Final Report - Strategic Environmental Research and Development ...

(Figure 3.8b). However, no effect on the polarization resistance was seen with Zn 2+ additions at
pH 3.
3.2.4.7 Surface Analysis. Surface analysis of the samples immersed in 5 mM Zn 2+ ion
concentrations for a period of 5 days in different environments (aerated, deaerated and
decarbonated) was done to characterize the differences in the surface film which led to the
differences in impedance measurements and hence to aid in the understanding of the mechanism
of inhibition by the Zn 2+ ion at pH 4 and pH 7. X-ray diffraction of the sample surface was done
to identify the chemical compounds and XPS, being a surface sensitive measurement was carried
out to obtain a chemical composition with a detailed individual elemental analysis.
Q
P - Zn Al (OH) CO .4H O
6 2 16 3 2
Q - Zn (OH) Cl .H O
5 8 2 2
R - Zn(OH) .0.5H O
2 2
S - Zn CO (OH) .H O
4 3 6 2
P
Q
Q
P
pH 7 decarbonated
PQ
PS
Q P
R Q
Q
R P
QP
pH 7 Deaerated
pH 7 Aerated
pH4 Decarbonated
pH 4 Deaerated
pH 4 Aerated
5 10 15 20 25 30 35
2 theta
Figure 3.20. X-ray diffractograms of AA 2024-T3 coupons after immersion for 5 days in
0.1M NaCl + 5 mM Zn 2+ solution in different environment
Table 3.3 shows the results for the elemental chemical quantifications of the surface film on AA
2024-T3 alloy immersed in the inhibited solutions in various environmental conditions. These
were obtained from the analysis of the high resolution spectra. The percentage of carbon (C)
shown correspond to the carbonate peak and the other carbon peaks were not accounted as they
might be obtained from contamination during measurements. Figure 3.20 shows the x ray
diffraction patterns of the samples exposed to various conditions. The assignment of the
compounds was done using the JCPDS software and with the help of literature sources [56, 58].
5.3.2.4.7.1 Exposure to pH 7 inhibited electrolyte. Localized film formation on the sample
could be seen visually. The XRD of the sample in Figure 3.20 shows peaks corresponding to two
compounds, zinc aluminum carbonate hydroxide (Zn 6 Al 2 (OH) 16 CO 3 .4H 2 O) and zinc chloride
hydroxide (Zn 5 (OH) 8 Cl 2 .H 2 O). The relative XRD peak intensity of the Zn-Al carbonate
hydroxide was higher than that of the zinc chloride hydroxide. As the XRD peak intensities are
directly proportional to the concentration of the compounds, the film formed in aerated solutions
at pH 7 contains predominantly Zn-Al carbonate hydroxide and low concentrations of Zn
chloride hydroxide.
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