Theory of metal passivation

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.

It is known that, there is no unified model of passivation. The most common and in first sight convincing conception of phase oxide is connecting passivation with mechanical formation of thin film on metal surface with oxide layer. However, potential of phase oxide formation differ from critical parameter of polarised curve (pic. 1), specially from potential 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.

Strong electric field. It define the transform of metal to metal oxide: .

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.

With this purpose it would be possible to use the thermodynamic theory of Gibbs- Folmer, according to which at formation of a new phase the free energy of system changes in the value .

,

Where q- the geometrical factor, l- the size of the cluster , M, r- molecular weight and density of a firm phase, - chemical potentials of supersaturated solution and firm phase with concentration C1,2 and coefficient of activity f1,2. In the point of maximum 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.

At calculation of probability it is accepted, that on the surface M spontaneously arise (or on the contrary, break up) twin crystal particles of various sizes of a and with inter nuclear distance r0. The sizes change as a result of the consecutive elementary acts (transitions) of the type such as , i.e. growth or disintegration of crystal particles.