Final Report - Strategic Environmental Research and Development ...

Corrosion rate ( 1/R
p
) ohm -1 cm -2
Corrosion rate ( 1/R
p
) ohm -1 cm -2
10 -4
10 -3 2 3 4 5 6 7
decarbonated and uninhibited case
10 -4
10 -5
7 h
22 h
44 h
65 h
83 h
94 h
10 -5
10 -3 2 3 4 5 6 7 8
decarbonated and inhibited case
6 h
27 h
43 h
64 h
86 h
108 h
10 -6
10 -6
pH
(a)
(b)
Figure 3.29. The corrosion rate of 2024-T3 coupons immersed in decarbonated (a)
sodium chloride solutions (b) zinc containing sodium chloride solutions at different pH
(obtained from the EIS measurements).
XPS surface analysis results are consistent with this idea where negligible carbonate hydroxide
compound was seen on the sample in decarbonated pH 4 Zn 2+ -containing solutions, in contrast to
the aerated pH 4 Zn 2+ -containing solution which had a relatively higher concentration of the
carbonate compound. Hence, CO 2 -bearing precipitation aids in retarding the corrosion reactions
in Zn 2+ -containing chloride solutions.
5.3.2.5.3 Mechanism of inhibition. A mechanism is proposed to explain the inhibition by the
Zn 2+ in acidic (pH 3), mildly acidic (pH 4) and neutral (pH 7) environments. AA 2024-T3 is
passivated at pH 7 except at the intermetallic particles as the oxide film is much thinner at the
intermetallic particles. The alloy is susceptible to localized corrosion due to galvanic coupling
between the Cu-rich sites within the intermetallic particle and the anodic constituents of the
active intermetallics or between the Cu-rich sites and the matrix surrounding the intermetallic
particles. These Cu-containing particles support ORR and dissolve the matrix surrounding the
particle leading to pitting or trenching of the alloy or dealloying of the active intermetallics [8].
ORR leads to an increase in alkalinity and drives the local pH on the intermetallic particle
upwards. This local increase in pH lowers the solubility of Zn 2+ cation adjacent to the
intermetallic particles resulting in massive precipitation, which eventually leads to exhaustion of
the Zn 2+ ion. Corrosion process is followed by precipitation. Higher corrosion rate is seen at
shorter periods of exposure time and then a decrease in the corrosion rate results due to increased
precipitation with increase in exposure time as seen in Figure 3.27b. This decrease in the
corrosion rate was observed after the formation of an effective physical barrier to ORR, which
causes localized inhibition. The massive localized precipitation was evidenced in the SEM
pictures of the free corrosion sample in pH 7 solution where the entire surface was covered with
crystalline deposits of Zn-Al carbonate hydroxide. The deposits were densely populated over the
intermetallics and had openings or defects at pits, trenches in and around the intermetallic
particles, through which corrosion can continue to occur.
Al dissolves in acidic pH seen in the Pourbaix diagrams. The entire surface of the Al alloy is
activated at pH 4 including the intermetallic particles and the matrix. The alloy is subjected to
pH
139