surface pretreatment by phosphate conversion coatings – a review

146 T.S.N. Sankara Narayanan
Fig. 6. X-ray diffraction pattern of zinc phosphate coating formed on steel (a) as-prepared; (b) partially
dehydrated at 150 °C for 1 hour; (c) partially dehydrated and rehydrated at 95% RH for 70 hours. (Reprinted
from Journal of Molecular Structure, Vol. 511-512, A. Stoch, Cz. Paluszkiewicz and E. Dlugon, An effect of
methylaminoethoxysilane on zinc phosphate rehydration, pp. 295-299 (1999) with permission from Elsevier
Science).
ished or disappeared and new lines were observed
at 10.90, 11.20 and 22° 2θ. During rehydration the
line at 11.20° 2θ diminished while that at 9.65° 2θ
grew (Fig. 6). It has been observed that the rehydration
of the phosphate crystals induces residual
stresses and reduce the phosphate-paint film adhesiveness
[150].
Numerous research reports are available to date,
which elaborate the behavior of phosphate coatings
during chemical and thermal aggressions. The dissolution
of zinc phosphate coating in high-pH environments
was first reported by Wiggle et al. [142].
Later, using sodium hydroxide as a simulated medium,
Roberts et al. [143] demonstrated the selective
leaching of phosphorous from the coating. Van
Ooij and de Vries [144] interpreted X-ray photoelectron
spectroscopic studies to show that hydroxyl
ions exchange phosphate ions in the first few layers
of the phosphate coating. Servais et al. [140]
determined the chemical stability of zinc phosphate
coatings by subjecting them to immersion treatment
in 0.8 g/l sodium hydroxide solution (pH 12.3) at 40
°C for 3 minutes. Similarly, the effects of high-pH
environments on zinc phosphate coatings were determined
by Sommer and Leidheiser Jr. [141], using
0.01, 0.1 and 1.0 M sodium hydroxide solutions.
Kwiatkowski et al. [151] recommended a borate
buffer solution containing 0.01 M ethylenediaminetetraacetic
acid (EDTA) as the medium to test the
alkaline stability of the coating and suggested its
usefulness in predicting the same at a shorter time
interval. All these studies uniformly agree that coatings
richer in phosphophyllite possess greater alkaline
solubility and can serve as effective bases
for cathodic electrophoretic painting.
Phosphate coatings undergo marked changes
when subjected to variation of temperature.
Thermogravimetric technique is usually used to
monitor these changes [152]. The changes occurring
in zinc phosphate coating on steel upon heating
was studied by Kojima et al. [148] and Sugaya
and Kondo [149]. When heated above room temperature,
a gradual loss in weight occurs. This was
attributed to a nonstructural weight loss. However,
with subsequent increase in temperature dehydration
of the constituent phases of the phosphate crystals,
occurs. The dehydration of hopeite
[Zn (PO ) . 4H2O] and phosphophyllite
3 4 2
[Zn Fe(PO ) . 4H2O] commences respectively at 80
2 4 2
and 110 °C; however, a more pronounced weight
loss occurs at approximately 150 °C, where hopeite