2.4.2. Anticorrosion performance evaluation
The epoxy coatings, applied on the surface treated samples,
were immersed in the 3.5 wt% NaCl solution up to 30 days. Then,
the electrochemical impedance spectroscopy (EIS) was utilized to
investigate the corrosion protection properties ofthe epoxy coating
on the steel substrates without and with Zn and Zn-PVA conversion
coatings. The experiment was done by an AUTOLAB G1 at
amplitude and frequency range of ±10 mV and 10 kHz–10 mHz,
respectively. Also, the measurements were performed in a conventional
three-electrode cell including coated steel specimen as
working electrode, platinum as counter electrode and Ag/AgCl (KCl
3 M) as reference electrode. The impedance data were obtained at
open circuit potential (OCP) on 1 cm2 area of each sample after different
immersion times in the 3.5 wt% NaCl solution. Each test was
done three times for confirming the accuracy of the results. Also,
the impedance data were analyzed by Ivium software. All of these
experiments were conducted at room temperature. Also, the test
was done on the three replications in order to ensure the repeatability
of the measurements.
2.4.3. Cathodic disbonding measurements
Epoxy coatings were applied on the steel substrates with and
without ZnandZn-PVAchemicaltreatments. The cathodicdisbondment
of the epoxy coating was determined. For this purpose, 5 mm
artificial hole was drilled at the center of 9 cm2 painted samples.
The measurements were done on the samples immersed in the
3.5 w/w NaCl solution until the epoxy coating was disbonded from
the steel substrate. The experiment was conducted at a polarization
potential of –1.34V (compared with Ag/AgCl) for 100 h. After
polarization, the cell was dismounted from the samples and the
average disbonded area was measured on three replications.
2.4.4. Pull-off adhesion strength measurements
The adhesion strengths of the organic coatings were measured
on the samples treated by zinc phosphate conversion coatings without
and with PVA. The test was done by Posi test pull-off adhesion
tester (DEFEELSKO) before and after 30 days’ immersion in the
3.5 wt% NaCl solution.
3. Results and discussion
3.1. Surface characterization
3.1.1. SEM analysis
The surface morphology of the steel samples treated by Zn and
Zn-PVA was studied by SEM. The SEM micrographs are displayed
in Fig. 2.
From the SEM micrographs it is clear that PVA changed the
morphology of the zinc phosphate conversion coating considerably.
Both size and shape of the phosphate crystals were changed
after addition of PVA. It can be seen from the SEM micrographs that
addition of PVA to the phosphate coating increased the population
2.4.2. Anticorrosion performance evaluation
The epoxy coatings, applied on the surface treated samples,
were immersed in the 3.5 wt% NaCl solution up to 30 days. Then,
the electrochemical impedance spectroscopy (EIS) was utilized to
investigate the corrosion protection properties ofthe epoxy coating
on the steel substrates without and with Zn and Zn-PVA conversion
coatings. The experiment was done by an AUTOLAB G1 at
amplitude and frequency range of ±10 mV and 10 kHz–10 mHz,
respectively. Also, the measurements were performed in a conventional
three-electrode cell including coated steel specimen as
working electrode, platinum as counter electrode and Ag/AgCl (KCl
3 M) as reference electrode. The impedance data were obtained at
open circuit potential (OCP) on 1 cm2 area of each sample after different
immersion times in the 3.5 wt% NaCl solution. Each test was
done three times for confirming the accuracy of the results. Also,
the impedance data were analyzed by Ivium software. All of these
experiments were conducted at room temperature. Also, the test
was done on the three replications in order to ensure the repeatability
of the measurements.
2.4.3. Cathodic disbonding measurements
Epoxy coatings were applied on the steel substrates with and
without ZnandZn-PVAchemicaltreatments. The cathodicdisbondment
of the epoxy coating was determined. For this purpose, 5 mm
artificial hole was drilled at the center of 9 cm2 painted samples.
The measurements were done on the samples immersed in the
3.5 w/w NaCl solution until the epoxy coating was disbonded from
the steel substrate. The experiment was conducted at a polarization
potential of –1.34V (compared with Ag/AgCl) for 100 h. After
polarization, the cell was dismounted from the samples and the
average disbonded area was measured on three replications.
2.4.4. Pull-off adhesion strength measurements
The adhesion strengths of the organic coatings were measured
on the samples treated by zinc phosphate conversion coatings without
and with PVA. The test was done by Posi test pull-off adhesion
tester (DEFEELSKO) before and after 30 days’ immersion in the
3.5 wt% NaCl solution.
3. Results and discussion
3.1. Surface characterization
3.1.1. SEM analysis
The surface morphology of the steel samples treated by Zn and
Zn-PVA was studied by SEM. The SEM micrographs are displayed
in Fig. 2.
From the SEM micrographs it is clear that PVA changed the
morphology of the zinc phosphate conversion coating considerably.
Both size and shape of the phosphate crystals were changed
after addition of PVA. It can be seen from the SEM micrographs that
addition of PVA to the phosphate coating increased the population
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