RU1795355C - Method for testing high-resistivity metal for resistance to electrochemical corrosion - Google Patents

Abstract

The invention relates to a test technique. The purpose of the invention is to increase the reliability of tests. A method of testing a metal with high electrical resistivity for resistance to electrochemical corrosion consists in making the sample rod, installing it coaxially inside the working cylindrical electrode, exposing the electrode and sample to an aggressive environment for a specified time and determining the parameter by which stability is judged metal. The length of sample I is chosen from the ratio, and the length L of the worker is from the ratio 0,, 81, where d is the inner diameter of the working electrode. 3 ill. ate with

Description

The invention relates to the chemical industry, in particular to testing electrode materials for apparatuses with direct electrical heating for electrochemical corrosion.

Nowadays, heating chambers (boilers), evaporators and electrode-type installations with direct electric heating, in which Joule, heat generated during the passage of current through the paciisop electrolyte, are used, are becoming more widespread.

J3, therefore, reliable methods are needed to study the corrosion of materials that take into account vapor and factors affecting the fracture process that meet the operating conditions in devices with direct electric heating at the stages of heating electrically conductive solutions, evaporation and evaporation desorption (e.g., hydrogen fluoride and from fluorosulphate solutions); dissolution of metals in acidic and alkaline media.

A known installation for testing materials for corrosion, comprising a test chamber filled with medium, means for attaching the test sample, a current source and recording equipment connected to the means for attaching the sample. The disadvantage is the inability to condense electrolyte vapor.

Closest to the invention, the technical solution is an installation for testing materials for corrosion.

enclosing a test chamber filled with egressin medium, means for attaching the test sample, a chamber for condensing the vapor of the medium; nozzles connecting chambers; current source; re-1. logging equipment, the test chamber being made in the form of a system of coaxial cylinders of different lengths without specifying the limits of the ratio of length and diameter; a source of energy for connecting the means for attaching the samples to the wall of the test chamber.

The disadvantage of this installation is the inability to stably ensure uniform electrochemical corrosion of materials with high electrical resistivity and, as a consequence, the inaccurate determination of the corrosion index, as well as the inability to control the power of the test chamber due to changes in surface area (electrodes).

The first of these drawbacks is found if copper electrodes working in solutions of nitric, sulfuric acid or sodium chloride are replaced by electrodes with high resistivity, for example, steel, nickel and their alloys, or any other materials whose resistivity is 5 - 10 times higher than the resistivity of copper. Such a replacement will lead to an uneven current density along the central electrode, and, as a consequence, to its uneven destruction. In this case, the electrodes take the form of a needle. .

The method is illustrated in Figs. 1-3.

As an example, FIG. 2 shows the distribution of current density over the central (test) electrode (curves 1) and the change in its radius (curves 2).

The experiment was carried out for two materials: steel 0X1049 and nickel, tested in a solution of sulfuric acid with a concentration of 38 wt.% For 2.3 hours. The geometric dimensions of the test chamber were adopted as in the analogue.

In order to replace the central electrode in the prototype chamber, it is necessary to install a new mechanical seal with the inevitable stop of the entire installation. The latter violates the conditions of the corrosion test and leads to incorrect results. The working length of the central electrode varies

It is also impossible to protrude in the prototype chamber, since it is necessary to change the length of the external electrode-cylinder, i.e. almost every experiment needs a new setup, which is unacceptable for real conditions.

The above indicates that the functionality of the prototype camera is limited: it is difficult to ensure the reliability of the testing of materials closer to the operating conditions of the heating chambers of the electrode type.

The aim of the invention is to increase the reliability of tests of metals with high resistivity for electrochemical corrosion.

The goal is achieved in that the length of the inner electrode - the test sample is selected within 1 - 2.5 of the inner diameter of the outer electrode, and the length of the outer electrode of the test chamber from a ratio of 0.5 - 0.8 of the length of the test sample.

By changing the length of the active portion of the central electrode involved in the reaction, we can further change the chamber power and current density at the central electrode.

A uniform corrosion rate of materials with high vnenbHbiM resistance up to 4 Ohms is ensured due to the fact that the working part of the external electrode is 0.5 to 0.8 in height the height of the internal electrode, located symmetrically relative to the ends of the chamber. A decrease or increase in the height of the external electrode leads to a change in the resistance of the interelectrode space and, as a result, to a decrease in the tangential component of the electric field strength over the surface of the test sample. This also explains the choice of the limit of variation of the working length of the test sample (inner electrode) in the range of 1 -2.5 of the inner diameter of the outer electrode of the test chamber, as well as the ratio of the inner diameter of the outer electrode to the diameter of the inner electrode in the range of 1.5-10.

The indicated choice was based on relations determining the law of distribution of current density over the internal electrode

dh

2U0-chox pp-d-lnD / d -chox

(1)

/ VlnD / d

obtained analytically for an electrode-type coaxial chamber (Fig. 1, 1). In equation (1) are indicated: 5 (x) is the normal component of the current density over the surface of the test sample with a height

21 at a distance X from its surface: U0 is the voltage of the source; / and / EPR are the resistivities of the solution and the material of the internal electrode (sample); d and D are the diameters of the inner (sample) and outer Electrode.

: The uniformity of the distribution of current density over the internal electrode was estimated using a coefficient characterizing the ratio of current densities at the end and beginning (the place of connection of the source) of the electrode:

TO

1

end

(2)

An analysis of expressions (1) and (2) shows that an increase in I / D, D / d, pnp decreases the value of K, and an increase in pp leads to an increase in K.

It is limited by the limiting values of IamoDo 2.1 Ohm m, Rpr A m and I D / d 10, we determine the boundary of Change K in accordance with Fig. 2 in the limits 1 I / O 2.5.

  . Within the specified boundary, | and I / B changes the uniformity of the current density is not lower than 0.999, which gives an almost uniform corrosion rate of the internal electrode.

An increase in the length of the sample above 2.5 D leads to a sharp decrease in the coefficient K. A decrease in I above D, although it leads to equalization of the current density, is not accompanied by a sharp decrease in the power of the installation:

P (l)

-d-Uj-tha-l V np-pp-lnDTd

(3)

Using a double-sided circuit n; itani from an energy source is equivalent to n; about its effect of halving the height of the test chamber without reducing its power (efficiency). The fundamental difference between the method is as follows: 1) the outer and inner electrodes are made of the test material, 2) the working length of the inner electrode is adjusted from the ratio of 1 - 2.5 of the inner diameter of the outer electrode, and the length of the outer electrode is 0.5 - 0.8 from the maximum length of the inner electrode, 3) the ratio of the inner diameter of the outer electrode to the diameter of the inner electrode can vary within I: 0.5 D / d 10.

The proposed method is implemented in the installation shown in Fig. 1. It contains a test chamber 1 filled with a high-temperature, highly aggressive medium 2, an internal electrode 3, coated with an electrically insulating material 5, a self-regulating diaphragm type seal 5. an external electrode 6. There are fluoroplastic gaskets between the camera body and the external electrode 7. The middle part of the chamber made of test subject

0 material, and its detailed part is made of electrically insulating corrosion-resistant material, while the inner diameter of the chamber along the entire length is constant. The chamber 1 contains the input pipe 8 and output

5 solution. The AC source 9 provides heating of the chamber 1 due to the alternating electric current. The recording equipment is made in the form of a potentiometer 10 with a filter 11 and supplied with a voltage stabilizer 12. The temperature at the inlet and outlet of the chamber is recorded by thermocouples 13. For condensation of vapor, there is a condenser 14 with phase separator 15. To maintain a constant temperature at the inlet to the evaporative The chamber has an additional thermostat 16. A pump 17 is installed for forced circulation of the solution. The tightness of the chamber is ensured by

0 tie rods 18, gaskets 7 and self-adjusting seals such as diaphragm 5.

These seals 5 are mounted in the end parts 19 of the test chamber 1 and

5 are devices that essentially copy the operation of the diaphragm of any camera, but with the only difference being that in the camera with the help of a diaphragm made in the form of rigid shutter-segments, the necessary hole is set for the passage of the light beam, and in the camera we offer - sectors (segments) of the diaphragm are elastic, they seem to fit (clasp) the central electrode

5 test chamber with stockings - insulators, allowing you to change the outer diameter of this electrode. Elastic segments not only play the role of a stuffing box seal similar to glands used in mechanical engineering (for water compaction), but also centers the test sample — the central electrode 3 with stockings — insulators 4 with respect to the external electrode b, which is very important for zoom in

5 conditions for testing materials for electrochemical corrosion to operational: and increasing the reliability of the obtained experimental data. At the same time, the diaphragm seals 5 ensure the tightness of the entire test chamber 1, therefore, the medium 2 does not go beyond the chamber 1. The elasticity of the diaphragm seals is such that the tightness of the chamber is ensured even in the absence of the test sample mounted in the center of the chamber 1. This allows no interruption experiment (when the solution is circulating in the circuit of constant process parameters), (without disassembling the test chamber), replace the test sample, namely: its material and dimensions (diameter, length and working part). Like all packing glands, self-regulating diaphragm type seals are installed in the landing sheets of the end faces of the chamber 19 according to the principle of blind fit.

The device operates as follows: In the end parts 19 of the test chamber 1, by means of fasteners 18, the test sample 3 is placed with stockings-insulators 4 and filled with a high-temperature highly aggressive medium 2. Using the AC source 9, the specified current density in the test chamber 1 on the outer electrode b and the test sample 3, which is the inner electrode, The material of the test sample 3 and the outer electrode 6 is the same. As a result of electrochemical corrosion, the cross section of the sample changes, as a result of which its ohmic resistance changes and the depth index of corrosion is determined. A constant concentration of the medium is achieved by condensation of steam in the condenser 14, from where it enters the separator 15 and thermostat 16. Using the pump 17, the solution is supplied through the pipe 8 to the chamber 1. The temperature of the solution at the inlet to the pipe 8 and the chamber 1 are constant. by using an additional thermostat 16.

The test results are shown in figure 4 for the same materials as in fig.Z,

but for a ratio of 0.5 I / O 10. It can be seen from the results of Fig. 4 that the selected boundary of variation provides a high uniformity in the distribution of current density and, as a result, an almost uniform corrosion rate along the inner electrode. After testing, the radius of the inner electrode does not change shape, remaining almost cylindrical,

Thus, the proposed method

ensures uniform distribution of current density along the length of the electrode and allows you to bring the conditions of testing of electrode materials closer to operational, extends the functionality

chamber, by changing the current density, the diameters of the inner electrode, i.e. test reliability is provided.

Formula A method for testing a metal with high resistivity for resistance to electrochemical corrosion, according to which the sample is made with a rod, set it coaxially inside the working cylindrical electrode, expose the electrode and the sample to aggressive

a specified time and determine the parameter by which metal stability is judged, characterized in that, in order to increase the reliability of the tests, the length of the sample I is selected from the ratio d I 2.5d, and the length L of the working electrode from the ratio 0.5 L 0.81, where d is the inner diameter of the working electrode.