INDUSTRIAL CHEMISTRY
CORROSION:-
Definition of corrosion
Corrosion is the gradual destruction of a metal brought about by its environment. Corrosion is an electrochemical process.
RUSTING OF IRON:When iron material is left in the atmosphere for a long time, it gets corroded gradually and get coated with reddish brown deposit called rust. This corrosion of iron is called Rusting.
Rust is hydrated ferric oxide: Fe2 03 • x • H2O
MECHANISM OF RUSTING OF IRON : The surface of iron metal due to mechanical strain or impurities lead to a difference of potential between the different regions on the surface which is in contact with moisture. Hence anodic and cathodic sites are developed on the surface.
At anodic part on the surface of iron the iron atoms undergo oxidation by losing electrons. These electrons move to cathodic part on the surface itself and taken by 02 and H2O of the atmosphere to form OH- ions.
Anode: Fe -------Ã Fe+2 +2e- (Oxidation)
Cathode : H2O + ½ O2 + 2e- -----Ã 2OH- (Reduction)
The Fe+2 ions and OH- ions when meet together nearer to cathode form Fe(OH)2. In presence of excess 02, Fe(OH)2 is oxidized to Fe(OH)3. It loses water to convert into reddish brown rust (hydrated Fe2O3).
Necessary conditions for RustingThe following are the conditions necessary for the corrosion of Iron to form rust.
1) Water
2) Oxygen gas
3) An electrolyte: for example carbon-di-oxide dissolved in water or sodium chloride solution can form the electrolyte.
4) Impurities on imperfections in the Iron.
In similar conditions rusting is very much more rapid for steel than for chemically pure Iron.
Electrochemical theory of corrosion:Many types of corrosion occur as electrochemical processes resulting from galvanic cells setup between two dissimilar metals or two dissimilar parts of the same metal. Corrosion occurs due to the existence of separate anodic and cathodic areas between which current flows through the conducting liquid.
At anodic areas the metal undergoes oxidation (loses electrons) to form metallic ions (called corrosion). At cathodic areas reduction (gaining of electrons) takes place for the dissolved constituents in the conducting medium like H+ ions, H2O and 02.
The metallic ions at anodic part and non-metallic ions formed at cathodic part diffuse towards each other through conducting medium and form a corrosion product.
At anode: M ----> M+n + ne- oxidation ( corrosion )
At cathod:
2H+ + 2e- ---Ã H2
H2O + ½ O2 + 2e- ---Ã 2OH-M+n + nOH- -----Ã M (OH)n
Example: Rusting of Iron.
A. GALVANIC CORROSION: [GALVANIC CELL OR COMPOSITION CELL]
When two different metals (Zinc and Copper) are electrically connected and exposed to an electrolyte the metal higher in electrochemical series (Zn) forms the anode and undergoes corrosion (oxidation). While the other metal (Cu) acts as cathode. This type of corrosion is called Galvanic corrosion.
Anode : Zn -------Ã Zn+2 +2e- oxidation (corrosion)Cathode : 2H+ + 2e- ---Ã H2
O2 + 4e- -------Ã 2O-2
H2O + ½ O2 + 2e- --Ã 2 OH-
B. CONCENTRATION CELL CORROSION:
The concentration cell corrosion is due to electrochemical attack on the metal surface exposed to an electrolyte of varying concentrations or of varying aeration.
When one part of metal is exposed to a different air concentration from the other part, it causes a difference in potential between the two parts. The poor oxygenated parts are anodic and undergo corrosion. The more aerated parts act as cathode.
Anode e : Zn -------Ã Zn+2 +2e- Fe -------Ã Fe+2 +2e- (Oxidation)Cathode: H2O + ½ O2 + 2e- -----Ã 2 OH- . H2O + ½ O2 +2e- -------Ã 2OH- (Reduction)
C. STRESS CELL CORROSION:
Stress cells are established between stressed and unstressed parts of the metal.
Presence of stress on the metal produces strains which result in localised zones of higher electrode potential, forming anodic areas with respect to unstressed cathodic parts. So at these stress points corrosion occurs.
Example: Nail: The head and pointed end are stressed parts, act as anode and undergo corrosion. The body part behaves as cathode and is protected from corrosion.
Example: Nail: The head and pointed end are stressed parts, act as anode and undergo corrosion. The body part behaves as cathode and is protected from corrosion.
Methods of prevention of corrosion:-
1) By modifying resistance of a metal can be increased by alloying it with other selected metals.
Example by alloying steel with chromium (18%) and Nickel (8%) we can make it rust proof. The product obtained is known as stainless steel.
2) By modifying the environment of metals by physical and chemical changes.
The rate of corrosion is greatly lowered by lowering the temperature and or by lowering the concentration of electrolytes in the surrounding atmosphere.
Protective coatingsCorrosion starts as an attack on the surface of the metal so by covering the surface of metal with a suitable protective coating, corrosion can be prevented .For example a painted surface isolates the underlying metal from the corroding electrolyte.
The protective coating applied can be metallic:
A) Metallic coatings
For some purpose steel is coated with chromium or Aluminum or Zinc because these metals form protective oxide films on their exposed surface.
Example: Galvanized Iron pipes (G.I pipes) contain a coating of zinc metal on Iron.
B) Organic coating
Temporary protection of small articles such as tools can be obtained smearing oil or grease on the surface.
Plastic coatings on metal surface are widely used to prevent corrosion. Polythene, p.v.c (poly vinyl chloride) and rubber are used for this purpose.
These coatings provide very effective barriers and can prevent corrosion even under various conditions such as in seawater and industrial atmosphere.
C) Ceramic coatings
Vitreous enamel oxide coatings are widely used to protect motorcar bodies and refrigerators. Ordinary oil based paints are not as effective as these enamels.
Cathodic protection or galvanic protection
The object, which is to be protected from corrosion, is made cathodic by coupling it with another more active metal. If you want to protect Iron pipes from corrosion, then we have to complete it with anyone of the metals, which is for e.g. Magnesium, can be used.
Here, Magnesium plate serves as a SACRIFICIAL ANODE because undergoes corrosion instead of the iron pipe, which becomes the cathode Thus Magnesium is sacrificed in the protection of the Iron.
In galvanized Iron, even when the protective Zinc coating is broken, Iron is protected because Zinc is more electropositive than Iron. In this case the Zinc coating acts as a sacrificial anode and protects the Iron. The exposed iron does not corrode because it becomes the cathode.
INDUSTRIAL WATER
IntroductionWater is nature's most abundant and most useful compound. Water is not only essential for lives of animals and plants but also occupies unique position in industries. It is used as engineering material in steam generation. Water is used as coolant in power and chemical plants .It is used in other fields and chemical plants .It is also used in other fields such as production of steel, rayon, paper, atomic energy, textiles and chemicals.
Water is of two types:
1. Soft water
2. Hard water
Soft water is the one, which do not, consists of impurities and gives free lather or Foam with soap solution. Example: Rain water and Distilled water
Hard water is the one which consists of impurities and which will not give lather or Foam with soap solution. Example: Seawater
The impurities are of several types,
Ex Suspended impurities such as clay silt, dissolved inorganic impurities such as Calicium, Magnesium, Sodium, Bicarbonates, Carboantes, Sulphates, chlorides and organic impurities.
Hardness of water
Hardness of water is that characteristic which prevents the lathering of soap. This is due to the presence of certain salts of calcium, magnesium and other heavy metals dissolved in it. A sample of hard water treated with soap (sodium or potassium sterate) does not produce lather but on the other hand forms a white scum or precipitate is formed due to the formation of insoluble soaps of calcium and magnesium. The reactions are shown below.
2C17H35COONA +Cacl2 ----> (C17H35COO) 2Ca + 2Nacl
Sodium sterate (soap) ---- -> Calcium sterate insoluble (ppt or curd)
2C17H35COONa + MgS04 ----> (C17H35COO) 2Mg + Na2S04
Sodium sterate (soap) ------> magnesium sterate insoluble (ppt or curd)
Thus water, which does not produce lather with soap solution readily but forms a white curd is called hard water, on the other hand water which produce lather easily on shaking with soap solution is called soft water.
Types of HardnessThere are two types of hardness
1. Temporary hardness
2. Permanent hardness
Temporary hardness
It is caused by the presence of dissolved bicarbonates of calcium, magnesium and other heavy metals. Temporary hardness can be destroyed by boiling on boiling the water, the bi carbonates of calcium and Magnesium decompose to form insoluble carbonates which can be easily removed by filtration or decantation and thus the water is softened.
The following reactions indicate the process of heating
Ca(HC03)2 -HEAT--> CaC03 + H2O+ C02
Calcium carbonate calcium bicarbonate (insoluble)
Mg(HCO3) -- HEAT ---> Mg CO3 + CO2 + H2O
Permanent hardness
This is caused due to the presence of chlorides, and sulphates of calcium, magnesium, iron and other heavy metals and permanent hardness cannot be destroyed on heating.
Disadvantages of using hard water in industry
Textile industry:Hard water is used in industry causes much of the soap (used in washing yarn fabric etc) to go as waste because hard water cannot produce good quality of lather without precipitating. Moreover these precipitates adhere to the fabrics. These fabrics when dyed later on do not produce exact shades of colour. Iron and Mn salts present in water cause spots on fabrics.
Sugar industry:Water containing sulphate. nitrate alkali carbonates etc if used in sugar refining causes difficulties in the crystallization of sugar. Such sugar undergoes decomposition when stored.
Dyeing industry:The dissolved Ca, Mg & Fe salts present in hard water may react with costly dyes forming undesirable precipitates which give impure shades & forming spots on the fabrics.
Paper industry: Calcium & magnesium salts in hard water tends to react with chemicals & other materials employed to provide a smooth & glassy finish to paper. The salts of iron may even affect the colour of the paper.
Laundry:Hard water containing dissolved calcium. Magnesium, iron, MN & aluminum salts if used in laundry causes much of the soap used in washing to go as waste.
Bakeries:Organic maters such as fungi, bacteria's, if present in water used in bakeries may effect the yeasts action & the quality of the materials produced is poor.
Concrete making:Hard water affects the hydration of cement & final strength of the hardened concrete.
Pharmaceutical industry:Hard water if used in producing pharmaceutical products like drugs, ointment, injections may produce certain undesirable products in them which harm the human being.
Degree of Hardness:Quantity of hardness causing salts in a certain volume or weight of water is called Degree of hardness.
Degree of hardness is expressed in equivalents of CaCO3. Though the hardness is caused due to various salts. The choice of CaCO3 is because its molecular weight is 100 (equivalent weight = 50) and it is the most insoluble salt that can be precipitated.
100 parts by weight of CaCO3 are equivalent to 162 parts by weight of Ca ( HCO3)2
146 parts by weight of Mg (HCO3)2
111 parts by weight of CaCl2
95 parts by weight of MgCl2
136 parts by weight ofCaSO4 120 parts by weight of MgSO4Eqt. Of CaCO3 =
Mass of hardness producing substance in mg x100
Mol. wt. of hardness producing substance
The degree of hardness of water is expressed in 3 systems.
A. Parts per million: (PPM)
The degree of hardness of water is expressed as the number of parts by weight of CaCO3 equivalents present in one million parts (106 ) by weight of water.
1 ppm = 1 part of CaCO3 eqt. Hardness in 106 parts of water.
B. French Degree :(°Fr)French degree is the parts of CaCO3 eqt. Hardness per 105 parts of water.
C. Clark's Degree :(°Cl)Cark's degree is the parts of CaCO3 eqt. Hardness per 70,000 parts of water. Relationship between various units of Hardness.
1 ppm = 1 mg. per litre
=0.1°Fr
= 0.07° Cl
Water treatment:
Softening of hard water:The process of removing hardness producing salts from water is known as softening of water.
3 methods are employed for softening of water.
A) Lime soda process
B) Zeolite process
C) Ion-exchange process
A. LIME SODA PROCESS: The Process involves through mixing of the calculated quantities of hydrated lime
[Ca (OH)2] and Soda ash (Na2 Co3) with the hard water.
The soluble Calcium and Magnesium salts in water will be converted into insoluble CaCO3 and Mg (OH)2 and are filtered off.
The chemistry of the method is indicated by the following reactions.
For Calcium and Magnesium bicarbonates, only lime is required.
Ca (HCO3)2 + Ca (OH)2 -------Ã 2CaCO3 + 2H2O
Mg(HCO3)2 + 2Ca(OH)2 ------Ã 2CaCO3 + Mg(OH)2 + 2H2O
Mg(HCO3)2 + Ca(OH)2 ------Ã MgCO3 + CaCO3 + 2H2O
MgCO3 + Ca(OH)2 ------Ã Mg(OH)2 + CaCO3
If CaSO4 and CaCl2 are present only soda is required.CaSO4 + Na2CO3 -------Ã CaCO3 + Na2SO4
CaCl2 + Na2 CO3 ------Ã CaCO3 + 2 NaCl
If MgSO4 and MgCl2 are present, Lime and Soda both are required.MgSO4 + Na2CO3 + Ca (OH)2 -------Ã Mg(OH)2 + CaCO3 + Na2SO4
MgSO4 + Na2CO3 -----Ã MgCO3 + Na2SO4
MgCO3 + Ca(OH)2 ----Ã Mg (OH)2 + CaCO3
MgCl2 + Na2CO3 + Ca(OH)2 ------Ã Mg (OH)2 + CaCO3 + 2 NaCl
MgCl2 + Na2 CO3 -------Ã MgCO3 + 2 NaCl
MgCO3 + Ca(OH)2 -----Ã Mg (OH)2 + CaCO3
1.Cold Lime - Soda Process:Calculated amount of Lime, Soda and alum (coagulant) at room temperature are mixed with water and fed into a chamber. Stirred vigorously by rotating paddles. Hardness causing ions will be converted into precipitates. The gelatinous precipitate of Al(OH)3 formed from alum entraps the finely divided precipitates to form sludge and settles down. 50 % hardness is removed in this process.
(2). Hot Soda lime process:
This process involves in treating water with softening chemicals at a temperature of 80 - 150 deg.centigrade. Since hot process is operated at a temperature dose to the B.P. of the solution the reaction proceeds faster, i.e. softening reactions are completed within 15min. The precipitate & sludge formed settle rapidly and hence no co agents are needed much of the dissolved gases such as C02 & air are driven out of the water, viscosity of the water is lowered & hence filtration of such water becomes much easier. Hot soda lime process produces water of comparatively lower residual hardness of 15 - 30 ppm.
Hot lime soda process plant consists of 3 parts.
a) A reaction tank in which raw water chemicals & steam are through mixed.
b) A conical sediment vessel in which sludge settles down.
c) A sand filter, which ensures complete removal of sludge from the softened water.
B. ZEOLITE (PERMUTIT) PROCESS:
Principle: Zeolites are naturally occurring Sodium alumino silicates which when prepared artificially are called permutits.
Na2O • Al2O3 • 2SiO2. (or Na2 Al2 Si2 08)
It is also called Sodium zeolite and represented as Na2Z.
When Sodium Zeolite comes in contact with hard water, it exchanges its Na+ ions with the Ca+2 and Mg+2 ions to form insoluble Calcium and Magnesium Zeolites.
Na2Z + CaCI2 -------> CaZ + 2 NaCI
Na2Z + MgSO4 ------> MgZ + Na2SO4
Process : Na2Z is placed as a bed in a suitable container and hard water is allowed to pass through it. The hardness causing Ca+2 and Mg+2 ions are retained by the Zeolite as CaZ and MgZ while the outgoing water contains Sodium salts and free from hardness.
Regeneration of Zeolite :After some time the Na2Z is completely converted into Calcium and Magnesium Zeolites and it ceases to soften water. Now 10. % NaCl solution (brine) is passed through this bed and Na2Z is regenerated for further use. The washings containing CaCl2 and MgCl2 are drained out.
CaZ + 2 NaCI -> Na2Z + CaCI2
MgZ + 2 NaCI -> Na2Z + MgCI2
C. ION - EXCHANGE PROCESS: (Demineralization or De-ionisation) :
Principle: In Ion-exchange process, all the cations and anions present in hard water are removed by Ion-exchange resins (Acid resins and basic resins). The water obtained is almost similar to distilled water.
Ion-exchange resins are insoluble cross-linked, long chain organic polymers with microporous structure. The functional groups attached to the chain are responsible for the exchange of ions.
Acid resins containing acidic groups ( - COOH, SO3H, etc.) are capable of exchanging their H+ ions with all other cations, which come in their contact. These are called cation exchange resins, eg. Sulphonated polystyrene.
2 RH + Ca+2 ----> R2Ca + 2 H+2 RH + Mg+ 2 ----> R2 Mg + 2 H+
Basic resins containing basic functional groups (- OH--) are capable of exchanging their OH-- ions with other anions which come in their contact. These are called anion exchange resins, eg. Amine formaldehyde co polymer.
R-OH + HC0--3 ----> RHCO3 + OH--
2R-OH+ SO4--2 ----> R2SO4 + 2 OH--
Process:
The hard water is first passed through acid resin bed, which removes all the cations like Ca+2, Mg+2, Na+ etc. from it, and equivalent amount of H+ ions are released from this column to water.Then this water is passed through basic resin bed which removes all the anions like HCO--3 ,Cl-,SO4--2 etc. Present in the water and equivalent amount of OH-- ions are released.H+ and OH- released from acidic and basic resins respectively combine to form water molecule.
H+ + OH------> H20
Regeneration of Resins:The exhausted acidic and basic resins are regenerated by passing dil. HCl solution and dil. NaOH solution respectively through their beds.
R2 Ca + 2HCl ------Ã 2RH + CaCl2
R2SO4 + 2 NaOH ----Ã 2 ROH + Na2SO4
(Generally both types of resins are mixed together and hard water is passed through it. The resulting water may be of high purity.)
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