Theory of metal passivation, Детальна інформація

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.

l of activation ja and passivation jП. 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.

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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:







lg ia(ja)









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 j. 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.

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,

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.