Where does electrochemical corrosion occur - Metals and their processing

Lecture 2. Electrochemical corrosion

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.

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.

Alloys with magnesium and silicon

Such materials are most often used in mechanical engineering and construction. Mg2Si makes alloys of this variety very strong. Sometimes copper is also a component of such elements. It is also introduced into the alloy for strengthening. However, copper is added to such materials in very small quantities. Otherwise, the anti-corrosion properties of the aluminum alloy may be greatly reduced. Intercrystalline rusting in them begins already with the addition of more than 0.5% copper.

Also, the susceptibility to corrosion of such materials may increase with an unjustified increase in the amount of silicon included in their composition. This substance is usually added to aluminum alloys in such proportions that after the formation of Mg2Si there is nothing left over. Only some materials of this variety contain silicon in its pure form.

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.

Natural materials

Does not work on aluminum:

Rock salt.
Petroleum distillation products: gasoline, kerosene, paraffin, oils, resins.
Wax.
Rubber.
Essential oils.
Hard coal, anthracite, brown coal.
Cellulose.
Squirrels.
Natural gypsum.
Particularly pure types of oil.

Effective on aluminum:

Oil, or more precisely, the contaminants it contains, mainly water with salts dissolved in it, which form acids as a result of hydrolysis.
Tannins.
Wood impregnation products.
Clay.

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):

M + nH2O = Mz+* nH2O + ze.

This part of the surface is the negative pole of the local microelement, where the metal is electrochemically dissolved.

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:

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.

Properties

There's fire all around! I don’t know the meaning of the word, but I feel it’s burning

Let's study the characteristics of aluminum. The described metal melts at a temperature of 659 degrees Celsius. The density of the substance is 2.69*103 kg/cm3. Aluminum belongs to the group of active metals. Resistance to corrosion processes depends on a number of factors:

Purity of the alloy. For the production of various equipment, metal is taken that is distinguished by its purity. It should not contain various impurities. Aluminum grades AI1 and AB2 are widely used.
Environment in which aluminum is located.
What is the concentration of impurities in the environment surrounding aluminum.
Temperature.
The pH of the environment has a great influence. You need to know that aluminum oxide can form when the pH is in the range between 3 and 9. In an environment where an oxide film immediately appears on the surface of an aluminum sheet, corrosion processes will not develop.

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–.

Using LMB

This method, like oxidation, is used to protect aluminum from rust quite often. This material can be painted using dry, wet or powder methods. In the first case, aluminum is first treated with a composition containing zinc and strontium. Next, the paintwork itself is applied to the metal.

When using the powder method, the working surface is first degreased by immersion in alkaline or acidic solutions. Next, chromate, zirconium, phosphate or titanium compounds are applied to the product.

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– or
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).

Organic compounds

Non-corrosive:

All saturated hydrocarbons, benzene, toluene, naphthalene.
Aliphatic alcohols (interaction with aluminum is possible only after evaporation of water and only when heated). Methyl alcohol causes minor corrosion in aqueous solution, most severely at 25% water content. Other alcohols, such as ethyl, propyl, glycerin and its derivatives, do not corrode aluminum.
Phenol and its aqueous solutions (minor destruction occurs at temperatures above 60°C and a concentration of 50%). Phenol derivatives behave similarly.
Esters.
Aldehydes, including formalin, acrolein, benzaldehyde.
Ketones.
Freons.
Amines and amides.
Certain halogen-containing compounds (eg polyvinyl chloride).
Carbohydrates: glucose, lactose, cellulose.

Lead to minor corrosion:

Some organic chlorine compounds that release hydrochloric acid due to hydrolysis in aqueous solutions.
Aqueous solutions of organic acids: acetic, formic, monochloroacetic, oxalic, tartaric, citric, malic, salicylic, ascorbic, etc.

Strong effect on aluminum:

Some organic acids in the complete absence of water.
Salts of organic acids can create sources of local corrosion.
Carbon tetrachloride is highly corrosive in the presence of traces of moisture.
Most halogen compounds attack aluminum, and the corrosion rate increases with temperature. The corrosive activity of halogen decreases in the following sequence: F-Cl-Br-I.
Some sulfur-containing organic compounds, such as ethyl sulfuric acid.

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:

Source: https://instanko.ru/elektrichestvo/elektrohimicheskaya-korroziya.html

Electrochemical corrosion - types and methods of protection

Among all existing types of destruction of metals, the most common is electrochemical corrosion, which occurs as a result of its interaction with an electrolytically conductive medium. The main reason for this phenomenon is the thermodynamic instability of metals in the environments that surround them.

Many objects and structures are susceptible to this type of corrosion:

gas and water pipelines;
elements of vehicles;
other structures made of metal.

Corrosive processes, that is, rust, can occur in the atmosphere, in the soil, and even in salt water. Cleaning metal structures from manifestations of electrochemical corrosion is a complex and lengthy process, so it is easier to prevent its occurrence.

Main varieties

During corrosion in electrolytes, chemical energy is converted into electrical energy. In this regard, it is called electrochemical. It is customary to distinguish the following types of electrochemical corrosion.

Intercrystalline

Intergranular corrosion refers to a dangerous phenomenon in which the grain boundaries of nickel, aluminum and other metals are destroyed in a selective manner. As a result, the strength and plastic properties of the material are lost. The danger of this type of corrosion is that it is not always visible visually.

Pitting

Pitting electrochemical corrosion is a point lesion of individual areas of the surface of copper and other metals. Depending on the nature of the lesion, closed, open, and superficial pitting are distinguished. The size of the affected areas can vary from 0.1 mm to 1.5 mm.

Slotted

Crevice electrochemical corrosion is commonly called the intensified process of destruction of metal structures in the locations of cracks, gaps and cracks. Crevice corrosion can occur in air, gas mixtures, and sea water. This type of destruction is typical for gas pipelines, the bottoms of sea vessels and many other objects.

Corrosion occurs in conditions of a small amount of oxidizer due to the difficult approach to the crack walls. This leads to the accumulation of corrosive products inside the gaps. The electrolyte contained in the internal space of the gap can change under the influence of hydrolysis of corrosion products.

In order to protect metals from crevice corrosion, it is common to use several methods:

sealing gaps and cracks;
electrochemical protection;
inhibition process.

As preventive methods, you should use only those materials that are least susceptible to rust, and also initially correctly and rationally design gas pipelines and other important objects.

Competent prevention in many cases is a simpler process than subsequent cleaning of metal structures from ingrained rust.

How different types of corrosion manifest themselves

An example of the corrosion process is the destruction of various devices, car components, as well as any structures made of metal and located:

in atmospheric air;
in waters - seas, rivers contained in the soil and under layers of soil;
in technical environments, etc.

During the rusting process, the metal becomes a multi-electron galvanic cell. So, for example, if copper and iron come into contact in an electrolytic medium, copper is the cathode and iron is the anode. By donating electrons to copper, iron in the form of ions enters the solution. Hydrogen ions begin to move towards copper and are discharged there. Becoming more and more negative, the cathode soon becomes equal to the potential of the anode, as a result of which the corrosion process begins to slow down.

Different types of corrosion manifest themselves in different ways. Electrochemical corrosion manifests itself more intensely in cases where the cathode contains inclusions of metal with less activity compared to the corroding one - rust appears on them faster and is quite expressive.

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Atmospheric corrosion occurs in humid air and normal temperatures. In this case, a film of moisture with dissolved oxygen forms on the surface of the metal. The process of metal destruction becomes more intense as air humidity and the content of gaseous oxides of carbon and sulfur increase, provided that:

cracks;
roughness;
other factors that facilitate the condensation process.

Soil corrosion most affects a variety of underground structures, gas pipelines, cables and other structures. The destruction of copper and other metals occurs due to their close contact with soil moisture, which also contains dissolved oxygen. Destruction of pipelines can occur as early as six months after their construction if the soil in which they are installed is characterized by high acidity.

Under the influence of stray currents emanating from foreign objects, electrical corrosion occurs. Its main sources are electric railways, power lines, as well as special installations operating on direct electric current. To a greater extent, this type of corrosion provokes destruction:

gas pipelines;
all kinds of structures (bridges, hangars);
electrical cables;
oil pipelines.

The action of the current provokes the emergence of electron entry and exit areas - that is, cathodes and anodes. The most intense destructive process is in areas with anodes, so rust is more noticeable there.

Corrosion of individual components of gas and water pipelines can be caused by the fact that the installation process is mixed, that is, it occurs using different materials. The most common examples are pitting corrosion that occurs in copper elements, as well as corrosion of bimetals.

With a mixed installation of iron elements with copper and zinc alloys, the corrosion process is less critical than with copper casting, that is, with alloys of copper, zinc and tin. Pipeline corrosion can be prevented using special methods.

Methods of rust protection

Various methods are used to combat insidious rust. Let's look at those that are the most effective.

Method No. 1

One of the most popular methods is electrochemical protection of cast iron, steel, titanium, copper and other metals. What is it based on?

Electrochemical processing of metals is a special method aimed at changing the shape, size and roughness of the surface by anodic dissolution in an electrolyte under the influence of electric current.

To ensure reliable protection against rust, it is necessary to treat metal products with special means, which contain various components of organic and inorganic origin, even before using them. This method allows you to prevent rust from appearing for a certain time, but later you will have to renew the coating.

Scheme of cathodic protection of pipelines

Electrical protection is a process in which a metal structure is connected to an external source of direct electrical current. As a result of this, polarization of cathode-type electrodes is formed on its surface, and all anode regions begin to transform into cathode ones.

Electrochemical processing of metals can occur with the participation of an anode or cathode. In some cases, alternating processing of a metal product with both electrodes occurs.

Cathodic corrosion protection is necessary in situations where the metal to be protected does not have a predisposition to passivation. An external current source is connected to the metal product - a special cathodic protection station.

This method is suitable for protecting gas pipelines, as well as water supply and heating pipelines.

However, this method has certain disadvantages in the form of cracking and destruction of protective coatings - this occurs in cases of a significant shift in the potential of the object in the negative direction.

Method No. 2

Electrospark processing of metals can be carried out using various types of installations - non-contact, contact, and also anodic-mechanical.

Method No. 3

To reliably protect gas pipelines and other pipelines from rust, a method such as electric arc spraying is often used. The advantages of this method are obvious:

significant thickness of the protective layer;
high level of performance and reliability;
use of relatively inexpensive equipment;
simple technological process;
possibility of using automated lines;
low energy costs.

Among the disadvantages of this method are low efficiency when processing structures in corrosive environments, as well as insufficient adhesion strength to the steel base in some cases. In any other situations, such electrical protection is very effective.

Method No. 4

To protect a variety of metal structures - gas pipelines, bridge structures, all kinds of pipelines - effective anti-corrosion treatment is required.

This procedure is carried out in several stages:

thorough removal of fatty deposits and oils using effective solvents;
cleaning the treated surface from salts soluble in water is carried out using professional high-pressure apparatus;
removal of existing structural errors, alignment of edges - this is necessary to prevent chipping of the applied paint coating;
thorough cleaning of the surface using a sandblaster - this is done not only to remove rust, but also to give the desired degree of roughness;
application of anti-corrosion material and an additional protective layer.

Correct pre-treatment of gas pipelines and all kinds of metal structures will provide them with reliable protection from electrochemical corrosion during operation.

Source: https://kraska.guru/specmaterialy/korroziya/elektroximicheskaya-korroziya.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

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.

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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.

Alexandrova M. | Metal corrosion | Journal "Chemistry" No. 17/2006

Electrochemical corrosion of one metal

With electrochemical corrosion, an electrical circuit occurs. In this case, there may be cases of corrosion of both one metal and metals in contact. Electrochemical corrosion requires the presence of oxygen and water to occur.

Let us consider the case when there is no metal-to-metal contact, and the metal (iron) is in the air.

Some areas of the iron surface serve as an anode on which its oxidation occurs
( E ° is the standard electrode potential):

Fe (solid) = Fe2+ (aq) + 2 e , E °oxide = 0.44 B.

The electrons generated in this case move through the metal to other areas of the surface, which play the role of a cathode. Oxygen reduction occurs on them:

O2 (g) + 4H+ (aq) + 4 e = 2H2O (l),

°recovery = +1.23 V.

This process is illustrated in Fig. 1.

H+ ions participate in the reduction of oxygen. If the H+ concentration decreases (as the pH increases), O2 reduction becomes more difficult. It has been noticed that iron in contact with a solution whose pH is above 9–10 does not corrode.

During the corrosion process, Fe2+ ions formed at the anode are oxidized to Fe3+:

4Fe2+ (aq) + O2 (g) + (2 n + 4)H2O (l) = 2Fe2O3• n H2O (sol) + 8H+ (aq).

Since the role of the cathode is usually played by that part of the surface that is best provided with an influx of oxygen, rust most often appears in these areas. If you carefully examine a shovel that has been left in the open air for some time with dirt stuck to the blade, you will notice that depressions have formed under the dirt on the surface of the metal, and rust has appeared wherever O2 could penetrate.

Increased corrosion in the presence of salts is often encountered by motorists in areas where roads are generously sprinkled with salt in winter to combat icy conditions. The effect of salts is explained by the fact that the ions formed create the electrolyte necessary for the formation of a closed electrical circuit.

The presence of anodic and cathodic sites on the surface of iron leads to the creation of two unequal chemical environments on it. They can arise due to the presence of impurities or defects in the crystal lattice (apparently caused by stress within the metal).

In places where there are impurities or defects, the microscopic environment of a particular iron atom can cause its oxidation state to slightly increase or decrease compared to "normal" atoms in the crystal lattice. Therefore, such places can play the role of anodes or cathodes.

Ultra-pure iron, in which the number of such defects is reduced to a minimum, is much less likely to corrode than ordinary iron.

A classic example is the famous Qutub Column in India near Delhi, which has stood for almost one and a half thousand years and has not collapsed, despite the hot and humid climate. It is made of iron, which contains almost no impurities. How the ancient metallurgists managed to obtain such pure metal still remains a mystery.

At the beginning of the last century, the luxury yacht “Call of the Sea” was built by order of an American millionaire. Its bottom was sheathed in Monel metal (an alloy of copper and nickel), and the steering frame, keel and other parts were made of steel. When the yacht was launched, a giant galvanic cell appeared, consisting of a cathode (Monel metal), a steel anode and an electrolyte solution - sea water.

The consequences were terrible! Even before entering the open sea, the yacht was completely out of order, so the “Call of the Sea” remained in the history of navigation as an example of design shortsightedness and arrogant ignorance. Let's try to figure out what happened.

Let's consider the contact of two metals using the example of tin and iron.

Iron is often coated with another metal, such as tin, zinc or chromium, to protect it from corrosion. The so-called “tinplate” is obtained by covering sheet iron with a thin layer of tin. The tin protects the iron as long as the protective layer remains intact.

Once it is damaged, air and moisture begin to affect the iron; tin even accelerates the corrosion process because it serves as a cathode in the electrochemical process.

A comparison of the oxidation electrode potentials of iron and tin shows that iron is oxidized more easily than tin:

Fe (solid) = Fe2+ (aq) + 2 e , E °oxide = 0.44 B,

Source: https://him.1sept.ru/article.php?ID=200601705

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:

Heterogeneity of the product, that is, the presence of foreign inclusions.
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.

https://www.youtube.com/watch?v=u7Dej6IV_28

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:

Constructive decisions.
Active methods.
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

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.

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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:

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.
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.
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.
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.
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.
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.
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:

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.
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:

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.
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.
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.
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.
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.
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.
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

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.

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 .

Electrochemical protection methods are effective in a conductive environment (a striking example is sea water). Therefore, protectors are often used to protect the underwater parts of marine vessels.

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