Corrosion

1. Corrosion

2. Lecture-1 • Introduction -causes and consequences of corrosion • Galvanic series • Types of corrosion. Dry corrosion- mechanism of dry corrosion due to oxygen • Pilling-Bedworth rule. • Corrosion by other gases.

3. Introduction • It is the degradation of a material due to a reaction with its environment. OR • Process of Destruction of the material through chemical or electrochemical attack by its environment. • Slow process • Corrosion rate is the amount of metal ions removed from the metal per unit area and unit time. https://youtu.be/jQoE_9x37mQ?list=PLqxe1FKqykZ9IvmFCW8Tj21oJF82tPzOt

4. Why Do Metals Corrode? Periodic Table • Out of 118 elements, there are 20 non-metals, 7 metalloids • and remaining are metals. • Out of metals 8 are precious/noble metals (Ruthenium, Osmium, Rhodium, Iridium, Palladium, Platinum, Silver and gold ). They do not undergo corrosion. They are found in pure state in nature.

5. All other metals except Noble metals exist in their thermodynamically stable lower energy state of ore. Corrosion is reverse of metallurgy • Once metals are extracted from their ores, we have to protect them from corrosion and increase their life

6. Consequences of Corrosion • The economic and social consequences of corrosion include • Due to corrosion the efficiency of the machine reduces drastically leading to plant shut down. • The corroded equipment must be replaced. • Corrosion releases toxic products. • 4. Health deteriorates

12. Dry corrosion Mechanism:- 1. Adsorption When temp increases the metal undergoes oxidation and loses e- 2M → 2M+n  + 2ne- (Oxidation) Metal Ion 2. Dissociation and formation of metal ion and oxide ion. Electron are gained by the oxygen atoms to form oxide ions nO2 + 4ne- → 2nO2- (Reduction ) Oxide Ion 3. Formation of metal oxide layer by combination of ions 2M + nO2 → 2M + 2nO2- (Net Reaction ) Metal Oxide

13. Pilling-Bedworth ratio (PBR) Pilling-Bedworth ratio: It is the ratio of the volume of the elementary cell of a metal oxide to the volume of the elementary cell of the corresponding metal from which film is formed. (PBR) = Volume of metal oxide If the PBR is greater than one but smaller than 1.45 then the metal oxide film is non-porous and protective CuO : Cu = 1.68 Ag2O:Ag=1.59 Al2O3: Al = 1.28 2Mo2O3:Mo=3.4 Na2O:Na=0.57 Volume of metal surface consumed

14. Corrosion by other gases like H2S, Cl2 • The degree of attack depends on the formation of protective or non-protective films. • AgCl film, resulting from the attack of Cl2 on Ag. Protects the metal from further attack. • Cl2 gas attacks on tin (Sn) forming volatile SnCl2. Does not protect metal from further attack. • H2S at high temperature attacks steel forming a FeS scale, which is porous and does not protect metal from further attack.

15. Lecture 2 • Wet corrosion – Types, Galvanic and concentration cell corrosion. • Mechanisms of wet corrosion. (H2 liberation mechanism and O2 absorption mechanism). • Other types of corrosion

16. Electrochemical or Wet corrosion Takes place under moist or wet conditions through the formation of short circuited galvanic cells. Electrochemical corrosion involves Separate 'anodic' and 'cathodic' areas between which current flows through the conducting medium. Occurrence of oxidation (corrosion) at anodic areas takes place which generates metallic ions Non-metallic ions like OH- or O2- are formed at cathodic areas

17. Mechanism of Wet Corrosion Metal atoms on the surface of anode, pass into conducting medium by forming metal-ions, leaving behind electrons. Reaction at anode (Oxidation) Anode: M M+n + ne Fe Fe +2 + 2e- (Oxidation) Reaction at cathode (Reduction) (i) H2 liberation / evolution ( acidic medium) Cathode : 2H+ + 2 e- 2H2 ↑ (Reduction) Fe + 2H+ Fe++ + H2 ↑ https://youtu.be/UpfG3HqhcWg

18. Mechanism of Wet Corrosion

19. Medium or electrolyte- neutral or basic Reaction at anode (Oxidation) Anode: M M+n + ne Fe Fe +2 + 2e- (Oxidation) Reaction at Cathode – O2 absorption (Reduction) If the corroding medium is neutral or slightly alkaline containing some oxygen gas dissolved, the reaction of oxygen absorption takes place on cathode. Cathode : O2 + 2H2O + 4 e- 4 OH- (Reduction) Overall reaction will be 2Fe + 2H2O + O2 2Fe(OH)2 ↓

20. Oxygen absorption

21. Stress Corrosion Occurs in the presence of tensile stress and corrosive environment E.g. brass get corrode in traces of ammonia. Pitting Corrosion Formed as a result of pit and cavities localized attack and formed by cracking protective coating

22. Water-line Corrosion When water is stagnant in a steel tank, the concentration of Oxygen above the water surface is greater than that under the surface. This generates an oxygen concentration cell. In this cell the metal just above the water level is cathodic w.r.t. the metal below the water level. Water-line Corrosion

23. Lecture 3 • Factors influencing corrosion (nature of metal and nature of environment). • Methods of corrosion control- cathodic protection and anodic protection

24. Factors influencing Corrosion • Nature of the Metal 2. Nature of the environment. Nature of Metal • Position in Galvanic Series • Relative areas of the anode and cathode • Nature of the oxide film

25. Nature of Environment • Temperature • Humidity • pH • Impurities and Suspended Particles

26. CORROSION PROTECTION METHODS • Cathodic Protection • Anodic Protection • Metallic Coating • Surface Conversion Coating • Electroless Plating • Corrosion Inhibitors • Powder Coating

27. Cathodic protection Principle of this method is, the metal to be protected is forced to behave as cathode. 1. Sacrificial anodic method 2. Impressed current method 1. Sacrificial Anodic method • The metallic structure to be protected from corrosion, is connected to anodic metal (active) by an insulated wire. • The more active metal like Zn, Al, Mg, etc. acts as anode and gets corroded, protecting the main metal, hence it is called as sacrificial anode. https://youtu.be/RAlC75xG4qU?list=PLqxe1FKqykZ_vOORtJL4z7OIxiHhMqXlb

28. Sacrificial Anodic Method

29. Impressed Current • In this method, an impressed current is applied in opposite direction to nullify the corrosion current and convert the corroding metal from anode to cathode. Applications Open water box coolers, water tanks, buried water or gas pipeline, condensers, transmission line towers, laid - up ships, marine piers, etc. Impressed Current technique

30. Anodic Protectic Method Principle A metal or alloy having wider range of passivity voltage, is made anodic and the voltage in the passivity range, is applied over it to control its corrosion even by strongly corroding media. Applications (i)Chemical reactors. (ii)Complex metallic installations on ground or under sea water. (iii)Industrial water coolers. (iv)Industrial metal condensers. (v)Pipe lines for carrying corrosive liquids or solutions. etc.

31. Anodic protection

32. Metallic coating 1.Anodic coatings 2.Cathodic coatings Differences between anodic coating and cathodic coating. • Anodic coating • If the coating metal is higher placed in galvanic series than the base metal, then the coating is called as anodic coating. • Anodic coating is mostly preferred. • Coating of Zn, Al, Cr on steel • It is carried out by galvanizing process • Cathodic coating • If the coating metal is lower placed in galvanic series than the base metal, it is known as cathodic coating. • Cathodic coating is less preferred. • Coating of tin on steel, Ag or Au on brass • It is carried out by tinning process.

33. Lecture 4 Protective coatings- • surface preparation • Metallic coatings: Types of coating- Methods of applications • hot dipping • Cladding • electroplating • cementation). https://youtu.be/UE7zY9JoVIc https://youtu.be/aDkpbB31lek

34. Methods of metallic coating (A)Hot dipping • Hot dipping is used for producing a coating of low melting metals such as Zinc (419°C) or tin (232°C), etc. on metal like steels, copper, brass, etc. (a) Galvanizing Applications of galvanizing • It is used widely for protection of iron. • Various galvanised iron (G.I.) articles are in use. e.g. G.I. Sheets (plane or corrugated), wires, pipes, buckets, tubes, screws, nails, etc. G.I. vessels cannot be used for storing foods as there is formation of poisonous products by action of foods on zinc. Zinc is anodic coating. galvanising

35. (b)Tinning Applications of tinning Tin has greater corrosion resistance and better glaze. (i)Tinned containers can be used for storing foods, ghee, oils, pickles, medicines. (ii)Copper wires before insulation by rubber, are tinned to avoid attack of sulphur of rubber. (iii)Tinned copper or brass vessels or sheets are used for making utensils and refrigeration equipments. Tinning

36. Metal Cladding • It is the process of sandwiching the base metal between two thin layers of coating metal by hot-rolling the composite to produce a firm bonding. • The coat metals are usually metals of least reactivity (Cu, Ni, Ag, Pt, Ti) • very thin and its thickness is only 5% of the total composite metal. • Duralumin sandwiched between Al sheets and hot rolled to produce Alcad composite which is free from stress corrosion

37. Electroplating (i)Definition • Electroplating is the method in which coating metal is coated on a base metal on the basis of electrolysis principles. (ii)Method https://youtu.be/aDkpbB31lek

38. At Cathode (article) Cr+3 + 3e- Cr (if chromium plating) Ag + e- Ag (if silver plating) At anode Cr Cr+3 + 3e- (if chromium anode) Ag Ag + + e- (if silver anode) Advantages Electroplating can be done on articles of any shape; the coating is strongly adhered, it can be applied on surfaces of plastic, glass, wood also (e.g. PCB), gives brighter look. Applications of electroplating (i)corrosion protection. (ii)decoration or better appearance. (iii)to have surface of plastic, glass, wood conducting. (iv)for electroforming of many parts of machines by electrodeposition of thick layer.

39. Cementation. • In the cementation technique, there is formation of strong layer of alloy of coating metal and base metal, on the surface of base metal. • Uses • Coating small articles line nuts, bolts, screws, spanners, screw drivers, tools furnace parts, and turbine blades. • Applicable for base metal like steel. • Strong coating and having good abrasion resistance. Cementation

40. Lecture 5 • Electroless coatings • Non-metallic coatings: chemical conversion coatings • phosphate coatings, • chromate coatings, • Chemical oxide coatings • anodized coatings powder coatings

41. Conversion Coatings – Phosphate Coating • Immersion in a Zn-P bath with Phosphoric acid causes growth of a crystalline zinc phosphate layer - Iron, Zinc or Manganese Phosphate layer formed • Typically applied to C-steel, low alloy steel and cast irons • -Sometimes applied to Zinc, Cadmium, Aluminum and Tin • Typically very thin ~ 2.5 mm Anodized Coating • Electrolytic passivation process. • This is produced on non-ferrous metals like Al, Zn, Mg. • In this method base metal is made as anode. • Process is carried out by passing moderate direct current through an electrolytic solution (acid solution) taken in a bath in which the metal is suspended as anode. • Coating is formed as a result of oxidation of the base metal due to the oxygen released at the anode.

42. Conversion Coatings – Chrome Coating • Food cans • Immersion in a chromic acid bath (pH ~ 1.8) with other chemicals to coat surface • Known carcinogen chemicals used, so alternatives are currently under research • Molybdate chemicals currently best subsititute for aluminum coatings • Many household goods – screws, hinges (yellow brown appearance) • Typically very thin < 2.5 mm

43. Oxide coating • Not all oxides are detrimental – many are tightly adhering leading to passivation and hardening of surface • Al2O3- Chromium in Stainless steel rapidly corrodes to passivate the surface • Gun-bluing • Heat steel to 370 oC in steam or oil • Chemical Baths – similar in nature to gun-bluing • Black Oxide – chemical application • Typically applied to steel, copper and stainless steel • Anodizing – electrochemical conversion • Usually done to Aluminum • 2-25 mm thick typically • Multiple colors possible • Improved Corrosion and Wear Resistance

44. Powder coating • Definition: Powder coating is the technique of forming a thin coat of a thermoplastic material along with some additives, on the surface on a metal. • There are two methods of powder coating. • A)Electrostatic spraying • B)Plasma spraying Applications of Powder Coating • (i)Decoration of domestic appliances, heating and air conditioning equipments, computer, car accessories, etc. • (ii)Corrosion resistance of metals in outdoor and indoor applications like military, transportation, aerospace applications. • (iii)Dry film lubricants like graphite, MoS2, tungsten disulphide backed by binders like epoxy, phenolic resins, are applied on metal surfaces for lubrication, by using powder coating method.

45. Plasma Spraying Method • Plasma arc spraying produces denser coatings. The coatings are less porous, strongly adhered. • The mixture of the fine powder of coating constituents is converted to plasma by the arc produced, having temperature 1500°C. The plasma is directed on the surface of work piece, to produce a thin coating of 250-450 mμ Electrostatic spraying Method • The dry constituents of powder coating i.e thermo softening plastic, colour pigments, filler, and other additives used to particle size 40-90 µm • These coating particles are dispersed in air stream and passed through a high voltage gun where the coating particles pick up negative electrostatic charge. • Then the base metal having the electrostatic powder deposited is baked where the powder particles melt to form uniform, strong coating.

46. Electrostatic spraying Method

47. Lecture 6 Organic coatings- paints, varnishes and enamels

48. Organic coatings • Organic coatings are inert organic barriers like paints, varnishes, lacquers and enamels applied on metallic surfaces and other constructional materials for both corrosion protection and decoration. • A good organic coating should process the following qualities. • Should be chemically inert to the corrosive environment. • Should have good surface adhesion. • iii) Should be impermeable to water, salts and gases. • iv) Should possess good spreading capability

49. Why do we need paints? Paints are surface coatings generally suitable for site use, marketed in liquid form. They may be used for one or more of the following purposes • To protect the underlying surface by exclusion of the atmosphere, moisture and insects. • To provide a decorative easily maintained surface. • To provide light- and heat-reflecting properties. • To give special effects • for example, inhibitive paints for protection of metals; electrically conductive paints as a source of heat; condensation- resisting paints. https://youtu.be/cui76v739VM

50. Coatings are materials that are applied to a surface which form a continuous film in order to beautify and/or protect the surface. Paint: Pigmented surface coating Varnish: Coating that lacks a pigment Lacquer: Thermoplastic solution paints or varnishes, term also used for all clear wood finishes Enamel: Hard, thermosetting paints