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What is Corrosion?

What is corrosion

What is corrosion engineering ?

In this article from Journal of Corrosion, we will discuss about what is corrosion and related topics.

Corrosion is a natural phenomenon that occurs when materials reach their lowest free energy level. This phenomenon is reason of destruction of material, by reacting with the environment in contact with it.

According to some experts, this definition should be confined to metals. However most corrosion engineers have to deal with both metals and non-metals.

Thus, this phenomenon can’t be dedicated to metals. Corrosion is associated with loss of gloss, appearance, weight and in many cases performance of the part in service. However, this won’t happen for all materials. The probability of its occurrence, is proportional to the behavior of the material and environmental conditions, which is in contact with.

In the following, a comprehensive article on what corrosion is will be discussed.

Here is what corrosion engineering is all about;

Corrosion engineering, is a practical combination of science and art, in order to prevent and control destruction economically and safely.

Therefore, in order to function properly, corrosion engineers must have sufficient skills in the following areas.

  • Corrosion measures and principles
  • Chemical, physical and mechanical properties of materials
  • Corrosion experiments
  • The nature of the corrosive environment
  • Availability of materials
  • Electronic equipment and design equipment

What is corrosion; behavior of material in the environment

What is corrosion; Corrosion of materials
What is corrosion; Corrosion of materials

To better understand what corrosion is, we need to know the behavior of materials reaction to the environment in which they are located. In this feature, we can examine the behavior of materials in three groups:

First group

Safety, is the first behavior that can be imagined for materials. For example, platinum can be repeatedly immersed in acidic solutions and use it again. Glass and plastics behave somewhat similarly and are resistant to some corrosive acids.

This is what is known as safe or noble conduct. A large group of metals such as gold, silver and platinum fall into this category. However, noble materials can behave differently depending on the environment they are in contact with.

Second group

Probably the most troublesome group that has always rushed to the aid of the industry due to the significant reduction in costs. This group consists active metals.

The presence of this group in the sun environment, along with loss of gloss, beauty, weight loss and the creation of uneven and porous surfaces. Corrosion of metal fences, iron doors and windows, especially those installed in the humid southern and northern regions of the country. These are some of the most tangible examples of corrosion around us.

Iron and steel are the most widely used raw materials in the industry in this category. Zinc, sodium, and magnesium behave similarly. That is why sodium can only be stored under oil.

Iron and steel are corroded in a wide range of aqueous and gaseous environments. The most common corrosion of iron and steel occurs in aquatic environments and in humid environments.

Third group

The most mysterious group are metals that can change from active to inactive or passive.

The most well-known material in this group is Aluminum. Aluminum is the most widely used metal in the industry after Iron due to its unique properties. One of its features is the formation of a resistant and thin layer of Aluminum Oxide ( Al2O3 ). This protects it from corrosion.

The stability of the passive layer can be affected by environmental factors. For example, unforeseen changes in the concentration of corrosive ions that overshadow all engineering calculations.

What are the types of corrosion?

The classification of forms of corrosion has been done for many years with different perspectives.

An extensive view categorized corrosion into two categories:

  • Corrosion that is affected by no other process.
  • Corrosion that is affected by another process, such as stress or erosion.

A more centralized view categorized corrosion as uniform or localized, watery or gaseous, wet or dry, and etc. .

The phenomenon of corrosion appears in different forms in equipment. Sometimes the appearance of the fragment remains intact and only certain points are cracked and perforated. Familiarity with the types of corrosion can be useful in correctly identifying what corrosion is and finding ways to deal with it.

In the following, we will become more familiar with the types of corrosion.

Uniform Corrosion

The most common, famous and well-known type of corrosion, is UniformCorrosion. Uniform corrosion processes involve many of the corrosion phenomena encountered in corrosion engineering and corrosion science. Most of these phenomena are related to water or gas corrosion.

This phenomenon is detected by a chemical or electrochemical reaction. This reaction, proceeds uniformly across the fragment or a large surface.

Due to this phenomenon, the metal becomes thinner and eventually destroyed.

For example, a piece of Iron or Zinc dissolves in dilute Sulfuric Acid at a uniform rate over its entire surface. However, technically, this type of corrosion does not cause much concern, because the life of the part can be accurately estimated by corrosion experiments . The chemical reaction that has occurred for corroded metal atoms, is as follows:

Fe → Fe+2 + 2e

In other words, during corrosion reactions, metal atoms turn into metal ions and enter the solution. The following picture, shows the uniform corrosion of a steel nail.

Uniform corrosion can be prevented or reduced by specific methods. These methods, include the use of suitable materials such as coatings, inhibitors and cathodic protection.

Related topics: Cathodic protection

What is corrosion; Uniform corrosion
What is corrosion; Uniform corrosion

Galvanic Corrosion

Another common type is galvanic corrosion or bimetallic. This type of corrosion occurs due to potential differences between two heterogeneous metals; When in a corrosive environment or conductive solution.

There is a potential difference even on the surface of a piece or metal. The reason is the presence of grain boundaries with different orientations, impurities and defects. This potential difference is usually small and can not cause severe corrosion. But heterogeneous metal generally exerts a greater potential difference. This potential difference causes electrons to flow.

In this case, the metal, which is more electrochemically active, dissolves and loses its atoms.

Following galvanic corrosion, corrosion of the more resistant metal decreases and corrosion of the less-resistant metal increases compared to when the two metals are not in contact with each other. Less metal – more resistant anode and more resistant metal is cathode.

Watch the video below about galvanic corrosion.

Term for corrosion of two metals

This corrosion is called galvanic (electrical) or bimetallic corrosion, because electric flow and bimetallic metals are involved. The propulsion of flow and corrosion is the potential difference between two metals. What is known as a dry cell is a good example of this phenomenon.

The Carbon electrode acts as a noble or corrosion-resistant metal and the Zinc acts as an anode and corrodes. The wet paste between the electrodes is the conductive medium that carries the flow. Use magnesium as an anode.

For a clearer picture of galvanic corrosion and to know what this type of corrosion is, pay attention to the image below.

What is corrosion; Galvanic corrosion of two heterogeneous metals
What is corrosion; Galvanic corrosion of two heterogeneous metals
What is corrosion; Dry cell
What is corrosion; Dry cell

What is EMF and Galvanic Series in Corrosion?

Potential differences between metals in reversible or non-corrosive conditions, form the basis for predicting corrosion susceptibility. The potential between metals exposed to solutions containing an atomic gram of their ions is measured at a constant. The following figure shows a table known as the electromotive force table or EMF .

What is corrosion; Galvanic series of metals in contact with the solution of related ions
What is corrosion; Galvanic series of metals in contact with the solution of related ions

In order to simplify, all potentials have been measured relative to H2/H+, which is considered zero. The potential between metals is determined by considering the absolute difference between their EMF potential. For example, there is a potential of 0.462 volts between the reversible electrodes of Copper and Silver and 1.1 volts between the reversible electrodes of Copper and Zinc.

In the real world, galvanic couples are rarely in the ionic medium. But because most engineering materials are alloys, the galvanic pair consists of one (or two) metal alloys. Under these conditions, the following galvanic series can be effective for predicting galvanic relationships.

What is corrosion; Galvanic series of metal alloys
What is corrosion; Galvanic series of metal alloys

What is corrosion; Environmental effects on galvanic corrosion

The nature and corrosiveness of the environment can greatly determine the amount of bimetallic corrosion. Usually, metals with less resistance to the environment are corrosive and the metal with more resistance is cathode. Sometimes, the potential of a metal pair is reversed. The table below, shows the different behavior of Zinc and Steel in different environments.

What is corrosion; The effect of temperature on galvanic corrosion

Zinc and Steel are usually corroded in the environment. But when they are placed next to each other or paired, the Zinc is corroded and the Steel is protected. In exceptional cases, such as domestic water at temperatures above 180◦ Fahrenheit, the pair will reversed and the Steel becomes anode.

Zinc corrosion products make it act like a nobler metal than Steel. Haney has also shown, that Zinc becomes less active in the presence of inhibitory ions such as Nitrates, Bicarbonates, and Carbonates in water.

Tantalum is also a corrosion-resistant metal that is relative to Platinum and Carbon anode. But this cell is active at high temperatures. For example, for the Tantalum-Platinum couple, the flow is not established until the temperature reaches 110 °C, and the current will flow at 100mA/ft 265 °C.

Tantalum is strongly cathode compared to Iron with a high Silicon percentage in strong Sulfuric Acid, but the flow quickly reaches zero. Because, at the temperature of 145 °C, the polarity of the cell is reversed. Tantalum should not be in contact with anodic metals, as Hydrogen absorbs the cathode and becomes brittle.

What is corrosion; Effect of atmosphere on galvanic corrosion

Galvanic corrosion occurs in the atmosphere too. Corrosion intensity depends on the type and amount of moisture. For example, corrosion near the coast is greater than dry atmosphere. Condensates created in the beach environment have more salt and easily direct the electrical flow and the electrolyte is better than what appears in dry environments.

Atmospheric tests have shown that Zinc is higher than anode Steel, but Tin and Nickel are always cathodes, and Aluminum behaves differently. Galvanic corrosion does not occur when the metals are in a dry place, as there is no electrolyte to carry the charge between the two electrodes. In high-resistance or pure water, corrosion attack can be a deep groove. Bimetallic corrosion occurs with localized attacks near the joints.

What is corrosion; Effect of distance on galvanic corrosion

Corrosion attacks are more common due to corrosion of galvanic metal near communications and are less occur when moving away from them. The effect of distance depends on the conductivity of the solution. Naturally, there is more resistance to load transfer when the trajectory is longer.

What is corrosion; Effect of surface on galvanic corrosion

Another important factor in galvanic corrosionو is the surface effect, or the ratio of cathode to anode. The surface that is a problem for us, is the surface at which, the cathode is high compared to the anode. For specific flow in cell, the flow density for a smaller electrode is greater in the anodic area. The higher the flow of density anode, will increase the corrosion rate. Anodic corrosion may be 100 to 1000 times greater than when the anode and cathode are level.

The galvanic series indicates that Iron is anode than Copper and therefore corrodes faster when in contact with it. This effect is exacerbated when the cathode surface is much larger than the anode surface. The following figures show the results of holding a Steel rivet in contact with a piece of Copper in 3% Sodium Chloride solution at different times.

What is corrosion; Steel rivet on a piece of copper immersed in 3% sodium chloride solution at the beginning of the test
What is corrosion; Steel rivet on a piece of copper immersed in 3% sodium chloride solution at the beginning of the test
What is corrosion; Steel rivet on a piece of copper immersed in 3% sodium chloride solution after 6 months
What is corrosion; Steel rivet on a piece of copper immersed in 3% sodium chloride solution after 10 months
What is corrosion; Steel rivet on a piece of copper immersed in 3% sodium chloride solution after 10 months

As can be seen, Steel nails are severely corroded over time. However, in reverse situation, when Iron surface is larger than Copper, Iron corrosion will intensifies.

What is corrosion; Copper rivet on steel piece immersed in 3% sodium chloride solution at the beginning of the test
What is corrosion; Copper rivet on steel piece immersed in 3% sodium chloride solution at the beginning of the test
Copper rivet on steel piece immersed in 3% sodium chloride solution at the beginning of the test
Copper rivet on steel piece immersed in 3% sodium chloride solution at the beginning of the test
What is corrosion; Copper rivet on steel piece immersed in 3% sodium chloride solution after 10 months
What is corrosion; Copper rivet on steel piece immersed in 3% sodium chloride solution after 10 months

Localized corrosion

localized Corrosion is a type of corrosion that occurs in distinct materials . Although this explanation is correct, and is compatible with the localized conditions of the environment; we can say, it is the localized conditions of the environment that cause corrosion attacks.

Therefore, the propagation of cavities on the metal surfaces of the patio is a severe attack that occurs in the grooves caused by the overlap of the bimetals. Also, the degradation in which two metals react with each other and create cracks, and the filament-like degradation of metal surfaces covered with thin organic layers, is due to differences in the metallurgy of alloys and due to different local chemistry of the environment.

Severe localized corrosion, which usually occurs between grooves or other coated surfaces of metal, which are exposed to corrosives, is called crevice corrosion

This type of corrosion is generally associated with small volumes of static solution; It is caused by the presence of holes, washers, overlapping surfaces, surface deposits, grooves under the screws and rivets.

Another name for this phenomenon is washer or sediment corrosion

Crevice corrosion mechanism

Until now, crevice corrosion has been thought to result from differences in ions or the concentration of oxygen between the crevice and its surroundings. Consequently, the term concentration cell was also used for this type of corrosion. It has recently been shown that although there is a difference in the concentration of ions and oxygen in the groove corrosion, it will not be a fundamental factor.

To reveal the corrosion mechanism, consider M riveted metal plate immersed in aerated seawater solution (pH7). The final reaction includes dissolving the M metal and revival oxygen to hydroxide ions.

Oxidation reaction

M →M2+ + 2e

Revival reaction

O2+ 2H2O+2e → 4OH

Investigation of oxidation reaction

Initially, these reactions occur uniformly across the surface, including the area inside the groove. Also, the charge balance in the metal and the solution is established.

Each electron produced during the formation of metal ions is rapidly consumed by the oxygen reduction reaction. Also, one hydroxide ion is produced per ion in solution. After a short time, the oxygen in the groove runs out due to the limited displacement of the solution and the reduction of oxygen in this area stops.

This in turn does not change the corrosion behavior. Since the area between a crevice is usually very small compared to the outer surface, there will be almost no change in the overall rate of oxygen reduction. Therefore, the corrosion rate inside and outside the crevice remains the same.

Oxygen depletion in the groove has a significant indirect effect. When oxygen is depleted, there is no reduction of oxygen in that area. But corrosion continues and the area inside the groove becomes heavily positive.

This creates a positive charge that attracts the chloride ions to balance the charge. This causes a significant amount of metal chloride to form inside the groove.

What is corrosion; Metal salt hydrolysis

With the exception of alkali metals, metal chlorides and sulfates are hydrolyzed in water to form HCl and metal hydroxide. The mechanism is shown in the figure below.

Metal salt hydrolysis

M+Cl +H2O → MOH +HCl

The acid formed in the groove continues the corrosion process and the dissolution reaction of the metal inside the groove continues.

The figure below shows “sub-sedimentary corrosion” due to the deposition of corrosion products and the formation of mineral crusts. This case is created on the inner surfaces of fire pipes.

What is corrosion; Groove corrosion
What is corrosion; Groove corrosion

Pitting corrosion

Another type of corrosion is severe local corrosion that causes holes in the metal. The diameter of the holes can be large or small, but in most cases they are relatively small.

The cavities are sometimes separate, or very close to each other, and can look like a rough surface. A cavity can be defined as an empty space or hole whose surface diameter is equal to or greater than its depth.

The following figure shows the impeller of a compressor blade and shows the effects of this corrosion.

What is corrosion; Cavitation of aluminum alloy
What is corrosion; Cavitation of aluminum alloy

Intergranular corrosion

By impurities in the grain boundaries, enrichment of an element or depletion of it occurs. A small percentage of iron in aluminum, where iron dissolution is low, can cause separation at the grain boundaries. This causes corrosion between the seeds.

Another type of corrosion is seen when discharging stainless steel grain boundaries from chromium. The following figure shows the sensitivity of stainless steel grains due to the formation of chromium carbide in steel grains.

Intergranular corrosion
Intergranular corrosion

What is selective separation in corrosion?

Separation of metal from solid alloy is by corrosion process. If we want to know what this type of corrosion is, we can refer to the most famous type, which is the separation of zinc from rice (removal of zinc).

A similar process took place in other alloying systems in which aluminum, iron, cobalt, and chromium were separated.

Erosion corrosion

The intensification and increase in the rate of degradation on the metal surface is due to the relative motion of a corrosive fluid and the metal surface. This movement is generally quite slow and is accompanied by mechanical abrasion and scratches.

The metal separates from the surface as dissolved iron ions, or forms solid corrosion products and is mechanically scraped off the surface.

What is corrosion; Abrasive corrosion of the compressor blade
What is corrosion; Abrasive corrosion of the compressor blade

Environmentally Induced Cracking

Environmental cracking is a type of corrosion that is externally affected by stress.

Two types of cracking or fracture phenomena are discussed:

  • Corrosion Cracking (SCC)
  • Hydrogen corrosion (often referred to as hydrogen failure).

These two phenomena have many similarities to each other. For example, both phenomena generally depend on yield stress and applied stress.

Increasing both of these factors reduces SCC resistance, hydrogen corrosion, molten metal brittleness, and solid metal trading.

Theories related to mechanisms:

What are the theories regarding the mechanisms of corrosion?

It should be noted that there is no satisfactory theory for any of these mechanisms. A theory that fully explains all the behaviors observed in the laboratory or field.

Many theories relate to the behavior of some alloy systems or environments. But none of them are comprehensive enough. For example, it cannot explain the development of different behaviors that explain hydrogen corrosion for systems that form hydrides versus iron alloys that do not form hydrides.

There are many of these contradictory factors that hinder the development of pervasive theories. In addition, there has been controversy since the 1960s about the true micromechanical causes of SCC. The controversy is that some researchers attribute it to hydrogen degradation rather than an active corrosion pathway.

There are specific convincing data on the role of hydrogen in the SCC of some alloys. But not enough data is available to explain this concept.

Satisfactory mechanical models have not been developed for any of the forms of environmental stress corrosion. Because there is basically no prediction of stress corrosion with the environment.

However, the need to anticipate this type of destruction is very important. Because observable and measurable corrosion usually does not occur before or during the onset or propagation of cracks.

It is very localized when corrosion occurs. Such as cavities or cracks that may be difficult to detect.

Tension corrosion cracking

Tension cracking
Tension cracking

It is another type of corrosion that occurs due to the simultaneous effect of tensile stress and corrosive environment. Many researchers call all cracking damage that occurs in corrosive environments SCC, such as damage due to hydrogen brittleness.

For example, cathodic protection is a good way to prevent SCC but causes hydrogen brittleness.

Hydrogen damage

Which is the mechanical degradation of metal due to the presence and opposition of hydrogen.

Hydrogen degradation is divided into four major categories:

  1. Hydrogen blistering
  2. Hydrogen embittlerment
  3. Decarburization
  4. Hydrogen attack

1.Hydrogen blistering

Hydrogen blistering results from the penetration of hydrogen into the metal. An example of blistering is shown in the figure below. The result is local deformation and eventual complete destruction of the part.

What is corrosion;
What is corrosion;
Hydrogen embrittlement

Hydrogen brittleness is also caused by the penetration of hydrogen into the metal, which results in loss of ductility (flexibility) and tensile strength.


Decarbonization or loss of steel carbon is caused by wet hydrogen at high temperatures. Decomposition reduces the tensile stress of steel.

4.Hydrogen attack

Hydrogen attack refers to the interaction between hydrogen and a part of an alloy at high temperatures.

What is microbial corrosion?

Micro-biologically influenced corrosion (MIC) is a type of corrosion caused by the activity and presence of microbes, bacteria, microalgae, minerals, and organic and chemical.

Microbial corrosion comes in many forms and can be controlled with traditional methods and pesticides.

MIC occurs on almost all metal surfaces exposed to non-sterile systems, which in fact leads to metal dissolution and loss of mechanical properties.

This type of corrosion is seen in power plants and the chemical industry, as well as in places that use cold water towers.

What is corrosion? Electron microscopic image of microbial corrosion
What is corrosion? Electron microscopic image of microbial corrosion

Various bacteria cause microbial corrosion, such as sulfate-reducing bacteria (SRB), nitrate-reducing bacteria, methanogenic bacteria,

Acid-producing bacteria, iron-oxidizing bacteria (IOBs), sulfur-oxidizing bacteria.

What is the following classification of microbes in the subject of corrosion ?; And we pay according to the need for oxygen.

Aerobic microbes

Microbes that need oxygen in their metabolic process are called aerobic. These microbes grow in nutrient environments containing dissolved oxygen.

Anaerobic microbes

They are microbes that grow best in low-oxygen or no-oxygen environments. Microbial corrosion occurs in both aquatic and terrestrial environments and is responsible for more than 30% of corrosion damage.

About 80-90% of known bacteria need oxygen; For this reason, in industrial environments, oxygen is removed from the system, but destructive anaerobic bacteria are still present and cause corrosion.

Sulfate-reducing bacteria (SRB) is one of the most important anaerobic microbes.

SRBs live in groups and produce gelatinous products and sediments, and the activity of these microbes lowers oxygen and pH, which intensifies corrosion.

Due to the abundance of ions (SO42 ) in aqueous media, SRB is known as the main culprit of anaerobic microbial corrosion. SRB ions (SO42 ) are used as an electron mediator to generate energy in their metabolism. Consumption of these ions indirectly causes the accumulation of corrosive sulfides and the production of acidic mineral products, which cause metals to become porous.

What are the theories proposed for SRB in corrosion?

Various mechanisms for MIC have been proposed by the SRB. These mechanisms include the depolarization mechanism, local corrosive tuberculosis, and metabolite-induced corrosion.

The theory of cathodic depolarization has been widely published in the literature, pointing to the role of bacteria in the MIC, which involves the transfer of electrons from the cathode region to the SO42 reduction process through the presence of intermediate hydrogen (such as hydrogenase).

Metal sulfide with SRB activity causes local corrosion by establishing a metal / sulfide galvanic pair. Even in the absence of H2S, which causes galvanic corrosion.

Recently, extracellular electron transfer (EET) theory has introduced a new concept of metal / microbial interaction. According to this theory, it is possible that electrons entered the cytoplasm by passing through the cell wall from outside the cell. This leads to more severe corrosion.

What is iron-oxidizing bacteria (IOBs) in corrosion?

IOBs, or what is known as metal scavenging microorganisms, are considered MIC agents. In an aerobic environment, IOB can precipitate iron hydroxide (Fe (OH) 3, FeOOH, Fe2O3) in extracellular membranes. It can also cause corrosion.

These microorganisms produce their growth energy from the oxidation of ferrous ions (Fe2 +) to ferric (Fe3+) and oxygen as the electron transporter, which eventually precipitates as ferric hydroxide.

What is corrosion? Picture of a sewer pipe that has suffered from microbial corrosion
What is corrosion? Picture of a sewer pipe that has suffered from microbial corrosion
Mechanism of microbial corrosion

The cathodic depolarization mechanism has been republished as the most well-known SRB corrosion mechanism in several journals. It was first proposed in 1934 by Kohr and Walat.

When metals are exposed to water, they are polarized by the loss of positive metal ions. Free electrons reduce water protons in the absence of oxygen (anaerobic conditions) (cathodic reaction) to form hydrogen that settles on the metal surface.

What is corrosion? Mechanism of microbial corrosion
What is corrosion? Mechanism of microbial corrosion

SRBs are expected to consume hydrogen according to the reaction in the following 4 tables, which can lead to iron oxidation. This process accelerates the anodic dissolution and subsequent formation of FeS and Fe(OH)2 as corrosion products.

According to this theory, continuous inhibition of the hydrogen film by SRB dissolves iron. Therefore, this theory suggests that any anaerobic bacterium that consumes hydrogen can cause iron corrosion.

What is corrosion? Description of reactions in SRB corrosion
What is corrosion? Description of reactions in SRB corrosion

Some important factors in corrosion must be present, such as:

  • Energy source
  • Nutrients, especially carbon source
  • Nitrogen
  • Hydrogen
  • Oxygen
  • Phosphate
  • Sulfur
  • Electron donors and receptors that perform the metabolic mechanism of corrosion by microbes.

What is tuberculosis in corrosion?

Here we examine what is the term wart in corrosion and what it means. Warting of the steel surface, also known as tuberculosis, is accompanied by the formation of reddish-brown iron oxide bumps on the inside of the tubes.

Tuberculosis is more common in steels made before 1940 and is usually an important part of the water distribution system. As this type of corrosion develops over time, consumers will realize its effects by the end of the 21st century.

This phenomenon is in the category of bacterial corrosion and is completely related to old pipes. Tuberculosis is associated with the activity of microorganisms in the tube environment. Bacteria in the tube feed on iron and manganese water, which eventually leads to the formation of sediment and accumulation of iron / manganese products on the inner surface of the tube.

Formation of tuberculosis on the inner surface of fire hoses
Formation of tuberculosis on the inner surface of fire hoses

Metallurgically Induced Corrosion

To better understand the phenomenon of metallurgical corrosion, we must first know what metallurgy means.

The meaning of metallurgical:

The science of metallurgy includes the following:
  • Metal recovery from ore (extractive metallurgy)
  • Formation of metals in different forms (mechanical metallurgy) and understanding the interaction of chemical species
  • Orientation of crystals and displacement in metals and alloys (physical metallurgy)

All of these processes can affect the composition, structure, stress levels, and impurities of metals and alloys, resulting in their corrosion behavior. Another process, welding, is usually used after the formation and formation of metals with metallurgical techniques.

Basic understanding of electrochemical corrosion kinetics:

The basic understanding of electrochemical corrosion kinetics is based on the uniform dissolution of pure and homogeneous metal, which is equally exposed to a homogeneous environment in all places. Corrosion engineers know that this description does not reflect reality in practical terms.

In many engineering structures, different alloys are joined together in different ways. There are often cracks and the environment is variable and depends on fluid conditions.

In addition, metals and alloys are usually coated with a surface layer. Their corrosion behavior is affected by the surface layer and a number of metallurgical factors.

Even pure monocrystals have imperfections that can affect corrosion. Impurities and alloying elements, granules, secondary phases and their components are often affected.

Finally, welded structures corrode sooner. Because there is metallurgical heterogeneity in and near the welds.

Most susceptible spot or defect on the surface:

In general, the most prone spot or defect on a metal surface is the first to be exposed to a corrosive environment. Depending on the alloying elements, the corrosion resistance of stainless steels and nickel-based alloys varies considerably.

In many cases, stainless steels undergo local corrosion, which is strongly influenced by the combination of alloys and metallurgical agents.

Cavity corrosion in stainless steels often begins with the addition of manganese sulfide (MnS). The cause can be attributed to the reaction of products containing sulfide (S) and the effect of dissolution of these products on the passive film.

Formation of carbides at the grain boundary can reduce corrosion resistance by forming crude (Cr) sensitive areas in the adjacent field.

A continuous network of such Cr-depleted areas can sensitize stainless steel. It can also be prone to intergranular corrosion or tension corrosion.

What is corrosion; Atmospheric corrosion

Atmospheric corrosion, erosion and degradation of metal materials and their properties are due to interaction with the Earth’s atmosphere at specific air temperatures, humidity, air chemistry and climatic values. Atmospheric corrosion is different from metal corrosion, which occurs at high temperatures and in the absence of moisture on the metal surface (dry or gaseous corrosion) and does not correspond to the humidity and atmospheric temperature of the earth.

The mechanism of corrosion at high temperatures is chemical and is quite different from atmospheric corrosion.

Most metal structures and equipment are somewhat exposed to atmospheric conditions and can cause atmospheric corrosion. In some cases where the corrosion is severe, the metal can be completely destroyed and become products of corrosion.

With sufficient background and knowledge of the main conditions of metal-atmosphere contact and its effect on metal corrosion, many serious corrosion problems can be avoided.

خوردگی چیست؛ خوردگی اتمسفری

Cause of atmospheric corrosion

Several industries face severe corrosionproblems due to highly aggressive atmospheres. Including power transmission and distribution industries, chemical / petrochemical plants and equipment, aircraft, automobile and related components, transportation infrastructure, offshore structures and equipment, building industry and electronics.

Atmospheric corrosion occurs spontaneously. It can be prevented and controlled, but it never stops. This is because the metals that are commonly used are not pure on earth except in some noble metals. Metals are usually found in minerals and ores, chemical compounds including oxygen, hydrogen and sulfur.

These mineral compounds are the thermodynamically stable states of metals in which the free energy of Gibbs is minimal. For the separation of metals from their ores and for metallurgical and production processes, energy in the form of heat, chemical, electrical or mechanical, brings the metal to a higher energy level.

Atmospheric corrosion mechanism

The metal product does not have its most thermodynamically stable state. This fact causes metals to become corrosion products that have the same chemical composition as the original ore and are more thermodynamically stable.

When metals come into contact with the atmosphere (oxygen) and water (moisture) in the presence of corrosive species such as chlorides or sulfur dioxide, the corrosion process begins and corrosion products such as oxides, hydroxides or oxyhydroxides are formed.

The main cause of atmospheric corrosion is an aqueous process and its mechanism is electrochemical. During a chemical reaction, not only mass transfer but also the exchange of charged particles (electrons and ions) takes place at the interface between the metal (electron conductor) and the electrolyte (ion conductor).

What is the role of anode and cathode in this type of corrosion?

Electron transfer (electric fluid flow) occurs due to the formation of a galvanic corrosion on the metal surface. Three elements are essential for the function of cell corrosion: Anode and cathode, electrolyte and oxidizing agent. The anode and cathode sites form several corrosion cells.

Anodes are areas on the metal that have a higher energy state due to various factors such as heterogeneous metal composition, grain boundaries, several metallurgical phases, local metal defects, and non-uniform metal operations (such as incorrect heat treatment).

Oxidation corrosion reaction takes place in anodic sites:

The metal dissolves at the anode to form cations (positively charged ions). They may originally appear as hydrated metal ions but eventually become oxides, hydroxides, and corrosion products of the metal. For example, atmospheric corrosion products made of steel usually contain g-FeOOH as the main components.

Atmospheric corrosion; What is the anodic and cathodic reaction in this type of corrosion?

Cathodes are metal places with lower energy states. For example, inert non-metallic components and less active active metal phases or structures. The cathodic reaction occurs at these sites and involves the reduction of oxidizing agents such as air, oxygen, or hydrogen ions.

The electrolyte is in contact with the metal surface like moisture. Humidity includes soluble ion species (atmospheric pollutants) as well as good ionic conductivity that can sustain electrochemical reactions.

An oxidizing agent such as oxygen and hydrogen ions (H+) is essential for the reception of electrons emitted by the metal in the anodic reaction:

Where Oxy is the oxidizing agent and Red is reduced.

What is corrosion; Atmospheric corrosion
What is corrosion; Atmospheric corrosion

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