Selected Definitions Corrosion: It is defined as the process of decomposition or destruction of metallic material in presence of environmen through chemical reactions. Galvanic corrosion: This occur when current flow from anode to cathode. The anode represent more active metal whil cathode represent less active metal. Intergranular Cladding:
corrosion:
This is a chemical or electrochemical attack on the grain boundaries of a metal.
It is the bonding of dissimilar metals.
Passivating potential: It is the potential develop at critical point. Passivating current:
The flow of current decrease to minimum value above the passivating potential.
12.1 INTRODUCTION A large number of equipments are used for manufacturing of drugs. When the solution which is corrosive in nature comes in contact with the equipment, it can destroy the performance and fabrication of the equipment. Therefore, great care should be adopt during selection of material for construction of equipment. Corrosion is defmed as the process of decomposition or destruction of metallic material in presence of environment through chemical reactions. Corrosion is a natural process. Corrosion may be dry or wet type. Dry corrosion occurs in the absence of an aqueous environment, usually in the presence of gases and vapors, mainly at high temperatures. Wet corrosion is due to attack of aqueous media.
Table 12.1: Difference between Dry and wet corrosion Dry corrosion Corrosion occurs in the absence of moisture.
Wet corrosion Corrosion occurs in presence of conducting medium It is a rapid process It depends on the size of the anodic part of metal. Corrosion occurs at anode but rust is deposited at cathode.
The process is slow. The process of corrosion is uniform. Corrosion products are produced at the site of corrosion
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12.2 THEORIES OF CORROSION There are three theories of corrosion: a. b. c.
Acid theory, Dry or Chemical theory of corrosion and Galvanic or Electrochemical or Wet theory of corrosion.
12.2.1 Acid theory: This theory states that the acid cause corrosion of a metal. Iron is corroded by atmospheric carbon dioxide, moisture and oxygen. Corrosion products are the combination of Fe (HC03 Fe(OH)C03 and Fe(OHk
n,
1
Fe+2C02 +H20+2"02
~
1 2Fe(HCOJ)2 +H20+2"02 2Fe(HCOJ)2 +2H20
~
Fe(HCOJ)2 ~
(12.1)
2Fe(OH)COJ +2C02 +2HP
(12.2) (12.3)
2Fe(OH)J +2C02
12.2.2 Dry or chemical theory of corrosion: This theory states that, corrosion is due to the reaction of atmospheric gases such as oxygen, halogens, sulphur oxides, nitrogen oxides, hydrogen sulfide and fumes of chemical products with metal. The chemical affinity of the metal to the reactive gas predict the extent of corrosion. Example: Alkali and alkaline earth metals react with oxygen at room temperature and form corresponding oxides
12.2.3 Galvanic or Electrochemical or Wet theory of corrosion: When the metal or two dissimilar metals dipped partly or fully in conducting liquid. This cause formation of a galvanic cell on the metals surface. Some parts of the metal surface act as anode and rest as cathode. The chemical in the environment and humidity acts as an electrolyte which cause oxidation of anodic part. This results in corrosion at anode, while reduction takes place at cathode. Therefore formation of corrosion product takes place on the surface of the metal between anode and cathode.
Corrosion reaction between metals:
Cathode (Cu) Reduction
AnodeZn Oxidation
Electrolyte (BCI)
Figure 12.1: Galvanic theory of corrosion Galvanic corrosion occur when current flow from anode to cathode. Anode represent more active metal while cathode represent less active metal. Two metals Zn (anode) and Cu (cathode) when dipped in
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electrolytic solution, they forms galvanic cells. When two electrodes are connected to wire, spontaneous reaction occur. At anode (oxidation): (Indicates rough surface) (12.4)
Zn ----+ Znt" + 2eAt cathode (reduction): (indicates formation of bubbles at surface)
(12.5) When current flows the anode metal get corroded and cathode metal is protected. Sometime cathodic polarisation (deposition of hydrogen layer on cathode lower the corrosion) occur.
12.3 TYPE OF CORROSION Corrosion
can be classified as Type of corrosion
------..------1 General corrosion
Localised corrosion Intergranular
1
--,
Structural corrosion
corrosion
[C"Phlte
Biological corrosion
e.' resion
Pitting corrosion Stress corrosion
Dezincification
Fretting corrosion Erosion corrosion Cavitation corrosion Galvanic corrosion Oxygen concentration cells Corrosion fatigue Impingement Attack Hydrogen Embrittlement
12.3.1 General corrosion It is also known as uniform attack corrosion. It is the most common type of corrosion and is caused by a chemical or electrochemical reaction that damages the entire exposed surface of a metal.
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12.3.2 Localised corrosion It occurs in various ways a. Intergranular corrosion: Intergranular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. Therefore it is also called Intergranular corrosion. It often occurs due to impurities in the metal, which tend to be present in higher contents near the grain boundaries. For example Austenitic stainless steel with nitric acid when properly heated, it cause intergranular corrosion due to precipitation of grain boundary. b. Pitting corrosion: Pitting corrosion occurs when a small hole or cavity is formed in the metal, usually as a result of de-passivation of a small area. This area becomes anodic, while a part of the remaining metal becomes cathodic, producing a localized galvanic reaction. The deterioration of this small area penetrates the metal and can lead to failure. This form of corrosion is often difficult to detect because it is generally relatively small and can be covered and hidden by compounds produced by corrosion. For example: Alloy of aluminium or stainless steel in presence of aqueous solution and chlorides cause cavities. c. Crevice corrosion: Similar to pitting, crevice corrosion occurs at a specific location or crevices. The solution get deposited to crevices and drying of liquid takes more time. As a result crevice corrosion occurs. This type of corrosion is often associated with a stagnant micro-environment. Acidic conditions or a depletion of oxygen in a crevice can cause crevice corrosion. d. Stress corrosion: When tensile stress is greater than yield stress, then cracks develop on the surface. For example: cold formed brass develop cracks in presence of ammonia environment. e. Fretting corrosion: Fretting corrosion occurs as a result of repeated wearing or due to sliding or vibration of metals on an uneven rough surface. The corrosion occur due to generation of frictional heat which produce oxide films which continuously removed and cause corrosion at fresh surface. This can be avoided by minimizing friction or by proper lubrication of equipment. f. Erosion corrosion: This occur due to destruction of metal by combination of abrasion and attrition caused by flow of liquid or gas. The factors, such as alloy content of steel, pipe system design, component geometry and water composition, influence erosion. The use of harder metals and velocity or environment changes are used to prevent erosion. g. Cavitation erosion: Cavitation erosion occurs due to formation of vacuum bubbles in liquid stream when passed over the surface. The bubbles on metal surface get collapsed and cause damage and further corrosion. This type of corrosion generally occur around propellers. This can be avoided by using more resistant metal and by using protective coating. h. Galvanic corrosion: Galvanic corrosion occurs when two different metals are found together in a corrosive electrolyte. A galvanic cell is formed between the two metals, where one metal becomes the anode and the other the cathode. The anode corrodes and deteriorates faster while the cathode deteriorates more slowly. Protective oxide film reduce galvanic corrosion. l, Oxygen concentration cell: This type of corrosion occur due to difference in amount of oxygen in solution at one point when compared with other. The least oxygen concentration increase rate of corrosion. Redesign of equipment avoid such type of corrosion. j. Corrosion fatigue: Due to repetition metal surface get corroded by corrosive media. The protective oxide mm that prevent corrosion get ruptured at higher rate as compared to new protective film. Therefore ,surface loose its fatigue resistance and fmally equipment get deteriorate. k. Impingement attack or corrosion: It is also called velocity accelerated corrosion. This type of corrosion is accelerated by removal of oxide films. I. Hydrogen Embrittlement: Penetration of hydrogen into metals cause loss in ductility. At elevated temperature, cracking of carbon steel occur. As a result, failure occur by embrittlement, cracking and blistering. The solution of hydrogen sulphide and hydrocyanic acid is susceptible to this form of attack.
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12.3.3 Structural Corrosion In this type of corrosion, structural strength of metal get reduced due to corrosion. It is of two type a. Graphite corrosion: Graphite corrosion generally involve Gray Cast iron in which metallic iron get converted into corrosion product. This produce residue of intact graphite which is mixed with iron corrosive product and insoluble components of cast iron. b. Dezincifi~tion: This is observed in brass which contain 15% zinc. The major constituent of corrosion is metallic copper. The mechanism behind corrosion is (1) Redeposition of copper over corrosion products (2) Formation of zinc corrosion products leaving copper residue. This type of corrosion can be decreased by adding little amount of arsenic, antimony, phosphorus to the alloy.
12.3.4 Biological Corrosion This type of corrosion occur due to metabolic activity of microorganism which cause deterioration of metal. The biological corrosion is due to a. Changing resistance to surface film b. Developing electrolyte-concentration cells on metal surface c. Developing corrosive environment d. Altering rate of anodic/cathodic reaction. For example: Anaerobes sulphate-reducing bacteria in the soil produce hydrogen sulphite and calcium sulphite. When these compounds come in contact with underground pipes, convert iron to iron sulphide.
12.4 PREVENTION OF CORROSION Although corrosion is a natural process, it can be controlled by using effective methods and strategies. There are different ways to control corrosion. These are 1. Materials Selection: The selection of the right and proper materials for particular corrosive environments is essential. Corrosion behaviour of each metal and alloy is unique. The corrosion of metal has a strong relation with the environment to which it is exposed.
. k R ate 0f corrOSIve attac
Corrosivity of environment = -------=--------
Corrosion resistance of metal
(12.6)
Proper knowledge of the nature of the environment to which the material is exposed is very important. In few cases such as in fine mesh wire screens and orifices in which changes are not permitted due to corrosion. In these cases non metallic materials should be selected for fabrication. 2. Design of equipment: The proper design is as equally important parameter because it reduce the time and cost required for corrosion maintenance and repair. Corrosion frequently happens in dead spaces or crevices and it is highly recommended to eliminate or minimize these areas while designing. The corrosion can be reduced if there are no crevices in the equipment or if equipment is properly cleaned or if liquid is completely drained after working or if proper inspection of equipments should be done at time. 3. Coating and lining: The metals are more prone to corrosion. To combat corrosion in metals, non metals coating or lining should be used. Electroplating, cladding, organic coating can also be used. Galvanic corrosion can be controlled by applying barrier coatings or insulating both the anodes and
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cathodes to prevent the flow of electrons across the joint. Organic coating are also used as lining of tanks, piping and shipping containers. Cladding is the bonding of dissimilar metals. It is achieved by rolling of two sheets of metal together. Cladding is also done for steel with an alloy is another approach to combat corrosion. 4.
By changing the environment: Corrosion can be prevented by removing air from boiler feed water which prevent steel from the corrosive effect of water. In case of nickel based alloy the pumping of inert gas reduce air or oxygen content. The corrosive effect of acidic media on stainless steel alloys can be minimized by aeration. Corrosion can also be reduced by decreasing the temperature, by reducing the moisture and also by decreasing the exposure time.
S.
Use of Corrosion Inhibitors: Corrosion inhibitors are used to decrease corrosion of metals. The inhibitors are used in critical amount (less than 0.1 % by weight). For example: Chromates, phosphates and silicates are used to protect iron and steel in aqueous solutions. Organic sulphides and arnines are used to protect iron and steel in acidic medium. Copper sulphate is used to protect stainless steel from corrosion in hot diluted solution of sulphuric acid.
6.
Cathodic and Anodic protection:
Cathodic protection is achieved by two methods.
a.
Sacrificial anode methods
b.
Impressed ernf methods
a.
Sacrificial anode methods: As the name indicates, anodes are kept in contact with protected metal (cathode), this cause sacrification of anode. For example: zinc, aluminium, magnesium and their alloys are used as sacrificial anode for protection of iron and steel tanks.
b.
Impressed emf methods: In this method, external voltage is applied between tanks and electrodes. The anode is maintained always at positive. The natural galvanic effect is avoided. Thus anode is non- consumed. So any metal or non corrodible alloys are used. For example: in case of sulphuric acid and deionised water, anodes are buried in ground while graphite and high silicone steel are compressed. The advantage of this methods are: simple, most effective, inexpensive and used to store mild corrosive liquors.
In contrast to cathodic protection, anodic protection is one of the more recently developed methods for controlling corrosion. In anodic protection, predetermined potential is applied to metal. At initial stage, as current increases, metal dissolution or corrosion occur. At critical point passivation occurs. The potential develop at critical point is called passivating potential. Above the passivating potential, current flow decrease to minimum value. This is called passivating current. The main advantage of anodic protection is that it require small current. This is used in transportation of concentrated sulphuric acid.
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4DEVIEW
QUESTIONS
VERY SHORT ANSWER QUESTIONS 1.
Define Corrosion Answer- It is defined as the process of decomposition or destruction of metallic material in presence of environment through chemical reactions.
2.
Define Galvanic corrosion Answer- This occur when current flow from anode to cathode. Anode represent more active metal while cathode represent less active metal.
3.
Give name of corrosion inhibitors Answer- Chromates, Phosphates and silicates, Organic sulphides and arnines, Copper sulphate.
4.
Write advantage of increased emf methods Answer- The advantage of this methods are: Simple, most effective, inexpensive and used to store mild corrosive liquors.
S.
Explain rate of corrosive attack. Answer- Rate of corrosive attack is directly proportional to corrosivity of environment and inversely proportional to corrosion resistance of metal.
SHORT ANSWER QUESTIONS 1.
Write difference between dry and wet corrosion. Answer Wet corrosion
Dry corrosion Corrosion occurs in the absence of moisture.
Corrosion occurs in presence of conducting medium
The process is slow.
It is a rapid process
The process of corrosion is uniform.
It depends on the size of the anodic part of metal.
Corrosion products are produced at the site of corrosion
Corrosion occurs at anode but rust is deposited at cathode.
2.
Which type of corrosion is difficult to detect and why? Answer- Pitting corrosion occurs when a small hole or cavity is formed in the metal, usually as a result of depassivation of a small area. because pits are relatively small and can be covered and hidden by compounds produced by corrosion.
3.
How proper design of equipment is important
to prevent corrosion?
Answer- Proper design of equipment is important because it reduce the time and cost required for corrosion maintenance and repair. Corrosion frequently happens in dead spaces or crevices and it is highly recommended to eliminate or minimize these areas while designing. The corrosion can be reduced if there are no crevices in the equipment or if equipment is properly cleaned or if liquid is completely drained after working or if proper inspection of equipments should be done at time.
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Define fretting corrosion and how it can be prevented? Answer- Fretting corrosion occurs as a result of repeated wearing or due to sliding or vibration of metals on an uneven rough surface. The corrosion occur due to generation of frictional heat which produce oxide films which continuously removed and cause corrosion at fresh surface. This can be avoided by minimizing friction by proper lubrication of equipment.
5.
Define biological corrosion. Write causes of biological corrosion. Answer- This type of corrosion occur due to metabolic activity of microorganism which cause deterioration of metal. The biological corrosion is due to
a.
Changing resistance to surface film
b.
Developing electrolyte-concentration cells on metal surface
c.
Developing corrosive environment
d.
Altering rate of anodiclcathodic reaction.
LONG ANSWER OUESTIONS 1.
Define corrosion. Explain in detail different types of corrosion. (Refer article 12.1 and 12.3)
2.
Explain in detail different theories of corrosion. (Refer article 12.2) Explain different methods used to tackle corrosion (Refer article 12.4) Write detailed note on different ways in which Localised corrosion occurs (Refer article 12.3.2) Write detailed note on a. Biological corrosion (Refer article 12.3.4) b. Structural corrosion (Refer article 12.3.3)
3. 4. 5.
MULTIPLE CHOICE OUESTIONS 1.
Corrosion can be prevented by a.
2.
Use of Corrosion Inhibitors
b.
Coating and lining
c.
By changing the environment
d. All of the above Which of the following lsI are used as sacrlftclal anode for protection of Iron and steel tanks. a.
Zinc
b.
Aluminium
c.
magnesium
d.
All of the above
Corrosion and its Prevention 3.
191
The process of decomposition or destruction of metallic material in presence of environment reactions is called
4.
S.
6.
7.
8.
9.
a.
corrosion
b.
degradation
c.
deposition
d.
deterioration
Which of the following is not theory of corrosion? a.
Acid theory of corrosion
b.
Chemical theory of corrosion
c.
galvanic theory of corrosion
d.
Bronsted theory of corrosion
Which of the following is also called uniform attack corrosion a.
General corrosion
b.
Localised corrosion
c.
Structural corrosion
d.
Biological corrosion
Fretting corrosion occurs due to a.
repeated wearing or due to sliding or vibration of metals on an uneven rough surface
b.
destruction of metal by combination of abrasion and attrition caused by flow of liquid or gas.
c.
formation of vacuum bubbles in liquid stream when passed over the surface.
d.
Formation of pits
Oxygen concentration cell occur due to a. difference in amount of oxygen in solution at one point when compared with other b.
when two different metals are found together in a corrosive electrolyte
c.
Penetration of hydrogen into metals
d.
formation of vacuum bubbles in liquid stream
Which of the following is also called velocity accelerated corrosion? a.
Impingement attack or corrosion
b.
Erosion corrosion
c.
Pitting corrosion
d.
Galvanic corrosion
During wet corrosion a. The anodic part undergoes oxidation b.
Cathodic part undergoes oxidation
c.
Anodic part undergoes reduction
d.
None of the above
10. The biological corrosion is due to a. Changing resistance to surface film b.
Developing corrosive environment
c.
Altering rate of anodic/cathodic reaction.
d.
All of the above
ANSWERS l.d 2.d 3.a 4.d
s.a
6.a 7.a
s.a
9.a
1O.d
through chemical