Corrosion
Corrrosion
Extraction of metal is the process in which the pure metal is obtained from its compounds through different metallurgical operations. The pure metal is in a higher energy state than its respective compounds. Therefore generally the pure metals are always having a tendency to convert in to its compound of lower energy by undergoing chemical reactions with different constituent present in its close environment. This phenomenon is known as corrosion. Corrosion is a process of destruction or disintegration of metallic surface and its environment and the nature of the corrosion product.
Corrosion may broadly classify in to two types:
(a) Chemical corrosion
(b) Electrochemical corrosion
Chemical corrosion
Chemical corrosion is due to direct chemical reaction of atmospheric gases like oxygen, hydrogen, hydrogen sulphide, sulphur dioxide etc. with metallic surfaces. Generally it is divided in to two types corrosion due to oxygen, i.e. oxidation corrosion and corrosion due to other gases. Chemical corrosion is also referred to as dry corrosion by some authors. It is misnomer because a number of liquid chemicals including acids can directly attack metals and cause corrosion.
Oxidation corrosion is due to the direct reaction of atmospherics oxygen with the metallic surfaces in the absence of moisture to form the corrosion product, i.e. metal oxides.
The metallic surface undergoes oxidation y the action of atmospheric oxygen. Metal is converted in to metal oxide at the metallic surfaces. Further oxidation depends on the nature of the first oxide film formed.
Stable oxide film
If the oxide film formed is stable in nature, i.e. it is continuous and properly adhering to the metallic surface, it will protect the metal surface from further corrosion by acting as a protective layer or barrier which will prevent contact between the underlying metal with the surrounding atmosphere. Therefore further corrosion will be arrested e.g. aluminum, copper, thorium, zinc etc.
Volatile oxide film
If metal oxide film is volatile in nature it will immediately volatilize off as and when it is formed, leaving behind the underlying fresh metal surface for further attack by oxygen. Therefore severe corrosion happens to these type of metals, e.g. Molybdenum.
Porous if the metal oxide film produced is porous in nature, i.e. it contains pores, gaps or cracks in its structure, the contact between the underlying metal and the surrounding atmospheres continues and hence the corrosion proceeds continuously, e.g. lithium, sodium, potassium, calcium, magnesium etc.
Piling-bedworth rule
The protective or non-protective nature of the oxide layer depends on the volume of the corrosion produced. The rule states that an oxide layer is said to be protective or non-porous, if the volume of the metallic oxide produced is equal to or greater than the volume of the metal from which it is formed. The metal oxide layer is said to be non-protective or porous in nature if the volume of the metal oxide is less than the metal from which it is derived. If the metal oxide is porous in nature, then the atmospheric oxygen diffuses through the pores and comes in contact with the underlying metal and corrosion continues.
Corrosion by other gases
Apart from oxygen, other gases like hydrogen, hydrogen sulphide, sulphur dioxide, chlorine, carbon dioxide etc. present in the atmospheric air causes chemical corrosion to the metallic parts. When silver is exposed to chlorine atmosphere, it is converted in to silver chloride which is stable in nature and therefore further corrosion is stopped. When tin is exposed to chlorine atmosphere, it is converted in to tin chloride which is highly volatile in nature and hence severe corrosion takes place for tin material.
Hydrogen embrittlement is a type of corrosion by the action of hydrogen/hydrogen sulphide gas over metallic surfaces. When iron is exposed to atmosphere containing hydrogen sulphide usually polluted Industrial atmosphere it reacts with hydrogen sulphide forming iron sulphide and atomic hydrogen or nascent hydrogen.
The atomic hydrogen is so active and it gets diffused in to iron metal surface at the pores, voids etc. and there it combines to form molecular hydrogen gas. Due to the pressure developed inside the metallic voids, gaps etc. metallic fissures and blisters are produced. This type of metallic corrosion is called hydrogen enbrittlement.
Electrochemical corrosion
The corrosion of metallic surfaces in wet environment is more common compared to corrosion due to dry environment. A metallic surface undergoes electrochemical corrosion when (i) a metal is in contact with a conducting wet medium and (ii) two dissimilar metals are in contact with each other and in contact with the wet surrounding.
Mechanism of electrochemical corrosion
Generally when a metal is in contact with a surrounding conducting solution, separation or segregation of anodic and cathodic areas takes place. this may be due to (i) the presence of another metal in contact with (ii) the difference in the stress at different regions (iii) the difference in the concentration of substances at different regions of the metal. The metal at the anodic region undergoes oxidation and starts dissolving in to the surrounding solution, leaving behind electrons in the metals.
These electrons flow through the metal towards the cathodic areas where it is consumed by the external agents present in the surrounding conducting medium since the same metal cannot be reduced further. This flow of electrons within the metal is called corrosion current. The electrons are consumed by the substances present in the surrounding conducting medium and the reduction reaction depends on the nature of the surrounding medium.
(i) If the surrounding medium is acidic in nature, the hydrogen ions in the medium will take up the electrons and evolution of hydrogen gas from this surrounding medium takes place.
(ii) If the surrounding medium is neutral or slightly alkaline in nature, absorption of oxygen from the atmosphere takes place by the surrounding medium and the dissolving oxygen and water molecules collectively absorb the electrons forming hydroxyl ions.
The hydroxyl ions present in the cathodic surface and metal cations dissolved at the anodic surface migrate towards each other and the metal hydroxide, the corrosion product is formed at any point on the metallic surfaces.
Galvanic corrosion
When two dissimilar metals are in contact with each other which in turn are in contact with a surrounding conducting medium, formation of electrochemical cell takes place. the more anodic metal undergoes oxidation and starts dissolving in to the surrounding solution leaving solution leaving behind the liberated electrons. These electrons flow through the metal towards the cathodic metal where it is cathodic metal where it is consumed by the external substances present in the surrounding conducting solution. The type of reduction reaction depends on the nature of the surrounding solution. If the surrounding medium is neutral or slightly alkaline in nature absorption of oxygen from atmosphere takes places and formation of hydroxyl ions takes place. The metal cations and hydroxyl ions combine to form the corrosion product. This type of corrosion is called galvanic corrosion.
Concentration cell corrosion
If a metallic surface is in contact with a surrounding solution of different concentration, then getting up of a concentration cell takes place. The metal in contact with solution having lower concentration acts as anode and the metal in contact with higher concentration act as cathode. If the difference in the concentration arises due to the difference in the amount of dissolved oxygen, then this types of corrosion is called as differential oxygen corrosion or differential aeration corrosion. The less oxygenated part where it is consumed by the external substance present in the solution at the cathodic parts. This will depend on the nature of the surrounding solution as explained earlier.
Factors affecting corrosion
Nature of metal
(i) Position in the electrochemical series: Metals having positive oxidation potentials as per electrochemical series are highly susceptible to corrosion than metal having negative oxidation potentials. Greater the oxidation potential of the anode and lower the oxidation potential of the cathode, greater will be the corrosion current and hence greater will be the rate of corrosion of the anode metal.
(ii) Relative anodic and cathodic areas: Corrosion is more severe and localized if the anodic area is smaller and the cathodic is larger. The reason is that the high demand for electrons by the larger cathodic area can be met by the smaller anodic area only by undergoing rapid and severe corrosion.
(iii) Over voltage: The difference between the potential of the electrode when gas evolution is actually observed and the theoretical value of the reversible electrode potential for the same electrode is called the over voltage. The reduction in over voltage at the cathode will accelerate the rate of corrosion of the anode metal.
(iv) Purity of the metal: Impurities present in the pure metal causes heterogeneity and form tiny electrochemical cells at the exposed metallic surfaces in presence of a conducting medium. The metal having higher oxidation potential will act as an anode which dissolves in the medium and corrosion occurs. For example impure zinc undergoes corrosion when it is in contact with lead or iron. Therefore corrosion resistance of a metal depends on its purity.
(v) Nature of the corrosion product: Extent of corrosion depends upon the nature of the corrosion product. The extent of corrosion depends upon the nature of the corrosion product. The extent of corrosion depends upon the nature of the corrosion products, such as its stability and solubility. If the corrosion product is insoluble and forms a film on the surface the corrosion may not proceeds further (lead in storage battery). If the corrosion product is soluble in medium corrosion proceeds smoothly,.
Nature of the environment
(i) Temperature: Rise in temperature of the environment increases the corrosion reaction due to increase in conduction of the medium and decrease and decrease in over voltage.
(ii) Humidity: The rate of corrosion of metals depends directly on the moisture content or humidity of the surrounding atmosphere. The impurities present in the atmosphere such as sulphur dioxide, carbon dioxide etc. dissolve in the water content of the atmosphere which will act as an electrolyte for setting up an electrochemical cell which leads to corrosion.
(iii) Impurities: Polluted atmosphere, usually in industrial belts containing impurities such as carbon dioxide, sulphur dioxide, hydrogen, fumes of hydrochloric acid, sulphuric acid etc., especially in a humid atmosphere enhance the rate of corrosion.
(iv)Effect of pH: Generally acidic environment is more harmful than alkaline or neutral environment. Therefore corrosion rate of metals by acidic surrounding can be minimized by increasing the pH of the solution.
(v) Conductance of the corroding medium: In the corrosion of undergoing and submerged metallic substances, the conductance of the medium i.e. soil or solution plays an important role. Conductance of dry sandy soil is minimum than that of marshy or clayey soils. Increase in concentration of salts in the medium increases the rate of corrosion of the anode metal.
Corrosion control
The harmful effect of corrosion is tremendous as it is estimated that about 15 to 20% of the iron manufactured worldwide is lost by corrosion. It is therefore essential to prevent corrosion. But this cannot be achieved hundred percent as we cannot attain an ideal environment, i.e. uniform environment and uniform metal compositions. Therefore we can only minimize the effect of corrosion. Even if it happens to be the metallic surfaces, it should be uniform through the surface and should not be a localized one. Important corrosion controlling methods are:
Proper selection and design
Proper selection and design of the metal are the best ways of controlling corrosion:
(i) Selection: If two dissimilar metals have to be used in a particular industrial setup, we have to select the metals in such a way that their electrode potentials should be as close as possible.
(ii) Design: As the mechanical properties of a metal depend mostly on the nature of the metal, proper designing of the metallic structure should be done. Sharp corners, joints, bends, sharp edges etc. should be avoided especially in a wet environment as they are corrosion prone areas. When two dissimilar whereas the areas of cathodic metallic parts have to be kept as minimum.
Use of pure metal
Pure metals are more corrosion resistant than impure metals as the impurities in metal lead to the formation of large number of tiny galvanic arranged in series.
Cathodic protection
In cathodic protection the metallic structure to be protected is forced to behave like a cathode and corrosion occurs only at the anode. There are two types of cathodic protection.
1. Sacrificial anodic protection: In this method, the metallic structure which is to be protected is connected to a more anodic metal (a metal having higher oxidation potential) through a conducting wire. Under this condition, corrosion occurs only in the more anodic metal and then it will protect the metallic structure sacrificially. Railway lines, ship hulls, steel structures of offshore petroleum wells, undergoing pipe lines etc., are protected from corrosion by the use of magnesium or zinc as the sacrificial anodes.
2. Impressed current cathodic protection: During the process of corrosion a potential is developed and the corrosion current is due to the flow of electrons from anodic areas of the metallic structure to cathodic areas. In order to prevent corrosion at the anodic region, a direct current having slightly higher potential is sent through the metallic structure in the opposite direction with the help of a battery, which will nullify the corrosion current. Generally an insoluble electrode like graphite or silica is used as the cathode. Iron rods in RCC structure, ship hulls, steel structures of off-shore petroleum wells etc. are protected by this method.
Use of corrosion inhibitors
This is a process of modifying the environment by adding certain chemicals to the corroding medium. These chemicals are known as corrosion inhibitors, as they arrest the anodic or cathodic reactions:
1. Anodic inhibitors: Anodic inhibitors are oxygen carriers and are added to the corroding medium. It combines with the metal ions produced at the anodic regions of the metallic surface and acts as a protective coating. They prevent further corrosion as they form a barrier between the metal and the surrounding, e.g. of anodic inhibitors: chromate, phosphate or tungstate of transition metals.
2. Cathodic inhibitors: These are substances capable of reacting with the agents present at the cathode regions which absorbs electrons coming from the anodic regions and precipitate them in to insoluble compounds. If the corroding medium is acidic in nature, the cathodic inhibitors remove the hydrogen cations, e.g. of cathodic inhibitors amines, substituted urea, the urea mercaptans etc. if the corroding material is neutral or slightly alkaline in nature, the cathodic inhibitors eliminate the dissolved oxygen content of the medium which is responsible for cathodic reaction, e.g. of cathodic inhibitors sodium sulphate, nitrogen hydrate etc.
3. Using metal alloys: Alloys are homogeneous solid solutions. They are more corrosion resistant than pure metals, e.g. stainless steel is an alloy with chromium and nickel , is highly corrosion resistant because the chromic oxide formed on the surface is of high self healing type.
Different types of corrosion
Pitting corrosion
Formation of a crack or pit on the cathodic coating on the metallic surface leads to severe localized corrosion of the metal at the exposed portion. The metal at the exposed portion will act as anode and it starts dissolving in to the surrounding medium and the extent of corrosion depends on the nature of the surrounding conducting medium, e.g. tin coated.
Water line corrosion
Water storage tank undergo corrosion at the metallic portion just below the water surface region. This is in accordance with differential aeration corrosion principle. The less oxygenated part will act as anode, which is the metallic part just below the water surface and corrosion occurs.
Stress corrosion
When a metal rod is subjected to different amounts of stress at the different places then the more stressed portion acts as an anode and less stressed portion acts as cathode. The anode portion corrodes faster. Mechanical failure of a metal is increased by a corrosive environment. In steam boiler due to caustic embrittlement, boiler corrosion occurs and the metallic parts become brittle. Stress corrosion occurs at the riveted joints which are subjected to more stresses.
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