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Prevention
of
Corrosion
Prof. Harish Chopra,
Department of Chemistry,
SLIET, Longowal (Pb) INDIA
Introduction
Prevention of corrosion is very important, as it is
very harmful for metals causing huge losses to
industry and country. As already explained
corrosion is of many types and hence different
treatments are needed to retard corrosion.
2
Change or Modification of Material
3
The corrosion may be controlled by partial or complete
change in the material used. But usually modification of
the material is preferred. This can be done by two ways as
given below:
1) Coating the metal, in order to introduce a corrosion
resistant coating between metal and environment, the
coating may consist of:
(a) another metal, E.g., zinc or tin coatings on steel, or
(b) a protective coating derived from the metal itself,
E.g., aluminium oxide on “anodised” aluminium,
(c) organic coatings, such as resins, plastics, paints,
enamel, oils, and greases.
1) Alloying the metal to produce a more corrosion
resistant alloy, E.g., stainless steel, in which ordinary
steel is alloyed with chromium and nickel. Stainless steel
is protected by an invisibly thin, naturally formed film of
chromium sesquioxide Cr2O3.
Corrosion Inhibitors
4
A corrosion inhibitor is a chemical
additive, which, when added to a
corrosive aqueous environment,
reduces the rate of corrosion.
The inhibitors are broadly
classified as following types:
❏ Anodic inhibitors
❏ Cathodic inhibitors
❏ Adsorption inhibitors
❏ Mixed inhibitors
Anodic Inhibitors
5
An anodic inhibitor interferes with the anodic
process. If an anodic inhibitor is not present
at a concentration level sufficient to block off
all the anodic sites, localized attack such as
pitting corrosion can become a serious
problem due to the anodic reaction as shown
below:
The anodic inhibitors react with the metal
ions formed during anode reaction to produce
sparingly soluble inorganic salts. These salts
are absorbed on the surface of metal and form
a protective coating which reduces corrosion.
The examples of anodic inhibitors include
orthophosphate, nitrite, ferricyanide,
molybdates, tungstates, chromates and
silicates.
Cathodic Inhibitors
6
The major cathodic reaction in cooling systems is the reduction of
oxygen as shown below:
The additives, which suppress the above type of reaction, are called as
cathodic inhibitors. The most commonly used cathodic inhibitors are salts
of Mg, Zn or Ni. These salts react with hydroxide ions formed during
cathode reaction to give corresponding insoluble hydroxides which get
deposited on cathode forming barriers resulting in prevention of
corrosion, E.g., zinc ions are used as cathodic inhibitors because of the
precipitation of Zn(OH)2 at cathodic sites as a consequence of the
localized high pH.
Adsorption Inhibitors
7
Many organic inhibitors work by
an adsorption mechanism. The
resultant film of chemisorbed
inhibitor is then responsible for
protection either by physically
blocking the surface from the
corrosion environment or by
retarding the electrochemical
processes. The main organic
functional groups capable of
forming chemisorbed bonds
with metal surfaces are amino
(NH2), carboxyl (COOH), and
phosphonate (PO3H2) although
other functional groups or atoms
can form coordinate bonds with
metal surfaces.
Mixed Inhibitors
8
Mixed inhibitors are compounds that
form a film or precipitate to reduce both
the cathodic and anodic reactions.
Sodium silicate and phosphates used in
domestic water softener salts to prevent
rust water are examples. Because of the
danger of pitting when using anodic
inhibitors alone, it became common
practice to incorporate a cathodic
inhibitor into formulated performance.
It was obtained by a combination of
inhibitors than from the sum of the
individual performances. This use of
combination of inhibitors is generally
referred to a “synergism” and
demonstrates the synergistic action which
exists between zinc and chromate ions.
Cathodic Protection
9
If electrons are passed into the metal and reach the metal/electrolyte
interface (a cathodic current) the anodic reaction becomes subdued
while the cathodic reaction rate increases. This process is called
cathodic protection and can only be applied if there is a suitable
conducting medium such as earth or water through which a current
can flow to the metal to be protected. In most soils or natural waters
corrosion of steel is prevented if the potential of the metal surface is
lowered by 300 or 400 mV. Cathodic protection is of two types:
1. Sacrificial Anodic Protection
2. Impressed Current Cathodic Protection
Sacrificial Anodic Protection
10
Sacrificial anodic protection may be achieved by obtaining electrons from the anodic
dissolution of a metal low in the galvanic series such as aluminium, zinc or
magnesium. A measure of protection is offered by driving a magnesium rod into the
ground near the pipe and providing an electrical connection to the pipe. Since, the
magnesium has a standard potential of 2.38 volts compared to 0.41 volts for iron, it
can act as an anode of a voltaic cell with the steel pipe acting as the cathode. With
damp soil serving as the electrolyte, a small current can flow in the wire connected
to the pipe. The magnesium rod will be eventually consumed by the reaction
while the steel pipe as the cathode will be protected by the reaction:
Impressed Current Cathodic Protection
11
Impressed-current-type cathodic
protection systems provide cathodic
current from an external power source. A
direct current (DC) power source forces
current to discharge from expendable
anodes through the electrolyte and on to
the structure to be protected. Although
the current is not generated by the
corrosion of a sacrificial metal/alloy, the
energized materials used for the
auxiliary anodes do corrode. The basic
components of an impressed- current-
type cathodic protection system are the
structure to be protected, a DC power
source, a group of auxiliary anodes
(ground bed or anode bed), and insulated
lead wires connecting the structure to
be protected to the negative terminal of
the power source and the ground bed to
the positive terminal of the power
source.
Protective Coatings
12
Protective coatings are probably the most widely used products for
corrosion control. They are used to provide long-term protection
under a broad range of corrosive conditions, extending from
atmospheric exposure to the most demanding chemical processing
conditions. Protective coatings in themselves provide little or no
structural strength, yet they protect other materials to protect their
strength.
The main function of a protective coating is to isolate structural
reactive elements from environmental corrosives. A coating must
provide to a substrate a continuous barrier and any imperfection can
become the focal point for degradation and corrosion of the substrate.
The various types of commonly used protective coatings are:
Metallic Coatings
Electroplating, Electroless plating, Zinc Coatings, Metal Cladding
Inorganic Coatings
Metallic Coatings
13
Metallic coatings provide a layer that changes
the surface properties of the corroding metal to
those of the metal being applied. The corroding
metal becomes a composite material exhibiting
properties generally not achievable by either
material if used alone. The coatings provide a
durable, corrosion resistant layer, and the core
material provides the load-bearing capability.
The deposition of metal coatings, such as
chromium, nickel, copper, and cadmium, is
usually achieved by wet chemical processes.
Alternative metal deposition methods have
replaced some of the wet processes and may
play a greater role in metal coating in the
future. Metallic coatings are deposited by
electroplating, electroless plating, spraying,
hot dipping, chemical vapour deposition and
ion vapour deposition.
Electroplating
14
Electroplating is the deposition of a metallic coating on to an object by
putting a negative charge on to the object and immersing it into a
solution which contains a salt of the metal to be deposited. Electroplating
is achieved by passing an electrical current through a solution containing
dissolved metal ions and the metal object to be plated. The metal object
serves as the cathode in an electrochemical cell, attracting metal ions
from the solution. Ferrous and non-ferrous metal objects are plated
with a variety of metals, including aluminium, tin, bronze, cadmium,
copper, chromium, iron, lead, nickel, zinc, as well as precious metals,
such as gold, platinum, and silver.
Nickel Plating
Electroless Plating
15
Electroless nickel (EN) plating is a chemical reduction process which
depends upon the catalytic reduction process of nickel ions in an aqueous
solution (containing a chemical reducing agent) and the subsequent
deposition of nickel metal without the use of electrical energy.
Due to its excellent corrosion resistance and high hardness, the process
finds extensive application on items such as valves, pump parts, etc., to
enhance the life of components exposed to harsh conditions of service,
particularly in the oil field and marine sector.
Electroless Plating
16
The advantages of EN plating are:
1. The deposits in EN plating are uniform, even on complex
shapes
2. The deposits are often less porous and hence provide better
barrier corrosion protection to steel substrates, much
superior to that of electroplated nickel and hard chrome
3. The deposits cause about 1/5th as much hydrogen
absorption as electrolytic nickel and about 1/10th as much
hard chrome
4. The deposits have natural lubricity unlike electrolytic
nickel
5. The deposits have good wettability for oils.
Electroless Plating
17
Electroless nickel plating shows excellent corrosion resistance, high-
wear resistance and uniformity of coating.. Some of the important
applications are:
1. Oil & Gas: Valve components (such as balls, gates, plugs, etc.),
pumps, pipe fittings, packers, barrels, etc.
2. Chemical Processing: Heat exchangers, filter units, pump
housing and impellers, mixing blades etc.
3. Plastics: Moulds and dies for injecting and low and blow
moulding of plastics components, extruders, machine parts
rollers, etc.
4. Textile: Printing cylinders, machine parts, spinneret's, threaded
guides etc.
5. Automotive: Shock absorbers, heat sinks, gears, cylinders, brake
pistons, etc.
6. Aviation and aerospace: Satellite and rocket components, rams
pistons, valve components, etc.
7. Food & pharmaceutical: Capsule machinery dies, chocolates
moulds, food processing machinery components, etc.
Inorganic Coatings
18
Inorganic coatings can be formed by chemical action, with
or without electrical assistance. The treatments change the
immediate surface layer of metal into a film of metallic
oxide or compound, which has better corrosion resistance
than the natural oxide film and provides an effective base
or key for supplementary protection such as paints. In
some instances, these treatments can also be a
preliminary step prior to painting.
Anodizing: Anodizing is an electrochemical process in
which aluminum is the anode. Anodizing involves the
electrolytic oxidation of a surface to produce a tightly
adherent oxide scale, which is thicker than the naturally
occurring film. The electric current passing through an
electrolyte converts the metal surface to a durable
aluminum oxide. The difference between plating and
anodizing is that the oxide coating is integral part of
the metal substrate as opposed to being a metallic
coating deposition. The oxidized surface is hard and
abrasion resistant, and it provides some degree of
corrosion resistance.
Anodizing Processes
19
Hardcoat anodizing: Hardcoating is a sulphuric
acid anodizing process, with the electrolyte
concentration, temperature, and electric current
parameters changed to produce the hardened surface.
A “hardcoat” finish is tough and durable. It is used
where abrasion, corrosion resistance and surface
hardness are critical factors.
Bulk anodizing: Bulk anodizing is an
electrochemical process for anodizing small,
irregularly shaped parts which are processed in
perforated aluminum, plastic or titanium baskets. The
large quantity of parts that can be finished in
relatively short time makes this technique highly
economical. Another advantage in processing such
large volumes at one time is the resulting consistency
in color and quality. Finishing items such as rivets,
ferrules, medical hubs, etc. using the bulk process
make production economically feasible.
Anodizing Processes
20
Sulphuric acid anodizing: This is the most
common method of anodizing. The part to be anodized is
subjected to a specified electric current through a
sulphuric acid electrolyte, converting the surface to an
aluminum oxide coating capable of absorbing dyes in a
wide range of colours. Abrasion and/or corrosion
resistance is enhanced, and the surface may also be used
as a base for applied coatings, such as paint, teflon, and
adhesives.
Factors Affecting Corrosion
21
Nature of the metal
I. Position in the galvanic series: If the metal is higher up in the galvanic series, the
oxidation potential is higher which makes the metal to have greater tendency to become anodic
and hence the rate of corrosion is higher. So, corrosion rate is directly related to the position of
the metal in the galvanic series.
II. Purity of metal: The rate of corrosion is higher in case of impure metals in comparison to
pure metals. The impurities present in metal cause heterogeneity and thus tiny electrochemical
cells are set up at the exposed part of the impurity and corrosion of metal around the impurity
takes place due to local action.
III. Physical state of the metal: The rate of corrosion is dependent on the physical state of
metal. The smaller the grain size of the metal or alloy, the higher will be its rate of corrosion.
IV. Nature of the oxide film: The ratio of the volumes of the metal oxide to the metal is called
as "specific volume ratio". Greater the specific volume ratio, lesser is the oxidation corrosion rate.
V. Relative areas of the anode and cathode: When two different metals or alloys are in
contact, the corrosion of the anodic part is directly proportional to the ratio of the cathodic part
and the anodic part. When cathodic area is smaller, the demand for electrons will be less and this
results in the reduced rate of dissolution of metal at anodic regions.
VI. Solubility of corrosion products: In electrochemical corrosion, corrosion proceeds at a
faster rate if the corrosion product is soluble in corroding medium, E.g., Pb in H2SO4 medium
forms PbSO4 which is insoluble in the corroding medium, thus corrosion proceeds at a slower rate.
VII.Volatility of corrosion products: Rapid and uninterrupted corrosion of metal take place
if corrosion product is volatile. This is due to the fact that as soon as corrosion product is formed,
it volatilizes, thereby leaving the underlying metal surface for further corrosion attack.
Factors Affecting Corrosion
22
Nature of the corroding environment
I. Effect of pH: Corrosion of that metal which is readily attacked by acids can
be reduced by increasing the pH of the attacking environment.
II. Humidity of air: The rate and extent of corrosion is directly proportional
to humidity. This is due to the fact that moisture acts as a solvent for O2,
H2S, SO2, NaCl, etc. to furnish the electrolyte essential for setting up a
corrosion cell. So, higher the humidity higher will be rate of corrosion.
III. Temperature: The corrosion rate is generally enhanced with increase of
temperature of environment due to the increase in diffusion rate.
IV. Presence of suspended particles in atmosphere: In case of
atmospheric corrosion (a) if the suspended particles are chemically active in
nature (like NaCl, (NH4)2SO4), they absorb moisture and act as strong
electrolytes, so corrosion rate is enhanced, (b) if the suspended particles are
chemically inactive in nature (for example, charcoal), they absorb both
sulphur gases, and moisture and slowly enhance corrosion rate.
V. Presence of impurities in atmosphere: The rate of corrosion of
metals is more in areas near to the industry and sea. This is due to the fact
that corrosive gases like H2S, SO2, CO2 and vapours of H2SO4 and HCI in the
industrial areas and NaCl of sea water lead to increased conductivity of the
liquid layer in contact with the metal surface, thereby increasing the
corrosion rate.
23
References
The some contents are taken from:
Chemistry For Engineers
By
Harish Chopra
Anupama Parmar
[In addition, Internet sources have also been used]
Thank You !
24

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Corrosion prevention

  • 1. Prevention of Corrosion Prof. Harish Chopra, Department of Chemistry, SLIET, Longowal (Pb) INDIA
  • 2. Introduction Prevention of corrosion is very important, as it is very harmful for metals causing huge losses to industry and country. As already explained corrosion is of many types and hence different treatments are needed to retard corrosion. 2
  • 3. Change or Modification of Material 3 The corrosion may be controlled by partial or complete change in the material used. But usually modification of the material is preferred. This can be done by two ways as given below: 1) Coating the metal, in order to introduce a corrosion resistant coating between metal and environment, the coating may consist of: (a) another metal, E.g., zinc or tin coatings on steel, or (b) a protective coating derived from the metal itself, E.g., aluminium oxide on “anodised” aluminium, (c) organic coatings, such as resins, plastics, paints, enamel, oils, and greases. 1) Alloying the metal to produce a more corrosion resistant alloy, E.g., stainless steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium sesquioxide Cr2O3.
  • 4. Corrosion Inhibitors 4 A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous environment, reduces the rate of corrosion. The inhibitors are broadly classified as following types: ❏ Anodic inhibitors ❏ Cathodic inhibitors ❏ Adsorption inhibitors ❏ Mixed inhibitors
  • 5. Anodic Inhibitors 5 An anodic inhibitor interferes with the anodic process. If an anodic inhibitor is not present at a concentration level sufficient to block off all the anodic sites, localized attack such as pitting corrosion can become a serious problem due to the anodic reaction as shown below: The anodic inhibitors react with the metal ions formed during anode reaction to produce sparingly soluble inorganic salts. These salts are absorbed on the surface of metal and form a protective coating which reduces corrosion. The examples of anodic inhibitors include orthophosphate, nitrite, ferricyanide, molybdates, tungstates, chromates and silicates.
  • 6. Cathodic Inhibitors 6 The major cathodic reaction in cooling systems is the reduction of oxygen as shown below: The additives, which suppress the above type of reaction, are called as cathodic inhibitors. The most commonly used cathodic inhibitors are salts of Mg, Zn or Ni. These salts react with hydroxide ions formed during cathode reaction to give corresponding insoluble hydroxides which get deposited on cathode forming barriers resulting in prevention of corrosion, E.g., zinc ions are used as cathodic inhibitors because of the precipitation of Zn(OH)2 at cathodic sites as a consequence of the localized high pH.
  • 7. Adsorption Inhibitors 7 Many organic inhibitors work by an adsorption mechanism. The resultant film of chemisorbed inhibitor is then responsible for protection either by physically blocking the surface from the corrosion environment or by retarding the electrochemical processes. The main organic functional groups capable of forming chemisorbed bonds with metal surfaces are amino (NH2), carboxyl (COOH), and phosphonate (PO3H2) although other functional groups or atoms can form coordinate bonds with metal surfaces.
  • 8. Mixed Inhibitors 8 Mixed inhibitors are compounds that form a film or precipitate to reduce both the cathodic and anodic reactions. Sodium silicate and phosphates used in domestic water softener salts to prevent rust water are examples. Because of the danger of pitting when using anodic inhibitors alone, it became common practice to incorporate a cathodic inhibitor into formulated performance. It was obtained by a combination of inhibitors than from the sum of the individual performances. This use of combination of inhibitors is generally referred to a “synergism” and demonstrates the synergistic action which exists between zinc and chromate ions.
  • 9. Cathodic Protection 9 If electrons are passed into the metal and reach the metal/electrolyte interface (a cathodic current) the anodic reaction becomes subdued while the cathodic reaction rate increases. This process is called cathodic protection and can only be applied if there is a suitable conducting medium such as earth or water through which a current can flow to the metal to be protected. In most soils or natural waters corrosion of steel is prevented if the potential of the metal surface is lowered by 300 or 400 mV. Cathodic protection is of two types: 1. Sacrificial Anodic Protection 2. Impressed Current Cathodic Protection
  • 10. Sacrificial Anodic Protection 10 Sacrificial anodic protection may be achieved by obtaining electrons from the anodic dissolution of a metal low in the galvanic series such as aluminium, zinc or magnesium. A measure of protection is offered by driving a magnesium rod into the ground near the pipe and providing an electrical connection to the pipe. Since, the magnesium has a standard potential of 2.38 volts compared to 0.41 volts for iron, it can act as an anode of a voltaic cell with the steel pipe acting as the cathode. With damp soil serving as the electrolyte, a small current can flow in the wire connected to the pipe. The magnesium rod will be eventually consumed by the reaction while the steel pipe as the cathode will be protected by the reaction:
  • 11. Impressed Current Cathodic Protection 11 Impressed-current-type cathodic protection systems provide cathodic current from an external power source. A direct current (DC) power source forces current to discharge from expendable anodes through the electrolyte and on to the structure to be protected. Although the current is not generated by the corrosion of a sacrificial metal/alloy, the energized materials used for the auxiliary anodes do corrode. The basic components of an impressed- current- type cathodic protection system are the structure to be protected, a DC power source, a group of auxiliary anodes (ground bed or anode bed), and insulated lead wires connecting the structure to be protected to the negative terminal of the power source and the ground bed to the positive terminal of the power source.
  • 12. Protective Coatings 12 Protective coatings are probably the most widely used products for corrosion control. They are used to provide long-term protection under a broad range of corrosive conditions, extending from atmospheric exposure to the most demanding chemical processing conditions. Protective coatings in themselves provide little or no structural strength, yet they protect other materials to protect their strength. The main function of a protective coating is to isolate structural reactive elements from environmental corrosives. A coating must provide to a substrate a continuous barrier and any imperfection can become the focal point for degradation and corrosion of the substrate. The various types of commonly used protective coatings are: Metallic Coatings Electroplating, Electroless plating, Zinc Coatings, Metal Cladding Inorganic Coatings
  • 13. Metallic Coatings 13 Metallic coatings provide a layer that changes the surface properties of the corroding metal to those of the metal being applied. The corroding metal becomes a composite material exhibiting properties generally not achievable by either material if used alone. The coatings provide a durable, corrosion resistant layer, and the core material provides the load-bearing capability. The deposition of metal coatings, such as chromium, nickel, copper, and cadmium, is usually achieved by wet chemical processes. Alternative metal deposition methods have replaced some of the wet processes and may play a greater role in metal coating in the future. Metallic coatings are deposited by electroplating, electroless plating, spraying, hot dipping, chemical vapour deposition and ion vapour deposition.
  • 14. Electroplating 14 Electroplating is the deposition of a metallic coating on to an object by putting a negative charge on to the object and immersing it into a solution which contains a salt of the metal to be deposited. Electroplating is achieved by passing an electrical current through a solution containing dissolved metal ions and the metal object to be plated. The metal object serves as the cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and non-ferrous metal objects are plated with a variety of metals, including aluminium, tin, bronze, cadmium, copper, chromium, iron, lead, nickel, zinc, as well as precious metals, such as gold, platinum, and silver. Nickel Plating
  • 15. Electroless Plating 15 Electroless nickel (EN) plating is a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy. Due to its excellent corrosion resistance and high hardness, the process finds extensive application on items such as valves, pump parts, etc., to enhance the life of components exposed to harsh conditions of service, particularly in the oil field and marine sector.
  • 16. Electroless Plating 16 The advantages of EN plating are: 1. The deposits in EN plating are uniform, even on complex shapes 2. The deposits are often less porous and hence provide better barrier corrosion protection to steel substrates, much superior to that of electroplated nickel and hard chrome 3. The deposits cause about 1/5th as much hydrogen absorption as electrolytic nickel and about 1/10th as much hard chrome 4. The deposits have natural lubricity unlike electrolytic nickel 5. The deposits have good wettability for oils.
  • 17. Electroless Plating 17 Electroless nickel plating shows excellent corrosion resistance, high- wear resistance and uniformity of coating.. Some of the important applications are: 1. Oil & Gas: Valve components (such as balls, gates, plugs, etc.), pumps, pipe fittings, packers, barrels, etc. 2. Chemical Processing: Heat exchangers, filter units, pump housing and impellers, mixing blades etc. 3. Plastics: Moulds and dies for injecting and low and blow moulding of plastics components, extruders, machine parts rollers, etc. 4. Textile: Printing cylinders, machine parts, spinneret's, threaded guides etc. 5. Automotive: Shock absorbers, heat sinks, gears, cylinders, brake pistons, etc. 6. Aviation and aerospace: Satellite and rocket components, rams pistons, valve components, etc. 7. Food & pharmaceutical: Capsule machinery dies, chocolates moulds, food processing machinery components, etc.
  • 18. Inorganic Coatings 18 Inorganic coatings can be formed by chemical action, with or without electrical assistance. The treatments change the immediate surface layer of metal into a film of metallic oxide or compound, which has better corrosion resistance than the natural oxide film and provides an effective base or key for supplementary protection such as paints. In some instances, these treatments can also be a preliminary step prior to painting. Anodizing: Anodizing is an electrochemical process in which aluminum is the anode. Anodizing involves the electrolytic oxidation of a surface to produce a tightly adherent oxide scale, which is thicker than the naturally occurring film. The electric current passing through an electrolyte converts the metal surface to a durable aluminum oxide. The difference between plating and anodizing is that the oxide coating is integral part of the metal substrate as opposed to being a metallic coating deposition. The oxidized surface is hard and abrasion resistant, and it provides some degree of corrosion resistance.
  • 19. Anodizing Processes 19 Hardcoat anodizing: Hardcoating is a sulphuric acid anodizing process, with the electrolyte concentration, temperature, and electric current parameters changed to produce the hardened surface. A “hardcoat” finish is tough and durable. It is used where abrasion, corrosion resistance and surface hardness are critical factors. Bulk anodizing: Bulk anodizing is an electrochemical process for anodizing small, irregularly shaped parts which are processed in perforated aluminum, plastic or titanium baskets. The large quantity of parts that can be finished in relatively short time makes this technique highly economical. Another advantage in processing such large volumes at one time is the resulting consistency in color and quality. Finishing items such as rivets, ferrules, medical hubs, etc. using the bulk process make production economically feasible.
  • 20. Anodizing Processes 20 Sulphuric acid anodizing: This is the most common method of anodizing. The part to be anodized is subjected to a specified electric current through a sulphuric acid electrolyte, converting the surface to an aluminum oxide coating capable of absorbing dyes in a wide range of colours. Abrasion and/or corrosion resistance is enhanced, and the surface may also be used as a base for applied coatings, such as paint, teflon, and adhesives.
  • 21. Factors Affecting Corrosion 21 Nature of the metal I. Position in the galvanic series: If the metal is higher up in the galvanic series, the oxidation potential is higher which makes the metal to have greater tendency to become anodic and hence the rate of corrosion is higher. So, corrosion rate is directly related to the position of the metal in the galvanic series. II. Purity of metal: The rate of corrosion is higher in case of impure metals in comparison to pure metals. The impurities present in metal cause heterogeneity and thus tiny electrochemical cells are set up at the exposed part of the impurity and corrosion of metal around the impurity takes place due to local action. III. Physical state of the metal: The rate of corrosion is dependent on the physical state of metal. The smaller the grain size of the metal or alloy, the higher will be its rate of corrosion. IV. Nature of the oxide film: The ratio of the volumes of the metal oxide to the metal is called as "specific volume ratio". Greater the specific volume ratio, lesser is the oxidation corrosion rate. V. Relative areas of the anode and cathode: When two different metals or alloys are in contact, the corrosion of the anodic part is directly proportional to the ratio of the cathodic part and the anodic part. When cathodic area is smaller, the demand for electrons will be less and this results in the reduced rate of dissolution of metal at anodic regions. VI. Solubility of corrosion products: In electrochemical corrosion, corrosion proceeds at a faster rate if the corrosion product is soluble in corroding medium, E.g., Pb in H2SO4 medium forms PbSO4 which is insoluble in the corroding medium, thus corrosion proceeds at a slower rate. VII.Volatility of corrosion products: Rapid and uninterrupted corrosion of metal take place if corrosion product is volatile. This is due to the fact that as soon as corrosion product is formed, it volatilizes, thereby leaving the underlying metal surface for further corrosion attack.
  • 22. Factors Affecting Corrosion 22 Nature of the corroding environment I. Effect of pH: Corrosion of that metal which is readily attacked by acids can be reduced by increasing the pH of the attacking environment. II. Humidity of air: The rate and extent of corrosion is directly proportional to humidity. This is due to the fact that moisture acts as a solvent for O2, H2S, SO2, NaCl, etc. to furnish the electrolyte essential for setting up a corrosion cell. So, higher the humidity higher will be rate of corrosion. III. Temperature: The corrosion rate is generally enhanced with increase of temperature of environment due to the increase in diffusion rate. IV. Presence of suspended particles in atmosphere: In case of atmospheric corrosion (a) if the suspended particles are chemically active in nature (like NaCl, (NH4)2SO4), they absorb moisture and act as strong electrolytes, so corrosion rate is enhanced, (b) if the suspended particles are chemically inactive in nature (for example, charcoal), they absorb both sulphur gases, and moisture and slowly enhance corrosion rate. V. Presence of impurities in atmosphere: The rate of corrosion of metals is more in areas near to the industry and sea. This is due to the fact that corrosive gases like H2S, SO2, CO2 and vapours of H2SO4 and HCI in the industrial areas and NaCl of sea water lead to increased conductivity of the liquid layer in contact with the metal surface, thereby increasing the corrosion rate.
  • 23. 23 References The some contents are taken from: Chemistry For Engineers By Harish Chopra Anupama Parmar [In addition, Internet sources have also been used]