Phenomenon of passivity of metal
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Answer: The phenomenon of passivity in metals was first described by
M. V. Lomonosov [3] in 1738; it was illustrated by the action of nitric
acid on iron. J. Keir [4], in 1790, independently of Lomonosov, described
and investigated in greater detail the phenomenon of passivity
of iron in nitric acid.
From then to the present, the phenomenon of passivity in iron and
other metals and alloys has been the subject of investigation by
physical chemists, corrosion scientists, and electrochemists, in
particular. Hundreds of papers relating to various aspects of the
passivity phenomenon have been published. Such widespread interest
in this phenomenon was provoked by its complexity and by the practical
importance of passivity for increasing the corrosion resistance
of metals and alloys.
DEFINITION OF PASSIVITY
Passivity can be defined as the stable inhibition or stifling of any
action, process, or reaction. In this respect, the term passivity can
be construed as any improved corrosion resistance of a metal in an
electrolyte, whatever the cause. However, the general opinion expressed
in the scientific literature clearly indicates that not all cases
of improved corrosion resistance can be considered as being caused
by passivity. Instead, the term usually applies only to metals thermodynamically
unstable under the given conditions, the improved corrosion
resistance of which is accompanied by a simultaneous shift of the
electrode potential in the positive direction. For this reason, the
resistances of noble metals in many electrolytes, of iron in neutral
solutions in absence of oxygen or during cathodic polarization, and of
spectroscopically pure zinc in dilute acid solutions, or iron in acid
9
N. D. Tomashov et al., Passivity and Protection of Metals Against Corrosion
© Plenum Press 1967
10 THE PHENOMENON OF PASSIVITY IN METALS
solutions to which arsenic compounds have been added, are, as a rule,
not called passive, since, in these cases, the corrosion resistance
is not associated with the shift of potential in the positive (cathodic)
direction. The increased corrosion resistance in these cases is
determined by an entirely different mechanism from that usually
involved in passivity.
Derived from modern concepts of electrochemical corrosion, the
phenomenon of passivity can be defined scientifically on the basis of
change in the factors controlling corrosion. It is known [1] that the
rate of a thermodynamically probable electrochemical corrosion
process is determined by equation (1) (p. 5), which shows that the
corrosion rate depends on the degree of thermodynamic instability,
i.e., the emf of the corrosion process equal to E~-E~, and on the
resistance of the electrochemical processes as a series of consecutively
connected steps. The principal resistive factors in the corrosion
system are usually the cathodic (P d and anodic (P a) polarization.
Analysis of numerous examples of passivity shows that in all
cases of improved corrosion resistance, there is a sharp increase of
anodic polarization, that is, a sharp increase in the inhibition of the
anodic process.
It is therefore logical to define passivity on the basis of the
controlling factor in the corrosion system [5]. In other words,
passivity can be defined as a state of improved corrosion-resistance
of a metal or alloy (under conditions where, from a thermodynamical
paint of view, the metal or the alloy is reactive), accounted for by
inhibition of the anodic process of metal dissolution.
In many cases, improved corrosion resistance of metals is
associated primarily with the thermodynamic stability of the system
or with cathodic polarization or ohmic resistance. These examples,
based on the present definition of passivity, cannot be associated with
passivity.
The present definition of passivity makes it possible to calculate
approximately* the degree of passivity of a metal in a given corrosive
medium from the magnitude of anodic control [ea =/). Va/(E~ - E~)]
or from the ratio of anodic inhibition (anodic polarization) to cathodic
inhibition (polarization) [1T = /). val/). V c].
Figure 2 shows curves for change in degree of passivity of iron in
neutral aerated solutions determined from the change in magnitude of
7T = /). Va/!1 V c and the magnitude of anodic control [ea = /). Va/(E~ - E&)]
as a function of the corrosion potential of iron. It can be seen that
*More precisely. the degree of passivity at each potential can be determined from the tangent
of the angle at a given point of the anodic polarization curve.
Copyright: Plennum Press 1967. This was taken from an actual research. and does not belong to me