(c) Dr. Payal B. Joshi. Corrosion

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Erika Carroll
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2 Corrosion

3 So Why Study Corrosion? Metals are precious resources Engineering design is incomplete without knowledge of corrosion Applying knowledge of corrosion protection can minimize disasters Artificial implants for the human body!!!

4 Examples Rusting of Iron: Reddish scale and powder of oxide (Fe 3 O 4 ) is formed Iron becomes weak Green film of basic carbonate [CuCO 3 + Cu(OH) 2 ] on surface of Cu when exposed to moist air containing CO 2

5 Any process of chemical or electrochemical decay or destruction of a metal/alloy due to the action of surrounding environment/medium is called corrosion

6 Corrosion: Metallurgy in Reverse ENERGY ORE METAL

7 Corrosion Dry/Atmospheric Wet/Electrochemical Corrosion due to Oxygen Corrosion due to other gases Evolution of Hydrogen Absorption of Oxygen

8 Dry/Atmospheric corrosion Presence of Oxygen Metallic surfaces exposed to air undergoes oxidation 2M + O 2 2MO, where M is a metal Metallic oxide (MO) formed on the surface of the metal is in form of a thin film

9 Nature of Oxide film stable unstable volatile porous Ex. Iron Oxide (FeO) Non porous Oxides of Al Ni, Cr Oxides of noble metals i.e. Au, Ag etc. Oxide of molybdenum

10 If oxide film is porous in nature, rate of further corrosion is not reduced. These pores give free access to oxygen, which attacks the underlying pure metal. Thus, corrosion does not stop till pure metal is available.

11 Gold does not get corroded on oxidation There is a formation of unstable oxide film Oxide layer formed decomposes back into metal and oxygen Consequently, oxidation corrosion is not possible, thus Ag, Au, Pt do not undergo oxidation corrosion

12 Volatile oxide film ex. Molybdenum oxide

13 Corrosion due to other Gases Corrosive effect of gases depends on chemical affinity of the metal for gases Cl 2, H 2 S, SO 2, CO 2, etc add to the corrosive effect to the metal Gases react with metals to form films of corresponding compounds get deposited on the metal surface This film may be porous (non-protective) or non-porous (protective) Ag + Cl 2 (dry gas) AgCl (protective film) Ag + H 2 S Ag 2 S (corrosive)

14 Contd Gases like H 2 attacks the metal at ordinary temperature and is called hydrogen embrittlement Fe + H 2 S FeS + 2[H] (nascent hydrogen) Nascent hydrogen diffuses into the metal and form voids/ holes Nascent hydrogen atoms combine to form molecular hydrogen [H] + [H] H 2 (g) Molecular hydrogen leaves metal in form of gas leaving a cavity in the metal When maximum number of H 2 molecules are evolved, metal loses its tensile strength, ductility and malleability becomes weak

15 Wet/ Immersed/Electrochemical Corrosion Based on Nernst theory Occurs when Metals are in contact with moist air/ liquid medium Two dissimilar metallic articles are dipped in a solution Anode gives of ions in the solution and gets corroded Cathode accepts ions and forms a protective coating and does not get corroded

Evolution of Hydrogen At the junction where Cu is in contact with steel tank, metallic Fe gets corroded Fe Fe 2+ + 2e

16 Evolution of Hydrogen At the junction where Cu is in contact with steel tank, metallic Fe gets corroded Fe Fe e -- Electrons flow from anode to cathode, H + ion are eliminated as H 2 ; 2H + + 2e -- H 2 (g) Fe + 2H + Fe 2+ + H 2 (g)

17 Absorption of Oxygen Steel plate is coated with an oxide film Entire coated steel plate acts as cathode Imagine a crack on one point of oxide film Fe is exposed to atmosphere and this point acts as anode Fe Fe e ½ O 2 + H 2 O +2 e 2OH - Fe OH - Fe(OH) 2 (rust)

18 Differential Aeration Iron corrodes under the drop of water Areas covered under the drop of water or salt solution have less access to oxygen and turn anodic than the remaining parts of the metal becomes cathodic Oxygen concentration cell hence increases corrosion rate

19 Differential Aeration Zn Zn e (oxidation) 1/2O 2 + H 2 O + 2e 2OH (reduction)

20 Waterline Corrosion

21 Pitting Corrosion Localized accelerated attack Resulting in formation of cavities in metal due to dirt, deposits, cracked oxide film Occurs in form of pinholes, pits/ cavities Bottom of pit-- less oxygenated anodic More oxygenated part- Cathodic-protected

22 Intergranular Corrosion (IGC) Microstructure of metals are made of grains separated by grain boundaries Corrosion occurs along grain boundaries (less noble) than grain centres (noble) Corrosive attack is localized at these less noble areas

23 IGC is the result of sensitization of the material due to inadequate heat treatment during welding. In presence of carbon in steel, chromium reacts with carbon during heat treatment to produce Cr-carbides 23Cr + 6C Cr 23 C 6

24 Stress Corrosion Combined effect of metal stresses & corrosive environment Localized electrochemical attack For stress corrosion to occur: Presence of stressed metal Specific corrosive environment is necessary (caustic alkali) Presence of stress produces strains (anodic), thereby possess higher electrode potential becomes chemically active easily attacked by corrosive environment

25 Example: Metals and alloys when subjected to welding and bending Results in stress of the metal At the stress portion, atoms in the stressed region get displaced becomes anodic corrosion

26 Galvanic corrosion It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic cell forms, corrosion of the anode will accelerate; corrosion of the cathode will decelerate or even stop. Seawater galvanic series is a list of metals and alloys ranked in order of their tendency to corrode in marine environments. If any two metals from the list are coupled together, the one closer to the anodic (or active) end of the series, the upper end in this case, will be the anode and thus will corrode faster, while the one toward the cathodic (or noble) end will corrode slower. Eg, Daniel cell (Zn-Cu couple) Zn = and Cu = 0.34

28 Factors Influencing Corrosion Nature of the Metal Position in Galvanic series Relative areas of anodic, cathodic parts Nature of Corroding Medium Temperature Humidity Purity of Metal Physical state of metal Nature of surface film ph Conductance of corroding medium Impurities in atmosphere Passive Character Solubility & volatility of corrosion products

Position in Galvanic series Extent of corrosion depends upon position of metal in galvanic series Greater the difference, faster is the corrosion of anodic metal/alloy

29 Position in Galvanic series Extent of corrosion depends upon position of metal in galvanic series Greater the difference, faster is the corrosion of anodic metal/alloy Higher up in galvanic series becomes anodic suffers corrosion When 2 metals/ alloys are in electrical contact in presence of an electrolyte, more active metal suffers corrosion

30 Relative areas of anodic and cathodic parts When two dissimilar metals/alloys are in contact, Corrosion of the anodic metal (part) is directly proportional to the ratio of the areas occupied by the cathode and anode. Thus, corrosion at anode area of cathode/ area of anode Purity of the metal Impurities cause heterogeneity forms tiny electrochemical cells Rate & extent of corrosion increases with increasing exposure and extent of impurities Corrosion resistance can be improved by increasing the purity of the metal

31 Physical state of the metal Areas of metal under stress --- anodic corrosion Higher the porosity of metal greater the corrosion Passive nature of metal Metals like Al, Cr, Mg, Ni, Co are passive Exhibit higher corrosion resistance Due to formation of highly protective, thin film of metal oxide self-healing in nature (if broken can repair itself) Corrosion resistance of stainless steel is due to passivating character of Cr in it

32 Nature of Corrosion products If corrosion product is soluble in corroding medium corrosion proceeds at a faster rate Insoluble corrosion product acts as physical barrier Suppressing corrosion PbSO 4 in case of Pb in H 2 SO 4 Nature of the corroding environment Increase in temperature of the environment, reaction as well as diffusion rate increases corrosion rate enhances Influence of humidity depends on physical characteristics of metal & nature of corrosion products

33 Nature of the corroding environment Percentage of oxygen Part of the metal having less availability of oxygen, becomes anodic and undergoes corrosion The other part with more content of oxygen acts as a cathode and is protected

34 Humidity in air Corrosion enhances in humid air as compared to in dry air. Humid air/moist air has tendency to dissolve gases such as O 2, CO 2 etc. and acidic vapours Due to this, it becomes easy to set up an electrochemical cell on the surface of metal.

35 Temperature Rate of corrosion is greater at higher temperature, because diffusion of ions increases with the rise in temperature.

36 Influence of ph (Hydrogen ion concentration) Acidic ph (ph < 7) are more corrosive than alkaline and neutral media Rate of corrosion is higher due to mechanism of electrochemical corrosion proceeds by evolution of H 2 gas at cathode Alkaline ph : Electrochemical corrosion by absorption of O 2 forming metal oxide film as cathodic product film gets adhered to metal surface rate of corrosion depends on nature of corrosion product

37 Presence of other gases/suspended particles SO 2, H 2 S gases or fumes of HCl, H 2 SO 4 tend to increase the electrical conductivity of metals, thus increasing corrosion Suspended particles like NaCl, (NH 4 ) 2 SO 4 which are hygroscopic in nature, absorb moisture and act as a strong electrolyte, thus increasing rate of corrosion

38 Conductance of the medium If the medium to which the metal is exposed is conducting, it causes severe corrosion Water pipe line buried underground exhibit corrosion, here the soil acts as a conducting medium

39 Corrosion Control

40 Methods to Prevent Corrosion Based on Treatment of Metal Selection of material and proper design Purification of metals-removal of Strain Alloy formation Application of metallic coatings Electroplating Hot Dipping Galvanizing Tinning Cathodic and Anodic protection

41 Methods based on treatment of the medium Dehumidification De-aeration Neutralization Corrosion inhibitors

42 Selection of the material Avoid contact of dissimilar metals in a corrosive environment While selecting dissimilar metals, select the ones which are close in the galvanic series

43 Proper Designing Designing of the parts should be such that it avoids sharp bends, stresses Screws, nuts, bolts should be avoided, rather welding should be preferred Surfaces of two joining parts should be smooth-- which avoids accumulation of corrosive liquids, suspended particles, dust, dirt, etc

44 Smooth bend Sharp corner Good design (eliminating sharp corners) can minimize corrosion

45 Purification of metals Impurities in the metal act as an anode and result in corrosion Pure metal offers corrosion resistance Iron Pillar

46 Alloy formation Alloys of Fe with Si, Ni, Cr, offer good resistance to corrosion Example: Stainless steel Alloy of Fe+C+Cr-- presence of Cr; Formation of Cr 2 O 3 has a self healing property protect stainless steel from corrosion

47 Cathodic protection Sacrificial anodic protection Metallic structure to be protected is connected by a wire to a more anodic metal Structure to be protected is made cathodic External anode will give off ions & get corroded --sacrificial anodic protection

48 Impressed current cathodic protection We know that metal in electrolyte anodic and cathodic areas are developed Anode gives off ions & cathode is protected So anode & cathode has a certain effective potential This potential is the current local action current Local action current is nullified or counter-balanced by applying current in opposite direction This is called the impressed current dc source

49 Impressed current from dc source is applied such that anode and cathode will possess the same/similar potential

50 Modifying the environment Removal of harmful constituents or neutralization of the harmful effects of any constituents that cause corrosion Few modifications are: a) De-humidification b)de-aeration and de-activation techniques c) Neutralization of acids present in environment

51 De-humidification Substances capable of absorbing the moisture are kept in vicinity. Silica gel or alumina can be used. By frequently changing silica gel or alumina corrosion can be minimized.

52 De-aeration Electrochemical corrosion by absorption of oxygen can be controlled In neutral and basic media, cathodic reaction is ½ O 2 + H 2 O +2 e 2OH - By adjusting the parameters like temperature or pressure, or with mechanical stirring/agitating to expel dissolved gases like O 2 and CO 2 etc. Purging N 2 or Ar inert gases through the solution to eliminate oxygen

53 Acid removal (Alkaline neutralization) Prevention of corrosion by neutralization of the acidic character of corrosive environment. (due to presence of gases such as H 2 S, HCl, CO 2, SO 2 etc.) This can be achieved by using NH 3, NaOH, lime injected in the system

54 Adding compounds like sodium sulphite, hydrazine removes oxygen that tends to accelerate corrosion 2Na 2 SO 3 + O 2 2Na 2 SO 4 (used at lower temperatures) N 2 H 4 + O 2 N 2 + 2H 2 O (at higher temperatures) Also called as scavenger compounds

55 Corrosion Inhibitors Any substance (organic or inorganic compounds) when added in small amounts to aqueous corrosive environment decreases the rate of corrosion of the metal They form film of corrosion product at surface that limits further corrosion Synergistic effects (2 or more inhibitors) Anodic and Cathodic inhibitors

56 Anodic inhibitor/passivator (Barrier type) Capable of suppressing anodic reactions Chromates (CrO 4 2- ), phosphates (PO 4 3- ), tungstates (WO 4 2- ), molybdates (MoO 4 2- ) of transition elements with high oxygen content are capable of suppressing anodic reaction and thus preventing dissolution of metal acting as an anode These compounds form a sparingly soluble compound with metal ion formed at anode due to anodic reaction (i.e. loss of electrons) This compound gets adsorbed on the surface of metal and forms a protective adherent films-passivating the metal surface; thereby reducing corrosion

57 Cathodic inhibitors (Barrier type) This type of inhibitors are used in acidic as well as neutral solution. In acidic solutions, cathodic reaction is evolution of hydrogen. 2H + + 2e H 2 Controlled by decreasing evolution of hydrogen at cathode. Arsenic compounds restrict cathodic action of liberating H 2 In neutral solution, cathodic reaction is due to absorption of oxygen gas and formation of OH ions as H 2 O + ½ O 2 + 2e 2OH Eliminating oxygen from the corroding medium by deaeration methods or by retarding the diffusion of O 2 to the cathodic areas Mg, Zn salts react with OH- forming hydroxides that acts as barrier for corrosion

58 Organic adsorption inhibitors Sulphides, mercaptans form an oily layer on the metal surface and prevent adsorption of hydrogen. It also prevents solvation of the metal ion Inorganic adsorption inhibitors Bicarbonates, phosphates form tough adherent layers on the metal and prevent corrosion Vapor phase inhibitors Dicyclohexylamine nitrite (DCHN) It has low vapor pressure (3 x 10-7 Atm at 25 o C). They vaporize and form a thin barrier film on the metal surface and inhibit corrosion

59 Metallic coatings Hot dipping Metals like Zn, Sn, Pb have low melting points Aim: Base metal X has to be coated with any one of the above metals Above metal (say Zn) is placed in a furnace which is maintained at temperature just above their melting points We get molten form of the metal (say Zn) Base metal (X) is dipped in molten form (Zn) and one gets the desired coating

60 Electroplating* Cathode: Base metal (to be electroplated) Anode: Coating metal Electrolyte: Soluble salt of coating metal in the electrolytic cell * Refer (c) Dr. Payal notes B. Joshi for description

61 Contd.. Metal coatings anodic to base metal is called anodic coating (Zn on Iron) Metal coatings of a more noble metal wrt base metal is cathodic coating (Sn on base metal)

62 Galvanizing (Anodic coating)* Application of a coat of Zn metal on the surface of the base metal X 650C * Refer (c) Dr. Payal notes B. Joshi for description

63 Tinning (Cathodic coating)* Application of a coat of Sn metal on the surface of the base metal X 260C * Refer notes for description

64 Metal Cladding Sandwiching the base metal (alloy sheets) permanently between two layers of dense, homogenous corrosion resistant pure metals is known as metal cladding. Sheets of protecting metal (Cu, Al, Sn, Ni) are pressed on top and below the base metal using hot rollers at high temperatures. Choice of cladding material depends on corrosion resistance of the metal in the working environment

65 Alclad It is a classic example of metal cladding protection used in aircraft industry. Alclad, as the name suggests, is a plate of duralumin (Al=95%, Cu=4%, Mn=0.5%, Mg=0.5%) sandwiched between two layers of 99.5% pure Aluminium metal using hot rollers.

66 Organic Coatings Inert organic barrier applied on metallic surfaces for corrosion protection and to enhance aesthetic appeal of surfaces Protective value of such coatings depend on Its chemical inertness to corrosive environment Good surface adhesion Its impermeability to water, salts, gas, etc Proper application method Paints, Varnishes

67 Organic Coating Definition Constituent Functions Paints Mechanical dispersion mixture of one/more pigments in a vehicle Drying oil/ vehicle (linseed oil, fish oil) Pigments (white pigments: ZnO, TiO 2 ; colored: chromes) Thinners (turpentine, spirit, kerosene) Driers (oxygen carrier catalysts like resinates & tungstates of Co, Mn, Zn) Help pigments to hold on surface, Provide dry film by oxidation, Provide tough, durable, water resistant film Provide opacity, color strength, protection, resistance to abrasion Minimize cracking on drying Increase elasticity of paint film, reduce viscosity of paint to suitable consistency, helps drying of paint Improve quality of oil film, accelerate drying process of the oil film thro oxidation, polymerisation and condensation

68 Extenders/Fillers Inert materials that improve the properties of the paint, eg, gypsum, chalk, talc, silica, etc They serve to fill voids in the paint film Reduce cracking of paint after drying & improve durability of the paint film Plasticizers: To give elasticity to the paint film and prevent cracking

69 Characteristics of a good paint Should be fluid enough with high covering power to spread easily over the protected surface Protect painted surfaces from corrosion effects of the environment Form tough, uniform, adherent and impervious film that does not crack on drying Color of the paint should be stable to effects of the atmosphere

70 Organic Coating Varnish Definition Constituent Functions Colloidal dispersion of natural/ synthetic resins in oil Resin (shellac, phenol, urea, polymers) Drying oil/ vehicle (linseed oil, fish oil) Thinners (turpentine, spirit, kerosene) Driers (Pb, Co, Mn, linoleates) Provide high resistance to weathering, chemical action, water Provides hardening to dried films Helps in drying varnish films by oxidation & polymerization Adjust/ reduce viscosity of varnish to suitable consistency Improve quality of oil film, accelerate drying process of the oil film thro oxidation, polymerisation and condensation Antiskinning agents (tertiary amyl phenols) Helps film to get adhered to underlying surface

71 Characteristics of good varnish Produce protective film hard, tough, durable and resistant film to wear and tear that does not crack on drying Dry quickly Aesthetic appeal of the film Color stable to exposure to atmosphere

72 Varnishes are of two types : Oil varnishes Spirit varnishes Oil Varnishes These are formed by dissolving resins in oil They dry very slowly They leave a tough film Spirit Varnishes These are formed by dissolving resins in spirits They dry very quickly They leave brittle film

73 Current Trends Functional coatings/ Self-cleaning coatings oil- and water repelling self-cleaning surfaces are fabricated superhydrophobic and superoleophobic Eg, SiO 2 -TiO 2 layered surfaces with polymers like PS-PDMS-PS (polystyrene- polydimethylsiloxanepolystyrene)

Corrosion. Cause of Corrosion: Electrochemical Mechanism of Corrosion (Rusting of Iron)

Corrosion. Cause of Corrosion: Electrochemical Mechanism of Corrosion (Rusting of Iron) Corrosion Any process of deterioration (or destruction) and consequent loss of a solid metallic material, through an unwanted (or unintentional) chemical or electrochemical attack by its environment, starting