density of needle like phosphate crystals and decreased the phosphate
particle size.Also, a combination of needle-like and plate-like
crystals can be seen on the SEM micrographs of the PVA containing
phosphate coating. It seems that PVA promoted the nucleation of
the phosphate crystal formation on the steel surface.
3.1.2. XPS analysis
XPS analysis was also utilized to better characterize the chemical
composition of the coating layer deposited on the surface of Zn-PVA
treated sample. The overview XPS spectrum of the Zn-PVA treated
sampke is shown in Fig. 3. Also, the high resolution XPS spectra
were obtained from deconvolution of multiple peaks in the Zn 2p,
P 2p and C 1s regions.
As can be seen in Fig. 3, two different regions were observed
in the Zn 2p and P 2p regions. The deconvolution of Zn 2p
region resulted in understanding of the presence of Zn3(PO4)2
and Zn(OH)2/ZnO components at biding energies of 1023.21 and
1022.17 eV, respectively [19]. It is clear that the peak intensity
related to Zn3(PO4)2 componentis much higher than Zn(OH)2/ZnO.
Phosphate components having PO4
3− and HPO4
2− groups are
detected at binding energies of 132.6 and 133.58 eV, respectively.
All of these observations show the presence of hopeite phase on
the steel surface. Also, three different zones are observed in the C
1s region. The binding energies for the peaks have been referenced
to C–C bond at 285 eV and C–O at 286.51 eV [42]. This can show the
presence of PVA in the zinc phosphate layer deposited on the steel
surface.
The zinc phosphate film precipitation on the steel surface in the
absence of the PVA can be defined by Eqs. (1) and (2) [40].
2Zn2+ + Fe2+ + 2PO4
3− + 4H2O → Zn2Fe(PO4)2·4H2O (phosphophyllite) ↓ (1)
3Zn2+ + 2PO4
3− + 4H2O → Zn3(PO4)2·4H2O (hopeite) ↓ (2)
It seems that PVA may promote the nucleation of the phosphate
conversion coating. It has many OH groups which highly tend to be
adsorbed on the anodic sits ofthe metal surface restricting the steel
dissolution rate. Also, PVA may form PVA-Zn2+ complexes on the
steel surface in the phosphate solution. PVA-Zn2+ complex may be
converted into PVA-Fe2+ complex on the anodic sites of the metal
surface by releasing Zn2+ ions according to Eq. (3) [41].
Zn2+-PVA + Fe2+ → Fe2+-PVA + Zn2+ (3)
The released Zn2+ could participate in the Zn3(PO4)2·4H2O film
formation on the cathodic sites.
3.1.3. Contact angle measurements
The water contact angle () values were measured on the surface
of bare steel and the samples treated by zinc phosphate (Zn) and
Zn-PVA. The results obtained are shown in Fig. 4.
Based on the results given in Fig. 4, the phosphate coating precipitation
on the steel surface resulted in a decrement in contact
angle of water. According to Fig. 4, Zn-PVA treated sample showed
a lower contact angle compared to the Zn treated one. This observation
indicates that Zn-PVA showed more hydrophilic nature than
the one without PVA. This may be attributed to both the increase
of roughness as a result of addition of PVA and the presence of PVA
in the phosphate coating structure making it more hydrophilic.
3.2. Pull-off adhesion strength measurements
Since the adhesion properties improvement of the organic coating
to the substrate is one of the important roles of the conversion