CORROSION Syllabus: Causes and effects of corrosion theories of corrosion (dry/chemical and electrochemical corrosion) Factors effecting corrosion Corrosion control methods Cathode protection Sacrificial Anodic, Impressed current methods Surface coatings Methods of application on metals (Hot dipping, Galvanizing, tinning, Cladding, Electroplating), Organic surface coatings Paints Their constituents and their functions. Objectives: The problems associated with corrosion and knowledge for control awareness is necessary for all branches of engineering. 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
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 (CuCO 3.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 O 2, CO 2, H 2 S, SO 2, halogens, etc., with metal surfaces in the immediate vicinity. Dry corrosion is classified into three types. i) Oxidation corrosion ii) iii) Corrosion by other gases 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. 2 M Mn + + 2 ne - (Loss of electrons) (Oxidation) n O 2 + ne- 2 no - 2 (Gain of electrons) (Reduction) 2 M + n 2 O 2 2 Mn + - + no 2 Engineering Chemistry Page 55
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. If unstable oxidation product is formed, the product decomposes readily and may allow further corrosion. 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 MoO 3 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 nonprotective. 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 SO 2, CO 2, Cl 2, H 2 S, and F2 etc. The degree of attack depends upon the formation of protective or non- Engineering Chemistry Page 56
protective films on the metal surface. Example, AgCl forms the protective films. SnCl 4 forms a volatile product, while attack of Fe by H 2 S 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 M n+ + 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 Fe 2+ + 2e (Oxidation) 2 H + + 2 e - H 2 (Reduction) Fe + 2 H + Fe 2+ + H 2 Engineering Chemistry Page 57
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 O 2 + H 2 O + 2 e - 2OH - (Reduction) The Fe 2+ ions formed at anode, and OH - ions formed at cathode, diffuse towards each other forming Fe (OH) 2 i.e., Fe 2+ + 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 [Fe 2 O 3 x.h 2 O]. 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 environment of the metal. 2. It involves the formation of large number of 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
3. Galvanic series In the electrochemical series the elements are arranged in the increasing order of their reduction potential values. Galvanic series or electrochemical series is an arrangement of metals in the increasing order of their reduction potentials. The metals with more anodic character occupy the top positions in the series whereas the bottom positions are occupied by more cathodic metals. A metal top in the series is more anodic and undergoes corrosion faster than the metal below in the series. Examples: Mg, Zn, Al, Cd, Duralumin, steel, lead tin (solder), Pb, Sn, Cu and its alloys, Cupro Nickel, bronze, passive stainless steel, Ag, Ti, Graphite, Au, Pt. The noble character increases down this series. Active (or anodic) Noble (or cathodic) 1. Mg 2. Mg alloys 3. Zn 4. Al 5. Cd 6. Al alloys 7. Mild steel 8. Cast iron 9. High Ni Cast iron 10. Pb-Sn Solder 11. Pb 12. Sn 13. Lconel 14. Ni-Mo-Fe alloys 15. Brasses 16. Monel 17. Silver solder 18. Cu 19. Ni 20. Cr stainless steel 21. 18-8 stainless steel 22. 18-8 Mo stainless steel 23. Ag 24. Ti 25. Graphite 26. Au 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. Engineering Chemistry Page 59
of the position of the alloy which contains the same metal in it. 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 6. Microbial corrosion 7. Intergranular 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. Engineering Chemistry Page 60
Zn Zn 2+ + 2e - (Oxidation) ½ O 2 + H 2 O + 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. 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. Engineering Chemistry Page 61
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 Na 2 CO 3, which decomposes to give caustic NaOH and liberate CO 2. Na 2 CO 3 + H 2 O 2 NaOH + CO 2 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 (Na 2 FeO 2 ) in cracks and crevices, where the metal is stressed. The sodium ferroate further decomposes giving Fe 3 O 4 (magnetite) with regeneration of NaOH, thereby enhancing further dissociation of Iron. 3 Na 2 FeO 2 + 4 H 2 O 6 NaOH + Fe 3 O 4 + H 2 6 Na 2 FeO 2 + 6 H 2 O + O 2 12 NaOH + 2 Fe 2 O 4 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 Na 2 SO 4 in place of Na 2 CO 3 for water treatment. 4.6. Microbial corrosion: Metals undergo corrosion due to microbial action both in aerobic and anaerobic conditions. There are mainly 4 types of microbes which cause corrosion in nature. a) Sulphate reducing bacteria (Sporovobrio desulphuricous) b) Sulphur bacteria (Thioracillus) c) Iron and manganese bacteria d) Film forming bacteria. a) Sulphate reducing bacteria: This bacteria as a part of its metabolic activity, takes sulphates present in the soil along with water and air. The typical equations involving corrosion by sulphate reducing bacteria are given below. Engineering Chemistry Page 62
Anodic solution of iron 8 H 2 O 8 H + + 8 OH - 4 Fe + 8 H + 4 Fe 2+ + 8 H Depolarization, due to activity of bacteria H 2 SO 4 + 8 H H 2 S + 4 H 2 O Corrosion products Fe 2+ + H 2 S FeS + 2 H + 3 Fe 2+ + 6 OH - 3 Fe(OH) 2 b) Sulphur bacteria (Thioracillus): This is a kind of bacteria which has sulphur present in its cell, which as a part of metabolic activity, picks up the oxygen and moisture present in the soil and excrete sulphates making the soil acidic. This eventually leads to corrosion of buried metals. c) Iron and manganese bacteria: These bacteria consume Iron and Manganese deposits directly and digest them converting them into sulphides and hydroxides at optimum conditions of 25 30 o C and ph 5-9. e) Film forming bacteria: These are usually algae and fungi which form a thin film on the surface of the metal accommodating the accumulation of dust, moisture leading to formation of differential concentration or differential aerations cell. 4.7. Intergranular corrosion All solids have grain structures which when exposed to corrosive environment undergo corrosion because of formation of potential zones of areas within the crystal lattice. During crystallization of the metal, the impurities present in the materials get accumulated near the boundaries of grains, while the pure form of metal occupies the grain proper. This leads to the formation of two areas of different potentials, which makes the corrosion current to flow from the active grain boundary (anode) towards grain proper (cathode). Such a material when it is exposed to corrosive environment, grain boundaries are attacked readily causing corrosion. 4.8. Passivation: The phenomenon in which a metal exhibits extra corrosion resistance than that is expected from its position in the electro chemical series or galvanic series is known as passivation. This extra resistance towards corrosion is obtained due to formation of a very thin film of oxide layer (0.0004 mm of thickness). This thin film is non-porous, highly protective and of self-healing nature. Engineering Chemistry Page 63
4.8.1. Soil corrosion: This type of corrosion depends on the presence of Salts, moisture, ph, bacteria, aeration and texture of the soil. Based on the texture of the soil, soils are of three types. 4.8.2. Graveled or sandy soil: These are loose soils having sufficient aeration. When an iron rod is buried in such a soil, it gets corroded because of undergoing differential aeration corrosion. The severity of corrosion also depends upon the type of product and the salts present. 4.8.3. Water logged soils: Corrosion of metals in waterlogged soils takes place due to microbial action following a wet mechanism. 4.8.4. Intermediate soils: These types of soils have gravel or sandy clay like matter along with moisture. Corrosion of metals occurs as a consequence of both differential aeration and microbial attack. 4.8.5. Erosion corrosion: It is caused by the combined effect of the abrading action of turbulent flow of gases, vapours and liquids and mechanical action of solids over a metal surface. 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. b) Over voltage: When a Zn rod (high in position in galvanic series) is placed in 1N H 2 SO 4, 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 CuSO 4 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 64
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. f) 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 (CO 2, O 2, 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 CO 2, H 2 S, SO 2 and fumes of HCl, H 2 SO 4 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 65
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 66
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 67
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 - H 2 (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 H 2 O + 2 1 O 2 + 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 2 SO 3 ) or by deaeration. 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 coatingmetal being anodic is attacked; leaving the underlying cathodic metal (iron) unattacked (Figure 17 ) Engineering Chemistry Page 68
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 ZnCl 2 ). 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 69
The base metal sheet of iron or steel is cleaned by acid pickling method with dilute sulphuric acid at 60-90 0 C, 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. Comparison between galvanization and tinning Engineering Chemistry Page 70
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 1. Protection continues even if the coating is broken 4. Food materials cannot be stored in zinc coated containers as zinc easily dissolves in acid food stuffs and converts into toxic compounds. 5. The galvanized sheet is subjected to the process of annealing 6. Galvanized articles are good engineering meterials 3. Protection is provided only when coating is continuous 2. Tin Coating is non-toxic. So food items can be stored. 3. No annealing is necessary. 6. Tinned articles are used only for storing food c) Electro plating: The process of depositing or coating a metal on the surface of base metal/ non metal by electrolysis is called electro plating. It is widely adopted to coat base metals with protective metallic coatings of Cu, Ni, Zn, Pb, Sn, Au and Ag. Process: The metal surface is cleaned thoroughly. The article to be electroplated is made as 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 current is passed through the electrolyte. The anode dissolves, depositing the metal ions from the solution on the article at cathode in the form of a fine thin metallic coating. Ex: Electroplating of gold: Cathode: Article to be electroplated Anode: A block of gold metal Electrolyte: Aqueous solution of AuCl 3 or potassium auro-cyanide K[Au(CN) 2 ] Factors affecting electroplating: Cleaning of the article is essential for strong adherent electroplating. Concentration of the electrolyte is a major factor in electroplating. Low concentration of metal in ions produces uniform, coherent metal deposition. Thickness of the deposit should be minimized order to get a strong adherent coating. Additives such as glue, boric acid etc. should be added to the electrolyte bath to get a strong adherent and smooth coating. Engineering Chemistry Page 71
Battery Gold (Anode) Cathode AuCl 3 or K[Au(CN) 3 ] The electrolyte selected should be highly soluble and should not undergo any chemical reaction. ph of the electrolytic bath must be properly maintained to get the deposition effectively. Applications: It is widely used technique in industries and consumer goods. It can be used for both metals and non metals. In metals it prevents corrosion and in non metals it increases the strength d) Electro less plating: The deposition of a metal form its salt solution on catalytically active surface by a suitable reducing agent without use of electrical energy is called electro less plating or chemical plating. The metal ions are reduced to the metal which gets plated over the catalytic surface the metal surface is treated with acid (etching) and treated with reducing agent like formaldehyde. Heat treatment may be adopted. Electro less plating can be done for on conducting surfaces like plastic or printed circuit boards. Some times complexing agents stabilizers and buffer solutions may also be necessary this technique is widely used in electronic decorative equipment, automobile industry etc., e) Metal Cladding: It is the process by which a dense, homogeneous layer of coating metal is bonded (cladded) firmly and permanently to the base metal on one or both sides. The choice of the cladding metal depends on the corrosion resistance required for any particular environment. Here, the metal to be protected is sandwiched between the two layers of the protecting metal. The whole combination is pressed by rollers under the action of heat and pressure. The cladding materials generally used are corrosion resistant. Examples: Al (Aluminium) Figure. 21) Ni, Cu, Pb, Ag etc. This method is widely adopted in air craft and automobile industry. Engineering Chemistry Page 72
f) Metal spraying: In this method, the molten metal is sprayed on the cleaned base metal with the help of a spraying gun. The metal surface must be rough. The metal to be sprayed in molten state is fed through a central barrel. A gaseous mixture (oxy acetylene) passing through a tube around the barrel burns at the orifice to melt the wire. The molten metal is then projected against the surface to be coated. This method is limited to low melting metals like Zn, Pb, Sn etc. Non metallic articles like glass, plastic and wood are also coated. g) Powder metal method: Here, finely divided powdered metal is sucked from the powder chamber and then heated, as it passes through the flame of the blow pipe. The blow-pipe disintegrates the metal into a cloud of molten globules, which are then adsorbed on the base metal surface. This method is limited to low-melting metals like Zn, Pb, Sn etc. This can be applied to fabricated structure and there is no possibility of damage. 7.2. Chemical Conversion Coating: These are inorganic surface barriers, produced by chemical or electro chemical reactions, brought at the surface of the base metal. Such coatings are particularly used as an excellent base for paints, lacquers, oils and enamels. 7.2.1. Phosphate coating: It is a conversion coating consisting of an insoluble crystalline metal-phosphate salt formed in a chemical reaction between the substrate metal iron and phosphoric acid solution containing ions of metals (Zn, Fe or Mn) The reaction for the formation of zinc phosphate coating on the surface of base metal iron may be represented as Zn (H 2 PO 4 ) 2 + Fe + 4H 2 O Zn 3 (PO 4 ) 2. 4 H 2 O + Fe HPO 4 + H 3 PO 4 + H 2 Base metal Coating These are usually applied either by immersion or spraying or brushing. Such coatings do not offer complete resistance to atmospheric corrosion and are principally used as an adherent base/primer-coat for paint, lacquers, oils etc. Phosphate coatings however impair the welding strength. 7.2.2. Chromate coating: These are specially used for the protection of Zinc, cadmium-plated parts, aluminium and magnesium. They are produced by immersion of the article in a bath of acid potassium chromate, followed by immersion in a bath of neutral chromate solution. Chromate films are amorphous, non-porous and more-corrosion resistant than phosphate coatings, but they possess comparatively low abrasion-resistance. 7.2.3. Anodized coatings: These are generally, produced on non-ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation process, in which the base metal is made as anode. It is carried out by passing a moderate direct electric current through a bath in which the metal or alloy is suspended from anode. The Engineering Chemistry Page 73
bath usually contains sulphuric, chromic, phosphoric, oxalic or boric acid. As the anodized coatings are somewhat thicker than the natural oxide films, so they possess improved resistance to corrosion as well as mechanical injury. 7.3 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 7.3.1. 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 7.3.2. 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. CH 2 COOR A simple glyceryl ester CHCOOR CH 2 COOR The most widely used drying oils are linseed oil, soybean oil, and dehydrate castor oil. 7.3.3. 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. Engineering Chemistry Page 74
7.3.4. 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. 7.3.5. 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 (BaSO 4 ), talc, asbestos, ground silica, gypsum ground mica, slate powder, china-clay, calcium sulphate. 7.3.6. 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. 7.3.7. 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. 7.4. Special paints 7.4.1. Emulsion paints: This paint contain pigment, extender and film forming substances such as linseed oil or synthetic styrene butadiene copolymers, polyvinyl acetate and acrylic polymers dispersed in water as oil in water emulsion, Emulsion paints can be diluted with water and have ease of application and quick drying. Distempers are emulsions containing suitable pigment suspended in a solution of casein. 7.4.2. Luminescent paints: These contain luminophor pigments, i.e., which fluorescence under U.V.light. Such pigments absorb U.V. or shorter wavelength radiations and emit radiations in visible region. Luminophor pigments include Zn S or sulphides of Zn & Co, titanium with small amounts of color modifiers like Cu, Ag, Mn and B called activators. 7.4.3. Fire retardant paints: These contain binders or other components, which break down at elevated temperatures. Producing non-inflammable gases like CO 2, NH 3, water vapor HCl, HBr, which dilute the inflammable gases. E.g.: PVC, Chlorinated rubbers, epoxides break down to give corresponding hydrogen halides. Urea-formaldehyde resin yield NH 3, Carbonate pigments yield CO 2. Engineering Chemistry Page 75
7.4.4. Temperature indicating paints: These contain the pigments, which undergoes color change at a specific temperature. Such an ingredient is a double salt (or) an amine salt of any Cu, Fe, Cr, Mn, CO, Ni and Mo or a combination of these salts. Such paint can indicate any temperature of environment. 7.4.5. Aluminium paints: in it the base material is a fine powder of aluminium. Finely powdered aluminium is suspended in either spirit varnish or oil varnish, depending on the requirement. Advantages: i) It possesses a quit good covering powder ii) It imparts very attractive and pleasing appearance to the surface iii) It is fairly good heat resisting iv) The painted surface visible even in dark. 7.4.6. Distempers: These are water paints. The ingredients are i) cheaper than paints, varnishes white chalk powder and plasters, cement concrete or wall surfaces in the interior of building. -ooo- Engineering Chemistry Page 76
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. Briefly discuss the various metallic coatings that prevent corrosion. 5. Explain the following factors influencing that rate of corrosion. a. Nature of corrosion product. b. Position in electro chemical series. c. ph 6. Distinguish between galvanizing and tinining. 7. 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? 8. Explain in brief various types of corrosion with suitable example. 9. Brief the cathodic protection of preventing corrosion. a. Sacrificial anodic protection b. Impressed current cathodic protection. 10. Distinguish between wet corrosion and dry corrosion. 11. Define corrosion. Write about the fundamental reasons on occurrence of corrosion 12. Write about dry and wet corrosion with their respective mechanisms. 13. Explain in brief about electrochemical theory of corrosion? 14. What is galvanic corrosion 15. Write the mechanism involved in Hydrogen evolution and oxygen absorption type corrosion. 16. Explain the various types of electrochemical corrosion. 17. Explain the factors influencing the rate of corrosion. 18. What is electro chemical corrosion how does it occur? Describe the mechanism. 19. Briefly discuss the various metallic coatings that prevent corrosion. 20. Distinguish between galvanizing tinning. 21. Explain in brief the methods and preparation of surface for coatings. 22. Explain in brief varioustypes of corrosion with suitable example. 23. Describe the methods involved in corrosion control 24. What are paints? Explain in brief about the constituents and their functions 25. Explain in brief about the nature of oxidation in acceleration of corrosion. Engineering Chemistry Page 77
26. What is pilling bed worth rule? Give one example 27. What is meant by passivation? How passive oxide film is superior than natural oxide film for controlling corrosion? 28. What are corrosion inhibitors? Discuss anodic and cathodic inhibitor with suitable example. 29. Explain the following factors influencing the rate of corrosion. a. Nature of corrosion product. b. Position in electro chemical series. c. ph d. Design 30. Brief the cathodic protection of preventing corrosion. a. Sacrificial anodic protection b. Impressed current cathodic protection. 31. Briefly discuss the various metallic coatings that prevent corrosion. 32. Brief out hot dipping and electroplating with suitable example 33. 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. 34. Why chromium anodes are not use in chromium plating. 35. Explain the role of current density and ph on the nature of electro deposit. Engineering Chemistry Page 78