Electrochemical corrosion is the most common form of corrosion. Electrochemical occurs when the metal comes into contact with the surrounding electrolytically conductive medium. In this case, the reduction of the oxidizing component of the corrosive medium does not proceed simultaneously with the ionization of metal atoms, and their rates depend on the electrode potential of the metal. The root cause of electrochemical corrosion is the thermodynamic instability of metals in their environments. Corrosion of pipelines, upholstery of the bottom of a sea vessel, various metal structures in the atmosphere are, and many more, examples of electrochemical corrosion.
Electrochemical corrosion includes such types of local destruction as pitting, intergranular corrosion, crevice. In addition, the processes electrochemical corrosion occur in soil, atmosphere, sea.
Mechanism of electrochemical corrosion can proceed in two ways:
1) Homogeneous mechanism of electrochemical corrosion:
Surface layer met. regarded as homogeneous and homogeneous;
The reason for the dissolution of the metal is the thermodynamic possibility of cathodic or anode acts;
K and A regions migrate over the surface in time;
The rate of electrochemical corrosion depends on the kinetic factor (time);
A homogeneous surface can be considered as a limiting case, which can also be realized in liquid metals.
2) Heterogeneous mechanism of electrochemical corrosion:
In hard metals, the surface is inhomogeneous, because. different atoms occupy different positions in the crystal lattice in the alloy;
Heterogeneity is observed in the presence of foreign inclusions in the alloy.
Electrochemical corrosion has some features: it is divided into two simultaneously occurring processes (cathodic and anodic), which are kinetically dependent on each other; in some areas of the surface, electrochemical corrosion can take on a local character; dissolution of the main met. occurs at the anodes.
The surface of any metal consists of many microelectrodes short-circuited through the metal itself. Contacting with a corrosive medium, the resulting galvanic cells contribute to its electrochemical destruction.
The reasons for the occurrence of local galvanic cells can be very different:
1) alloy heterogeneity
Heterogeneity met. phases due to the inhomogeneity of the alloy and the presence of micro- and macro-inclusions;
Unevenness of oxide films on the surface due to the presence of macro- and micropores, as well as uneven formation of secondary corrosion products;
The presence of crystal grain boundaries on the surface, the appearance of a dislocation on the surface, the anisotropy of crystals.
2) inhomogeneity of the medium
Area with limited access oxidizing agent will be the anode in relation to the area with free access, which accelerates electrochemical corrosion.
3) heterogeneity of physical conditions
Irradiation (irradiated area - anode);
The impact of external currents (the place of entry of the stray current is the cathode, the place of exit is the anode);
Temperature (in relation to cold areas, heated ones are anodes), etc.
During the operation of a galvanic cell, two electrode processes occur simultaneously:
Anodic- metal ions go into solution
Fe → Fe 2+ + 2e
An oxidation reaction takes place.
Cathode- excess electrons are assimilated by molecules or atoms of the electrolyte, which are then reduced. A reduction reaction takes place at the cathode.
O 2 + 2H 2 O + 4e → 4OH - (oxygen depolarization in neutral, alkaline media)
O 2 + 4H + + 4e → 2H 2 O (oxygen depolarization in acidic environments)
2 H + + 2e → H 2 (during hydrogen depolarization).
The inhibition of the anodic process leads to the inhibition of the cathodic process as well.
Corrosion of metal takes place at the anode.
When two electrically conductive phases come into contact (for example, met. - Medium), when one of them is positively charged and the other is negatively charged, a potential difference arises between them. This phenomenon is associated with the appearance of a double electric layer (EDL). Charged particles are located asymmetrically at the phase boundary.
Potential jumps in the process of electrochemical corrosion can occur due to two reasons:
When enough great energy hydration, metal ions can break off and go into solution, leaving an equivalent number of electrons on the surface, which determine its negative charge. A negatively charged surface attracts meth cations to itself. from a solution. Thus, a double electric layer appears at the phase boundary.
Electrolyte cations are discharged on the metal surface. This leads to the fact that the surface of the met. acquires a positive charge, which forms a double electric layer with the anions of the solution.
Sometimes a situation arises when the surface is not charged and, accordingly, there is no DEL. The potential at which this phenomenon is observed is called the potential of zero charge (φ N). Each metal has its own potential of zero charge.
The magnitude of the electrode potentials has a very great influence on the nature of the corrosion process.
The potential jump between two phases cannot be measured, but using the compensation method, it is possible to measure the electromotive force of the element (EMF), which consists of a reference electrode (its potential is conventionally taken as zero) and the electrode under study. A standard hydrogen electrode is taken as the reference electrode. The EMF of a galvanic cell (a standard hydrogen electrode and the element under study) is called the electrode potential. Reference electrodes can also be silver chloride, calomel, saturated copper sulfate.
International convention in Stockholm 1953. it was decided to always put the reference electrode on the left when recording. In this case, the EMF is calculated as the potential difference between the right and left electrodes.
E = Vp - Vl
If a positive charge inside the system moves from left to right - the EMF of the element is considered positive, while
E max \u003d - (ΔG T) / mnF,
where F is the Faraday number. If positive charges move in the opposite direction, then the equation will look like:
E max =+(ΔG T)/mnF.
During corrosion in electrolytes, the most common and significant are adsorption (adsorption of cations or anions at the phase boundary) and electrode potentials (transition of cations from metal to electrolyte or vice versa).
The electrode potential at which the metal is in equilibrium with its own ions is called equilibrium (reversible). It depends on the nature of the metal phase, solvent, electrolyte temperature, activity of met ions.
The equilibrium potential obeys the Nernst equation:
E=E ο + (RT/nF) Lnα Me n+
where, E ο - standard potential met.; R is the molar gas constant; n is the degree of oxidation of the met ion; T - temperature; F - Faraday number; α Me n+ - activity of met ions.
At the established equilibrium potential, electrochemical corrosion is not observed.
If an electric current passes through the electrode, its equilibrium state is disturbed. The electrode potential changes depending on the direction and strength of the current. A change in the potential difference, leading to a decrease in current strength, is commonly called polarization. The decrease in the polarizability of the electrodes is called depolarization.
The rate of electrochemical corrosion is the lower, the greater the polarization. Polarization is characterized by the magnitude of the overvoltage.
Polarization is of three types:
Electrochemical (when slowing down the anodic or cathodic processes);
Concentration (observed when the rate of approach of the depolarizer to the surface and removal of corrosion products is low);
Phase (associated with the formation of a new phase on the surface).
Electrochemical corrosion is also observed when two dissimilar metals come into contact. In the electrolyte, they form a galvanic couple. The more electronegative of these will be the anode. The anode will gradually dissolve in the process. In this case, there is a slowdown or even complete cessation of electrochemical corrosion at the cathode (more electropositive). For example, when contacting sea water duralumin with nickel will intensively dissolve exactly duralumin.
Among all existing species 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 subject 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 ground, 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 electrochemical corrosion.
Intergranular
Intergranular corrosion refers to such 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 main danger of this type of corrosion is that it is not always visible visually.
Pitting
Pitting electrochemical corrosion is a point damage to individual areas of the surface of copper and other metals. Depending on the nature of the lesion, there are closed, open, and also superficial pitting. The size of the affected areas can vary from 0.1 mm to 1.5 mm.
slotted
Crevice electrochemical corrosion is called an enhanced process of destruction metal structures at the locations of cracks, gaps and cracks. Crevice corrosion can occur in an air atmosphere, gas mixtures as well as sea water. This type of destruction is typical for gas pipelines, the bottoms of ships and many other objects.
The occurrence of corrosion under conditions of a small amount of oxidizing agent is common due to the difficult approach to the walls of the slot. This leads to the accumulation of corrosive products inside the gaps. The electrolyte contained in the internal space of the gap may change under the influence of hydrolysis of corrosion products.
In order to protect metals from crevice corrosion, it is customary to apply several methods:
- sealing gaps and cracks;
- electrochemical protection;
- the process of inhibition.
As preventive methods, only those materials should be used that are least degree prone to rust, as well as initially competently and rationally design gas pipelines and other important facilities.
Competent prevention in many cases is a simpler process than the subsequent cleaning of metal structures from stubborn rust.
How does corrosion manifest itself?
As an example of the course of a corrosive process, one can cite the destruction of various devices, car components, as well as any structures made of metal and located:
- in atmospheric air;
- in the waters - the seas, rivers contained in the soil and under the layers of soil;
- in technical environments, etc.
In the process of rusting, the metal becomes a multielectronic 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. 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 equals 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 is more intense when there are inclusions of metal with less activity in the cathode compared to the corroding one - rust appears on them faster and is quite expressive.
Atmospheric corrosion occurs in conditions of humid air and normal temperature. In this case, a film of moisture with dissolved oxygen is formed on the metal surface. The process of destruction of the metal becomes more intense as the humidity of the air and the content of gaseous oxides of carbon and sulfur increase, provided that:
- cracks;
- roughness;
- other factors provoking the facilitation of 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. The destruction of pipelines can occur already six months after their construction, if the soil in which they are installed is characterized by increased acidity.
Under the influence of stray currents emanating from foreign objects, electrical corrosion occurs. Its main sources are electrical railways, power lines, and special installations operating on direct current. To a greater extent, this type of corrosion provokes the destruction of:
- gas pipelines;
- all kinds of structures (bridges, hangars);
- electrical cables;
- oil pipelines.
The action of the current provokes the appearance of areas of entry and exit of electrons - that is, cathodes and anodes. The most intense destructive process is precisely in the areas with anodes, so rust is more noticeable on them.
Corrosion of individual components of gas pipelines and water pipelines can be caused by the fact that the process of their installation is mixed, that is, it occurs using various materials. The most common examples are pitting in copper elements and bimetal corrosion.
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. Corrosion of pipelines can be prevented using special methods.
Rust Prevention Methods
To combat insidious rust are used various methods. Consider those of them that are the most effective.
Method number 1
One of the most popular methods is the 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 surface roughness by anodic dissolution in an electrolyte under the influence of an electric current.
To ensure reliable protection against rust, it is necessary even before the start of operation metal products process them with special means, which in their composition contain various components of organic and inorganic origin. This method prevents the appearance of rust on certain time, however, you will have to update the coverage later.
Electrical protection is a process in which a metal structure is connected to an external source of constant electric current. As a result, polarization of electrodes of the cathode type 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 the anode or cathode. In some cases, alternating processing of a metal product by both electrodes occurs.
Cathodic corrosion protection is necessary in situations where the metal to be protected does not show a tendency to passivate. 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 pipelines for water supply and heating. 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 number 2
Electrospark processing of metals can be carried out using installations various types- non-contact, contact, as well as anode-mechanical.
Method number 3
For reliable protection of 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;
- the use of relatively inexpensive equipment;
- simple technological process;
- the possibility of using automated lines;
- low energy costs.
Among the shortcomings this method- low efficiency in processing structures in corrosive environments, as well as insufficient adhesion to the steel base in some cases. In any other situation, such electrical protection is very effective.
Method number 4
To protect a variety of metal structures - gas pipelines, bridge structures, all kinds of pipelines - an 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 of the treated surface from water-soluble salts is carried out using professional high-pressure apparatuses;
- removal of existing structural errors, alignment of edges - this is necessary to prevent chipping of the applied paintwork;
- thorough cleaning of the surface with 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.
Proper pre-treatment of gas pipelines and various metal structures will provide them with reliable protection against electrochemical corrosion during operation.
Lecture 4
Electrochemical corrosion is a spontaneous destruction of metals as a result of electrochemical interaction with liquid electrolytes with electrical conductivity. These electrolytes can be water, aqueous solutions acids, alkalis, molten salts. Electrochemical corrosion is widespread and has many varieties. The reason for electrochemical corrosion is the reduced thermodynamic stability of most metals and their tendency to go into an ionic state.
During electrochemical corrosion, the interaction of a metal with the environment is characterized by anodic and cathodic processes occurring in different parts of the metal surface. Corrosion products are formed only in the anode areas.
Electrochemical corrosion is the result of the work of corrosive galvanic cells. It occurs as follows: an oxidation reaction proceeds at the anodic sites with the formation of Fe 2+ metal ions, and hydroxide is formed at the cathode sites under the influence of oxygen (as a result of the oxygen depolarization reaction). Ions Fe 2+ and OH - are sent to each other and form an insoluble precipitate Fe (OH) 2, which can decompose into iron oxide and water. (Fe (OH) 2 -> Fe 2 O 3 + H 2 O). The electrons released during the oxidation reaction from the anode section over the metal of the product flow to the cathode section and participate in the reduction reaction.
The model of a corrosion microelement is shown in Figure 4.
The following types of corrosion processes proceed with the electrochemical mechanism:
Corrosion in electrolytes - corrosion of metals in liquid media that conduct electricity. Depending on the type of electrolyte, corrosion is distinguished in solutions of acids, alkalis and salts (acid, alkali, salt), in sea, river water.
The composition of the electrolyte determines the mechanism of the corrosion process, affects its kinetics and rate. For example, the corrosion rate decreases if the electrolyte contains anions or oxidizing agents, as a result of interaction with which a film of sparingly soluble salts is formed on the metal surface.
Oxygen dissolved in the electrolyte has an inhibitory or accelerating effect on the corrosion process of metals.
The electrolyte concentration also affects the corrosion process. Practically in all natural environments with an increase in the concentration of salts in the electrolyte, the corrosion rate first increases to a certain maximum, and then decreases as a result of a decrease in the solubility of oxygen and the difficulty of the cathodic process.
The temperature of the electrolyte also affects the rate of corrosion. This is due to the fact that with increasing temperature, the electrical conductivity of the electrolyte increases. With an increase in the electrolyte temperature, the corrosion rate can sometimes increase by tens and hundreds of times.
Soil corrosion - corrosion of underground metal structures under the influence of soil electrolyte. On the surface of metal products in contact with soil electrolyte, due to local inhomogeneities of the metal or electrolyte, a large number of corrosive cells, the nature of which is similar to the nature of galvanic cells.
Soil corrosion is the most common type of electrochemical corrosion affecting underground metal structures. Soils and soils are extremely diverse not only within large regions, but even within the same small area. Between the soil, the most surface layer land, and the ground has no clear boundary.
Biocorrosion - special case soil corrosion, occurring under the influence of microorganisms, as a result of the vital activity of which substances are formed that accelerate corrosion processes. In nature, sulfate-reducing anaerobic bacteria are the most widespread, as a result of which hydrogen sulfide is formed, which, when combined with iron, gives iron sulfide. Sulfate-reducing anaerobic bacteria commonly found in water, mud, sewage, oil wells, bottom sediments, soil, cement. The most favorable environment for the development of these bacteria are soils with (optimally 6-7.5) at 25-30 0 C. The vital activity of iron aerobic bacteria is accompanied by the release of an insoluble film of iron hydroxide as corrosion products.
Biocorrosion (fouling of underwater structures with marine plant and animal organisms - bryozoans, balanus, diatoms, corals) destroys protective coatings and accelerates the destruction of metals. Some living organisms (for example, mussels) slow down the corrosion process, as they consume a lot of oxygen.
Atmospheric corrosion - corrosion of metals in the atmosphere of air or in the environment of any moist gas. In the absence of moisture in the air, iron corrodes at a negligible rate.
It depends on the degree of moisture content of the metal surface and, on this basis, is divided into three types:
1) wet atmospheric corrosion (at relative humidity air ~ 100%) in the presence of a visible moisture film on the metal surface;
2) wet atmospheric corrosion in the presence of an invisible film of moisture on the metal surface, which is formed during capillary adsorption or chemical condensation;
3) dry atmospheric corrosion in the complete absence of moisture on the metal surface.
Atmospheres differ significantly in humidity, temperature and pollution, so the rate of atmospheric corrosion in different areas is not the same, the closer to the sea coast, the more the air is saturated with sea salt, especially NaCl. AT industrial areas significant amounts of SO 2 appear in the air, which turns into sulfuric acid. When engines are running internal combustion NO is formed in large quantities, which is released into the atmosphere. Large amounts of H2S are released into the atmosphere in cities and industrial centers.
According to the aggressiveness of the atmosphere, it can be divided into the following main types: marine, industrial, urban, rural, arctic.
Specific factors affecting the aggressiveness of the atmosphere are (except for gases) dust and moisture.
Rust films formed under atmospheric conditions may have protective properties, so the rate of corrosion decreases with time.
In real conditions, all these types of corrosion mutually transform into each other. The nature of the change in the rate of atmospheric corrosion depending on the thickness of the moisture film is shown in the figure. This rate varies from zero for dry atmospheric corrosion, reaches a maximum for wet atmospheric corrosion, and decreases to some constant value characterizing the corrosion rate of a given metal in the electrolyte.
The rate of atmospheric corrosion of metals is affected by big number factors:
1) air humidity (electrolyte creation);
2) air impurities (gases SO 2, SO 3, K 2 S, NH 3, Cb-HC1, etc. in contact with water act as depassivators, complexing agents or cathodic depolarizers; solid particles increase the electrical conductivity of the electrolyte film and facilitate the adsorption of gases and moisture from the air);
3) the nature of the atmosphere (clean, dirty, dry, wet);
4) geographical area(tropics, subtropics, middle lane, empty, pole);
5) the state of the surface of the corroding material (presence of corrosion products);
6) the presence of foreign inclusions in the metal (some of them protect against corrosion - cathodic inclusions, such as Cu, Pb, Pd, while others contribute to the destruction of the metal);
7) temperature (as the temperature rises, the corrosion rate and humidity decrease).
Methods for protecting metals from atmospheric corrosion are as follows:
a) applying protective coatings (lubricants, varnishes, films, zinc plating, nickel plating, chromium plating, phosphating, oxide films); b) influence on control processes (anode passivation of Cr, Al, Ti, Ni, cathodic inclusions of Cu, Pd); c) reduction of the electrolyte layer on the surface of the corroding metal (drying and air purification); d) the use of corrosion inhibitors (NaNO 2 , nitrites, carbonates, benzoates of dicyclohexylamine and monoethanolamine) mainly during the storage of metals and their transportation in containers or packaging from wrapping materials.
Electrocorrosion - corrosion of underground metal structures caused by the penetration of leakage currents from the rails of electrified vehicles or other industrial electrical installations and structures (electrified railways, subways, DC transmission lines, cathodic protection installations of underground metal structures) into the structures.
These currents are called stray currents, their magnitude and direction can change over time.
The main quantity characterizing the intensity of the electrocorrosion process is the strength of the current flowing from the underground structure into the ground, per unit surface.
The magnitude of the leakage current from an underground structure depends on many factors, in particular:
Resistivity land;
The magnitude of stray currents in the earth;
Mutual arrangement sources of stray currents and underground structures;
The condition of the external insulating coating on the underground structure;
Longitudinal resistance of an underground structure.
Contact corrosion is corrosion caused by electrical contact between two metals with different electrochemical potentials.
Stress corrosion occurs when the corrosive environment and mechanical stresses in the metal are simultaneously exposed.
Crevice corrosion - acceleration of corrosion destruction of metal by electrolyte in narrow gaps and crevices (in threaded and flanged joints).
Corrosive erosion - with simultaneous exposure to a corrosive environment and friction.
Corrosive cavitation occurs with simultaneous corrosion and impact environment(corrosion of the blades of impellers of centrifugal pumps, destruction of propeller blades on ships).
Fretting - corrosion is a local corrosion destruction of metals when exposed to aggressive environment in conditions of oscillatory movement of two rubbing surfaces relative to each other.
Structural corrosion is due to the structural inhomogeneity of the alloy. In this case, an accelerated process of corrosion destruction occurs due to increased activity any component of the alloy.
Thermal contact corrosion occurs due to the temperature gradient, due to uneven heating of the metal surface.
Chemical corrosion is a process consisting in the destruction of a metal when interacting with an aggressive external environment. The chemical variety of corrosion processes has no connection with the impact of electric current. With this type of corrosion, an oxidative reaction occurs, where the material being destroyed is at the same time a reducing agent of the elements of the environment.
The classification of a variety of an aggressive environment includes two types of metal destruction:
- chemical corrosion in non-electrolyte liquids;
- chemical gas corrosion.
Gas corrosion
The most common type of chemical corrosion - gas - is a corrosive process that occurs in gases at elevated temperatures. This problem is typical for the operation of many types of technological equipment and parts (furnace fittings, engines, turbines, etc.). In addition, over high temperatures used in the processing of metals high pressure(heating before rolling, stamping, forging, thermal processes, etc.).
Features of the state of metals at elevated temperatures are determined by their two properties - heat resistance and heat resistance. Heat resistance is the degree of stability of the mechanical properties of a metal at ultrahigh temperatures. Under the stability of mechanical properties is meant the retention of strength for a long time and resistance to creep. Heat resistance is the resistance of a metal to the corrosive activity of gases at elevated temperatures.
Development speed gas corrosion driven by a number of indicators, including:
- atmospheric temperature;
- components included in the metal or alloy;
- parameters of the environment where gases are located;
- duration of contact with the gaseous medium;
- properties of corrosive products.
The corrosion process is more influenced by the properties and parameters of the oxide film that appears on the metal surface. Oxide formation can be chronologically divided into two stages:
- adsorption of oxygen molecules on a metal surface interacting with the atmosphere;
- the contact of a metal surface with a gas, resulting in a chemical compound.
The first stage is characterized by the appearance of an ionic bond, as a result of the interaction of oxygen and surface atoms, when the oxygen atom takes away a pair of electrons from the metal. The resulting bond is distinguished by exceptional strength - it is greater than the bond of oxygen with the metal in the oxide.
The explanation for this connection lies in the action of the atomic field on oxygen. As soon as the metal surface is filled with an oxidizing agent (and this happens very quickly), at low temperatures, due to the van der Waals force, the adsorption of oxidizing molecules begins. The result of the reaction is the appearance of the thinnest monomolecular film, which becomes thicker over time, which complicates the access of oxygen.
At the second stage, there is chemical reaction, during which the oxidizing element of the medium takes valence electrons from the metal. Chemical corrosion is the end result of the reaction.
Characteristics of the oxide film
The classification of oxide films includes three types:
- thin (invisible without special devices);
- medium (temper colors);
- thick (visible to the naked eye).
The resulting oxide film has protective capabilities - it slows down or even completely inhibits the development of chemical corrosion. Also, the presence of an oxide film increases the heat resistance of the metal.
However, really effective film must meet a number of characteristics:
- be non-porous;
- have a solid structure;
- have good adhesive properties;
- differ in chemical inertness in relation to the atmosphere;
- be hard and wear resistant.
One of the above conditions - a continuous structure has a particularly importance. The continuity condition is the excess of the volume of oxide film molecules over the volume of metal atoms. Continuity is the ability of the oxide to cover the entire metal surface. If this condition is not met, the film cannot be considered protective. However, there are exceptions to this rule: for some metals, for example, for magnesium and elements of the alkaline earth group (excluding beryllium), continuity is not a critical indicator.
Several techniques are used to determine the thickness of the oxide film. The protective qualities of the film can be determined at the time of its formation. To do this, the rate of oxidation of the metal, and the parameters of the change in rate over time, are studied.
For an already formed oxide, another method is used, consisting in studying the thickness and protective characteristics films. To do this, a reagent is applied to the surface. Next, experts fix the time it takes for the penetration of the reagent, and based on the data obtained, they draw a conclusion about the film thickness.
Note! Even the finally formed oxide film continues to interact with the oxidizing environment and the metal.
Corrosion development rate
The rate at which chemical corrosion develops depends on temperature regime. At high temperatures, oxidative processes develop more rapidly. Moreover, the decrease in the role of the thermodynamic factor of the reaction does not affect the process.
Of considerable importance is cooling and variable heating. Due to thermal stresses, cracks appear in the oxide film. Through the gaps, the oxidizing element enters the surface. As a result, a new layer oxide film, and the former peels off.
The components of the gaseous medium also play an important role. This factor is individual for different types of metals and is consistent with temperature fluctuations. For example, copper quickly corrodes if it comes into contact with oxygen, but is resistant to this process in a sulfur oxide environment. For nickel, on the contrary, sulfur oxide is destructive, and stability is observed in oxygen, carbon dioxide and the aquatic environment. But chromium is resistant to all of the listed media.
Note! If the oxide dissociation pressure level exceeds the pressure of the oxidizing element, the oxidizing process stops and the metal becomes thermodynamically stable.
The alloy components also affect the rate of the oxidative reaction. For example, manganese, sulfur, nickel, and phosphorus do nothing to oxidize iron. But aluminum, silicon and chromium make the process slower. Cobalt, copper, beryllium and titanium slow down the oxidation of iron even more. Additions of vanadium, tungsten and molybdenum will help to make the process more intensive, which is explained by the fusibility and volatility of these metals. The slowest oxidation reactions proceed with the austenitic structure, since it is most adapted to high temperatures.
Another factor on which the corrosion rate depends is the characteristics of the treated surface. A smooth surface oxidizes more slowly, while an uneven surface oxidizes faster.
Corrosion in non-electrolyte liquids
Non-conductive liquid media (i.e. non-electrolyte liquids) include such organic matter, how:
- benzene;
- chloroform;
- alcohols;
- carbon tetrachloride;
- phenol;
- oil;
- petrol;
- kerosene, etc.
In addition, a small amount of inorganic liquids, such as liquid bromine and molten sulfur, are considered non-electrolyte liquids.
It should be noted that organic solvents themselves do not react with metals, however, in the presence of a small amount of impurities, an intense interaction process occurs.
The sulfur-containing elements in the oil increase the corrosion rate. Also, corrosive processes are enhanced by high temperatures and the presence of oxygen in the liquid. Moisture intensifies the development of corrosion in accordance with the electromechanical principle.
Another factor in the rapid development of corrosion is liquid bromine. At normal temperatures it is especially destructive to high-carbon steels, aluminum and titanium. The effect of bromine on iron and nickel is less significant. Lead, silver, tantalum and platinum show the greatest resistance to liquid bromine.
Molten sulfur reacts aggressively with almost all metals, primarily lead, tin and copper. Sulfur affects carbon steels and titanium less and almost completely destroys aluminum.
Protective measures for metal structures in non-conductive liquid media are carried out by adding metals resistant to a particular environment (for example, steels with high content chromium). Also, special protective coatings are used (for example, in an environment where there is a lot of sulfur, aluminum coatings are used).
Corrosion protection methods
Corrosion control methods include:
The choice of a specific material depends on the potential efficiency (including technological and financial) of its use.
Modern principles of metal protection are based on the following methods:
- Improving the chemical resistance of materials. Chemically resistant materials (high-polymer plastics, glass, ceramics) have successfully proven themselves.
- Isolation of the material from the aggressive environment.
- Reducing the aggressiveness of the technological environment. Examples of such actions include the neutralization and removal of acidity in corrosive environments, as well as the use of various inhibitors.
- Electrochemical protection (imposition of external current).
The above methods are divided into two groups:
- Chemical resistance enhancement and insulation are applied before the steel structure is put into service.
- Reducing the aggressiveness of the environment and electrochemical protection are used already in the process of using a metal product. The use of these two techniques makes it possible to introduce new methods of protection, as a result of which protection is provided by changing operating conditions.
One of the most commonly used methods of metal protection - galvanic anti-corrosion coating - is economically unprofitable with large surface areas. Cause in high costs for the preparatory process.
The leading place among the methods of protection is occupied by the coating of metals paintwork materials. The popularity of this method of combating corrosion is due to a combination of several factors:
- high protective properties (hydrophobicity, repulsion of liquids, low gas permeability and vapor permeability);
- manufacturability;
- ample opportunities for decorative solutions;
- maintainability;
- economic justification.
At the same time, the use of widely available materials is not without drawbacks:
- incomplete wetting of the metal surface;
- impaired adhesion of the coating to the base metal, which leads to the accumulation of electrolyte under the anti-corrosion coating and, thus, contributes to corrosion;
- porosity, leading to increased moisture permeability.
And yet, the painted surface protects the metal from corrosion processes even with fragmentary damage to the film, while imperfect galvanic coatings can even accelerate corrosion.
Organosilicate coatings
Chemical corrosion practically does not apply to organosilicate materials. The reasons for this lie in the increased chemical stability of such compositions, their resistance to light, hydrophobic properties and low water absorption. Organosilicates are also resistant to low temperatures, have good adhesive properties and wear resistance.
The problems of metal destruction due to the effects of corrosion do not disappear, despite the development of technologies to combat them. The reason is the constant increase in the production of metals and more and more difficult conditions exploitation of their products. It is impossible to finally solve the problem at this stage, so the efforts of scientists are focused on finding ways to slow down corrosion processes.
