UNIT III: SCIENCE OF CORROSION AND ITS CONTROL
IntroductionThe term corrosion is used to denote a change. A metal changes from its elementary state to the combined state, more or less rapidly, when it comes into contact with the gaseous/liquid medium. This is actually owing to the chemical interaction between the metal and the environment.
Definition
Corrosion* “The spontaneous destruction and consequent loss of a metal/alloy due to unavoidable chemical/electrochemical attack by the environment”
Example:
1. When Cu is exposed to the industrial environment it forms an adherent protective green deposit which isolates the metal from the environment, hence the further action is very slow. 2. When iron metal is exposed to the industrial environment, the metal forms a loosely adherent
product of hydrated ferric oxide called rust, which is relatively non-protective.
Hence, the fundamental approach to the phenomena of corrosion, the structural features of the metal, reactions which occur at the interface and nature of the environment are to be considered. Electrochemical theory of corrosion
Most of the corrosion cases are electrochemical in nature, taking place by an electrochemical attack on the metal in the presence of moisture/conducting medium- called wet corrosion.
According to the theory, when a metal is in contact with the conducting medium or when dissimilar metals/alloys are either immersed partially/completely in a solution, a large number of galvanic cells with the existence of anodic and cathodic area on the metal, are formed.
In this corrosion, oxidation of the metal and reduction of species present in solution takes place at anodic and cathodic parts, respectively.
The electrons are transferred through the metal from anode to cathode.
The anodic part of the metal suffers from corrosion and cathode is protected from corrosion.
Corrosion reactions are:
At anode (oxidation reaction): M →Mn+ + ne
-The reaction at cathode (reduction reaction) depends on the nature of the environment: If the medium is acidic,
(a) In the presence of dissolved oxygen : 2H+ + ½O
2 + 2e-→H2O (b) In the absence of dissolved oxygen: 2H+ + 2e-→ H
2 If the medium is alkaline/neutral,
(a) In the presence of dissolved oxygen : H2O+½ O2 + 2e-→2 (b) In the absence of dissolved oxygen : 2H2O+2e-→2 + H2 Example: Rusting of an Iron in the presence of moist air
At anode: Fe→Fe+2 + 2e
-At cathode: H2O+½ O2 + 2e-→2 Net reaction: Fe+2 +2 →Fe (OH)
2
In the presence of excess of oxygen: 2Fe (OH) 2+ ½ O2→Fe2O3.2H2O (rust) In the limited supply of oxygen: 3Fe (OH) 2+½ O2→Fe3O4.3H2O
Types of corrosion
Differential metal Differential aeration Stress corrosion
corrosion corrosion
Ex: Galvanic corrosion Ex: Pitting corrosion Ex: caustic embrittlement Waterline corrosion
Differential metal corrosion
When two dissimilar metals are in direct contact with one another and exposed to a corrosive conducting medium, the metal higher up in the electrochemical series behaves as anode and suffers from corrosion, whereas the metal lower in the electrochemical series becomes cathode and protected from corrosion. This type of corrosion is also known as Galvanic corrosion.
If the potential difference between the electrodes is high, greater the extent of corrosion.
Oxidation /reduction takes place at anode/cathode respectively.
The reduction at cathode depends on the nature of the corrosive environment. In acidic medium, corrosion occurs by hydrogen evolution; while in alkaline/neutral solution, oxygen absorption takes place.
When Zn and Cu metals are electrically connected and exposed to an electrolyte, Zn (higher in electrochemical series) forms anode and suffers from corrosion whereas Cu (lower in electrochemical series) forms cathode and protected from corrosion.
Differential aeration corrosion
This type of corrosion is due to the formation of differential aeration cell or oxygen concentration cell.
When a metal surface is exposed to differential air or oxygen concentrations- forms differential aeration cell.
The more oxygenated part of the metal behaves as cathode and less oxygenated part becomes cathode.
Differential aeration of metal causes a flow of current called the differential current and the corrosion is called differential aeration corrosion. Example (a): Rusting of an iron.
Example (b): Consider a piece of Zn metal is partially immersed in a dilute solution of neutral salt (NaCl), and the solution is not agitated properly. The part of the metal above and closely adjacent to the water-line are more oxygenated, because of easy access of oxygen and hence become cathodic. The part of the Zn metal immersed to greater depth, which have less access of oxygen and becomes anode. Hence a difference in potential between the electrodes is created, which causes a flow of current between the two differentially aerated areas of the same metal and causes corrosion at anode.
Differential aeration accounts for the corrosion of metals partially immersed in a solution, just below the water line. This type of differential aeration corrosion is also known as water line corrosion.
Consider a steel tank containing water. The maximum corrosion takes place along a line just beneath the level of water meniscus. The area above the waterline is highly oxygenated and acts as the cathodic and completely unaffected by corrosion. (Eg. Marine plants attacking themselves in the sides).
Poor oxygenated more oxygenated
Area (anode) area (cathode)
Pitting corrosion
Pitting is very destructive and frequently ruins the tubes, pipes etc.
Pitting is due to breakdown or cracking of the protective film on a metal at specific points. The presence of impurities like sand, dust, scale, etc., on the surface of metal leads to pitting.
Pitting corrosion is due to the formation of differential aeration cell.
This attack becomes more intensified with time.
Grain boundary:
This type of corrosion occurs along grain boundaries and only where the material especially sensitive to corrosive attack exists and corrosive liquid possesses a selective character of attacking only at the grain boundaries. This type of corrosion is due to the fact that the grain boundaries contain material which shows electrode potential is more anodic than that of the grain centre in the particular corroding medium. This may be due to the precipitation of certain compounds at the grain boundaries, thereby leaving the solid metal solution impoverished in one constituent. For example during welding of stainless steel consists 18% Cr, 8% Ni and 0.1% C , Chromium carbide is precipitated at the grain boundaries, thereby region just adjacent to grain boundaries become depleted in chromium composition and is more anodic with respect to the solid solution within the grain ( which is richer in chromium). For the same reason, it is also anodic to the particles of the chromium carbide so-precipitated.
Boiler corrosion
The process of degradation of the boiler surface by the attack of boiler feed water is called as boiler corrosion. The dissolved gases like oxygen and carbon dioxide present in the boiler feed water cause boiler corrosion.
Reactions causing boiler corrosion Corrosion due to dissolved oxygen.
Natural water usually contains 8-9 ppm of dissolved oxygen. Generally oxygen levels of more than 7 ppm present in the boiler feed water cause boiler corrosion. The dissolved oxygen present in boiler feed water can attack boiler surface and produce rust as follows.
2Fe + 2H2O + O2 → 2Fe (OH) 2
4Fe (OH) 2 + O2 + 2H2O → 2 [Fe2O3. 3H2O] Rust
The above reaction indicates the corrosion in boilers due to DO. When water containing DO is heated, it liberates oxygen which in turn causes corrosion.
Corrosion due to Carbon dioxide
Carbon dioxide is present naturally in the water and also come from residual temporary hardness. Carbon dioxide dissolves in water forming carbonic acid (H2CO3), which is slightly acidic and it causes intense local corrosion called “pitting corrosion” and hence its elimination is necessary to prevent boiler corrosion.
Ca (HCO3)2 CaCO3↓ + CO2+ H2O
Mg (HCO3)2 Mg (OH) 2↓ + CO2 H2O + CO2 H2CO3
Corrosion due to MgCl2 (Acid corrosion)
This type of corrosion mainly due to the formation of acids when salts like, MgCl2, CaCl2 are dissolved in water. These salts, particularly MgCl2 undergo hydrolysis at high temperature (200°C) forming HCl in boiler, which leads to corrosion as mentioned in the below reactions;
MgCl2 + 2H2O → Mg (OH) 2 + 2HCl
The HCl liberated, attacks iron and forms rust as follows;
Fe + 2HCl → FeCl2 + H2
4 Fe (OH) 2 + O2 + 2H2O → 2 [Fe2O3. 3H2O], Rust
Controlling of boiler corrosion
Boiler corrosion can be controlled by reducing the quantities of oxygen, carbon dioxide and any acid from boiler feed water. This can be done by following methods.
Methods for the removal of DO
Dissolved oxygen is eliminated by any of the following methods;
Using mechanical deaerators: The solubility of oxygen directly proportional to pressure and inversely to temperature and this principle is used in mechanical deaerators. In this, hot water is sprayed to remove DO.
Use of sodium sulfite (Na2SO3): The complete removal of DO is achieved by adding Na2SO3, which reacts with DO forming sulphates;
Na2SO3 ½ O2 → Na2SO4
Use of Hydrazine: Hydrazine (N2H4) is used at 40% concentration in aqueous form due to its explosive nature.
N2H4 + ½ O2 → N2 + H2O
One of the advantage of using hydrazine is, it does not form any salt.
Methods for the removal of CO2
Mechanical removal of carbon dioxide can be done by deaeration. Chemical removal of carbon dioxide can be done by treating with lime or ammonium hydroxide.
Ca (OH) 2 + CO2 CaCO3 + H2O
NH4OH + CO2 (NH4)2 CO3 + H2O
Any acidic impurities can be removed by treating with alkaline agents like ammonium hydroxide.
Factors affecting the rate of corrosion
The rate and extent of corrosion is depends on the following factors 1. Primary factors
Nature of the metal
The position of the metal/alloy in the galvanic series decides the rate and extent of corrosion.
The rate of corrosion depends upon the difference in the position of the metals in the galvanic series. Greater the difference, faster is the corrosion at anode.
Exception: metals and alloys which show passivity are exception for this general trend. Such metals form a protective coating on surface which prevent corrosion
Nature of the corrosion product
In aerated atmosphere almost all metals get covered with a thin surface film of metal oxide(corrosion product).
The thickness of the oxide layer varies with respect to the nature of the metal and the environment.
If the oxide film (corrosion product) is nonporous, protective in nature, prevents the further corrosion. The layer acts as a barrier between the fresh metal surface and corrosive environment.
If the oxide film (corrosion product) is porous, unstable in nature, continues the corrosion processes.
Example: Aluminium Titanium and chromium form a protective film of metal oxide on the surface. Stainless steel forms a protective film of Cr2O3 on the surface. But in the case of Zinc and Iron, the corrosion product formed do not have a protective value
Ratio of anode to cathode
The rate of corrosion (x) is directly proportional to the ratio of area of cathode to the area of anode. i.e., x = area of cathode/ area of anode
Higher the value of x, greater is the rate of corrosion.
When anode is small and cathode is large all the electrons liberated at anode, are consumed at the cathodic region. Therefore, the rate of anodic reaction is greater and increases the extent of corrosion.
Example: A broken coating of Tin on Iron surface results in intense corrosion at the broken region. Iron is anodic to tin. Exposed region has small area and acts as anode. Tin acts as a cathode which has a large area.
Polarization
The anodic and cathodic reactions take place simultaneously during corrosion, and causes polarization of the electrodes.
Anodic polarization occurs due to accumulation of metal ions in the vicinity of anodic region. This retards the formation of new metal ion. Thus corrosion processes is retarded.
The presence of depolarizers reduces the polarization effect and thereby increases the rate of corrosion.
The addition of complexing agents around anode and/or the presence of oxidizing agents around cathode, acts as depolarizers.
2. Secondary factors: pH of the medium
Acidic media are generally more corrosive than alkaline/neutral media. The pH of the solutions decides the type of cathodic reaction.
The corrosion of iron in oxygen free water is slow, until the pH<5, the corresponding corrosion rate is much higher in presence of oxygen.
The metals which are amphoteric in nature viz. Al, Zn, etc., dissolve in alkaline solutions as complex ions.
Corrosion of metals readily attacked by acid can be reduced by increasing the pH of the environment. Example: Zn suffers from severe corrosion even in the presence of mild acidic medium, whereas corrosion is minimum at pH=11.
Temperature:
The velocity of a chemical reaction increases with increase in temperature.
If the medium is acidic, hydrogen evolution takes place at cathode. The rate of diffusion of H+ towards cathode increases with increase in temperature and enhances the rate of corrosion.
If the medium is alkaline / neutral, oxygen absorption takes place at cathode. Since the solubilities of the dissolved gases decreases with increase in temperature, the rate of corrosion also decreases.
Passive metals become active at high temperature and increases the rate of corrosion with increasing temperature. Ex. Caustic embrittlement in high pressure boilers.
Humidity:
The rate of corrosion increases with increase in humidity. As the humidity increases in medium, the corrosion rate gradually increases following parabolic law (weight gain against relative humidity).
Above a certain value of relative humidity is called critical humidity.
Ex: Iron rusts very slowly in an atmosphere with less than 60% relative humidity. Beyond this value it rusts very faster. This is attributed to the porous capillary structure of rust particles on iron. The capillaries get filled at low relative humidity and at critical humidity the pores are full. An increase beyond the critical humidity would make the water to penetrate the surface resulting in severe rusting of iron.
Corrosion control
Corrosion can be completely avoided only under ideal conditions. Since it is impossible to attain such conditions, it can be minimized by using various corrosion control methods. They are:
The principle is to force the metal to be protected, to behave as cathode. There are two types of cathodic protections namely,
1) Sacrificial anodic protection
2) Impressed current cathodic protection Sacrificial anodic protection
The metallic structure to be protected is connected to a more anodic metal using a metallic wire.
The more active metal gets corroded, while the parent structure is protected from corrosion.
The more active metal so employed is called sacrificial anode.
The sacrificial anodes to be replaced by fresh ones as and when it is required.
Commonly used sacrificial anodes are: Mg, Zn, Al etc.
This method is generally used for the protection of buried pipelines, ship hulls, water tanks, etc.
Example: steel pipe is protected by connecting it to a block of Mg or Zn. In such cases steel acts as a cathode and is unaffected Mg or Zn acts anode and undergoes sacrificial corrosion
Impressed current cathodic protection
The metallic structure to be protected is made as cathode by impressing the current.
The current is applied in the opposite direction to nullify the corrosion current.
The impressed current is obtained from a source like battery.
An insoluble anode (ex: graphite, high silicon content iron, etc.) is buried in the soil, and
The anode is usually placed in a backfill, to provide a better electrical contact with the surroundings.
This method is suitable for large structures and for long term operations
Anodic protection
The principle is to force the metal to be passive, to behave as anode.
Metals like titanium, chromium, iron, nickel and their alloys show passivity.
Metals become passive on applying potential greater than passivating potential by maintaining constant current using potentiostat.
Passivity is due to the formation of oxide film on the surface in an oxidizing environment The passivation can be explained by plotting a graph
The plot of corrosion current as a function of applied potential shows as the potential is increased the corrosion current increases first (region AB),then decreases (region BC),then remains constant(region CD),then increases again(region DE)
Region CD is passive region at this region rate of corrosion is very slow, but not
Protective coatings
Corrosion is prevented by the application of protective coating on the surface of metal, thereby the metal surface is isolated from the corrosive environment.
The coatings being chemically inert to the environment under specific conditions of temperature and pressure, forms a physical barrier between the coated surface and its environment.
Coatings are not only preventing corrosion but also decorate the surface of the metal.
Important types of protective coatings are: (i) Metal coatings
Metal coatings
Metal coatings can be applied on the base metal by hot dipping process.
This method is used for producing a coating of low melting metals such as Zn, Al, Sn etc., on iron / steel metals which have relatively high melting point.
The process involves immersing of the base metal in a molten bath of coating metal covered by a flux layer. The flux cleans the surface of the metal base metal and prevents the oxidation of molten coating metal. The coating metal may be anodic or cathodic to the base metal.
Example: Galvanizing and Tinning
Galvanizing Tinning
Coating of zinc on iron or steel, by hot dipping process is called galvanizing. (M.P of Zn = 419oC)
The article is washed with organic solvents to remove oil/grease, with sulphuric acid to remove scale/rust, then with water and dried, before coating.
Coating metal is anodic to iron/steel, called anodic coating.
The molten metal bath is covered with a flux of Ammonium chloride, which prevents the oxidation of the coated metal.
The article is dipped in a molten bath of Zn. The excess of coated metal is removed by passing through a pair of hot rollers and cooled gradually.
Galvanizing is applied to nails, bolts, pipes, roofing sheets etc.
Galvanized sheets cannot be used for preparing/storing food stuffs, since Zn dissolves in acidic medium and forms toxic compounds.
If any crack is produced on the galvanized sheets, causes severe corrosion on the coated Zn metal and the base metal is protected.
Zn is chosen as a protective coating for iron/steel because of its natural resistance against corrosion in most atmospheric conditions, and Zn is electronegative to iron and can protect it sacrificially.
Coating of tin on iron or steel, by hot dipping process is called tinning. (M.P of Sn = 232oC).
The metal surface is washed with organic solvents to remove oil/grease, with sulphuric acid to remove scale/rust then with water and dried, before coating.
Coating metal is cathodic to iron/steel, called cathodic coating.
The molten metal bath is covered by a flux of Zinc chloride.
The clean and dry sheet is passed through flux layer, molten tin, finally removed out through palm oil, which prevents the oxidation of the coated tin.
It possesses more resistance against atmosphere.
It is non-toxic in nature and more noble than the base metal.
Tinning is widely used for coating the steel sheets, Cu and brass sheets used for manufacturing containers for storing/packing food materials, cooking utensils, refrigeration equipments, etc.
If any crack is produced on the tinned sheets, causes severe corrosion of the base metal.
METAL FINISHING Introduction:
Metal finishing covers the wide range of processes carried out in order to modify the surface properties of a metal. These processes involve deposition of a layer of another metal or a polymer, conversion of a surface layer of atoms into oxide films which ultimately modify the surface of the metal.
The principle of metal finishing are used in electroplating of metals, alloys and composites, manufacture of electronic components such as PCBs, capacitors, connectors etc.
Definition: “It is a process of modifying surface properties of metals by deposition of a layer of another metal or polymer on its surface, by the formation of an oxide film”.
Technological importance of metal finishing:
The main technological importance of metal finishing include
1. Imparting the metal surface to higher corrosion resistance. 2. Imparting improved wear resistance.
3. Providing electrical and thermal conducting surface. 4. Imparting thermal resistance and hardness.
5. Providing optical and thermal reflectivity.
6. In the manufacture of electrical and electronic components such as PCB’s, capacitors contacts, etc.
7. In electro framing of articles, electrochemical machining, electro polishing and electrochemical etching.
8. To increase the decorativeness of metal surface.
9. In electrotyping and to build up material or restoration. 10. To improve wear resistance or solder ability.
Principles of metal finishing: 1) Polarization:
The polarization is an electrode phenomenon. The electrode potential is determined by the Nernst equation,
Where Mn+ is the concentration of the metal ions surrounding the electrode surface at
surface decreases due to the reduction of some of the metal ions into metal atoms. There exist a concentration gradient between electrode surface and bulk concentration. Therefore there is a shift in the equilibrium and a change in electrode potential.
As a result there will be a change in the electrode potential; however equilibrium is
re-established due to the diffusion of metal ions towards the electrode. If the diffusion is slow the electrode potential changes and the electrode is said to be polarized. Polarized electrode uses more negative potential than required in order to maintain given current.
M n+ + n e- M
Therefore polarization can be defined as the process where there is a variation of electrode potential due to the inadequate supply of species from the bulk of the solution to the electrode is known as “polarization”
Factors affecting the electrode polarization:
1. Nature of the electrode [size, shape & composition] 2. Electrolyte concentration
3. Temperature
4. Rate of stirring of the electrolyte 5. Products formed at the electrode
Large electrode surface, low [Mn+] concentration, continuous stirring decreases Significance of polarization:
1) Polarization is a condition caused by changes in bath concentration or changes within either the anode or cathode films. It is caused by the movement and discharge of ions. 2) Polarization is always present during electrolysis of a solution but is of concern when its
effect becomes excessive. It is caused by a depletion of the electrode surface, as in concentration polarization, or resistance polarization which is caused by the formation of a diffusion layer surrounding the electrode.
2) Decomposition potential:
It is defined as “the minimum voltage that must be applied in order to bring about continuous electrolysis of an electrolyte”.
Electrolysis of an electrolyte occurs only when applied voltage is above certain value below which electrolysis do not occur. This can be determined by an electrolytic cell, if dilute acid or bases are used as electrolytes, it required more than 1.7V. The decomposition potential for Zn and iodine cell is experimentally found to be 4.3 volts. The decomposition potential is
represented as
Determination of decomposition potential:
The decomposition potential is determined using an electrolytic cell as shown in the figure.
The cell consists of two platinum electrodes immersed in a dilute solution of an acid or a base. The voltage is varied along the wire and the current passing through the cell is measured using an ammeter. At low voltage no reaction occurs and there is a very slight increase in the current & small amount of hydrogen & oxygen are liberated at the cathode & anode respectively.. This hydrogen gas adsorbed on the cathode electrode & produces back emf which opposes the applied emf. On increasing the voltage to slightly above 1.7V, there is an abrupt increase in the current and process of electrolysis begins. A plot of the current against the applied voltage as below.
Significance of decomposition potential:
It is impossible to predict the order of discharge of ions from an electrolytic solution containing several ions.
3) Over voltage:
It is defined as “The excess voltage that has to be applied above theoretical
decomposition potential to bring the continuous electrolysis of an electrolyte” is known as over voltage.
The theoretical voltage required for the decomposition of aqueous solution of an acid is equal to the emf of the reversible cell with hydrogen and oxygen gases at one atmosphere. This is known to be about 1.23 volt at ordinary temperature with Platonized platinum electrodes. It is however found that the observed decomposition potential is always higher than the theoretical value. Thus with platinum and lead electrodes, a voltage of 1.7 and 2.2 is respectively required for the electrolysis of dilute sulphuric acid as against a theoretical value of 1.23 volt. This difference between the observed and theoretical decomposition potential is called “over voltage”.
Over voltage = (Experimental decomposition potential – theoretical decomposition
potential)
Over voltage of an electrolyte depends on
1)
Nature and physical state of the metal employed for the electrodes.2)
Nature of the substance deposited.3)
Current density.4)
Temperature.5)
Rate of stirring of electrolyte.Electroplating
Definition: “it is a process of deposition of a metal by electrolysis, over the surface of substrate. The substrate may be another metal, polymer, ceramic, or a composite”.
Theory of electroplating:
Electroplating process being electrolysis, the amount of metal getting deposited and the amount of current passing through the electrolytic cell are related to each other by the law of electrolysis called Faraday’s laws.
“The amount of substance deposited, or liberated at an electrode is directly proportional to the quantity of electricity passing through the electrolyte solution during electrolysis”.
Mathematically, if W gram of a substance deposited when Q coulombs of electricity is passed, then
W α Q or W = Z I t (since Q=I t)
Where I is the current in amperes, t is the time in seconds for which current has been passed, Z is the proportionality constant called electrochemical equivalent.
Faraday’s second law:
“When same quantity of electricity passes through solutions of different electrolytes, the amount of substances deposited, or liberated at the electrodes are directly proportional to their
electrochemical equivalents”.
When the same quantity of electricity is passed through different electrolytic solutions, the masses of the different substances (m1 and m2) deposited or liberated at the electrodes are directly proportional to their equivalent masses (E1 and E2).
m1/ m2 = E1/E2
According to Faraday’s laws one mole of electrons deposits or liberates one equivalent of any substance at an electrode.
The principal components of electroplating process are:
1) Electroplating bath 2) Cathode 3) Anode 4) Electroplating tank 5) Reactions at anode and cathode
1) Electroplating bath: The plating bath contains solution for plating process. It is normally a mixtures of metal ion solution, other electrolytes, complexing agents and various organic additives added to improve the nature of deposit.
2) Cathode: The substrate to be plated is made as cathode and suspended as separate bars. These cathode electrodes are placed in electrolytic bath solution.
3) Anode: The metal which is to be plated on the other metal is made as anode. Anode is used in the form of a rod, a plate or pellets. The anode is enclosed inside an anode bag to retain
impurities.
5) Reaction at anode and cathode: Electroplating is the process of electrolytically depositing a layer of metal on a surface. The anode metal undergoes oxidation liberating metal ions and electrons. The electrolytic bath containing metal ions undergoes reduction to metal atoms and gets deposited on the metal or substance to be plated.
At cathode: M n+ + n e- M At anode: M M n+ + n e-
Factors (plating variables) influencing nature of electrodeposits:
The nature of the electrodeposit is affected by numbers of factors which are discussed below.
1) Metal ion concentration. 2) Electrolyte concentration. 3) Complexing agents. 4) Organic additives. 5) Current density. 6) pH.
7) Temperature.
8) Throwing power of plating bath.
1) Metal ion concentration:
A higher concentration of metal ion increases the mass transfer leading to poor deposit. For a good adherent deposit, the metal ion concentration should be low; it is normally 1-3 mol dm-3. The low metal ion concentration can be achieved by adding compounds having common ions or by the formation of complex compounds and ions. In general a decrease in metal ion
concentration decreases the crystal size and result in a fine adherent coating films.
Example: when copper is deposited from CuSO4 bath, H2SO4 is added to the solution. Due to the common ion effect of SO42- , the concentration of Cu2+ ion in the solution is reduced.
CuSO4 Cu2+ + SO4 2-H2SO4 2H+ + SO4
2-2) Electrolyte concentration:
A good adherent deposition can be obtained with higher electrolyte concentration. The
3) Complexing agents: These are added
1) to maintain low metal ion concentration.
2) to prevent the chemical reaction between the cathode metal and plating ions. 3) to improve the throwing power the plating bath.
4) to increase the solubility of slightly soluble metal salts.
5) to prevent the passivation of anode so that anode dissolves easily and improve the current density.
Ex: Cyanides, hydroxides, sulphamates.
4) Organic additives:
These are added to improve the nature of electrodeposits. They modify the structure, morphology and properties of the electrodeposits. The different organic additives used are as follows:
a) Brighteners c) Structure modifiers
b) Levelers d) Wetting agents
a) Brighteners:
These are added to obtain a bright and microscopically fine deposit. The brighteners are
adsorbed on the nuclei of the metal and forms new nuclei, i.e., large number of smaller crystals resulting in the formation of good deposit.
Ex: Aromatic sulphones or sulphonates & compounds containing C=O, N=C=S, etc.
b) Levelers:
Levelers are added to produce an even (level) deposit by getting adsorbed at regions where rapid deposition takes place. The levelers reduce the rate of deposition at those points.
Ex: Sodium alkyl sulphonates.
c) Structure modifiers or Stress relievers:
These additives modify the structure of the deposit and orientation in such a way as to alter the deposit properties. These substances avoid the development of internal stress in the deposits.
Ex: Saccharin.
Wetting agents are used to release hydrogen gas bubbles from the surface. The wetting agents also improve the uniformity of the deposit.
Ex: Sodium lauryl sulphate.
5) Current density:
Current density is the current per unit area expressed in amperes per m2. At low current density, a bright, crystalline deposit is produced but the rate of deposition is slow. At higher current
density, hydrogen evolution occurs and deposits are spongy, irregular and loosely held. Deposits may have a burnt appearance.
For good deposit the current density should be optimum (10- 70 A m-2).
6) P H :
The nature and appearance of the electrodeposit depends on pH of the plating solution. If pH of the medium is low H2 gas is evolved at the cathode causing brittle and burnt deposit. At higher pH values, deposits of metallic oxides or hydroxides may form. Hence an optimum pH (4- 8) is employed.
7) Temperature:
A good deposit is formed at slightly higher temperature (35-60oC). At very low temperature hydrogen evolution takes place at the cathode forming a burnt deposit. Therefore moderate temperature is used to get good deposit.
8) Throwing Power of a plating bath:
The ability of a plating bath to give a uniform and even deposit on the entire surface of the object, irrespective of its shape is known as “throwing power”.
Haring Blum cell consists of plating bath solution whose throwing power is to be determined. Anode is placed at the centre and two cathodes are placed on the either side of the anode at a distance x1 and x2. Electroplating is carried out and weight of electro deposit on the two cathodes is weighed. The weight of electro deposit (w1) on x1 which is placed far from anode is less than another x2, which is very nearer to the anode. Then throwing of plating bath is calculated from the equation.
% throwing power =
Where K = x1 / x2 (when x1 > x2); M = w2 / w1
Throwing power is said to be very good (100%) when w1 ═ w2
Application of electroplating:
For better appearance
Plating for protection
Plating for special surface
Plating on non metals
Plating for engineering effect
Gold electroplating
Gold plating is widely used in the following applications; i) In semiconductors.
ii) In Printed or etched circuits. iii) In Contacts and Connectors.
Based on the PH range maintained during plating, gold plating has been classified into three baths namely;
a) Acid cyanide bath, b) Alkaline cyanide bath, c) Neutral cyanide bath a) Acid cyanide bath
PH : 3.8-4.3
id : 100-400 ASF
Temperature : 210c-490c
Anode : Platinum clad
Cathode : Specimen/ Article
Duration : 15 seconds at 400 ASF
Application : Used in the manufacture of PCBs and Connectors. b) Alkaline cyanide bath
Plating bath : Potassium gold cyanide 8-20g/l Dipotassium Phosphate 22-45g/l KCN 15-30g/l
PH : 12
id : 3-5 ASF
Temperature : 490c-710c
Anode : Stainless Steel
Cathode : Specimen/ Article
Duration : 08mins
Application : Used for the Semiconductors.
c) Neutral cyanide bath
Plating bath : Potassium gold cyanide 8-20g/l, Monopotassium Phosphate 22-45g/l Potassium Citrate 70g/l
PH : 6.0-8.0
id : 1-3 ASF
Temperature : 710c
Cathode : Specimen/ Article
Duration : 12mins
Application : Used for the Semiconductors
Alkaline bath gives thin & porous deposit and preferred for decorative purposes. Neutral bath is less porous and used in manufacture of PCBs. Acidic bath gives nonporous, pure and ductile deposit and used in electronic industry.
In gold plating, insoluble anode is preferred over soluble anode, because, anode efficiency is greater than that of cathode and leads to bulk deposit.
Direct gold plated silver / copper surface tarnish due to diffusion of silver / copper atoms to the surface; an undercoat is preferred.
Applications: Gold plating is used in jewelry, printed circuits, electrical contacts, transistors, IC parts, reactors and heat exchangers, etc.
Electroless plating
“Electroless plating is an autocatalytic reduction of metal ions with the help of a reducing agent on a catalytically active substrate without using electricity”.
Metal ion + Reducing agent catalytically activated surface Metal (d) + oxidized product It is an autocatalytic reaction as the deposited metal atoms catalyses further reduction of metal ions. To initiate electroless plating, substrate surface should be catalytically active.
Catalytically active surfaces: Pd, Pt, Cr, Ni, Fe, steel
Catalytically inactive surfaces: Cu, Al, brass, insulators
Inactive surface can be converted into active Pd surface by treating with a solution of palladium chloride (in HCl) followed by stannous chloride solution (in HCl).
Comparison between electroplating and electroless plating
Factor Electroplating Electroless plating
Anode and Cathode Anode – Soluble / insoluble
Cathode – substrate
Substrate acts both as anode and cathode
Nature of substrate Must be conductor Conductor / semiconductor / insulator
Throwing power Low High
Nature of deposit Relatively pure and does not exhibit unique surface properties
Impart unique surface (mechanical, electrical and magnetic) properties due to incorporation of oxidized or reduced product
Advantages of electro less plating are
Plating can be carried out on any kind of substrate (conductors / semiconductors / insulators)
Impart unique mechanical, electrical and magnetic properties
High throwing power
Does not require electrical energy
Composition of electro less plating
Metal salt : Suitable quantity of metal salt is added to furnish metal ions Reducing agent : Responsible for reduction of metal ions & plays vital role in
imparting unique surface properties Complexing agent : Monitor the free metal ion concentration
Buffer : Prevent alteration in pH and helps to get desired quality deposit Stabilizer : Enhances the stability of the bath by preventing unwanted
reaction/s
Exaltant / Accelerator : Increase the deposition rate Copper electro less plating
Plating is generally carried out in alkaline medium and plating bath composition depends upon the quality and thickness of the deposit. Following bath gives a thick, rapid deposit.
Electroless plating of Copper
Before electroless plating, the surface is cleaned thoroughly. Insulators such as plastics and printed circuit boards are activated by dipping first in stannous chloride (SnCl2) and then in palladium chloride (PdCl2). Then, the electroless plating is done under the following conditions:
1 Plating bath solution CuSO4 [12g/lt]
2 Reducing agent Formaldehyde (HCHO)
3 Complexing agent EDTA
4 Buffer Sodium hydroxide and Rochelle salt
(Na-K-tartarate)
5 pH 11
6 Temperature 25℃
Reactions: 2HCHO− + 4OH− → 2HCOO− + 2H
2O + H2 + 2e (oxidation) Cu2+ + 2e → Cu (reaction)
Formaldehyde and copper sulphate are added to the plating bath periodically Applications:
1. Used for metalizing printed circuit boards. 2. Used to produce through-hole connections.
Through-hole connection is PCB’s:
For PCB’s with double sided circuits, through-hole connection is required. The through-hole connection
is made by electroless plating technique.
Preparation of PCB by electroless plating:
1. The base material is made up of glass reinforced plastic[GRP’S] or a epoxy polymer. 2. The base material which is double sided, is electroplated with copper
4. The rest of copper is removed by etching with suitiable etchent to get circuit pattern
UNIT III: SCIENCE OF CORROSION AND ITS CONTROL Question bank
2 marks questions (20)
1. What is the effect of pH on rate of corrosion?
2. Define passivation. Give an example.
3. List the technological importance of metal finishing.
4. What is the effect of temperature and conductance on the rate of corrosion.
5. What is throwing power? Mention any two factors influencing throwing power.
6. Differentiate between dry and wet corrosion.
7. Identify the type of corrosion the following.
i) Bolt and nut made from different metals are in contact with each other.
ii) Deposition of small particles of dust on iron surface.
8. Define electroplating.
9. Summarize the applications of electroplating of gold.
10. Differentiate between galvanic series and electrochemical series.
11. List all primary and secondary factors influencing rate of corrosion.
12. Give examples for the metals can be protected by cathodic and anodic protection.
13. List any two applications of electroless plating of copper.
14. Pin holes on tin coated iron are more prone to corrosion than those on zinc coated iron. Why?
15. List the any 4 functions of addition of complexing agent during electroplating.
16. List the characteristics of a good electrodeposit.
17. What are the steps involved in the surface cleaning.
19. For a electrolytic mixture containing Zn2+ , Cu2+ , Ag+ which ion is going to discharge first. Give
reason.
20. Why soluble anodes are not preferred during electroplating of g
5 marks questions (12)
1. Differentiate between electroplating and electroless plating.
2. Write a note on grain boundary corrosion.
3. Summarize on boiler corrosion.
4. What is metal finishing? Discuss theory of electroplating.
5. What is decomposition potential? Explain its significance.
6. Define over voltage (over potential). Mention the factors affecting it.
7. What is electroless plating? Mention its advantages.
8. Explain how electroplating of gold is carried out.
9. Write a note on anodic protection.
10.What is polarization potential? Explain its significance.
11.What is Differential metal corrosion? Give example.
12.What is cathodic protection and explain impressed current method.
8 marks questions (6)
1. Define corrosion. Discuss the electrochemical theory of corrosion by taking rusting of iron as an example.
2. Explain in detail the factors affecting the rate of corrosion.
3. Distinguish between galvanizing and tinning. How metals can be protected by sacrificial anode method.
4. List all the factors affecting the nature of electrodeposit. Explain any 3.
