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НазваTheory of metal passivation (реферат)
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РозділІноземна мова, реферати англійською, німецькою
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Тип документуРеферат
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Research concept

 

Theory of metal passivation

 

Metal surface contacting with solution of electrolyte in some definite

condition transformed to so called passive state. Study of this

phenomena on the border of metal – electrolyte plays an important role,

as they define the process of destruction of metal. And it is

thermodynamically favourable for metal to dissolve as a result of these

process. Such phenomenon was first observed by M. Faraday. This is one

of the main factor of stability of metal in aggressive environment.

 

П. In case of iron this difference is 0,63 v. For this reason the phase

film conception of passivation cannot be taken in that from.

 

In case of metal passsivation determining role plays water molecule.

Some part of water molecule dissociate in the process of adsorption and

ion of oxygen breaking the bond with proton firmly block the most active

centre of metal surface. This may be considered as start of passivation.

 

In the theory of passivation some physical factor must be taken in

account. Most important of those are stated bellow.

 

.

 

Equilibrium exchange on the border with solution in which take part the

ion OH- and Oox.

 

Number of nonequilibrium vacancy in the passivaing oxide lattice.

 

Energetic inhomogeneity of metal surface.

 

Major factor of the process is inter phase difference of potential,

which is defined by composition of the solution. Depending on its value

the current of dissolution take the form:

 

Breaks on this curve is connected with the formation of thin protection

layer in sector II. Reaction of this passive layer formation is

 

 

The oxygen undertakes from molecules of water, and half metal from the

substrate of metal surface. As a result of formation of this layer the

current falls on 4-7 orders in a very narrow interval of potential

change (. After formation of a continuous monolayer there occur the

state of passivity III.

 

The question, how this passive layer is formatted was not studied. We

shall try to explain the process of passive layer formation and the

kinetic of the process.

 

.

 

,

 

the cluster is equilibrium, its critical size lkp surpasses few times

the sizes of building particles (molecules) of the layer. The

probability of its formation is defined by the work A of this process

 

,

 

With the condensation of the factor of crystallisation Wc , the

probability of crystal cluster formation Wk is

 

.

 

It is defined by the classical approaches, according to which the

formation of equilibrium crystal take place by consecutive connection of

building particles to the complexes, already available on surface M.

 

, i.e. growth or disintegration of crystal particles.

 

. These values represent quantity of the acts taking place in 1 cm2 of

the surface M for 1 sec. They are proportional to superficial

concentration na of particles of the given size and probabilities of the

elementary acts

 

.

 

Resulting speed of direct and return transitions

 

.

 

At balance state

 

 

Proceeding from this it is possible to find out

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