Corrosion

Август 8, 2022

Содержание

2. Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with their environment. In
3. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in
4. Many structural alloys corrode merely from exposure to moisture in air, but the process can be
5. Metal corrosion is a spontaneous thermodynamic destruction (anodic oxidation) metal as a result of exposure to
6. In terms of redox reactions: nature of this interaction is reduced to the oxidation of the
7. Iron oxidation by atmospheric oxygen: 4Fe + 3O2 = 2Fe2O3 Corrosion of iron in aqueous solution
8. Steel Corrosion 1) Initial Oxidation Reaction: 2) Secondary Oxidation Reaction: rust
9. Classification of Corrosion By type of corrosive environment By the nature of destruction By the types
10. By operating conditions known the following types of electrochemical corrosion crevice corrosion contact corrosion corrosion fatigue
11. Crevice corrosion - electrochemical corrosion in cracks and gaps between the two metals, which enters the
12. Gas corrosion Atmospheric corrosion Liquid corrosion Soil corrosion Stray currents
13. Gas Corrosion - chemical corrosion of metals in gases at high temperatures (e.g., in the combustion
14. Continual Corrosion Uniform corrosion Uneven corrosion
15. Local corrosion By ulcers By points By spots
16. Causes of localized corrosion Intergranual corrosion Sea water Road salt
17. CHEMICAL CORROSION is the interaction of the metal with the environment in which the oxidation of
18. 1. Oxidation corrosion (Reaction with oxygen) Some of the metals directly react with oxygen in the
19. During oxidation of a metal, metal oxide is formed as a thin film on the metallic
20. Oxides of Pb, Al and Sn are stable and hence inhibit further corrosion. They form a
21. 2. Corrosion by other gases (Cl2, SO2, H2S, NOx) In dry atmosphere, these gases react with
22. 3. Liquid - metal corrosion In several industries, molten metal passes through metallic pipes and causes
23. ELECTROCHEMICAL CORROSION is anodic oxidation of the metal by environment elements in which the oxidation of
24. Oxidation of anodic part takes place and it results in corrosion at anode, while reduction takes
25. Mechanism: Electrochemical corrosion involves the flow of electron current between anodic and catholic regions. The anodic
26. Thus, electrochemical corrosion occurs: where a conducting liquid (water, acid, salt solutions) is in contact with
27. Oxygen reduction (acidic medium pH Fe + O2 + 4H+ = Fe2O3 * nH2O Oxygen reduction
28. Steel (Fe, E°=-0,44V) Steel (Fe, E°=-0,44V) Zn, E°=-0,76V) Sn, E°=-0,15V) tin-plated steel At anode: Fe° –
29. Factors Influencing Corrosion The nature and extent of corrosion depend on the metal and the environment.
30. 1. The position of metals in the electrochemical Series Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc
31. potential area where any corrosion impossible, example Аu (1,5 В) area of corrosion with oxygen depolarization
32. Affecting Nature of the Corroding Environment Temperature: Corrosion generally increases with rise in temperature of environment.
33. Group 1 - alkali metals - the lowest corrosion resistance 1 group of the sub-groups -
34. Group 3 - Aluminium - forming stable oxide film (but it is destroyed in solutions of
35. 5,6,7,8 group - by metals of subgroups have high ability to passivate and hence high resistance
36. PROTECTION METHODS Material selection Improvements in material Design of structures Alteration of environment Add inhibitors Galvanic
39. Скачать презентацию

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Corrosion is the gradual destruction of materials (usually metals) by chemical

reaction with their environment.
In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidants such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal.

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Corrosion can also occur in materials other than metals, such as

ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

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Many structural alloys corrode merely from exposure to moisture in air,

but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

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Metal corrosion is a spontaneous thermodynamic destruction (anodic oxidation) metal as

a result of exposure to chemical and electrochemical environment, it is heterogeneous redox process that occurs at the interface.

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In terms of redox reactions: nature of this interaction is reduced

to the oxidation of the metal and restore oxidant. When metal corrosion occur at its surface take place simultaneously two independent electrochemical reactions:
at anode: Me° - ne = Me+n
at cathode: Ox + ne = Red
Corrosive environment called surfactants, are present around the structural member, its impact on the material and cause it to corrode. Corrosive medium may be air, industrial atmosphere, gases, water, sea climate, land - soil, acids, alkalis, water and salt solutions.

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Iron oxidation by atmospheric oxygen:
4Fe + 3O2 = 2Fe2O3
Corrosion of iron

in aqueous solution containing hydrogen ions (acid medium, pH<7):
2Fe + 6H+ = 2Fe3+ + 3H2
Corrosion of iron in water containing oxygen (neutral medium, pH=7):
4Fe + 3O2 + 2H2О = 2Fe2O3·H2О

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Steel Corrosion
1) Initial Oxidation Reaction:
2) Secondary Oxidation Reaction:
rust

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Classification of Corrosion
By type of corrosive environment
By the nature of
destruction
By the

types of corrosive process

Chemical corrosion

Electrochemical corrosion
continual
local
intergranual

gas

liquid

soil

atmospheric

stray currents

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By operating conditions known the following types of electrochemical corrosion
crevice corrosion
contact

corrosion

corrosion fatigue

corrosion
cracking

Corrosion friction
(Fretting)

Corrosion in the collision
(Cavitation)

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Crevice corrosion - electrochemical corrosion in cracks and gaps between the

two metals, which enters the electrolyte.

Contact corrosion - electrochemical corrosion of any two metals with different potentials in contact, dipped in an electrolyte.

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Gas corrosion
Atmospheric corrosion
Liquid corrosion
Soil corrosion
Stray currents

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Gas Corrosion - chemical corrosion of metals in gases at high

temperatures (e.g., in the combustion chamber jet).
Atmospheric corrosion - corrosion in air (in the presence of a condensed film of moisture and precipitation).
Liquid corrosion - the chemical destruction of metal in fluids, electrolytes which are not.

Soil corrosion - corrosion of metal products in contact with the soil - soil electrolyte.
Stray currents corrosion is destruction of metal structures due to the ingress of corrosion of the conductive medium (soil, sea water) electric (so called stray) current. Source stray currents in the soil often electrified railways, as well as current generators (eg, welders), grounded on the ground.

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Continual Corrosion
Uniform corrosion
Uneven corrosion

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Local corrosion
By ulcers
By points
By spots

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Causes of localized corrosion
Intergranual corrosion
Sea water
Road salt

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CHEMICAL CORROSION
is the interaction of the metal with the environment in

which the oxidation of the metal and restoring the oxidant corrosive environment occurs in one act without causing an electric current.
This type of corrosion occurs due to direct chemical attack of environment or atmospheric gases like oxygen halogens, hydrogen sulphide, sulphur dioxide, nitrogen or anhydrous inorganic liquid with metal surfaces in immediate proximity.
Chemical corrosion is of three main types as:
Oxidation Corrosion (with Oxygen)
Corrosion by Other Gases
In non-electrolyte Liquid Metal Corrosion

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1. Oxidation corrosion
(Reaction with oxygen)
Some of the metals directly react with

oxygen in the absence of moisture:
Alkali and alkaline earth metals react with oxygen at room temperature and form corresponding oxides, while some metals react with oxygen at higher temperature.
Metals like Ag, Au and Pt are not oxidized as they are noble metals.

4Fe + 3О2 = 2Fe2О3

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During oxidation of a metal, metal oxide is formed as

a thin film on the metallic surface which protects the metal from further corrosion:
If diffusion of either oxygen or metal is across this layer, further corrosion is possible. Thus, the layer of metal oxide plays an important role in the process of corrosion.

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Oxides of Pb, Al and Sn are stable and hence inhibit

further corrosion. They form a stable, tightly adhering oxide film.
In case of porous oxide film, atmospheric gases pass through the pores and react with the metal and the process of corrosion continues to occur till the entire metal is converted into oxide.
Porous oxide layer is formed by alkali and alkaline earth metals. Molybdenum forms a volatile oxide film of MoO3 which accelerates corrosion.
Au, Ag, Pt form unstable oxide layer which decomposes soon after the formation, thereby preventing further corrosion.

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2. Corrosion by other gases (Cl2, SO2, H2S, NOx)
In dry atmosphere,

these gases react with metal and form corrosion products which may be protective or non-protective.
Dry Cl2 reacts with Ag and forms AgCl which is a protective layer, while SnCl4 is volatile.
In petroleum industries at high temperatures, H2S attacks steel forming FeS scale which is porous and interferes with normal operations.

2Fe + 3Cl2 = 2FeCl3

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3. Liquid - metal corrosion
In several industries, molten metal passes through

metallic pipes and causes corrosion due to dissolution or due to internal penetration.
For example:
liquid metal mercury dissolves most metals by forming amalgams, thereby corroding them.
In oil
In sulphur
In organic substances

Cu + S = CuS

2 Ag + S = Ag2S

2Al + 6ССl4 = 3C2Cl6 + 3AlCl3

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ELECTROCHEMICAL CORROSION
is anodic oxidation of the metal by environment elements

in which the oxidation of the metal and restoring the oxidant corrosive environment is a result of the flow of several elementary acts, accompanied by the appearance of a galvanic couple and electric current.
This type of corrosion occurs when the metal comes in contact with a conducting liquid or when two dissimilar metals are immersed or dipped partly in a solution. There is the formation of a galvanic cell on the surface of metals. Some parts of the metal surface act as anode and rest act as cathode. Water must be present to serve as a medium for the transport of ions.
The most common depolarizers are oxygen, acids, and the cations of less active metals.

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Oxidation of anodic part takes place and it results in corrosion

at anode, while reduction takes place at cathode:
The corrosion product is formed on the surface of the metal between anode and cathode.

At cathode:
Hydrogen depolarization

Oxygen depolarization

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Mechanism: Electrochemical corrosion involves the flow of electron current between anodic and

catholic regions. The anodic reaction involves dissolution of the metal as corresponding metallic tops with the liberation of tree electrons.
Anodic area:  Me°-ne- = Me+n (Oxidation)
Cathodic area: Reduction reaction consumes electrons and depending on the nature, of corrosive environment the following reactions may take place:
Hydrogen evolution: 2H+ + 2e- = H2
Oxygen reduction (acidic medium pH<7):
           O2 + 4H+ + 4e- = 2H2O
Oxygen reduction (neutral or alkaline medium):
           O2 + 2H2O + 4e- = 4OH-
Metal ion reduction:   M3+ + e- = M2+
Metal deposition:       M+ + e- = M↓

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Thus, electrochemical corrosion occurs:
where a conducting liquid (water, acid, salt solutions)

is in contact with a metal or
when two dissimilar, metals or alloys are either immersed or dipped partially in a aqueous solution.
In electrochemical or wet corrosion the following observations are made:
Formation of anodic and catholic part or parts in contact with each, other.
Presence of conducting medium
Corrosion of anodic areas only.
Formation of corrosion product between anodic and catholic areas.

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Oxygen reduction (acidic medium pH<7):
Fe + O2 + 4H+ = Fe2O3 *

nH2O
Oxygen reduction (neutral or alkaline medium):
2Fe + O2 + 3H2O = 2Fe(OH)3

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Steel (Fe, E°=-0,44V)
Steel (Fe, E°=-0,44V)
Zn,
E°=-0,76V)
Sn,
E°=-0,15V)
tin-plated steel
At anode: Fe° –

2e = Fe2+
At cathode: 2H+ + 2e = H2
Fe + H2SO4 = FeSO4 + H2

galvanized steel

At anode: Zn° – 2e = Zn2+
At cathode: Fe+2 + 2e = Fe
Zn + H2SO4 = ZnSO4 + H2

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Factors Influencing Corrosion
The nature and extent of corrosion depend on the

metal and the environment. The important factors which may influence the corrosion process are:
Nature of the metal (position Me in ECS)
Environment (oxidant type)
Concentration of electrolyte
Temperature
Hydrogen over voltage (pH)

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1. The position of metals in the electrochemical Series
Lithium
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Nickel
Tin
Hydrogen--------------- 0.00
Copper
Silver
Platinum
Gold---------------------- +1.15

Metal SRP, Eo

Decreasing tendency
to loose
electrons

Increasing order of std reduction potential

Less Reduction

More oxidation

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potential area where any corrosion impossible, example Аu (1,5 В)
area of

corrosion with oxygen depolarization possible, example Cu (0,34 В)

area of corrosion with hydrogen depolarization possible, example Fe (-0,44 B)

-0,41 B

+ 0,82 В

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Affecting Nature of the Corroding Environment
Temperature: Corrosion generally increases with rise

in temperature of environment.
Humidity of the air: Rate of corrosion increases with presence of moisture in the atmosphere.
Impurities: Presence of impurities like CO2, H2S, SO2, acid fumes etc increases corrosion rate.
Influence of pH: In acid medium corrosion is more and in alkaline medium it is less.

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Group 1 - alkali metals - the lowest corrosion resistance
1 group

of the sub-groups - very resistant metals
Group 2 – are unstable,
Group 2 sub-groups – are more stable (in the presence of oxygen to form a strong oxide film protecting from further destruction)

corrosion properties of metals

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Group 3 - Aluminium - forming stable oxide film (but it

is destroyed in solutions of acids and bases). In concentrated nitric and sulfuric acids passivated aluminum.
Group 4 – tin (Sn) and lead (Pb) – corrosion-resistant metals, thanks to strong oxide film.

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5,6,7,8 group - by metals of subgroups have high ability to

passivate and hence high resistance to corrosion.
Osmium, iridium, platinum - the most resistant to corrosion
Iron passivated by concentrated sulfuric and nitric acids

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PROTECTION METHODS
Material selection
Improvements in material
Design of structures
Alteration of environment
Add inhibitors
Galvanic Cathodic

or Anodic protection
Active electrochemical protection
Coatings

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Corrosion

Содержание

Corrosion is the gradual destruction of materials (usually metals) by chemical

Corrosion can also occur in materials other than metals, such as

Many structural alloys corrode merely from exposure to moisture in air,

Metal corrosion is a spontaneous thermodynamic destruction (anodic oxidation) metal as

In terms of redox reactions: nature of this interaction is reduced

Iron oxidation by atmospheric oxygen:4Fe + 3O2 = 2Fe2O3Corrosion of iron

Steel Corrosion 1) Initial Oxidation Reaction: 2) Secondary Oxidation Reaction:rust

Classification of CorrosionBy type of corrosive environmentBy the nature ofdestructionBy the

By operating conditions known the following types of electrochemical corrosioncrevice corrosioncontact

Crevice corrosion - electrochemical corrosion in cracks and gaps between the

Gas corrosion Atmospheric corrosionLiquid corrosionSoil corrosionStray currents

Gas Corrosion - chemical corrosion of metals in gases at high

Continual CorrosionUniform corrosionUneven corrosion

Local corrosionBy ulcers By pointsBy spots

Causes of localized corrosionIntergranual corrosionSea waterRoad salt

CHEMICAL CORROSIONis the interaction of the metal with the environment in

1. Oxidation corrosion(Reaction with oxygen) Some of the metals directly react with

During oxidation of a metal, metal oxide is formed as

Oxides of Pb, Al and Sn are stable and hence inhibit

2. Corrosion by other gases (Cl2, SO2, H2S, NOx)In dry atmosphere,

3. Liquid - metal corrosion In several industries, molten metal passes through

ELECTROCHEMICAL CORROSION is anodic oxidation of the metal by environment elements