Corrosion for Students PDF

Unit No.6: Corrosion Milestone 1 1. Introduction to Corrosion 2. Causes of Corrosion 3. Types of Corrosion 4. Dry Corrosion 5. Oxidation corrosion 6. Pilling – Bed Worth ratio 7. Corrosion by other gases 8. Liquid metal corrosion 9. Wet Corrosion 10. Electrochemical T...

Unit No.6: Corrosion Milestone 1 1. Introduction to Corrosion 2. Causes of Corrosion 3. Types of Corrosion 4. Dry Corrosion 5. Oxidation corrosion 6. Pilling – Bed Worth ratio 7. Corrosion by other gases 8. Liquid metal corrosion 9. Wet Corrosion 10. Electrochemical Theory of Wet Corrosion 11. Different Forms of Wet Corrosion 12. Galvanic Corrosion 13. Factors Affecting Corrosion 14. Corrosion Control (or Prevention) 15. Cathodic Protection 16. 17. Techniques of Metal Coating 18. Inorganic Coating 19. Organic Coating 20. Cathodic Protection 21. Corrosion Inhibitors Milestone 2 1. Methods of corrosion prevention 2. Metallic protection (Cathodic protection and anodic protection methods) 3. Application of protective metallic coating (anodic coating and cathodic coating) 4. Methods of application of a metallic coating Hot dipping method b. Electroplating c. Cementation d. Metal cladding 1. Introduction Corrosion can be defined as the slow degradation or deterioration of metallic surface due to unwanted attack by the atmosphere gases, soil, chemical, or electrochemical reaction with the surrounding environment. Degradation or deterioration means loss or reduction in the useful properties of the material because of corrosion. Some common effects observed are- (i) Change in surface texture and surface roughness of metals (ii) Weakening of joints due to loss of material. (iii) Loss of useful material properties such as heat transfer, malleability, ductility etc. (iv) Reduction in dimension. The best example of the process of corrosion can be seen in the case of ‘rusting of iron’, in which the metal is deteriorated due to the formation of a layer of reddish scale and powder of Fe 3O4on the surface. Another example is of copper article corrosion. Formation of a green layer of basic copper carbonate consisting of [CuCO3 + Cu (OH)2] on the surface of copper when exposed to a corrosive environment. 2. Causes of Corrosion  Except for few metals like gold, platinum (noble metal), other metals are found in nature in the combined state such as oxides, hydroxides, carbonates, chlorides etc. These compound form of metals are called their ore.  In nature, metals exist in the combined state, they are stable and are in a lower energy state.  However, during the extraction of metal from their ores, energy is added to them. Thus, the pure form of metals is less stable as they are in a higher energy state.  So, the pure metals have a natural tendency to go back to their combined state.  For this, metals interact with their environment either chemically or electrochemically to form a stable compound by the process of corrosion.  Life Cycle of Iron 3. Types of Corrosion  The process of corrosion of metal depends on the environment in which the metal is placed.  Various types of corrosion processes along with their respective mechanism are given below. 4. Dry or Chemical Corrosion This type of corrosion occurs when metals are exposed to a dry environment. Due to the direct chemical action of atmospheric gases like O2, CO2, H2S, halogens (like Cl2, Br2, I2) on metal resulting in the formation of compounds such as oxides, halides, sulphates and sulphides is known as chemical corrosion. The products which are formed due to dry corrosion may be insoluble, soluble or liquid in nature. Dry Corrosion The rate of dry corrosion depends on two factors. a) Chemical affinity between the metal and atmospheric gases.  b) The nature of corrosion product ( which is protecting the original metal by layer or film formation). Dry Corrosion is of three types: (i) Oxidation corrosion (Corrosion by oxygen) (ii) Corrosion by other gases (iii) Liquid metal corrosion 5. Oxidation corrosion Oxidation corrosion occurs by the direct action of dry oxygen (absence of moisture) on a metal surface at low or high temperature. Oxidation corrosion is sensitive to temperature. Most of the metals have a slow oxidation rate at low temperature. At high temperature, almost all metals except silver, gold, platinum is oxidized. Mechanism of Corrosion by Oxygen When a metal is exposed to air, oxygen is absorbed even at ordinary temperatures. This absorption is due to vander Waal’s forces. This absorbed oxygen may gradually chemically combine with the metal by electron transfer. Loss of electron by metal atom (being electropositive) to an oxygen atom, resulting in the formation of metal ion and oxide ion. This process can be represented by following chemical equation-  Formation of metal oxide by the union of the ions and finally leading to the formation of a thin layer or film of metal oxide on the surface of the metal.  Iron forms Fe2O3, chromium forms Cr2O3, zinc forms ZnO, aluminium forms Al2O3, copper forms CuO, on corrosion by oxygen gas.  The nature of oxide film plays an important role in deciding further the rate of corrosion. Once the primary oxide layer is formed on the metal surface the rate of reaction will be guided by the property of the film formed. Types of oxide films a) porous film / non-protective oxide layer If the oxide film formed on the metal surface is porous, then through the pores, oxygen gas molecules penetrate and continue the corrosion of the underlying metal until the whole metal gets destroyed. Example: Oxide film of iron, magnesium, sodium, potassium, calcium metals etc. are porous. 4 Na + O2 2Na2O 2 Fe + O2 2FeO 2 Mg + O2 2MgO b) Protective or non-porous oxide Layer  If the oxide film formed on the metal surface is stable, nonporous, strong, well adhered to the metal surface and self-healing, then further corrosion stops, as the oxide film formed after some initial corrosion of the surface, acts as a barrier. It detaches the underlying metal from the corroding oxygen or other gases. Example: Oxide film of chromium, tin, zinc, lead, aluminium, nickel, copper, bronze alloy. 2 Al + 3/2 O2 Al2O3 2 Cu + O2 2CuO 2 Zn + O2 2ZnO 4 Cr + 3O2 2Cr2O3 c) Unstable Oxide Film  Generally, such types of oxide films are observed on noble metal surfaces. In this case, the oxide film formed is unstable. i.e., conversion of oxide into metal and oxygen takes place spontaneously, M + O2⇌ MO2  It can be seen that although the corrosion of the metals takes place, the metal does not get affected because of the reverse reaction. Example: 2Ag + O2 ⇌ Ag2O 2Au + O2 ⇌ Au2O d) Volatile oxide Film  This type of oxide film, gets evaporated as soon as it is formed. Thus, it exposes underlying metal for further corrosion.  In this type of corrosion rate of corrosion of metal is very high. Example: 2Mo + 3O2 2MoO3 6. Pilling – Bed Worth ratio (PBR)  This ratio gives an idea about the nature of oxide film formed after the corrosion of surface metal. Pilling Bed- Worth ratio = Volume of metal oxide formed Volume of metal consumed  The rule state that, if the volume of oxide is smaller than the volume of metal consumed in the metal oxide formation, then the oxide film is porous. i.e. If P.B.R. is less than 1 then porous oxide film is formed.  On the other hand, the film is non-porous if metal oxide formed has a greater volume than metal consumed during corrosion.  Oxide-metal volume ratio for some metals is given. CuO : Cu 1.68 Al2O3 : Al 1.28 FeO : Fe 1.77 Ag2O : Ag 1.59 MoO3 : Mo 3.4 Na2O : Na 0.57  However, if 1< PBR < 1.45 then the metal oxide film is non-porous and protective from further corrosion by any gaseous medium.  PBR1 Compressive stresses in the oxide film Uniformly cover the metal surface and is protective PBR>>1 Too much compressive stresses in the oxide film Oxide cracks 7. Corrosion due to Gases  Other than oxygen gas, other dry gases which cause a corrosive effect on the metals are SO2, CO2, Cl2, H2S, F2 etc.  The rate of corrosion caused due to other gases depends on the chemical affinity between metal and gas. It also depends on the nature of corrosion products formed. Corrosion by Hydrogen  The hydrogen gas can attack on metals in two ways: 1. Hydrogen embrittlement 2. Hydrogen attack. Hydrogen embrittlement At ordinary temperature, hydrogen gas is formed as a result of a chemical or electrochemical reaction at the metal surface. Formed hydrogen gas diffuses into metal and decreases its strength, this effect is called hydrogen embrittlement. Example: When HCl (aq. solution) comes in contact of metal surface, hydrogen gas comes out in nascent form Fe + 2HCl FeCl2 + 2H  Then nascent hydrogen , diffuses freely into the metal and traped in voids and their it combines to form hydrogen gas. H + H H2 As the concentration of hydrogen increases inside the metal due to high pressure the blisters are formed on the surface of metal. This whole process affect the strength of metal and metal is subjected to brittleness. Hydrogen attack  Every metal contains a small number of impurities of elements like C, S, O, N. At high temperature when hydrogen gas attacks the metal, it reacts with the impurities present in the metal.  At high temperature, hydrogen gas attacks metal to form atomic hydrogen by the thermal dissociation process. This atomic hydrogen diffuses into metal.  The formed gaseous reaction products decrease the strength of the metal. This process of corrosion is called hydrogen attack.  For example, at high temperature hydrogen in atomic state reacts with the carbon present in steel.It results in the formation of methane with high concentration and intergranular cracking or blistering appears on metal surface and the whole metal. This whole process affect the strength of metal and metal is subjected to brittleness, internal cracking and blisters.. C + 4H CH4  Here the carbon content in the steel is decreases, this process is called decarburization. This steel looses its strength due to this process. 8. Liquid metal corrosion  This type of corrosion occurs when a molten liquid is continuously passed on a solid metal surface or an alloy.  This type of corrosion mainly occurs in nuclear powers devices where the temperature is very high.  This behaviour may be due to the following. (a) dissolution of the solid metal by liquid metal. (b) penetration of molten metal into solid metal.  Certain metals like Al and stainless steel undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb, Cd etc.  Liquid metals penetrate the grain boundaries and fracture the solid metal. 9. Wet corrosion  Wet corrosion is also known as Electrochemical corrosion or immersed corrosion.  It can be defined as the corrosion caused by exposure of metal or two dissimilar metals in contact with an electrolyte solution (acid, base or salt) leading to the formation of an electrochemical or galvanic cell.   Wet corrosion occurs under the following conditions: a) When conducting solution is in contact with metal. b) When two, unlike metals, are immersed or single metal is dipped partially in an aqueous solution of acid, base or salt. Electrochemical Theory of Corrosion  Wet corrosion can be easily explained by electrochemical theory.  According to the electrochemical theory of corrosion, when a metal (or dissimilar metals) is exposed to an acidic environment medium, the process of corrosion sets in by the formation of separate ‘anodic’ and ‘cathodic’ areas within the metal surface.  One metal (or a part of single metal) act as an anode and the other metal (other part of single metal) act as a cathode when come in contact with an electrolyte.  Corrosion always occurs at the ‘anodic area’ of the metal due to the oxidation process. Anodic area corrodes or oxidizes leading to the formation of metallic ions or cations and the electrons are set free by the reaction. M Mn+ + ne- …….Oxidation  The metal ions [Mn+] formed during the destruction of metal either dissolves in the medium or forms a thin film of oxide on the metal surface.  Reduction takes place at the cathode. The electrons from the anode are accepted by the dissolved oxygen forming ions such as OH- or O2- ions depending upon the nature of conducting or corroding medium (electrolyte).  The type of cathodic reaction taking place depends on the nature of the electrolyte in contact with the cathodic metal as follows: i) In acidic medium a) In the absence of oxygen Hydrogen ions (of acidic medium) combine with electrons and form hydrogen gas. 2H+ + 2e-1 H2 (gas) b) In the presence of oxygen 2H+ + O2 + 4 e-1 2H2O ii) In a neutral or alkaline medium (presence of oxygen) Oxygen present in water combines with electrons and forms hydroxyl ions. 2H2O + O2 + 4 e-1 4OH- The electron is set free at the anodic area consumed at the cathodic area by the following two processes. i) Hydrogen evolution mechanism ii) Oxygen absorption mechanism Hydrogen evolution mechanism  This type of electrochemical corrosion occurs usually in an acidic environment like industrial waste, a solution of non-oxidizing acids.  Hydrogen evolution from cathodic metal occurs when they are exposed to an acidic environment. It is nothing but a displacement of hydrogen ions from the solution by metal ions which can be written as, At Anode - M M2+ + 2e- At Cathode - 2H+ + 2e- H2 The cathodic reaction involves the evolution of hydrogen gas. All the metals above hydrogen in the electrochemical series will oxidize in acid solution with the evolution of hydrogen gas when coming in contact with acidic electrolyte. When two dissimilar metals are in contact with acidic electrolyte, the metal which is placed higher in the electrochemical series or galvanic series acts as an anode while the other metal placed below act as a cathode. Consider the example, A steel tank containing acid industrial waste and small copper scrap is in contact. The steel tank acts as an anode and copper scrap acts as a cathode. Oxidation reaction takes place at anode releasing electrons. These electrons flow towards the cathode where a reduction reaction takes place. Anode: Steel tank (Fe) Cathode: copper scrap (Cu) Electrolyte: non-oxidizing acidic solution  Reactions: At anode: Fe Fe2+ + 2e- At cathode: 2H+ + 2e- H2 Oxygen absorption mechanism This type of electrochemical corrosion occurs usually in a neutral or alkaline environment like seawater, moist soil etc. In this mechanism, the anodic metal dissolves in solution and at cathode reduction of electrolyte takes place with the absorption of oxygen. Reactions: At anode: M Mn+ + ne- At cathode: 2e- + H2O + ½ O2 2OH- Example: Consider the steel plate lying on the ground. There is rust formation on its surface and a dew drop in contact with the metal through the pores is present. Here, anodic area (part of metal below pores), iron will dissolve by oxidation. Fe Fe2+ + 2e- The electrons will move to the cathodic area (rust present on the metal) through iron and will be accepted by electrolyte water. 2e- + H2O + ½ O2 2OH- Fe2+ + 2OH- Fe(OH)2 2Fe (OH)2+ 1/2O2 Fe2O3 + 2H2O Brown rust  Thus, the O2 from the aqueous medium is utilized for reaction and leads to the formation of metal hydroxide as the corrosion product. The metal hydroxide if soluble in the aqueous medium, remains dissolved and if not soluble, then it deposits on the cathode area near the anode. Difference between chemical and electrochemical corrosion Chemical corrosion Electrochemical corrosion It occurs only in dry conditions. It occurs in wet conditions in the presence of moisture and electrolyte. It involves a chemical attack of oxygen or other It involves an electrochemical attack of a corrosive gases. environment on the surface of the metal. Chemical corrosion products accumulate at the site Corrosion products accumulate somewhere of attack i.e. anode. between the area of anode and cathode. It is a self-controlled process. It is a continuous process. In this process, oxidation and reduction sites are In this process, oxidation and reduction take place same. at different sites. It is a slow process taking place by the chemical It is a fast electrochemical process. It proceeds reaction of atmospheric gases. through the cells. In chemical corrosion, the product may be In electrochemical corrosion, the product is always unstable, volatile or porous in nature. stable. Example: Fe3O4, Zn(OH)2 etc. In chemical corrosion, the path for electron flow is In electrochemical corrosion, the path for an not required. electron is always required. 11. Different Forms of Wet Corrosion The aqueous conducting medium may be like water, seawater, dew, fog, wet soil, high humidity. The corrosion involves anodic and cathodic areas formation and corrosion occurs on the anodic area only, while corrosion product either remains dissolved in medium or deposit on the cathode. Electrochemical corrosion involves either a galvanic cell formation or a differential aeration (concentration cell) formation. Different forms of wet or electrochemical corrosion are:  i) Galvanic Corrosion ii) Differential Aeration Cell Corrosion Galvanic Corrosion  The electrochemical corrosion of a metal, by the attack of an aqueous conducting medium, with the formation of a galvanic cell, is known as Galvanic corrosion.  For galvanic corrosion to occur, three conditions must be met. a) The difference in electrical potentials of the two metals in contact with one another. b) An electrical path between the two metals. c) A fluid is able to break down the metals.  All metals have an electrical potential assigned to them, based on their nobility. Metals such as platinum, silver, and Monel have lower corrosion potentials whereas metals such as copper, aluminium, and tin have much higher potentials. Any two dissimilar metals will have a galvanic mismatch and therefore a change of corrosion.  An electrical path must exist to allow metal ions to move from the active metal to the less active metal. Typically, the metals merely touch one another.  A fluid able to break down the metals, examples of such fluids can include atmospheric humidity or salt fog environments, ordinary seawater, citric acid, and bases. As this mist or moisture condenses and collects at the joint or interface, it will create the electrolyte needed to start breaking down the metals.  The galvanic cell is formed by i) Two dissimilar metals in contact  ii) Stressed and unstressed part of the same metal iii) Impurities in metal Galvanic Corrosion - Two dissimilar metals in contact When two dissimilar metals (Example- zinc and copper) are in contact with each other and are exposed to an aqueous conducting medium (electrolyte), the galvanic cell is formed. Metal or alloy higher placed in galvanic series, act as anode and gets corroded. Other metals act as a cathode and it does not corrode. At cathodic metal reduction of electrolyte takes place. Galvanic Corrosion of Two Dissimilar Metals Example: Brass pipe fitted to iron pipe. Teflon tape to break the electrical path between the two metals Corrosion Sank the Titanic?  The titanic was held together by 3 million rivets made with a different type of iron than the hull plates, he notes. and once the hull was finished, the ship sat in seawater for a year while the inside was furnished.  The dissimilar metals of the hull and rivets, bathed in electrically conductive seawater, might have created a circuit that slowly flecked away and weakened the rivets.  The titanic collision with the iceberg could have popped the weakened rivets, which would explain a clinking sound reported by survivors. Galvanic Corrosion -Stressed and unstressed part of the same metal Stress-corrosion occurs when a material exists in a relatively inert environment but corrodes due to applied stress. The stress may be externally applied or residual. Working on a part of metal (hammering, bending, cutting or turning) disturbs the grain structure of the metal at worked part. Thus, worked part is stressed and the non-worked part is unstressed. This creates a potential difference in the metal itself. Whenever an aqueous conducting medium comes in their contact, stressed or worked part acts as anode and gets corroded. While the un-stressed part acts as a cathode. Examples: - Threaded end of the pipe, bent part of a rod or plate, the tip of a pin, threaded part of iron nail. Galvanic Corrosion -Impurities in metal A large number of tiny galvanic cells are formed on the surface of an impure metal or improperly made alloy in an aqueous medium. Either the impurity acts as an anode or the main metal, depending upon whose position in galvanic series is higher.  If the impurity acts as an anode, the corrosion is highly localized and fast which causes small pits formation on the surface. Differential Aeration Cell Corrosion The concentration cells formed due to varying O2 concentrations on a metal surface are called differential aeration cells and the corrosion is called differential aeration corrosion. It is a type of electrochemical corrosion that affects metals such as steel and iron. When a poorly oxygenated area is adjacent to an area with a good supply of oxygen, an anodic/cathodic reaction occurs. The reaction occurs because oppositely charged electrons flow between the smaller anode and the larger cathode. Positively charged cations meeting negatively charged anions forming corrosion product and a resulting pit in the metal, otherwise known as pitting corrosion. When the ions between the cathodic and anodic areas meet iron hydroxide forms, precipitates and oxidise to form the corrosive material or rust. The diagram shows an instance of waterline corrosion. The equation is O2 + 2H2O + 4e– → 4OH– (cathode) and Fe2+ + 2e– → Fe– (anode). Partial immersion of metal The partial immersion of metal in water causes such cell formation, the immersed part of the metal is poor oxygenated and gets corroded by acting as an anode. The metal part exposed to higher oxygen concentration acts as a cathode. Examples: - water in metal tank or pot, ship, metallic installation in water, water drops on metal etc. Partial burial of metal in the ground A metal partially buried in the ground (poles, metallic installation on ground) or metal kept on the ground, have aerial part more oxygenated and metal in contact of moist soil poor oxygenated (anode). The anodic part gets corroded. Crevices When two plates or pieces of the same metal are not fitted properly, there is a small gap between them. The metallic surface in the gap is poorly oxygenated and gets corroded by acting as an anode. Oxide film on metal surface partially ruptured As long as the non-porous oxide film on a metal surface is uniform, no cell is formed but if the film gets ruptured partially then the ruptured part is more oxygenated and the covered part is poor oxygenated. Thus, at the ruptured area corrosion starts as it acts as an anode in the highly humid atmosphere. 13. Factors influencing the rate of corrosion Corrosion is the destruction of metal through chemical or electrochemical action with its surrounding. The rate of corrosion depends on i) Nature of the metal ii) Nature of environment Factors related to nature of metal a) Position of Metal in Galvanic Series The rate of corrosion of metals depends upon their position in the electrochemical series and galvanic series. When two metals or alloys are in electrical contact in a conducting medium, then the higher placed metal/alloy in galvanic series (more active) acts as anode and gets corroded. The rate and severity of corrosion further depend upon their difference in positions. More the metals are apart in galvanic series faster is the corrosion anodic metal and more is the current generated during corrosion. Dry corrosion is also faster in more active metals. b) Relative Areas of Cathode and Anode The important factor in galvanic corrosion is the area effect i.e. ratio of cathodic to anodic areas. If the ratio of cathodic to the anodic area is greater, then the rate of wet corrosion is faster, severe and localized on the anodic area. The rate of corrosion is more if the size of the cathodic area is large. Hence, a better design by the use of two metals is one in which anodic area is much larger than cathodic area i.e. the metal lowered placed in galvanic series is used in smaller proportion than the metal placed higher in galvanic series. c) Purity of Metal Impurities in metal form large numbers of a minute or tiny galvanic cells when there is conducting medium around the impure metal. If the impurity is passive then the metal undergoes corrosion but if the impurity is more active, then faster corrosion of impurity takes place with small pits formation on the surface, due to local action. TABLE Zn purity Relative corrosion rate 99.999 1 99.99 2650 99.95 5000 d)Physical State of Metal The smaller the grain size of the metal or alloy, the greater is found the rate of corrosion. Example: Steel gets corroded more easily than cast iron because grains in steel are of smaller size. Further, areas under stress in the same metal tend to be anodic. e) Nature of oxide film If the oxide film formed on the metal surface is protective, non-porous, strongly adhered, then neither dry corrosion nor electrochemical corrosion takes place. But if the film is porous, loosely adhered (steels) then the corrosion of the metal takes place until a complete destruction, by dry or wet corrosion mechanism. f) Over Voltage In the case of metals having a higher position in the EMF series than hydrogen, the corrosion of the metal in an acidic medium liberates hydrogen gas. The tiny bubbles of the hydrogen gas adhere to the metal surface and then the acidic solution is not in direct contact with the metal. This fact leads to decreasing the rate of corrosion of metal after the initial corrosion reaction begins. In other, such metal develops resistance to some extent or the metal behaves as if it is placed lower than its actual position in the EMF series. To restore the position of metal, some extra voltage is necessary. This extra voltage is known as overvoltage. However, all metals do not have hydrogen adsorption and their corrosion takes place in the usual way. The metals having an overvoltage nature are zinc, lead, chromium, nickel etc. Factors related to Nature of Environment a) Temperature The rate of atmospheric or dry corrosion is faster at a higher temperature because the attacking gas, as well as metal, get activated at a higher temperature.  In the case of electrochemical corrosion, the corrosion of anode depends upon the temperature. As per Nernst’s equation for electrode potential, the potential of metal is high at a higher temperature. Thus, the rate of wet corrosion also increases with an increase in temperature. Steel corrodes at faster rates at higher temperatures than at lower temperature. b) Moisture Moisture is a key factor in dry corrosion and wet corrosion. Critical humidity is the relative humidity above which atmospheric corrosion increases sharply. Oxygen and other reactive gases in the atmosphere do not have much corrosion action in the absence of moisture but the corrosion of metals takes place at a much higher rate above the critical humidity. Higher relative humidity, fog, mist in the air provides the aqueous conducting medium and causes galvanic or differential aeration cells formation. The rate of corrosion (of anodic part) by cell formation is fast. Moisture in soil, water, aqueous solutions also cause such cell formation and corrosion of metals. c) Effect of pH An acidic environment causes more corrosion than alkaline or neutral environments. Some metal shows corrosion resistance in an alkaline medium. Example: Zn corrodes minimum at pH 11, but at higher pH (more than 11) it corrodes faster. At pH 5.5, Al corrodes minimum and above 5.5 corrosion is fast. Many metals are readily attacked by acid but are resistant to alkali. By alternating the chemical character of the corroding medium i.e., pH, the corrosion rate of a given metal can be controlled. d) Conductance of Medium If soil contains moisture and soluble salts, it will increase the conductivity of the soil. An increase in conductivity will increase stray current corrosion. Moisture present in the soil also increases the conductance of the medium, which increases underground soil corrosion of metals. e) Nature of Electrolyte The electrolyte itself is a source of potential difference. The solution potential of metal depends on the type of ions and their concentration in the solution. The Cl-, NO3-, like ions have the ability to break the non-porous oxide films on the metal surface and cause enhanced wet corrosion. On the contrary, the presence of oxalate ions, phosphate ions, silicate ions etc. have the ability to slow down the rate of wet corrosion. 14. Methods of Corrosion Control Corrosion of the metal can be mitigated by using different methods such as cathodic or anodic protection methods, application of different protective coatings on the surface of the metal, by altering the environmental factors. Why control Corrosion? Materials are precious resources Engineering design is incomplete without knowledge of corrosion Applying knowledge of corrosion protection can minimize disasters Corrosion- may contaminate stored food, dairy products etc. 15. Cathodic Protection  During corrosion, the metal which is anodic gets corroded and the cathodic metal is protected from corrosion. The basic principle used in cathodic protection is the metal that needs protection from corrosion is forced to behave as a cathode.  There are two methods of cathodic protection: a) using sacrificial anodic protection b) using impressed current cathodic protection a) Using Sacrificial Anodic Protection or galvanic protection The metallic structure which needs to be protected from corrosion is connected to anodic metal (active) by an insulated wire. For this method, first a more active metal (more active metal like Zn, Al, Mg etc) from the galvanic series is selected. Then this metal is installed and an insulating metallic path is provided. This active metal will act as a sacrificial anode and gets corroded, protecting the pipe/structure working as a cathode, hence it is called a sacrificial anode To increase electrical contact, the active metal is placed in the backfill (coal and NaCl). When the sacrificial anode is consumed completely it is replaced by a fresh piece. Applications Small Pipelines with good Coating Harbor Facilities, Steel piles, Jetties, etc. (Mg or Zn rods are bolted along the sides of the ship) Vessels, Tanks, etc. (Zinc rods are inserted into the boiler or hot water tanks to prevent corrosion)  Buried cables..Plant facilities and Equipment, Seawater intakes, Screens, Condensers, Heat Exchangers, etc. Advantages  Simple in Installation.  No External Power Source.  Very few operations or maintenance requirements.  No Power Bills.  Easy to Design.  No expensive accessories like cables etc.  Economical for small structures. Limitations  Low Driving Voltage.  Poor performance due to passivation.  Limited Current. An extremely small current is available in higher resistivity electrolytes.  Low life. b) Using Impressed Current cathodic protection (ICCP) The flow of corrosion current is observed from anode to cathode. In the Impressed Current method, Direct Current is applied in the opposite direction to nullify the corrosion current which converts the corroding metal from anode to cathode. The impressed current is derived from a D.C. source and its positive terminal is connected to insoluble anode like graphite, stainless steel or scrap iron, buried in the soil. The negative terminal of D.C.is connected to the metallic structure to be protected. The anode is kept in a back-fill (composed of gypsum or coke) to increase electrical contact with the surrounding soil. DC source can be a solar cell, rectifier, generator, battery, or some other DC power.  Impressed Current Systems is designed for larger structures, or where electrolyte resistivity is higher and galvanic anodes cannot deliver enough current to provide protection. Applications Open water tank coolers Water tank Buried water or gas pipelines. Condenser Transmission line tower Laid-up ships Marine piers etc. Advantages  Current and Voltage can be varied.  Can be used in almost any resistivity Environment.  Can be designed for remote monitoring and control.  Can be designed for measurement of Instant OFF / ON.  No limitation of driving Voltage.  Economically feasible to replace anode system when required.  The system is extremely flexible Limitations of Cathodic protection  High capital investment and maintenance cost. Because of stray current, a pipeline or metallic installation near the protected metallic structure, get corroded rapidly. Special care needs to be taken to see the structure is not overprotected, otherwise, problems associated with H2 liberation or OH- formation will occur. Regular monitoring and maintenance required Requires Main supply or another source of electric Power Interference Problems must be considered. Relatively large chance of premature failure or breakdown. 16. Anodic Protection Anodic protection is a technique to control the corrosion of a metal surface by making it anode of an electrochemical cell and controlling the potential in a range where the metal is passive. Anodic protection is based on the formation of a protective film on metals by the externally applied anodic current. i.e., by passivating the metal. Passivity or Passivation is defined as a phenomenon in which a metal or an alloy exhibits outstanding higher corrosion resistance than its position in the electrochemical or galvanic series. Passivity is the result of the formation of a highly protective but very thin invisible film on the surface of metal or an alloy, which makes the metal more noble. A metal or alloy which have passive nature (formation of highly protective and very thin invisible film on the surface of the metal for example aluminium), is made more anodic and the voltage is applied in the same direction of corrosion current over it to control its corrosion even in strongly corroding media. Actually, the application of anodic current to a structure increases the dissolution rate of the metal. Thus, a thin layer of metal oxide is formed on the surface of the metal. This layer protects the metal from corrosion making it passive. This type of behaviour is observed for metals that exhibit active- passive transitions eg. Al, Cr. Ni etc. Anodic protection offers extreme corrosion protection in equipment where other methods won’t work because of the harshness of the environment. Anodic protection is used for carbon steel storage tanks where the environment is extremely acidic including concentrated sulfuric acid and 50 per cent caustic soda where cathodic protection is not suitable due to very high current requirements. Method  To protect the structure anodically, an electronic device potentiostat is used. Potentiostat maintains a metal at a constant potential with respect to the reference electrode. Out of the three terminals of the potentiostat, one is connected to the metal i.e. tank to be protected, another to an auxiliary cathode and the third terminal to reference electrode. Potentiostat maintains a constant potential between tank and reference electrode. Application of anodic current, causes dissolution of metal and formation of protective anodic oxide layer on its surface making it passive to the corrosion process. To create the anodic layer on the surface of the metal, corrosion protection experts apply a controlled electric current to charge the protective film with the use of an anode. The current can increase or decrease the thickness of the protective layer through the use of an external power supply. Applications This method is used for corrosion protection of: Aerospace application Chemical reactors  Complex metallic installations on the ground or under seawater. Industrial water coolers. Industrial metal condensers. Pipeline for carrying corrosive liquids or solutions etc. Advantages Low operating cost Applicable to highly corrosive media. Reliable to protect complex structures. The feasibility of the method can be predicted in the laboratory. Protection current gives an idea about corrosion rate. Limitations Applicable to only those metals which show active-passive behaviour. High installation cost. High starting current required. In case if the system goes out of control, then a very high corrosion rate occurs. Cathodic protection Anodic protection The basic principle is to force the metal to The basic principle is to increase the be protected to behave like a cathode passivity of the base metal by applying current in the direction in which metal would become more anodic The cathodic protection involves reversing The anodic protection involves the flow of current between two dissimilar suppression of anodic reaction by electrodes adjusting the potential of the more reactive metal The cathodic protection can be achieved There are no different methods for anodic by a sacrificial method and impressed protection current method It protects long length structure for long It can protect complicated structures duration It is not very costly The initial cost is high, though the operating cost is low It can provide steady, consistent protection It may not provide steady protection. If it for long term goes out of control, the rate of corrosion increases suddenly Applicable in a moderately corrosive Applicable in an extremely corrosive environment. environment 17. Metallic Coatings Application of protective coating on metal surface act as a barrier between metal and environment. Surface preparation refers to the various methods that can be used to treat the surface of material before coating application, the use of adhesives and other procedures. Surface preparation is essential for treating steel and other substrates before they are painted, coated or lined. Metal surface preparation for coating The surface preparation of the base metal undergoes the following steps: i. Remove Old Coatings ii. Remove Oils, Chlorides, Acids and Other Surface Contaminants iii. Remove oxide layer, corrosion products iv. Applying Etching treatment for better adhesion of coat 1. Remove Old Coatings Loose rust, loose mill scale and deteriorated coatings can be removed by the effective use of hand and power tools. Brush-Off Grade Blasting cleans to the same requirements and may be used as an alternative to scraping and wire brushing. 2. Remove Oils, Chlorides, Acids and Other Surface Contaminants i) Solvent Cleaning: Grease, oil, fatty substances are removed by treating the surface with Naphtha, CCl4, toluene, xylene etc. then cleaning is done by using hot water. ii) Alkali cleaning: For removal of dust, dirt, wax, grease, oil, fat, salt, acid residue, etc., scrub surface with a strong commercial detergent solution such as trisodium phosphate (TSP), then flush thoroughly with fresh water. The surface must be completely dry and free of any residue before it is coated. 3. Remove oxide layer, corrosion products a) Mechanical cleaning: In this loose rust and other impurities are removed with the help of a chisel, scraper etc b) Flame cleaning: In this, the surface is heated with a hot flame to remove moisture and scale. c) Sand blasting: This is used to remove the oxide layer and for getting a rough surface. The process involves introducing sand into the air stream under the pressure of 25 to 100 atm pressure. 4. Etching treatment for better adhesion of coat To achieve better adhesion, the metallic coating pickling method is used. It involves immersing the metal in acid or alkali to provide a clean, smooth surface for electrode deposition. Types of Metallic Coating i) Anodic Coating If the position of coating metal is higher (more active) in galvanic series as compared to the base metal (base structure), then the coating is called anodic coating. Example: Zn, Al, Cr coated on steel. If any pores, breaks or discontinuities occur in such anodic coating and there is a conducting medium around it, then galvanic cell set up.  In the galvanic cell, coating metal being anode starts getting attacked. As long as the coating metal is on the surface of the base metal, the base metal will remain protected. Therefore, anodic coatings are preferred. ii) Cathodic Coating If the position of coating metal is below (less active) in galvanic series as compared to the base metal (base structure), then the coating is called cathodic coating. Example: Coating of tin on steel, Silver or Gold on brass. As long as the coating is uniformly present on base metal, it is protected from corrosion. But if the cathodic coating is ruptured, the base metal acts as anode and it gets corroded due to galvanic cell formation. It causes an intense local attack on the exposed part of base metal resulting in pitting /cracking. Anodic coating Cathodic coating The base metal is at a lower place in The coating metal is at a lower the galvanic series than the coating place in galvanic series than metal. base the metal. The base metal will remain safe until The base metal will undergo fast the coating metal is on a metal surface corrosion if the coating rupture. (when the coating ruptures). It is a preferred coating as compare to This coating is less preferred. cathodic coating Methods of applying Metallic Coatings - i. Hot dipping ii. Electroplating iii. Cementation iv. Cladding 18. Hot Dipping  The hot dipping method is used for coating of low melting metals such as zinc (419°c) or tin (232°c), lead etc. on base metal like steels, copper, brass etc.  Metal is kept in a molten state and the base metal is dipped into it. The process consists of immersing the base metal in a bath, containing a molten form of coating metal, covered by a molten flux layer (usually ZnCI2. 2NH4CI).  The flux prevents the oxidation of melting metal.  For good adhesion, base metal should be clean and well prepared.  Most commonly used hot dipping methods include- (a) Galvanizing (b) Tinning or Tin plating a) Galvanizing The process of coating zinc on iron or steel is called galvanizing.  The steel article is cleaned well with dil. H2SO4, washed well with water and dried. It is dipped in a bath of molten zinc maintained at 425-450°C. The surface of the bath is covered with flux like NH4Cl. After taking it out, the article (sheet, pipe, wire) is passed through the hot roller to make a coating of uniform thickness and to remove any excess zinc. Then it is cooled slowly. Applications of galvanized articles It is used widely for the protection of iron. Various galvanized iron articles are in use in different articles. Example: Galvanized iron (G.I.) sheets, wires, pipes buckets, tubes, screws, nails etc. Galvanized iron vessels cannot be used for storing foods as there is the formation of poisonous products by the action of foods on zinc. Galvanizing is a type of anodic coating. b) Tinning The process of coating tin on steel is known as tinning. The cleaned steel article (sheet) passes through a tin bath maintained at about 240°C. (bath contains flux floating over the molten tin to avoid oxidation of molten metal) and then through palm oil.  Palm oil protects the hot tin-coated surface against corrosion. The rollers finally remove the excess tin and make a uniform coating. Tinning is a type of cathodic coating. Applications of Tinned articles Tinned containers can be used for storing foods, ghee, oils, pickles, medicines because they have high corrosion resistance and do not form poisonous products after action by foods. Copper wire before insulation by rubber is tinned to avoid the attack of sulphur on rubber. Tinned copper or brass vessels or sheets are used for cooking utensils and refrigeration equipment. Difference between Galvanizing and Tinning Galvanizing Tinning Zinc coating on iron. Tin coating on iron. The temperature for coating is about The temperature for coating is about 450°C. 250°C. This is an Anodic coating. This is a Cathodic coating. Applicable for general use like sheets, Applicable for containers for storage pipes, wire, angles. of edible material. Zinc coating on iron is an anodic Tin coating on iron is cathodic coating. coating. Thus, better protection of Thus, rapid corrosion of base metal, if base metal. the coating is ruptured. Galvanizing is a cheaper process. Tinning is a costlier process. 19. Electroplating Electroplating is the method in which coating metal is coated on a base metal. This process is based on the electrolysis principle. The article to be electroplated is cleaned well. There is a nonconducting tank containing a salt solution of the coating metal. The article is connected to the negative terminal of a direct source supply to make it cathode. The coating metal rod or strip should be kept as anode. After adjusting the suitable pH and current density on article surface, the electroplating begins with redox reactions as follows: At anode Cr Cr3+ + 3e- (if chromium anode) Ag Ag+ + e- (if silver anode) Ni Ni2+ + 2e- (if nickel anode) The coating metal (anode) passes in solution as its ions. The number of metal atoms discharged on an article in a certain time is equal to the number of anode metal ions passed in solution. At Cathode (Article) Cr+3 + 3e- Cr (if chromium plating) Ag+ + e- Ag (if silver plating) Ni2+ + 2e- Ni (if nickel plating) The metal ions in solution migrate towards the article and capture electrons on reaching there to get discharged as metal atoms. There is no change in the concentration of the salt solution in the pot. The decrease in weight of the anode is equal to the increase in weight of the article due to plating (coating) formed. Advantages Electroplating can be done on articles of any shape, the coating is strongly adhered to the article, it can be applied on the surface of plastics, glass, wood and also gives a brighter look. Applications of Electroplating Corrosion protection. Decoration or better appearance. Coating metal can be copper, nickel, chromium, gold, silver, platinum, zinc, aluminium etc. Base metal can be iron, steel, or non-metal like plastic, wood, glass, ceramics.