UNIT 2
ELECTRO CHEMISTRY AND CORROSION
The electrode potential is defined as the Potential difference developed between the metal ions from metal to the solution or from solution to the metal. At equilibrium the potential difference remains constant. The electrode potential of a metal is defined as the direct measure of its tendency to get reduced is called reduction potential, its value is +x volts. Similarly the tendency of an electrode to lose electrons is a measure of its tendency to get oxidized is called oxidation potential, its value is –x volts.
Expression of electrode potential:
Consider the following redox reaction
Mn+ + ne- ↔ M
For such a redox reversible reaction, the free energy change (∆ G) and its equilibrium constant (K) are related as;
∆ G = -RT ln k + RT ln [product]/[Reactant]
∆ G0 + RT ln [product]/[Reactant]………………..(i)
Where
∆ G 0 = standard free energy change.
The above equation is known as Van’t Hoff Isotherm.
The decrease in free energy in the reversible reaction will produce electrical energy i.e.
-∆ G = nEF and ∆ G 0 = -nE0F…………………………(ii)
Where
E = Electrode potential
E0 = Standard electrode potential
F = Faraday (96,500 coulombs)
Comparing equation 1 & 2
-nEF = -nE0F + RT ln [M]/[Mn+]
= -nE0F + Rt ln 1/ [Mn+]
Where, concentration of the metal is unity or
-nEF = -nE0F - RT ln [Mn+]
Dividing the equation by –nF
E= E0 + RT ln [Mn+]/nF
E= E0 + 2.303RT log [Mn+]/nF
E= E0 + 0.0591 log [Mn+]/n ……………(iii)
This equation-3 is known as “Nernst Equation” for electrode potential
Reference electrode are the electrode with reference to those, the electrode potential of any electrode can be measured. It can acts both as an anode or cathode depending upon the nature of other electrode.
The Reference Electrodes can be classified in to two types
i) Primary reference electrodes Ex: Standard hydrogen electrode
Ii) Secondary reference electrodes Ex: Calomel and Ag/Agcl electrodes
Construction and working of Standard Calomel electrode (SCE)
1. Calomel electrode is a metal-metal salt Ion electrode.
2. It consists of mercury, mercurous Chloride and a solution of KCl. Mercury is placed at the bottom of a glass tube.
3. A paste of mercury and mercurous chloride Is placed above the mercury. The space above the paste is filled with a KCl solution of known concentration.
4. A platinum wire is kept immersed into the mercury to obtain electrical contact.
5. Calomel electrode can be represented as:
Hg|Hg2Cl2|KCl
Calomel Electrode:
Calomel electrode is particularly very simple to construct, free from surface sensitivity and accurate to use even in a very normal laboratory.
The calomel electrode consists of an inner glass tube and an outer jacket. In the inner glass tube a platinum wire is dipped into mercury which rests on a paste of mercurous chloride, Hg2Cl2 (commercially known as calomel) and mercury. This paste is in contact with KCl present in the outer jacket, through the glass frit plug fixed at the bottom of inner glass tube. The calomel electrode comes in contact with the experimental solution through a frit arranged to the outer jacket. The potential of this electrode depends on the concentration of KCl taken in the outer jacket.
Glass electrode:
Most often used pH electrodes are called glass electrodes and belong to the family of ISE. They are sensitive only to H+ ions. Typical glass electrode is made of glass tube engaged with small glass bubble sensitive to protons. Inside of the electrode is usually filled with buffered solution of chlorides in which silver wire covered with silver chloride is immersed.
PH of internal solution varies- E.g.; it can be 1.0(0.1M HCl) or 7.0 Active part of the electrode is the glass bubble. While tube has strong and thick walls, bubble is made to be as thin as possible. Surface of the glass is protonated by both internal and external solution till equilibrium is achieved. Both sides of the glass are charged by the adsorbed protons, this charge is responsible for potential difference. This potential in turn is described by the Nernst equation and is directly proportional to the pH difference between solutions on both sides of the glass. The majority of pH electrodes available now a day are combination electrodes that have both glass H+ ion sensitive electrode and reference electrode compartments, conveniently placed in one housing.
This is a redox electrode reversible to protons and often replaces the hydrogen electrode. Quinhydrone is a 1:1 molar mixture of quinone and hydroquinone. The electrode consists of a shiny platinum electrode dipped in a acid / base test solution, which is saturated with quinhydrone.
Advantages:
- The quinhydrone electrode is simple to set up and needs no removal of air.
- The reversibility equilibrium is achieved faster than hydrogen gas electrode thereby allowing a quicker measurement.
- PH values of solutions containing reducible substances like Cu2+, Cd2+, unsaturated acids, NO3 - , etc., and catalytic poisons can be measured using quin-hydrone electrode.
Limitations:
- The electrode can not be used at pH values greater than 8.
This electrode also fails in the presence of strong oxidizing and reducing agents.
Batteries:
The electrical interconnection of two or more electrochemical cells, each of which contain two electrodes and an electrolyte is called a Battery. The condition at which battery is properly working is supply of electric power, the positive terminal is cathode while the negative terminal is anode.
Primary (Lithium cell)
It consists of lithium anode with solid electrolyte or liquid electrolyte and solid or liquid cathode. A thin protective insulating film is formed on lithium anode protecting the anode against corrosion as it is conductive to lithium ions but not electrons while water and alcohol never form such film.
Lithium iodide solid cathode cell consists of iodine PVP cathode with 3V voltage. It is highly stable and dependable and hence used in medical source for electronic flash guns of cameras.
Lithium Ion Cells
Anode: Graphite, Carbon compound.
Cathode:Oxide of Lithium
Uses:
Used in Laptops, cellular phones, electronic vehicles.
Secondary Batteries:
Lead-Acid Storage Battery & Lithium Ion Battery:
Lead storage battery is the most common device used to store energy in the portable form. This is also called as lead acid battery. Although the batteries are reliable, which contain acidic material inside that required a proper disposal method after its complete use. These batteries have moderate power density and good time. The battery consists of lead grids on its electrodes. The anodic grid opening is filled with spongy lead while the cathodic grid consists of lead oxide (PbO2).
Charge Chemistry of the battery:
Charge batteries are those batteries which can be recharged after single use. In this type of battery each plate contain negative as well as the positive end. The negative plate is of lead while the positive plate is made up of lead oxide in an electrolyte of approx 4.0M sulphuric acid.
Negative plate reaction:
PbSO4(s) + H+(aq) + 2e– →Pb(s) + HSO4–(aq)
Positive plate reaction:
PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4–(aq) + 3H+(aq) + 2e–
Combining these two reactions, the overall reaction is the reverse of the discharge reaction:
2PbSO4(s) + 2H2O(l) →Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4–(aq)
Discharge Chemistry of the Battery:
The positive and negative plate of the batteries becomes lead sulphate. Due to the loss of sulfuric acid from electrolytes it becomes the water.
Negative plate reaction:
Pb(s) + HSO4–(aq) → PbSO4(s) + H+(aq) + 2e–
Positive plate reaction:
PbO2(s) + HSO4–(aq) + 3H+(aq) + 2e– → PbSO4(s) + 2H2O(l)
Combining these two reactions, one can determine the overall reaction:
Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4–(aq) → 2PbSO4(s) + 2H2O(l)
Causes of Corrosion:
Corrosion is the electrochemical process that occurs in various forms such as chemical forms and atmospheric forms. On the contact of acid sucstance with iron it pretend to form rust. Rust is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and moisture. When steel is exposed to water, the iron particles are lost to the water’s acidic electrolytes. The iron particles then become oxidized, which results in the formation of Fe⁺⁺. When Fe⁺⁺ is formed, two electrons are released and flow through the steel to another area of the steel known as the cathodic area.
Oxygen causes these electrons to rise up and form hydroxyl ions (OH). The hydroxyl ions react with the Fe2+ to form hydrous iron oxide, better known as rust. Where the affected iron particles were, has now become a corrosion pit, and where they are now, is called the corrosion product (rust).
Consequences of corrosion:
Corrosion has many consequences on economic, health, safety, to our society.
Economic Effects: To determine the economic cost on the country’s economy there are several studies conducted by different country. Among all the most extensive study was carried out by United State in 1975 and gathered that it cost about $70 billion.
Health Effects:Recent years have seen an increasing use of metal prosthetic devices in the body, such as pins, plates, hip joints, pacemakers, and other implants. New alloys and better techniques of implantation have been developed, but corrosion continues to create problems. E.g.- inflammation caused by corrosion products in the tissue around implants, fracture of weight-bearing prosthetic devices.
Technological effects:Technology sector is also very badly affected by the corrosion. A great deal of the development of new technology is held back by corrosion problems because material is required to withstand in many cases simultaneously higher temperature, higher pressure. Corrosion problems that are less difficult to solve affect solar energy systems, which require alloys to withstand hot circulating heat transfer fluids for long periods of time, and geothermal systems, which require materials to withstand highly concentrated solutions of corrosive salts at high temperatures and pressures.
Chemical Corrosion:The reaction of metal with water vapour or gas at high temperature causes the metal to corrode chemically. This is the redox process in which the electron of the metal are passed directly to the substance in the environment. The metal corrodes generally in the metal which is in higher contact with water.
3Fe + 4H2O Fe3O4 + 4H2
3Fe + 2O2 Fe3O4
Electrochemical Corrosion:Corrosion occurs due to the electrochemical process of oxidation and reduction process. In the corroding solution electrons are released by the metal and that is gained by teh elements in the corroding solution. The release of electron from metal is called as the oxidation while vice-versa that is gain of electron by elements is reduction. The regular electron flow in the corrosion reaction can be measured and controlled electronically. This is why controlled electrochemical experimental methods are used to characterize the corrosion properties of metal.
For example,
(i) a thin film of moisture on a metal surface forms the electrolyte for atmospheric corrosion.
(ii) when wet concrete is the electrolyte for reinforcing rod corrosion in bridges. Although most corrosion takes place in water, corrosion in non-aqueous systems is not unknown.
Corrosion process showing the anodic and cathodic component of current
Mechanism of dry corrosion due to O2 gas there are 4 types: -
- Absorption of oxygen molecules on the metal surface
- Dissociation of oxygen atom into metal atom
- Loss of e- by metal atom
- Formation of oxide layer on the metal surface.
Types of corrosion: Galvanic, water-line and pitting corrosion:
There are several types of corrosion:
(i) Galvanic Corrosion:Galvanic corrosion is the most common corrosion which can be get in notice. This corrosion occurs when two different type of metals are in contact with each other in the presence of electrolyte. In this type of corrosion noble metal are safe while the active metals corrodes.
(ii) Pitting Corrosion: This type of corrosion occurs at certain conditions, there is a accelerated corrosion in some areas rather than the uniform corrosion over the substance. This condition includes low level of concentration of oxygen or high concentration of chlorides.
(iii) Microbial Corrosion: Microbial Corrosion is caused by micro-organisms. They commonly referred to as microbiologically influenced corrosion. It applies to both metallic and non-metallic materials with or without oxygen. In the presence of oxygen there are some bacteria that directly oxidize iron to iron oxides and hydroxides while in the absence of oxygen sulphate reducing bacteria are active and produce hydrogen sulphide causes sulphide stress cracking.
(iv) High Temperature Corrosion: The deterioration caused on the metal due to heating. This can cause when the metal is kept in hot atmosphere that too in the presence of oxygen, sulphur, or with any other compound which is capable of oxidizing the material.
(v) Crevice Corrosion:This occurs in confined spaces where access of fluid from the environment is limited such as gaps and contact areas between parts, under gaskets or seals, inside cracks and seams and spaces filled with deposits.
Factors affecting rate of corrosion:
(i) Nature of metal
Position of metal in galvanic series.
If position is higher in galvanic series then it carrode faster
While for 2 metal the difference between them shows the corrosion ratio.
(ii) Potential Difference
If the difference at the electrode potential between two metal is high then the rate of corrosion would be also high while vice versa for lesser difference.
(iii) Purity of metal
Corrosion never took place in pure metals. While if metal itself has a impurity then galvanic cell set up easily which intend increases the rate of corrosion.
(iv) Relative areas of cathode and anode parts
Rate of corrosion is directly depends on the area of cathode and inversely depends on the area of anode. If the area of cathode is larger then there is more demand of electrons while in the smaller anode area the corrosion took place very fast.
(v) Nature of corrosion Product
Metal oxide film is formed on the surface of metal by corrosion due to oxygen. The formed film would be stable, unstable, volatile.
(vi) Temperature
At high temperature the rate of corrosion increases as because there is a consistent increase in the ionization and mobility difference rate while in some cases rate of corrosion decreases at high temperature as the solubility of O2 gas increases.
(vii) Presence of moisture
The rate of corrosion decreases in dry while increases in presence of moisture. Moisture act as the solvent for setting up of electrochemical corrosion.
(viii) Effect of pH
Rate of corrosion is high at acidic pH due to the evolution of H2 gas at cathode.
(ix) Concentration of electrolytes
This is also called as the Oxygen concentration cell. The rate of corrosion would be directly depend on the supply of oxygen on air.
(x) Over Voltage
The difference between the actual value and theoretical value of decomposition potential of electrode.
Cathodic Protection:
The prevention of corrosion by making metallic structure as cathode in the electrolytic cell is called cathodic protection. Since there will not be any anodic area on the metal, corrosion does not occur. There are two methods of applying cathodic protection to metallic structures.
a) Sacrificial anodic protection (galvanic protection)
b) Impressed current cathodic protection
Sacrificial anodic protection:
The metal structure can be safe from corrosion by connecting it with a copper(Cu) wire to a more active metal so that all the corrosion is concentrated at the more active metal as the more active metal is sacrificed in the process of saving metal from corrosion . Hence it is known as sacrificial anode. The most common sacrificial anodes are Magnesium, Zinc and thier alloys. Zinc is used as sacrificial anode in good electrolytes such as sea water. Magnesium is used as a sacrificial anode in high resistivity electrolytes such as soil.
Impressed Current cathodic protection:
In this method current from an external source is applied in the opposite direction to nullify the corrosion current. This is done to convert corroding metal from anode to cathode. Once the metal become cathodic it is protected from corrosion. The anode may be either inert material or active material. The anodic materials are Platinum, Graphite, Stainless steel, Scrap iron. The anode is buried in Backfill such as Gypsum to increase the electrical contact between itself and surrounding soil.
Metallic Coatings:
Iron and steel are the most commonly used construction materials due to their low cost, easy availability, ease of fabrication into desired structures. Iron and steel structures can be protected from corrosion by covering their surfaces with metallic coatings . The metallic coatings often used are Zn , Al , Tin , Cu , Cd , Cr etc ., these coatings separate the base metal from corrosive environment and also functions as an effective barrier for the protection of base metal .
Hot dipping: It is used for producing a coating of low-melting metals such as Zn (m.p.=419 deg C), Sn (m.p.=232 deg C), Pb, Al etc., on iron, steel and copper which have relatively higher melting points. The process in immersing the base metal in a bath of the molten coating-metal, covered by a molten flux layer (usually zinc chloride).
Metal Cladding:It is a process of coating of brass metal on dense homogeneous layer of cladding material. Cladding metals can be pure metals or alloys. In this process arranging Tin sheets of coating metal and the base metal sheet in the form of sandwich which is then passed through rollers under the action of heat and pressure.
E.g.: Duralumin is sandwiched between two layers of 99.5% pure aluminum. This sheet is called as Alclad used in Aircraft industry.
Electro Plating:It is the process of coating of base metal by a dense homogeneous layer of a coating material by passing a direct current through electrolytic solution consists of soluble salts of coating metal.
