ME 1 2 Chemistry 2nd Semester (1)
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
CHEMISTRY
Second semester
KîshØr PåshÅ
122076
1|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Chemical Corrosion:
metal converts to its Oxides when they exposed into a reactive gas
Electro-chemical Corrosion : when metal
emerged in conducting liquid
Factors Influence Corrosion:
1.
2.
3.
4.
5.
6.
7.
nature of the metal
temperature
concentration of elecrolyte
electrode potential
aeration
agitation
hydrogen over volteage & pH of the electrolyte
Eight Forms of Corrosion:
Uniform attack:
Uniform attack is the most common form of corrosion. It is normally
characterized by a chemical or electrochemical reaction that proceeds
uniformly over the entired exposed surface or over a large area. The metal
becomes thinner and eventually fails.
Example : A piece of steel or zinc immersed in dilute sulfuric acid
will normally dissolve at a uniform rate over its entire surface.
Galvanic / Two-metal corrossion:
A potential difference is usually exists between two dissimilar metals when
they are immereged in a corrosive or conductive solution. If these metals
are placed in contact this potential difference produces electron flow
between them. Corrosion of the less corrosion-resistance metal is usually
increased and attack of the more resistance material is decreased, as
compared with the behavior of these metals when they are not in contact.
2|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
The less resistance metal becomes anodic and the more resistance metal
becomes cathodic. Usually the cathode / cathodic metal corrodes very
little or not at all in this type of corrosion. This type of corrosion is
called galvanic corrosion or two-metal corrosion. It’s an electrochemical
corrosion.
Crevice corrosion:
Intensive localized corrosion frequently occurs within crevices (ফাটল) and
other shielded areas on metal surfaces exposed to corrosives. This type
attack is usually associated with small volume of stagnant (বদ্ধ) solution
caused by holes, gasket surfaces, lap joints, surface deposits and crevice
under bolt or rivet heads. As a result, this form of corrosion is called
crevice corrosion or diposit / gasket corrosion.
Filiform corrosion:
It is a special type of crevice corrosion. In most instances it occurs under
protective films and for that reason it is often referred to as underfilm
corrosion.
Example: The attack of enameled surfaces of food or beverage
cans that have been exposed to the atmosphere.
Pitting Corrosion:
Pitting is a form of extremely localized attack that results in holes in the
metal. Those holes may be small or large in diameter – but in most cases
they are relatively small. Pits are sometimes isolated or so close together
so that they look just like a rough surface.
Generally a pit may be described as a cavity (গহ্বর) or hole with the surface
diameter about the same as or the less than the depth.
Pitting is one of the most destructive and insidious forms of corrosion. It
causes equipment to fail because of perforation (ছিদ্র করা) with only a small
percent of weight loss of the entire structure. It is often difficult to
detect pits because of their small size and because the pits are often
covered with corrosion products. In addition it is difficult to measure
3|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
quantitatively and compare the extent of pitting because of varying depths
and number of pits.
Intergranular corrosion:
Localized attack at and adjacent to grain boundaries with relatively little
corrosion of the grains, is Intergranular Corrossion. The alloy disintegrates
(grains fall out) and/or loses its strength.
If a metal corrodes – uniform attack results since the grain boundaries are
usually only slightly more reactive than the matrix.
Intergranular corrosion
can be caused by
Impurities at the grain boundaries.
Enrichment of one of the alloying elements
Depletion (শূ ন্যতা) of one of those elements in the grain boundary
areas
Erosion corrosion:
It is the acceleration or increase in rate of deterioration or attack on a
metal because of relative movement of a corrosive fluid and the metal
surface.
Generally, this movement is quite rapid and mechanical wear effects or
abrasion (ঘর্ষণ) are involved. Metal is removed from the surface as dissolved
ions or it forms solid corrosion products that are mechanically swept from
the metal surface. Sometimes movement of the environment decreased this type
of corrosion.
Erosion corrosion is characterized in appearance by grooves, gullies, waves,
rounded holes and valleys and usually exhibits a directional pattern.
Stress corrosion:
Stress corrosion cracking (SCC) refers to cracking caused by the simultaneous
presence of tensile stress and a specific corrosive medium.
4|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
The stresses may be internal such as those caused by cold work, welding, and
heat treatment or external forces caused by mechanical stresses set up by
assembly practices. A good example of this form of corrosion is 316 stainless
steel in marine environments. 316 stainless steel was developed to withstand
attacks in chloride environments, but if stressed the steel will fail by
stress corrosion cracking.
Electrochemical Theory of Corrosion:
Electrochemistry is the branch of chemistry dealing with relationships
between electricity and chemical reactions. It involves oxidation and
reduction reactions.
Corrosion is an example of a type of electrochemical reaction. In the natural
environment – oxygen gas is a good oxidizing agent. Most metals has lower
reduction potentials than O2 . Therefore they are easily oxidized in the
presence of oxygen.
[Metals such as gold, silver and platinum are not so easily oxidized and are
sometimes referred to as noble metals. The reason for lack of oxidation in
these noble metals are varied and sometimes complex.]
Metal works as Cathode in presence of O2
Metal works as Anode in absence of O2
Rusting – Electrochemical Theory of Corrosion:
Iron metal is spontaneously oxidized in the presence of O 2 and an aqueous
electrolyte solution.
5|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Physical strains (scratches, dents, bends etc) present on the iron are more
easily oxidized than other areas. This directly relates to physics, i.e., the
way electric fields are generated at the surface of the metal. Stronger
fields are generated at the physically strained parts of the metal. The
result is that these regions are anodic (oxidation occurs) and simultaneously
different areas are cathodic regions at which a reduction reaction (usually
of O2 ) occurs.
Fe2+ (aq) + 2e
Fe(s)
4OH-
O2 (g) + 2H2O + 4e
(anodic)
(cathodic)
These two half reactions together give the overall reaction:
Fe(s) + ½ O2(g) + H2O(l)
Fe2+(aq) + 2OH-(aq)
Common experience with this process (e.g., car fenders) tends to show that
Fe2+ is eventually oxidized further to Fe3+, in the compound iron (III) oxide
(rust):
4Fe2+(aq) + O2(g) + 4H2O(l)
2Fe2O3(s, red colour) + 8H+(aq)
In case of pure metal
If there is any strain – that part will act as anode
Rest of the parts will act as cathode
Explanation: Strong electric field is created around the strain and that
makes the strain part anode.
6|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Speed of Reaction
1. Area effects are important, especially in galvanic and localized
corrosion. Consider the difference between a cell with a very large
anode area compared to the cathode and the opposite.
Since metal is corroded at the anode – the rate of corrosion will be
proportional to the rate at which the anodic reaction proceeds.
For anodic reaction to proceed, however, there must be corresponding
cathodic reactions
The cathodic reaction therefore controls the rate of the overall
reaction
With a large cathode and a small anode there is more surface area on
which cathodic reactions may proceed. So, anodic reaction proceeds at
much faster rate than the reverse (i.e., large anode, small cathode)
2. Pure metal’s corrosion rate is way much lower than impure metal.
Impure metal / dissimilar metal’s corrosion is high .
So, where use of dissimilar metal is unavoidable – it is desirable to
use the more noble (cathodic) metal in the smallest possible exposed
area relative to the anode.
Economic losses
Economic losses are divided into two1. Direct loss
a. Replacement of corroded equipment
b. Preventive maintenance – like painting
c. Inability to use otherwise desirable materials
7|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
d. Damage of equipment adjacent to that in which corrosion failure
occurs
2. Indirect loss
a. Shutdown of equipment due to corrosion failure
b. Contamination of a product
c. Loss of valuable product
d. Loss of efficiency
e. Overdesign to a allow a corrosion
Cells
Primary Cell : Directly products electricity . Non-conducting liquid/gaseous
Anode – negative ; Cathode – positive
Secondary Cell : Stored . Conducting liquid . External Source
Anode – positive ; Cathode – negative
Anodic reaction = Oxidation
reaction
Cathodic reaction = Reduction
reaction
Corrosion cell:
Corrosion cell is an electrochemical cell there cathodic and anodic reactions
take place.
8|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Three types of corrosion cells:
1. Dissimilar electrode cell:
Cell with different metal of electrode such as Zn and Cu in an
electrode form is known as a dissimilar electrode cell.
Fe++ + 2eCu
Oxidation: Fe
Reduction: Cu++ + 2eCu++ + Fe
Fe++ + Cu
These cells also include cold worked metal in contact with the same metal
annealed, grain boundary metal in contact with grains and a single metal
crystal of definite orientation in contact with other crystal od different
orientation.
2. Concentration cell:
These are having two identical electrodes each in contact with a
solution of different composition.
There are two types of cell –
a. Salt concentration cell: The cell with two identical electrodes
each in contact with a solution of different concentration is
known as salt concentration cell.
The electrochemical theory of corrosion has the conditions i)
An electric source and an electron consumer
ii)
A potential difference between source and consumer
iii) A continuous conductive pattern to flow electron from
the source to consumer
The electrode is contact with dilute solution known as
anode
The electrode is contact with dilute concentration solution
is known as cathode
b. Differential concentration cell: Aerated electrode is cathode &
deaerated electrode is anode
Both dilute solutions
9|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
3. Differential Temperature cell : same metal electrode with a different
temperature
Inhibitors:
An inhibitor is a chemical substance which when added in small concentration
to add environment effectively decreased the corrosion rate.
Chromates, silicates and organic ammines are common inhibitors.
In case of organic amines – inhibitors are adsorbed on anodic and cathodic
sites and stifle the corrosion current.
Other inhibitors specifically affect on the cathodic or anodic.
The effectiveness of the action of an inhibitor is often expressed as
inhibitor effect (Z) which represents the ratio of the metal dissolution.
Metal dissolution rate in an uninhabited corrosion medium (S1) to the
dissolution rate of the same metal under same condition but in inhabited
corrosion medium (S)
Z = S1/S
Classifications:
1. Anodic inhibitors: Al & Al-alloys ; silicon can be used
2. Cathodic inhibitors: Reduce the surface area of cathode
a. Cathodic inhibitors that absorbs oxygen ex: H2N-NH2, sodium
sulphide
Na2SO3 + ½ O2 = Na2SO4
NH2-H2N + O2 = 2H2O + N2
10 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
b. Cathodic inhibitors that reduce the area of cathode ; ex: ZnSO4,
Ca(HCO3)2
Ca(HCO3)2 + NaOH = CaCO3 + NaHCO3 + H2O
c. Cathodic inhibitors that increase the over potential of the
cathodic process
3. Organic inhibitors: Amines and their salt
4. Vapor phase inhibitors: gaseous phase with high vapor pressure ; also
an organic inhibitor – metallic surface adsorbs it
Ex: Morpholine
Thermal Cracking
Decomposition or pyrolysis of higher hydrocarbon into lower hydrocarbons at
high temperature.
Thermal Cracking Plants:
3 elements – Furnaces, Hot pumps & Evaporator
Process:
i)
ii)
Raw material to rectification column
There raw materials mixed with heavy fraction of cracking products
to TUBE FURNACE
iii) Cracking takes place at 470-480
iv)
Vapor and liquid mixture formed in the partial cracking flows into
reaction chamber for the completion of cracking process at 500°C and
a pressure of 0.2-0.25 Mpa
v)
The heavy cracking residue is separated in the EVAPORATOR
vi)
The vaporous products flow consecutively through two rectifications
column
vii) In the lower part of a column the solar fraction (gas oil) is
stripped and delivered into the tube furnace for cracking at 510-530
viii) The gas oil fed into the reaction chamber and the cycle is repeated
11 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Sulphur compounds:
These compounds are decomposed during cracking and H2S gas is liberated
.
C4H9SH >> C4H8 + H2S
Cyclic Sulphur compounds such as thiophane and thiophene – very stable
against decomposition. H2S and S [formed by oxidation of H2S] which can be
formed in a cracking of sulphurous and high sulphurous petroleum grades may
cause serious corrosion of the process equipment.
Process: ?
Urea:
Industrial production:
Two stages:
1. Formation of ammonium carbamate
2NH3 + CO2 >>> NH2CO-NH4 + 159.1 KJ
2. NH2COONH4 >>> (NH2)2CO + H2O – 285kj
Conditions for good yield:
i)
ii)
iii)
iv)
12 | P a g e
Carbon dioxide be free from oxygen and hydrogen to avoid hazard of
corrosion or explosion
NH3-CO2 ratio varies widely, ranging from about 10% excess NH3 over
the stoichiometric amount to 100% or more. The larger excess gives
better result
Preheating of ammonia is essential for better results
Reaction Temperature should be as high as possible, but must be
limited because of corrosion. The maximum temperature that can be
attend in stainless steel lined reactor s is about 380 F
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
v)
The average pressure is about 2700 psi – but the pressure as low as
2400 and as high as 6000 psig have been reported
Rubber:
Vulcanization:
Vulcanization (or vulcanisation) is a chemical process for converting natural
rubber or related polymers into more durable materials via the addition of
sulfur or other equivalent curatives or accelerators. These additives modify
the polymer by forming cross-links (bridges) between individual polymer
chains. Vulcanized materials are less sticky and have superior mechanical
properties.
Although the curing of rubber has been carried out since prehistoric times,
the modern process of vulcanization, named after Vulcan, the Roman god of
fire, was not developed until the 19th century. Today, a vast array of
products is made with vulcanized rubber including tires, shoe soles, hoses,
and conveyor belts. Hard vulcanized rubber is sometimes sold under the brand
names ebonite or vulcanite, and is used to make articles such as clarinet and
saxophone mouth pieces, bowling balls and hockey pucks.
Vulcanization depends upon i) the amount of sulphur used; by increasing the
amount of sulphur the rubber can be hardened ii) Temperature, iii) Duration
of heating
Five types of curing systems are in common use. They are:
1. Sulfur systems.
13 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
2. Peroxides
3. Urethane cross linkers
4. Metallic oxides
5. Acetoxysilane
Vulcanization with sulfur:
By far the most common vulcanizing methods depend on sulfur. Sulfur, by
itself, is a slow vulcanizing agent and does not vulcanize synthetic
polyolefins. Even with natural rubber, large amounts of sulfur, as well as
high temperatures and long heating periods are necessary and one obtains an
unsatisfactory crosslinking efficiency with unsatisfactory strength and aging
properties. Only with vulcanization accelerators can the quality
corresponding to today's level of technology be achieved. The multiplicity of
vulcanization effects demanded cannot be achieved with one universal
substance; a large number of diverse additives, comprising the "cure
package," are necessary.
The combined cure package in a typical rubber compound consists of sulfur
together with an assortment of compounds that modify the kinetics of
crosslinking and stabilize the final product. These additives include
accelerators, activators like zinc oxide and stearic acid and antidegradants.
The accelerators and activators are catalysts. An additional level of control
is achieved by retarding agents that inhibit vulcanization until some optimal
time or temperature. Antidegradants are used to prevent degradation of the
vulcanized product by heat, oxygen and ozone.
Reclaimed rubber:
Reclaimed rubber is the product obtained from miscellaneous waste rubber
articles like worn out tyres, tubes, gaskets, hoses, foot wear etc. which are
heated and treated with chemical.
By reclaimed is meant a chemical treatment (depolymerisation) by which a
waste rubber product gives back its rubber content through separation of
other materials such as fibres, but combined sulphur is not removed
14 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Process:
1. The miscellaneous waste rubber articles such as tyres, tubes, scrap
etc. are cut to small pieces and ground to particles of fine dimensions
in a cracker, which exert powerful grinding and tearing action. The
ground scrap is fed into fast moving screens which separate the fine
particles and divert the large scrap pieces back to the cracker for
further grinding to fine powder.
2. Finely, ground scrap is then passed under a magnetic separator for
removing ferrous impurities.
3. The purified waste powdered rubber is then digested in a steam
jacketed digester fitted with agitation blades, with caustic soda
solution containing chlorides of zinc and calcium at about 200 under a
pressure of 200lbs per sq. inch. For 8-15 hours – depending upon raw
material composition.
4. By this process fibres are hydrolysed and rubber becomes devulcanised.
5. After the removal of fibres, reclaiming agents such as petroleum and
coal, tar oils and softeners are added.
6. Sulphur is removed as sodium sulphide and polysulphide and so rubber
becomes devulcanized
7. After digestion the charge is forced into a blow down tank where the
cooked up or digested rubber is washed and on emerging meets a hot
blast of air to get dried to requisistewater content.
8. Finally the dried rubber is mixed up with processing and reinforcing
agents such as clay, carbon black etc. and softeners in small
proportions in BANBURY MIXTURE and forced through hot rolls which shape
and extrude the rubber in the form of a continuous sheet to be cut at
regular lengths at regular intervals.
Advantages:
1.
2.
3.
4.
5.
6.
7.
Less costly & uniform in composition
Mixing time is less
Has good ageing properties
Free from scorching problems
Extrusion and calendaring takes little time
Fast curing
Less thermoplastic
15 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
COD (Chemical Oxygen Demand):
In environmental chemistry, the chemical oxygen demand test is commonly used
to indirectly measure the amount of organic compounds in water.
BOD(Biochemical Oxygen Demand):
Biochemical oxygen demand or B.O.D is the amount of dissolved oxygen needed
by aerobic biological organisms in a body of water to break down organic
material present in a given water sample at certain temperature over a
specific time period.
Viscosity index:
Viscosity index (VI) is an arbitrary measure for the change of viscosity with
variations in temperature. It is used to characterize viscosity changes with
relation to temperature in lubricating oil.
The viscosity of liquids decreases as temperature increases. The viscosity of
a lubricant is closely related to its ability to reduce friction. Generally,
the least viscous lubricant which still forces the two moving surfaces apart
is desired. If the lubricant is too viscous, it will require a large amount
of energy to move (as in honey); if it is too thin, the surfaces will come in
contact and friction will increase.
Many lubricant applications require the lubricant to perform across a wide
range of conditions, for example, automotive lubricants are required to
reduce friction between engine components when the engine is started from
cold (relative to the engine's operating temperatures) up to 200 °C or 392 °F
when it is running. The best oils with the highest VI will remain stable and
not vary much in viscosity over the temperature range. This allows for
consistent engine performance within the normal working conditions.
16 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
The VI scale was set up by the Society of Automotive Engineers (SAE). The
temperatures chosen arbitrarily for reference are 100 and 210 °F (38 and 99
°C). The original scale only stretched between VI=0 (lowest VI oil,
naphthenic) and VI=100 (best oil, paraffinnic) but since the conception of
the scale better oils have also been produced, leading to VIs greater than
100.
Classification
-35 - Low
35 - 80 - Medium
80 - 110 - High
110+ - Very High
V = 100
(L-U)/ (L-H)
where V indicates the viscosity index, U the kinematic viscosity at 40 °C
(104 °F), and L & H are various values based on the kinematic viscosity at
100 °C (212 °F) available in ASTM D2270
Pigments in Paint:
White: White lead, titanium dioxide, zinc oxide, lithopone
Red: Read lead, iron oxides, cadmium reds, rogue
Blue: Ultramarine, cobalt blues, iron blues
17 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Green: Chromium oxides, chrome green, phthalocyanine green etc.
Yellow: Litharge, lead/zinc chromates, ochre
Black: Carbon black, lamp black, furnace black
Orange: Basic lead chromate, cadmium orange
Brown: Burnt umber, burnt sienna etc.
Metallics: Copper powder, zinc dust, aluminums
Metal protective: Red lead, blue lead, zinc and basic lead
18 | P a g e
Roll: 122076
Section B
CHEMISTRY
Second semester
KîshØr PåshÅ
122076
1|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Chemical Corrosion:
metal converts to its Oxides when they exposed into a reactive gas
Electro-chemical Corrosion : when metal
emerged in conducting liquid
Factors Influence Corrosion:
1.
2.
3.
4.
5.
6.
7.
nature of the metal
temperature
concentration of elecrolyte
electrode potential
aeration
agitation
hydrogen over volteage & pH of the electrolyte
Eight Forms of Corrosion:
Uniform attack:
Uniform attack is the most common form of corrosion. It is normally
characterized by a chemical or electrochemical reaction that proceeds
uniformly over the entired exposed surface or over a large area. The metal
becomes thinner and eventually fails.
Example : A piece of steel or zinc immersed in dilute sulfuric acid
will normally dissolve at a uniform rate over its entire surface.
Galvanic / Two-metal corrossion:
A potential difference is usually exists between two dissimilar metals when
they are immereged in a corrosive or conductive solution. If these metals
are placed in contact this potential difference produces electron flow
between them. Corrosion of the less corrosion-resistance metal is usually
increased and attack of the more resistance material is decreased, as
compared with the behavior of these metals when they are not in contact.
2|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
The less resistance metal becomes anodic and the more resistance metal
becomes cathodic. Usually the cathode / cathodic metal corrodes very
little or not at all in this type of corrosion. This type of corrosion is
called galvanic corrosion or two-metal corrosion. It’s an electrochemical
corrosion.
Crevice corrosion:
Intensive localized corrosion frequently occurs within crevices (ফাটল) and
other shielded areas on metal surfaces exposed to corrosives. This type
attack is usually associated with small volume of stagnant (বদ্ধ) solution
caused by holes, gasket surfaces, lap joints, surface deposits and crevice
under bolt or rivet heads. As a result, this form of corrosion is called
crevice corrosion or diposit / gasket corrosion.
Filiform corrosion:
It is a special type of crevice corrosion. In most instances it occurs under
protective films and for that reason it is often referred to as underfilm
corrosion.
Example: The attack of enameled surfaces of food or beverage
cans that have been exposed to the atmosphere.
Pitting Corrosion:
Pitting is a form of extremely localized attack that results in holes in the
metal. Those holes may be small or large in diameter – but in most cases
they are relatively small. Pits are sometimes isolated or so close together
so that they look just like a rough surface.
Generally a pit may be described as a cavity (গহ্বর) or hole with the surface
diameter about the same as or the less than the depth.
Pitting is one of the most destructive and insidious forms of corrosion. It
causes equipment to fail because of perforation (ছিদ্র করা) with only a small
percent of weight loss of the entire structure. It is often difficult to
detect pits because of their small size and because the pits are often
covered with corrosion products. In addition it is difficult to measure
3|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
quantitatively and compare the extent of pitting because of varying depths
and number of pits.
Intergranular corrosion:
Localized attack at and adjacent to grain boundaries with relatively little
corrosion of the grains, is Intergranular Corrossion. The alloy disintegrates
(grains fall out) and/or loses its strength.
If a metal corrodes – uniform attack results since the grain boundaries are
usually only slightly more reactive than the matrix.
Intergranular corrosion
can be caused by
Impurities at the grain boundaries.
Enrichment of one of the alloying elements
Depletion (শূ ন্যতা) of one of those elements in the grain boundary
areas
Erosion corrosion:
It is the acceleration or increase in rate of deterioration or attack on a
metal because of relative movement of a corrosive fluid and the metal
surface.
Generally, this movement is quite rapid and mechanical wear effects or
abrasion (ঘর্ষণ) are involved. Metal is removed from the surface as dissolved
ions or it forms solid corrosion products that are mechanically swept from
the metal surface. Sometimes movement of the environment decreased this type
of corrosion.
Erosion corrosion is characterized in appearance by grooves, gullies, waves,
rounded holes and valleys and usually exhibits a directional pattern.
Stress corrosion:
Stress corrosion cracking (SCC) refers to cracking caused by the simultaneous
presence of tensile stress and a specific corrosive medium.
4|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
The stresses may be internal such as those caused by cold work, welding, and
heat treatment or external forces caused by mechanical stresses set up by
assembly practices. A good example of this form of corrosion is 316 stainless
steel in marine environments. 316 stainless steel was developed to withstand
attacks in chloride environments, but if stressed the steel will fail by
stress corrosion cracking.
Electrochemical Theory of Corrosion:
Electrochemistry is the branch of chemistry dealing with relationships
between electricity and chemical reactions. It involves oxidation and
reduction reactions.
Corrosion is an example of a type of electrochemical reaction. In the natural
environment – oxygen gas is a good oxidizing agent. Most metals has lower
reduction potentials than O2 . Therefore they are easily oxidized in the
presence of oxygen.
[Metals such as gold, silver and platinum are not so easily oxidized and are
sometimes referred to as noble metals. The reason for lack of oxidation in
these noble metals are varied and sometimes complex.]
Metal works as Cathode in presence of O2
Metal works as Anode in absence of O2
Rusting – Electrochemical Theory of Corrosion:
Iron metal is spontaneously oxidized in the presence of O 2 and an aqueous
electrolyte solution.
5|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Physical strains (scratches, dents, bends etc) present on the iron are more
easily oxidized than other areas. This directly relates to physics, i.e., the
way electric fields are generated at the surface of the metal. Stronger
fields are generated at the physically strained parts of the metal. The
result is that these regions are anodic (oxidation occurs) and simultaneously
different areas are cathodic regions at which a reduction reaction (usually
of O2 ) occurs.
Fe2+ (aq) + 2e
Fe(s)
4OH-
O2 (g) + 2H2O + 4e
(anodic)
(cathodic)
These two half reactions together give the overall reaction:
Fe(s) + ½ O2(g) + H2O(l)
Fe2+(aq) + 2OH-(aq)
Common experience with this process (e.g., car fenders) tends to show that
Fe2+ is eventually oxidized further to Fe3+, in the compound iron (III) oxide
(rust):
4Fe2+(aq) + O2(g) + 4H2O(l)
2Fe2O3(s, red colour) + 8H+(aq)
In case of pure metal
If there is any strain – that part will act as anode
Rest of the parts will act as cathode
Explanation: Strong electric field is created around the strain and that
makes the strain part anode.
6|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Speed of Reaction
1. Area effects are important, especially in galvanic and localized
corrosion. Consider the difference between a cell with a very large
anode area compared to the cathode and the opposite.
Since metal is corroded at the anode – the rate of corrosion will be
proportional to the rate at which the anodic reaction proceeds.
For anodic reaction to proceed, however, there must be corresponding
cathodic reactions
The cathodic reaction therefore controls the rate of the overall
reaction
With a large cathode and a small anode there is more surface area on
which cathodic reactions may proceed. So, anodic reaction proceeds at
much faster rate than the reverse (i.e., large anode, small cathode)
2. Pure metal’s corrosion rate is way much lower than impure metal.
Impure metal / dissimilar metal’s corrosion is high .
So, where use of dissimilar metal is unavoidable – it is desirable to
use the more noble (cathodic) metal in the smallest possible exposed
area relative to the anode.
Economic losses
Economic losses are divided into two1. Direct loss
a. Replacement of corroded equipment
b. Preventive maintenance – like painting
c. Inability to use otherwise desirable materials
7|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
d. Damage of equipment adjacent to that in which corrosion failure
occurs
2. Indirect loss
a. Shutdown of equipment due to corrosion failure
b. Contamination of a product
c. Loss of valuable product
d. Loss of efficiency
e. Overdesign to a allow a corrosion
Cells
Primary Cell : Directly products electricity . Non-conducting liquid/gaseous
Anode – negative ; Cathode – positive
Secondary Cell : Stored . Conducting liquid . External Source
Anode – positive ; Cathode – negative
Anodic reaction = Oxidation
reaction
Cathodic reaction = Reduction
reaction
Corrosion cell:
Corrosion cell is an electrochemical cell there cathodic and anodic reactions
take place.
8|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Three types of corrosion cells:
1. Dissimilar electrode cell:
Cell with different metal of electrode such as Zn and Cu in an
electrode form is known as a dissimilar electrode cell.
Fe++ + 2eCu
Oxidation: Fe
Reduction: Cu++ + 2eCu++ + Fe
Fe++ + Cu
These cells also include cold worked metal in contact with the same metal
annealed, grain boundary metal in contact with grains and a single metal
crystal of definite orientation in contact with other crystal od different
orientation.
2. Concentration cell:
These are having two identical electrodes each in contact with a
solution of different composition.
There are two types of cell –
a. Salt concentration cell: The cell with two identical electrodes
each in contact with a solution of different concentration is
known as salt concentration cell.
The electrochemical theory of corrosion has the conditions i)
An electric source and an electron consumer
ii)
A potential difference between source and consumer
iii) A continuous conductive pattern to flow electron from
the source to consumer
The electrode is contact with dilute solution known as
anode
The electrode is contact with dilute concentration solution
is known as cathode
b. Differential concentration cell: Aerated electrode is cathode &
deaerated electrode is anode
Both dilute solutions
9|Page
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
3. Differential Temperature cell : same metal electrode with a different
temperature
Inhibitors:
An inhibitor is a chemical substance which when added in small concentration
to add environment effectively decreased the corrosion rate.
Chromates, silicates and organic ammines are common inhibitors.
In case of organic amines – inhibitors are adsorbed on anodic and cathodic
sites and stifle the corrosion current.
Other inhibitors specifically affect on the cathodic or anodic.
The effectiveness of the action of an inhibitor is often expressed as
inhibitor effect (Z) which represents the ratio of the metal dissolution.
Metal dissolution rate in an uninhabited corrosion medium (S1) to the
dissolution rate of the same metal under same condition but in inhabited
corrosion medium (S)
Z = S1/S
Classifications:
1. Anodic inhibitors: Al & Al-alloys ; silicon can be used
2. Cathodic inhibitors: Reduce the surface area of cathode
a. Cathodic inhibitors that absorbs oxygen ex: H2N-NH2, sodium
sulphide
Na2SO3 + ½ O2 = Na2SO4
NH2-H2N + O2 = 2H2O + N2
10 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
b. Cathodic inhibitors that reduce the area of cathode ; ex: ZnSO4,
Ca(HCO3)2
Ca(HCO3)2 + NaOH = CaCO3 + NaHCO3 + H2O
c. Cathodic inhibitors that increase the over potential of the
cathodic process
3. Organic inhibitors: Amines and their salt
4. Vapor phase inhibitors: gaseous phase with high vapor pressure ; also
an organic inhibitor – metallic surface adsorbs it
Ex: Morpholine
Thermal Cracking
Decomposition or pyrolysis of higher hydrocarbon into lower hydrocarbons at
high temperature.
Thermal Cracking Plants:
3 elements – Furnaces, Hot pumps & Evaporator
Process:
i)
ii)
Raw material to rectification column
There raw materials mixed with heavy fraction of cracking products
to TUBE FURNACE
iii) Cracking takes place at 470-480
iv)
Vapor and liquid mixture formed in the partial cracking flows into
reaction chamber for the completion of cracking process at 500°C and
a pressure of 0.2-0.25 Mpa
v)
The heavy cracking residue is separated in the EVAPORATOR
vi)
The vaporous products flow consecutively through two rectifications
column
vii) In the lower part of a column the solar fraction (gas oil) is
stripped and delivered into the tube furnace for cracking at 510-530
viii) The gas oil fed into the reaction chamber and the cycle is repeated
11 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Sulphur compounds:
These compounds are decomposed during cracking and H2S gas is liberated
.
C4H9SH >> C4H8 + H2S
Cyclic Sulphur compounds such as thiophane and thiophene – very stable
against decomposition. H2S and S [formed by oxidation of H2S] which can be
formed in a cracking of sulphurous and high sulphurous petroleum grades may
cause serious corrosion of the process equipment.
Process: ?
Urea:
Industrial production:
Two stages:
1. Formation of ammonium carbamate
2NH3 + CO2 >>> NH2CO-NH4 + 159.1 KJ
2. NH2COONH4 >>> (NH2)2CO + H2O – 285kj
Conditions for good yield:
i)
ii)
iii)
iv)
12 | P a g e
Carbon dioxide be free from oxygen and hydrogen to avoid hazard of
corrosion or explosion
NH3-CO2 ratio varies widely, ranging from about 10% excess NH3 over
the stoichiometric amount to 100% or more. The larger excess gives
better result
Preheating of ammonia is essential for better results
Reaction Temperature should be as high as possible, but must be
limited because of corrosion. The maximum temperature that can be
attend in stainless steel lined reactor s is about 380 F
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
v)
The average pressure is about 2700 psi – but the pressure as low as
2400 and as high as 6000 psig have been reported
Rubber:
Vulcanization:
Vulcanization (or vulcanisation) is a chemical process for converting natural
rubber or related polymers into more durable materials via the addition of
sulfur or other equivalent curatives or accelerators. These additives modify
the polymer by forming cross-links (bridges) between individual polymer
chains. Vulcanized materials are less sticky and have superior mechanical
properties.
Although the curing of rubber has been carried out since prehistoric times,
the modern process of vulcanization, named after Vulcan, the Roman god of
fire, was not developed until the 19th century. Today, a vast array of
products is made with vulcanized rubber including tires, shoe soles, hoses,
and conveyor belts. Hard vulcanized rubber is sometimes sold under the brand
names ebonite or vulcanite, and is used to make articles such as clarinet and
saxophone mouth pieces, bowling balls and hockey pucks.
Vulcanization depends upon i) the amount of sulphur used; by increasing the
amount of sulphur the rubber can be hardened ii) Temperature, iii) Duration
of heating
Five types of curing systems are in common use. They are:
1. Sulfur systems.
13 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
2. Peroxides
3. Urethane cross linkers
4. Metallic oxides
5. Acetoxysilane
Vulcanization with sulfur:
By far the most common vulcanizing methods depend on sulfur. Sulfur, by
itself, is a slow vulcanizing agent and does not vulcanize synthetic
polyolefins. Even with natural rubber, large amounts of sulfur, as well as
high temperatures and long heating periods are necessary and one obtains an
unsatisfactory crosslinking efficiency with unsatisfactory strength and aging
properties. Only with vulcanization accelerators can the quality
corresponding to today's level of technology be achieved. The multiplicity of
vulcanization effects demanded cannot be achieved with one universal
substance; a large number of diverse additives, comprising the "cure
package," are necessary.
The combined cure package in a typical rubber compound consists of sulfur
together with an assortment of compounds that modify the kinetics of
crosslinking and stabilize the final product. These additives include
accelerators, activators like zinc oxide and stearic acid and antidegradants.
The accelerators and activators are catalysts. An additional level of control
is achieved by retarding agents that inhibit vulcanization until some optimal
time or temperature. Antidegradants are used to prevent degradation of the
vulcanized product by heat, oxygen and ozone.
Reclaimed rubber:
Reclaimed rubber is the product obtained from miscellaneous waste rubber
articles like worn out tyres, tubes, gaskets, hoses, foot wear etc. which are
heated and treated with chemical.
By reclaimed is meant a chemical treatment (depolymerisation) by which a
waste rubber product gives back its rubber content through separation of
other materials such as fibres, but combined sulphur is not removed
14 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Process:
1. The miscellaneous waste rubber articles such as tyres, tubes, scrap
etc. are cut to small pieces and ground to particles of fine dimensions
in a cracker, which exert powerful grinding and tearing action. The
ground scrap is fed into fast moving screens which separate the fine
particles and divert the large scrap pieces back to the cracker for
further grinding to fine powder.
2. Finely, ground scrap is then passed under a magnetic separator for
removing ferrous impurities.
3. The purified waste powdered rubber is then digested in a steam
jacketed digester fitted with agitation blades, with caustic soda
solution containing chlorides of zinc and calcium at about 200 under a
pressure of 200lbs per sq. inch. For 8-15 hours – depending upon raw
material composition.
4. By this process fibres are hydrolysed and rubber becomes devulcanised.
5. After the removal of fibres, reclaiming agents such as petroleum and
coal, tar oils and softeners are added.
6. Sulphur is removed as sodium sulphide and polysulphide and so rubber
becomes devulcanized
7. After digestion the charge is forced into a blow down tank where the
cooked up or digested rubber is washed and on emerging meets a hot
blast of air to get dried to requisistewater content.
8. Finally the dried rubber is mixed up with processing and reinforcing
agents such as clay, carbon black etc. and softeners in small
proportions in BANBURY MIXTURE and forced through hot rolls which shape
and extrude the rubber in the form of a continuous sheet to be cut at
regular lengths at regular intervals.
Advantages:
1.
2.
3.
4.
5.
6.
7.
Less costly & uniform in composition
Mixing time is less
Has good ageing properties
Free from scorching problems
Extrusion and calendaring takes little time
Fast curing
Less thermoplastic
15 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
COD (Chemical Oxygen Demand):
In environmental chemistry, the chemical oxygen demand test is commonly used
to indirectly measure the amount of organic compounds in water.
BOD(Biochemical Oxygen Demand):
Biochemical oxygen demand or B.O.D is the amount of dissolved oxygen needed
by aerobic biological organisms in a body of water to break down organic
material present in a given water sample at certain temperature over a
specific time period.
Viscosity index:
Viscosity index (VI) is an arbitrary measure for the change of viscosity with
variations in temperature. It is used to characterize viscosity changes with
relation to temperature in lubricating oil.
The viscosity of liquids decreases as temperature increases. The viscosity of
a lubricant is closely related to its ability to reduce friction. Generally,
the least viscous lubricant which still forces the two moving surfaces apart
is desired. If the lubricant is too viscous, it will require a large amount
of energy to move (as in honey); if it is too thin, the surfaces will come in
contact and friction will increase.
Many lubricant applications require the lubricant to perform across a wide
range of conditions, for example, automotive lubricants are required to
reduce friction between engine components when the engine is started from
cold (relative to the engine's operating temperatures) up to 200 °C or 392 °F
when it is running. The best oils with the highest VI will remain stable and
not vary much in viscosity over the temperature range. This allows for
consistent engine performance within the normal working conditions.
16 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
The VI scale was set up by the Society of Automotive Engineers (SAE). The
temperatures chosen arbitrarily for reference are 100 and 210 °F (38 and 99
°C). The original scale only stretched between VI=0 (lowest VI oil,
naphthenic) and VI=100 (best oil, paraffinnic) but since the conception of
the scale better oils have also been produced, leading to VIs greater than
100.
Classification
-35 - Low
35 - 80 - Medium
80 - 110 - High
110+ - Very High
V = 100
(L-U)/ (L-H)
where V indicates the viscosity index, U the kinematic viscosity at 40 °C
(104 °F), and L & H are various values based on the kinematic viscosity at
100 °C (212 °F) available in ASTM D2270
Pigments in Paint:
White: White lead, titanium dioxide, zinc oxide, lithopone
Red: Read lead, iron oxides, cadmium reds, rogue
Blue: Ultramarine, cobalt blues, iron blues
17 | P a g e
CHEMISTRY - Second semester - KîshØr PåshÅ
Roll: 122076
Section B
Green: Chromium oxides, chrome green, phthalocyanine green etc.
Yellow: Litharge, lead/zinc chromates, ochre
Black: Carbon black, lamp black, furnace black
Orange: Basic lead chromate, cadmium orange
Brown: Burnt umber, burnt sienna etc.
Metallics: Copper powder, zinc dust, aluminums
Metal protective: Red lead, blue lead, zinc and basic lead
18 | P a g e