Determination of corrosion of metals by a stray current
Corrosion of metals by stray currents is a particular case of the effect of the electric field in the electrolyte on the process of electrochemical corrosion of metals, which has a very practical significance.
The electric current in the ground, branched off from its main path, is called wandering. The source of the wandering current can be electric railways, electrolyzers, welding machines, cathodic installations, transmission lines, etc. The ground is a parallel conductor, for example, of rails, and depending on the magnitude of the electrical resistance of rails and ground, the current is sometimes very large (hundreds of amperes) on the ground. Wandering currents have a range of up to tens of kilometers away from current-carrying structures.
Consider the influence of wandering currents from electrified rail transport on the corrosion of underground structures (Figure 7.6). When the traction current flows along the rails along them, a voltage drop occurs and the rails receive some positive potential relative to the ground, and imperfect rails insulation from the ground causes leakage of current to the surrounding ground. And the greater the longitudinal resistance and the less the transient electrical resistance, the greater the total leakage current to the ground. Near the substations and points of connection of the cables of the current collectors to the rails, there are sections of constant negative polarity of the rails, and at a considerable distance from them there is a constant positive polarity. Wandering currents flowing in the earth, encountering metal structures on their way, penetrate into them and flow along them on some extent, creating anode and cathode zones.
Fig. 7.6. Corrosion of the pipeline from stray currents electrified
The peculiarity of the process of metal corrosion in the field of stray currents is that it is an electrolytic process that proceeds according to the laws of electrolysis. At the cathode sites of the structure, the hydrogen reduction reaction will take place, and the oxidation reaction of the metal will occur at the anode sites.
The presence of cathode and anode sites on the structure is determined by the direction of the current. The site of the current input or output of the electron input is a cathode, the current output or electron input structure is an anode.
If the electric current is constant, then the sections of the metal from which the positive ions enter the electrolyte are anodes and subjected to electrocorrosion; The areas on which the positive ions pass from the electrolyte to the metal are the cathodes on which the reduction process takes place. The corrosion rate is proportional to the flowing current.
Variable stray currents are also corrosive, but less so than permanent. The intensity of the corrosion of metal in the field of stray currents is usually much greater than the intensity of soil corrosion. These two processes are superimposed on each other. In this case, the coincidence of the anode zones of corrosion elements and wandering currents leads to increased corrosion. For sufficiently large potentials of wandering currents, the current suppresses the current of the cathode microelement that occurs during the corrosion process, spreading the destruction to all microstructures of the structure in the anode zone of stray currents.
Under the influence of wandering currents, the value of the polarization potential of the structure shifts in the anode zones in a positive direction with respect to the stationary potential, in the cathode ones, into the negative one. The magnitude of the corrosion rate is determined by the leakage current density .
Determination of the presence of stray currents in the earth . The necessary equipment for determining stray currents includes: voltmeters with an internal resistance of at least 1 MΩ with measurement limits: 0.5 ... 0 ... 0.5 V; 1.0 0 ... 1.0 V; 5.0 ... 0 ... 5.0 V or
other, close to the indicated limits, two copper-sulfate comparison electrodes. Copper-sulfate electrodes are located parallel to the future route of the structure, and then perpendicular to the axis of the track.
The potential difference on the route of the projected structure is measured between two points of the earth every 1000 m along two mutually perpendicular directions when the measuring electrodes are spaced by 1 (10 m to detect stray currents) .The voltmeter reading is taken at each point after 10 s for 10 min If the measured value exceeds (in absolute value) 0.040 V or the largest swing of the measured value (the difference between the largest and the smallest values) in time exceeds 0.040 V (in both cases, taking into account the difference in potential between the applied reference electrodes), then the presence of stray currents is recorded at this measurement point.
Determination of the dangerous influence of a wandering direct current . Samples to determine the hazardous effect of a wandering DC are parts of underground structures. The following equipment is used: a voltmeter with an internal resistance of ns less than 1 MΩ, a copper-sulfate comparison electrode, an electrode in the form of a steel rod.
Measurements are made at control points, wells, pits or from the ground surface at the minimum possible distance (in plan) from the pipeline. The positive terminal of the voltmeter is connected to the construction, the negative terminal to the reference electrode. The duration and the measurement mode, as well as the step between the measurement points along the route, shall be established in accordance with the regulatory documents. When measuring in the areas of action of stray currents, where the amplitude of the oscillations of the measured potential difference exceeds 0.5 V, steel electrodes can be used instead of copper-sulfate comparison electrodes, with the exception of measurements at communication facilities.
The stationary potential of the underground structure is determined when the electrochemical protection means are switched off by continuously measuring and recording the potential difference between the construction and the copper-sulfate comparison electrode for a sufficiently long time, up to detecting the practically unchanged potential value (within 0.04 AT). Typically, this refers to a period of interruption in the movement of electrified transport, for example in cities at night, when there is no stray current. For the stationary potential of the structure, the average value of the potential is taken with the difference of the measured values not more than 0.04 V.
If the stationary potential is not measurable, its value relative to the copper-sulfate reference electrode is assumed to be:
• minus 0.70 V - for steel;
• minus 0.4 V - for lead;
• minus 0.70 V - for aluminum.
The difference Δ (i, B, between the measured potential of the structure and the stationary potential is calculated by the formula
where C/is the most negative or most positive instantaneous potential difference between the structure and the copper-sulphate reference electrode. AT; U CT - The stationary potential of the structure, V.
If the maximum swing in the potential of a structure measured relative to a copper-sulfate reference electrode (the absolute potential difference between the largest and the smallest values) does not exceed 0.04 V, the displacement of the potential does not characterize the dangerous action of stray currents.
Determination of signs of biochemical corrosion
Biochemical corrosion is caused mainly by bacteria found in soils, reservoirs, groundwater. Biocorrosion of metals can also cause various genera of microscopic fungi. Biochemical corrosion is seasonal in nature and most intense in the spring, when the best conditions for the development and existence of bacteria are created. Bacteria that cause corrosion are divided into aerobic and anaerobic. Aerobic bacteria exist and develop only if there is oxygen, anaerobic bacteria - in its absence.
Aerobic bacteria , contributing to corrosion, are subdivided into sulfur-oxidizing bacteria and ferrous bacteria precipitating iron. The first develop best in an acidic environment (pH = 0 ... 6). The essence of their action is that they can oxidize sulfur to sulfuric acid, the local concentration of which can reach up to 10%. The iron bacteria develop well in the pH range 4 ... 10. They absorb iron in the ionic state and are isolated as insoluble compounds. These compounds are unevenly distributed over the surface of the metal and have an electrode potential different from the parent metal. The resulting electrochemical heterogeneity of the surface enhances corrosion.
Anaerobic bacteria are basically sulfate-reducing and restore sulfate ions to sulfide ions. These bagers develop at pH = 5 ... 9, actively absorb the hydrogen released during the cathodic process. The presence of sulphides and hydrogen sulphide in a corrosive environment accelerates corrosion. The ions of ferrous iron in the solution bind the liberated hydrogen sulphide, a deposit of iron sulphide is formed on the surface of the metal, which facilitates the course of the cathodic depolarization reaction. Corrosion has a pitting character .
Biocorrosive aggressiveness of the soil  at the depth of the underground structure is determined by the following qualitative features:
• the color of the soil (greyish, greenish and gray tones indicate an anaerobic situation with excessive moistening and the predominance of reduced forms of iron, aluminum, manganese);
• the presence in the soil of reduced sulfur compounds, which are the products of the life-activity of sulfate-reducing bacteria.
Determination of the presence of reduced sulfur compounds . For a sample of soil, corrosion products, or deposits of 1 cm in volume, 2-3 drops of hydrochloric acid, diluted with distilled water in a ratio of 1: 3 (initial acid density 1.47 g/cm), are applied using a pipette. - According to the characteristic odor of the released hydrogen sulfide, the presence of reduced sulfur compounds (hydrosulfides, iron sulfides, certain organic sulfur compounds).
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