Corrosion 2019-20 PDF

UNIT III-B CORROSION 2019-20 CORROSION Syllabus: Introduction to corrosion, mechanism of dry and wet corrosion, Pilling Bedworth ratios and uses, Types of corrosion – Differential aeration corr...

UNIT III-B CORROSION 2019-20 CORROSION Syllabus: Introduction to corrosion, mechanism of dry and wet corrosion, Pilling Bedworth ratios and uses, Types of corrosion – Differential aeration corrosion, galvanic corrosion, pitting corrosion, waterline corrosion and stress corrosion, Factors affecting the rate of corrosion – metal based factors and environmental based factors, protection techniques – metal coatings – galvanization and tinning, cathodic protection, inhibitors – cathodic and anodic, organic coatings – paints – constituents and their functions. Objectives: To familiarize the dry and wet corrosion, corrosion prevention methods and factors affecting corrosion Outcomes: Students gain the knowledge on mechanism of corrosion, factors responsible, types corrosion and methods of protection. OUTLINES  Introduction  Theories of corrosion  Galvanic series  Types of corrosion  Factors influencing corrosion  Corrosion control methods  Protective coatings  Constituents of paints and their functions Engineering Chemistry Page 54 UNIT III-B CORROSION 2019-20 1. Introduction: “The phenomenon of deterioration and destruction of matter by unwanted, unintentional attack of the environment leading to loss of matter starting at its surface is called corrosion”. Examples are rusting of iron, formation of mill scales, tarnishing of silver, formation of a green film of basic carbonate (CuCO3.Cu (OH)2) on the surface of copper etc. The basic reason for corrosion is that metals are more stable as their minerals/compounds than in pure state with few exceptions like gold etc. Corrosion is a challenge for engineering materials due to enormous loss of material in corrosion. 2. Theories of corrosion- types Corrosion is broadly classified into two types. 1. Dry or chemical corrosion 2. Wet or electrochemical corrosion 2.1 Dry or chemical corrosion This type of corrosion takes place by the direct attack of gases present in atmosphere such as O2, CO2, H2S, SO2, halogens, etc., with metal surfaces in the immediate vicinity. Dry corrosion is classified into three types. i) Oxidation corrosion ii) Corrosion by other gases iii) Liquid metal corrosion 2.1.1 Oxidation corrosion: This is brought about by the direct action of oxygen on the metal surface in the absence of moisture. The oxygen atoms of the air are held close to the surface by means of weak Vander waal forces. Over a period of time, these forces results in the formation of weak bonds converting the metal into its corresponding metal oxide. The phenomenon is known as chemisorption. The following reactions are involved in oxidation corrosion. 2M Mn+ + 2 ne- (Loss of electrons) (Oxidation) n O + ne- 2 2 nO2- (Gain of electrons) (Reduction) 2 M + n O2 2 Mn+ + nO2- 2 Engineering Chemistry Page 55 UNIT III-B CORROSION 2019-20 Mechanism: Oxidation occurs at the surface of the metal first and forms a layer of deposit (oxide) that tends to restrict further oxidation. The nature of the oxide film formed plays an important role on the surface of the metal as it may be stable, unstable, volatile and porous. If a stable layer is formed on the surface, such a product prevents the exposure of the metal for further corrosion (example formation Al2O3layer on Aluminium). If unstable oxidation product is formed, the product decomposes readily and may allow further corrosion (Gold). If the product formed is volatile in nature, it readily volatilizes, leaving behind fresh metal surface. This leads to rapid and excessive corrosion. Ex: Molybdenum oxide MoO3 It a porous product is formed, an unobstructed and uninterrupted oxidation corrosion reaction takes place. 2.1.2. Pilling Bedworth Rule: According to this, “an oxide product is protective or non-porous, if the volume of oxide is at least as great as the volume of metal from which it is formed”. On the other hand, if the volume of oxide formed is less than the volume of the metal, the oxide layer is porous and non- protective. Thus smaller is the specific volume ratio (Volume of metal oxide/Volume of the metal), greater is the oxidation corrosion. Ex: Alkali& alkaline earth metals (Li, K, Na, and Mg) form oxides having volume less than the volume of metals. While Al forms oxides which is non-porous and protective. The specific volume ratios of Ni, Cr and W are 1.6, 2.0 and 3.6 respectively. Hence, the rate of oxidation of tungsten (W) is least, even at elevated temperatures. 2.1.3 Corrosion by other gases: This type of corrosion takes place by the chemical affinity of gases such as SO2, CO2, Cl2, H2S, and F2 etc. The degree of attack depends upon the formation of protective or non- Engineering Chemistry Page 56 UNIT III-B CORROSION 2019-20 protective films on the metal surface. Example, AgCl forms the protective films. SnCl4 forms a volatile product, while attack of Fe by H2S gas forms a porous FeS film. 2.1.4. Liquid metal corrosion: This type of corrosion takes place due to chemical action of a flowing liquid metal on another solid metal surface or an alloy. Such corrosion occurs in devices used for nuclear power. The corrosion involves either dissolution of solid metal by a liquid metal or internal penetration of liquid metal into solid metal, which weaken the solid metal. 2.2. Wet corrosion This type of corrosion occurs when a conducting liquid is in contact with metal or when two dissimilar metals or alloys are either immersed or dipped partially in a solution. It involves the formation of two areas of different potentials in contact with a conducting liquid. One is named as anodic area where oxidation reaction takes place, the other is referred to as a cathodic area involving reduction. The metal at anodic area is destroyed either by dissolving or by forming a combined state, such as oxides. Hence corrosion always occurs at anodic areas. At cathode, the dissolved constituents gain the electrons forming non-metallic ions. The metallic ions and non-metallic ions diffuse towards each other forming product somewhere between anode and cathode. 2.2.1. Mechanism of wet or electro chemical corrosion: Electro chemical corrosion involves flow of electric current between anodic and cathodic areas. At anode, dissolution of metal takes place forming corresponding metallic ions. M Mn+ + ne- On the other hand, at cathode, consumption of electrons takes place either by i) Evolution of hydrogen type ii) Absorption of oxygen type i) Evolution of hydrogen type: This type of corrosion occurs if the conducting medium is acidic in nature. For example, Iron dissolves and forms ferrous ions with the liberation of electrons. These electrons flow from anode to cathode, where H+ ions are eliminated as hydrogen gas. Fe Fe2+ + 2e (Oxidation) 2 H+ + 2 e- H2 (Reduction) Fe + 2 H+ Fe2+ + H2 Engineering Chemistry Page 57 UNIT III-B CORROSION 2019-20 ii) Absorption of oxygen type: A cathodic reaction can be absorption of oxygen, if the conducting liquid is neutral or aqueous and sufficiently aerated. Some cracks developed in iron oxide film cause this type of corrosion. The surface of iron is always coated with a thin oxide film. The crack developed will create an anodic area on the surface while the well coated metal parts act as cathode. The anodic areas are small and the cathodic areas are large. Corrosion occurs at the anode and rust occurs in between anode and cathodic areas. When the amount of oxygen increases corrosion is accelerated. ½ O2 + H2O + 2 e- 2OH- (Reduction) The Fe2+ ions formed at anode, and OH- ions formed at cathode, diffuse towards each other forming Fe (OH)2 i.e., Fe2+ + 2 OH- Fe(OH)2 If enough oxygen is present, the Fe (OH)2 is oxidized further to Fe(OH)3. This eventually is converted in to rust [Fe2O3 x.H2O]. 2.2.2. Difference between chemical Corrosion and electrochemical corrosion Chemical Corrosion Electrochemical Corrosion 1. It takes place in dry condition 1. It takes place in wet condition such as in the presence of electrolytes. 2. It involves the direct chemical attack of 2. It involves the formation of large number of environment of the metal. galvanic cells. 3. It takes place on homogeneous and 3. It takes place on heterogeneous surfaces only. heterogeneous surfaces. 4. Corrosion product accumulates at the same 4. Corrosion product accumulates at cathode, but place where corrosion is taking place. corrosion takes place at anode. 5. Uniform corrosion takes place. 5. Non – Uniform corrosion takes place. Engineering Chemistry Page 58 UNIT III-B CORROSION 2019-20 3. Galvanic series In the electrochemical series the elements are arranged in the Active 1. Mg increasing order of their reduction potential values. Galvanic (or anodic) 2. Mg alloys series or electrochemical series is an arrangement of metals 3. Zn 4. Al in the increasing order of their reduction potentials. The 5. Cd 6. Al alloys metals with more anodic character occupy the top positions 7. Mild steel 8. Cast iron in the series whereas the bottom positions are occupied by 9. High Ni Cast iron 10. Pb-Sn Solder more cathodic metals. A metal top in the series is more 11. Pb 12. Sn anodic and undergoes corrosion faster than the metal below 13. Lconel in the series. 14. Ni-Mo-Fe alloys 15. Brasses Examples: Mg, Zn, Al, Cd, Duralumin, steel, lead – tin 16. Monel 17. Silver solder (solder), Pb, Sn, Cu and its alloys, Cupro – Nickel, bronze, 18. Cu 19. Ni passive stainless steel, Ag, Ti, Graphite, Au, Pt. 20. Cr stainless steel 21. 18-8 stainless steel The noble character increases down this series. 22. 18-8 Mo stainless steel 23. Ag 24. Ti 25. Graphite Noble 26. Au (or cathodic) 27. Pt Although electrochemical series gives useful information regarding the chemical reactivity of metal it does not predict the corrosion behaviour of the metal several side reactions may take place which influence the corrosion reaction hence oxidation potentials of various metals and alloys are determined with SCE, immersing the metal and alloys in sea water when these oxidation potentials are arranged in the decreasing order of their activity the galvanic series arises. 3.1 Differences between electrochemical series and galvanic series: Galvanic series Electrochemical series 1. This series was developed by the study 1. This was developed by dipping of corrosion of metals and alloys in sea pure metals in their 1M salt solution water without their oxide film. 2. The position of the given metal may shift 2. The position of the metal is fixed 3. The corrosion of alloys can be studied 3. No information regarding alloys. from the series. 4. The position of a metal is different from that 4. The position of the metal is fixed. of the position of the alloy which contains the same metal in it. Engineering Chemistry Page 59  UNIT III-B CORROSION 2019-20 5. The series predicts relative corrosion nature 5. The series predicts relative displacement nature. 6. The series comprises metals & alloys 6. This comprises metals & non-metals. 4. Types of corrosion 1. Galvanic corrosion 2. Concentration cell corrosion 3. Pitting corrosion 4. Waterline corrosion 5. Stress corrosion 4.1. Galvanic corrosion When two dissimilar metals are electrically connected and exposed to an electrolyte, the metals higher in electrochemical series have a tendency of forming anode and undergo corrosion. For example, when zinc and copper are electrically connected either in acidic solutions or in their respective salt solution, zinc being more anodic by virtue of its position in electro chemical series, forms anode and copper automatically becomes cathode. Ex: Steel screws in a brass marine hardware, steel pipe connected to copper etc. ,,,, 4.2. Concentration cell corrosion: This type of corrosion takes place, when a metal surface is exposed to an electrolyte of varying concentrations or varying aerations. The poorly oxygenated parts are more prone to become anodic areas. For example, when a zinc rod is partially immersed in neutral salt solution, the metal above the water line is more oxygenated, while the portion that is immersed has smaller oxygen concentration and thus become anodic. Hence a potential difference is created, which causes the flow of current between two differentially aerated areas of same metal. Zn Zn2+ + 2e- (Oxidation) ½ O2 + H2O + 2e- 2 OH- (Reduction) The circuit is completed by migration of ions through the electrolyte and flow of electrons through the metal from anode to cathode. Engineering Chemistry Page 60 UNIT III-B CORROSION 2019-20 4.3. Pitting corrosion It is defined as intense, localized, accelerated attack resulting in the formation of a pinholes, pits and cavities on the metal surface. Such a type of corrosion takes place when there is a breakdown, peeling or cracking of a protective film due to scratches, abrading action, sliding under load etc. 4.4. Waterline corrosion: When water is stored in a container or a steel tank, it is generally found that most of the corrosion takes place just beneath the line of water level. The area above waterline is highly oxygenated and acts as cathode, while the area just beneath the waterline is poorly oxygenated and becomes anodic site. This type of corrosion is also a consequence of differential aeration. 4.5. Stress corrosion: It is a combined effect of static tensile stress and the corrosive environment on a metal. An important example of this type of stress corrosion is caustic embrittlement. Corrosion due to caustic embrittlement A high pressure boiler is used for generation of steam. The water used for steam generation, usually contains small quantities of Na2CO3, which decomposes to give caustic NaOH and liberate CO2. Engineering Chemistry Page 61 UNIT III-B CORROSION 2019-20 Na2CO3 + H2O 2 NaOH + CO2 This makes the water alkaline and NaOH thus formed flows into minute air cracks and crevices present on the boiler surface and get deposited as caustic soda. NaOH thus deposited dissolves iron as sodium ferroate (Na2 FeO2) in cracks and crevices, where the metal is stressed. The sodium ferroate further decomposes giving Fe3O4 (magnetite) with regeneration of NaOH, thereby enhancing further dissociation of Iron. 3 Na2FeO2 + 4 H2O 6 NaOH + Fe3O4 + H2 6 Na2FeO2 + 6 H2O + O2 12 NaOH + 2 Fe2O4 Caustic embrittlement can also be represented by means of an electro chemical equation. + Iron | Conc. NaOH | dil NaOH | Iron - The caustic embrittlement can be prevented by adding tannin or lignin to the boiler water or by using Na2SO4 in place of Na2CO3 for water treatment. 5. Factors influencing corrosion The rate and extent of corrosion, depends on the following characteristics i) Metal based factors ii) Environment based factors 5.1. Metal based factors a) Position in the galvanic series: When two metals or alloys are in electrical contact, in presence of an electrolyte, the more active metal (or higher up in the series) suffers corrosion. The rate and severity of corrosion depends upon the difference in their positions and greater is the difference, the faster is the corrosion of anodic metal/alloy. a) Over voltage: When a Zn rod (high in position in galvanic series) is placed in 1N H2SO4, it undergoes corrosion forming a film and evolving hydrogen gas. The initial rate of corrosion is slow, because of over voltage (0.7V). However, if few drops of CuSO4 are added, the corrosion rate of Zn is accelerated, as Cu gets deposited on Zn metal, there by the over voltage is reduced to 0.33V. The reduction is over voltage of the corroding metal/alloy accelerates the corrosion rate. c) Relative areas of cathodic and anodic parts: When two dissimilar metals or alloys are in contact, the corrosion of the anodic part is directly proportional to the ratio of areas of the cathodic part and the anodic part. Corrosion is more rapid, severe and highly localized, if the anodic area is small, because the current density at a smaller anodic area is much greater, and the demand for electrons (large cathodic area) can be met by smaller anodic areas only by undergoing “corrosion more briskly”. Engineering Chemistry Page 62 UNIT III-B CORROSION 2019-20 d) Purity of the metal: Impurities in a metal, cause heterogeneity, and forming electrochemical cells (at exposed parts) and the anodic part gets corroded. Example, Zinc metal containing Pb or Fe as impurity gets corroded. The rate and extent of corrosion increases with the increase in exposure and the extent of the impurities present. Corrosion resistance of a metal is increased by increasing its purity. a) Physical state of the metal The rate of corrosion is influenced by physical state of metal. The smaller the grain size of the metal or alloy, the greater will be its solubility and hence, greater will be its corrosion. 5.2. Environment based factors a) Temperature: With increase of temperature of environment, the reaction as well as diffusion rate increases, thereby corrosion rate is generally enhanced. b) Humidity of air: It is the deciding factor in atmospheric corrosion. “Critical humidity” is defined as the relative humidity above which the atmospheric corrosion rate of metal increases sharply”. The corrosion of metal becomes faster in humid atmosphere, since the gases (CO2, O2, etc.) and water vapour present in atmosphere furnish water to the electrolyte leading to the setting up of an electrochemical cell. c) Presence of impurities in atmosphere: Atmosphere in the industrial areas contains corrosive gases like CO2, H2S, SO2 and fumes of HCl, H2SO4 etc. In the presence of these gases and water vapour present, the acidity of the liquid, adjacent to the metal surface increases and electrical conductivity also increases. Consequently, the corrosion increases. d) Influence of pH: Generally, acidic media are more corrosive than alkaline and neutral media. Amphoteric metals (Al, Pb) dissolve in alkaline solutions as complex ions. For example, corrosion of Fe is slow in oxygen – free water, but is increased due to the presence of oxygen. Corrosion of metals, readily attacked by acid, can be reduced by increasing the pH of the attacking environment. 6. Corrosion control (Protection against corrosion) Some of the corrosion control methods are described as follows. 6.1. Proper designing: The design of the material should be such that corrosion, even if it occurs, is uniform and does not result in intense and localized corrosion”. Important design principles are: Avoid the contact of dissimilar metals in the presence of a corroding solution, otherwise the corrosion is localized on the more active metal and less active metal remains protected. Engineering Chemistry Page 63 UNIT III-B CORROSION 2019-20 a. When two dissimilar metals are to be in contact, the anodic material should have as large area as possible; whereas the cathodic metal should have as much smaller area as possible. b. If two dissimilar metals in contact have to be used, they should be as close as possible to each other in the electro chemical series. c. Whenever the direct joining of dissimilar metals is unavoidable, an insulating fitting may be applied in between them to avoid the direct metal to metal contact. d. The anodic metal should not be painted or coated, when in contact with a dissimilar cathodic metal. e. A proper design should avoid the presence of crevices between adjacent parts of structure, even in case of the same metal, since crevices permit concentration differences. f. Sharp corners and recesses should be avoided, as they are favorable for the formation of stagnant areas and accumulation of solids. g. The equipment should be supported on legs to allow free circulation of air and prevent the formation of stagnant pools or damp areas. 6.2. Use of pure metal: Impurities in a metal cause heterogeneity, which decrease corrosion resistance of the metal. Hence corrosion resistance of any metal is improved by increasing its purity. Ex: Al, Mg. Engineering Chemistry Page 64 UNIT III-B CORROSION 2019-20 Ex: the corrosion resistance of Al depends on its oxide film formation, which is highly protective only on the high purity metal. 6.3. Using metal alloys: Corrosion resistance of most metals is best increased by alloying them with suitable elements. For maximum corrosion resistance, the alloy should be completely homogeneous. 6.4. Cathodic protection: The principle involved here is to force the metal to be protected as to behave like a cathode. There are two types of cathodic protections. i) Sacrificial anodic protection method: The metallic structure to be protected is connected by a wire to the more anodic metal, so that active metal itself get corroded slowly, while the parent structure is protected. The more active metal is called “sacrificial anode”, which must be replaced, when consumed completely. Metals commonly used as sacrificial anodes are Mg & Zn. ii) Impressed current cathodic protection: An impressed current is applied in opposite direction to nullify the corrosion current, and convert the corroding metal from anode to cathode. Usually a sufficient D.C. is applied to an insoluble anode, buried in the soil and connected to the metallic structure to be protected (Fig. 16.). The anode is usually in a backfill (composed of cock breeze or gypsum), so as increase the electrical contact with the surrounding soil. This kind of protection technique is useful for large structures for long term operations. 6.5. Use of inhibitors: A corrosion inhibitor is “a substance when added in small quantities to the aqueous corrosive environment effectively decreases the corrosion of the metal”. Engineering Chemistry Page 65 UNIT III-B CORROSION 2019-20 i) Anodic inhibitors: Anodic inhibitors stop the corrosion reaction, occurring at anode, by forming a precipitate with a newly produced metal ion. These are adsorbed on the metal surface in the form of a protective film or barrier. Examples are chromates, phosphates, tungstates and other transition metals with high oxygen content. ii) Cathodic inhibitors: In acidic solutions, the main cathodic reaction is evolution of hydrogen. a) 2H+ (aq) + 2e- H2(g) Corrosion may be reduced either by slowing down the diffusion of hydrated H+ ions to the cathode and/or by increasing the over voltage of hydrogen evolution. The diffusion of H+ ions is considerably decreased by organic inhibitors like amines, mercaptans, heterocyclic nitrogen compounds, substituted urea and thiourea, heavy metal soaps, which are capable of being adsorbed at metal surfaces. b) In neutral solutions, the cathodic reaction is 1 H2O + 2 O2 + 2e- 2 OH-(aq) Corrosion is controlled either by eliminating oxygen from the corroding medium or by retarding its diffusion to the cathodic areas. The oxygen is eliminated either by reducing agents (like Na 2SO3) or by de- aeration. The inhibitors like Mg, Zn or Ni salts tend to retard the diffusion of OH- ions to cathodic areas. 7. Protective coatings It is the oldest of the common procedures for corrosion prevention. A coated surface isolates the underlying metal from the corroding environment. i) The coating applied must be chemically inert to the environment under particular conditions of temperature and pressure. ii) The coatings must prevent the penetration of the environment to the material, which they protect. There are mainly three types of protective coatings a) Metallic coatings: b) Inorganic coatings (chemical conversion) ; c) Organic coatings (paints etc.,) 7.1. Metallic coatings: A metal is coated on the other metal, in order to prevent corrosion. These are of two types a) Anodic coatings: These are produced from coating-metals, which are “anodic” to the base metal. This provides the complete protection to the underlying base metal as long as the coating intact. However, the formation of the pores or cracks on the protective layer can set up severe galvanic corrosion leading to complete destruction of the base metal. E.g.: In case of galvanized steel, zinc, the coating- metal being anodic is attacked; leaving the underlying cathodic metal (iron) unattacked (Figure 17 ) Engineering Chemistry Page 66 UNIT III-B CORROSION 2019-20 b) Cathodic coatings: These are obtained by coating a more noble metal having higher electrode potential than the base metal. The cathodic coating provides effective protection to the base metal only when they are completely continuous and free from pores, breaks or discontinuities. An example of cathodic coating is Tinning, coating of tin on iron (Figure 18 ). 7.1.2. Methods of application of metallic coatings: a) Hot dipping: It is used for producing a coating of low-melting metal such as Zn, Sn, Pb, Al etc. on iron, steel and copper, which have relatively higher melting points. The process consists of immersing the base metal in a bath of the molten coating – metal, covered by a molten flux layer (usually ZnCl2). The flux cleans the base metal surface and prevents the oxidation of the molten – coating metal. For good adhesion, the base metal surface must be very clean; otherwise it cannot be properly wetted by the molten metal. The two most widely applied hot dipping methods are: i) Galvanizing and ii) Tinning i) Galvanizing: It the process of coating iron or steel sheets with a thin coat of metallic zinc to prevent the sheets from rusting. (Figure. 19 ) Engineering Chemistry Page 67 UNIT III-B CORROSION 2019-20 The base metal sheet of iron or steel is cleaned by acid pickling method with dilute sulphuric acid at 60- 900C, washed and dried. It is then dipped in a bath of molten zinc and after taking out of bath it is passed between hot rollers to remove excess zinc and annealed (slow cooling). Galvanized utensils cannot be used for storing foods as zinc dissolves and forms toxic substances. ii) Tinning: The process of coating metallic tin over the iron or steel articles (Figure. 20) is called tinning. The surface the base metal i.e., iron sheet is cleaned by acid pickling with dilute sulphuric acid and passed through a bath of zinc chloride flux. The flue helps the molten metal to adhere to the iron metal sheet surface. Then the sheet is passed through the molten tin bath and pressed between two rollers with a layer of palm oil. The oil will help to protect the tin coated layer from any oxidation. The rollers also remove excess tin and produce a thin film of coating with uniform concentration. The tinned metal possesses good resistance against atmospheric corrosion and tin is nontoxic. Hence such containers can be safely used for storing food material. Engineering Chemistry Page 68 UNIT III-B CORROSION 2019-20 Comparison between galvanization and tinning Galvanization Tinning 1. Coating of iron with zinc to prevent 1. Coating is done with tin corrosion 2. It protects the metal sacrificially 2. Protection is due to noble character of tin 3. Protection continues even if the coating is 3. Protection is provided only when coating broken is continuous 4. Food materials cannot be stored in zinc 4. Tin Coating is non-toxic. So food items coated containers as zinc easily dissolves in acid can be stored. food stuffs and converts into toxic compounds. 5. The galvanized sheet is subjected to the process of 5. No annealing is necessary. annealing 6. Galvanized articles are good engineering materials 6.Tinned articles are used only for storing food 7.2 Organic coatings (Paints) Organic coatings are inert barriers applied on metallic surfaces and other construction material for both corrosion protection and decoration. The most important organic surface coating is paint. Paint is a mechanical dispersion of mixture of one or more pigments in a vehicle. This vehicle is a liquid consisting of non-volatile film forming material, and a volatile solvent (thinner). Constituents of Paint 7.2.1. Pigment: It is a solid substance, which provide colour to the paint. It is also used to improve the strength and adhesion of the paint, protect against corrosion. It imparts impermeability to moisture and increases weather-resistance. Example: Common Pigment Colour 1. White lead, Zinc oxide, li9thophone White 2. Red lead, ferric oxide, Chrome red Red 3. Chromium oxide Green 4. Prussian blue Blue 5. Carbon black Black 6. Umber Brown Brown 7.2.2. Vehicle (or) drying oil: It is a film forming constituent of paint. These are the glyceryl esters of high molecular-weight fatty acids. This vehicle or binder provides desired chemical and physical properties. It determines the adhesion, cohesion and flexibility of the paint. Engineering Chemistry Page 69 UNIT III-B CORROSION 2019-20 CH2COOR A simple glyceryl ester CHCOOR CH2COOR The most widely used drying oils are linseed oil, soybean oil, and dehydrate castor oil. 7.2.3 Thinner: It reduces the viscosity of the paint to a suitable consistency, suspends the pigments, dissolves the vehicle and other additives. It increases the penetration power of vehicle and elasticity of the paint film. It also helps in drying of the paint as it evaporates easily. Eg: The common thinners are turpentine, mineral spirits, benzene, naphtha, toulol, xylol, kerosene, methylated naphthalene. 7.2.4 Driers : These are the oxygen carrier catalysts. They accelerate the drying of the oil-film through oxidation, polymerization and condensation. The main function of the drier is to improve the drying quality of the oil film. Eg: Resinates, linoleates, tungstates and naphthenates of Co, Mn, Pb and Zn. 7.2.5. Extenders or fillers: These are low refractive indices materials. These are added to reduce the cost, increase durability, to provide negligible covering power to the paint and to reduce the cracking of dry paint film. These fill the voids in the film, increase random arrangement of pigment and acts as the carrier for pigment color. Eg: Barytes (BaSO4), talc, asbestos, ground silica, gypsum ground mica, slate powder, china-clay, calcium sulphate. 7.2.6 Plasticizers: Plasticizers are added to the paint to provide elasticity to the film and to minimize its cracking. Eg: Tri cresyl phosphate, tri phenyl phosphate, tri butyl phthalate. 7.2.7 Anti skinning agents: These are added to prevent gelling and skinning of the paint film. Eg: Poly hydroxy phenol In case of non-conjugated hydrocarbons, interaction of oxygen with the double bonds results in the formation of hydro peroxides. Engineering Chemistry Page 70 UNIT III-B CORROSION 2019-20 Exercise Question 1. Give suitable reasons. a. Zn gets corroded vigorously when connected to Cu than with Te. b. Copper equipment should not possessive a small steel bolt. c. Small anodic area results in intense local corrosion. 2. What is electro chemical corrosion how does it occur? Describe the mechanism. 3. What are corrosion inhibitors? Discuss anodic and cathodic inhibitor with suitable example. 4. Distinguish between galvanizing and tinining. 5. Explain the role of current density and pH on the nature of electro deposit. a. Explain the anodic oxidation process within example. b. What pre treatment technique is use for removing oxide scale on metal surface? 6. Explain in brief various types of corrosion with suitable example. 7. Brief the cathodic protection of preventing corrosion. a. Sacrificial anodic protection b. Impressed current cathodic protection. 8. Distinguish between wet corrosion and dry corrosion. 9. Define corrosion. Write about the fundamental reasons on occurrence of corrosion 10. Write about dry and wet corrosion with their respective mechanisms. 11. Explain in brief about electrochemical theory of corrosion? 12. What is galvanic corrosion 13. Write the mechanism involved in Hydrogen evolution and oxygen absorption type corrosion. 14. Explain the various types of electrochemical corrosion. 15. Explain the factors influencing the rate of corrosion. 16. What is electro chemical corrosion how does it occur? Describe the mechanism. 17. Briefly discuss the various metallic coatings that prevent corrosion. 18. Distinguish between galvanizing tinning. 19. Explain in brief the methods and preparation of surface for coatings. 20. Explain in brief various types of corrosion with suitable example. 21. Describe the methods involved in corrosion control 22. What are paints? Explain in brief about the constituents and their functions 23. Explain in brief about the nature of oxidation in acceleration of corrosion. 24. What is pilling bed worth rule? Give one example 25. What are corrosion inhibitors? Discuss anodic and cathodic inhibitor with suitable example. 26. Brief out hot dipping and electroplating with suitable example 27. Give suitable reasons. a. Zn gets corroded vigorously when connected to Cu than with Te. b. Copper equipment should not possessive a small steel bolt. c. Small anodic area results in intense local corrosion. 28. Explain the role of current density and pH on the nature of electro deposit. Engineering Chemistry Page 71