surface pretreatment by phosphate conversion coatings – a review

158 T.S.N. Sankara Narayanan
Table 9. The assignments of the FTIR spectra of nickel and manganese modified zinc phosphate coating
on zinc coated steel (after choudhery and vance [313]).
Wavenumber Shape/Intensity Assignment
(cm -1 ) Before heating After stoving After exposure
for 20 minutes at 180 °C to 100%
RH for 14 days
3545 Sharp/Strong Free O-H Lost Water stretching
3290 V. broad/Strong OH/H 2 O stretching Broadens peaks intensify.
1635 Sharp/Strong H 2 O bending Shifts to 1649 Spectrum begins
1135 Sh./Strong P-O stretching Shoulder 1184 to resemble the
appears and one obtained for
shoulder at 1026 sample before
now a peak heating
1098 Sharp/Strong P-O stretching Shoulder 1184
appears and
shoulder at 1026
now a peak
1069 Sharp/Strong P-O stretching Shoulder 1184
appears and
shoulder at 1026
now a peak
1026 Sh./Strong P-O stretching Shoulder 1184
appears and
shoulder at 1026
now a peak
1003 Sharp/Strong H 2 O rocking Lost
925 Sharp/Strong P-O stretching –
638 Sharp/Medium O-P-O bending/ Broadens
H 2 O waging
metry of the regular tetrahedron structure by interaction
with the surrounding crystalline structure so
that the symmetry will become distorted. As a result,
the degeneracy of the vibration modes will be
united and split and hence results in four peaks.
Three peaks, except for the main peak, correspond
to the stretching vibration mode of ν , which
3
split into three peaks; the main peak around 900
cm-1 corresponds to the symmetrical stretching vi-
3- bration of ν . The level of interaction of PO with its
1 4
surroundings affects the number of basic vibrations,
the Raman band or the intensity of the Raman spectrum.
With the incorporation of manganese or nickel
in the hopeite films, the Raman band of the main
peak as well as the three peaks of ν are shifted to
3
a lower wavelength. Increasing the metal content in
hopeite films has an increasing effect on the data of
Raman band. Raman spectra showed a sensitivity
to the degree of modification of hopeite films by metal
components to a greater extent than did IR spectra.
AFM is used to study the effect of surface preconditioning
on phosphatability of zinc coated steel.
AFM is useful in getting a better understanding of
the activation process [328].
Quartz crystal impedance system (QCIS) was
used to study the formation of phosphate coating
from a manganese modified low-zinc phosphating
bath [329]. It involves rapid and simultaneous measurement
of admittance spectra of the zinc coated
piezoelectric quartz crystal resonator (PQC). By
measuring the equivalent circuit parameters and frequency
shift of the zinc coated PQC resonator, the
growth kinetics of the phosphating process can be