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CHEMISTRY
Second semester
KîshØr PåshÅ
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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. nature of the metal 2. temperature
3. concentration of elecrolyte 4. electrode potential
5. aeration 6. agitation
7. 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:
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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.
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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 (শূন্যতা) 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:
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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 O2 and an aqueous
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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.
Fe(s) Fe2+ (aq) + 2e (anodic)
O2 (g) + 2H2O + 4e 4OH- (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
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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 two-
1. Direct loss
a. Replacement of corroded equipment
b. Preventive maintenance – like painting
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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
Corrosion cell:
Corrosion cell is an electrochemical cell there cathodic and anodic reactions take place.
Anodic reaction = Oxidation reaction
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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.
Oxidation: Fe Fe++ + 2e-
Reduction: Cu++ + 2e- Cu
Cu++ + 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
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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
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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) Raw material to rectification column
ii) 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
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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) Carbon dioxide be free from oxygen and hydrogen to avoid hazard of corrosion or explosion
ii) 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
iii) Preheating of ammonia is essential for better results
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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:
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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.
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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. Less costly & uniform in composition
2. Mixing time is less
3. Has good ageing properties
4. Free from scorching problems
5. Extrusion and calendaring takes little time
6. Fast curing
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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
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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
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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
