What is electrochemical corrosion

Electrochemical corrosion: description of the process, objects of destruction, methods of combating

Thousands of years of development of civilization would have been impossible without the metal from which both arrowheads and spears of the prehistoric period, and the most complex machines of our time were made.

Entire eras bear “metal” names: bronze, copper, iron. Metallurgical plants work around the clock to provide the industry with the necessary quantity of metal blanks.

Machine-building enterprises make a huge range of products from them, from pipes, rails and sheets, to needles and pins.

Metal corrosion , especially its main type - electrochemical, has always created difficulties in the operation of any metal products, destroying them untimely. The simplest tools (knife, ax, plow) quickly became unusable in a humid environment. It took numerous and lengthy studies of chemical destruction processes before technical solutions were found that stopped the corrosion of metals.

Process description

Electrochemical corrosion is a process that occurs in the presence of:

• electrolyte;
• metals with low and high redox potentials (electrode potentials).

The electrolyte is formed by water, condensate, and any natural precipitation. The presence of two types of metal almost always does not happen, and is due to two factors:

1. Heterogeneity of the product, that is, the presence of foreign inclusions.
2. Direct contact with products made of various metals.

In an electrolyte, inhomogeneous metals form a short-circuited galvanic cell called a corrosion cell. This combination leads to the dissolution of metal with a lower electrode potential, which is called electrochemical corrosion. The speed of this process strongly depends on the presence of salts in the solution and its temperature.

Main objects of corrosion

Inhomogeneous metal areas are randomly located on the surface of the product and depend on the technology and quality of their manufacture, so corrosion damage is often local in nature. In addition, the locality of corrosion depends on heterogeneity:

• protective oxide films;
• electrolyte;
• influence of external factors (heating, irradiation);
• internal stresses causing uneven deformation.

Welded and riveted joints are prominent representatives of the contact of foreign metals subject to active electrochemical corrosion. Welding and riveting are the most common technologies in the construction of permanent joints in all leading industries and large pipeline systems:

• mechanical engineering;
• shipbuilding;
• oil pipelines;
• gas pipelines;
• water pipes.

The most significant damage to welds and rivet joints occurs in sea water, the presence of salt in which significantly accelerates the corrosion process.

A catastrophic situation arose in 1967 with the ore carrier Anatina , when sea water from high storm waves entered the ship’s holds. Copper structures in the interior of the holds and the steel hull contributed to the creation of a corrosive element in the seawater electrolyte. Transient electrochemical corrosion caused softening of the ship's hull and the creation of an emergency situation, leading to the evacuation of the crew.

The positive effect of electrochemical corrosion is very rare. For example, when installing new pipes in hot heating systems of residential buildings. The threaded connections of the couplings begin to flow during the initial start-up until corrosion products consisting of hydrated iron fill the micropores in the threads.

Regardless of the type of corrosion, chemical or electrochemical, its consequences are the same - the destruction of products of enormous value . Moreover, in addition to direct losses from materials that have become unusable, there are indirect losses associated with product leaks, downtime when replacing unusable materials and parts, and violations of technological process regulations.

Numerous studies and the development of technical progress have led to the creation of a whole system of methods and means to combat corrosion. There are three main directions in corrosion protection:

1. Constructive decisions.
2. Active methods.
3. Passive methods.

Design solutions consist in choosing materials that are minimally susceptible to corrosion due to their physical properties:

• stainless steels;
• alloy steels;
• non-ferrous metals.

Active methods of struggle were suggested by electrochemical corrosion itself. A constant voltage is applied to the protected metal structure in such a way as to increase its electrode potential and slow down the process of electrochemical dissolution. The second option for active protection is a sacrificial anode, which has a low electrode potential, as a result of which it is destroyed instead of the protected object.

Passive methods consist of applying protective coatings. Technical progress in this area began to develop with the application of simple paint and varnish coatings that prevent the penetration of oxygen, moisture and condensation onto the surface of metals. Then electroplated coatings appeared based on:

• zinc - galvanizing;
• chromium - chrome plating;
• nickel - nickel plating.

Galvanized iron, nickel-plated and chrome-plated cutlery, food cans serve for many years, not succumbing to electrochemical corrosion, maintaining a beautiful appearance, and preventing food spoilage.

Technical progress in the development of corrosion control methods

Since the loss of metal from corrosion is astronomical, technological progress continues to offer new methods of combating it as research advances and hardware improves . These include:

• thermal spraying, forming ultra-thin protective coatings;
• thermal diffusion coatings that create durable surface protection;
• cadmium plating, which protects steel in sea water.

The growth of industrial production occurs with a constant increase in the production of metal products. Electrochemical corrosion, regardless of the historical era, poses a constant threat to a huge number of structures and critical structures. Therefore, the creation of new methods and means of struggle is one of the tasks of research into technological progress.

Source: https://tokar.guru/metally/elektrohimicheskaya-korroziya-opisanie-processa-i-metody-borby.html

Metal corrosion, electrochemical corrosion. Rust

Iron oxidation. Rust

Rust is a complex substance formed by the chemical reaction of water and iron. Already at an air humidity of 50%, the surface of the iron is covered with a layer of water 15 molecules thick. Corrosion intensifies in the presence of chloride ions Cl-, which contribute to the dissolution of rust and its transition into solution in the form of a chemical compound [Fe(H2O)2Cl4].

NaNO2 solution, glycerin, amines (butylamine) and their salts are used as corrosion inhibitors (substances that slow down metal corrosion). The white precipitate of Fe(OH)2 iron II hydroxide, present in the rust solution, quickly oxidizes in air, turning into yellow-brown iron(III) hydroxide.

Unlike Fe(OH)2, the new iron (III) hydroxide Fe(OH)3, which is amphoteric, although its acidic properties are much weaker than basic; it dissolves only in concentrated alkalis when heated:

Fe(OH)3+3KOH→ K3[Fe(OH)6].

When iron is oxidized, the reaction intermediate is a dirty green precipitate of hydroxide containing Fe(II) and Fe(III).

An interesting but unpleasant property of rust is that it occupies a much larger volume than iron occupied. This can cause catastrophic consequences: although rust appears loose and soft in appearance, as it grows, gigantic forces develop. When in London, according to the design of Christopher Wren in 1675-1710.

When they built the grandiose St. Paul's Cathedral, the stone blocks of the bell tower were connected for strength with iron brackets, which were laid in grooves hollowed out in the stone. Over hundreds of years, the brackets rusted, increased in volume and began to lift the stonework, causing the bell tower to warp.

Engineers have calculated: the pressure developed by rust is so great that it would lift even a two-kilometer layer of stones! I had to dismantle the masonry and insert new stainless steel brackets into the grooves.

The rate of iron oxidation ( the appearance of rust ) very much depends on the general contact surface between metal and air. Thus, an ordinary nail, even if it is very hot, does not soon turn into scale.

When exposed to high heat, small sawdust quickly burns, and when introduced into a flame, it flares up in the form of sparks. By a chemical method - the reduction of iron oxides with hydrogen - you can obtain even finer iron powder; it is called pyrophoric. It bursts into flames in air even at normal temperatures.

Many other metals in a finely crushed state, as well as the oxide FeO, can also be pyrophoric.

Metal corrosion

Corrosion of metals - (from the Latin corrodere - to gnaw ) processes occurring as a result of chemical exposure to the environment, as a result of which their destruction occurs.

Another process differs from this process - erosion of metals , which is the destruction of their surface under the mechanical influence of the environment.

Corrosion of metals means corrosion, which also begins on their surface. There is a chemical interaction with the environment.

This process is spontaneous and is also a consequence of redox reactions with environmental components.

As a result of the destruction of a metal, its oxidation products are formed, namely: oxides, hydroxides, and sometimes it simply dissolves in the medium to an ionic state. This transformation is accompanied by a significant change in properties.

Various types of metal corrosion . One of its main types is chemical , which is sometimes also called gas corrosion , since sometimes it occurs under the influence of gaseous components from the environment at high temperatures.

Chemical corrosion can also occur under the influence of certain aggressive liquids. The main thing of this process is that it occurs without the occurrence of electric current in the system.

It affects parts and components of machines operating in an oxygen atmosphere at high temperatures, such as turbine engines, rocket engines and some others, and also parts and components of chemical production equipment.

Another common type of destruction of metals is electrochemical corrosion - surface destruction in an electrolyte environment with the appearance of electric current in the system. Electrochemical corrosion is destruction in the atmosphere, on soil, in water bodies, and in soils.

The nature of destruction of the metal surface can be different and depends on the properties of this metal and the conditions of the process. Now let's take a closer look at electrochemical corrosion.

Types of corrosion

Types of corrosion: uniform, uneven, selective, spotty, pitting, cracking, intergranular

The main types of corrosion observed include:

Electrochemical corrosion occurs more intensely if the cathode contains a metal that is less active than the corroding one. For example, if steel corrodes (and steel is an alloy of iron and carbon in which iron carbide is partially formed), the role of such areas is played by iron carbide (FeC).

Atmospheric corrosion

- occurs in moist air at normal temperatures. The metal surface is covered with a film of moisture containing dissolved oxygen. The intensity of metal destruction increases with increasing air humidity, as well as the content of gaseous oxides of carbon and sulfur in it, if there are roughnesses and cracks in the metal that facilitate moisture condensation.

Soil corrosion

— pipelines, cables, and underground structures are susceptible to it. In this case, the metals come into contact with soil moisture containing dissolved oxygen. In damp soil with high acidity, pipelines are destroyed within six months after they are laid (of course, if measures are not taken to protect them).

Electrical corrosion

- occurs under the influence of stray currents arising from extraneous sources (power lines, electric railways, various electrical installations operating on direct electric current).

Stray currents cause destruction of gas pipelines, oil pipelines, electrical cables, and various structures. Under the influence of electric current, electron entry and exit areas—cathodes and anodes—appear on metal objects on the ground.

The most intense destruction is observed in the anodic areas.

Electrochemical corrosion

- the most common type of destruction of metals. An example of electrochemical corrosion is, for example, the destruction of machine parts, instruments and various metal structures in soil, groundwater, river and sea waters, in the atmosphere, under moisture films, in technical solutions, under the influence of cutting fluids, etc.

As already noted, electrochemical corrosion occurs on the surface of metals under the influence of electric currents, that is, redox chemical reactions occur, characterized by the release of electrons and their transfer, as cathode and anodic sites are formed.

The formation of cathodes and anodes is facilitated by the chemical heterogeneity of metals (impurities and inclusions), the presence of areas of residual deformation, the heterogeneity of protective films covering the metal, etc. Most often, not one factor, but several, is involved in the formation of this type of metal destruction.

When the metal begins to corrode, it turns into a multi-electron galvanic cell.

For example, consider what happens if copper Cu comes into contact with iron Fe in an electrolyte environment. Such a system is a galvanic cell, where iron is the anode (“+”) and copper is the cathode. Iron donates electrons to copper and goes into solution in the form of ions. Hydrogen ions move towards the copper, where they are discharged. The cathode gradually becomes more negative, eventually becoming equal to the anode's potential and corrosion slows down.

You can conduct an experiment on this topic at home. We will need three glasses with a solution of table salt (table salt), 3 iron nails, a piece of zinc and copper wire (without insulation). So let's get started. Place the first nail in a glass of salt solution. Screw a copper wire to the second nail, and a piece of zinc to the third. Then dip each nail in a glass of salt solution (there were 3 of them) and leave them for 2-3 days.

What happens: All of our nails will show signs of rust (corrosion). The worst condition will be the nail that was in the solution together with the copper wire, and the least corroded is the one that is tied to zinc! Explanation: All metals have different abilities to donate electrons. You can compare them in this ability by familiarizing yourself with a number of metal voltages :

Li← K← Rb← Cs← Ba← Ca← Na← Mg← Al← Mn← Cr← Zn← Fe← Cd← Co← Ni← Sn← Pb← H2← Cu← Ag← Hg← Pt← Au

Those metals that are to the left in the voltage series (for example, Zn - zinc is to the left of Fe - iron) give up their electrons more easily than the metal on the right (for example, Cu - copper is to the right of Fe - iron). This means that as soon as both metals enter the electrolyte (current conductor - salt solution), they immediately form a galvanic couple. The more active metal (to the left) is charged positively, and the less active one is negatively charged.

Let's return to our experience: the same thing happened in our glasses with solutions. Iron (Fe) is to the left of copper (Cu), so it charges positively while oxidizing quickly. In a glass of zinc, zinc (Zn) is more active than iron. Therefore, until all the zinc rusts, the iron will not collapse (which is often used for technical purposes).

Source: https://www.kristallikov.net/page25.html

How to prevent galvanic corrosion

Corrosion is the process of destruction of metal under the influence of moisture, aggressive substances with which the coating or product comes into contact during operation. This phenomenon is common and can be seen at home, on the street and at work. When metal finds itself in an alkaline, acidic environment, oxidation begins, which over time turns into rust. In other words, chemical corrosion occurs.

Today we’ll talk about another type of corrosion, which is formed due to “stray currents”. It is what appears on taps, heated towel rails and other plumbing equipment. The destruction of metal by “stray currents” is called electrochemical corrosion.

Many metals are susceptible to this corrosion process, including reliable and durable stainless steel. Under the influence of electromechanical influence, the product quickly loses its attractiveness, and seams, joints, and metal walls are destroyed. As a result, there is a threat of emergency situations, including serious leaks, threatening neighbors below with flooding and property damage.

How to Determine Electrochemical Corrosion

Let's consider an example of the formation of signs of electrochemical corrosion on a heated towel rail.

Equipment is made from all kinds of raw materials. Stainless steel models are considered the best option, since the operational period of such a product is much longer than the period of use of analogues made of other metals. However, stainless steel is not always able to cope with the aggressive effects of electrochemical corrosion.

The initial process of destruction is indicated by almost imperceptible spots of rust that form on the surface of the product; then they increase in size, which means the process progresses and becomes deeper.

If you clean rust with a coarse abrasive, then a black dot is hidden under it, indicating the active development of destruction. Thus, rust “eats” the metal from the outside and inside, creating a small hole. In such cases, the entire product, including fittings, is damaged. Corrosion, as a rule, begins to develop in weak areas, which are welds.

If rust is found on a product made of high-grade steel, then the problem most likely lies in the presence of electricity in the water.

Electrochemical corrosion works with the support of “helpers”, for example, chlorine, which is used by enterprises to disinfect water, oxygen oxidizer, calcium salts, magnesium and other substances. Rust actively spreads under the influence of hot water - if the liquid inside the pipe heats up above 70°C, the destructive effect accelerates.

Why does corrosion occur?

To understand the whole pattern of rust on plumbing equipment, it is worth knowing where electricity comes from in water.

Among the reasons:

1. If the house is an old building, it may not have high-quality grounding, which is mandatory if the apartment has many household appliances and modern plumbing, for example, a whirlpool bath, water heaters, etc.

   Grounding ensures the safety of using equipment.

   When operating equipment that has certain defects, leakage of currents occurs, which are sent to the risers and interact with water.

   Thus, pinpoint “breakdowns” appear, which provoke the formation of rust.

   If the grounding is carried out in accordance with the standards, such troubles will not appear.

2. If people who like to save money on utility bills for electricity live in the house, the problem of electrochemical corrosion arises. Thus, unscrupulous residents use the riser as a neutral wire or use special devices to “twist” the electricity meter readings. As a result of these and other manipulations, the risk of corrosion processes is high. The situation is also unsafe for life - if you touch the pipe, there is a risk of electric shock.
3. The potential difference between metals provokes the formation of trouble. Currents occur when two different metals come into close contact with each other. If the design of the house is carried out in compliance with norms and standards, such a problem should not arise, because conductive components are grounded. Protective measures help equalize potentials.
4. Today, new materials used for the manufacture of pipes and plumbing equipment are widespread: metal-plastic, polyethylene, etc. Along with this, new problems have arisen. So, if a plastic analogue is used when replacing part of a pipe, a potential difference may form, contributing to the appearance of “stray currents”. Despite the fact that plastic belongs to the group of dielectrics, it is located in an aqueous environment, which becomes a conductor - the appearance of rust in this case is almost inevitable.
5. Not only risers, but also communications located underground are susceptible to the formation of currents. Stray electricity in them can also enter the apartment. Underground systems are affected by electric transport (trams, trolleybuses, metro trains). Leakage currents appear, and only operating companies that are obliged to carry out thorough checks can fix the problem.
6. Problems often arise if, not far from the pipes responsible for water supply and heating, there is electrical wiring with poor quality insulation. Damage provokes the appearance of currents that wander through the water supply pipes.
7. Among the causes of corrosion is static electricity that accumulates on metal when it comes into contact with water.

It is not possible to monitor compliance with recommended standards and compliance with the rules for installation and use of equipment in a high-rise building. Often, such work is performed by craftsmen with no or minimal experience.

The result is emergency situations and unpleasant consequences for the residents of the house. If the heated towel rail is corroded, you may not have anything to do with it, and also you will not suspend the process due to the destruction of engineering equipment for reasons beyond your control.

There are several measures that will help prevent metal destruction:

1. When purchasing a heated towel rail, do not install it. The work should be entrusted to experienced craftsmen who have permission to carry out such events.
2. The structure necessarily requires grounding, which is especially important for those who have installed plastic pipes located between the common riser and the heated towel rail.

There are several options available for grounding equipment:

1. If the riser and its elements are made of metal, it is necessary to connect the riser, for example, using a clamp and a copper wire with a cross-section of at least 4 mm2. Then the hot water supply riser is connected by wire to the PE bus and the electrical panel located on the floor. To ensure complete safety during operation of the heated towel rail, other conductive objects are grounded, including bathtubs made of stainless steel and cast iron.
2. When the riser and its components are made of polymers, it is necessary to install a metal insert corresponding to the diameter of the pipe. The part is inserted between the heated towel rail connection and the ball valve. A grounding clamp is mounted on the insert. Using a copper wire, connect to the electrical panel.
3. In the case where the riser consists of plastic and metal elements, install a metal insert between the mounted equipment and the ball valve.

   A clamp is attached to it and connected to the switchboard via a copper cable.

4. Another solution to the problem is to install a system for equalizing potentials. Installation is carried out directly in the bathroom. This step will avoid corrosion and also minimize the risks associated with electric shock. If polymer pipes are responsible for the water supply system, then grounding leads are also connected to the nearest electrical panel.
5. There are many models of heated towel rails on sale.

   Experts recommend not purchasing designs that are too cheap. For safe and long-term operation, you should pay attention to equipment equipped with protection against “stray currents”. Heated towel rails work on the basis of a polymer that is used to treat the inside of the pipe. It eliminates contact of water with metal.

6. The polymer is completely safe, as it does not contain harmful substances that can harm humans. It is not afraid of excessively high temperatures and is not subject to destruction.

   The cost of such products is slightly higher than standard products, while the service life exceeds the operating period of analogues several times.

7. Electrical corrosion can also be eliminated by replacing the equipment with electrical equipment. In this case, the heating of the heated towel rail begins when the device is connected to the outlet. As a rule, there is a button on the case that allows you to turn it on or off. This is a good solution for many people looking for a safe and durable option.

   The device operates autonomously and does not depend on the functionality of the water supply system, which means you can warm up the room and dry things even if there is no hot water in the tap. The disadvantage of the device is the consumption of electricity, for which you will have to pay.

Important! If the choice fell on electrical equipment, you should know that it has low power, so it can operate from a stationary power source - an outlet.

However, keep in mind that the device operates in a bathroom where high humidity prevails, which means its connection must be made through an RCD and an automatic circuit breaker.

We tried to tell you all the most important things about electrochemical corrosion and methods for its prevention/elimination. You will probably not encounter such a problem, but if it has already been discovered, it is better to immediately report the problem to an engineer at the management company.

The specialist should tell you what measures should be taken and what equipment will be the best option for your bathroom. After installation, the engineer will check the heated towel rail for leaks and also sign the certificate of commissioning of the device.

It is advisable to entrust the installation of a heated towel rail to an experienced technician, because the durability of the device, as well as the safety and comfort of users, largely depends on the correct installation.

Source: https://sanline.by/articles/jelektrohimicheskaja-korrozija

Causes of electrochemical corrosion and methods of protecting metal

Electrochemical corrosion of metals has been one of the pressing problems of humanity since the moment metal products began to be used in the production of various objects necessary for humans. The problem of protecting metals from corrosion has always been acute, because under the influence of destructive oxidation processes, objects lost functionality, became deformed and became unusable, and it was necessary to look for ways to protect them.

When chemistry emerged as a separate science, and the use of metals began to acquire widespread industrial significance, humanity began to explore these processes and look for ways to combat destruction from external influences.

What is corrosion

The process of destruction of the top layer of a metal material under the influence of external influences is called corrosion in the broad sense.

The term corrosion in this case is only a characteristic of the fact that a metal surface enters into a chemical reaction and loses its original properties under its influence.

There are 4 main signs by which you can determine that this process exists:

• a process that develops on the surface and eventually penetrates into the metal product;
• the reaction occurs spontaneously because the stability of the thermodynamic balance between the environment and the system of atoms in the alloy or monolith is disrupted;
• chemistry perceives this process not simply as a destruction reaction, but as a reduction and oxidation reaction: when entering into a reaction, some atoms replace others;
• the properties and characteristics of the metal undergo significant changes during such a reaction, or are lost where it occurs.

Metal corrosion

Corrosion - destruction of the surface of steels and alloys under the influence of various physical and chemical factors - causes enormous damage to parts and metal structures. Every year this invisible enemy “eats” about 13 million tons of metal. For comparison, the metallurgical industry of the European Union countries produced only 0.5 million tons more in 2014. And these are just direct losses. And long-term operation of steel products without their effective protection against corrosion is generally impossible.

What is corrosion and its types

The main reason for intense oxidation of the surface of metals (which is the main cause of corrosion) are:

1. Increased environmental humidity.
2. Presence of stray currents.
3. Unfavorable atmospheric composition.

Accordingly, the chemical, tribochemical and electrochemical nature of corrosion is distinguished. It is they, in the totality of their influence, that destroy the bulk of the metal.

Chemical corrosion

This type of corrosion is caused by active oxidation of the metal surface in a humid environment. The undisputed leader here is steel (excluding stainless steel). Iron, being the main component of steel, when interacting with oxygen forms three types of oxides: FeO, Fe2O3 and Fe3O4.

The main trouble is that the oxide corresponds to a certain range of external temperatures, so practical protection of steel from corrosion is observed only at temperatures above 10000C, when a thick film of high-temperature oxide FeO itself begins to protect the metal from subsequent rust formation.

This process is called bluing, and is actively used in technology to protect the surface of steel products. But this is a special case, and in most cases it is impossible to actively protect the metal from corrosion in this way.

Chemical corrosion is activated at elevated temperatures. The tendency of metals to chemical oxidation is determined by the value of their oxygen potential - the ability to participate in redox reactions. Steel is not the worst option: lead, cobalt, and nickel, in particular, oxidize more intensely.

Electrochemical corrosion

This type of corrosion is more insidious: the destruction of the metal in this case occurs due to the combined influence of water and soil on the steel surface (for example, underground pipelines).

Wet soil, being a slightly alkaline environment, promotes the formation and movement of stray electrical currents in the soil. They are a consequence of the ionization of metal particles in an oxygen-containing environment, and initiates the transfer of metal cations from the surface to the outside.

The fight against such corrosion is complicated by the difficulty of diagnosing the condition of the soil at the site of laying steel communications.

Electrochemical corrosion occurs when the contact devices of power lines oxidize when the gaps between the elements of the electrical circuit increase. In addition to their destruction, in this case the power consumption of devices increases sharply.

Tribochemical corrosion

Metalworking tools that operate at elevated temperatures and pressures are susceptible to this type. Anti-corrosion coating of cutters, punches, dies, etc. is impossible, since the part requires high surface hardness.

Meanwhile, during high-speed cutting, cold pressing and other energy-intensive metal processing processes, mechanochemical reactions begin to occur, the intensity of which increases with increasing temperature at the “tool-workpiece” contact surface.

The resulting iron oxide Fe2O3 is characterized by increased hardness, and therefore begins to intensively destroy the surface of the tool.

Corrosion control methods

The choice of an appropriate method for protecting the surface from rust formation is determined by the conditions in which the part or structure operates. The following methods are most effective:

• Application of surface weather-resistant coatings;
• Surface metallization;
• Alloying the metal with elements that are more resistant to participation in redox reactions;
• Changes in the chemical composition of the environment.

Mechanical Surface Coatings

Surface protection of metal can be done by painting it or applying surface films that are neutral in composition to the effects of oxygen. In everyday life, as well as when processing relatively large areas (mainly underground pipelines), painting is used.

Among the most durable paints are enamels and paints containing aluminum.

In the first case, the effect is achieved by blocking access to oxygen to the steel surface, and in the second, by applying aluminum to the surface, which, being a chemically inert metal, protects the steel from corrosion destruction.

The positive features of this method of protection are the ease of its implementation and relatively low financial costs, since the process is quite simply mechanized. At the same time, the durability of this method of protection is low, since, not having a high degree of affinity with the base metal, such coatings begin to mechanically deteriorate after some time.

Chemical surface coatings 

Corrosion protection in this case occurs due to the formation on the surface of the metal being processed of a chemical film consisting of components that are resistant to oxygen, pressure, temperature and humidity. For example, carbon steels are treated with phosphating.

The process can be performed either cold or hot, and consists of forming a layer of manganese and zinc phosphate salts on the metal surface. An analogue of phosphating is oxalation - the process of treating metal with oxalic acid salts.

   The use of precisely such technologies increases the resistance of metals against tribochemical corrosion.

The disadvantage of these methods is the labor intensity and complexity of their application, which requires special equipment. In addition, the final surface changes its color, which is not always acceptable for aesthetic reasons.

Alloying and metallization

Unlike previous methods, here the end result is the formation of a layer of metal that is chemically inert to oxygen. These metals include those that are as far as possible from hydrogen on the oxygen activity line. As efficiency increases, this series looks like this: chromium→copper→zinc→silver→aluminum→platinum.

The difference in the technologies for obtaining such anti-corrosion layers lies in the method of their application.

During metallization, an ionized arc flow of finely dispersed sprayed metal is directed to the surface, and alloying is carried out during the process of metal smelting, as a consequence of the occurrence of metallurgical reactions between the base metal and the introduced alloying additives.

Changes in the composition of the environment

In some cases, a significant reduction in corrosion can be achieved by changing the composition of the atmosphere in which the protected metal structure operates. This can be evacuation (for relatively small objects), or work in an environment of inert gases (argon, neon, xenon).

This method is very effective, but requires additional equipment - protective chambers, suits for operating personnel, etc.

It is used mainly in research laboratories and pilot production, where the necessary microclimate is specially maintained.

Whoever hinders us will help us

In conclusion, we will point out a rather unusual method of corrosion protection: with the help of iron oxides themselves, more precisely, one of them - iron oxide Fe3O4. This substance is formed at temperatures of 2505000C and, in terms of its mechanical properties, is a highly viscous technological lubricant.

Being present on the surface of the workpiece, Fe3O4 blocks access to air oxygen during semi-hot deformation of metals and alloys, and thereby blocks the process of initiation of tribochemical corrosion. This phenomenon is used for high-speed upsetting of hard-to-deform metals and alloys.

The effectiveness of this method is due to the fact that with each technological cycle the contact surfaces are renewed, and therefore the stability of the process is adjusted automatically.⁠

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Electrochemical corrosion: methods and devices for protecting pipelines or a car from chemical rust, its types and examples

Corrosion is the process of spontaneous destruction of the surface of materials due to interaction with the environment. Its cause is the thermodynamic instability of chemical elements to certain substances.

Formally, polymers, wood, ceramics, and rubber are susceptible to corrosion, but the term “aging” is more often applied to them. The most serious damage is caused by rusting of metals, to protect which high-tech countermeasures are being developed. But we'll talk about this later. Scientists distinguish between chemical and electrochemical corrosion of metals.

Chemical corrosion

It usually occurs when the metal structure is exposed to dry gases, liquids or solutions that do not conduct electric current. The essence of this type of corrosion is the direct interaction of the metal with an aggressive environment.

Elements chemically corrode during heat treatment or as a result of prolonged use at sufficiently high temperatures. This applies to gas turbine blades, smelting furnace fittings, internal combustion engine parts, and so on.

As a result, certain compounds are formed on the surface: oxides, nitrides, sulfides.

Electrochemical corrosion: examples

It is divided into:

• Atmospheric, which occurs when there is a liquid film on the metal surface in which gases contained in the atmosphere (for example, O2, CO2, SO2) are able to dissolve to form electrolyte systems.
• Liquid, which flows in a conductive liquid medium.
• Ground water, which flows under the influence of groundwater.

Causes

• Since usually any metal that is used for industrial needs is not ideally pure and contains inclusions of various types, electrochemical corrosion of metals occurs due to the formation of a large number of short-circuited local galvanic elements on the surface of the iron.
• Their appearance can be associated not only with the presence of various (especially metal) impurities (contact corrosion), but also with surface heterogeneity, crystal lattice defects, mechanical damage, and the like.

Interaction mechanism

The process of electrochemical corrosion depends on the chemical composition of materials and the characteristics of the external environment.

If the so-called technical metal is covered with a wet film, then two independent reactions occur in each of the indicated galvanic microelements that form on the surface.

The more active component of the corrosion pair gives up electrons (for example, zinc in a Zn-Fe pair) and passes into the liquid medium as hydrated ions (that is, it corrodes) according to the following reaction (anodic process):

1. M + nH2O = Mz+* nH2O + ze.
2. This part of the surface is the negative pole of the local microelement, where the metal is electrochemically dissolved.
3. On the less active area of ​​the surface, which is the positive pole of the microelement (iron in the Zn-Fe pair), electrons are bonded due to the occurrence of a reduction reaction (cathode process) according to the following scheme:
4. Ox + ze = Red.

Thus, the presence of oxidizing agents in the water film, which are capable of binding electrons, provides the possibility of further progress of the anodic process. Accordingly, electrochemical corrosion can develop only if both anodic and cathodic processes occur simultaneously. Due to inhibition of one of them, the oxidation rate decreases.

Polarization process

Both of the above processes cause polarization of the corresponding poles (electrodes) of the microelement. What are the features here? Typically, electrochemical corrosion of metals is more significantly slowed down by polarization of the cathode. Therefore, it will increase under the influence of factors that prevent this reaction and are accompanied by the so-called depolarization of the positive electrode.

In many corrosion processes, cathodic depolarization is carried out by the discharge of hydrogen ions or the reduction of water molecules and corresponds to the formulas:

• In an acidic environment: 2H+ + 2e = H2.
• In alkaline: 2H2O + 2e = H2 + 2OH–.

Potential range

The potential that corresponds to these processes, depending on the nature of the aggressive environment, can vary from -0.83 to 0 V. For a neutral aqueous solution at temperatures close to standard, it is approximately -0.41 V.

Consequently, hydrogen ions contained in water and in neutral aqueous systems can only oxidize metals with a potential less than -0.41 V (located in the voltage series up to cadmium).

Considering that some of the elements are protected by an oxide film, the number of metals susceptible to oxidation in neutral environments by hydrogen ions is insignificant.

If the wet film contains dissolved oxygen in the air, then it is capable, depending on the nature of the medium, of binding electrons through the effect of oxygen depolarization. In this case, the electrochemical corrosion scheme looks like this:

• O2 + 4e + 2H2O = 4OH–
• O2 + 4e + 4H+ = 2H2O.

The potentials of these electrode reactions at temperatures close to standard vary from 0.4 V (alkaline medium) to 1.23 V (acidic medium). In neutral environments, the potential of the oxygen reduction process under the specified conditions corresponds to a value of 0.8 V. This means that dissolved oxygen is capable of oxidizing metals with a potential of less than 0.8 V (located in the voltage series up to silver).

The most important oxidizing agents

Types of electrochemical corrosion are characterized by oxidizing elements, the most important of which are hydrogen ions and oxygen.

At the same time, a film containing dissolved oxygen is much more dangerous in terms of corrosion than moisture, where there is no oxygen, and which is capable of oxidizing metals exclusively with hydrogen ions, since in the latter case the number of types of materials capable of corroding is much smaller.

For example, steel and cast iron contain carbon impurities mainly in the form of iron carbide Fe3C. In this case, the mechanism of electrochemical corrosion with hydrogen depolarization for these metals is as follows:

• (-) Fe - 2e + nH2O = Fe2+ · nH2O (rust may form);
• (+) 2H+ + 2e = H2 (in an acidified environment);
• (+) 2H2O + 2e = H2 + 2OH– (in a neutral and alkaline environment).

The mechanism of corrosion of iron, which contains copper impurities, in the case of oxygen depolarization of the cathode is described by the equations:

• (-) Fe - 2e + nH2O = Fe2+ nH2O;
• (+) 0.5O2 + H2O + 2e = 2OH– (in an acidified environment);
• (+) 0.5O2 + 2H+ + 2e = H2O (in a neutral and alkaline environment).

Electrochemical corrosion occurs at different rates. This indicator depends on:

• potential difference between the poles of a galvanic microcell;
• composition and properties of the electrolyte medium (pH, presence of corrosion inhibitors and stimulators);
• concentration (intensity of supply) of the oxidizing agent;
• temperature.

Protection methods

Electrochemical protection of metals from corrosion is achieved in the following ways:

• Creation of anti-corrosion alloys (alloying).
• Increasing the purity of an individual metal.
• Applying various protective coatings to the surface.

These coatings, in turn, are:

• Non-metallic (paints, varnishes, lubricants, enamels).
• Metal (anodic and cathodic coatings).
• Formed by special surface treatment (passivation of iron in concentrated sulfuric or nitric acids; iron, nickel, cobalt, magnesium in alkali solutions; formation of an oxide film, for example, on aluminum).

Metal protective coating

The most interesting and promising is electrochemical protection against corrosion by another type of metal. Based on the nature of their protective effect, metallized coatings are divided into anodic and cathodic. Let's look at this point in more detail.

An anodic coating is a coating formed by a more active (less noble) metal than the one being protected. That is, protection is carried out with an element that is in the voltage range before the base material (for example, coating iron with zinc or cadmium).

With local destruction of the protective layer, the less noble metal coating will corrode. In the area of ​​scratches and cracks, a local galvanic cell is formed, the cathode of which is the metal being protected, and the anode is the coating, which is oxidized. The integrity of such a protective film does not matter.

However, the thicker it is, the slower the electrochemical corrosion will develop, and the longer the beneficial effect will last.

Cathodic coating is a coating with a metal with a high potential, which in the series of voltages comes after the protected material (for example, spraying low-alloy steels with copper, tin, nickel, silver). The coating must be continuous, since when it is damaged, local galvanic cells are formed in which the base metal will be the anode and the protective layer will be the cathode.

How to protect metal from oxidation

Electrochemical protection against corrosion is divided into two types: sacrificial and cathodic. The protective coating is similar to the anodic coating. A large plate of a more active alloy is attached to the material to be protected.

A galvanic cell is formed, in which the base metal serves as the cathode, and the protector serves as the anode (it corrodes). Typically, zinc, aluminum or magnesium-based alloys are used for this type of protection.

The protector gradually dissolves, so it must be replaced periodically.

Electrochemical corrosion of pipelines causes a lot of trouble in public utilities and in industry in general. In the fight against it, the cathodic polarization method is most suitable.

To do this, a metal structure, which is protected from destructive oxidation processes, is connected to the negative pole of any external direct current source (it then becomes a cathode, and the rate of hydrogen evolution increases and the corrosion rate decreases), and a low-value metal is connected to the positive pole .

Source: https://xn----8sbna6aihebzq3cl.xn--p1ai/sposoby-borby-s-korroziej/metody-zashhity-ot-elektrohimicheskoj-korrozii.html

Electrochemical corrosion of metals examples

Electrochemical corrosion of metals. Not everyone knows exactly what it is, although the sad consequences of its manifestation are encountered in our everyday life, at almost every step.

We habitually call it rust, without particularly delving into the essence of the process. Theoretical formulations explaining the mechanism of occurrence and action of electrochemical corrosion, due to the invisibility of the ongoing processes, remain beyond our perception of reality.

Of course, you can approach this phenomenon (and not only) this way, perceiving corrosion (destruction of metal) as inevitable, but you can do it differently, understand the essence of what is happening and, if not change it, at least predict the end result.

Why does iron rust?

Everyone knows that metal (iron and not only) rusts in a humid environment, i.e., layer after layer is destroyed, gradually becomes thinner and ultimately turns into a useless oxidation product, brown Fe2O3 powder.

I will show this using the example of steel. It contains crystallites of ferrite (iron) and cementite (iron carbide). The potential of ferrite is less than the potential of cementite crystallites, so in this bundle iron will be the anode and iron carbide the cathode. The ions formed as a result of electrochemical corrosion combine to form insoluble hydroxide Fe(OH)2, which, when dried, becomes ordinary rust.

In pseudo-scientific language, we are talking about an oxidation-reduction process, in which on the surface of a metal spontaneously, due to the difference in potential, areas are formed, points of galvanic pairs, in which positively charged areas, anodes, give up their electrons to cathodes, negatively charged areas, where the reverse occurs. process, recovery.

This is a general description of the mechanism of occurrence and progression of electrochemical corrosion; in fact, depending on the environmental conditions, the corrosion process in each specific case has its own characteristics. I will give typical examples.

Corrosion due to uneven aeration of the metal surface

If you apply a drop of ordinary drinking water to a carbon alloy of iron, then after a while a visible circle of rust will clearly appear along the edges of the drop, and the metal that is under the drop will remain clean, but some time will pass and on the metal that is under the drop, they will become visible. small sores.

Based on these observations, we can conclude that the cathodic reduction site is located at the periphery of the drop, because in this place the flow of air (oxygen) is more accessible, and the anode is located in the center of the drop, where there is less oxygen.

How does a nail hammered into a wall rust?

That is why a nail hammered into a wall under the influence of corrosion will thin out first of all in the wall, where air access is difficult, but this statement is not entirely true. First, the section of metal that is in a borderline state is destroyed, not on the surface, but not too deep in the wall.

This is explained by the fact that electrons located in the immediate vicinity of the cathode migrate first. There is simply no time to reach the metal crystals located deep in the wall.

Corrosion due to direct contact of different metals

If two plates of different metals are placed on top of each other and pressed tightly enough, and to speed up the process, place them in a humid environment, then after some time, you will notice that one of the plates began to corrode, i.e. it turned out to be an anode and began to collapse.

Which plate will be the anode depends on the potential of the metal. For example, in a copper-iron pair, the anode will be iron, because the potential of iron (–0.44) is less than the potential of copper (+0.34).

By the way, you can unwittingly provoke the occurrence of corrosion by placing plate on plate, even from the same metal. True, in this case, completely different processes will be involved. Depending on the specific circumstances, this may be crevice or intergranular corrosion, but the result (destruction) will be the same.

Corrosion due to stray currents

Here is an example of such corrosion, vivid and from an electrician’s point of view, even somewhat dramatic, the functioning of the grounding system according to the TT system. Although in this case favorable conditions are deliberately created for the charge to flow to the ground, in essence this does not change anything. The input point is always the cathode, the output point, the anode, i.e. this is the section of the metal that is destroyed most intensively.

Judge for yourself, 1 A of daily flowing current per year consumes about 10% of the mass of the grounding structure in contact with the ground.

Corrosion under the influence of stray currents has one distinctive feature that sets it apart from all corrosion processes. Volume and intensity of destruction. The processes are occurring on such a large scale and rapidly that eliminating their consequences will require a lot of time and material investments.

The effect of water on rust formation

Although the atoms of a water molecule do not participate in the ion exchange of individual sections of the metal, its presence in one form or another is a prerequisite for the occurrence of electrochemical corrosion. After all, even chemically pure water, albeit weak, is an electrolyte, a medium that conducts electric current.

At what relative air humidity does the corrosion process begin?

A relative air humidity of 75% is considered critical, but this does not mean that at lower humidity corrosion is impossible, but only that at such air humidity the thickness of the liquid film on the metal reaches the optimal size for the occurrence and occurrence of electrochemical corrosion.

Source: https://masterkvartira.ru/jelektrohimicheskaja-korrozija-metallov-primery.html

Electrochemical corrosion and protection against it:

Corrosion is the process of spontaneous destruction of the surface of materials due to interaction with the environment. Its cause is the thermodynamic instability of chemical elements to certain substances.

Formally, polymers, wood, ceramics, and rubber are susceptible to corrosion, but the term “aging” is more often applied to them. The most serious damage is caused by rusting of metals, to protect which high-tech countermeasures are being developed. But we'll talk about this later.

Scientists distinguish between chemical and electrochemical corrosion of metals.

Treatment of aggressive environment

This method is effective when electrochemical corrosion of iron occurs in a small volume of conductive liquid. There are two ways to cope with destructive processes in this case:

• Removing oxygen from a liquid (deaeration) as a result of purging with an inert gas.
• By introducing inhibitors into the environment - so-called corrosion inhibitors. For example, if the surface is destroyed as a result of oxidation with oxygen, organic substances are added whose molecules contain certain amino acids (imino-, thio- and other groups). They are well adsorbed on the metal surface and significantly reduce the rate of electrochemical reactions leading to destruction of the surface contact layer.

Conclusion

Of course, chemical and electrochemical corrosion causes significant damage both in industry and in everyday life. If the metal did not corrode, the service life of many objects, parts, units, and mechanisms would increase significantly.

Now scientists are actively developing alternative materials that can replace metal and are not inferior in performance characteristics, but it is probably impossible to completely abandon its use in the near future.

In this case, advanced methods of protecting metal surfaces from corrosion come to the fore.

Source: https://www.syl.ru/article/174339/new_elektrohimicheskaya-korroziya-i-zaschita-ot-nee

What is electrochemical corrosion of metals - Metalworker's Handbook

Electrochemical corrosion of metals. Not everyone knows exactly what it is, although the sad consequences of its manifestation are encountered in our everyday life, at almost every step.

We habitually call it rust, without particularly delving into the essence of the process. Theoretical formulations explaining the mechanism of occurrence and action of electrochemical corrosion, due to the invisibility of the ongoing processes, remain beyond our perception of reality.

Of course, you can approach this phenomenon (and not only) this way, perceiving corrosion (destruction of metal) as inevitable, but you can do it differently, understand the essence of what is happening and, if not change it, at least predict the end result.