Corrosion and Its Control PDF

ECH/BSH/SFIT2024-25 Corrosion and Its Control Destruction or deterioration or gradual eating away or disintegration of a metal by chemical or electrochemical reaction with its environment is called corrosion. Any process of deterioration & consequent loss of a solid metalli...

ECH/BSH/SFIT2024-25 Corrosion and Its Control Destruction or deterioration or gradual eating away or disintegration of a metal by chemical or electrochemical reaction with its environment is called corrosion. Any process of deterioration & consequent loss of a solid metallic material through an unwanted (or unintentional) chemical or electrochemical attack by its environment, starting at its surface, is called corrosion. Thus, it is a process of reverse of extraction of metals. Metallic Corrosion (oxidation) Metal compound Metallurgy (reduction) A state of higher energy A state of lower energy The definition of corrosion includes both metallic & nonmetallic material but the term corrosion invariably use to denote the destruction of metals. The most familiar examples of corrosion are the rusting of iron when exposed to the atmospheric conditions. During this a layer of reddish scale & powder of oxide (Fe3O4) is formed & the iron becomes weak. Another example is the formation of green film of basic carbonate [CuCO3 + Cu (OH)2] the surface of copper when exposed to moist air containing CO2. Electrochemical series: When metals are arranged in order of their standard electrode potential (E0) with respect to standard hydrogen electrode potential (E0 red=0). Elements Electrode reaction S.E.P.(Eo)volts at Anodic 25 oC Lithium Li + e- Li(base) -2.96 Potassium K+ + e K -2.93 Calcium Ca++ + 2e- Ca -2.87 Sodium Na+ + e- Na -2.71 Magnesium Mg++ + 2e- Mg -2.37 Aluminium Al+++ + 3e- Al -1.69 Zinc Zn++ + 2e- Zn -0.76 Chromium Cr+++ + 3e- Cr -0.71 Iron Fe++ + 2e- Fe -0.44 Nickel Ni++ + 2e- Ni -0.25 Tin Sn++ + 2e- Sn -0.14 Hydrogen 2H1 + 2e- H2 -0.00 Reference: 1  ECH/BSH/SFIT2024-25 Copper Cu++ + 2e- Cu + 0.34 Mercury Hg++ + 2e- Hg + 0.79 Silver Ag+ + e- Ag + 0.80 Platinum Pt++ + 2e- Pt + 1.20 Gold Au+++ + 3e- Au +1.50 Cathodic The above table represents the electrode potential of common metals against standard hydrogen electrode at 25oC in molar solutions of their ions together with anode reaction of the metal. The ability of metals to resists corrosion is to some extent depends upon their position in the electrochemical series. A greater negative potential indicates greater tendency of metal to pass into the solution as well as metal ions (M+). Therefore one metal displaces another metal from the solution when if it’s standard electrode potential is more negative. All metals above hydrogen in the series displace hydrogen from a solution of its ions. Metal with high negative potential are chemically very active. Galvanic Series The values of S.E.P were measured under certain chemical conditions in which ions concentration & temperature were closely associated & the electrodes were completely clean & free from oxide films. Thus the electrochemical series though useful does not give a basis for predicting in a particular environment which metals will be anode or cathode & the magnitude of cell voltage. For practical purpose, the electric potentials of many materials are obtained in a single environment mostly sea water (by using calomel electrode). Such data in a tabular is called a ‘galvanic series’ If a pair of metals from these series were connected together in sea water, the metal which is higher in the series would be anode & corrode of the further they are apart, the greater the corrosion tendency. Therefore galvanic series is of more direct help in corrosion processes than electrochemical series. From which the relative tendencies of metals & alloys can be judged. Galvanic series in Sea water of metals & alloys. In the galvanic series chemically more active metals are given higher position. These metals easily react with oxygen, sulphur etc. hence corrode readily. The noble metals at the lower end do not easily react & therefore corrode so easily under ordinary conditions when two metals are in electrical contact, in the atoms presence of an electrolyte, the one higher up in galvanic series corrode & the other is protected. The greater the difference in their position in galvanic series the faster will be the corrosion. Metals close to each other in the galvanic series show fewer tendencies to corrode when in contact with each other. 2  ECH/BSH/SFIT2024-25 1. Immersed corrosion. (Wet or Electro chemical corrosion) (Electrochemical Theory) This type of corrosion occurs i) When conducting liquid is in contact with metal or ii) When two dissimilar metals or alloys are either immersed or dipped partially in aqueous solutions of acids, basis of lack of involves a transfer of electron. The modern electrochemical theory is based on Nernst theory according to which all metals have a tendency to pass into solution. e.g. In Zn – Cu system, Zn electrodes is dipped into 7  ECH/BSH/SFIT2024-25 ZnSO4 solution & Cu electrode is dipped in CuSO4 solution. The positive Zn ions (Zn++) continuously pass in the solution lying behind negatively charged electrons. Zn Zn++ + 2e- These free electrons will move form Zinc (oxide) to Copper plate (Cathode) where they will be bound by copper ions that are being reduced and settle on the copper plate (Cathodic process), Cu++ + 2e- Cu This tendency of metal to pass into solution when immersed in a solution of its salts is measured in terms of electrode potential. In terms of corrosion tendency reactivity is compared using Galvanic series as reference data. If a metal having higher position in series(more reactive) comes in contact with another less reactive metal having lower position in the series, galvanic cell is set up & the reactive metal becomes anodic and goes into the solution to a measurable extent. If the surrounding liquid is sufficiently acidic, H2 gas will be evolved at the cathodic metal while anodic metal dissolves. If the acidity of surrounding liquid falls below a certain value, the rate of dissolution of the anodic metal slows down & is controlled by the rate at which oxygen can diffuse to the cathode & depolarize the corrosion cell. This corrosion occurs due to the existence of separate anodic and cathodic parts between which current flows through the conducting solution. At anodic area oxidation reaction (i.e. liberation of free electrons) takes place, so anodic metal is destroyed by either dissolving or assuming combined state ( such as oxide) Hence corrosion always occur at anodic areas. At anode: M M++ + ne- (oxidation) Metal Metal ion Mn+ Dissolves in solution metal ion forms compounds such as oxide. On the other hand at cathodic area, reduction reaction (i.e. gain of electrons) takes place. Usually cathode reaction does not affect the cathode. Since most metal can not be further reduced. 8  ECH/BSH/SFIT2024-25 So at cathodic part dissolved constituent in the conducting medium accepts the electrons to form some ions (like OH- , O2-). The metallic ions (at anodic part) & nonmetallic ions (formed at cathodic part) diffused towards each other through conducting medium & form a corrosion product somewhere between anode & cathode. The electrons set free at the anode flow through the metal & are finally consumed in the cathode reaction. To summaries for electrochemical corrosion the three essential are:- i) The formation of anodic & cathodic part anode corrodes by forming ions of the cathode is protected. ii) Electrical contact between anode & cathode for the conduction of electrons iii) Electrolyte is usually provided by the presence of moisture. Thus the corrosion which is brought about through ionic reaction in presence of moisture or solution as a conducting medium is called electrochemical corrosion or immersed corrosion as it occurs where they are immersed in the solution. 9  ECH/BSH/SFIT2024-25 Hence according to electrochemical theory any corrosion process may be regarded as an electrochemical process in which cathode & anode are formed on the surface of the metals & electrolyte which may be water & a salt solution must be present to allow flow of negative current to form the corrosion products. Mechanism of electrochemical corrosion Corrosion of metal in aqueous solution is an electrochemical phenomenon, involving flow of electrons (electric current) between the anodic & cathodic areas. The 10  ECH/BSH/SFIT2024-25 anodic reaction involves dissolution of metal as corresponding metallic ions with the liberation of free electrons. M M+ + ne- (at anodic area) On the other hand, the cathodic reaction consumes electrons with either i) evolution of hydrogen or ii) absorption of oxygen, depending on the nature of corrosive environment. i) Evolution of Hydrogen: - This type of corrosion occurs usually in acidic environment like industrial waste, solution of non oxidizing acids (like HCl). e.g. A steel tank containing acidic industrial waste of small pieces of copper scrap in contact with the steel. The portion of the steel tank in contact with copper is corroded the most with the evolution of hydrogen gas. The reactions are, Fe Fe2+ + 2e- (oxidation) These electrons flow from anode to cathode through the metal. At cathode positive ions ( of acidic solution) are eliminated as hydrogen gas. 2H+ + 2e- H2 reduction Hydrogen Evolution Mechanism Thus overall reaction is Fe + 2H+ Fe++ + H2 11  ECH/BSH/SFIT2024-25 This type of corrosion is, therefore nothing but displacement of hydrogen ion from the acidic solution by the metal ions. In general all metal above hydrogen in the electrochemical series have a tendency of dissolving in acidic solution with the simultaneous liberation of hydrogen. In hydrogen evolution type of corrosion cathode are small areas & anodes are usually large areas. ii) Absorption of Oxygen :- An excellent example of this type of corrosion is the rusting of iron in neutral aqueous solution of electrolytes such as NaCl in presence of atmospheric oxygen. Let us consider a steel pate lying on the ground & is exposed to the atmosphere. The surface of iron (steel plate) is generally coated with a thin film of iron oxide. However of this iron oxide film develops some cracks, anodic areas are created on the surface while scale protected steel acts as a cathode & water acts as an electrolyte. At the anode area steel goes into solution as Fe++ ion with the liberation of electrons. Fe Fe++ + 2e- ( oxidation) These free electrons are flow from anode to cathode through iron metal where the electrons are intercepted by oxygen atoms present in the atmosphere & in presence of water drop. They form hydroxyl (OH-) ions as follows 2H2O + O2 + 2e- 4(OH)- (Reduction) The Fe++ ions at anode & OH- ions at cathode differ & combine to form compound ferrous hydroxide (brown rust) 12  ECH/BSH/SFIT2024-25 Fe++ + 2 OH- Fe(OH)2 If enough oxygen is present, ferrous hydroxide is easily oxidized to formic hydroxide (yellow rust) actually (corresponding Fe2O3.H2O) 4Fe(OH)2 + O2 + 2H2O 4Fe(OH)3 Fe2O3.H2O yellow Rust If supply oxygen is limited the corrosion product is black anhydrous magnetite (Fe3O4) Classification of Corrosion:- Corrosion occurring in an environment can be classified broadly with respect to the following factors. a) Nature of the corrodent :- Corrosion can be dry or wet. Dry corrosion occurs in absence of moisture & usually involves reaction of metal with dry gases at higher temperature. Wet corrosion occurs in presence of water or a conducting liquid. b) Mechanism of corrosion:- Corrosion can be direct chemical corrosion or electrochemical corrosion i.e. (differential aeration) due to indirect electrochemical attack. In direct chemical attack metal corrodes in presence of reacting chemicals. 3  ECH/BSH/SFIT2024-25 In electrochemical attack one metal undergoes corrosion in presence of another metal & a contact liquid. In differential aeration corrosion i.e. in electrochemical corrosion different parts of the same metal are different aeration (or concentration) of the corroding medium. c) Appearance of the corroded metal:- Corrosion may be either uniform or localized. In uniform corrosion metals gets corroded over the entire surface at the same rate, while in localized corrosion only small & specific area of the metal get corroded. Other different forms of the corrosion are,  Galvanic Corrosion  Pitting Corrosion  Intergranular Corrosion  Waterline corrosion  Stress corrosion  Microbial corrosion  crevice corrosion  Atmosphere corrosion  Soil corrosion  Fretting Corrosion Different Types of corrosion Galvanic Corrosion: - When two metallic materials (dissimilar metals) are electrically connected and exposed to an electrolyte, the metal higher in electrochemical series undergoes corrosion, is called ‘galvanic corrosion’. 13  ECH/BSH/SFIT2024-25 When two dissimilar metals as Zn & Cu are electrically connected & exposed to electrolyte, Zinc which is higher in Galvanic series forms the anode & undergoes corrosion (attacked & get dissolved) while copper which is lower in the series acts as the cathode. This type of corrosion is known as galvanic corrosion. The type of cathode reaction depends upon the nature of corrosive environment. In acidic solution the corrosion occurs by hydrogen evaluation method while in slightly alkaline or neutral solution the corrosion takes place by oxygen absorption method. (The electrons current flows from the anodic metal (Zn) to cathodic metal (Cu) where the electrons are intercepted by the positive ion deposited there) Zn Zn++ + 2e- (oxidation) Thus it is evident that corrosion occurs at the anodic metal, while the cathodic part is protected from attack. Examples of Galvanic corrosion:- 1. Rusting of fencing wire under joints. 2. Copper sheet joined by iron nails. 3. Steel pipe connected to iron nails. 4. Steel screw in marine brass hardware. 5. Lead antimony solder around copper wire. Thus Galvanic or bimetallic corrosion occurs when two or more dissimilar metals are connected together in a corrosive, electrically conducting fluid. Galvanic corrosion can be very mild, causing no extra corrosion of engineering significance. It can also be very severe leading to rapid catastrophic failure. The severity of galvanic corrosion is affected by the potential difference, the cathodic efficiency, the area ratio and the conductivity and composition of the fluid. One of the most common fluids in which galvanic corrosion occurs is seawater. 4. Concentration cell theory of corrosion (Differential aeration theory of corrosion.) Simple electrochemical theory cannot explain corrosion occurring on single metal surface as it demands the presence of two dissimilar metals in the presence of an electrolyte. For this purpose, formation of concentration cell on the metal surface is created. A piece of pure metal in contact with an electrolyte which has different concentration at different parts which is in contact with dilute electrolyte. This is due to the formation of concentration cell & can be called as concentration cell corrosion. 14  ECH/BSH/SFIT2024-25 Concentration cell corrosion is due the electrochemical attack on the metal surface exposed to an electrolyte of varying concentration or of varying aeration. Differential aeration: - It is the most important & most corrosion type of concentration cell corrosion which occurs when one part of a metal is exposed to a different air concentration from the other part of the metal. As a result, difference in the potential takes place between differently aerated areas. It has been found experimentally that poor oxygenated parts of metals are anodic. If a piece of metal, Zinc is partially immersed in dilute solution of a salt NaCl & the solution is not agitated properly, the parts above & closely adjacent to the water line are most strongly aerated because of the easy access of oxygen to this area & hence become cathodic. On the other hand, parts immersed in greater depth have less access of, oxygen & so they are poorly aerated i.e. they show a smaller oxygen concentration & thus become anodic. As a result, difference of potential is created which is responsible for the flow of current between two differently aerated areas of same metal Zinc. Zinc will dissolve at the anodic area & oxygen will take up electrons at the cathodic area to form hydroxyl ions. Zn Zn++ + 2e- (Oxidation) 1/2O2 + H2O + 2e- 2OH- (reduction) The circuit is completed by migration of ions, through the electrolyte & flow of electrons through the metal from anode to cathode. 15  ECH/BSH/SFIT2024-25 In the similar way iron corrodes under drops of water ( or salt solution). Areas covered by droplets having no access of oxygen, become anodic with respect to the other areas, which are freely exposed to air. It is therefore well evident that oxygen concentration cell increases the corrosion, but it takes where these is less access of oxygen i.e the oxygen concentration is lower. General facts about the differential aeration corrosion. i) Corrosion may be accelerated in apparently inaccessible places, because the oxygen deficient areas serve as anodes & therefore cracks & crevices serve as centers for corrosion. ii) Corrosion is accelerated under accumulation of dirt, sand, scale or other contamination. This is because of accumulation of rust or scale or sand etc. restricts the access of oxygen & establishes on anode to promote still greater accumulation. These causes localized corrosion because of non uniform corrosion. iii) Metal exposed to aqueous media corrode under blacks of wood or pieces of glass. The differential aeration type of corrosion is a localized attack on some oxygen deficient areas & results in localized pitting. iv) This attack becomes more intensified with time because of the corrosion product accumulated around a small anodic area thereby making that part inaccessible more effective. 16  ECH/BSH/SFIT2024-25 Stress Corrosion Stress corrosion or stress cracking is the type of corrosion which is common in fabrication articles of some alloys such as high Zinc brasses of Nickel brasses & is due to the presence of stress in metal structure caused by heavy working like rolling drawing or insufficient annealing. This type of corrosion is the combined effect of tensile stress and the corrosive environment on an alloy. Generally this type of corrosion occurs in specific environment such as, 1. Strong caustic alkalis or nitrates (which affected mild steel) 2. Traces of ammonia (Affecting brass) 3. Marine water or H2S gas or solution of acid halides (chlorides particularly) affecting stainless steel etc. Stress corrosion is probably due to localized electrochemical reaction, occurring along a narrow path forming local anodic & cathodic areas on the metal surface. The presence of stress gives rise to strain which develops localized zones of higher electrode potential. Stressed area of metal becomes weak and undergo corrosion. Some typical examples of stress corrosion are, 1. Season cracking:- It occurs in brass alloys (Cu + Zn) when exposed to ammonia (such as cartridge cases).Both Cu and Zn form complex cation in presence of NH3. 2. Caustic embrittlement: - This is the most dangerous form of stress corrosion which occurs in mild steel when it is exposed to alkaline solution at high temperature as well as stress. Caustic embrittlement is the phenomenon during which the boiler material becomes brittle due to the accumulation of caustic substances. This type of boiler corrosion is caused by the use of highly alkaline water (caustic medium) in the high-pressure boiler. During the water treatment process Na2CO3 is mixed. The residual or remaining Na2CO3 in the softened water cause trouble. In high pressure boilers Na2CO3 decomposes to give sodium hydroxide and CO2, and sodium hydroxide thus produced makes the boiler water “caustic”. Na2CO3 + H2O 2NaOH + CO2 This caustic water flows into the minute hair-cracks, present in the inner side of boiler, by capillary action. On evaporation of water the dissolved caustic soda concentration increases progressively which attacks the surrounding area, thereby dissolving iron of boiler as Sodium ferroate (Na2FeO2). 17  ECH/BSH/SFIT2024-25 Accumulation of Sodium ferroate causes embrittlement of boiler walls, more particularly at stressed parts like bends, joints, rivets, etc., causing even failure of the boiler FACTORS INFLUENCING CORROSION:- The rate & extent of corrosion depends upon the following factors:- i) Nature of the metal ii) Nature of the environment. I) Nature of metal:- 1) Position of metal in galvanic series (oxidation potential):- The extent of corrosion depends upon the position of metal in the electrochemical series & galvanic series. When two metals or alloys are in electrical contact in presence of an electrolyte, the metal or alloy higher up in the galvanic series (or more active metal) becomes anode & undergoes corrosion. Further the more two metals apart in the galvanic series the greater is the difference in their oxidation potential & hence faster will be the corrosion of anodic metal. 2) Purity of metal (impurities):- The content in molecular level of a metal is very important in the case of Corrosion. Pure metal has lesser chance of corrosion. But in the case of impure metals due to the presence of minute electrochemical cells, there are high chances of corrosion. If the metal itself contain impurity metals, tiny galvanic cell can be set up easily. This increases rate of corrosion. The effect of impurities on the rate of corrosion of Zinc can be seen from the following table. Metal % purity Corrosion rate(comparative) Zn 99.999 1 Zn 99.99 2.650 Zn 99.95 5000 3) Relative areas of cathodic & anodic parts :- When two dissimilar metals are in contact, “the corrosion of anodic part is directly proportional to the ratio of areas of cathodic part & anodic part. Consequently, corrosion is more rapid serve & localized if the anodic area is small. when the cathodic area is larger demand for electrons is more (increases) which can only be met by the anodic area by undergoing faster corrosion. Thus smaller area of the anode faster is the rate of corrosion. Rate of corrosion α area of cathode area of anode 18  ECH/BSH/SFIT2024-25 4) Physical state of metal:- The rate of corrosion is influenced by physical state of the metal (such as grain size orientation of crystals, stress etc.) For instance the smaller the grain size of the metal or alloy the grater will be the solubility & greater (as compared to the macroscopic crystals) will be its corrosion. The corrosion rate may also be influenced by the orientation of crystal at the metal surface e.g. the corrosion rate of copper ions was found to be different on different faces of a pure copper crystal. Moreover, areas under stress, even in a pure metal tend to be anodic & corrosion takes place at these areas. This is due to the fact that “atoms of a metal at the boundaries having electrode potential different than that of the atoms in the proper grain size. Consequently anode & cathode is developed. 5) Nature of oxide film:- In aerated atmosphere, practically all metal gets covered with an oxide & its thickness depends upon the nature of the metal.Rate of further corrosion depends upon type of oxide layer. Metals such as Mg, Ca Ba, Li, Na & K (alkali & alkaline earth metal) form oxide whose specific volume is lesser than that of the metal atom. Hence oxide film form will be porous through which oxygen can diffuse & bring about further corrosion. Al & heavy metals form oxide whose specific volume is greater than that of the metal atom & the passive film so formed will protect the metal from further oxidation unless a cracks or fissure in the film develops. Metals like Ag, Au and Pt do not show oxidation corrosion due to formation of unstable oxide layer. If the oxide layer is volatile, it volatilizes as soon as it is formed, there by leaving the underlying metal surface exposed for further attack. This causes rapid & continuous corrosion leading to excessive corrosion. MoO3, the oxidation corrosion product of Mo is volatile. 6) Solubility of corrosion products: - Solubility of corrosion product formed is an important factor in electrochemical corrosion. If the corrosion product is soluble in the corroding medium, then corrosion proceeds at a faster rate. On the contrary if the corrosion product is insoluble in the medium or it interacts with the medium to form another insoluble product (eg PbSO4 formation in cast of Pb in H2SO4 19  ECH/BSH/SFIT2024-25 medium) than the corrosion product formation as physical barriers thereby suppressing further corrosion. 8) Volatility of corrosion products:- II) Nature of the environment:- 1. Temperature:- The rate of chemical reaction increases with temperature. Increase in temperature increases ionization & mobility of reacting ions & molecules, it also increases diffusion rate. Hence increased temperatures generally enhanced corrosion rate. However solubility of gases such as O2 which affect corrosion decreases with temperature so in some environment rate of corrosion is less at high temperature. e.g. Rate of corrosion of copper & monel metal is less in boiling sulphuric acid while steel corrodes more in boiling sulphuric acid. Dissolved O2 is absent in boiling H2SO4 for corrosion of copper O2 is necessary. In absence of O2 corrosion of Cu is less. But in case of steel O2 is necessary to keep steel in passive condition steel corrodes in absence of oxygen. 2. Presence of moisture:- (Humidity of air) Atmospheric corrosion of iron is rather slow in dry air but increases rapidly in the presence of moisture. This is mostly due to the fact that moisture acts as the solvent for oxygen & other gases or such which furnish water to electrolyte for setting up an electrochemical corrosion cell. Nature of moisture source also plays an important role. Rain water apart from supplying the necessary moisture source for electrochemical attack may also wash away good parts of oxide from the metal surface. 3. Presence of impurities in atmosphere:- Atmosphere in the vicinity of industrial areas contains corrosive gases like CO2, H2S, SO2 & fumes of HCl, H2SO4 etc. In the presence of these gases the acidity of the liquid adjacent to the metal surface increases & its electrical conductivity also increases. This consequently results in an increase of corrosion current flowing in the local electrochemical cells on the exposed metal surfaces. Similarly in the marine atmosphere presence of sodium & other chlorides leads to increased conductivity thereby corrosion is speeded up. 4. Effect of PH:- The hydrogen ion concentration of the medium is another important factor in corrosion reaction as well as corrosion control. Acidic media are generally more corrosive than alkaline & neutral media. However passive metals like Al, Ti form passive layer and remains protected in acidic solution & undergo rapid into alkaline solution. 20  ECH/BSH/SFIT2024-25 The corrosion of iron is slow in oxygen free water until the PH falls below 5. The corrosion rate is much higher in presence of oxygen at PH 4. In less acidic solution (by absorption of O2 gas) the excess of OH- ions in the area combine with Fe++ to form Fe(OH)2 which undergoes further oxidation to form rust. Zinc is rapidly corroded even in weakly acidic solution such as carbonic acid. Even fermenting organic matter in a galvanized container can corrode the Zinc from the container. Zn suffers minimum corrosion at PH11 but at higher alkalinities it goes into solution as complex. 5. Nature of ions :- (anions & cations ) Chlorides ions present in the medium destroy the passive film & corrode many metal & alloys. On the contrary, some anions like silicates may form an insoluble reaction product (eg silica gel) which inhibits corrosion. Many metals including iron corrode more rapidly in ammonium salts than in sodium salts of identical concentration. 6. Conductance of the medium:- The conductance of the medium is of profound importance in the corrosion of underground or submerged stricture because of the corrosion current depends on this factor. Conductance of clay & mineralized soils is higher than that of dry & sandy soils. Prevention & control of corrosion:- Corrosion is one of the major causes of increasing maintenance cost of all the industries therefore either prevention or subsequent control of corrosion becomes all the more important. The methods for corrosion control vary from condition to condition. The following are the methods which can be used either singly or in combination in order to protect the metals from corrosion. 1. Proper selection of materials. 2. Improvement of design (proper design) 3. Use of alloys/ Pure metal. 4. Modifying the environment 5. Use of corrosion inhibitors. 6. Cathodic & Anodic protection 7. Application of protective coating. e.g. i) Non Metallic coatings & ii) Organic coatings. 21  ECH/BSH/SFIT2024-25 1. Proper selection of material:- Proper selection of material for manufacturing machine parts or joining different parts to construct a unit helps tremendously to avoid or control corrosion. Thus selection of right type of material is the main factor for corrosion control. The choice of metal should be made not only on its cost & structure but also on its chemical properties & environment. This can be done by taking following precaution. a) Avoid contact of dissimilar metals especially if the working environment is corrosive. Example:- Joining different part to make a ship, if iron metal strips are used to join the wooden parts together & of screws used are of brass ( Cu & Zn alloys) when ship floats on marine water , localized corrosion enhanced tremendously & highly active metal starts (Zinc) acting as anode. b) Suppose if it is unavoidable to choose two dissimilar metals, then area of anodic metal should be larger than that of cathodic metal. c) If two dissimilar metals are to be selected, then metals should be chosen in such a way that they are as close as possible in the galvanic series. d) When two dissimilar metals are required to be joined their direct contact should be avoided by inserting a piece of hard plastic or rubber into the joint. The piece acts as an insulator & current flow should be reduced & minimizes galvanic corrosion. e) When two dissimilar metals are used in contact the anodic metal should not be painted or coated. Any crack lead to rapid localized corrosion. 2. Proper designing:- Corrosion resistance of metal parts or machines can be improved by improving the design. The design of the metal should be such that “corrosion even if it occurs is uniform & does not result in intense & localized corrosion. A better design should always avoid sharp bends, corners, lap joints, projected parts etc. or as far as possible use of screws, nuts, and bolts should be avoided (to avoid differential aeration), rather welding can be used; Welded joints should be preferred over riveted or bolted lap joints since welded joints do not allow the entry of gases & liquids inside, which cause corrosion. As far as possible, crevices & obstruction should avoided in tanks & pipelines or between adjacent parts of structure to avoid formation of concentration cells. 22  ECH/BSH/SFIT2024-25 As far as possible, the metal washer should be replaced by rubber or plastic washer as they do not absorb water & do not generate galvanic couples. The surfaces of two joining parts should be as smooth as possible, which avoids accumulation of the corrosive liquids, suspended particle, dust ,dirt , grit, stagnation of water etc. The equipment should be kept free from dust & dirt to reduce rusting due to pitting & differential aeration effect. Wherever possible the equipment should be supported on legs instead on large block to allow free circulation of air & prevent the formation of stagnant pool or damp areas. Using of joints is such that the chance of water entering inside is limited. 23  ECH/BSH/SFIT2024-25 6) Cathodic & Anodic protection: - a) Cathodic protection:- In this method, the desirable metal is protected by forcing them to behave like a cathode, thereby corrosion is prevented. Anodic metal is converted in to cathode ,there are two types of cathodic protection: i) Sacrificial anode method ii) Impressed current method i) Sacrificial anode method:- In this protection method, the metallic structure to be protected is converted in to cathode by connecting with a more anodic metal(having higher position in galvanic series) Corrosion is concentrated at this more active metal. The more active metal itself gets corroded while the parent structure is protected. Since the more active metal sacrifices itself, by undergoing corrosion and saving the base metal, the method is named as Sacrificial anode method or auxillary anode method. When the piece of more active metal gets corroded completely, it is simply replaced by new piece. Most of the common sacrificial anodes are based on the alloys of magnesium, zinc and aluminium. Applications of this method are seen to protect cables or iron pipelines, by connecting them to Mg- block: and in case of marine structures ships are protected by using Zn-plates as sacrificial anode. Even water tanks, boilers are protected by using Zn metal. 24  ECH/BSH/SFIT2024-25 Sacrificial anode method 2) Cathodic protection by impressed current :- In this method, to convert the corroding metal from anode to cathode an impressed current is applied in opposite direction to that of corrosion current, thereby nullify the effect of corrosion current. Such an impressed current is obtained by using d.c. source such as battery or dry cell along with an insoluble anode such as platinum, stainless steel, graphite. In this method, the insoluble anode(graphite) is normally embedded underground and connected to desirable structure. The anode is usually kept in the backfill made up of gypsum, which can help in increasing electric contact with soil. D.C. current is applied to this buried anode as a result of this impressed current, the anode deteriorates and desirable structure remains as protective end. 25  ECH/BSH/SFIT2024-25 impressed current method Applications:- Impressed current systems are appropriate for protection of larger structures and are more effective in handling the more complicated corrosion problems than are sacrificial anodes This type of cathodic protection by impressed current is applied to open water box coolers, water tanks, buried water or pipelines, condensers, transmission line towers, marine piers, laid-up ships etc. This type of protection mechanism is, particularly well suited for large structures and long term operation. They can be automatically controlled which reduces the maintenance and operating costs. 26  ECH/BSH/SFIT2024-25 Use of protective coatings In order to protect the metal from corrosion effects, it is possible to cover their surface in various ways. The main types of protective coatings may be classified as 1) metallic coatings and 2) organic coatings 1) Metallic coatings: Metallic coatings can be divided into anodic and cathodic coatings. The metal to be protected is called the base metal; where as the metal used for protection is called the ‘coating metal’. 1. Anodic coating ( coating with more active metal) Anodic coatings are produced from metals which are anodic to the base metal. For example, Zn, Al and Cd are anodic on steel, because their reactivity is more than that of the base metal, (iron). If any discontinuities, pores or breaks occur in such anodic coatings, a galvanic cell is formed between the coating metal and the exposed part of base metal. ( in case of galvanized steel, between Zn and the exposed iron). Zinc, being anodic to iron will dissolve and the iron being cathodic is protected. Thus, no attack on iron occurs until all the zinc first corroded in the vicinity of exposed iron parts. Hence, zinc coating protects iron “sacrificially”. 27  ECH/BSH/SFIT2024-25 2. Cathodic coatings: Cathodic coatings are obtained by the application of coating of more noble metals (i.e. having lower reactivity) than the base metal. They protect the base metal, because they have higher corrosion resistance than steel or iron. Metals used as cathodic coatings are Cu, Sn, Ni, Cr, Ag and to a smaller extent, precious metals used as gold and platinum. Cathodic coatings give effective protection to the base metal till the coatings are completely continuous and free from pores. If such coatings are broken or punctured much more damage can be done to a base metal than to metal without protection. For example, a tin coating on sheet of iron provides protection only as long as the surface of the metal is completely covered, since the position of tin is lower than iron in electrochemical series. However, if the surface coating is broken then tin becomes the cathode, while the exposed part of iron acts as anode. (fig.) A galvanic cell is set up and an intense localized attack at the small exposed part occurs, resulting in severe pitting and perforation of the base metal, (iron). Such combination of a small anode and large cathode area is always very dangerous. 28  ECH/BSH/SFIT2024-25 Applications:- The process of galvanizing is commonly used for coating the iron or steel sheets, tubes, pipes, screws, nuts, bolts and wires for protection against corrosion. Advantages:- Zinc offers better protection against corrosion because even if any cracks are developed over the film of deposit, at a later stage, zinc being more reactive than iron or steel, undergoes corrosion keeping the base metal article unaffected. Disadvantages:- Galvanized zinc coated utensils or containers can not be used for preparing or storing food stuffs, particularly those which are acidic in nature. This is because zinc dissolves to form poisonous zinc compounds which are injurious to human health. Distinction between galvanizing and tinning Galvanising Tinning 1. It is the process of coating iron or steel 1. It is a process of coating steel sheets sheets with a thin coat of zinc by hot with a thon layer of tin to prevent them dipping to prevent them from rusting from corrosion. 2. Zinc metal protects iron as it is more 2. Tin protects the base metal as it is anodic to iron. less electropositive than iron. It is more resistant to chemical attack. 29  ECH/BSH/SFIT2024-25 3. Zinc continues to protect the metal by 3. In tinning, tin protects the metal galvanic action, even if the coating is till the coating is perfect. A break broken. in coating causes rapid corrosion. 4. Galvanized containers cannot be used 4. Tin containers can be used for storing for storing acidic food stuffs, as acids acidic food stuff as tin does not form react with zinc to form poisonous any poisonous products. compounds. Application of corrosion engineering in electronic and photonic devices. Corrosion Of Electronic Devices Electronic corrosion, or electronic component corrosion, is degradation or deterioration of electronic devices and components due to chemical reactions with the environment. Common causes include exposure to moisture, high humidity, corrosive gases, and airborne contaminants. Corrosion can lead to electronic devices malfunctioning or failing, which can affect their performance, reliability, and lifespan. It can also increase maintenance costs, reduce productivity, and lead to data loss. Cause of Corrosion Of Electronic Devices Oxidation of Metals: Metals like copper and aluminum, commonly used in circuits, oxidize and form insulating layers that impede conductivity. Moisture and Contaminants: Humidity, salts, and pollutants can create electrolytic conditions that accelerate corrosion. Thermal Effects: Elevated temperatures can increase reaction rates and corrosion. Poor Storage Conditions: Inadequate storage conditions, such as storing electronics in damp or corrosive environments, can accelerate the corrosion process. 30  ECH/BSH/SFIT2024-25 Coastal Areas: Electronics located near coastal regions are at a higher risk of corrosion due to the presence of high humidity in the air. Industrial Facilities: Many industrial processes involve the use of chemicals or high temperatures that can lead to the production of corrosive gases. For example, metal refining, combustion processes in power plants can release corrosive gases such as SO2, H2S, and NOx which can be corrosive. Examples: Printed Circuit Boards (PCBs) Copper traces on PCBs are prone to corrosion, especially in humid environments. Moisture can lead to copper oxidation, forming a non-conductive layer that interrupts electrical connections. Example: Devices exposed to outdoor conditions without protective coatings often exhibit corrosion, leading to short circuits and failures. Connectors and Contacts Metal connectors (e.g. nickel, or tin) can corrode over time due to oxidation or galvanic corrosion, when exposed to moisture / contaminants. Example: Automotive electronics, where connectors may experience exposure to road salt and moisture, leading to increased resistance and potential failure. Battery Corrosion Corrosion can occur at battery terminals due to leakage of electrolytes or due to galvanic corrosion between dissimilar metals in contact. Example: Corroded battery terminals in devices like remote controls or toys can cause poor connections, leading to intermittent operation or complete failure. Sensors Environmental sensors can corrode due to exposure to the elements, affecting their accuracy and reliability. Example: Humidity sensor may fail if moisture penetrates Corrosion in Photonic Devices Photonics is the science and technology of light and other radiant energy, and includes devices like Optical components, lasers, fiber optics, and electro-optical instruments 31  ECH/BSH/SFIT2024-25 Effects on Optical Components: Corrosion can degrade coatings on lenses and mirrors, affecting transmission and reflection of light. Wavelength Dependence: Corrosion may alter the refractive index of materials, leading to changes in optical performance. Examples Optical Coatings Anti-reflective and other optical coatings on lenses can degrade due to environmental factors, leading to reduced transmission and increased scattering. Example: Camera lenses used in marine environments can show signs of corrosion on coatings, diminishing image quality. Laser Diodes The optical components of laser diodes can corrode, especially when exposed to high humidity or corrosive gases. Example: Laser systems in industrial applications may fail if moisture enters leading to corrosion of critical optical components Fiber Optic Connectors Corrosion can occur at the metal parts of fiber optic connectors, affecting the signal quality. Example: Outdoor fiber optic installations may experience connector degradation due to environmental exposure, impacting telecommunications reliability. Photovoltaic Cells Corrosion of metal contacts in solar panels can lead to performance degradation and decreased energy output. Example: Solar panels installed in coastal areas are particularly vulnerable to salt-induced corrosion, reducing their efficiency and lifespan. 32  ECH/BSH/SFIT2024-25 Corrosion Detection Methods Visual Inspection: Look for discoloration, delamination, or physical damage. Electrochemical Techniques: Use of techniques like cyclic voltammetry and spectroscopy to assess corrosion rates. Non-destructive Testing: Methods like X-ray fluorescence (XRF) to analyze material composition and integrity. Prevention Methods: Environmental Considerations: Many of these examples illustrate the importance of protecting devices from environmental factors like humidity, salt, and pollutants. Need to maintain low humidity and clean environments. Impact of Design: Proper sealing and the use of corrosion-resistant materials can greatly enhance the durability of both electronic and photonic devices. Material Selection: Use of corrosion-resistant materials (e.g., gold-plated contacts, stainless steel). Protective Coatings: Application of conformal coatings (e.g., polymers) to insulate and protect circuit boards. Electronic & photonic devices can fail due to several failure modes depending on inherent factors related to: Materials, Design, Cleanliness under exposure to humidity and other gaseous conditions. Conclusion Addressing corrosion is critical for maintaining the functionality and reliability of electronic and photonic devices. Ongoing research into novel materials and coatings, as well as improved detection methods, is essential for advancing technology resilience. 33