My Profile
Active Members
TodayLast 7 Days
more...
Awards & Gifts
Online Exams
Fresher Jobs
Our fresher job section is exclusively for fresh graduates! Find jobs for freshers in major Indian
cities including Bangalore, Chennai, Hyderabad, Pune or Kochi
Resources
Find educational articles, blogs, discussion threads and other resources.
Colleges
Find details about any college in India or search for courses.
|
Resources » Articles/Knowledge Sharing » Education »
Vtu chemistry notes
|
CORROSION
It is a common experience that most of the metals (except least reactive like ,Gold, platinum) are slowly destructed on long exposure to atmosphere.
Eg: (i) Rusting of iron, when exposed atmospheric conditions, a layer of reddish scale and powder of oxide is (F3O4) is formed and iron becomes weak.
(ii) Formation of green film of basic carbonates (CuCO3 + Cu(OH)2) on the surface of cupper, when exposed to moist-air containing carbon dioxide.
Defination:
It is the destruction or deuteriotion and consequent loss of metals due to reaction with environment.
It can be classified as;
1) Dry corrosion(chemical corrosion) : This involves direct attack of metals by corrosive gases such as carbon dioxide, sulphur dioxide, ammonia, hydrogen chloride.etc present in the corrosion environment. Since this is is observed in the absence of moisture or conducting electrolyte, it is called dry corrosion.
e.g. If a metal is attacked by oxygen, then metallic oxide is the corrosion product.
M? Mn+ + ne-
½ O2 + 2e- ? O2-
Mn+n + O2- ? M2On
2) Wet corrosion(electrochemical corrosion):
If the corrosion of the metal takes place through the ionic reactions in the presence of moisture or electrolytes it is called wet corrosion. The presence of small amount of moisture can change the corrosion picture completely.
e.g. Dry chlorine is non corrosive to steel but moist chlorine leads to corrosion.
Theory of electrochemical corrosion: (Theory of rusting of iron)
According to this theory corrosion of the metal takes place through the formation of anodic or cathodic regions within the same metal surface in the presence of conducting medium.
Corrosion reactions:
Anodic reaction (i.e. at the anodic area) involves the dissolution of metal as corresponding metallic ions with the liberation of free electrons.
At anodic area: M ? Mn+ + ne- (oxidation)
Eg. When iron undergoes corrosion, Fe ? Fe2+ + 2e-
Cathodic reaction: Consumes electrons with either by (i) evolution of hydrogen or (ii) absorption of oxygen depending on the nature of corrosive environment.
(i) Evolution of hydrogen takes place in the absence of oxygen.
(a) In acidic medium and in the absence of oxygen, the cathodic reaction is,
2H+ + 2e- ? H2?
(b) In neutral or alkaline medium and in the absence of oxygen, hydroxide ions are formed with simultaneous liberation of hydrogen
2H2O + 2e- ? 2OH- + H2?
(ii) Absorption of oxygen takes place in the presence of oxygen
a) In acidic medium and in the presence of oxygen
2H+ + ½ O2 + 2e- ? H2O
(b) In neutral or alkaline medium and in the presence of oxygen, hydroxide ions are formed by the following reaction
H2O + ½ O2 + 2e- ? 2OH-
Corrosion of iron produces Fe2+ ions and OH- ions at the anode and cathode sites respectively. These ions diffuse towards each other. Since smaller Fe2+ ions diffuse more rapidly than OH- ions, their combination occurs more commonly near the cathodic region to produce insoluble Fe(OH)2.
Fe2+ + 2OH- ? Fe(OH)2
In an oxidizing environment, it is oxidized to ferric oxide and the yellow rust is hydrated ferric oxide.
2Fe(OH)2 + ½ O2 + H2O ? Fe2O3.3H2O.
In the presence of limited oxygen, ferrous hydroxide is converted into magnetic oxide of iron (F3O4) and is known as black rust.
3Fe(OH)2 + ½ O2 ? Fe3O4.3H2O.
Types of corrosion:
A. Galvanic corrosion(differential aeration corrosion):
1. This occurs when two dissimilar metals are in contact with each other in corrosive medium.
2. The potential difference between the two electrodes are driving force for corrosion.
3. The more active metal i.e metal with lower electrode potential(higher in electrochemical series) acts as anode hence undergoes corrosion. (e.g metals like Zn, Mg )
4. The less active metal i.e metal with higher electrode potential (lower in electrochemical series) acts as cathode.
5. The anodic metal undergoes corrosion and the cathodic metal generally unattacked.
6. e.g. 1. in steel pipe connected to copper plumbing,
2. in steel screws in copper plating.
3. Steel screws in brass marine hardware
4. Lead-antimony solder around copper wire
B. Differential aeration corrosion: (concentration cell corrosion)
1. This type of corrosion occurs when metal is exposed to different concentrations of air or oxygen.
2. This constitutes oxygen concentration cell,due to which corrosion occurs.
3. Portion of the metal which is less oxygenated acts as anode hence undergoes corrosion.
4. More oxygenated part acts as cathode hence it is unattacked.
e.g. In partially buried pipeline, the area below the soil is less oxygenated acts as anode hence undergoes corrosion. Where as exposed region is more oxygenated is free from corrosion as it acts as cathode.
5. At the anode (less O2 concentration), M ? Mn+ + ne
At the cathode (more O2 concentration), H2O + ½ O2 + 2e- ? 2OH-
Waterline corrosion:
1.It is a case of differential aeration corrosion.
2. This takes place due to the formation of oxygen concentration cell.
3. When water is stored in a metallic tank , it is observed that corrosion takes place along the line just below the level of water in the container. The area above water is more oxygenated , acts as cathode. Area below water is less oxygenated, acts as anode. Therefore area below Water is corroded and area above water is protected. Hence a distinct brown line is formed below the water line due to corrosion.
It is seen in steel water storage tanks, floating ships etc.
C. Pitting corrosion:
1. It is form of extremely localized attack, the rate of attack being greater at some places than the other resulting in holes or pits in the metal.
2. It is a destructive form of corrosion and results in the sudden failure of the equipments due to the formation of the pits.
3. Pitting corrosion is generally initiated by the presence of the impurities like sand , dust, scale or due to the breaking down of the protective films on the metal surface.
4. The area of the metal below the deposit is exposed to less oxygen concentration hence acts as anode. Where as the uncovered region acts as cathode. So the area below the deposit undergoes corrosion leading to small pinholes.
5. Because of small anodic and large cathodic area corrosion becomes faster leading to pits. Once the pit is formed, due to corrosion product formed, the rate corrosion increases, as it provides conditions for differential aeration corrosion and as a result pit grows.
D . Stress corrosion(cracking corrosion):
1.It is a cracking of a metal due to combined effect of i) stressed metallic areas ii) corrosive environment.
2. It is also highly localized and distructive form of corrosion.
3. During manufacture of the metallic articles, they are subjected to processes like, bending pressing rolling, quenching etc. Which causes stress on the certain areas of metal.
4. The stressed regions have higher energy compared to unstressed region. As a result an electrochemical cell is set up. The stressed region being more active acts as anode, and unstressed region acts as cathode.
5. In the presence of corrosive environment, the stressed part undergoes corrosion which ultimately propagates, leads to crack in the metal.
e.g. Caustic embrittlement
Caustic embrittlement
1.It is a form of stress corrosion that occurs in boilers at high pressure leading to the failure of the boiler.
2. This is initiated at stressed parts like, cracks, joints, bends etc.
3. In case of high pressure bouilers, excess of sodium carbonate is added during the process of removal
of hard water. As a result the the boiler feed water is slightly alkaline. Which at high pressure
decomposes to give sodium hydroxide.
Na2CO3 +H2O ? 2NaOH + CO2
4. This sodium hydroxide enters minute cracks in the boiler walls. Water evaporates and concentration of sodium hydroxide increases. The concentrated sodium hydroxide dissolves iron forming sodium ferrate(Na2FeO2).
5. Which decomposes forming magnetite as follows.
3Na2FeO2 + 4H2O ? Fe3O4 + 6NaOH+ H2
Thus sodium hydroxide is regenerated, which continues to corrode further. The continuous corrosion leads deposition of magnetite on th wall, which makes it brittle.
6. The continuous embrittlement of boiler parts results in failure of the boiler or boiler explosion.
Factors affecting rate of corrosion:
1. Nature of the metals:
Metals with higher electrode potentials do not corrode easily. They are noble metals like, gold , platinum, silver.Whereas metals with lower electrode potentials , readily undergo corrosion.e.g. metals like, zinc, magnesium, aluminium.
When two metals are in contact with each other,higher the difference in electrode potentials greater is the
corrosion.For eg., the potential difference between iron and copper is 0.78V which is more than that between iron and tin (0.3V). Therefore, iron corrodes faster when in contact with copper than that with tin. On this account, the use of dissimilar metals should be avoided wherever possible. For example, bolt and nut or screw and washer should made of the same metal (or alloy) to avoid galvanic corrosion.
2. Ratio of cathodic to anodic region:
The rate of corrosion is influenced by relative size of cathodic to anodic area. If the metal has small anode and large cathodic region, corrosion rate is very high. As the ratio decreases corrosion rate further increases. This is because at anode electrons are liberated. Which are consumed at cathodic region. If cathodic region is larger the liberated electrons are rapidly consumed at cathode. This futher enhances, the anodic raction leading to increase in overall rate of corrosion.
When two dissimilar metals are in contact, Corrosion is more rapid and severe, if the anodic area is small and cathodic area is large (e.g., a small steel pipe fitted in a large copper tank), If during plating of tin on iron, some areas are not covered or some pin holes are left, there results a small anode and large cathode . An intense localized corrosion occurs at the exposed small anodic surface. On the other hand, zinc plating on iron gives an anodic coating to iron since zinc is above Fe in the electrochemical series. Even if zinc plating peels off at some points intense corrosion of iron would not occur. This is because of formation of large anodic and small cathodic areas .
3. Nature of the corrosion product:
The corrosion product formed like metal oxide may act as protective film, if it is stable, insoluble, and non porous. If it acts as protective film it prevents further corrosion by acting as barrier between metal surface and corrosion medium. On the other hand if corrosion product is unstable, porous, and soluble , it further enhances corrosion.
Eg. in oxidizing environments, metals like aluminium, chromium, titanium, etc., are highly passive as their oxides as corrosion products form protective films on the metal surface, preventing further corrosion. Metals such as iron, zinc, magnesium etc., do not form any protective film and are highly susceptible for continuous corrosion, when exposed to oxidizing environments.
4. pH of the medium:IN general rate of corrosion is higher in acidic pH tha in neutral and alkaline pH.In case of iron, at very high pH protective coating of iron oxide is formed which prevents corrosion. Where as at low pH severe corrosion takes place. But for metals like Aluminium, corrosion rate is high even at high pH.
5..Temperature:
Rate of corrosion increases with increase in temperature. This due to the increase in conductance of the medium with increase in temperature and hence an increase in the diffusion rate. As a consequence, corrosion progresses faster at higher temperatures. In some cases, rise in temperature decreases passivity, which again leads to an increase in the corrosion rate.
6 Effect of oxygen:
Oxygen plays an important corroding medium. If the metal is exposed to different concentration of oxygen, differential aeration corrosion takes place. The region which is less exposed to oxygen acts as anode hence undergoes corrosion and region exposed to high oxygen concentration acts cathode hence doesn’t undergo corrosion.
7) Hydrogen over voltage:
The metal with lower hydrogen over voltage on its surface is more succeptible for corrosion, when cathodic reaction is hydrogen evolution type. Since at lower hydrogen over voltage, liberation of hydrogen gas is easy. Therefore cathodic reaction is very fast, which inturn makes anodic reaction very fast. Thus increasing rate of corrosion. Higher the over voltage lesser is the corrosion.
Corrosion control:
There are varios ways by which corrosion can be controlled. They are,
1. Protective coatings:
i).Metallic coatings:
In this method protective metal is coated on the surface of the metal which can be done by electroplating, flamespray, hot dipping etc. They can be anodic coating or cathodic coatings.
1) anodic coating:
In this base metal is coated with more active or metal with lower electrode potential which are anodic to the base metal. The coated metal undergoes corrosion, where as the base metal is protected. Thus anodic coating protects the underlying base metal sacrificially. The one of the important characteristics of anodic coating is that, even if the coating is ruptured, the base metal does not undergoes corrosion. The exposed surface of the metal is cathodic with respect to the coating metal and the coating metal preferentially undergoes corrosion. The protection is ensured as long as the anodic coating metal is still present on the surface. Therefore, anodic coating is also known as sacrificial coating. Galvanization is a familiar example for anodic coating and is extensively used to protect iron and steel objects.
Galvanisation:
The process of coating base metal like steel, copper etc. with zinc is called galvanisation.It consists of following steps:
1.Articles to be galvanized are dipped in organic solvent to remove dust or grease.
2. Then dipped in dilute sulphuric acid to remove any rust formed. And washed with water(pickiling).
3. Then dipped in solution of zinc chloride and ammonium chloride which acts as a flux to have firm
coating..
4.Then dipped in molten zinc at 430?C
5. Excess of zinc is removed by rolling over rollers.
Uses: Uses: It is most widely used for protection of iron from atmospheric corrosion in the form of roofing sheets, wires, pipes, nails, bolts, screws, buckets, tubes etc. It may be pointed here that zinc gets dissolved in dilute acids, to form highly toxic (or poisonous) compounds. Hence, galvanized utensils cannot be used for preparation and storing foodstuffs, especially acidic ones.
2).Cathodic coating:
In this base metal is coated with less reactive metals (metals with high electro
e potential) which are more cathodic to the base metal. The coating metal being less reactive than base metal does not undergo corrosion, but acts as a barrier between the base metal and the corrosive environment. The cathodic coating provides protection only when it is undamaged, continuous and non porous. When the coating is discontinuous, an intense localized corrosion occurs due to the setting up of a galvanic cell that consists of a large cathodic area and small anodic area.
e Tinning is a familiar example for cathodic coating
Tinning:
It is a process of coating base metal with tin. Tinning of iron or steel is carried out by hot dipping method and the process involves the following steps.
1) Articles to be tinned are dipped in organic solvent to remove dust or grease.
2) Then dipped in dilute sulphuric acid to remove any rust formed. And washed with water
3) Then dipped in solution of zinc chloride and ammonium chloride . which acts as a flux to have firm coating
4) Then dipped in molten tin.
5) Dipped in palm oil to prevent oxidation of tin.
6) Excess of tin is removed by rolling over rollers.
Uses:
Uses: Because of non-toxic nature of tin, tinning is widely used for coating steel, Cu, and brass sheets, used for manufacturing containers of storing foodstuffs, ghee, oils, kerosene and packing food materials. Tinned-copper sheets are employed for making cooking utensils and refrigeration equipments.
ii). Inorganic coating
In this protective coating of inorganic metal oxide is formed on the surface which prevents the metal from corrosion acting as barrier between the underlying metal surface and corrosion environment. Such coatings are particularly used an excellent base for paints, laquers oils and enamels.
.
1) Anodised oxide coatings:
In this type metals like Al, Zn , Mg are made to develop protective metal oxide coating artificially on their surfaces. It carried by making base metal anode and inert metal like lead is made cathode. The electrolytic bath containing sulphuric acid ,phosphoric acid,chromic acid, or boric acid is taken. On passing current the oxide film is formed on the base metal. These oxide coatings prevent the corrosion of the base metal.
e.g. Anodisation of Aluminium, in which Aluminium develops protective Aluminium oxide coating
which prevents corrosion of Aluminium. .
Anodized articles are used as soap boxes, tiffin carriers, window frames, etc.
2) Phosphate coatings (Phosphating):
In this type base metal is made to develop protective metal phosphate coating on its surface. This done by dipping base metal in electrolytic bath containing an aqeous solution of phosphoric acid. The modern phosphate bath contains metal phosphate, phosphoric acid and accelerators such as nitrates, nitrites.
Phosphate coatings are frequently appied on iron, steel, and zinc.
Phosphate coatings are used as primer coat for painting of automobiles, other equipments like fridge, washing machine etc.
It is coated over metal parts like nuts, bolts of machines which cannot be readily plated.
iii).Organic coating
Organic coatings are inert organic barriers applied on metel surfaces and other constructional materials for both corrosion protection as well as for decoration.They include paints, Enamels, Emulsions, Plastic coating.
2 . corrosion inhibitors:
These are the substances added in small amounts to the corrosive medium to decrease the corrosion rate. They provide corrosion protection by retarding either cathodic or anodic ractions.On this basis they are classified as:
i)Anodic corrosion inhibitors:
These are the substances which decrease the corrosion ratr by decresing the rate of anodic reaction.
During corrosion oxidation occures at anode. These electrons are taken up by cathodic regions. When inhibitors like chromate, molybdate, tungstate are added , these anions will combine with metal ions formed at anode forming sparingly soluble salt. This deposits at anode as thin film which acts as a protective barrier between base metal and corrosive medium thereby decreasing the rate of corrosion.
ii)Cathodic inhibitors:
These are the substances which decrease the corrosion ratr by decresing the rate of anodic reaction. Based on the nature of the cathodic reactions two types of inhibitors occure. If the cathodic rection is hydrogen evolution type, then inhibitors like, amines urea,thiourea,mercaptans and heterocyclic nitrogen compounds are added. These are absorbed at cathodic surface and prevent diffusion of hydrogen ions. If rection is oxygen absorption type, addition of sodium sulphite, hydrazine removes oxygen from aqueous solution by formation of salts.
Na2SO3 + O2 ? Na2SO4
NH2NH2 + O2 ? N2 + 2H2O
Sulphates of Magnesium , Zinc etc. can also be added as cathodic inhibitors., since they combine with OH- ions at the cathode and forms insoluble metallic hydrides which deposits on cathode. These reactions prevent diffusion of oxygen to cathode.
Mg2+ + 2 OH- ? Mg(OH)2
3.Cathodic protection:
It is a technique of protecting a metal from corrosion by converting it to cathode. It can be done by,
i)sacrificial anodic method:
In this metal to be protected is converted into cathode by connecting it to more active metal. The active metal acts as auxillary anode. The more active metal itself gets corroded slowly; while the parent structure (cathodic) is protected. Metals like zinc Aluminium generaly used as auxillary anodes. A metallic block of Al, zinc or Mg are connected to meatallic structure of iron to be protected. Anode being more reactive undergoes corrosion, where as the metallic structure is unaffected.Since anode is sacrifised to prevent the corrosion of anode, It is called sacrificial anode method.
1) A magnesium block connected buried oil storage tankacts as sacrificial anode, and prevents the tank from corrosion.
2) When Magnesium blocks arfe connected to the sides of the ship, it acts as anode, prevents ship from corrosion.
3) Magnesium blocks connected to buried pipelines prevents structure from corrosion , by acting as anode.
ii)Impressed current method:
In this method electrons are supplied from external source to the metallic structure to be protected. Metal to be protected is connected to the negative terminal of D.C source. A rod made up of inert graphite or palladium is converted to positive terminal.The electrons are released from anode prevents the basic structure from corrosion.
4.Anodic protection:
1. It is a technique of protecting a metal from corrosion by converting it to anode.
2. Metal is made anode by supplying anodic current externally using potentiostat.
3. This method is applicable for passive metals ,like Ni, chromium, titanium, Al,etc.
4. When known potential is applied using potentiostat, initially current also increases, indicating corrosion of
the metal
5. After particular potential current starts decreasing showing the formation of protective layer.
6. If the potential is further increased current remains constant, showing the protection of the metal due to the complete formation of the protective layer.This region of constant potential , where metal is unattacked is called passive region. The potential is called passivating potential.
Limitations:(i) the corrosion rate cannot be reduced to zero as in cathodic protection.
(ii) It cannot be applied to metals which donot passivate.
Applications: Stainless steel containers used for transporting corrosive chemicals (e.g concentrated acids) are protected from corrosion by passivating them by maintaining at its passivating potential.
|
|
Responses to the resource: "Vtu chemistry notes"
|
No responses found. Be the first to respond and make money from revenue sharing program.
|
|
|