Definition:-The Process of destruction or decoration of metal or its alloys by chemical or electrochemical attack through its environment starting at its surface is called corrosion.
Ex. Rusting of iron (Fe2O3.2H2O)
Types of Corrosion:-
Corrosion of the metals occurs by the attack of surrounding environment on the surface of the metals. The attack occurs in two ways
1. Dry Corrosion or chemical corrosion 2. Wet or electrochemical Corrosion
Dry or Chemical theory of Corrosion
This type of corrosion occurs mainly through the direct chemical action of atmospheric gases such as oxygen, halogens, hydrogen sulphide, SO2 or anhydrous inorganic liquids with metal surfaces in
immediate proximity. The extent of corrosion depends on
1. The chemical affinity between the corrosive environment and solid metals and
2. Ability of reaction product on metal surface to form a protective film.
There are three main types of corrosion.
1. Oxidation Corrosion: It Occurs by the direct action of oxygen at low temperature on alkali and alkaline earth metals. At high temperature almost all metals are oxidized to form metal oxides. Metals like Ag, Au, and Pt are not oxidized as they are noble metals.
2M 2Mn++
ne-1/2O2+ 2ne- nO 2-
---2M + 1/2O2 M2O
During oxidation of metal, metal oxide is formed as a thin film on the metallic surface which protects the metal from further corrosion. If diffusion of either oxygen or metal is across these layer, further corrosion is possible. Thus the layer of metal oxide plays an important role in the process of corrosion. Oxides of Pb, Al and Sn are stable and hence inhibit further corrosion. Depending the layer formed it can be classified as Unstable oxide layer, Stable non-porous layer, Stable porous layer and volatile layer.
In case of porous and volatile oxides, the rate of further corrosion is faster. MoO3 is a volatile oxide and accelerates corrosion. 2. Reaction with other gases:- In dry corrosion, Cl2, SO2, H2S and
NOx gases react with metal and form corrosive products. Corrosion
product AgCl is a protective layer while SnCl4 is volatile.
3. Liquid metal corrosion :- In several industries, molten metal passes through metallic pipes and causes corrosion due to dissolution or due to internal penetration. For example, liquid metal mercury dissolves gold form corresponding amalgam, thereby corroding gold.
Wet or Electrochemical Theory of Corrosion:-
Oxidation of metal (ex. Fe) takes place at anode while the reduction process takes place at cathode. It may occur in two ways
(i) Evolution of H2 (ii) Absorption of O2
At anode : Fe Fe2+ + 2 e-
While at cathode reduction reaction is carried by evolution of H2 or
absorption of O2.
Case-I : Evolution of H2
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2H+ + 2e- H2
In this case metal react in the acidic environment and are dissolved (Undergo corrosion) to release H2 gas. All metal above hydrogen in
electrochemical series can show this type of corrosion. In hydrogen evolution type of corrosion, anodic area is large as compared to its cathodic area.
Case –II : Absorption of O2
This type of corrosion takes place in neutral or basic medium in the presence of oxygen. The oxide of iron covers the surface of the iron. The small scratch on the surface creates small anodic area and the rest of the surface acts as cathodic area. The following reaction occurs at anode and cathode
At anode: 2Fe 2Fe2++ 4e- At cathode: O2 +2H2O + 4e- 4OH-
---
2Fe + O2 +2 H2O Fe2+ + 4 OH- or Fe(OH)2
--- Ferrous oxide is easily oxidized to form Fe(OH)3 in the presence of
oxygen.
4Fe(OH)2 + O2+ H2O 4 Fe(OH)3
Ferric hydroxide is actually hydrated ferric oxide, Fe2O3.H2O, which
is yellowish rust. Anhydrous magnetite, Fe3O4 [a mixture of (FeO +
Fe2O3)], is also formed, which is brown-black in colour. It is mark
able that the corrosion occurs at anode but the corrosion product is formed near cathode. It is because of the rapid diffusion of Fe2+ as compared to anion.
Factors Influencing Corrosion:-
Since corrosion is a process of destruction of metal surface by its environment. It is influenced by two factors (i) Nature of the metal (ii) Environmental
Nature of Metal:-
(a) Position of metal in galvanic series:- (b) Hydrogen over voltage:-
(c) Purity of metal
(d) Relative areas of anode and cathode (e) Physical state of the metal
(f) Nature of metal oxide film
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Effect of Environment:
(a) Temperature (b) Humidity
(c) Impurity of atmosphere (d) pH value
(e) Conducting effect (f) Oxygen concentration
Cathodic
Protection
:-
The principle involved in cathodic protection is to force the metal to be protected to behave like a cathode. Cathodic protections are two types
1. Galvanized or Sacrificial Anodic method and
2. Impressed Current methods.
GALVANIZE OR SACRIFICIAL ANODIC METHOD
In galvanic protection, a more active metal is connected to the metal to be protected, so that all corrosion is concentrated on the more active metal and thus saving the metal structure from the corrosion. The more active metal used is called auxiliary anode or Sacrificial anode.
Three metals are commonly utilized for cathodic protection of steel. The selection of the anodic metal is dependent upon resistivity and electrolyte. General application guides for these metals are
• Magnesium – non-sea soil and freshwater • Zinc – sea soils and saltwater
• Aluminum - saltwater and limited freshwater applications
Application
1. Protection from soil corrosion of underground cables and pipelines
2. Protection from marine corrosion of cables, ship hulls, piers, water tanks etc.,…
IMPRESSED CURRENT METHODS
In this method, an impressed current is applied in opposite direction to nullify the corrosion current and convert the corroding metal from anode to cathode. Usually the impressed current is derived from direct current source with an insoluble anode like graphite or platinum. Usually a sufficient D.C. current is applied to an insoluble anode buried in the soil and connected to the metallic structure to be protected. The anode is usually in a black fill(coke), so as to increase the electrical contact with the surrounding soil.
APPLICATION:
1. Protection of underground pipe lines carrying oil or water 2. Applied to open water – box coolers, water tank transmission
towers, marine piers etc.,
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purpose is spoiled, hence sacrificial anodic method is superior to this method.
Corrosion
Inhibitors:A corrosion inhibitor is a substance, which when added in small quantities to the aqueous corrosive environment effectively decrease the corrosion of a metal.
They are two types
(1) Anodic Inhibitors:-These are oxygen carriers or oxidizing agents. They combine with anodic metal forming an oxide film or layer which snaps the galvanic circuit. Although this type of control is affective, it may be dangerous since severe local attack can occur, if certain areas are left unprotected by the depletion of the inhibitor Ex:- Transition metal chromates, Phosphates and Tungstates.
(2) Cathodic Inhibitors:-
a) In Acidic solution, the main cathodic reaction is evolution of hydrogen.
2H+ (aq) + 2e- H2(g)
Consequently corrosion may be reduced by either slowing down the diffusion of H+ ions to the cathode or by increasing the
over voltage of hydrogen evolution. The diffusion of H+ ions is decreased by adding organic inhibitors like amines, mercaptans, heterocyclic compounds, substituted urea and thio-urea, metal soaps
Antimony and arsenic oxides are used as inhibitors, because they deposit adherent film of arsenic or antimony at the cathodic areas, thereby increasing hydrogen voltage.
B) In neutral solution, the cathodic reaction is H2O + 1/2O2 + 2e- ---- 2OH- (aq)
The corrosion can be controlled by elimination of oxygen from the corroding medium, this carried by adding reducing agen6ts like Na2S,
Na2SO3 by deareation or by retarding its diffusion to the cathodic
areas by adding inhibitors like Mg, Zn and Ni salts.
Metallic coatings:-
a.Anodic Metal Coating:-
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Cathodic Metal Coating:-
The surface of a metal can be protected from corrosion by coating a base metal with a more noble metal whose tendency for corrosion is lower than the base metal in the galvanic series. A thin film coating of metal like tin , copper, nickel, etc are deposited on the base iron metal by cathodic coating techniques to prevent corrosion. Cathodic coating provide effective protection to the bsae metal only when they are completely continuous, non-porous and free from crack or breaks. If such coating are puncture more corrosion and severse damage can be done to the base metal than to the metal without it.
Galvanizing:-
Galvanizing is the process of coating of zinc on iron or steel to prevent rusting. The process is carried as follows
The iron article is first cleaned by acid pickling with dil.H2SO4
solution for 15 -20 minutes at 60-70ᴼC, removes any scale, rust or other impurities.
• The article is then washed well and dried.
• It is then dipped in molten zinc bath at 425-450 ᴼC. The surface of the bath is covered with NH4Cl flux to prevent oxide formation, then
the oxide is passed through a pair of hot rollers. The process removes any excess of zinc and produces a thin film of uniform thickness. • Then it is annealed at 650ᴼC and finally cooled slowly by annealing
process.
Uses:-
It is used for protection of iron in the form of roofing sheets, wires, pipes, nails, bolts, screws, tubes etcs.,
Galvanizing utensils cannot be used for preparing and storing food stuffs.
Tinning:-
*Tinning is the coating of tin over iron or steel articles. The process consists of treating steel sheet in dil.H2SO4 to remove any oxide.
*After this it is passed through a bath of zinc chloride flux. The flux helps molten metal to adhere to the metal sheet.
*Then the sheet is passed through a molten tin tank and then through a series of rollers.
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Uses:-
Tinning is widely used for food storage cans, tin coated copper sheets for cooking utensils
Anodizing
orAnodised oxide coating:-
These coating are made by anodic oxidation process this leads to the formation of protective, corrosion resistant, electrically insulating oxide film on the surface of non-ferrous metals like aluminum, zinc, magnesium and their alloys.
Anodic oxidation or anodizing is an electrolytic process of oxidation for thickening the natural oxide film on the surface of metal to achieve more protection against corrosion and mechanical injury.
Anodizing on aluminum is done by electrolysis in suitable acid electrolyte (sulphuric, chromic, oxalic or phosphoric acid) at 35 to 40ᴼC and moderate current densities. Aluminum is made the anode and a plate of lead or stainless steel is made the cathode. When the current is passed the oxygen liberated at the anode combines with
it to form the oxide, thus thickening the natural oxide film. The outer part of the oxide film formed is very porous and considerably softer than the layer directly adjoining the metal surface. The anodized surface is made impermeable by treatment with hot water. This treatment converts aluminium oxide into its monohydrate(Al2O3.H2O) which occupies more volume, thereby the pores are sealed.
Anodized coating on Zn can be obtained by making Zn as anode in an electrolytic bath containing chromic acid solution or chromates.
Organic Coatings:-
Organic coatings are inert organic barriers applied on metallic surface and other construction materials for corrosion protection and decoration. A good organic coating must have
1. Chemically insert to corrosive environment 2. Good surface adhesion
3. Good spreading power
4. Impermeability to water salts and gases 5. Proper application method
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Paints:
Paint is a mechanical dispersion mixture of one or more pigments in a vehicle. Vehicle is a liquid consisting of non-volatile film forming material with drying oil and a highly volatile solvent thinner.
When paint is applied on a metal surface the thinner evaporates while the drying oil slowly oxidises forming a light pigmented film
Characteristics of a good paint:
1. Good chemical stability, 2.Good surface adhesion 3.Good spreading power. 4. Resistance to corrosion 5.Attractive in colour 6. Insoluble In ordinary solvents 7.Non-poisonous, 8.Good hiding power, 9.Glossy film and 10. Reflect the uv light
CONSTITUENTS OF PAINTS AND THEIR AND FUNCTIONS:
Pigments: Pigments are solids. They give desire colour ,opacity and strength . these effect the UV radiation and protect the paint, pigments used for various colours are
Ø White :-zinc oxide, white lead , titanium oxide, litho phone Ø Red:- red lead, ferric oxide, chrome red
Ø Green:- chromium oxide Ø Blue:- Prussian blue Ø Black :- carbon black
Vehicle or drying oil : Vehicle is a film forming constituents of pain. These are glycerol esters of high molecular weight fatty acids, generally present in animal and vegetable oil. drying oil contains high percentages of unsaturated fatty acids and they dry quickly by
absorbing oxygen and polymerizes, so liquid oils is converted into solid film. The most widely used drying oils are linseed oil, soybean oil, and de hydrated castor oil.
Functions:
i. Main film forming constituents ii. Vehicle or medium
iii. Good toughness iv. Good adhesion
v. Good durability and vi. Good water-proofness
III. Thinners: thinners reduce the viscosity of paint and hence increase the covering area. These also increase the elasticity of paint film and increase the penetration power of the vehicle. Ex. Turpentine oil, mineral sprits, benzene, naphtha, kerosene, methylated naphthalene. etc.,
IV. Driers: driers are oxygen-carrying catalyst. They accelerate the drying of oil through oxidization, polymerization and condensation Thus their main function is to improve the drying quality of oil film. Too much of drier tends to produce hard and brittle films. Ex. Resinates, linoleates, tungastates and naphthanates of Cr, Mn, Pb and Zn.
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VI. Plasticizers: Platizers are incorporated into the paint to provide elasticity and to minimise cracking . Ex. Tricrysol phosphate , triphenyl phosphate , tributyl phosphate , dibutyl tartarate etc. VII. Anti-skinning agents : These are added to some paints for preventing gelling and skinning of the paint film . Important anti skinning agents are polyhydroxy phenols.
Varnishes:-
Varnish is a homogeneous colloidal dispersion solution of natural or synthetic resin in oil or thinner or both. It is used as a protective and decorative coating at suitable surfaces and dries by evaporation, oxidation and polymerization of its constituents leaving behind a hard transparent glossy, lustrous and durable film. There are two types of varnishes
1. Oil varnish, 2. Spirit varnish.
1. Oil Varnish: - is a homogeneous solution of one or more natural or synthetic resins in a drying and a volatile solvent. This type of varnish dried up by the evaporation of the volatile solvent, followed by oxidation and polymerization of drying oil. Ex. Copal varnish-used for exterior as well as interior works
2. Spirit Varnish – contain a resin dissolved in a completely volatile solvent. Spirirt varnishes dries quite rapidly, but leaves behind a film, which is brittle and so has a tendency to crack or peel off. Ex- Shellac- used for polishing wooden furniture.
Constituents of a Varnish:
a. Resins: Resin in varnish provides an element of hardening, resistant
to weathering, durability and resistance to chemical action and water proofness.
Natural resins: shellac, rosin, copal, dammer etc.,
Synthetic resins: phenol formaldehyde, urea formaldehyde etc.,
b. Drying oils: Drying oils are dried by oxidation and polymerization
and give a thin film. Ex. Linseed oil, dehydrated castoroil, soya bean oil, fish oil, etc.,
c. Solvents or thinners: these reduce the viscosity and increase coving
area. The solvent used are kerosene, turpentine oil, acetone etc.,
d. Antiskinning agents: These are used to prevent gelling. Ex. Tertiary
amyl phenol. Uses:
1. For protection against corrosion 2. As a brightening coat to paint surface
3. For improving the appearance and intensifying the ornamental grains of wood surfaces.
Questions:
1. Define corrosion? Present the mechanism of electrochemical corrosion?
2. Write at least 5 differences between dry corrosion and wet corrosion?
3. Describe the factors that influence corrosion with respect to metal and nature of environment?
4. What are varnishes and how are they used in corrosion prevention. 5. Define cathodic protection? Explain different methods of cathodic
protections with applications?
