ACS1110 (Course Title: Applied Chemistry Unit III: Corrosion and its Prevention
Corrosion may be defined as the destruction of materials by interaction with their environment. Materials refer to the metals, polymers, ceramics, composites etc. However, the term corrosion is generally used for deterioration of metals.
In fact, most of the metal except Au, Pt (platinum) and Pd (palladium) exist in nature in combined form such as oxides, sulphides, halides, carbonates, bicarbonates, hydroxides etc.
Since the metallic compounds are highly stable and during the extraction of metals, large amount of energy is required. Accordingly the isolated atom may be regarded as to exist in a much higher energy state than their corresponding ores. Thus the metals have a tendency to revert back to stable compound of lower energy state. Thus, when a metal is exposed to the environment, it reacts with gases and moisture present in the environment to form undesirable stable compounds. In most of the cases oxides are formed.
However, depending upon impurities present, carbonate, sulphate, sulphide may also be formed. Thus corrosion may be regarded as “reverse of extraction of metals”
Examples: Some common examples are (i) rusting of iron (Fe3O4)
(ii) development of green coating on Cu surface due to formation of basic carbonate of copper [CuCO3 + Cu(OH)2] when it is exposed to air containing moisture and CO2.
(iii) discolouration of silver (tarnishing of silver) (iv) loss of luster of lead
Harmful Effects of Corrosion (Consequences of Corrosion): Although the process of corrosion is very slow and occurs only on the surfaces of metals but losses due to corrosion are very high. The main harmful effect is the high cost of replacement of materials and equipments deteriorated due to corrosion. Some consequences are:
(i) replacement of corroded materials and equipments (ii) preventive maintenance (e.g. painting, coating etc.) (iii) loss of efficiency of machines
(iv) safety (e.g., fire hazards, explosion, release of toxic products, collapse of construction due to sudden structural failure)
(v) contamination of food products kept in corroded container (vi) health – pollution due to corrosion products
(vii) reduced cost of materials due to their disappearance (viii) plant shut down due to failure
Classification of corrosion: Corrosion may be classified on the basis of Nature of corrodent Mechanism of corrosion
1. Dry corrosion 2. Wet corrosion
1. Chemical corrosion
2. Electrochemical corrosion Dry or Chemical corrosion: The dry corrosion occurs mainly through the direct chemical attack of gases such as O2, H2, H2S, SO2 or anhydrous inorganic liquids on metal surface. It is of three types
1. Oxidation corrosion 2. Corrosion by other gases 3. Liquid metal corrosion
Oxidation corrosion: This type of corrosion occurs by the direct attack of oxygen on metal surface. Alkali metals (Li, Na, K, Rb and Cs) and alkaline earth metals (Be, Mg, Ca, Sr, Ba etc.) have high tendency of oxidation and therefore oxidized even at low temperature. However, other metals with the exception of Ag, Pt and Pd are oxidized only at high temperatures.
Mechanism
Types of oxide layers: On the basis of its nature oxide layers may be classified into four types:
1. Stable layer: A stable layer is fine-grained in structure and coated tightly on parent metal surface and acts as protective layer. Consequently, further oxidation corrosion is prevented. E.g., layer formed on Al, Sn, Cu, Pb etc.
2. Unstable layer: The oxide layer formed decomposes back into metal and oxygen: Metal oxide = metal + oxygen
Thus oxidation corrosion is not possible, e.g., Ag, Au and Pt
3. Volatile layer: The layer volatilizes (escapes) as it forms and fresh metal surface is available for further corrosion
e.g., MoO3 layer is highly volatile
4. Porous layer: If oxide layer is porous in nature the atmospheric oxygen can easily reach to metal surface through the pores or cracks. This result in continuous corrosion till entire metal is converted to its oxide e.g., alkali and alkaline earth metals.
Pilling–Bedworth rule: The protective and non-protective nature of the oxide layer at metal surface is decided by PB rule.
According to this rule
Oxide layer is protective if volume of Oxide > volume of Metal Oxide layer is non-protective if volume of Oxide < volume of Metal
Oxides of alkali and alkaline earth metal – non-protective – fast corrosion Oxide of Al – protective layer – slow corrosion of metal
(Porous oxide layer permits oxygen to metal surface through pores and cracks)
Electrode potential: When a metal is placed in a solution containing its own ions, a dynamic equilibrium exists b/w metal (M) and its ions (Mn+) according to either of the following reactions:
(i) M → Mn+ + n e– (oxidation) (ii) Mn+ + n e– → M (reduction)
Accordingly metal acquires a negative charge (case i) or positive charge (case ii) and attracts the positive or negative ions from solution, respectively.
Thus a difference in potential is developed b/w metal and the solution. At equilibrium, the potential difference b/w metal and solution containing its own ions is constant and called electrode potential.
For example, if Zn and Cu rods are dipped in solution containing their ions of molar concentration, the reduction electrode potential are – 0.76 and + 0.34 V at 25 °C, respectively. Hence Zn has tendency of oxidation while Cu has tendency of reduction.
Measurement of electrode potential ( Use of S.H.E.)
To measure electrode potential, a half cell is coupled with SHE and emf of the complete cell is taken as electrode potential of the half cell. e.g. emf of cell
Zn(s) | Zn+ (1.0 M) || H+(1.0 M) | H2 (1 atm.), Pt is + 0.76 V at 25 °C. The half dell reactions are:
Zn → Zn2+ + 2 e–
2 H+ + 2 e– → H2 (g)
Thus electrode potential of Zn|Zn2+ electrode is + 0.76 V. Since according to convention the electrode potential is represented as reduction potential and so the reduction potential of Zn|Zn2+ electrode is – 0.76 V. Accordingly emf of the cell Pt, H2 (1 atm) |H+ (1 .0 M) || Zn2+ | Zn(s) is – 0.76 V.
Similarly emf of cell Pt, H2 (1 atm) |H+ (1 .0 M) || Cu2+ | Cu(s) is + 0.34 V. Thus electrode pot of Cu2+|Cu cell is 0.34 for half-cell reaction is Cu2++2e– → Cu
Standard electrode potential: The electrode potential of a half cell containing molar concentration of its ion at 25 °C/298 K is called as standard electrode potential.
Importance of electrochemical series:
(a) Predicting anodic and cathodic behavior (b) Predicting replacement tendency
(c) Predicting feasibility of redox reaction
Electrochemical Cell: In the Daniell cell, copper and zinc electrodes are immersed in a solution of copper (II) sulfate and zinc sulfate, respectively.
At the anode, zinc is oxidized while at cathode Cu2+ ions are reduced as per the following half reactions:
Zn (s) → Zn2+ (aq) + 2e− (E° = - 0.76 V) Cu2+(aq) + 2e− → Cu (s) (E° = 0.34 V)
The overall cell reaction: Zn (s) + Cu2+(aq) → Zn2+(aq) + Cu(s) (E° = 1.10 V) Cell is represented as: (-) / Zn (s) / Zn2+ (aq) // Cu2+(aq) / Cu (s) / (+)
Nernst Equation (Dependence of emf on concn. and temperature)
For a general reaction:
Mn+ (aq) + n e– → M (s) The Nernst equation is given as:
] [
] ln [
o n
M M nF
E RT E
where E = electrode pot, E° = standard electrode pot, T = temp in Kelvin scale, R= gas constant (8.314 J/K/mol), F = Faraday constant (96500 coulombs) and n
= number of electrons transferred in reaction. Further
) 1 ] M [ solid pure for because (
] M [ nF ln E RT
E o n
Nernst equation for overall cell reaction: The Nernst equation can also be used to calculate emf of cell provided that the concentrations of ions and stand emf of the cell are known. For example, for cell
Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s), the cell reaction is
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s) and Nernst eqn. is
] ][
[
] ][
ln[
2
cell n
cell Zn Cu
Cu Zn
nF E RT
E o
] [
] log[
303 .
2 2
cell n
cell Cu
Zn nF
E RT
E o
) 298 ] (
[
] log[
2 0591 .
0 2
K Zn at
E Cu
Ecell cello n
Mechanism of electrochemical corrosion: The reactions occur during corrosion are:
At anodic area: At the anodic area metal undergoes oxidation i.e. associated with loss of electrons:
M (s) → Mn+ + n e– e.g., Fe (s) → Fe2+ + 2 e–
The process continues as long as the electrons and metallic ions are removed from the environment. If they are not removed, corrosion discontinues.
Cathodic reactions: Electrons released at anode are consumed at cathode.
Depending upon the nature of corrosive environment, either of the following reaction occurs:
1. evolution of hydrogen 2. absorption of oxygen
Electrochemical Corrosion with Evolution of Hydrogen: This type of corrosion occurs in acidic environment (H+ + e– → ½ H2). (e.g., corrosion of Fe in HCl): This type of corrosion may be represented as:
Fe (s) → Fe2+ + 2 e– (oxidation, anodic) 2 H+ + 2 e– → H2 (g) ↑ (reduction, cathodic) Overall reaction: Fe (s) + 2 H+ → Fe2+ + H2 (g) ↑
Roasting of Iron (Electrochemical Corrosion with Absorption of Oxygen):
Rusting of iron in neutral aqueous solution of electrolytes (like NaCl solution) in presence of atmospheric oxygen is a common example of this type of corrosion.
At the anodic area metal dissolves:
Fe (s) → Fe2+ + 2 e– (oxidation, anodic)
The liberated electrons flow from anodic to cathodic area through iron metal where they are consumed by oxygen:
½ O2 + H2O + 2 e– → 2 OH– (reduction)
The Fe2+ ions (at anode) and OH– (at cathode) are diffused away to form Fe(OH)2 as ppt.:
Fe2+ + 2 OH– + Fe(OH)2 ↓
1. If oxide supply is high, Fe(OH)2 is oxidized to Fe(OH)2: 4 Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3
This product is yellow rust correspond to Fe2O3.xH2O
2 Fe(OH)3 ≡ Fe2O3.3H2O
2. If supply of oxygen is low the corrosion product is black anhydrous Fe3O4 (magnetite)
Galvanic series
Difference b/w electrochemical and galvanic series Advantages of galvanic series):
1. Method of development 2. Position of metal
3. Information about alloys
4. Scope
5. Nature of information
Factors influencing corrosion rate: The rate and extent of corrosion depends on the following factors –
1. Nature of the metal 2. Nature of corroding environment 1. Nature of the metal
(a) Position in galvanic series (b) Reduction potential
(c) Nature of Surface film (d) Purity of metals
(e) Over voltage
(f) Relative areas of anodic and cathodic parts (g) Passive character of metals
(h) Volatility of corrosion products 2. Nature of corroding environment (a) Temperature
(b) Humidity of air
(c) Presence of impurities in (d) Conductance of the medium (e) pH of the medium
Corrosion control: Some corrosion control methods are : 1. Proper designing
2. Materials selection (i) use of pure metal (ii) use of corrosion resistant alloys
3. Cathodic and anodic protection 4. Protective coatings
Proper designing: The corrosion may be controlled by proper designing of the materials. Some of the important design principles are
(a) The contact b/w two dissimilar metals in presence of corrosive medium should be avoided.
(b) If two different metals are in contact they should be selected in such a way that they are as close as possible in electrochemical series.
(c) If two different metals are in contact the designing should be made in such a way that the anodic material should have as large area as possible while cathodic metal should have as small area as much as possible.
(d) If an active metal is used it should be insulated from cathodic materials i.e.
their direct contact should be avoided. (insulator – rubber, plastic etc.)
(e) The anodic metal should not be painted or coated when in contact with different cathodic metal b/c any crack in coating would result in rapid localized corrosion.
(f) Sharp corners, slots etc. should be avoided as they promotes corrosion (g) Joints should be minimized
(h) Joints should be made in such a way that liquid should not get a chance to enter and stay there.
Materials selection (a) Use of pure metal ( b) Use of corrosion resistant alloys Cathodic protection: The principle of the method is to force the metal to behave like cathode. These are of two types –
(i) galvanic or sacrificial anodic protection (ii) impressed current cathodic protection
Galvanic or sacrificial anodic protection
Impressed current cathodic protection
Protective coatings: Protective coatings are of two types – i. Metallic coating
ii. Non-metallic coatings , such as organic coatings, paints, varnishes, etc Methods of Applications of metal coatings: Metallic coatings are usually performed by the different methods such as Hot dipping, Electroplating, Metal sprying, Metal cladding, Cementation etc.
Anodic coatings Cathodic coatings
Hot dipping: In the process of hot dipping, the metal to be coated is dipped in the molten bath of the coating metal. This method is used for producing coating of low melting metals such as Zn (419 °C), Sn (232 °C), Pb (327 °C), Al (660
°C) etc on iron, steel and copper which have relatively higher melting points.
There are two methods of hot dipping are commonly used, namely galvanizing and tinning.
Galvanizing
Uses – Galvanizing is used to protect roofing sheets, wires, pipes, tanks, nails, screws, etc.
Tinning
Uses – B/c of non toxic natutre of tin, tinning is widely usecdd for coating steel, copper and brass sheets used for manufactring the containers for soring foodstuffs, ghee, oils, kerosene and packing food materials. Tnned copper sheets afre used for making cooking utensils and refrigeration equipments.
Organic Coatings: In the organic coatings, an inert organic material is applied to the surface of base metals for corrosion resistance and decoration. Paints, varnish, lacquers and enamels are the main organic coatings.
Requirements of a good paint: A good paint should essentially satisfy the following requirement:
[1] It should form a good impermeable and uniform film on the metal surface so that effective protection from coorosion is achieved
[2] It should have a high covering power [3] The film should not crack on drying
[4] A good paint should adhere well to the surface [5] It should spread on the metal surface easily.
[6] Its film should give be glossy film (having shine or lusture) [7] It should be corrosion resistant
[8] A good paint should give a stable and decent colour on the metal surface
[9] It should be easily applicable (with brush or spraying device) on metal surface so that it produces umiform and smooth surface
Constituents of paint and their functions: The important constituents of paint are – (1) Pigments (2) Vehicle or drying oils or medium (3) Thinners (4) Driers (5) Fillers or extenders (6) Plasticizers (7) Anti-skinning agents
