Corrosion and Its Corrosion Control

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CORROSION AND ITS CORROSION CONTROL

A wide variety of metals/alloys are being used in daily life, equipment, plantmachineries, industrial structures, bridges, building etc and all these have naturaltendency to get tarnished or deteriorated by the environment to which they areexposed.

Definition: Corrosion is the process of gradual deterioration of a metal from itssurface due to an unwanted chemical or electrochemical interaction of metal with itsenvironment.

The surfaces of almost all metals begin to decay more or less rapidly, whenexposed to gaseous atmosphere, water or another reactive liquid medium. As aresult of the decay, the metals are converted to their oxides, hydroxides, carbonates,sulphides etc. The process of decay of metal by environmental attack is calledcorrosion.

Ex: formation of reddish brown scale and powder of rust (Fe 2O3 .3H 2O) on thesurface of iron and formation of green film of basic carbonate [CaCO 3+Cu(OH) 2] on

the surface of copper.

Cause of corrosion: Most metals (with the exception of noble metals such as Au,Pt, etc.) exist in nature in combined forms as their oxides, carbonates, hydroxycarbonates, sulphides, chlorides and silicates. These chemically combined states of metal known as ores.

These chemically combined states of metal are thermodynamically morestable states for metal. These are reduced to their metallic states from their ores,during their extraction processes. During extraction of metals, considerable amountsof energy are required in metallurgy. Consequently, isolated pure metals can beregarded in a higher energy state which are thermodynamically unstable than theircorresponding ores, and they have a natural tendency to revert back to combinedstate ( thermodynamically more stable state ).

Metallurgy CorrosionOre ------------------ Metal --------------- Corroded metal

( Thermodynamically stable state) (Thermodynamically unstable) ( Thermodynamically stable state)

As a result when metals are put into use, in various forms, they are exposedto environment such as dry gases, moisture, liquids, etc. the exposed metal surfacesbegin to decay and forms more stable compounds of metals like oxides, carbonates,etc. Thus, corrosion is a process reverse of extraction of metals.

Effects of corrosion:

The valuable metallic properties like conductivity, malleability, ductility etc.are lost due to corrosion and thus loss of efficiency.

The process of corrosion is not incurred and is responsible for the enormouswastage of machines, equipment and different type of metallic products.

Losses occurring due to corrosion cannot be measured in terms of the cost of metals alone, but the high cost of fabrication into equipment/machine tool/structures should also be considered.

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The approximate estimate of loss of metal due to corrosion, as 2 to 2.5 billiondollars per annum all-over the world.

In view of the above, an engineer must understand the mechanism of corrosionso that the effects of corrosion can be minimized by avoiding the corrosion conditionsand provide simultaneous protection against corrosion.

Types of corrosion (Theories of corrosion): The corrosion process proceeds intwo types by chemical and electrochemical attack of environment.

CHEMICAL CORROSION (DRY CORROSION):

This type of corrosion occurs mainly through the direct chemical action of environment/atmospheric gases such as oxygen, halogen, hydrogen sulphides,Sulphur dioxide, nitrogen or anhydrous inorganic liquid with metal surfaces inimmediate proximity.

A tarnished surface of a metal is the best example of dry corrosion. Metalslike Al, Zn and even Fe etc. have a natural tendency to get oxidized and form a thin

film of their oxides on the surface of metal when they exposed to atmosphericoxygen.

Dry corrosion may be due to a) Oxidation corrosion, b) Corrosion by othergases.

1. Oxidation corrosion: Direct action of oxygen at high or low temperatures onmetals in the absence of moisture is called oxidation corrosion. The alkali (Li,Na, K, Rb etc.) and alkaline earth (Be, Ca, Sr etc.) metals are rapidlyoxidized at low temperatures, while metals like Fe, Al etc. (except noblemetals Au, Pt etc.) are oxidized at high temperatures by interaction of oxygen.

2 M -------- 2 M n+ + 2n e - (Loss of electron: Oxidation) Metal ionsn/2 O 2 + 2n e - -------- n O 2- (Gain of electron: Reduction)

-----------------------------------------------------2 M + n/2 O 2 -------- 2 M n+ + n O 2-

-----------------------------------------------------

Mechanism: The oxidation of the metal occurs at the surface first resulting theformation of metal oxide layer. The nature of oxide layer formed on the metalsurface decides the further action with the environment.

If the oxide film formed is continues and rigidly adhered to the surface of metal is impervious in nature, and is called stable oxide layer . It is protective andshields the metal from further corrosion.

Ex: oxide films on Cu or Al acts as a protective coating and further corrosionis prevented.

If unstable oxide film is formed decomposes back into the metal and oxygen.Ex: noble metals like Pt, Au etc. corrosion is not possible, since oxide layer formed isvery unstable and decomposes back to metal and oxygen.

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If oxide layer formed is porous or volatile is non-protective and more feasiblefor further attack of environment, hence corrosion continues till the entire metal isconverted to metal oxide. Ex: Mo forms a volatile oxide layer.

Pilling Bedworth rule : Greater is the specific volume ratio, lesser it the rate of corrosion. If the volume of the metal oxide layer is at least as great as the volume of metal from which its is formed is non-porous and becomes protective layer by tightlyadhering to the base metal.

Volume of metal oxide formed Specific volume ratio = ----------------------------------

Volume of the metal For example, the specific volume ratios of W, Cr and Ni are 3.6, 2.0 and 1.6

respectively. Hence, the rate of corrosion is least in Tungsten (W).

If the volume of metal oxide is less than the volume of the metal, the oxidelayer is porous, non continuous and non-protective and faces strains. Hence cracksand pores are developed in the layer, creating access to atmospheric oxygen toreach the underlying metal. In this case corrosion is continuous and rapidlyincreases.

For example: Li, Na and K.

2. Corrosion by other gases: In the absence of moisture a few gases likeSO 2 , CO 2 , Cl 2, H2S and F 2 etc. attack the metal. The degree of corrosiondepends on the formation of protective or non-protective films on the metalsurface.

If the film formed is protective or non-porous, the intensity or extent of attack decreases, because the film formed protects the metal from further attack.

Ex: AgCl film, resulting from the attack of Cl 2 on Ag.

If the film formed is non-protective or porous, the surface of the whole metalis gradually destroyed.

Ex: dry Cl 2 gas attacks on tin (Sn) forming volatile SnCl 4.

ELECTROCHEMICAL CORROSION (WET CORROSION):

Wet corrosion or electrochemical corrosion takes place under wet or moistconditions through the formation of short circuited tiny electrochemical cells. Wetcorrosion is more common than dry corrosion.

This type of corrosion can be observedi) when a metal is in contact with conducting liquid.ii) When two dissimilar metals are dipped partially in a solution.

This corrosion occurs due to the existence of separate anodic and cathodic areas. Impact of corrosive environment on a portion of metal, changes its electrodepotential compared to its original value of the electrode potential of the metal. Thiscreates a potential difference within the metal system. The area of lower reduction

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potential created in the metal is known as the anodic area and that with higherreduction potential value is called cathodic area.

The current flows from the anodic area to the cathodic area through theconducting solution. A galvanic cell is created in the metal.

At anodic area, oxidation reaction takes place, so metal is destroyed by eitherdissolving or assuming combined state. Hence, corrosion always occurs at anodicarea.

At anodic area: M -------- Mn+ + n e - (Oxidation)

Mn+ ------ dissolves in solution or forms compound such as oxide.

At cathodic area, reduction reaction takes place. Usually, cathode reactions donot affect the cathode, since most metals cannot be further reduced. So, at cathodicpart, dissolved constituents in the conducting medium accept the electrons to formsome ions like OH - , O 2- .

The metallic ion formed at anodic part and non-metallic ion formed at cathodepart diffuse towards each other through conducting medium and form a corrosion

product some where between anode and cathode. The electrons set free at theanode flow through the metal and are finally consumed in the cathodic reaction.

Thus, we may sum up that electrochemical corrosion involves:

i) the formation of anodic and cathodic areas or parts in contact witheach other,

ii) presence of a conducting medium,iii) corrosion of anodic areas only,iv) Formation of corrosion product somewhere between anodic and

cathodic areas.

Electrochemical corrosion involves flow of electron-current between the

anodic and cathodic areas. The anodic reaction involves in dissolution of metal ascorresponding metallic ions with the liberation of free electrons.

At anodic area: M -------- Mn+ + n e - (Oxidation)

The anodic reaction consumes electrons with either by i) evolution of hydrogen, or ii) absorption of oxygen depending on the nature of the corrosiveenvironment.

i) Evolution of hydrogen: In absence of oxygen.

In acidic medium: 2 H + + 2 e - H2 (Reduction)

In neutral or alkaline medium: 2H 2O + 2 e- H2 + 2OH -

Considering metal like Fe,

At the anodic reaction is dissolution of iron as ferrous ion with the liberation of electrons.

Fe ------- Fe 2+ + 2 e- (oxidation)

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These electrons flow through the metal, from anode to cathode, whereH+ ions are eliminated as hydrogen gas from acidic solution.

2 H + + 2 e - H2 (Reduction)

Mechanism of wet corrosion by hydrogen evolution

Thus, this type of corrosion causes displacement of hydrogen ions from theacidic solution by metal ions. Consequently, all metals above hydrogen in theelectrochemical series have a tendency to get dissolved in acidic solution withsimultaneous evolution of hydrogen.

The over all reaction: Fe + 2H + Fe 2+ + H 2

ii) Absorption of oxygen: in presence of dissolved oxygen.

In acidic medium: 4H + + O 2 + 4e - 2H 2O

In neutral or alkaline medium: 2H 2O + O 2 + 4e - 4OH -

By considering Rusting of iron in neutral aqueous solution of electrolytes inthe presence of atmospheric oxygen. At the anodic areas of the metal (iron) dissolveas ferrous ions with liberation of electrons.

Fe -------- Fe 2+ + 2e - (Oxidation)The liberated electrons flow from anodic to cathodic areas, through iron

metal, where electrons are intercepted by the dissolved oxygen as:

O 2 + H 2O + 2e-

------- 2 OH-

(Reduction)

The Fe 2+ ion at anode and OH - ions at cathode diffuse and when they meet,ferrous hydroxide is precipitated.

Fe 2+ + 2OH - -------- Fe(OH) 2

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Mechanism of wet corrosion by oxygen adsorption( rusting of iron)

If enough oxygen is present, ferrous hydroxide is easily oxidized to ferric hydroxide.

4 Fe(OH) 2 + O 2 + H 2O -------- 4 Fe(OH) 3

This product is called yellow rust, actually corresponds to Fe 2O3 . H 2O

If the supply of oxygen is limited the corrosion product may be even blackanhydrous magnetite, Fe 3O4.

Galvanic corrosion: When two dissimilar metals are in contact with each other in acorrosive conducting medium. The two metals differ in their tendencies to undergooxidation; the one with lower electrode potential or the more active metal (higher upin electrochemical series) acts as anode and the one with higher electrode potentialacts as cathode. The potential difference between two metals is the cause or drivingforce for corrosion. The anodic metal undergoes corrosion and the cathodic metal isgenerally unattacked.

Ex: When Zn and Cu are connected and exposed to corroding atmosphere Znbecomes anodic because of its higher oxidation potential or higher position in theelectrochemical series. Zn undergoes oxidation and corroded, where as Cuundergoes reduction and protected. The electrons released by Zn reach Cu throughthe metal.

Galvanic corrosion

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Anodic metal: Zn ---------- Zn 2+ + 2 e - (Oxidation)( More active metal)

At cathodic metal: depending on nature of the corrosion environment thecathode reaction (reduction) may be either hydrogen evolution or oxygen absorption.

i) 2 H + + 2 e - H2

ii) O 2 + H 2O + 2e - ------- 2 OH -

Galvanic corrosion can be minimized by

1. Avoiding galvanic couples.2. Providing an insulating material between two metals.

Some more examples: Steel screws/ rivets in copper sheet. Steel pipe connected to copper plumbing. Lead antimony solder around copper wire.

Differential aeration corrosion: When a metal is exposed to different airconcentrations or oxygen concentrations. The part of the metal exposed to higheroxygen concentration acts as cathodic region, and part of the metal exposed to loweroxygen concentration acts as anodic region. Consequently, the poorly oxygenatedregion undergoes corrosion. Corrosion of metals arising as a result of the formationof a oxygen concentration cell due to the uneven supply of air on the metal surface.

Corrosion due to partial immersion of a metal in a solution

Ex: The Zn metal partially immersed in a NaCl solution. The parts above thewaterline more oxygenated are more strongly aerated and hence, become cathodic.On the other hand, parts immersed to greater depth show a smaller oxygen

concentration and thus become anodic. So, a difference of potential is created, whichcauses a flow of current between the two differently aerated areas of the samemetal. Zn will dissolve at anodic area, and oxygen will take up electrons at thecathode areas to form hydroxyl ions. Thus, Zn undergoes corrosion at poorlyoxygenated part.

At anode: Zn ---------- Zn 2+ + 2 e - (poorly oxygenated part)

At cathode: O 2 + H2O + 2e - ------- 2OH - (well oxygenated part)

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Examples: Parts of the nail inside the wall, being exposed to lower oxygen

concentration than the exposed part, undergo corrosion. Window rods inside the frame suffer corrosion but not the exposed

region. Paper pins inside the paper gets corroded, and the exposed parts is

free from corrosion. Metal under dirt, dust, scale or water undergoes corrosion. Partially buried pipeline in soil or submerged in water undergoes

corrosion below the soil or water where as the exposed part is freefrom corrosion.

Waterline corrosion: It is a case of differential aeration corrosion. More prevalentin cases such as ocean going ships, water storage steel tanks etc, in which a portionof metal is always under water.

The water line corrosion takes place due to the formation of differentialoxygen concentration cell. The part of the metal below the water line exposed only tothe dissolved oxygen while the part above the water is exposed to higher

concentration of the atmosphere of the atmosphere. Thus, part of the metal belowthe water acts as anode and undergoes corrosion and the part above the water lineis free from corrosion.

A distinct brown line is formed just blow the water line due to the depositionof rust.

At anode: Fe -------- Fe 2+ + 2e - (Oxidation)

At cathode: O 2 + H 2O + 2e - ------- 2 OH -

Water storage steel tank

Ocean going ship

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to pitting is of the little significance when compared to indirect losses such asproduction, repairs and replacements.

FACTORS AFFECTING THE RATE OF CORROSION:

Position of metal in the Galvanic series: When the metal is higher up in thegalvanic series, greater is the oxidation potential. Thus, greater is its tendency tobecome anodic and hence greater is the rate of corrosion. Thus, extent of corrosiondepends upon the position of the metal in the galvanic series. When two metals arein electrical contact, greater is the difference in their positions in the electrochemicalseries, faster is the corrosion of anodic metal.

Relative areas of anode and cathode: If a metal has small anodic area and largecathodic area, then the corrosion is more intensive and faster is the corrosion rate atthe anodic region.

Cathodic areaRate of corrosion of anodic region -------------------------

Anodic area

As this ratio increases, the corrosion rate increases rapidly. At anode,

oxidation takes place and electrons are liberated. At the cathode, these electrons areconsumed. When anode is smaller and cathode region is large all the electronsliberated at the anode are rapidly consumed at the cathode. This process makes theanodic reaction to take place at its maximum rate, thus increasing the corrosion rate.If the cathode is smaller, the consumption of electrons will be slower and thecorrosion reaction as a whole will be slower.

Nature of corrosion product: The nature of the corrosion product largely decidesthe rate of further corrosion. The corrosion product formed on the surface of themetal may or may not act as a protective film.

If the corrosion product deposited is insoluble, stable, uniform and nonporous,it acts as a protective film preventing the further corrosion of metal. A thin, invisible,impervious, continuous film formed on the surface acts as a barrier between thefresh metal surface and the corrosion environment.

Ex; in oxidizing environments, metals like Al, Cr, Ti etc. are highly passive astheir oxides as corrosion products form protective films on the metal surfacepreventing further corrosion.

If the corrosion product is soluble, unstable, non uniform and porous, thecorrosion continues unabated. In such cases the fresh metal surface is continuouslyexposed to the corrosion environment and corrosion of the metal surface takes placecontinuously. When the corrosion product formed is either soluble in the medium orformed away from the anodic and cathodic sites, and then also the corrosion product

does not have any protective value.

Ex; metals such as Fe, Zn, Mg etc. do not form any protective film and arehighly susceptible for continuous corrosion when exposed to oxidizing environments.

Temperature: The rate of a chemical reaction, in general, increases with rise intemperature. Corrosion process is one such chemical reaction. Therefore, the rate of corrosion increases as the temperature increases. Increase in temperature increases

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the conductance of the corrosion medium, which also contributes to the increase rateof corrosion.

Humidity: The greater is humidity, the greater is the rate and extent of corrosion.This is due to the fact that the moisture or vapours present in atmosphere acts as asolvent for O 2 , H 2S, SO 2 and NaCl etc. to furnish the electrolyte essential for settingup an electrochemical cell.

PASSIVITY or PASSIVATION: Passivation is the phenomenon by which a metal oralloy shows high corrosion resistance due to the formation of highly protectivelayers, very thin (about 0.0004mm thick) and quit invisible surface film. It is thespontaneous formation of a hard non-reactive surface film that inhibits furthercorrosion. This layer is usually an oxide or nitride that is a few nanometers thick.

Passivation of metal takes place only in certain environments, which tend tomaintain protective film on the surface. Ex: Ti, Cr and Al containing stainless steelalloys exhibit outstanding corrosion resistance in presence of oxygen. This is due toformation of thin, protective oxide film on their surfaces. Whenever any damageoccurs, this film is automatically repaired in oxidizing environments.

Ex; Nickel can be used for handling elemental fluorine , owing to the formationof a passivation layer of nickel fluoride .

Pure aluminium naturally forms a tough resistant oxide, alumina , almostimmediately that protects it from further oxidation in most environments.

In the area of microelectronics, the formation of a strongly adheringpassivating oxide is important to the performance of silicon.

CORROSION CONTROL METHODS:

Cathodic protection: The method of protection given to a metal by forcibly makingit to behave like a cathode is called cathodic protection. There are two types of cathodic protection.

a) Sacrificial anodic protectionb) Impressed current cathodic protection

Sacrificial anodic protection : In this method of protection, the metallic structureto be protected called base metal is connected to more anodic metal through awire, so that all the corrosion is concentrated at the more anodic metal.

As the more anodic metal is sacrificed in the process of saving base metalfrom corrosion. Hence, it is known as sacrificial anod e. The corroded sacrificial anode

block is replaced by a fresh one.

Metals commonly used as sacrificial anodes are Zn, Al, Mg and their alloys.Mg is used in high resistivity electrolytes such as soils due to its most negativepotential and it can provide highest current out put.

Example: Railway tracks are protected from corrosion by this method.

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http://en.wikipedia.org/wiki/Nitridehttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Fluorinehttp://en.wikipedia.org/wiki/Nickel(II)_fluoridehttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Aluminahttp://en.wikipedia.org/wiki/Nitridehttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Fluorinehttp://en.wikipedia.org/wiki/Nickel(II)_fluoridehttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Alumina


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Applications: Important application of this technique are

a) Protection of underground cables and pipelines from soil corrosion.

b) Protection of ships and boat hulls from marine corrosion. Sheets of Mg or Znare hung around the ship hull, these sheets being anodic to corrosion so thesesheets get corroded, when consumed completely, these are replaced by freshone.

c) The formation of rusty water is prevented by the insertion of Mg sheets orrods into domestic water boilers or tanks.

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Impressed current cathodic protections: In this method current from an externalsource is impressed (applied in the opposite direction) to nullify the corrosioncurrent. Thus the anodic corroding metal becomes cathodic and protected fromcorrosion.

The anode may be either an inert metal or one which deteriorates and willhave to be replaced periodically. The commonly used anodic materials are graphite,carbon, stainless steel, scrap iron, high silica iron and platinum. The anode is buriedin back fill such as gypsum to increase the electrical contact between itself and thesurrounding soil.

Application : This protection method is useful when electrolyte resistivity and currentrequirements are high. It is well suited for large structures and long-termapplications.

This protection technique is employed in the case of open water box coolers,water tanks, buried pipe-lines, marine pipes etc.

METAL COATINGS: Deposition of protective metal over the surface of a base metalis called metallic coating. It is a valuable and well tried method for improving thecorrosion life of the underlying metal. Metal coatings are applied by electrodeposition, flame spraying, cladding, hot dipping, diffusion coating and vapordeposition. From the corrosion point of view, metal coatings can be divided into twoclasses anodic coatings and cathodic coatings.

Anodic coatings: Anodic coatings are produced by coating a base metal with moreactive metals which are cathodic to the base metal.

Ex: irons is coated with anodic and more active metal such as zinc,magnesium and aluminium.

The one of the important characteristic of anodic coating is that, even if thecoating is ruptured, the base metal does not undergo corrosion. The exposed surfaceof the metal is cathodic with respect to the coating metal and the coating metalpreferentially undergoes corrosion: The protection is ensured as long as the anodiccoating metal is still present on the surface. Therefore, anodic coating also known assacrificial coating.

Galvanization is a familiar example for anodic coating and is extensively usedto protect iron and steel objects.

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Galvanizing: It is a process of coating iron or steel sheets with a thin coat of Zn to prevent iron from rusting.

The base metal iron or steel sheet is cleaned by acid pickling method withdil.H 2SO 4 for 15-20 minutes at 60-90 oC. The sheet is then washed well and dried. Itis dipped in a bath of molten zinc maintained at 425-435 oC. The surface of the bathis kept covered with ammonium chloride flux to prevent oxide formation. The sheetis taken out and excess Zn is removed by passing it between a pair of hot rollers.Then the sheet is subjected to annealing process at 650 oC and cooled slowly. Analloy of iron and zinc were formed at the junction of the base metal and coatingmetal.

Applications: It is mostly used to protect iron used for roofing sheets, wires, pipes,nails, bolts, screws, buckets and tubes.

Galvanizing utensils cannot be used for preparing and storing food stuffsespecially acidic in nature, because zinc dissolves to form highly toxic or poisonouscompounds.

Cathodic coating: Cathodic coatings are produced by coating a base metal with amore noble metal which is cathodic. The metal such as copper, nickel, tin, silver iscommonly used as cathodic coatings on steel. These coating metals are less reactivethan the base metal and are less susceptible for corrosion.

The cathodic coating provides protection only when it is undamaged,continuous and non porous. When the coating is discontinuous, an intense localizedcorrosion occurs due to the setting up of a galvanic cell that consists of a largecathodic area and a small anodic area. Tinning is most commonly used surfacecoating on steel surfaces.

Tinning: Coating tin over iron or steel articles is called tinning. In thisprocess the surface of the base metal i.e., iron sheet is cleaned by acidic picklingwith dil.H 2SO 4 and passed through a bath of zinc chloride flux. The flux help themolten metal to adhere to the metal surface. Then the sheet is passed throughmolten tin bath and pressed in between two rollers from a layer of palm oil. Palm oilhelps to protect the tin coated surface against oxidation. The rollers remove excessof tin and produce a thin film of uniform concentration. An alloy of the base metalcoating metal at their junction.

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Applications: Tin metal possess good resistance against atmospheric corrosion. Tin isnon toxic and widely used for coating steel, copper, and brass sheet.

The containers coated with tin is used for storing food stuffs, ghee, oils etc.and packing food materials.

Tinned copper sheets are used for making cooking utensils and refrigerationequipment.

ELECTROPLATING: The process of deposition of a metal on to the surface of another metal, alloy or any conductor metal by the electrolysis is calledelectroplating.

It consists of a process, by which a metal is deposited on another metal oralloy by passing a direct current through an electrolyte solution containing the metalions to be deposited. The common coating metals used is Zn, Cu, Ni, Cr, Ag, Pt, Au,etc.

The electroplating device is essentially an electrolytic cell, in which twoelectrodes, anode and cathode are dipped in an electrolyte solution. The principalcomponents of an electroplating process are:

An electroplating bath containing a conducting salt and the metal to be platedin a soluble form as well as a buffer and additives.

The electronically conducting cathode, i.e., the article to be plated. The electronically conducting anode, the coating metal itself or an inert

material of good electrical conductivity like graphite. An inert vessel to contain the above mentioned materials, made up of either

rubber lined steel, plastic, concrete or wood.

In this process the object to be plated is made the cathode in an electrolytebath containing a metal ion, M n+ so that the simplest reaction at the cathode is

Mn+ + n e- M

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Wherever possible, the preferred anode reaction is the dissolution of the samemetal in the solution.

M Mn+ + n e-

In such cases, there is a continuous replenishment of electrolyte duringelectrolysis. However, if the anode is made of an inert material that does not passinto the solution, the electrolyte salt is added continuously in order to maintainoptimum metal ion concentration in the solution.

Ex: Nickel electroplating

Electroplating of Nickel: Nickel is a widely plated metal in plating industry, forboth decorative and protective applications. To electroplate nickel on other surfacesdifferent bath compositions are used.

Watts bath: It is an all purpose bath, used both for decorative and protectiveapplications of nickel deposit, as well as for making up worn out parts and forelectroforming.

Composition:NiSO 4 + NiCl 2 + boric acid + sodium lauryl sulfate Sulfamate bath: For heavy nickel deposits are required, sulfamate bath is

preferred.

Composition: Nickel sulfamate + NiCl 2 + Boric acid

The article to be electroplated is subjected to solvent cleaning to remove oil,

greases etc. then it is subjected to acid pickling with dil.HCl or dil.H 2SO 4 to removeany scales, oxides etc.

The cleaned article is made cathode of the electrolytic cell and the anode canbe made of the high purity Nickel pellets or pieces in titanium meshbasket/cotton/terylene bath. The electrodes are dipped in solution of an electrolyticbath.

When direct current is passed Ni metal is dissolved into the solution as Ni 2+

ions at anode.At anode: Ni Ni2+ + 2 e-

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Ni2+ ions migrate to the cathode and deposit on the base metal article in theform of a thin layer.

At cathode: Ni 2+ + 2 e- NiNickel plating forms a corrosion resistant, wear resistant, hard surface. When

nickel is co-deposited with insoluble materials such as alumina, zirconia, siliconcarbide, etc., the wear and abrasion resistance is enhanced.Application:

For decorative as well as protective applications Used as an undercoat for the decorative chromium finish on components

made of steel, brass, zinc die castings, plastics and to a limited extent on Aland Mg alloys.

Used as an undercoat for brass, gold an rhodium plating. Thick deposits of nickel heavy nickel are used in electroforming, for

corrosion and wear resistance purpose, for high temperature oxidationresistance, and to build up worn out or mismachined parts.

Black nickel plating is used for making name plates, type writer parts,camera components, optical and electrical instruments, military hardware.

ELECTROLESS PLATING: It is a method of depositing a metal or alloy over a

substrate by controlled chemical reduction of the metal ions by a suitable reducingagent without using electrical energy. The process is a chemical reaction and isautocatalytic.

The reduction of metal ions by the reducing agent is catalyzed by the metalatoms being plated. Therefore, electroless plating is also termed as autocatalyticplating.

The surface to be plated should be catalytically active so that in the beginningof the plating reaction, the surface of the substrate catalyses the reduction. Once thesubstrate is covered by the metal or alloy coating, the coating catalyses thereduction. The process continues with building of successive layers of the metal arealloy.

The electroless plating process can be represented as,Catalytic surface

Metal ions + Reducing agent ---------------- Metal + Oxidized product

The catalytic metals such as Ni, Co, Steel, Fe, Rh, Pd, Al, etc., do not requireany surface preparation before electroless plating on them. Non-catalytic metal suchas Cu, Brass, Ag, etc., need activation.

Electroless plating of Copper:

The composition of an electrolyte bath for electroless plating comprises the

following:

Salt : CuSO 4 .5H 2O, Reducing agent: Formaldehyde (HCHO)

Buffer :NaOH + Rochelle salt, pH=11( Potassium sodium tartrate, NaKC 4H4O 6) )

Complexing agent : EDTA, Temperature: 298 0K

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The redox reaction takes place as

At anode: 2HCHO + 4OH - 2HCOO - + 2H 2O + H 2 + 2e-At cathode: Cu 2+ + 2e- Cu

Over All reaction: 2HCHO + 4OH - + Cu 2+ 2HCOO - + 2H 2O + H 2 + Cu

The metallic copper obtained as a reduced product in the bath deposits on themetal surface.

However, the reduction proceeds through a cuprous state. An excess of cuprous oxide formation will cause the reduction reaction to proceed out of control.To inhibit the formation of cuprous oxide, air is bubbled slowly through theelectroless copper solution and small complexing agents are added to the solution.The presence of these complexing agents turns the electroless copper bath into achelated solution. Further more, in order for the reaction to progress, certainconcentration levels of copper sulphate, formaldehyde, sodium hydroxide, andchelating agent, and the correct chemical ratios of these chemical components mustbe maintained. The electroless copper bath must be analyzed at atleast every four

hours to make sure that optimum concentration and ratios are maintained.Replenishment of copper sulfate. Formaldehyde, sodium persulfate, and chelatingagent must be provided as needed. Constant monitoring and maintenance arerequired to properly operate the electroless copper bath.

Applications: Widely used for metalising printed circuit boards. For producing through-hole connections. For plating on non conductors. As a base for subsequent conventional electroplating. Applied on wave guides and for decorative plating on plastics.

ORGANIC COATINGS PAINT CONSTITUENTS AND FUNCTIONS:

Pigments: Pigments is one of the essential constituent of a paint.

The essential function of pigments are:a) To provide desired colour, opacity and strength to the paint,b) To give aesthetical appeal to the paint film,c) To give protection to the paint film by reflecting harmful ultraviolet light,d) To provide resistance to paint film against wear abrasion,e) To improve the impermeability of paint film to moisture andf) To increase the weather-resistance of the film.

Characteristics of a good pigment: A good pigment must opaque, chemically

inert, non toxic, freely mixable and cheap.

Vehicle or Drying oil: It is a film-forming constituent of the paint. The liquidportion of the paint in which the pigment is dispersed is called a medium orvehicle or drying oil.

Functions:a) They hold the pigment on the metal surface,b) They form the protective film,

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c) They impart water-repellency, durability and toughness to the film, andd) They give better adhesion to the metal surface.e) Drying oils are glyceryl esters of high molecular weight-fatty acids present

in animal and vegetable oils.Thinners: Viscosity of the paints is reduced by the addition of thinners. This isessentials so that the paint can be easily applied on the metal surface.

Functions:a) Thinners reduce the viscosity of the paint to suitable consistency, so that

it can be easily handled and applied to the metal surface.b) They dissolve the film-forming material an also the other desirable

additives in the vehicle.c) They evaporate rapidly and help in the drying of the paint film.d) They suspend the pigments in the paint.e) They increase the elasticity of the paint film.f) They also increase the penetration power of the vehicle

Driers: The drying of the oil film is accelerated or catalyzed by driers. They dothis by oxidation, polymerization and condensation. In fact, driers are oxygencarriers catalysts.

Fillers or extenders: fillers are inert material which are used to improve theproperties and reduce the cost of the paint.

Functions:a) They reduce the cost of the paint.b) They serve to fill the voids in the film,c) They increase random arrangement of the primary pigment particles, and

act as carriers for the pigment colour,d) They improve the durability of the film by reducing the cracking of the

paints after drying.

Plasticizers: plasticizers are added to the paint film to give elasticity to the paint

film and to prevent cracking of the film.Antiskinning agents: prevent the gelling and skinning of the paint film.

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Documents

Corrosion and Its Corrosion Control