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Seminar topic onOXIDATION

Sub:- electrometallurgy corrosion

Prepared by :- Arjun R prajapati

Year :- 2011-12

Roll no. :- 919

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Mechanism:

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What is Oxidation ?

Oxidation is the process of losing electrons. The metalatoms release electrons (are oxidized) and become

positive ions. The site at which this occurs is known as the anode.

Scale formation, tarnishing, & dry corrosion may be

used to describe this phenomena.

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Oxidation reactions:- (e.g. anodizing.)

(1) H2So4 ----> 2H+ + SO4--

(2) 2H+ + 2e- ----> H2

(3) SO4-- + 2H2O ----> H2SO4 + O

-2

(4) Al ----> Al3+ + 3e-

(5) Al

3+

+ O

-2

----> Al2o3

http://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.htmlhttp://www.mf.mpg.de/en/abteilungen/dosch/projects/stierle_oxidation_bild01.html

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REAL CASE :-

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Most metals are oxidized by the oxygen in air. Electrons leap from the metal to the oxygen molecules.

The negative oxygen ions which are thus formedpenetrate into the metal, causing the growth of an oxidesurface.As the oxide layer grows the rate of electron transferdecreases. The corrosion stops and the metal is madepassive.The oxidation process may continue, however, if the

electrons succeed in entering the metal through cracks orimpurities in the metal or if the oxide layer is dissolved.

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Sometimes oxidation is not such a bad thing, as in the

formation of super-durable anodized aluminum. Other times, oxidation can be destructive, such asthe rusting of an automobile or the spoiling of fresh fruit. We often used the

words oxidation and rustinterchangeably. When oxidation occurs in copper, on the other hand,the result is a greenish coating called copper oxide. When it involves oxygen, the process

of oxidation depends on the amount of oxygen present inthe air and the nature of the material it touches.

http://www.wisegeek.com/what-is-aluminum.htmhttp://www.wisegeek.com/what-is-rust.htmhttp://www.wisegeek.com/what-is-copper-oxide.htmhttp://www.wisegeek.com/what-is-copper-oxide.htmhttp://www.wisegeek.com/what-is-rust.htmhttp://www.wisegeek.com/what-is-aluminum.htm

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Oxidation is the most important corrosion reaction.

In most In most industrial environments, oxidationis often one of the reactions, irrespective of thepredominant mode of corrosion.

the oxidation reaction to develop a protective oxidelayer to resist corrosion attack.

Oxidation is the major problem in the chemicalindustries, metal production, and fabrication.

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Low temperature high temperature

oxidation: oxidation:

Metals exposed to air

Form a surfacecompound

Depending upon thecontamination of air

Because of metal is not

stable as such in theenvironment

Tends to undergoreaction for stable form

Material degradation athigher temperature thanambient temperature

When exposed tocorrosive environment

Temperature depend onthe metal and type ofenvironment to be used

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Need of HTO:

How fast metal react with environment at hightemperature;

What is the mechanism of reaction?

How corrosive is the environment;

How to control the reaction or how to protectmetal from degradation;

How to select material for such high temperatureapplication.

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stages of oxidation phase:

When clean metal surface is exposed to oxygengas the initial oxidation phase can be subdividedinto three stages.

Adsorption of the gas on the metal surface.

Individual separated oxide nuclei are formed on thesurface and lead to formation of continuous film on

the metal surface.

Reaction proceed by solid state diffusion of one orboth of reactants through the oxide film.

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The oxide film separates the metal from the gas

and the reaction mechanism changes completely. no. of reactions considered to account for manytemperature dependant oxidation phenomena

observed. For thin films the driving force for thesetransport of reactants due to electric fields in or

across the film whereas for thick films theoxidation kinetic determined by the chemicalpotential gradient across the scale.

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Oxidation kinetics :-

In general reaction mechanism is a function of

Time

surface characteristic of sample

temperature

The most important parameter of the metal oxidationfrom the engineering point of view is the reaction rate,and is usually measured as weight gain per unit area.

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Linear rate law:The oxide reaction product is generally retained on the

Metal surface and so the rate of oxidation is usually

measured and expressed as weight gain per unit area.

The simplest empirical relationship is the linear law.W =kLtWhere W = weight gain per unit area

t = timek

L= linear rate constant

linear oxidation is the characteristic of metal for which

a porous or cracked scale is formed so that the scale

does not represent a diffusion barrier between the two

reactants.

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sodium & potassium oxidize linearly and have oxide

to metal volume ratio less than one.

While it is about 2.5 times than in the case of

tantalum and niobium.

The linear rate constant represents the rate at which

some molecular dissociation or some other reaction

step at the interface is controllingthe total reaction rate.

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Role of diffusion in HTO

High temperature oxidation is basically thereaction of a metal with oxygen to form an oxide

Thin layer regime is that in which oxidation occursat low temperatures. Under these conditions,diffusion is generally not considered.

absorption of oxygen results in creating a field

which is enough to cause of the migration of cationfrom the metal surface.

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Such a process is restricted to only a fewmonolayers.

At high temperatures, the phenomenon ofmonolayer formation is insignificant.

The initial layer form too fast to record its sequenceof formation and the whole process of oxidationdepends upon how this monolayer grows to form thicklayer.

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Oxidation prevention :-

conditioning metalby coating the metal.by alloying the metal.

by retarding air-metal contact.

conditioning corrosive environment.by removal of oxygen.by corrosion inhibitors.

electrochemical control.

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Advantages of oxidation:

Good corrosion resistance s/c film.(specially inthe case of anodizing )

As a decorative purpose.

Surface hardening Electrical insulation

Thermal insulation

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