1070-4272/03/7602-0223 $25.00 C 2003 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 76, No. 2, 2003, pp. 223!225. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 2, 2003,
Original Russian Text Copyright + 2003 by Ovchiyan.
AND CORROSION PROTECTION OF METALS
On Passivating Action of Silicate-Alkaline Solutions on Metals
V. N. Ovchiyan
Institute of General and Inorganic Chemistry, National Academy of Sciences of the Republic of Armenia, Yerevan, Armenia
Received October 10, 2002
Abstract-A mechanism for enhancing the stability of the passive state of metals in silicate solutions is
described on the basis of a study of the corrosion-electrochemical behavior of the metals in concentrated
alkaline and silicate-alkaline solutions.
Concentrated silicate-alkaline solutions find use
in many branches of industry. These solutions serve
as technological medium in integrated processing of
silicate and aluminosilicate rocks and are used to
manufacture silicon dioxide, detergents, etc.
Corrosion of metals, caused by the media men-
tioned above, is characterized by a number of specific
features [1, 2]. Of interest in this context is a study
of the electrochemical behavior of steel in solutions
of this kind. However, only few investigations have
been carried out in this direction .
It is known that metallic construction materials
used in technology are mostly thermodynamically
unstable. They react with the ambient and are fre-
quently passivated through formation on their surface
of insoluble (or difficultly soluble) films of oxide,
hydroxide, or salt nature.
The stability of the passive state of metals depends
on the nature of a protective film and its heterogeneity.
Structural defects in the film are either due to im-
perfection of the surface structure of a metal or arise
in the course of film formation. One of ways to im-
prove the corrosion stability of a metal is to form on
its surface a less defective protective.
In the present study, the electrochemical behavior
of 08kp steel in concentrated alkaline (AS) and sili-
cate-alkaline (SAS) solutions was analyzed. Polariza-
tion curves were measured by changing the potential
stepwise, from the range of active dissolution to re-
passivation, at 0.1-V intervals and keeping the steel
at each potential for 15 min. The measurements were
done at elevated temperature in an autoclave whose
design was described in .
The solutions employed were fabricated using
sodium hydroxide of analytically pure grade and high-
purity amorphous silicon dioxide with impurity con-
tent of less than 10
Corrosion cracking was studied under continuous
stretching stress on wire samples passed through a ves-
sel with a solution.
A study of the corrosion-electrochemical behavior
of iron and steel in AS and SAS at 80oC has shown
[5, 6] that, under the given conditions, the metals tend
to undergo passivation. The region of the passive state
is rather wide and lies within 30.8_30.4 V (relative
to silver chloride electrode). The stationary potential
of iron and steel in both the solutions is within the
range 30.3_30.4 V, which points to the possibility
of a spontaneous transition of metals to the passive
state. Therefore, under standard conditions, they spon-
taneously undergo passivation in an alkaline medium
at low temperature and dissolve at a slow rate.
It has been shown [5, 7, 8] that, although the pas-
sivating layer formed on iron in AS has complex com-
position and structure, the passivation of iron is due
to formation of an exceedingly thin surface layer of
iron oxide g-Fe
The equilibrium potential of the reaction of g-Fe
formation in an AS containing 300 g l
30.896 V at 20oC. If account is taken of a minor
overvoltage, this value is in good agreement with the
experimental passivation potential of iron, equal to
30.85 V .
IR spectral and thermogravimetric studies of prod-
ucts formed in reaction of iron with alkaline solutions