The deposits produced in solutions with addition of
allyl alcohol (baths 5 and 6) have a more regular and
refined topography, with small grains and without
cracks (Figure 9(d)). These coatings also showed the
best corrosion performance (Figure 10). Shinier and
more uniform coatings than those produced in the
polyligand electrolyte (bath 2) were obtained by direct
addition of allyl alcohol to the base pyrophosphate
solution (bath 5). This suggests that allyl alcohol may act not only as a levelling agent, but also affects alloy
electrodeposition behaviour. It has already been verified
by the partial polarization curves (Figure 7). Furthermore,
this result also indicates that allyl alcohol may be
used as a new and efficient auxiliary ligand to produce a
more stable copper-pyrophosphate-based complex.
4. Conclusions
Copper–zinc alloy coatings can be produced in pyrophosphate
solutions, with different chemical compositions
and current density conditions. The properties of
the coatings may be improved by using an auxiliary
ligand and/or a proper organic additive. The use of
monobasic phosphate as an auxiliary ligand produces
stable copper complexes and homogeneous coatings. On
the other hand, the addition of saccharin and butynediol
affects the stability of copper complexes and the quality
of the coatings. The addition of allyl alcohol improves
the morphology of the coatings, as well as their
corrosion resistance, with or without monobasic phosphate
in the electrolyte. This organic compound seems
to act not only as a levelling agent, but also as a new and
efficient auxiliary ligand, since it directly affects the
copper partial polarization curve. This fact may be
attributed to an increase in the stability of pyrophosphate-based
copper complexes.
Fig. 9. Morphology of coatings from the solutions of Table 1: (a) bath 1; (b) bath 2; (c) bath 4; (d) bath 5.
0.01 0.1 1 10 100
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
E vs SCE / V
I / A m-2
Fig. 10. Anodic polarization curves of coatings produced in the
solutions of Table 1. Electrolyte: 0.5 M NaCl, pH 5.5, 25 C. Key:
(n) copper, (h) zinc, (d) bath 1, (s) bath 2, (m) bath 3, (n) bath 4,
(.) bath 5 and (,) bath 6.