Notes that will help you understanding the concepts in the easiest way, Schemes and Mind Maps of Chemistry

Download Notes that will help you understanding the concepts in the easiest way and more Schemes and Mind Maps Chemistry in PDF only on Docsity! What is corrosion? Cathode Anode METAL 0] 51; (0h) (0) Any process of decay and destruction of metal by unwanted chemical or electrochemical attack by its environment or surrounding starting at surface is know corrosion. Example: 1. Rusting of iron 2. formation of green layer on copper surface. CORROSION ENGINEERING The branch of Engineering that deals with the study of corrosion mechanisms and to prevent or control it economically and safely is known as Corrosion Engineering. FACTS ABOUT CORROSION Corrosion is a natural process. Corrosion is an electrochemical process. Corrosion leads to tremendous loss. It can’t be eliminated completely. However, its extent can be minimized. Corrosion is exactly the reverse of extraction of metals and also known as weeping of metals. state) Shu oRN Mee Ree K ON i) Oxidation Corrosion aN 2M™ + 2ne (oxidation) n/20, + 2ne —>+ nO* (reduction) 2M + n/20, —>, 2M" + nO Metal oxide Mechanism of corrosion: 1)Adsorption of O, on metal surface 2) Dissociation of O, molecule into atoms 3) Loss of electron by metal atom (being electropositive to oxygen atom resulting in the formation of metal ion and oxide ion. 4) Formation of metal oxide by anion of the ions and leading finally to form thin layer of film of metal oxide on the surface of metal. Nature of metal oxide layer They are four type a)Porous film, b)Non Porous(stable) c)Unstable d) Volatile a)Porous film: Porous film-Having pores or cracks The alkali metals like Li, K, Na and alkali earth metals like Ca, Mg, Sr form oxides of this type. Oxides of this type form volume less than the volume of metal form oxide is formed Permits free access of oxygen to fresh metal surface for action and corrosion nonstop. Examples:Iron,steel,magnisum,sodium,potassium,calcium metals are porous. c) Volatile film- oxide layer volatilizes leaving the underlying metal surface for further attack. E.g. molybdenum oxide (MoO;) is volatile. BE clatilizes when the oxide layer is volatile it volatilize(or vaporizes) as soon as it is formed. Therefore the fresh metal surface is exposed continuously to the atmosphere 111) Liquid metal corrosion- occurs due to the chemical action of flowing liquid metal at high T on solid metal or alloy. Possible processes are: * Dissolution of a solid metal by a liquid metal * Internal penetration of the liquid metal into the solid metal Corrosion by other gases- by CO,, SO,,Cl, H,S, aia Extent of corrosion depends upon the chemical affinity between metal and the gas involved and the nature of the film formed on the surface. They are 1) Halogen/Chlorine PAE OTe (ON Meme ile-vbeVbI rca eNyOeLUy 3) Decarburization and hydrogen embrittlement 4) CO and Co2 PE MMLC MEI (b)In neutral or alkaline medium in the presence of O, oer Ok aes 4OH- (such type of corrosion involving O, is called oxygen type corrosion) bare surface exposed to enviroment (anode) O, + 2H,O + 4e — 40H) Cathode "Aq. Sol of electroyle (Neutral or alkaline) Fig. 5.3 Small Anodic area and large Cathodic area e.g. Rusting of iron occurs by O, in the presence of aqueous solution At anode Fe ——— Fe?+ 2e FANE Tere (2 1/20, + H,O + 2e —> 20H Overall reaction Fe + 1/20, + HO ——=> Fe** + 20H or Fe(OH), (i)In excess supply of oxygen: In excess supply of oxygen, ferrous hydroxide is easily oxidized to ferric hydroxide. 2Fe(OH), +H,0 + 1/20, ——> 2Fe(OH), Fe,0,.xH,O NACo mat hye (ii) In limited supply of oxygen: In limited supply of oxygen, black magnetite Fe,O, or ferroferric oxide is formed. Fe(OH), ——— Fe,0,.FeO.6H,O Black Unlike other types of structural corrosion, galvanic corrosion does not depend on the metal’s texture, temper, etc. Galvanic corrosion may occur with any metal, as soon as two are in contact in a conductive liquid. It works like a battery. The appearance of galvanic corrosion is very characteristic. It is not dispersed like pitting corrosion, but highly localized in contact zone with the other metal. The zone affected by galvanic corrosion often has a shinier aspect than the rest of the surface. 25 Zn Zn” +2¢ Electronic current Fig. 5.7 Galvanic Corrosion eae oe The galvanic corrosion is possible in following cases: * Two dissimilar metals in contact ¢ Stressed and unstressed parts on the same metal ¢ Impurities in metal Factors affecting galvanic corrosion: (i) Potential difference between the two metals coupled (ii) Relative area of cathode and anode e.g. a) Steel pipe connected to copper plumbing. b) Steel screw in brass marine hardware FORMS OF CORROSION ¢ There are basically eight types of corrosion |. Underground corrosion and soil corrosion Il. Erosion Ill. Crevice corrosion IV. Pitting corrosion V. Transgranular and Intergranular (Intercrystalline ) corrosion Vl. Water line corrosion VII. Stray line corrosion VIII. Microbiological il FORMS OF CORROSION Following are the special names given to various examples of electrochemical corrosion taking plae under different situations: (1)Underground or soil corrosion: Corrosion of underground metallic structures by soil action electrochemically, is soil corrosion. It depends upon Soil, electrical conductivity, moisture and soil texture cr eererercr Cathode eae crercl H,0 + 40, + 2e > 20H” H,0 + 140, + 2e 3 el (Large cathodic area) M Anode & M3M"+ne (Small anodic area) Fig. 5.8 Mechanism of Pitting Corrosion [V] Waterline Corrosion |_— Water line ations pee Anode ae er ear a S (Less oxygenated Ca part) — = 4 More oxggenated part Cathode ae wa a a Ba ace oacas ec ama) poe Sa em | eae (SD em emme DE SY A Lo s/f es maak Vaan cet mum oo) aa Ooms LT a oa a ee Se eas | es) came sy) oe e ccoe| AS es 20 TT e Fa cae ema ae a ee as om ee Se wee A a | an poe A, oe SN) aa eros ‘gia came cms | twee enn) pes ae moun ARN (OB) wet) Pr eae Oo ee ee Re ee en eee ceca es aes a ge aoe) ae nome me oS —— Water Fig, 5.13 Water line Corrosion [vi] Micro-Biological Corrosion : Due to metabolic activity of various micro- organisms [vil] Stray-Current corrosion Metal structure such as water pipes,gas, pipes,poles etc. near power line,may get corroded rapidly due to eletron leakages from main circuit power line. protective or non-porous. E.g. AgCl layer formed by the attack of Cl, on Ag in y non-protective or porous. E.g. i) formation of volatile SnCl, by the attack of dry Cl, on Sn. ii) in petroleum industry, H,S at high T attacks steel forming porous FeS scale Factors Influencing Corrosion 1. Nature of the metal 2.Nature of Environment Factors affecting chemical corrosion 1. Nature of the metal 1) Position in the Galvanic series 11) Relative areas of anode and cathode ii) Purity of metal iv) Physical state of metal v) Nature of oxide film vi) Over voltage (111) Effect of pH (iv)Nature of ions present: Chloride ions present in the medium destroy the passive film, while silicate in the medium leads to the formation of insoluble film over the metal surface. C (v) Conductivity of corroding medium i Aetiva- Pasive ar Corrosion Corrosion Metals behavior as function of oxidizing power (E) and pH CORROSION CONTROL Electrochemical protection or cathodic protection The Electrochemical protection or cathodic protection evolves the supply of electrons to metal there by forcing the metal behave as cathode and as the result metal is not corroded. Cathodic protection can be achieved in two ways: Sesaeriticial Anodic protection In this protection the metallic equipment to be protected is connected to a more anodic metal (Zn,Al,Mg)so the metallic equipment acts as cathode. As a result corrosion take place at anodic metal and cathodic metallic equipment is saved from corrosion. *The more active metal so employed is called sacrificial anode *The purpose of increasing electrical contact , the active metal is placed in blackfill *Corroded scrifical anode is replaced by fresh one. Application(1)Buried steel pipelines (2) buried cables etc. [11] Cathodic Protection (i) By sacrificial anode: Flow of electron through external circuit > e Sacrificial anode Structure Mg -» Mg" + 2e Fig. 5.21 Sacrificial Cathodic Protection | CATHODE 2 of \ : oT) | IMPRESSED = SS oee = “, ANODE CATHODE 5 ye CORROSION CURRENT STOP FLOWING Wo OF CORROSION CURRENT SUPPRESSED BY PROTECTM NT DISCHARGED FROM IMPRESSED CURRENT SYSTE Advantages over sacrificial cathodic fe) cel xzted ela) ¢ It is controlled from outside. *No anode has to be replaced. Anodic protection TRANS PASSIVE REGION (8) PASSIVE REGION ACTIVE-PASSIVE TRANSITION APPLIED F POTENTIAL, E Oy CORROSION REGION (ACTIVE) a | eS See nd LOG (CURRENT DENSITY) }9———————_}>>- 4. Metallic coatings i. Electroplating ii. Hot dipping(Galvanising and tinning) ili. Cementation iv. Metal spraying Cementation: The base metal articles are packed in the powdered coating metal and is heated to a temperature just below the m.p. of more fusible metal, so that an alloy layer is formed over the surface. 1. Metallic coating Metallic coatings provide a layer that changes the surface properties of the work piece to those of the metal being applied. ® Hot dipping The base metal to be coated is immersed in a bath of the molten coating metal © Electroplating It is the process by whish a coating metal is deposited on the base metal by passing a direct current through an electrolyte solution. ® Metal cladding * The base metal to be protected and coating metal are sandwiched by pressing through rollers under the action of heat and pressure. & Cementation In this process a uniform surface coating is obtained by heating the base metal in a power of coating metal. SeMvietal Spraying The coating metal in molten state is sprayed on base metal by means of spraying gun. PROTECTIVE COATINGS (A) METALLIC COATINGS e Metallic coatings can be divided in two categories: * ANODIC COATINGS * CATHODIC COATINGS Electroplating Rectifier Metal cladding Cladding metal A Base Metal =. ~ Rolls Metal cladding Solid Metal Cladding Copper-Clad Steel (CCS) Process *» Carbon steel rod >Two strips of high purity copper > Pressure & heat > Metallurgical Bond