CORROSION SCIENCE Introduction
The 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.
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+ + ½O2 + 2e-→H2O (b) In the absence of dissolved oxygen: 2H+ + 2e-→ H2 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
Pitting corrosion
is a localized accelerated attack in which only small areas of the metal surface are attacked whilst the remainder is largely unaffected. This localised attack results in pitting. The pits may initiate and propagate to a certain depth resulting in the formation of cavities and becomes inactive.
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.
Stress corrosion
It is a highly localised attack on the metal.
This corrosion occurs only in the presence of specific corrosive environment and the presence of tensile stress on the metal.
Stress may be produced on the metal during fabrication of the article or during etching, drawing, servicing etc.
This corrosion involves an attack along the narrow paths forming local anodic areas with respect to more cathodic area of the metal surface.
The stress produces strains, resulting localised zones, which are chemically active and easily attacked even by a mild corrosive environment results in the formation of fissures.
The fissures lead to crack in the presence of stress.
The crack grows and propagates perpendicular to the operating stress, and failure occurs after progressing a finite distance.
It is a stress corrosion occurring in mild steel when exposed to alkaline solutions at high temperature and stress. The boiler water, usually contains a certain proportion of sodium carbonate added for water softening purposes. In high pressure boilers, the carbonates breaks up to give respective hydroxide and carbon dioxide, and make boiler water alkaline.
Na2CO3+H2O→NaOH+CO2
by capillary action. The water evaporates and increases the concentration of the alkali. This concentrates alkali dissolves iron as sodium ferroate in crevices, cracks and the metal under stress. The sodium ferroate decomposes into magnetite and alkali is regenerated in the process as per the following reactions.
NaOH +Fe→ Na2FeO2+H2
Na2FeO2+H2O→ NaOH +Fe3O4
This type of stress corrosion in boilers in the presence of alkaline medium, called caustic embrittlement. This can be prevented by the addition of the substances such as sodium sulphate, tannin, etc., which blocks the cracks and crevices, thereby prevents the penetration of alkali.
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.
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 metals with lower electrode potential values are more reactive and more susceptible for corrosion than the metals with higher electrode potential values.
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.
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.
Cathodic polarization occurs due to accumulation of hydroxyl ions in the vicinity of cathodic region. This accumulation retards the movement of oxygen towards the cathodic surface. Thus the reduction of oxygen is decreased. Therefore polarization of anode or cathode decreases the corrosion rate substantially
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 becomes 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).
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.
Pourbiax diagram:
A Pourbaix diagram provides information about the stability of a metal as a function of pH and potential. These diagrams are available for over 70 different metals. Pourbaix diagrams have several uses, including in corrosion studies.
A Pourbaix diagram is also known as a potential/pH diagram, equilibrium diagram describing various chemical phase formation at different pH and potential. Pourbaix diagrams are plotted by using the Nernst equation (an equation used to calculate electrode potential). As the Nernst equation is derived entirely from thermodynamics, the Pourbaix diagram can be used to determine which species (metals or alloys) is thermodynamically stable at a given electrode potential (E) and pH.
A Pourbaix diagram is divided in regions of “immunity”, “corrosion” and “passivity”. These regions provide information about the stability of a particular metal or alloy in a specific aqueous electrochemical environment under certain pH, E vs SHE, pressure and temperature conditions.
The immunity region is the region in which there is no metal dissolution. The corrosion region is the region in which there is active metal dissolution.
The passivation region is the region in which a protective metal-oxide film that prevents metal dissolution is formed.
Characteristics of Pourbaix diagram:
Horizontal lines represent electron transfer reactions. They are pH-independent, but potential-dependent.
Vertical lines are potential-independent but pH-dependent and not accompanied by any electron transfer.
Sloping, straight lines give the redox potentials of a solution in equilibrium. This equilibrium indicates electron transfer as well as pH.
The main objectives of the Pourbaix diagrams are:
1. To show the directions of the various reactions at given pH and potential.
2. To make a basis for estimation of the corrosion product compositions at various pH and potential combinations.
3. To show which environmental pH and potential changes will reduce or prevent corrosion.
As water is the medium in the electrochemicali or wet corrosion we must understand the stability region of H2O in the Pourbaix diagram. From the diagram below we see the stability region of water. When the E is increased or decreased the water undergoes oxidation or reduction. This appears along with all metal system.
Pourbaix diagram of water
Pourbaix diagram of Aluminium
Limitations of Pourbaix diagram
The validity of the diagrams is limited to reactions between pure metals, pure water and the species that can be formed from these. Small amounts of impurities and alloying elements may change the diagram.
These diagrams are purely based on thermodynamic data and do not provide any information on the reactions and not possible to measure the corrosion rates.
Consideration is given only to equilibrium conditions in specified environment, and factors such as temperature and velocity are not considered, which may seriously affect the corrosion rate.
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:
a) by protective coatings b) by corrosion inhibitors c) by cathodic protection Cathodic Protection:
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.
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
connected to the structure to be protected.
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.
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
(ii) Inorganic coatings and (iii) Organic 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.
Galvanising Tinning Coating of zinc on iron or steel, by hot
dipping process is called galvanising. (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.
Galvanising is applied to nails, bolts, pipes, roofing sheets etc.
Galvanised 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 galvanised 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.
Tin coatings form a useful preparation for protective painting in general applications.
Inorganic coatings (Chemical conversion coatings)
These coatings are produced at the surface of the metal by chemical / electrochemical reactions.
These coatings are applied on the article for decorative effect and to increase the corrosion resistance of the base metal.
Anodising Phosphating These coatings are generally produced
on non-ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation (electrochemical) process.
The base metal is made as anode. Anodising of Al: It is carried out to produce a porous/nonporous coating. The porous coating is obtained by anodic oxidation. The electrolysis is conducted in an acid bath, at moderate temperature 30-40oC, using moderate current densities, in which the base metal is made as anode. The commonly used baths are H2SO4 / Chromic acid / Phosphoric acid /oxalic acid. The thickness of the film increases with progressive oxidation. Outer most layer of the oxide film is very porous and soft, these pores are sealed by exposing to the boiling water. In this process the metal oxide layer changes into its mono hydrate.
The non porous coatings are produced by using non-corrosive electrolytes like boric acid and borax. These coatings are applied on electrolytic condensers.
The anodised coatings are thicker than the natural oxide film and possess improved corrosion resistance as well as resistance to mechanical injury.
These coatings are generally applied frequently to iron, steel and zinc and to a lesser extent on Al, Cd and Sn. These are produced by the chemical
reaction of the base metal with aqueous solution of phosphoric acid(pH-1.8-3.2) and phosphate of Fe, Mn or Zn.
The reactions are slow, hence it is enhanced by using accelarators along with the phosphating mixture.
The most common mode of acceleration is by addition of oxidizing agents, such as nitrate, nitrite, chlorate and hydrogen peroxide.
The chemical reaction between the base metal and the phosphating mixture results in the formation of surface film consisting of crystalline Mn-Fe Phosphate, Zn-Fe Phosphate etc. These coatings are applied by immersion or spraying or brushing. These coatings do not offer
complete resistance to the atmospheric corrosion.
These are used as a primer coat for paints, enamels, etc.
METAL FINISHING Introduction:
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 equilibrium. When there is a passage of current, the metal ion concentration near the electrode 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
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 which affect polarization:
Nature of the electrode (i.e. size, shape, composition).
Electrolyte concentration and its conductivity.
Temperature.
Rate of stirring of the electrolyte.
Product formed at the electrode. 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 ED = Ecathode - Eanode
Determination of decomposition potential:
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:
Minimum external voltage that must be applied in order to bring about electrolysis of electrolyte.
It is impossible to predict the order of discharge of ions from an electrolytic solution containing several ions.
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 platonised 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.
Faraday’s first law:
“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”.
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.
4) Electroplating tank: It is made of wood or steel. If steel tanks are used these are thermally insulated with ceramic or polymeric materials. The volume of tank may vary from 20 – 100 dm3.
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.
2-2) Electrolyte concentration:
A good adherent deposition can be obtained with higher electrolyte concentration. The
electrolytes used do not participate in the electrode reactions but increases the conductivity of the plating bath and cathode efficiency. It also controls the change in pH.
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.
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.
d) Wetting agents:
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:
Throwing power of plating bath can be determined by using “Haring Blum Cell”.
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 Methods of cleaning metal surface: (surface preparation):
The object before subjecting to electroplating process, it is essential to clean the surface of object. The following methods are used to clean the metal surface.
1) Solvent cleaning: Solvent cleaning involves the cleaning of metal by using organic solvents like ccl4, Toluene etc. These solvents remove organic matter, oil and grease.
3) Mechanical cleaning: The object is subjected for mechanical cleaning to remove oxide, rust and other impurities on the metal surface. The mechanical cleaning methods involves, cleaning with Bristle brushes, mechanical polishing, grinding using polishing machines, buffering and sand blasting.
4) Pickling: In this method the oxides scales are removed by dipping the base metal in a dilute acids. Pickling of steel involves dipping in dil.Hcl or dil.H2SO4 to remove rust and other oxide scales.
5) Electro polishing: In this method metal to be cleaned is made as anode in suitable solution. During the process the surface layer of metal gets dissolved along with impurities. Most commonly used baths for electro polishing are H2SO4, H3PO3, HNO3, etc.
Requirements of electrolyte solution for electroplating:
Metal content should be high but metal ion concentration should be low.
Conducting power of solution should be high.
Electrolyte solution should be stable under operating condition. 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 rangemaintained during plating, gold plating has been classified into three baths namely;
a) Acid cyanide bath
Plating bath : Potassium gold cyanide 8-16g/l, Citric acid 90g/l
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
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
Anode : Platinum clad Columbium 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.
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
Driving force Applied external potential Autocatalytic reduction reaction
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
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
Copper electro less plating provides highly conducting, corrosion resistant, nonporous surface. If, the substrate surface is catalytically inactive, then treatment with palladium chloride and stannous chloride in acid medium provides active Pd surface.
Reactions: 2HCHO− + 4OH− → 2HCOO− + 2H2O + H2 + 2e (oxidation) Cu2+ + 2e → Cu (reaction)
Plating bath composition
Metal salt – Copper sulphate ( g / L) Reducing agent – Formaldehyde ( mL / L)
Buffer – Sodium hydroxide ( g / L) – Sodium carbonate (g / L) Complexing agent – Rochelle salt ( g / L) – EDTA (g / L)
Stabilizer – Methanol (mL / L)
30 170
40 25 140
17 125
Temperature Room
pH 11.5 – 12.0
