unit iv corrosion and protective ... - drmgriyearbtech.com
TRANSCRIPT
UNIT – IV
Corrosion and protective coatings
Corrosion
Corrosion is defined as the gradual destruction or deterioration of metals or
alloys by the chemical or electrochemical reaction with its environment.
Causes of Corrosion
Metals have natural tendency to revert back to their compounds which are more
stable.
Hence they react with the agents in the environment and get converted into metallic
compounds.
The attack by agents begins at the surface and slowly travel inward.
Gases like oxygen, SO2, H2S etc are responsible for chemical corrosion.
Oxygen and moisture are responsible for electrochemical corrosion.
Consequences of Corrosion:
Enormous waste of money and materials.
Loss due to corrosion runs into 2 to 3 billion dollars/ year.
Machinery, pipes breakdown.
Replacement cost increases.
Maintenance cost increases.
Fire hazard due to holes in gas pipes.
Constructions become unusable due to corrosion.
The factors affecting corrosion.
1. Nature of the metals
a. Position in the galvanic series:
The extent of corrosion depends upon the position of the metal in the galvanic
series. Greater the oxidation potential, greater is the rate of corrosion. When two
metals are in electrical contact, the metal higher up in the galvanic series becomes
anodic and suffers corrosion. Further the rate and severity of corrosion depends
upon the difference in their positions in the galvanic series. Greater the difference
faster is the corrosion of anodic metal.
b. Relative anodic and cathodic areas:
Corrosion is more severe and localized if the anodic area is smaller and the
cathodic is larger. The reason is that the high demand for electrons by the larger
cathodic area can be met by the smaller anodic area only by undergoing rapid and
severe corrosion.
c. Over Voltage:
The difference between the potential of the electrode when gas evolution is
actually observed and the theoretical value of the reversible electrode potential
for the same electrode is called the over voltage. The reduction in over voltage at
the cathode will accelerate the rate of corrosion of the anode metal.
d. Purity of the metal :
Impurities present in the pure metal causes heterogeneity and form tiny
electrochemical cells at the exposed metallic surface in presence of a conducting
medium. The metal having higher oxidation potential will act as an anode which
dissolves in the medium and corrosion occurs. For example impure Zinc
undergoes corrosion when it is in contact with lead or iron. Therefore corrosion
resistance of a metal depends on its purity.
e. Nature of the corrosion product:
The extent of corrosion depend on the nature of the corrosion products, such
as its stability and solubility. If the corrosion product is insoluble and forms a
film on the surface the corrosion may not proceed further (Pb in storage battery).
If the corrosion product is soluble in medium, corrosion proceeds vigorously.
Nature of the Environment:
1. Temperature:
Rise in temperature of the environment increases the corrosion reaction, due to increase in conduction of the medium and decrease in over voltage.
2. Humidity:
Atmospheric corrosion of iron is slow in dry air but increases rapidly in the
presence of moisture. This is due to the fact that moisture acts as a solvent for the
oxygen in the air to furnish the electrolyte essential for setting up a corrosion cell.
Rusting of iron increases when the relative humidity of air reaches from 60 to 80 %.
3. Impurities:
Polluted atmosphere usually in industrial belts, containing impurities such as
CO2,SO2,H2, fumes of HCl, H2SO4etc, especially in a humid atmosphere, enhance the rate of corrosion.
4. Effect of pH:
Generallyacidic environment is more harmful than alkaline or neutral environment.Therefore corrosion rate of metals by acidic surrounding can beminimized by increasing the PH of the solution.
CLASSIFICATION (OR) THEORIES OF CORROSION Based on the environment, corrosion is classified into
(i) Dry or Chemical corrosion, and (ii) Wet or Electrochemical corrosion
DRY OR CHEMICAL CORROSION Dry corrosion is due to the attack of metal surfaces by the atmospheric gases such as
oxygen, hydrogen sulphide, sulphur dioxide, nitrogen, etc. There are 3 main types of dry corrosion.
1. Oxidation corrosion (or) corrosion by oxygen.
2. Corrosion by hydrogen.
3. Liquid-metal corrosion. Oxidation Corrosion (or) Corrosion by Oxygen
Oxidation corrosion is brought about by the direct attack of oxygen at low or high
temperatures on metal surface in the absence of moisture. Alkali metals (Li, Na, K, etc.)
and alkaline-earth metals (Mg, Ca, Sn, etc.) are rapidly oxidised at low temperature. At
high temperature, almost all metals (except, Ag, Au and Pt) are oxidised.
Mechanism of Dry Corrosion (i) Oxidation occurs first at the surface of the metal resulting in the formation of metal
ions
(M2+), which occurs at the metal / oxide interface.
M −−−−−> M2+ + 2e-
(ii) Oxygen changes to ionic form (O2-) due to the transfer of electron frommetal,which
occurs at the oxides film / environment interface
½ O2 + 2e- −−−−−> O2-
(iii) Oxide ions reacts with the metal ionto form the metal-oxide film.
M + ½ O2 −−−−−> M2+ + O2- ≡ MO
(Metal-oxide film)
Once the metal surface is converted to a monolayer of metal-oxide, for further
corrosion (oxidation) to occur, the metal ion diffuses outward through the metal-oxide
barrier. Thus the growth of oxide film commences perpendicular to the metal surface.
Nature of Oxide Film The nature of oxide film formed on the metal surface plays an important role in oxidation corrosion. (i) Stable Oxide Layer
A stable oxide layer is a fine-grained in structure, and gets adsorbed tightly to the
metal surface. Such a layer is impervious in nature and stops further oxygen attack through
diffusion. Such a film behaves as a protective coating and no further corrosion can develop. Example:
Oxides of Al, Sn, Pb, Cu, etc., are stable oxide layers. (ii).Unstable Oxide Layer
Unstable oxide layer is mainly produced on the surface of noble metals, which
decomposes back into the metal and oxygen.
Metal Oxide Metal + Oxygen Example:
Oxides of Pt, Ag, etc., are unstable oxide layers. (iii) Volatile Oxide Layer
The oxide layer volatilizes as soon as it is formed, leaving the metal surface for
further corrosion.
Example: Molybdenum oxide (MoO3) is volatile.
(iv) Protective (or) Non-Protective oxide film (Pilling- Bedworth rule)
(a) According to Pilling-Bedworth rule, if the volume of the oxide layer formed is less
than the volume of the metal, the oxide layer is porous and non-protective. Example: The volume of oxides of alkali and alkaline earth metals such as Na, Mg, Ca,
etc., is less thanthe volume of the metal consumed. Hence the oxide layer formed is porous
and non-protective.
(b) On the other hand, if the volume of the oxide layer formed is greater than the volume
of the metal, the oxide layer is non-porous and protective. Example :The volume of oxides of heavy metals such as Pb, Sn, etc., is greater than the
volumeof the metal. Hence the oxide layer formed is non-porous and protective. Pilling-Bedworth Ratio
The ratio of the volume of the oxide formed to the volume of the metal consumed is called
“Pilling-Bedworth ratio”
PBR
If O is greater than M, then O/M is high. This is protective or non-
porous layer If M is greater than O, then O/M is low. This is non-
protective or porous layer Corrosion by Hydrogen (a) Hydrogen embrittlement(at ordinary temperature) When metals contact to H2S at ordinary temperature causes evolution of atomic
hydrogen. Fe + H2S −−−−−>FeS + 2H
This atomic hydrogen diffuses readily into the metal and collects in the voids, where
it recombines to form molecular hydrogen.
H + H −−−−−> H2↑
Collection of these hydrogen gases in the voids develop very high pressure, which
causes cracks and blisters on metal. Thus, the process of formation of cracks and blisters
on the metal surface, due to high pressure of hydrogen gas is called hydrogen
embrittlement. (b) Decarburisation(at Higher Temperature)
At higher temperature atomic hydrogen is formed by the thermal dissociation of molecular hydrogen.
H 2 2 H
when steel is exposed to this environment, the atomic hydrogen readily combines with carbon of steel and produces methane gas.
C + 4H −−−−−> CH4↑ Collection of these gases in the voids develop very high pressure, which causes cracking.
Thus the process of decrease in carbon content in steel is termed as “decarburisation” of
steel
Liquid - Metal Corrosion
This is due to the chemical action of flowing liquid metal at high temperature. The
corrosion reaction involves
(i) either dissolution of a solid metal by a liquid metal. (or)
(ii) liquid metal may penetrate into the solid metal.
Mechanism of electrochemical corrosion
Wet or electrochemical corrosion is the more common type of corrosion.
This type of corrosion occurs when a conducting solution is in contact with a metal (or) when two dissimilar metals in contact with each other and also in contact with a conducting solution. Since this type of corrosion can occur only in presence of a liquid it is called as wet corrosion. A number of tiny electrochemical cells are set up on the corroding metal and this is responsible for corrosion. Hence this type of corrosion is called as electrochemical corrosion. Mechanism
Due to setting up of small electrochemical cells, the metal surface is
separated into cathodic and anodic areas. Anodic reaction: The anodic reaction involves the dissolution of the metal to produce metallic ions with the liberation of electrons.
𝐹𝑒 → 𝐹𝑒2+ + 2𝑒− The electrons flow through the metal to reach the cathodic area. Cathodic reaction: These electrons can be given to oxygen atoms if the metal is in contact with atmospheric oxygen and water to form hydroxide ions.
𝑂2 + 𝐻2𝑂 + 2𝑒− → 2𝑂𝐻−
The 𝐹𝑒2+ ions produced at the anode and the 𝑂𝐻− ions produced at the cathode diffuse towardseach other and when they meet ferrous hydroxide is precipitated.
𝐹𝑒2+ + 2𝑂𝐻− → 𝐹𝑒(𝑂𝐻)2
If enough oxygen is present, the ferrous hydroxide is converted to yellow rust.
2𝐹𝑒(𝑂𝐻)2 + 12⁄ 𝑂2 → 𝐹𝑒2𝑂3. 2𝐻2𝑂
In limited supply of oxygen black rust is formed
2𝐹𝑒(𝑂𝐻)2 + 12⁄ 𝑂2 + 𝐻2𝑂 → 𝐹𝑒2𝑂3. 3𝐻2𝑂
The corrosion products are found in between the cathode and the anode. It is important to note that the loss of metal takes place only at the anodic area and the cathodic area metal is not corroded.
Corrosion control
1. Sacrificial anode protection method and 2. Impressed current cathodic protection.
(i) Sacrificial anode protection method
In this method, the metallic structure which is to be protected is connected
to a more anodic metal (a metal having higher oxidation potential) through a conducting wire. Under thiscondition, corrosion occurs only in the more anodic metal and then it will protect the metallic structure sacrificially. Railway lines, ship hulls, steel structures of offshore petroleum wells, underground pipe lines etc., are protected from corrosion by the use of Mg or Zn as the sacrificial anodes.
Applications of Sacrificial anodic protection
1. This method is used for the protection of ships and boats. Sheets of Mg and
Zn are hung around the hull of the ship. Zn or Mg will act as anode
compared to iron (ship or boat is made of iron), so corrosion concentrates
on Zn or Mg. Since they are sacrificed in the process of saving iron, they are
called sacrificial anodes. 2. Protection of underground pipelines, cables from soil corrosion. 3. Insertions of Mg sheets into the domestic water boilers to prevent the
formation of rust. 4. Calcium metal is employed to minimize engine corrosion.
Impressed current cathodic protection method In this method, an impressed current is applied in the opposite direction
of the corrosion current to nullify it, and thus we prevent the formation of anodic sites in the metallic structures which are responsible for corrosion.
This can be done by connecting negative terminal of the battery to the
metallic structure to be protected and positive terminal of the battery is
connected to an inert anode. Inert anodes used for this purpose are graphite,
platinised titanium. The anode is buried in a “back fill” (composed of gypsm). The
“back fill” provides good electrical contact to anode. Since in this method current
from an external source is impressed on the system, this is called as impressed
current method.
Applications of impressed current protection
Structures like tanks, pipelines, transmission line towers, underground water pipelines, oil pipelines, etc., can be protected by this method.
Corrosion Inhibitor
Corrosion Inhibitor is a substance which when added in small quantities to the
corrosive aqueous environment, decreases the corrosion of the metal.
Inhibitors are of two types. They are Anodic inhibitors and Cathodic Inhibitors
Anodic inhibitors:
The inhibitors such as chromates, Phosphates, Tungstates or ions of transition
element with high oxygen content suppress the corrosion reaction, occurring at
the anode, by forming a sparingly soluble compound with the metal ion. They
form a protective film or barrier on the metal surface by adsorbtion, thereby
reducing the corrosion rate.
This type of control is effective but if certain areas are left unprotected, sever
local attack can occur.
Cathodic Inhibitors:
a) In acidic solutions, the evolution of Hydrogen takes place as the cathodic
reaction.
2𝐻+ (𝑎𝑞) + 2𝑒− → 𝐻2(𝑔)
Corrosion may be reduced either by slowing down the diffusion of H+ ion or by
increasing the overvoltage of hydrogen evolution. Organic inhibitors like amines,
mercaptans, ureas and thioureas decrease the diffusion of H+ ion by getting
adsorbed at the metal surface. Antimony and arsenic oxides increase the
hydrogen overvoltage by depositing an adherent film of metallic arsenic or
antimony at the cathode areas
b) In neutral solutions, the following cathodic reaction takes place
𝐻2 𝑂 (𝑒) + 1
2 𝑂2 + 2𝑒− ↔ 2𝑂𝐻− (𝑎𝑞)
The corrosion may be controlled by eliminating oxygen either by adding reducing
agents like sodium sulphite or by dearation. It may also be controlled by
retarding the diffusion of oxygen into the cathodic area by the addition of
inhibitors like Mg, Zn, Ni salts.These reacts with the hydroxyl ions at the cathode
forming corresponding insoluble hydroxides, which are deposited on the cathode
forming impermeable self- barriers.
Protective coatings
Metallic structures and machine parts undergo corrosion and therebylose their
strength. In order to decrease the corrosion and protect the base metals
protective coatings are applied on the surface of the metal. The protective
coatings act as a physical barrier between the metal and the atmosphere and thus
prevent corrosion. In addition these coatings also possess a decorative value.
Types of protective coatings:
Protective coatings are of the following three types:
a) Metallic coatings
b) Chemical conversion coatings
c) Organic coatings viz paints and varnishes.
Metallic coating:
Corrosion of the base metal can be prevented by either of the two types of
coatings given below.
a) Anodic coating
b) Cathodic coating
Anodic coating:
If metals anodic to the base metal are coated, it is called as anodic coating.
The metals which have lower electrode potential than the base metals are
employed for providing an anodic coating. For example, metals such as Zn, Al
and Cd have lower electrode potential than iron and hence can be coated to
protect steel.
If Zn metal is coated on steel, Zn becomes the anode because of its lower
electrode potential and steel remains as the cathode. Due to corrosive attack by
the agents in the atmosphere(O2,H2O), zinc starts dissolving, whereas iron is
protected.
Thus zinc coating sacrifices itself to protect steel.
Cathodic coating:
In this method, the base material is protected by coating with a metal
which has higher electrode potential. This means that the coating metal is more
corrosion resistant than the base metal.
Example: coating of tin on iron sheets protects iron from corrosion.
But, the tin coating on iron provides protection as long as the surface of
iron is completely covered with the tin coating. Even a small puncture in the tin
coating exposing the iron sets up an electrochemical cell in which iron is the
anode and tin is the cathode. This results in severe corrosion of the exposed iron
part leading to pitting.
Differences between anodic and cathodic coatings
S.no Anodic coating Cathodic coating
1 It protects the base by sacrificing itself It protects the base metal by being more corrosion resistant
2 Electrode potential of coating metal is lower than that of the base metal.
Electrode potential of the coating metal is higher than that of the base metal
3 Small punctures in the coating does not severely affect the base metal
Even small punctures in the coating results in severe corrosion of the base metal leading to pitting
4 Example: coating Zn on iron Example: coating of tin on iron
COATING PROCESS
The important methods employed in metal coating are
1. Hot dipping
2. Electroplating
3. Electroless plating
4. Metal spraying
5. Metal cladding
6. Cementation
Hot dipping:
Metals with low melting points can be coated on iron or steel by this
method. The base metal sheets(iron or steel) is dipped into a bath of molten Zn or
Sn.
a) Galvanizing:
Coating of zinc or iron or steel is called galvanizing. The steel article(sheet,
pipe or wire) is first cleaned by dipping it in dil.H2SO4 for 15 to 20 mins at
60 to 90oC.This process is called pickling and is done in order to remove
any rust, scale or other impurities. The article is then washed in water and
then dried. It is then dipped in molten zinc at 425-435oC. The surface of
molten zinc is covered by ammonium chloride(flux) to prevent oxidation of
zinc. The zinc coated article is passed through rollers to remove excess zinc
and to give a coating of uniform thickness. Then it is annealed by heating it
to650oC and gradually cooling it.
Uses:Galvanization protects iron sacrificially and also gives it a good
appearance.
Disadvantages: zinc dissolves in dilute oxide to give toxic compounds.
Hence, galvanized cans cannot be used for storing food items.
b) Tinning:
Coating tin over steel sheet is called tinning. The steel sheet is first cleaned
using dilute H2SO4(pickling) and is passed through a bath of zinc
chloride(flux). The role of flux is to make tin adhere to steel strongly.
Then the sheet is passed through two chambers containing molten
tin. The tin coated steel sheet is then passed through a chamber containing
hot palm oil. The palm oil forms a thin layer over tin and protects it from
oxidation. Then the sheet passes through hot rollers to remove excess tin.
Fig. Tinning of Sheet Steel
Uses: Due to non toxic nature of tin the tin coated steel is used for making
contains for storing food materials.
Electro plating:
A easily corroded base metal is protected by a coating of a metal resistant
to corrosion by the passage of electricity.
The base metal is made as the cathode and the coating metal is made as the
anode. The electrolyte used is a salt of the coating metal dissolved in water. When
electricity is applied the coating metal gives a thin coating on the article to be
coated.
Theory of electroplating:
Let us consider copper plating to illustrate the theory behind
electroplating. Copper sulphate solution is taken in a electrolytic cell. Copper is
made as the anode. The steel article which is to be coated with copper forms the
cathode.
Copper sulphate ionizes as follows
CuSO4 ↔Cu2+ + SO42-
When current is passed cn2+ ions moves towards the cathode and gets
deposited on the cathode.
Cu2+ +2e- Cu
The SO4 2-ions moves towards the anode and combines with Cu2+ ions
which are produced by dissolution of copper anode. Thus, the Cu2+ lost from the
electrolyte is replenished.
Cu + SO4 2- CuSO4 + 2e-(at anode)
Thus the copper anode dissolves away and has to be changed.
Instead of copper anode an inert electrode like graphite can be used. But,
CuSO4 has to be continuously added to make up for the loss of Cu2+ ions lost in
making the coating.
Electroplating of nickel:
Nickel plating provides a less porous, corrosion resistant and wear
resistant surface .
Anode: nickel (or inert electrode)
Cathode: article
Temp: 40-70oC
Current: 20-30mA/cm2
Electroplating of chromium:
It gives a shiny metallic coat to the base material. Since, chromium plating
is porous, the article is given an undercoat of nickel before chromium plating.
Anode: graphite electrode
Medium: acidic
Cathode: article to be coated
Plating bath: H2CrO4 and H2SO4 in 100:1 proportion by volume.
Current: 100-200mA/cm2
Cleaning of articles before electroplating:
The surface of the articles must be cleaned thoroughly to obtain uniform
and adherent coating. Solvent cleaning of the surface is done by organic solvents
to remove oil, grease etc. this is followed by cleaning with steam or with alkali
solution. Acid cleaning is used to remove oxides and other metals on the surface.
For better results sand blasting method is used.
Factors affecting electroplating:
1) Low electrolytic concentration gives better coatings.
2) Thin deposits are more adherent than thick ones.
3) Minimum current density should be used.
4) pH should be maintained at the correct level throughout.
5) Mild stirring is helpful.
Electroless plating:
A noble metal is uniformly coated on a base metal(like iron, steel) without
the use of electricity. The salt solution of the noble metal is reduced using a
reducing agent.
Noble metal ions + reducing agent metal + oxidized products
The metals thus produced gives a uniform coating on the article placed
inside the solution. The surface of the article (base metal) must be catalytically
activated for such coating to be made.
Electro less nickel plating:
1) The surface to be coated is degreased by using organic solvents and alkali.
2) This is followed by acid treatment to remove oxide deposits.
3) Metals and alloys of Al, Cu, Fe, brass etc can be directly nickel coated
without any activation.
4) Stainless steel surface should be activated by dipping it in a hot solution of
50% H2SO4.
5) Activation of non-metallic articles (like glass, plastics, quartz etc) is carried
out first by dipping in SnCl2 solution containing HCl. This is followed by
dipping in palladium chloride solution. On drying a thin active layer of Pd
is formed on the surface.
Method:
A plating bath solution of the following composition is made.
a) NiCl2 solution 20g/l
b) Buffer (sodacetate)- 10g/l
c) Complexing agent cumexaltant- sodium succinate
d) Optimum pH- 4.5
e) Optimum temp- 93oC
The article to be coated is placed in this path and the reducing agent
sodium hypophospite(20g/l) is added and stirred.
The complete cell reaction is
Ni 2+ + 2e- Ni
HPO2- + H2O HPO3- + 2H+ + 2e-
By adding the above equations we get,
Ni 2+ + 2e-+ HPO2- + H2O Ni+ HPO3- + 2H+ + 2e-
The Ni produced ion the reaction gets coated on the article uniformly.
Advantages:
1. Even article of intricate shapes can be coated uniformly.
2. Nickel deposits are non porous and better than electro plated nickel
coating.
3. Gives harder surface and better wearability.
Copper coating on printed circuit board:
PCB’s are made out of epoxy or phenolic polymers.
These phenolic sheets are activated by first dipping in SnCl2 containing
HCl followed by dipping in palladium chloride solution and drying, the surface is
found to have a thin layer of Pd.
The bath solution for copper plating consists of
1. Copper sulphate 12g/l
2. Buffer – NaOH + Rochelle salt
3. Complexing agent cum exaltantEDTA (20 g/l)
4. Optimum pH – 11.0
5. Temperature – 25oC
6. Reducing agent - Formaldehyde
Reduction occurs and Cu metal is produced.
Cu 2+ + 2e- Cu
2 HCHO + 4 OH- 2HCOO- + 2 H2O + H2 + 2e-
By adding the above equations we get,
Cu 2+ + 2 HCHO + 4 OH- Cu + 2HCOO- + 2 H2O + H2
The PCB is electroplated with copper. Then, the selected areas are
protected by employing electroplated image(or photoresist). Then the reminder
of Cu coating is etched away so as to get the required type of circuit pattern (or
track). Finally, the connection of two sides of PCB is made by drilling holes
followed by electro less copper plating through holes.
Metal spraying:
In this process, the coating metal in the molten condition is sprayed on the
surface of the article to be coated. The coated metal adheres to the metallic
surface.
The sprayed coatings are continuous but porous. Hence, a sealer oil or
paint is applied on such a coating to provide a smooth surface.
Advantages of the method:
1. Greater speed of working
2. Applicability to large surfaces
3. Ease of application
Method used:
Wire gun method:
A wire of the coating metal is melted by oxy-acetylene flame and atomized
and sprayed by a blast of compressed gas.
Powder metal method:
A finely powdered coating metal is sucked from the powder chamber. It is
heated and melted when it passes through the flame of the blow pipe. The blow
pipe breaks the metal into a cloud of molten globules which are sprayed over the
article. This is absorbed by the surface of the article and a fine coating is
obtained. This method can be employed for low melting metals like Zn, Pb, Sn
etc.
Metal cladding:
In this process a base metal sheet like steel is covered on one side or on
both sides by a thin sheet of noble metal. The sandwiched base metal is passed
through sheet rollers under heat and pressure. The sheets bond with each other
and a corrosion resistant metal sheet is obtained.
Base metals thus protected are mild steel, Al, Cu, Ni and their alloys. Cladding
metals – corrosion resistant metals like Ni, Cu, Pb, Ag& Pt.
Use:In aircraft and automobile industry.
Example: Al clad – in this duralium is sandwiched between two layers of 99.5%
pure aluminum.
Cementation or diffusion coating:
A rotating drum is filled with articles to be coated and the powder of the
coating metal. The drum revolves and is continuously heated to a suitable
temperature.
The coating metal diffuses into the surface of the base metal and forms
layers of alloys of varying composition. The layer adjacent to the base metal may
be an intermediate compound or solid solution. The outer layers are rich in
coating metal. The coating thickness is controlled by varying time of treatment
and temperature. This process is suitable for coating small articles( like bolts,
screws and valves). The coating metals used are those which can alloy with iron
(like Zn, Cr & Al).
Sherardizing- (developed by sherard) is a cementation process in which Zn
powder used as the coating metal.
Colorizing:The steel articles are sand blasted and tightly packed in a drum with
Al powder, Al oxide & NH4Cl flux. A reducing atmosphere is maintained using
hydrogen gas. The drum is made to revolve while heating it. This method is
mainly used for protection of furnace parts.
Chromizing:It is carried out by heating metal articles with a mixture of 55%
chromium powder& 45% of alumina to 1300-1400oC for 3 to 4 hours. This is
mainly used for protection of gas turbine blades.
CHEMICAL CONVERSION COATINGS
By chemical or electro chemical reactions the surface of the base metal is
converted into its inorganic compounds which act as barrier for corrosion. These
surface coating act as a good base for paints, lacquers and oils.
Important conversion coatings are
1) Phosphate coating
2) Chromate coating
3) Chemical oxide coating
4) Anodized coating
Phosphate coating:
This is done by immersing the base metal or articles into a bath of mixture
of phosphate and phosphoric acid. It can also be applied by spraying or brushing.
A chemical reaction occurs at the surface and a corrosion resistant coating is
crated at the surface.
The phosphate used are iron, manganese and zinc along with
accelerators(like copper salts). As these reactions are slow, accelerators are used.
The surface film consist of zinc-iron or manganese iron-phosphate.
Phosphate coatings are porous and do not offer complete protection
against corrosion. However they serve as excellent primer coat for applying
paints, lacquers and oils.
Chromate coating:
It is done by dipping the article in a bath of acidic potassium chromate
followed by its immersion in neutral chromate solution. The protective chromate
film formed on the surface is due to the presence of trivalent and hexavalent
chromates.
The chromate coatings are non porous and more corrosion resistant than
phosphate coating. Chromate coatings are mainly used in the protection of zinc,
cadmium plated parts, aluminium and magnesium.
Chemical oxide coating:
This is made by treating base metal with alkaline oxidizing solution or gas. This
treatment increases the thickness of the original oxide layer on the surface and
thereby increasing the protection.
Oxide coating form a good primer base for paints, lacquers and oils.steel
products are obtained in a range of colors yellow to light blue by heating steel in
air at 200-450oC.
Anodized coating:
Anodized coatings are generally produced on non-ferrous metals like Al,
Zn, Mg and their alloys by anodic oxidation process.
Anodized aluminium:
The aluminium article is made the anode in a cell containing a bath
consisting of sulphuric, chromic, oxalic or phosphoric acids. DC is passed at
moderate current densities at 35-40oC. Progressive oxidation occurs at the
surface of the anode and a thick layer of the oxide coating forms on the surface.
These films are further sealed by exposing them to boiling water. Anodized
coatings are more resistant to corrosion.
ORGANIC COATINGS
Organic materials which can form a protective film on a metallic surface is
called an organic coating.
Paints, varnishes, lacquers and enamels are organic coatings used both for
protection of metals as well as for decoration.
Paints:
A paint is a dispersion of one or more pigments in a liquid medium which
consists of a vehicle and thinner. The ‘vehicle’ is a liquid consisting of a non-
volatile, film forming material. The ‘thinner’ is a volatile solvent.
When a paint is applied to a metal surface the thinner evaporate while the
vehicle(drying oil) slowly oxidizes and polymerizes forming a dry pigmented film.
Requisites of a good paint:
1) Good spreadability or applicability
2) Should stick to the surface strongly
3) Should possess high covering power
4) Should not crack on drying
5) Should posses a stable color
6) Should be corrosion and water resistant
7) Should give a glossy film
Constituents of paints and their functions:
Pigments: pigments are solid colored substances and the most essential part of
a paint.
Functions:
1) Gives opacity to the film
2) Gives color to the paint
3) Protects the metal surface from the effects of weather
4) Protects the film from UV radiations which can crack the film
Examples:
Black pigment: lamp black, carbon black
White pigment: white lead(2PbCO3.Pb(OH)2)
Red pigment: Indian red Fe2O3
Green pigment: chromium oxide
Blue pigment: Prussian blue
Vehicles or drying oils:This is the film forming liquid in the paint. These are
glyceryl esters of high moleculer weight fatty acids. That is, they are vegetable or
animal oils.
Functions:
1) They form adherent and protective film on the surface by oxidation and
polymerization.
2) They impact water repellency, toughness and durability to the film.
Eg: linseed oil, soyabean oil, dehydrated castor oil.
The oil after application on a surface absorbs O2 at the double bonds. This results
in the formation of peroxides which polymerizes to form a tough, cross linked
film.
Thinners:Thinners are solvents which are very volatile. They make the paint
less viscous so that the paint can easily be applied on the surface. After
application the thinner dries out quickly leaving behind the film.
Functions:
1) Dissolve the additives added to the paint.
2) Increase the penetration power of the vehicle.
3) Increase the elasticity to the film.
Eg: Turpentine, mineral spirits from petroleum.
Extenders or fillers:These are white or colorless pigment which forms the
bulk of the paint.
Functions:
1) Reduces the cost of paint
2) Prevents shrinkage of paint
Eg: Talc, china clay, gypsum, silica etc
Driers: These are used to accelerate the process of drying.
Functions: They act as oxygen carriers or catalyst.
Eg: metallic soaps of Co, Mn, Pb
Plasticizers: Added to paints to provide elasticity to the film and to prevent
cracking of the film called plasticizers.
Eg: triphenyl phosphate, tricresyl phosphate etc
Antiskinning agents: These are chemicals added to prevent skinning of the
paint.
Eg: polyhydroxy phenol
Failure of paints:
1) Chalking: It is the gradual powdering of the applied paint. This is due to
improper dispersion of the pigment in the vehicle.
2) Cracking: Paint film can crack due to unequal expansion and contraction
and also due to action of UV radiation. This is avoided by giving a hard
primer coat.
3) Color change:The color of the paint changes due to action of light and
atmospheric action.