theory of metal passivation


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.
      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  (a  and  passivation  (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.
     1. Strong electric field. It define the transform of  metal  to  metal
        oxide: [pic].
     2. Equilibrium exchange on the border with solution in which take part
        the ion OH- and Oox.
     3. Number of nonequilibrium vacancy in the passivaing oxide lattice.
     4. 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
           [pic]
      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.
      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 [pic].
           [pic],
      Where q- the geometrical factor, l- the size of the cluster  ,  M,  (-
molecular weight and density of a firm phase, [pic]- chemical potentials  of
supersaturated  solution  and  firm  phase  with  concentration   C1,2   and
coefficient of activity f1,2. In the point of maximum [pic] 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
           [pic],
With the condensation of the factor of crystallisation Wc , the  probability
of crystal cluster formation Wk is
           [pic].
      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 [pic], i.e. growth or disintegration of crystal particles.
      Probability of elementary transitions we shall designate  Pa(  a(r0  ,
[pic]. Their speeds [pic]. 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
           [pic].
Resulting speed of direct and return transitions
           [pic].
At balance state
           [pic]
Proceeding from this it is possible to find out
           [pic]
Further we shall define A1 and A2. Proceeding from this it  is  possible  to
calculate the speed of cluster formation
            [pic]
      With the help of this formula it is possible to  define  the  laws  of
formation of the passive layer on the sector II (in pic.1).
      Then taking in to account the energetic inhomogeneity of metal surface
it is possible to find out the integrated current density
      [pic]
where (- bond energy.
      To each pair value of [pic] corresponds the certain probability  I((,
(i) of formation of twin cluster and the local degree of filling [pic]  by
them ith platforms of the surface  M.  With  the  growth  of  potential  (
formation of cluster becomes more  and  more  intensive.  And  accordingly
grows the integrated degree of its filling[pic] by cluster,
           [pic]
      Thus the processing of the first thin superficial layer  of  metal  in
oxide is finished. Take place complete passivation of  the  surface  M,  the
sector II on the curve (fig. 1) is replaced by the  sector  III,  for  which
the  new  physical  conditions  must  be  taken  in  account.  And   further
researched may be done.
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pic. 1

(ПП

(0

(П

IV

III

II

I

lg ia((a)

(а