RU2533344C1 - Installation for electrochemical survey of metal corrosion - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: installation for electrochemical survey of metal corrosion (Fig. 1) includes a circuit measuring electrode potentials, a circuit measuring corrosion current and a temperature regulator. The circuit measuring electrode potentials consists of electrodes (1) submersed into solutions placed in containers (4). Solutions are coupled by a salt bridge (3). A comparison electrode (e.g., silver-chloride electrode) (2) is submerged into the solution. The switch (6) and millivoltmeter (7) allow measuring potentials of metal electrodes in regard to the comparison electrode. The circuit measuring corrosion current consists of electrodes (1) submersed into solutions placed in containers (4). Solutions are coupled by a salt bridge (3). The following equipment is coupled in series between electrodes: a toggle switch (5), a calibrating resistor (8) with high-ohmic digital millivoltmeter (9) connected to it in parallel, a resistor box (10). The temperature regulator consists of a container filled with heat medium, for example water (13), with submerged vessels with surveyed electrodes as well as a stirrer (11) and thermometer (12). Electrochemical survey of the corrosion cell is made in the following way. The installation is assembled in compliance with the diagram (Fig. 1). Metal samples are isolated partially along its length by heat shrink tube or lacquer in order to create the unite area surface and to prevent contact of metal surface with solution-air phase boundary. Then the surface is treated in compliance with GOST 9.305-84. At the open toggle switch (5) potentials of the surveyed metal samples are measured when current is unavailable in the circuit (stationary potential of the metal electrode), which is countered in regard to standard hydrogen electrode (SHE). At closure of the circuit by the toggle switch (5) the required value of electric resistance is set by the resistance box (10) and voltage drop is measured at the calibrating resistor (8) by millivoltmeter (9). The obtained voltage value is used for calculation of current intensity in the in the surveyed circuit as per Ohm's law. According to obtained experimental data corrosion diagram is plotted in coordinates E(SHE)=f(I). In Fig. 2 there is an example of such diagram. Then calculation is made for degree of anode, cathode and ohmic control as well as for weight index of corrosion.

EFFECT: simplifying installation diagram to measure current intensity of practically short-circuited corrosion cell.

2 dwg

Description

A device for electrochemical research of metal corrosion relates to the field of studying the corrosion behavior of materials in various environments by constructing corrosion diagrams, which makes it possible to assess the nature of the influence of individual factors on the corrosion rate, as well as to identify the most significant (limiting) process (establish the degree of anodic, cathodic, and ohmic control).

This device allows the study of corrosion processes by the graphical method, which has several advantages over analytical methods, namely: analytical methods are in good agreement with the experimental result when studying simple cases of a corrosion process, while graphical dependences are easily established experimentally even for the most complex cases of corrosion, which meet the actual operating conditions of metal structures [1].

The most convenient, visual and therefore common is the Evans corrosion diagram. In these diagrams, the potential value is plotted along the ordinate axis, and the values of the cathode and anode currents are plotted on the abscissa axis, regardless of the fact that they have the opposite direction.

The electromotive force (EMF) of a galvanic cell is the difference between the equilibrium (stationary) potentials of the cathode and anode, measured in the absence of current in the circuit

. $$E={\varphi }_{\mathrm{to}}^0-{\varphi }_{\mathrm{but}}^0$$

.

When the current flows in the system in accordance with the kinetic features of the cathodic and anodic processes in the system, anodic and cathodic polarization is observed, therefore, the potential difference between the electrodes decreases to U. If the ohmic losses in the corrosion system are negligible, then the potential value common to the electrodes is established and corrosion current reaches a maximum.

To calculate the corrosion rate (corrosion current density), it is necessary to take into account the kinetic features of the processes at the anode and cathode. For this, it is necessary to use anodic and cathodic polarizabilities:

и $$P_{к}=\frac{\Delta {\varphi }_{к}}{i}$$

. $$P_{\mathrm{but}}=\frac{\Delta {\varphi }_a}{i}$$

and $$P_{\mathrm{to}}=\frac{\Delta {\varphi }_{\mathrm{to}}}{i}$$

.

To calculate the corrosion rate, you can use the following equation:

$$i=\frac{{\varphi }_{\mathrm{to}}^0-{\varphi }_{\mathrm{but}}^0}{R+P_{\mathrm{to}}+P_{\mathrm{but}}}=\frac{E}{R+P_{\mathrm{to}}+P_{\mathrm{but}}}$$

Thus, the rate of the corrosion process is determined by the resistance of its individual components: ohmic control, cathodic and anodic polarizability.

The degree of anodic, cathodic and ohmic control can be calculated using the following formulas:

; $$C_A=\frac{\Delta E_A}{\Delta E_A+\Delta E_K+\Delta E_R}\cdot \mathrm{one\; hundred}\%=\frac{\Delta E_A}{E_K^0-E_A^0}\cdot \mathrm{one\; hundred}\%$$

;

; $$C_K=\frac{\Delta E_K}{\Delta E_A+\Delta E_K+\Delta E_R}\cdot \mathrm{one\; hundred}\%=\frac{\Delta E_K}{E_K^0-E_A^0}\cdot \mathrm{one\; hundred}\%$$

;

, $$C_R=\frac{\Delta E_R}{\Delta E_A+\Delta E_K+\Delta E_R}\cdot \mathrm{one\; hundred}\%=\frac{\Delta E_R}{E_K^0-E_A^0}\cdot \mathrm{one\; hundred}\%$$

,

where C _A is the degree of anode control;

C _K is the degree of cathodic control;

C _R is the degree of ohmic control;

- анодная поляризация, т.е. смещение потенциала анода при протекании через него тока, В; $$\Delta E_A=E_A-E_A^0$$

- anode polarization, i.e. the displacement of the anode potential when current flows through it, V;

- начальный потенциал анода, В; $$E_A^0$$

- initial potential of the anode, V;

E _A is the effective potential of the anode (potential of the anode when a current is passed through it), V;

- начальный потенциал катода, В; $$E_K^0$$

- initial potential of the cathode, V;

E _K is the effective potential of the cathode (the potential of the anode when a current is passed through it), V;

- поляризация катода, В; $$\Delta E_K=E_K^0-E_K$$

- polarization of the cathode, V;

- омическое падение потенциала при величине тока пары I, В; $$\Delta E_R=IR=E_K-E_A$$

- ohmic potential drop at the current value of the pair I, V;

R is the ohmic resistance, Ohm;

E _K -E _A is the effective potential difference of the electrodes at a pair current of I, V.

:Then, knowing the value of the corrosion current, calculate the weight index of corrosion $$(K_{\mathrm{at}e\mathrm{from}}^{-})$$

:

$$K_{\mathrm{at}e\mathrm{from}}^{-}=\frac{I\cdot A}{26.8\cdot n\cdot S};\left[\frac{g}{m^2}\right],$$

where I is the strength of the corrosion current, A; A is the atomic mass of the metal, g; n is the number of electrons participating in the electrochemical act, S is the surface area of the anode sample, m ² .

Determining the degree of control of each of the above components is of great practical importance, because To effectively control the corrosion process, it is necessary to influence the limiting stage.

A device is known for experimental construction of a corrosion diagram [2], with which, according to the authors, it is possible to study a closed corrosion element.

The studied electrodes are immersed in a solution, reference electrodes are supplied to them. Closing the circuit with a toggle switch, measure the corrosion current using a "zero ammeter". At the same time, the potential of one and the second studied electrodes is measured using a potential meter and a switch.

The advantages of this scheme is the simplicity of the execution and conduct of the experiment.

The disadvantages include:

1) the impossibility of adjusting the current in the circuit, therefore, there is no way to measure the potentials of the electrodes at different currents;

2) the inclusion of a milliammeter in the circuit does not allow measuring the current of the short-circuited corrosion element due to the presence of resistance of the device shunt;

3) the use of a milliammeter does not allow to investigate corrosive elements with low currents, the inclusion of a microammeter in the circuit will increase the electrical resistance of the shunt, and therefore lead to even greater errors in determining the maximum current of the corrosive macrocell.

There are other schemes designed to measure the potentials of the electrodes of the corrosion element and the current flowing under various conditions [3]. These circuits make it possible to determine the currents of a short-circuited element, as well as eliminate the influence of the internal resistance of a corrosion pair.

The closest in technical essence is the device [4], which allows you to determine the potentials of the electrodes in the absence of current in the circuit, as well as potentials at different values of current strength.

The studied electrodes and reference electrodes are placed in an electrolyte solution located in the vessels. The solutions in the vessels are connected by an electrolytic key. The potentials of the electrodes in the absence of current in the circuit are measured using a millivoltmeter and a switch. To measure the potentials of the electrodes at a certain value of the strength of the corrosion current with a closed circuit on the resistance store, set the required value of the electrical resistance and measure the current using a milliammeter, while measuring the steady-state values of electrode potentials using a millivoltmeter and a switch. The installation involves conducting research at a constant and adjustable temperature, which is achieved by using a thermostat in combination with a stirrer and a thermometer.

Based on the obtained experimental data, a corrosion diagram is constructed, the analysis of which allows us to calculate the degree of anodic, cathodic and ohmic control, as well as the weight index of corrosion.

However, this scheme has a number of significant drawbacks, namely:

- measuring the current strength with a milliammeter or microammeter leads to the fact that a shunt having its own resistance, which depends on the selected range of measurement of the current strength, is connected sequentially to the additional resistance in the circuit.

- measurement of the strength of the corrosion current in a wide range of values is impossible, because when switching the measuring range, the resistance of the shunt changes and this will certainly affect the course of processes in the investigated corrosion element.

The technical result of the proposed device is to simplify the installation scheme for measuring the current strength of a practically short-circuited corrosion element.

This is achieved by the fact that in the device for electrochemical research of metal corrosion, including a resistance store for adjusting the current and devices for recording potentials and current strength, according to the invention, instead of a milliammeter or microammeter, a high-resistance digital millivoltmeter connected in parallel to a calibrated resistor (8) is used (Fig. .3) connected in series with a resistance magazine and having a small value of electrical resistance (tenths and units of Ohms). By measuring the voltage drop across this resistor, it is possible to calculate the magnitude of the current flowing through the circuit under study from Ohm's law.

The advantages of this scheme are:

- the ability to study various corrosion elements, giving both very small values of current strength (microamps), and much larger (milliamps and above);

- the ability to measure the potentials of the electrodes of the corrosion element at different values of the current strength in the circuit up to a current close to the short circuit current;

- installation for research is easy to manufacture and use.

Not identified solutions that have signs of the proposed method.

Figure 1 presents a diagram of the proposed device.

Figure 2 shows an example of a corrosion diagram (along the ordinate axis - the value of the potential of the electrodes, and on the abscissa axis - the values of the cathode and anode currents, regardless of the fact that they have the opposite direction).

Installation for electrochemical research of metal corrosion (figure 1) includes a circuit for measuring the potentials of the electrodes, a circuit for measuring corrosion current, and a thermostat.

The circuit for measuring potentials consists of electrodes (1) immersed in solutions located in vessels (4). The solutions are connected with an electrolytic key (3). A reference electrode (e.g., a silver chloride electrode) is immersed in each solution (2). The switch (6) and the millivoltmeter (7) allow you to measure the potentials of the metal electrodes relative to the applied reference electrode.

The circuit for measuring corrosion current consists of electrodes (1) immersed in solutions in vessels (4). The solutions are connected with an electrolytic key (3). Between the electrodes are connected in series: a toggle switch (5), a calibrated resistor (8) with a high-resistance digital millivoltmeter (9) connected in parallel to it, a resistance store (10).

The thermostat consists of a vessel filled with coolant, for example water (13), in which vessels with the studied electrodes are immersed, as well as a stirrer (11) and a thermometer (12).

The electrochemical study of the corrosive element is as follows. In accordance with the scheme (figure 1) is going to install.

Metal samples are partially insulated along the length with a heat shrink tube or varnish to create a specific surface area and prevent the metal surface from contacting the solution-air interface. Then produce surface treatment in accordance with GOST 9.305-84.

With the toggle switch open (5), the potentials of the studied metal samples are measured in the absence of current in the circuit (stationary potential of the metal electrode), which is then recounted with respect to the standard hydrogen electrode (SHE). When the circuit is closed, the toggle switch (5) sets the necessary value of the electrical resistance with the help of the resistance store (10) and the voltage drop across the calibrated resistor (8) is measured with a millivoltmeter (9). The obtained voltage value is used to calculate the current strength in the circuit under study from Ohm's law.

. На фиг.2 приведен пример такой диаграммы.Based on the experimental data obtained, a corrosion diagram is constructed in the coordinates $$E_{(\mathrm{FROM}\mathrm{AT}E)}=f(I)$$

. Figure 2 shows an example of such a diagram.

:Then, the degree of anodic, cathodic and ohmic control is calculated, as well as the weight index of corrosion $$(K_{\mathrm{at}e\mathrm{from}}^{-})$$

:

. $$K_{\mathrm{at}e\mathrm{from}}^{-}=\frac{I\cdot A}{26.8\cdot n\cdot S};\left[\frac{g}{m^2}\right]$$

.

In conclusion, we can conclude about the contact corrosion mechanism of the studied metals under the test conditions (control factor), the corrosion resistance of the anode, and the efficiency of the tread.

Information sources

1. Tomashov N.D. Theory of corrosion and metal protection. - M.: Publishing. USSR Academy of Sciences, 1960 .-- 591 p.

2. Stanjala S., Kitovski A., Shafranska I. Assessment of the electrochemical properties of aluminum coatings by immersion as an anti-corrosion protection of low-carbon steels in the environment of livestock facilities // Metal Protection. - 1987. - Vol. XXIII, No. 3. - S. 419-423.

3. Rosenfeld I.L., Vashkov O.I. To the method of measuring currents of corrosion elements

4. Tomashov N.D., Zhuk N.P., Titov V.A., Vedeneeva M.A. Laboratory work on corrosion and metal protection. - M.: Metallurgy, 1961, 239 p.

Claims (1)

A device for electrochemical research of metal corrosion, including a resistance shop for adjusting current and instruments for recording potentials and current strength, characterized in that a millivoltmeter is used to measure the corrosion current strength, connected in parallel to a calibrated resistor with a known electrical resistance.

Images