2. Experimental details
2.1. Cathodic polarization curves
Cathodic polarization curves were galvanostatically
obtained in a current density range of 1 to
1000 A m)2
. Square plates of mild steel AISI 1008
(7.40 · 10)5 m2
) were used as working electrodes, while
a 1.90 · 10)2 m2 brass plate (63% Cu, 37% Zn) acted as
counter electrode. The reference electrode was a saturated
mercury (I) sulphate electrode (SSE).
The polarization curves were obtained in the electrolytes
shown in Table 1. In these solutions, bath 1 is the
base pyrophosphate bath, where potassium pyrophosphate
(K4P2O7) is the main ligand. The other components
(additives and ligands) were added to this base
solution. The organic additives consisted of levelling
agents (butynediol and allyl alcohol) and a stressrelieving
compound (saccharin). All experiments were
carried out at room temperature, under stirring and at
pH 8.0.
2.2. Alloy electrodeposition experiments
Cu–Zn alloy electrodeposition experiments were also
performed in the solutions shown in Table 1. Fourcurrent density values were chosen from the polarization
curves previously obtained in order to produce coatings
with different compositions: 77, 116, 153 and 193 A m)2
.
The coatings were produced using the same system and
deposition conditions described in item 2.1. Plates of
mild steel AISI 1008 with an exposed area of
4.45 · 10)4 m2 were used as working electrodes.
Each alloy coating was analysed by atomic absorption
spectroscopy (AAS) to determine the content of the
elements. Partial polarization curves were also plotted
by calculating the effective corresponding currents for
copper and zinc deposition from the element contents in
the alloy coating and the current efficiency [10]. In each
case, the partial currents were associated with the
corresponding potential response of the total applied
current density.
The influence of deposition parameters on the morphology
of copper–zinc coatings was analysed by
scanning electron microscopy (SEM).
2.3. Corrosion experiments
The coatings obtained in the solutions shown in Table 1,
at 116 A m)2
, were electrochemically evaluated by
anodic potentiostatic polarization curves. The experiments
were performed in a 0.5 M NaCl solution, at
pH 5.5 and room temperature. The counter electrode
was platinum, while the reference electrode was saturated
calomel (SCE).
3. Results and discussion
3.1. Cathodic polarization curves
Figure 1 presents cathodic polarization curves of steel
electrodes in the solutions described in Table 1. The
main objective of these experiments was not only to
select the current density values to obtain alloy deposits
but also to detect the effects of the addition of the
auxiliary ligand (H2PO
4 ) and/or the organic additives
on the cathodic behaviour of the solutions.
The curves in Figure 1 are almost coincident, regardless
of the bath composition used. Earlier reports [14,
19] showed that the discharge of metallic pyrophosphate-based
complexes might occur as follows:
½MðP2O7Þ2
6 Ð ½MðP2O7Þ2 þ P2O4
7 ð2Þ