SU927862A1 - Method for protecting carbon steel against corrosion - Google Patents

Description

(54) METHOD FOR PROTECTING CARBON STEEL AGAINST CORROSION IN NEUTRAL AQUEOUS MEDIA

The invention relates to the protection of metals against corrosion in neutral aqueous media, in particular through the combined use of cathodic (or sacrificial) protection and corrosion inhibitors, and can be used in the chemical, energy, and other industries for the corrosion protection of aggregates, internal metallic whose surfaces are in contact with the aqueous medium, for example, heat exchangers, storage facilities, circulating water supply systems, power units, steam generators, etc.

There is a known method of protecting carbon steels from corrosion, which consists in introducing oxidizing additives (in particular, oxygen or hydrogen peroxide) into an aggressive medium 1.

However, the protective effect is negligible (the corrosion rate is reduced by 25 times). In addition, this method requires preliminary operations on deep desalination of the water used.

The known method of cathode protection of metal structures in water, comprising

. in cathodic polarization of the protected surface 2.

However, with the help of this method it is possible to reduce the corrosion rate of carbon steel only by a factor of 3-5.

The closest to the proposed technical essence and the achieved positive effect is a method of corrosion protection of carbon steel, based on a combination of cathodic polarization of the surface, in particular from the protector and inhibiting additives introduced into the solution, which use potassium chromate and dichromate 3.

However, the effectiveness of this method is also relatively small, and the widespread use of this method is limited by the high toxicity of salts of light-valent chromium used as inhibitors.

The purpose of the invention is to increase the effectiveness of anti-corrosion protection of carbon steel in a neutral aqueous medium.

The goal is achieved so that, according to the method, carbon steel is protected from corrosion in neutral aqueous media by simultaneous cathodic polarization of the surface and the introduction of a corrosion inhibitor, hydrogen peroxide is introduced as a inhibitor at a concentration of 0.7-3.0 g / kg and the potential of steel is maintained in the range of (-0.05) - (-0.60) V relative to its corrosion potential in the presence of hydrogen peroxide. The method is carried out as follows. I pass an electric current through the metal to be protected in an aqueous neutral medium so that the metal in the circuit works as a cathode. At the same time entering into the solution of hydrogen peroxide. With the help of special electrochemical equipment under-. keep the specified protective potential of steel constant. Periodically control the consumption of hydrogen peroxide and, as necessary, adjust the composition of the solution. The electrical conductivity of water required for cathodic protection (if necessary) is ensured by the addition of mineral salts (for example, sodium sulfate) to the solution. The drawing shows the dependence, Confirming the effectiveness of the proposed method. For consistency, the example compares the corrosion-electrochemical behavior of carbon steel (Art. 3) at different potentials in a pure solution of sodium sulfate and with the addition of hydrogen peroxide. The sample (Art. 3) is included in the electrical circuit as an electrode and a current is passed through it, for which purpose a second auxiliary (platinum) electrode is used. The electrolyte used in the circuit is first paicTBo 7.0 g / kg sodium sulfate (solution 1), and then the same solution with the addition of 2 g / kg hydrogen peroxide (solution 2). Using special electrochemical instrumentation, the constant constant potentials of the steel electrode are sequentially set and the values of the currents corresponding to each potential (cathodic or anode) are measured. According to the measurement results, current densities are calculated (current per surface unit), then polarization curves are plotted (potential dependences of current densities: curves A

The protection method for solution I and B for solution 2 (see figure). The rate of dissolution (corrosion) of steel (branches 1 and 1) is judged by anode currents. At potentials at which cathode currents flow through the electrode (branches 2 and 2), the dissolution rates are determined using an analytical method, for which samples of the solution are analyzed (for example, by the radiometric method) for the content of iron ions in them. The results of the analysis are recalculated | g in electrical units and the dependencies of the steel dissolution rate on the potential (voltage 3) in a wide potential range, including the areas of cathode currents (for solution 1 from (-0.1) to (-0.8) B, for solution 2 from (+0.8) to (-0.6) B). In addition, the corrosion potential of Ecor is measured and, respectively, for solutions 1 and 2. The corrosion rate in the absence of protection corresponds to the value of ij. With cathodic protection in solution 1 (potential shift, 05B from Ecor), the corrosion rate is i2, and at Е 0.20V - ij. The corrosion rate in solution 2 is expressed as (4 (for Ekor) - Points $ 1, b and characterize the corrosion rates under the conditions of implementation of the proposed method (together: the cathode is fed at different DE to H2O2 additives). From curve B it follows that the effectiveness of protection as LE grows, it first increases and then (starting from AE 0.6V) does not change. This determines the upper potential shift potential at cathode flattening. The lower limit of the DE is determined by the magnitude of the effect to be obtained, for example, for 10- fold reduction in corrosion rate is 0.05 V. The use of a hydrogen peroxide concentration above 3 g / kg, as follows from special experiments, is impractical because the protective effect does not increase. This determines the upper limit of the concentration of hydrogen peroxide (3 g / kg). the boundary (providing effects similar to those indicated in the table) is 0.7 g / kg. The effectiveness of various ways to protect Article 3 against corrosion in an aqueous medium containing 7 g / kg of sodium sulfate is given in the table.

Without protection

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Cathode zaicit

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Cathode protection HjOz, 2 g / kg

HiOj / O g / kg

Cathode protection NgOg, 0.7 g / kg From the data in the table it can be seen that when using the proposed method, the effectiveness of protection increases by about 200 times. It was established that the consumption of hydrogen peroxide as a result of its thermal and catalytic decomposition under the conditions of the proposed method is relatively high. So, when the ratio of the solution volume to the surface of the contacting metal is 100 l / m at 20 ° C and DE 0.50 V, the consumption of hydrogen peroxide is 1.65 mg / kg per hour, and at AE 0.20 V it is 0.034 mg / kg per hour. Applied the proposed method -; It increases the corrosion capacity of carbon steel by a factor of 200, which is significantly higher than the efficiency of the known methods. This makes it possible to significantly prolong the service life of the equipment, which leads to a high economic effect. The proposed method will also make it possible to simplify the technological process and reduce the risk of environmental pollution by means of hazardous substances.

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Table continuation

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1-7

4.20.10

4.39.10

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7.21.10

7.54.10

1-6

Claims (3)

1.28.101, 43.10. Formula of the Invention. A method for protecting carbon steel from corrosion in neutral aqueous media by simultaneously cathode polarizing a surface and introducing a corrosion inhibitor, o.t. and in order to increase the effectiveness of protection as an inhibitor. hydrogen peroxide at a concentration of 0.7-3.0 g / kg, and the potential of the steel is maintained in the N1-interval (-0.05) - (- 0.60) B relative to its corrosion potential in the presence of hydrogen peroxide. information taken into account in the examination 1. Ya. M. Kolotyrkin et al. Corrosi reactor materials. M., Atomizdat, 1960, p. 29. 2. Krasnoyarsk V.V. Electrochemical method of protecting metals from corrosion. M., Mashgiz, 1961, p. 26 3. Rosenfeld I. L. Delay of corrosion in neutral media. M., Publishing House of the Academy of Sciences of the USSR, 1953, p. 114