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The study was executed by potentiodynamic polarization method
to investigate : (1)the effect of sodium nitrate on the
corrosion of carbon steel in sodium chloride aqueous solutions;
(2) the in- hibition effect of chromate on the corrosion of
carbon steel in sodium nitrate aqueous solution at pH.dbond.3;
(3) the effect of organic solvent of isopropyl alcohol on the
corrosion of carbon steel in a similar system described above.
The results are summarized as follows : 1.The corrosion rate
increases with the increasing concentration of chloride ion,at
pH.dbond.3 and the concentration of sodium nitrate .gtoreq.0.2M
,it decreases with the increasing concentra- tion of chloride
ion. That may be caused by that the concentra- tion of nitrate
ion(powerful corrosive) on iron metal surface is decreased by
the competitive adsorption of chloride ion. 2.At pH.dbond.3,
the corrosion rate increases with the increasing concentration
of nitrate ion. This may attribute to that iron is oxydized to
ferrous ion by nitrate ion, and which is reduced to nitrogen
monoxide . At pH.dbond.5 and 11, the corrosion rate decreases
with the increasing concentration of nitrate ion, which may be
caused by nitrate ion was adsorbed and formed a passive film on
the surface of iron. At pH.dbond.7 and 9, if the con-
centration of sodium nitrate .gtoreq. 0.2M the corrosion rate
is increased by the additional crevice corrosion, which is
caused by the defects of passive film and decreasing oxygen
concentration in the crevice between the iron specimen and the
rubber cushion. 3.Chromate acts as an inhibitor in sodium
nitrate solution, probably due to the formation of a protective
filmof ferrous dichromate. 4.The effects of isopropyl alcohol
on the corrosion system are : (1) to decrease the conductivity
and oxygen concentration in solution; and (2) to reduce the
concentrations of nitrate ion and hydroxyl ion on the surface
of iron.
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