course : b.e & b. tech subject: applied chemistry unit: i corrosion & its control syllabus:...
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Course : B.E & B. Tech Subject: Applied Chemistry Unit: I
Corrosion & its Control
Syllabus:Corrosion – IntroductionChemical CorrosionElectrochemical CorrosionGalvanic CorrosionDifferential aeration CorrosionStress CorrosionSoil (Microbial) CorrosionFactors influencing Corrosion By Nature of Metal By Nature of EnvironmentCorrosion Control Selection of Materials Design Principles Cathodic Protection Sacrificial Anodic Protection Corrosion Inhibitors
Before Corrosion
After Corrosion
Dr. K. SIVAKUMARDepartment of
ChemistrySCSVMV University
Factors Influencing Corrosion
Extent of Corrosion depends mainly on,
Nature of the metal Nature of the environment
Nature of the metal
Position in the galvanic series:Extent of corrosion depends upon the position of the metal in the galvanic series.
Greater the oxidation potential, greater is the rate of corrosion.
When two metals are in direct contact, the metal higher up in the galvanic series becomes anodic and suffers corrosion.
Galvanic series:EMF series does not account for the Corrosion of all metals & alloys. So, a more practical series, called galvanic series is prepared.Anodic
Cathodic
Factors Influencing Corrosion
Rate of corrosion is more if the size of cathodic area is large
Relative areas of the anode and cathode:
If cathodic area is larger, the demand for electrons will be more and this results in the increase of rate of oxidation of metal at anode. i.e, high rate of corrosion at anode.
Large Cathode(Steel) Small Anode
(Aluminium)
e-n flow
Factors Influencing Corrosion
Purity of the metal:Impurities present in a metal create heterogeneity and thus galvanic cells are set up with distinct anodic and cathodic areas in the metal.
Higher the percentage of impurity present in a metal, faster is the rate of corrosion of the anodic metal.
Eg:- Impurities like Pb, Fe, etc. in Zn lead to the formation of tiny electrochemical cells at the exposed part of the impurity and the corrosion of Zn around the impurity takes place.
Factors Influencing Corrosion
Physical state of the metal:
Metallic materials with unevenly distributed stresses are easily corroded.
Even in a pure metal, the areas under stress acts as anode and get corroded. Caustic embrittlement takes place in stressed parts such as bends, joints, rivets, etc. in boilers.
Caustic embrittlement takes place in stressed parts such as bends, joints, rivets, etc. in boilers.
Factors Influencing Corrosion
Nature of the oxide film:Piling Bed-Worth Rule
If, Volume of Oxide < Volume of metal
Oxide film is porous and non-protective CORROSION CONTINUES due to penetration of O2 through pores Eg:- Metals such as Mg, Ca, Ba, etc.
If, Volume of Oxide > Volume of metal
Oxide film is non-porous and protectiveNO FURTHER CORROSIONEg:- Metals such as Al, Cr, Ni, etc.
Factors Influencing Corrosion
Nature of the Environment
Temperature:
Atmospheric corrosion of Fe is slow in dry air but the rate of corrosion increases rapidly in presence of moisture. This is due to the fact that moisture acts as a solvent for the oxygen in the air to form electrolyte and setting up an electrochemical cell.
Example: rusting of iron increases when the moisture content increases from 60% to 80%.
Corrosion increases with temperature due to the increase in rate of diffusion of ions
Humidity :
Factors Influencing Corrosion
Effect of pH :Rate of corrosion is maximum when the corrosive
environment is acidic.
In general, acidic environment is more corrosive than alkaline or neutral medium.
Corrosion rate can be reduced by increasing the pH of the medium. But metals such as Al, Zn dissolves in alkaline medium.
Factors Influencing Corrosion
Nature of the electrolyte :If the electrolyte consists of silicate ions, they form
insoluble silicates (that acts as protective coating) and prevent further corrosion.
If the electrolyte consists of chloride ions, they destroy the protective film and the surface is exposed for further corrosion.
If the conductance of electrolyte is high then, the corrosion current is easily conducted and hence the rate of corrosion is increased.
Factors Influencing Corrosion
Less oxygenated part of the metal acts as anode and more oxygenated part of metal acts as cathode.
Concentration of O2 and formation of O2 concentration cells:
Rate of corrosion increases with increasing supply of oxygen.
Factors Influencing Corrosion
Differential aeration:
Due to differential aeration, Corrosion often takes place under, metal washers buried pipelinescables passing through different types of soil
Eg:- Lead pipelines passing through clay and sand. Pipeline buried under clay get corroded easily because clay is less aerated than sand.
Corrosion Control
Corrosion can be controlled by two methods,
By modifying the metal By modifying the
environment
Corrosion control by modifying the metal Selection of the metal:Selection of right type of metal is the main factor for corrosion control.
Eg:- Noble metals that are immune to corrosion are used for producing ornaments and surgical instruments.
Ornaments
Surgical Instruments
Corrosion Control
Presence of impurities in a metal cause heterogeneity and decreases the corrosion–resistance of the metal.
Eg:- Presence of just 0.02% of iron in aluminium decreases its corrosion resistance.
Using pure metal:
Using metal alloys:Corrosion resistance can be improved by alloying the metal with suitable metal.
Eg:- Presence of chromium in stainless steel produces a protective oxide film.
Corrosion Control
Metallic materials should be designed in such a way to resist corrosion
Important Design Principles:
The contact of two dissimilar metals in the presence of the corroding solution should always be avoided, otherwise the more active metal will act as anode and corrosion will be localized.
Iron
Iron
Stainless steel bolt & nut
Corrosion Control
Important Design Principles:
If the contact between two dissimilar metals is unavoidable two metals are chosen in such a way that the anodic metal should be as large as possible and cathodic metal should be as small as possible.
Large Cathode(Steel)
Small Anode (Aluminium)
e-n flow
Large Anode(Aluminium)
Small Cathode (Steel)
e-n flow
Severe Corrosion Negligible Corrosion
Corrosion Control
Important Design Principles:
The (contacting) two dissimilar metals should possess close positions in the EMF series.
If direct contact is not possible to avoid, the metals can be insulated well before connecting to avoid the direct electric contact of metals.
Corrosion Control
Important Design Principles:The anodic metal should be used without any paint coatings for connecting with cathodic metal. Because even a slight break in paint coating will lead to localized corrosion.
On joining different metals, welded joints should be used instead of riveted joints.
Corrosion Control
Important Design Principles:
The design should provide easy cleaning and flushing of the corner and bends in the metallic equipment.
i.e., Sharp corners and recess should be avoided.
Corrosion Control
Important Design Principles:
Avoid angles and pockets in which water can collect.
The risk of dirt build-up is reduced with radiused corners.
Stagnant water is avoided by providing drain holes
Instead, use a shape that promotes draining
Corrosion Control
Important Design Principles:
Metallic equipments should be supported on legs to permit air circulation and to avoid the formation of damp areas under the equipment.
Corrosion ControlCathodic Protection:
Principle: Corroding anode is forcibly converted into cathode. Types:
1. Sacrificial anodic protection2. Impressed current cathodic protectionSacrificial anodic protection method
metallic equipment to be protected is electrically connected to more active anodic metal
SACRIFICIAL ANODE
Ship Without Sacrificial anode
Initially
Finally
Then..
At Seashore…
RUSTY SHIP
RUSTY SHIP WRECK
RUSTY ANCHORS
Corrosion Control
Impressed current cathodic protection:
current is applied (from a battery) in opposite direction to nullify the corrosion current
(+ ) (– )
D C
B u rie d p ip e
B a c k fi ll
G ra ph ite a n o d e S o il
Corrosion Control Impressed current cathodic protection:
Corrosion ControlCorrosion control by modifying the environment
Corrosion Inhibitors:Substance which on addition in small quantities to the corrosive environment reduces the corrosion of metal is called inhibitors.
Types of Inhibitors: 1. Anodic inhibitors 2. Cathodic inhibitors
Anodic inhibitors:Chemicals like chromates, phosphates, tungstates, when added to corrosive environment produces sparingly soluble compounds by reacting with metal ions (produced because of corrosion).
The sparingly soluble compounds get absorbed on the metal surface forms a protective film and thereby reduces corrosion rate.
This type of corrosion control is not fully reliable because, certain areas of metal are not covered by the film and this leads to severe corrosion.
Corrosion ControlCathodic inhibitors:
In acidic solution, the main cathodic reaction is liberation of H2.
In an acidic solution, the corrosion can be controlled by slowing down the diffusion of H+ ions through the cathode.This can be done by adding organic inhibitors like amines, pyridine, etc. They absorb over the cathodic metal surface and act as a protective layer.
In a neutral solution, the cathodic reaction is,
The formation of ions is only due to the presence of oxygen. By eliminating the oxygen from the medium, the corrosion rate can be reduced. Oxygen can be removed by adding some reducing agents like Na2SO3 or by deaeration.
22H 2e Haq g
2 21
H O O 2e 2OH2
Corrosion Control
Application of protective coatings:
Metallic surface can be protected from corrosion by covering it by organic or inorganic coating.
Corrosion Control
Deaeration
Presence of more O2 is harmful and it increases the corrosion rate. Dissolved oxygen can be removed by deaeration using mechanical agitation.
mechanical agitator
Corrosion Control
DeactivationRemoval of oxygen can be attained also by adding chemicals like Na2SO3, hydrazine etc.,
2 3 2 2 42Na SO O 2Na SO
2 4 2 2 2N H O N + 2H O
Corrosion ControlDehumidification
In this method, moisture from the air is removed by lowering the relative humidity of the surrounding air.
This can be achieved by adding silica gel which can absorb moisture.
