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theory of metal passivation

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Research concept

Metal surface contacting with solution of electrolyte in some definite
condition transferred 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.), specially from
potential of activation (a and passivation (p. 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 passsivation of metal 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.

Energetically inhomogeneity of 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 the 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 the 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.

.

, (1)

The germ is equilibrum, his(its) critical size lkp on some orders
surpasses the sizes of building particles (molecules). The probability
of his(its) formation(education) is defined(determined) by work And this
process

, (2)

Aeie?eoe/aneea germs (oetheooaoeee of density) disappear, cae?eoe/aneea
grow.

The thermodynamic theory aeaana- Oieueia?a takes into account
condensation of particles, which however is sufficient for occurrence
only of liquid germs. But at formation(education) of crystal structure
of only one condensation a little. It is necessary still to take into
account the factor no?oeoo?e?iaaiey. In view of this factor we find
probability Wk crystal ca?iaeuoaia?aciaaiey

, (3)

It(he) is defined(determined) by the classical approaches, according to
which the formation(education) equilibrum e?enoaeeeea occurs by
consecutive connection of building particles to already available on a
surface I to complexes.

At calculation of probability (3) it is accepted, that on a surface I
spontaneously arise (or on the contrary, break up) aeaoia?iua crystal
particles of the various sizes and with internuclear distance r0. The
sizes change as a result of the consecutive elementary
certificates(acts) (transitions) of a type and Ua ? r0, i.e. growth or
disintegration of crystal particles.

. These sizes represent quantity(amount) of the given
certificates(acts) occurring for 1 nae. On 1 ni2 the surfaces I. they
are proportional to superficial concentration na of particles of the
given size and probabilities of the elementary certificates(acts)

(4)

Resulting speed of direct and return transitions

. (5)

At balance

(6).

Proceeding from this it is possible to receive

(7)

Further we shall define(determine) A1 and A2, for this purpose we shall
take into account, that as shown in work No?aineee Eaeoaa, at occurrence
e?enoaeeeea the free energy of system changes on size

, (8)

Where the summation will be carried out(spent) on all micropositions of
particles in a crystal lattice.

Proceeding from this it is possible to receive,

(9)

With the help by this formula it is possible to deduce(remove) laws of
formation(education) of this layer on a site II. thus it appears the
power heterogeneity is important to take into account a surface of
metal, i.e. on a different site of a surface of different energy of
communication(connection). For its(her) account a surface I we shall
present by set of platforms dSi, within the limits of each of which the
energy yi is constant. The general(common) area S of a metal surface I
is equal

(10)

The statistical distribution of these platforms is described in density
of probability f (yi), so probability dWi to find on I
meaning(importance) yi is equal

The function f (y) is possible to choose by different ways, distributed
from them,

(11)

Through f (yi) and local density of a current i = i (j, yi), generated
the set i- uo of platforms dSi, expresses integrated density of a
current

(12)

Its(her) filling by germs, equal, accordingly grows

(13)

Account of these formulas give good concurrence to experimental data.
Thus the processing of the first thin superficial layer of metal in
ieneae is finished. It(him) aeaoia?iua the centers merge, forming a
continuous monolayer. Occurs complete ianneaaoeey of a surface I, the
site II curve fig. 1 is replaced by a site III, for which the new
physical conditions are characteristic.

pic. 1

(ПП

(0

IV

III

II

I

lg ia((a)

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