On the other hand, a phosphate coating that consists of a low P ratio, a major hopeite structure, coarse crystals (25-30 [micro]m), a comparatively thicker coating (3-3.5 g/m2), and higher porosity (2.6%) gives poor corrosion resistance.
The phosphate coating enriched with a phosphophyllite structure showed small globular crystals with less porosity, whereas a hopeite structure showed coarse crystals with high porosity and comparatively thicker coating.
But only hopeite is formed on galvanized steel and is measured as the ratio of hopeite (020)/hopeite (311) (151) (241) plane.
The [P] and [H] were measured at a peak intensity of 9.86 and 9.59[degrees], respectively, for a 20 value of phosphophyllite (100) and hopeite (020).
3[Zn.sup.2+] + 2[H.sub.2]P[O.sub.4.sup.-] + 4[H.sub.2]O [right arrow] [Zn.sub.3] [(P[O.sub.4]).sub.2].4[H.sub.2]O (Hopeite) + 4[H.sup.+] (1)
Hopeite ([Zn.sub.3][(P[O.sub.4]).sub.2].4[H.sub.2]O) has a higher alkaline solubility than phosphophyllite ([Zn.sub.2]Fe[(P[O.sub.4]).sub.2].[H.sub.2]O) [29, 32, 33].
The relatively high weight of Ni in the coating as confirmed by the EDX analysis may indicate the substitution of [Zn.sup.2+] by [Ni.sup.2+] in hopeite or the deposition of Ni in the coating.