in the 0–1300 eV range. The atomic composition, after a
Shirley type background subtraction [24], was evaluated using
sensitivity factors supplied by Perkin [25]. The assignments of
the peaks were carried out by using the values reported in the
reference handbook [25], in the NIST XPS Database [26, 27]
and in the references reported in [28–31].
The corrosion resistance of the coating was analyzed by
anodic polarization tests and electrochemical impedance
spectroscopy (EIS) at ambient temperature. The anodic
polarization tests were performed in a solution containing
0.1 M Na2SO4 and 0.05 M NaCl, with an AMEL 7060
Potentiostat, using a saturated calomel electrode as reference
electrode (SCE) and a platinum electrode as counter
electrode with a scan rate of 1 mV s-1
. The EIS measurements
were carried out in the previous described
solution at the value of the open circuit potential and in a
frequency range between 105 and 10-2 Hz with a perturbation
amplitude of 5 mV. The impedance measurements
were recorded with a Solartron Schlumberger 1255 FRA
Spectrometer coupled with a EG&G 273A Potentiostat and
the ZView software was used for the fitting of impedance
spectra.
3 Results and discussion
During the PEO process a large number of sparks/microdischarges
occurred on the surface of the samples; the
number of these sparks increased with the current density
and the treatment time. The treatments were performed
working at constant current density, and monitoring the
potential. A potential increase was observed during the
process of 60 s, and depended quantitatively on the current
density applied. In fact, for low current densities a potential
increase of about 10–20 V was measured, whereas the
increase was more important (about 40–50 V) for the
treatments carried out at high current densities (see
Table 2).
The morphology of the surface layers, on commercially
pure magnesium, obtained with PEO process at different
current densities for the same time (60 s), are shown in
Fig. 1. The surfaces of all samples showed numerous craters
and pores and the number of pores decreased with the
increase of the current density applied during the treatment.
In detail, passing from the sample treated at 0.1 A cm-2 to
the sample treated at 0.45 A cm-2
, there was a significant