CORROSION CHATAP

CORROSION : 

CORROSION

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Metals exposed to atmospheric gases, water and liquid medium. Converted to their ores. Eg : Rusting of iron “ The process of decay of metal by environmental attack”

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Effects of corrosion Reduced Strength Life time is reduced Metallic properties are lost Wastage of metal

Theories of Corrosion: 

Theories of Corrosion

CHEMICAL (OR)DRY CORROSION: 

CHEMICAL (OR)DRY CORROSION THE DIRECT CHEMICAL ATTACK OF THE ATMOSPHERIC CONTENT ON METAL SURFACES IN THE ABSENCE OF MOISTURE . OXIDATION CORROSION: DIRECT ACTION OF OXYGEN AT HIGH(OR)LOW TEMPARETURES ON METAL SURFACE. CORROSION BY OTHER GASES: ATTACK OF GASES LIKE SO 2 , CO 2 , H 2 S, Cl 2 ,F 2 etc ON METAL SURFACE. LIQUID METAL CORROSION: ATTACK OF INORGANIC LIQUID METALS ON SOLID METALLIC SURFACE

OXIDATION CORROSION: 

OXIDATION CORROSION Atmospheric O2 2M 2 M2+ 2 e- ( Oxidation by loss of electrons ) O2 2 e- 2 O2- ( Reduction by gain of electrons ) Total reaction 2 O2- O2 2M 2 M2+ 2 MO

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If the formed metal oxide is stable further corrosion of metal is prevented by the formed metal oxide.

If the formed metal oxide is unstable corrosion is not occur.: 

If the formed metal oxide is unstable corrosion is not occur.

If the formed metal oxide is volatile ,fresh metal surface is rapidly exposed and converted into metal oxide and evaporated.: 

If the formed metal oxide is volatile ,fresh metal surface is rapidly exposed and converted into metal oxide and evaporated.

If the formed metal oxide layer is porous , under laying metal is attacked and converted in to metal oxide. the total metal is converted into metal oxide form.: 

If the formed metal oxide layer is porous , under laying metal is attacked and converted in to metal oxide. the total metal is converted into metal oxide form.

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12 Three types of oxides may form, depending on the volume ratio between the metal and the oxide: (a) magnesium produces a porous oxide film, b) aluminum forms a protective, adherent, nonporous oxide film, and (c) iron forms an oxide film that spills off the surface and provides poor protection. Pilling- Bedworth rule - Describes the type of oxide film/layer that forms on a metal surface during oxidation. specific ratio= volume of metal oxide/volume of metal

Corrosion by other gases: 

Corrosion by other gases The gases such as SO 2 , CO 2 , H 2 S, Cl 2 ,F 2 etc., when come in direct contact with metal surface corrosion is occurs. The extent of corrosion depends on the chemical affinity between the metal and the gas concerned. The prevention of metal corrosion can be known from the nature of corrosion product ., i.e. whether the layer of corrosion product is protecting or non protecting in nature .

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If the formed corrosion product is protecting (or) non porous metal is prevented . Ex: AgCl layer on metallic silver by the action of Cl2 gas. 2. If the formed corrosion product is non protecting (or) porous , the corrosion of metals occurs non stop. Ex: H2S gas attacks on steel at high temperature forming FeS , a corrosion product which is porous.

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Liquid metal corrosion The chemical action of the flowing liquid metal at high temperature , on a solid metal or alloy produces liquid metal corrosion. There are two reasons for this corrosion Dissolution of the solid metal by liquid metal Internal penetration of the liquid metal into the solid phase, weakening the solid metal. Ex: liquid Na or liquid Nitrogen used as a coolant in a nuclear plants, these causes cadmium corrosion.

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wet corrosion (or) Electro chemical corrosion Anodic reaction : Oxidation of metal Cathodic reaction : consumption of electrons The metal surface undergoes an electrochemical reaction with the moisture and oxygen in the atmosphere. This theory is known as electrochemical theory of corrosion . Mechanism :

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M(s)  M 2+ (aq) + 2e - (Dissolution or corrosion of metal takes place) 2 H + (aq) + 2e -  H 2 (Acidic medium) ½ O 2 (g) + H 2 O (aq) +2 e - OH - (Neutral medium) Mechanism of wet corrosion

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Wet corrosion takes by the following two ways based the medium: 1. Evolution of H2 : Anode: Fe(s)  Fe 2+ (aq) + 2e - 2 H + (aq) + 2e -  H 2 Cathode:

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Absorption of O2 : Anode: Fe(s)  Fe 2+ (aq) + 2e - Cathode: ½ O 2 (g) + H 2 O (aq) +2 e -  2OH -

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Water Rust Iron Dissolves- Fe ® Fe +2 e - Salt speeds up process by increasing conductivity O 2 + 2H 2 O +4e - ® 4OH - Fe 2+ + O 2 + 2H 2 O ® Fe 2 O 3 + 8 H + Fe 2+

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Dry or chemical corrosion This occurs at dry conditions Corrosion is uniform It is a slow process It involves direct chemical attack Explained by absorption mechanism Wet or electrochemical corrosion This occurs at wet conditions (electrolytic medium) Corrosion Is not uniform It is a rapid process It involves formation of electro chemical cells Explained by mechanism of electro chemical reactions

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Galvanic corrosion is an electrochemical corrosion. It is due to a potential difference between two different metals connected through a circuit for current flow to occur from more active metal (Anode) to the more noble metal (Cathode) Galvanic corrosion Galvanic coupling is a galvanic cell in which the anode is the less corrosion resistant metal than the cathode

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Zn (s)  Zn 2+ + 2e - Anode (oxidation): Cathode (reduction): ½ O 2 +H 2 O +2 e -  2OH - Zn 2 + + 2OH -  Zn(OH)2 (Corrosion product) e - e - e - e - e -

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HCl Fe +2 e - e - e - e - e -

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Galvanic Cells 19.2 spontaneous redox reaction anode oxidation cat hode red uction - +

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Concentration cell corrosion High conc.O2 Low conc.O2 e -  Zn (s)  Zn 2+ + 2e - oxidation : ½ O 2 +H 2 O +2 e -  2OH - Reduction : Zn 2 + + 2OH -  Zn(OH)2 (Corrosion product) Because of differential aeration the concentration of O2 is varied and caused for Oxidation of metal . This is two types : Pitting corrosion Water line corrosion

Factors influencing corrosion: 

Factors influencing corrosion Solution pH. Oxidizing agent. Temperature. Velocity. Surface Films. Other Factors.

Solution pH: 

Solution pH Metals such as iron dissolve rapidly in acidic solution. In the middle pH range (4 to 10), the conc of H+ ions is low. Hence, the corrosion rate is controlled by the rate of transport of oxygen. Certain amphoteric metals dissolve rapidly in either acidic or basic solution. E.g. Al and Zn. Noble metals are not affected by pH. E.g. gold and platinum. H + ions capture electrons and promote anodic corrosion.

Oxidizing agents: 

Oxidizing agents Oxidizing agents accelerate the corrosion of one class of materials, whereas retard another class. Oxidizing agents such as oxygen react with hydrogen to form water. Once hydrogen is removed, corrosion is accelerated. E.g. copper in NaCl Oxidizing agent retard corrosion due to formation of surface oxide films, which makes the surface more resistant to chemical attack. Thus a balance between the power of oxidizing agent to preserve the protective layer and their tendency to destroy the protective film determine the corrosion of metal.

Temperature : 

Temperature Rise in temp increase rate of corrosion. Increase in temp reduce the solubility of oxygen or air. The released oxygen enhances the corrosion. Increase in temp induces phase change, which enhance the rate of corrosion. At high temp organic chemicals are saturated with water. as temp decreases, water gets condensed. Oxygen is needed for maintaining iron oxide film. In the absence of O2 corrosion of S.S. increases. Copper based alloys do not depend on oxide film for corrosion.

Velocity : 

Velocity High velocity of corrosive medium increases corrosion. Corrosion pdts are formed rapidly, bcz chemicals are brought to the surface at a high rate. The accumulation of insoluble film on the metallic surface is prevented. So corrosion resistance of these films decreases. The corrosion pdts are easily stifled and carried away, thereby exposing the new surfaces for corrosion.

Surface Films: 

Surface Films The oxide films are formed on the surface of S.S. these films absorb moisture, which delay time of drying and hence increases the extent of corrosion. Insoluble slats such as carbonates and sulphates may be precipitated from hot solution on the metal surfaces. These protects the metal surfaces. If the film is porous (e.g. ZnO ) corrosion continues. Nonporous films ( CrO on iron) prevents further corrosion. Oil and grease films may occur on the surface either intentionally or naturally. These films protect surface from direct contact with corrosive substance. E.g. metals submerged in sewage .

Other factor : 

Other factor The conc of corrosive chemicals. In distillation columns, evaporators, the conc can change continuously, so difficult to predict the corrosion rate. The presence of moisture that collects during cooling can turn innocuous chemicals into dangerous corrosives.

Type of Corrosion: 

Type of Corrosion Four Type of corrosion 1. Fluid corrosion, General 2. Fluid corrosion, Localized 3. Fluid corrosion, Structural 4. Fluid corrosion, Biological.

1. Fluid corrosion, General : 

1. Fluid corrosion, General When corrosion is generally confined to a metal surface, it is known as general corrosion. It occurs in uniform fashion over the entire exposed surface area. Two type general corrosion 1. Physicochemical corrosion 2. electrochemical corrosion

1. Fluid corrosion, General : 

1. Fluid corrosion, General

2. Fluid corrosion, Localized : 

2. Fluid corrosion, Localized It is most commonly observed on diff location. Four type 1. Specific site corrosion 2. Stress induced corrosion 3. Liquid flow related corrosion 4. Chemical reaction related corrosion

A)Specific site corrosion: 

A)Specific site corrosion Mechanically weak spots or dead spots in a reaction vessel cause sp site corrosion. Three type : 1) Inter-granular corrosion 2) Pitting corrosion 3) Crevice corrosion

a)Inter-granular corrosion : 

a)Inter-granular corrosion Selective corrosion that occurs in the grain boundaries in a metal/alloy is called as inter-granular corrosion. When it is severe it causes loss of strength and ductility. E.g. Austenitic S.S + HNO3 grain boundary ppt. S.S is stabilized by adding niobium/titanium (less than 0.03 %).

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Pitting Corrosion is an extremely localized corrosion mechanism that causes destructive pits. b)Pitting corrosion

c)Crevice corrosion : 

c)Crevice corrosion Here, corrosion take place in crevices bcz solutions retained at this place and takes longer time to dry out. When this occurs, the severity of attack is more severe at crevices. Crevices are formed bcz of the metal contact with another piece of the same or other metal or with a nonmetallic material. Corrosion in crevice is due to deficiency of O2, Acidity changes, Depletion of inhibitor.

Pitting and Crevice Corrosion : 

Pitting and Crevice Corrosion

B) Stress induced corrosion: 

B) Stress induced corrosion Residual internal stress in metal external applied stress accelerate the corrosion. Residual internal force is produced by: Deformation during fabrication Unequal rate of cooling from high temp. Internal stress rearrangement involving volume changes Stress induced by rivets, bolts and shrink fits. Eliminating high stress areas prevent this type of corrosion.

a)Stress Corrosion Cracking : 

a)Stress Corrosion Cracking At the surface, if the tensile stress is equal to or more than yield stress, the surface Develops crack is known as stress Corrosion cracking. E.g. cold formed brass develops crack in the environment of ammonia. Embrittlement of cracking of steel is observed in caustic solution.

b)Corrosion fatigue: 

b)Corrosion fatigue Corrosion fatigue is the ability of metal surface to withstand repeated cycle of corrosion. The metal surface is stressed and simultaneously attacked by the corrosive media. Pits indicating corrosion are formed initially, which further develops in to cracks. The protective surface oxide film reduces corrosion. Under cycling or repeated stress conditions, rupture of protective oxide films takes place at a higher rate than at which new protective films can be formed. So the rate of corrosion is enhanced.

c)Fretting corrosion: 

c)Fretting corrosion Fretting corrosion occurs when metals slide over each other and cause mechanical damage to one or both. During relative movement of metals, two process may occur, (i) frictional heat is generated, which oxidize the metal to form oxide films. (ii) removal of the protective films resulting in exposure of fresh surface to corrosion attack. This can be avoided by using harder materials, minimizing friction by lubrication or by proper designing of the equipment.

C)Flow related corrosion: 

C)Flow related corrosion Liq. Metals can cause corrosion. The driving force is the tendency of the liq. To dissolve solids or penetrating the metal along the grain boundaries at place of wetting. E.g. mercury attack on Al alloy Molten Zinc on S.S.

a)Impingement corrosion: 

a)Impingement corrosion Also referred as erosion corrosion or velocity accelerated corrosion. It is accelerated by removal of corrosive products, which would otherwise tend to stifle the corrosion reaction.

b)Erosion corrosion : 

b)Erosion corrosion Erosion is the destruction of metal by abrasion and attrition caused by the flow of liq./gas. Factors that influence erosion 1) Alloy content of the steel (e.g. Cr, Cu, Mn ) 2) Pipe system design and component geometry. 3) Water and steam composition (especially pH and oxygen content). The use of harder metals and changes in velocity or environment are used to prevent erosion.

c)Cavitation corrosion: 

c)Cavitation corrosion Formation of transient voids or vacuum bubbles in a liq stream passing over a surface is known as cavitation. The bubble may collapse on the metal surface thereby causing severe impact or explosive effect. So considerable damage and corrosion is observed. Cavitation corrosion is also observed around propellers, rudder in pumps etc.

D)Chemical Reaction related corrosion: 

D)Chemical Reaction related corrosion Corrosion involves chemical reactions such as oxidation and reduction. Galvanic corrosion Oxygen conc cell Hydrogen embrittlement

a)Galvanic corrosion: 

a)Galvanic corrosion It is associated with the flow of current to a less active metal from a more active metal in the same environment. Coupling of two metals, which are widely separated in the electrochemical series, generally produces an accelerated attack on the more active metal, zinc.

b) Oxygen conc cell: 

b) Oxygen conc cell It is due to the presence of oxygen electrolytic cell. i.e. diff in the amt of oxygen in solution at one point exists when compared to another. Corrosion is accelerated when the O2 is least, for example, under gasket, stuffing boxes etc.

c)Hydrogen embrittlement: 

c)Hydrogen embrittlement hydrogen can penetrate carbon steel and react with carbon to form methane. The removal of carbon result in decreased strength. Corrosion is possible at high temp as significant hydrogen partial pressure is generated. This cause a loss of ductility, and failure by cracking of the steel. Resistance to this type of attack is improved by allowing with chromium / molybdenum.

c)Hydrogen embrittlement: 

c)Hydrogen embrittlement Hydrogen damage can also result from H2 generated by electrochemical corrosion reaction. The result is failure by embrittlement , cracking, and blistering. This is observed in solution of sp weak acids such as hydrogen sulphide and HCN.

3.Fluid corrosion: Structural: 

3.Fluid corrosion: Structural Here, the strength is reduced on account of corrosion. This may occur when one component of the alloy is removed or released into solution. The corrosion pdt may retain in the plant. E.g. Graphite corrosion Dezincification

a)Graphite corrosion: 

a)Graphite corrosion Graphite is allotropy of carbon. Graphite corrosion may occur in gray cast iron. Metallic iron is converted in to corrosive pdts leaving a residue of intact graphite mixed with iron corrosive pdts and other insoluble constituent of cast iron. When the layer of corrosion is impervious corrosion will cease. If layer is porous corrosion will be greater.

a)Graphite corrosion: 

a)Graphite corrosion When carbon steel is heated for prolonged periods at temp greater than 455 C, carbon may segregated, which is then transformed in to graphite. So the structural strength of the steel is affected. Employing killed steels of Cr and Molybdenum or Cr and Ni can prevent this type of corrosion.

b)Dezincification :: 

b)Dezincification : It is seen in brass containing more than 15 % zinc. In brass the principle pdt of corrosion is metallic copper, which may redeposit on the plant. Another mechanism involves the formation of zinc corrosion pdts . Corrosion may occur as a plug filling pits or as a continuous layer surrounding the unaffected core of brass. It can be reduced by addition of small amt of arsenic, antimony or phosphorus to the alloy.

4. Fluid corrosion : Biological: 

4. Fluid corrosion : Biological The metabolic action of M.O. can either directly or indirectly cause deterioration of a metal. Such a process is called as a biological corrosion. The cause of biological corrosion are: Producing corrosive environment or altering environment composition. Creating electrolyte conc cells on the metal surface. Altering resistance to surface films. Influencing the rate of cathodic/ anodic reaction.

4. Fluid corrosion : Biological: 

4. Fluid corrosion : Biological The role of biological corrosion may be explained by sulphate reducing bacteria in slightly acidic or alkaline soils. Sulphate Hydrogen Sulphite Calcium Sulphite Iron Sulphide Corrosion pdt Reducing bacteria Anaerobic On Iron in Soil

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Factors affecting the rate of corrosion Nature of metal : Position in galvanic series Over voltage Relative areas of cathodic and anodic parts Purity of metal Physical state of metal Nature of surface film Passive character of metal Volatility of corrosion product Solubility of corrosion product

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2. Nature of corroding environment : Temperature Humidity of air Presence of impurities in atmosphere Nature of ions present in environment Conductance of corroding medium Amount of oxygen in atmosphere Velocity of ions which flow in the medium PH value of the medium Suspended impurities

Corrosion control methods: 

Corrosion control methods Proper design Selection of metals Modifying the environment Sacrificial anodic protection Corrosion inhibitors Surface coatings (Paints, Electroplating, etc.)

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Corrosion control methods Cathodic protection: The method of protection given to a metal by forcibly making it to behave like a cathode. Sacrificial anodic protection Impressed current cathodic protection Use of inhibitors: A substance which when added in small quantities to the aqueous corrosive environment , effectively decrease the corrosion of a metal .

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1.Sacrificial anodic protection : In this method of protection , the metallic structure to be protected called “base metal” is connected to more anodic metal through a wire. 2.Impressed current cathodic protection : In this method an impressed current is applied in the opposite direction to nullify the corrosion current , thus the anodic corroding metal becomes cathodic and protected from corrosion.

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Cathodic protection of a buried steel pipeline: (a) A sacrificial magnesium anode assures that the galvanic cell makes the pipeline the cathode. (b) An impressed voltage between a scrap iron auxiliary anode and the pipeline assures that the pipeline is the cathode. Corrosion inhibitors # Anodic inhibitors: These inhibitors avoid the corrosion reaction occurring at the anode. Ex: Chromates , Phosphates , Tungstates

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# Cathodic inhibitors: I n acidic medium corrosion may reduced by : slowing down the diffusion of H+ ions by adding organic inhibitors like amines, mercaptans, hetero cyclic nitrogen compounds. By increasing the over voltage of hydrogen evolution by adding inhibitors like Antimony and Arsenic oxides I n neutral medium corrosion may reduced by : Eliminating oxygen from the corrosion medium by the addition of Na2SO3, Na2S ; Retarding the diffusion to the cathodic area by the inhibitors like Mg , Zn or Ni salts.

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Cathodic Protection of an Iron Storage Tank

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Electroplating Electroplating is a process of depositing a thin layer of a fine and superior metal (like chromium, zinc, nickel, gold etc.) over the article of a baser and cheaper metal (like iron, copper, brass), with the help of electric current. Electroplating with Nickel Electroplating with copper

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Before electroplating the metal surface is cleaned thoroughly. Firstly, an alkaline solution is used to remove grease and then it is treated with acid to remove any oxide layer. It is then washed with water. The article to be electroplated is made cathode since metallic ions are positive and thus get deposited on the cathode. The anode is made of pure metal, which is to be coated on the article. The electrolyte is the salt of the metal to be coated on the article. A direct (D.C.) current is passed through the electrolyte. The anode dissolves, depositing the metal ions from the solution on the article in the form of a metallic coating. The process of electroplating involves the following steps:

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Electroplating with Nickel : Electrolyte - Aqueous solution of nickel sulphate Cathode - Article to be electroplated (nail) Anode - Block of nickel metal. Dissociation of nickel sulphate

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Electroplating with copper

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Electroless plating Electroless plating is a process in which no electrical energy is needed. In this process a noble metal from its salt solution is deposited on the surface of a less noble metal which is catalytically active , using a suitable reducing agent. The reducing agent reduces the metallic ions to metal which gets deposited on the metal surface to be protected. Thin and uniform coating is formed in this process.

Application of corrosions: 

Application of corrosions

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