid chemistry. - M .: Chemistry, 1995, 336 p.).

- measure the value of the surface tension of the medium at the liquid-gas interface by a known method; (see Dulitskaya R.A., Feldman R.I. Workshop on physical and colloid chemistry. - M.: Higher School, 1978, 100 pp.)
- calculate the work of the adhesion of the medium-metal by the formula:
Wa = σ • (1 + cosθ),
where W _a is the work of adhesion in the metal-medium system (mJ / m ² );
σ is the surface tension (N / m);
θ is the wetting angle.

- measure the electrode potential by a known method (see Tomashov ND, Zhuk N. P. et al. Laboratory work on corrosion and metal protection. - M .: Metallurgy, 1971, 280 p.);
- the effectiveness of the protective action of the corrosion inhibitor is estimated at least 90% if the values of the adhesion work are not more than 100 mJ / m ² and the potential difference between the base metal and the metal coated with the corrosion products should be no more than 10 mV.

In the method carried out according to the second embodiment:
- as the metal sample, the base metal and metal samples are used that imitate portions of the heat affected zone of the welded joint (HAZ);
- take a sample of the environment from the existing equipment without stopping it;
- a sample of the medium (with a syringe or pipette) is applied to the surface of the base metal or sections of the HAZ;
- determine the wetting angle by a known method (see Zimon A. D., Leshchenko NF Colloid chemistry. - M .: Chemistry, 1995, 336 p.);
- measure the value of the surface tension of the medium at the liquid-gas interface by a known method (see Dulitskaya R.A., Feldman R.I. Workshop on physical and colloidal chemistry. - M.: Higher School, 1978, 100 pp.); .

- calculate the work of the adhesion of the medium-metal by the formula:
Wa = σ • (1 + cosθ),
where W _a is the work of adhesion in the metal-medium system (J);
σ is the surface tension (N / m);
θ is the wetting angle.

- measure the electrode potential by a known method (see Tomashov ND, Zhuk N. P. et al. Laboratory work on corrosion and metal protection. M: Metallurgy, 1971, 280 p.);
- by the largest value of the work of adhesion and the maximum negative value of the electrode potential of the metal of the sample simulating the portion of the heat affected zone of the welded joints, the least corrosion-resistant portion of the welded joint is fixed;
- enter a corrosion inhibitor;
- re-measure the contact angle, the surface tension of the test medium at the liquid-gas interface;
- calculate the work of adhesion according to the above formula;
- measure the electrode potential of the HAZ sites and the base metal;
- the effectiveness of the protective effect of the corrosion inhibitor is estimated to be at least 90% if the values of the adhesion work are not more than 100 mJ / m ² and the electrode potential difference between the base metal samples and metal samples imitating the name of the corrosion-resistant section of the welded joint should be no more than 10 mV .

In a preferred embodiment:
- additionally measure the electrode potential of metal samples simulating various sections of the welded joint,
- they consider effective at least 90% inhibitory protection in all areas of the welded joint, if the potential difference does not exceed 10 mV.

 The invention claimed by the group meets the requirements of the unity of the invention, since the group of single-object inventions forms a single inventive concept, the application relates to objects of the invention of the same type, of the same purpose - to a method for assessing the effectiveness of inhibitory protection of a metal in a corrosive environment, providing the same technical result - increasing the reliability of assessing the reliability of inhibitory metal protection in corrosive environments.

 Below are examples of the implementation of the proposed method.

 Example 1

According to the first embodiment of the invention, the effectiveness of inhibitory protection is assessed on the base metal - steel 09G2S and on the same steel coated with corrosion products, for which test specimens are exposed to the following environments:
1 - Acidic water with a hydrogen sulfide content of 2 g / l, pH 4.1,
2 - Neutral medium in the absence of hydrogen sulfide, pH 7.5 with a total mineralization of 3-5 g / l,
3 - Alkaline medium (pH 9.8) in the presence of sulfide ions (2 g / l) with a total salinity of 3-5 g / l.

 Compounds based on imidazolines (trademark SEPACORR-5478 AM), pyridines (I-55-D), tertiary amines (INCORAX, patent No. 2111955) and INCORGAS (patent No. 2061098) are investigated as corrosion inhibitors. These inhibitors are widely used in the oil and gas industry or have undergone extensive testing and are recommended for use to protect metal equipment in various technological environments.

Table 1 presents the results of testing the effectiveness of corrosion inhibitors in acidic water (medium 1). Assessment of the effectiveness of the protective effect of corrosion inhibitors was carried out by the gravimetric method, as well as by measuring the electrode potential. At the same time, the values of the contact angle and surface tension at the gas-liquid interface were measured, according to which the adhesion work was calculated. The tests were carried out on metal without a pre-deposited film of corrosion products, the concentration of corrosion inhibitors ranged from 1 to 300 mg / l (table 1) and from 1 to 1000 mg / l (table 2)
Table 2 presents the test results of an INCORGAS inhibitor in a neutral medium of pH 6.1 - 7.2 (with a total mineralization of 3-5 g / l) (medium 2) and in an alkaline with the addition of sulfide ions, pH 9.8, with a total salinity of 5 g / l (medium 3).

From the above data it follows that if reliable inhibitor protection of the metal is provided and the corrosion rate values determined in accordance with GOST 9.506-87 comply with regulatory requirements, that is, they provide a protective effect of at least 90%, the amount of adhesion determined by the proposed method will be no more 100 mJ / m ² . This is true for all studied media (tab. 1, 2).

 Table 3 presents the results of measurements of the electrode potential before the introduction of corrosion inhibitors and after their introduction into medium 1 (pH 4.1 and a concentration of hydrogen sulfide 2 g / l). The concentration of inhibitors was 300 mg / L. Electrode potentials are measured on samples without a pre-deposited sulfide film and on samples with a pre-formed sulfide film on them.

 From the results of table 3 it follows that the difference between the electrode potential of a metal without a film of corrosion products and on a metal coated with a film can be significant (300 mV). The method allows you to choose the most effective corrosion inhibitor - in this case, INCORAX-S, the protective effect of which is manifested both in the presence of corrosion products on the metal, and without them.

 Table 4 shows the results of evaluating the effectiveness of the protective effect of corrosion inhibitors, performed by the proposed method and the methodology used in the prototype.

As can be seen from the above data, if to evaluate the effectiveness of inhibitory protection we apply the method of assessing the corrosion rate by polarization resistance (prototype of the invention), then it does not allow to obtain reliable data. So, according to measurements made in this way, the data is not stable. The corrosion rate 10 minutes after the start of the measurement is 0.09 g / m ² • h, that is, 90% protection is provided, but after 30 minutes the corrosion values are fixed already 0.4 g / m ² • h, which indicates a low efficiency of the protective inhibitor action.

 When applying the proposed method, ΔΦ values remain much higher than 10 mV both after 10 minutes and 30 minutes from the start of the test. This indicates a lack of effectiveness of the inhibitor.

 Example 2

According to a second embodiment of the invention, the effectiveness of inhibitory protection on welded joints of 09G2S steel is evaluated. In order to study corrosion processes and inhibitory protection, a series of prismatic samples 10 × 10 × 55 mm in size were made from 09G2S steel, on which thermal cycles of individual sections of the HAZ were simulated. The choice of specific values of the maximum heating temperatures (from 1350 to 750 ^o C) for samples simulating HAZ sections is due to the need to create a structure that allows determining the magnitude of the adhesive interaction of the metal-medium system from the ferrite content. The structure and corrosion properties of 7 HAZ sections are studied, the maximum heating temperature of which varied from 1350 to 750 ^o C at a constant heating and cooling rate. The samples are used to determine the ratio of phase components: ferrite and perlite, as well as the dispersion of the structure. The adsorption interaction (adhesion work) of the metal and the medium is carried out on media with different ion-salt composition and concentration of corrosion inhibitors. The following environments were selected:
I. Acidic water with a hydrogen sulfide content of 2 g / l, pH 4.1.

 II. Neutral medium in the absence of hydrogen sulfide, pH 7.5, total salinity 4 g / l.

 III. Alkaline medium (solution of monoethanolamine and methyldiethanolamine in water at a total concentration of 40%) in the presence of hydrogen sulfide, 15 g / l, pH 9.8.

 Table 5 presents the characteristics of the studied HAZ sites, while the base metal is considered as the HAZ site that has not been subjected to temperature influence.

 Stage 1. Table 6 presents the results of testing the corrosion resistance of the samples according to the indicator "corrosion rate", determined according to GOST 9.506-87, simulating HAZ sections (N sample 1-8) in environments I-III. At the same time, we measured the values of the contact angle and surface tension at the gas-liquid phase boundary, according to which the adhesion work was calculated in accordance with the proposed method.

 It is known that sections of welded joints with maximum values of corrosion rate will be the least resistant to the influence of a corrosive environment. From the data of table 6 it is seen that using the guest method, such areas are defined.

 The least stable section for medium I is section 1; for medium II, sections 1.4 are those; for medium III, sections 4 and 5.

The same areas correspond to the maximum values of the work of adhesion (W _a ) and more electronegative values of the electrode potential, determined by the proposed method.

 Thus, the method allows the values of the potential and work of adhesion to identify the least corrosion-resistant section of the HAZ, which is confirmed by measurements of the corrosion rate in these areas.

 Stage 2.

 Investigations are carried out in corrosive environments I, II, III of the HST sections that are least resistant to corrosion and selected in Example 1 in the presence of corrosion inhibitors: A - for medium I, B - for II, III environments, at optimal inhibitor concentrations that amounted to 100 mg / l.

From the presented values it follows that the addition of a corrosion inhibitor to each medium leads to refinement of the electrode potential values of the studied HAZ sites, the values of adhesion work are no higher than 100 mJ / m ² , while the corrosion rate decreases by about 10 times (protective effect is 90% )

 A preferred embodiment is illustrated by an example.

 Example 3

 The studies were carried out on the most characteristic HAZ sites in order to assess the spread of the electrode potentials values along HAZ sites in the presence and without corrosion inhibitors in medium I (Table 8).

 It can be seen from the presented data that at ΔΦ between the HAZ sections with the minimum and maximum potential values of more than 10 mV, there is a risk of contact corrosion, this is confirmed by an increase in the values of corrosion currents and the corrosion rate, which additionally appears between these sections (Table 8).

The drawing shows changes in electrode potential and corrosion rate along the HAZ of a welded joint, where:
- 1, 2 - curves of changes in corrosion rate,
- 3, 4 - curves of changes in the electrode potential in HAZ areas without additives (1, 3) and in the presence of a corrosion inhibitor (2, 4) in medium III (a mixture of amino alcohols containing H ₂ S.

 From the presented data it follows that in the absence of a corrosion inhibitor, the spread in the values of electrode potentials over HAZ sites in medium III is a significant value (up to 35 mV), which leads to contact corrosion. In the presence of a correctly selected corrosion inhibitor, the spread is no more than 10 mV, which ensures the equipotentiality of the surface. Moreover, the corrosion rate in all areas of the HAZ is minimal. Thus, the proposed method allows with a high degree of reliability to evaluate the effectiveness of inhibitory protection of metals, including welded joints, in corrosive environments.

Claims (3)

1. A method for evaluating the effectiveness of inhibitory protection of a metal in a corrosive environment, including applying a sample of a corrosive environment to samples of the base metal and measuring the electrode potential, characterized in that metal samples coated with corrosion products are used as metal samples, additionally measure the wetting angle and surface tension of the test medium at the liquid-gas interface, which determine the work of adhesion, and the value of the work of adhesion ii of not more than 100 mJ / m ² and the electrode potential difference between the base metal and the metal coated with corrosion products, no more than 10 mV indicates the metal inhibitor protection reliability in corrosive environments. 2. A method for evaluating the effectiveness of inhibitory protection of a metal in a corrosive environment, including applying a sample of the investigated corrosive environment to the samples of the base metal and subsequent determination of the electrode potential, characterized in that metal samples simulating metal are used along with the base metal These are sections of the heat-affected zone of welded joints; in addition, they measure the wetting angle and surface tension of the test medium at the phase boundary viscosity is the gas by which the adhesion work is determined, and by the largest value of the adhesion work and the maximum value of the negative electrode potential between the base metal sample and metal samples simulating the areas of the heat-affected zone of welded joints, the least corrosion-resistant section of the welded joint is fixed, after which use a corrosion inhibitor and re-measure the wetting angle, the surface tension of the test medium at the liquid - ha phase interface h, by which the adhesion work is determined, as well as the electrode potential difference between the samples of the base metal and the least corrosion-resistant section of the welded joint, the adhesion work value of not more than 100 mJ / m ² and the electrode potential difference of not more than 10 mV indicates the reliability of inhibitor protection metal in a corrosive environment.  3. The method according to claim 2, characterized in that to increase the reliability of the assessment of the inhibitory protection of the metal, the electrode potential of metal samples simulating different sections of the welded joint is additionally measured, and the potential difference between the sections of the welded joint is not more than 10 mV, which indicates the reliability of inhibitory protection for all areas of the welded joint.

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