Final Report - Strategic Environmental Research and Development ...

Corrosion rate, i
corr
(Amps/cm 2 )
increase in Zn 2+ ion concentration, thus decreasing the susceptibility of the alloy to localized
corrosion.
10 -5 0 80 160 240 320 400 480 560 640
La
Pr
Ce
10 -6
Zn
Inhibitor concentration (ppm)
Figure 3.24. Comparison of the rates of corrosion at different inhibitor chloride
concentrations (obtained from polarization studies)
The effect of Zn 2+ concentration on the cathodic polarization response from 2024-T3 alloy and
the extent of corrosion inhibition by Zn 2+ ion when compared to REM cations is summarized in
Figure 3.24. Corrosion current was obtained by the extrapolation of cathodic current from the
cathodic polarization curves done in natural pH solutions with the help of Gamry Echem
Analyst TM software (explained in part 3.1). The plot shows the corrosion rate of 2024 samples
versus the concentration of cationic inhibitors added as metal chlorides in 0.1M NaCl solution.
Zn 2+ inhibits ORR when its concentration is as low as 6 ppm (0.1 mM) and reaches a maximum
at around 325 ppm (5 mM) unlike the Ce 3+ and Pr 3+ cations which exhibit concentration windows
in which their inhibition is maximum. This range is typically 100 – 300 ppm. This means that
excessive REM cation concentrations are not effective inhibitors while the inhibition by Zn 2+
cation is still at its maximum at concentrations above 5 mM. This critical concentration of 5 mM
for Zn 2+ has been chosen for the speciation calculations and electrochemical experiments as
mentioned above in solutions at different pH. Among all the cationic inhibitors, Zn 2+ inhibited
the cathodic kinetics to the maximum extent by almost an order of magnitude while the
lanthanides suppress the kinetics by less than order of magnitude of current.
5.3.2.5.1.1 ORR inhibition on Cu RD. 1 mM Zn 2+ additions to 0.1 M NaCl solution were
observed to suppress the mass transport limited ORR kinetics on Cu RDE by about two orders of
magnitude at 2000 rpm. Levich representations of ORR current as a function of rotation rate at a
bias of 0.75 V, 0.8 V, 0.85 V, 0.9 V and1 V vs SCE for the chloride-only solutions and at a bias
of 0.65 V, 0.675 V, 0.7 V, 0.725 V, 0.750 V vs SCE for with 1 mM Zn 2+ additions is shown in
Figure 3.25. As described in section 10.3.1.4.2, the ratio, R, which is the ratio of the slope of the
Levich plots of the Zn 2+ -bearing solution (k Zn ) to that of the slope of chloride-only solution (k),
is a quantification factor where low value of R implies good corrosion inhibition. The slope of
the fit of the chloride-only solution k is written as,
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