surface pretreatment by phosphate conversion coatings – a review

156 T.S.N. Sankara Narayanan
Fig. 9. DSC curves of unmodified zinc phosphate
coating (a) before stoving; and (b) after stoving and
rehydrated at 100%RH for 63 hours. (Source: R.A.
Choudhery and C.J. Vance in Advances in Corrosion
Protection by Organic Coatings, D. Scantelbury
and M. Kendig (Eds.), The Electrochemical Society,
NJ, 1989, p.64. Reproduced by permission of
The Electrochemical Society, Inc.).
to similar conditions (Fig. 10) [313]. The main limitation
of DSC is that the phosphate coating has to
be physically removed from the substrate.
Handke has demonstrated the use of FTIR to
study the mechanism of phosphate coating formation
[327]. FTIR is sufficiently sensitive to distinguish
between different types of coatings in terms
of composition and crystallinity [313]. FTIR is also
sensitive to the degree of hydration of phosphate
coatings. The FTIR spectra of nickel and manganese
modified zinc phosphate coating on cold rolled
steel (CRS) and zinc coated steel (ZCS) under conditions
namely, before heating, after stoving at 180
°C for 20 minutes and after exposure to 100% RH
for 14 days was analyzed (Figs. 11 and 12). The
assignments of the FTIR spectra for CRS and ZCS
Fig. 10. DSC curves for nickel and manganese
modified zinc phosphate coating. Solid line represents
before stoving and broken line represents after
stoving and subsequent rehydration. (Source:
R.A. Choudhery and C.J. Vance in Advances in
Corrosion Protection by Organic Coatings, D.
Scantelbury and M. Kendig (Eds.), The Electrochemical
Society, NJ, 1989, p.64. Reproduced by
permission of The Electrochemical Society, Inc.).
are given in Tables 8 and 9, respectively. The FTIR
spectra of phosphate coatings on CRS and ZCS
are similar before heating. On heating there are significant
changes in the FTIR spectra of ZCS while
the changes for steel are less noticeable. On exposing
to 100% RH it is evident that phosphate coatings
formed on ZCS shows greater tendency to rehydrate,
indicated by the increase in intensity of all
peaks associated with water vibrations. On the other
hand phosphate coating formed on CRS exhibit very
little tendency to rehydrate; the spectrum essentially
remains constant.
The infrared and Raman spectra of α-hopeite and
phosphophyllite are shown in Figs. 13-15 [341]. They
resemble the room temperature spectra reported by
Hill and Jones [342] and Hill [343]. The infrared and
Raman spectra of both compounds display a large
number of bands. In addition, the infrared and Raman
spectra of these compounds resemble each other
very strongly. Hence distinction of the respective
peaks in phosphate coatings is not easy. However,
there are some peaks, which are different for these
compounds, are useful in identifying these compounds.
For example the Raman bands at 1150,
1055 and 310 cm -1 are typical for α-hopeite whereas
the Raman bands at 1135, 1070 and 118 cm -1 are
useful for identifying phosphophyllite in phosphate
coating.