Improving the Corrosion Behavior of Ductile Cast Iron in Sulphuric Acid by Heat Treatment

Corrosion behaviour of DCI in H
2
SO
4
 solution 
The anodic potentiodynamic polarization curves of DCI in H
2
SO
4
solution was carried out under different conditions of 
acid concentration, scan rate and temperature. 
Figures 2 and 3
illustrate the anodic potentiodynamic E-i curves of the 
as-received DCI (specimen No. 0) and the heat- treated specimen's (Nos. 1-5) in 1.0 M and 2.0 M H
2
SO
4
respectively 
at 30
º
C. It was found that, the heat treatment as well as the change in acid concentration has no significant influence on 
the general features of the polarization curves. Inspections of these curves reveal that: on positive going scan, cathodic 
current densities, i
c
corresponding to hydrogen evolution reaction decreases gradually and reach zero values at E
corr
(zero current potential). However, the anodic parts of the curves are complicated, and each curve exhibits an active 
to passive transition state. In the active dissolution region, the anodic current density i
a
corresponding to the anodic 
dissolution of Fe, increases exponentially with the applied potential according to the following equation: 
i
a
= K e
(αzFE/RT) 
(1) 
where, K is the rate constant, α is the transfer coefficient, E is the anodic potential, z is the number of electrons and 
F is the Faraday's constant. The active region is followed by a narrow limiting current region associated with current 
oscillations. At certain potential (passivation potential), E
pass
, the anodic current drops to a very small current value 

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Ibrahim MAM, et al.
Der Chemica Sinica, 2017, 8(6):513-523
Pelagia Research Library
516
i
pass
indicating the onset of permanent passivation. In E-i curves, the height of the anodic dissolution current before the 
current oscillation region represents the critical current density i
p
. Current oscillation phenomenon is usually observed 
during the anodic electro dissolution-passivation of iron and steel in sulphuric acid solutions [15].