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Fe-H2O SYSTEM INPOURBAIX DIAGRAMSEMINAR TOPIC ON

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Potential-pH diagrams are also called pourbaixdiagram after the name of the originatorMarcel Pourbaix , a Belgium electrochemist

and corrosion scientist.

Pourbaix Diagram

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These diagrams represent the stability of metalas a function of potential and pH.

They are analogous to phase equilibrium

diagrams, where stability of various phases isshown as a function of tempreture andpercentage composition of a metal.

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At a particular tempreture and composition astable phase can be easily determined.Similarly at a particular combination of pHand potential , a stable phase can bedetermined from POURBAIX DIAGRAM.

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These diagrams are plotted for the variousequillibria on normal Cartesian coordinates withpotential on vertical axis or ordinate and pH on

horizontal axis or abssica.Means The vertical axis is labeled Eh for thevoltage potential with respect to the standard

hydrogen electrode (SHE) as calculated by theNernst equation. The "h" stands for Hydrogen,although other standards may be used.(this is

for room temperature only)

http://en.wikipedia.org/wiki/Standard_hydrogen_electrodehttp://en.wikipedia.org/wiki/Standard_hydrogen_electrodehttp://en.wikipedia.org/wiki/Nernst_equationhttp://en.wikipedia.org/wiki/Nernst_equationhttp://en.wikipedia.org/wiki/Standard_hydrogen_electrodehttp://en.wikipedia.org/wiki/Standard_hydrogen_electrode

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The horizontal axis is labeled pH for the -log

function of the H+

ion concentration.

pH = log[H+ ]

http://en.wikipedia.org/wiki/PHhttp://en.wikipedia.org/wiki/PH

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The diagram takes account ofelectrochemical and chemical equillibria of

metals in conjunction with water and sincethere are several such equillibria for eachmetal , only one metal can be clearly

represented on one diagram.

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Oxidizing conditions are described by the toppart of the diagram (high positive electrode

potential).

Reducing conditions are described by the

bottom part of the diagram (high negativeelectrode potential).

http://www.substech.com/dokuwiki/doku.php?id=electrode_potentialshttp://www.substech.com/dokuwiki/doku.php?id=electrode_potentialshttp://www.substech.com/dokuwiki/doku.php?id=electrode_potentialshttp://www.substech.com/dokuwiki/doku.php?id=electrode_potentials

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A typical diagram for Fe-H2O

system is shown:-

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The diagram defines the following zonesof the equilibrium states:

below the line a-b-j: Solid iron (immunityzone). The electrochemical reactions in thiszone proceed in the direction of reduction

of iron ions. No corrosion occurs in thiszone.

a-b-n-c-d-e: Aqueous solution of ion Fe2+(corrosion zone). Metallic iron oxidizes inthis zone.

e-d-f-g-k: Aqueous solution of ion Fe3+(corrosion zone). Metallic iron oxidizes(corrodes) in this zone.

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h-f-g-m: Aqueous solution of ion FeO42-

(corrosion zone).

c-d-f-h-i: Solid ferrous oxide Fe2O3(passivation zone). Iron oxidizes(corrodes) in this zone however theresulted oxide film depresses theoxidation process causing passivation(corrosion protection of the metal due toformation of a film of a solid product ofthe oxidation reaction).

n-c-i-p: Solid oxide Fe3O4 (Fe2O3*FeO)

(passivation zone). The oxide filmcauses passivation

b-n-p-j: Solid hydroxide (II) Fe(OH)2 /FeO*nH2O / green rust (passivation

zone).

http://www.substech.com/dokuwiki/doku.php?id=galvanic_corrosionhttp://www.substech.com/dokuwiki/doku.php?id=galvanic_corrosion

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Here are some of the reactions and the

corresponding lines of the Fe-H2O Pourbaix

diagram:a-b: Fe(s) = Fe2+(aq) + 2e- Redox reactionindependent of PH. The equilibrium occurs athe electrode potential value -0.44V, which is

equal to the standard electrode potential ofiron (see the Electrochemical series).

e-d: Fe2+(aq) = Fe3+(aq) + e- Redoxreaction independent of PH.d-f: 2Fe3+(aq) + 3O2- = Fe2O3(s) Non-redoxreaction dependent on PH.

http://www.substech.com/dokuwiki/doku.php?id=electrode_potentialshttp://www.substech.com/dokuwiki/doku.php?id=electrode_potentialshttp://www.substech.com/dokuwiki/doku.php?id=electrode_potentialshttp://www.substech.com/dokuwiki/doku.php?id=electrode_potentials

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b-n: Fe2+(aq) + 2OH-(aq) = Fe(OH)2(s) Non-redox reaction dependent on PH.

c-d: 2Fe2+(aq) + 3H2O = Fe2O3(s) + 6H+(aq)

+2e- Redox reaction dependent on PH.b-j: Fe(s) + 2OH-(aq) = Fe(OH)2(s) + 2e

-Redox reaction dependent on PH.

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The horizontal , vertical and the sloping lines in the diagram

form the following three types of domains or regions:-

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1.} Immunity Domain : This region represents the

state in which corrosion is thermodynamicallyimpossible in a particular environment. In this region the

activity of metal ion doen not exceed 10-6

gram ion/litre.

2.} Passive Domain: This region represents the

state in which the metal is thermodynamically unstablein a particular environment but corrosion is prevented

by the modification of the surface due to formation ofoxidelayer.In this region solid metal compound are

thermodynamically stable and in equilibrium with metalions of 10-6 gram ion / litre activity.

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3.} Corrosion Domain: In this region the metal corrosion

is thermodynamically possible. The area is separated fromthe immunity domain by lines with represent solubility equal

to 10-6

gram ion / litre.These three domains are shown for Fe in above diagram.

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Application of E/pH Diagrams:-Potential/pH diagrams are useful in electrometallurgy.The diagrams are particularly important in study ofcorrosion of metals in aqueous media andelectrodeposition of metals and oxides. These diagramsare useful for predicting:

1.} Spontaneous direction of reactions.

2.} Stability and composition of corrosion products.

3.} Environmental changes which will prevent or reducethe corrosive attack.

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According to E/pH Diagram for Fe-

H2O system, following methods are

used to reduce corrosion:

1.} By lowering the electrode potential down intothe region of immunity e.g. by cathodic protection.

2.} By raising the electrode potential up to theregion of passivity. E.g by anodic protection.

3.} By raising the pH of the alkanity of solution sothat a passive film is formed.

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Limitations of E/pH Diagrams:-1.} These diagrams give no information oncorrosion rate since they are based on

thermodynamic and not on the kinetic data.

2.} The term passivity in these diagrams isapplied to field of existence for oxides ,hydroxides or other sparingly solublesubstances,irrespective of their protectiveproperties.

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An Introduction To Electrometallurgy BySatya Narayan and Rajendra Sharan

Corrosion Engineering By Mars. GFontana

An Introduction To Metallic Corrosionand its Prevention by Raj Narayan

www.google.com

Referances

http://www.google.com/http://www.google.com/

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Thank You