Characteristics of plasma electrolytic oxidation coatings on 6061 al alloy prepared at different current densities

CD increased from 5 A/dm
2
 to 12.5 A/dm
2
, respectively. 
Figure 2. Voltage transients during PEO processing under various 
applied current densities (5 ~ 12.5 A/dm
2
). 
Characteristics of plasma electrolytic oxidation coatings on 6061 AL alloy... 
703 
Table 1. PEO coating characteristics under the different applied current densities. 
PEO Current Density (A/dm
2
) blank 5.0 7.5 10.0 12.5 
Respond voltage after 30 min. (V) - 481 484 493 502 
Thickness (m) - 12.7 1.5 16.4 3.3 33.4 5.1 44.8 8.2 
Growth rate (m/min) - 0.423 0.547 1.113 1.493 
Micro-Hardness (HV) 60 1211 47 1247 36 1290 32 1248 66 
3.2. Effects of current density on structure and hardness of the PEO layers 
Figure 3 shows surface morphology of Al-PEO coatings fabricated by different CDs. The 
images indicate that the ratio of agglomerate-bumps-region/flatten-region is higher with lower 
applied CD. This phenomenon is due to incomplete process occurring under lower applied 
energy. However, cracking occurs on coating surface under 12.5 A/dm
2
 applied CD due to the 
thermal stress. On the other hand, the relatively large pores with less porosity only occur at the 
condition of 12.5 A/dm
2
. 
Figure 3. SEM images of surface morphology on PEO coating of 6061 Al-alloy under current 
density of (a) 5.0 A/dm
2
, (b) 7.5 A/dm
2
, (c) 10.0 A/dm
2
 and (d) 12.5 A/dm
2
. 
Quang-Phu Tran, Van-Da Dao, Van-Hoi Pham 
704 
Figure 4. XRD patterns of PEO coatings on 6061 Al-alloy 
under various current densities: a) 5.0 A/dm
2
, b) 7.5 A/dm
2
, 
c) 10.0 A/dm
2
, and d) 12.5 A/dm
2
. 
XRD patterns of Al-PEO 
coatings under various current 
densities are shown in Figure 4. The 
XRD patterns indicate the main 
phases of Al (JCPDS card No. 04-
0787), -Al2O3 (JCPDS card No. 
10-0425), W (JCPDS card No. 04-
0608), and α-Al2O3 (JCPDS card 
No. 10-0173). α-Al2O3 phase can be 
slightly identified when increasing 
CD, in which the input energy is 
high enough to cause 
recrystallization [2], however, the 
signal of α-Al2O3 pattern was still 
feeble even under the 12.5 A/dm
2
condition of this study. The 
intensity of metallic W phase 
increases obviously with increasing 
CD. With more α-Al2O3 and 
metallic W phase present, ideally 
the coating hardness should also be 
improved. However, as shown in 
Figure 5. Cross-sectional SEM image with elements mapping of PEO layer at applied current 
density of 10 A/dm
2
: (a) Si; (b) O; (c) Al; (d) W; e) P elements; and f) SEM micrograph image. 
Characteristics of plasma electrolytic oxidation coatings on 6061 AL alloy... 
705 
Table 1, the coating with the highest hardness of 1290 HV is fabricated by using CD = 10 
A/dm
2. It can be explained by the cracking found on the coatings and the effect of tiny α-Al2O3 
phase containing is negligible. As CD is higher than 10 A/dm
2
, stronger thermal stresses will 
occur due to more energy input, larger coating thickness and/or the layer-formation rate. 
The SEM cross-sectional images with EDX mapping of Si, Al, P, O, and W elements on 
PEO coating at applied CD 10.0 A/dm
2
 were shown in Figure 5. It is seen that the Al and O 
elements are distributed homogeneously in the coating. The distribution of Si and P gathered on 
the top region of the PEO layer. However, the component of tungsten was scarce to represent the 
trend of distribution. 
3.3. Corrosion behaviors of the PEO coatings obtained by different current densities 
Figure 6 represents polarization curves of the bare and PEO coated 6061 Al alloy produced 
under various applied CDs. The extracted parameters of corrosion potential Ecorr, corrosion 
current density Icorr, cathodic and anodic Tafel slopes (c and a) are collected in Table 2. The 
polarization resistance values Rp (in cm
2
) were calculated by using Stern-Geary equation [31] 
2.302 
c a
p
Corr c a
R
I
 
 
   (1) 
and also collected in Table 2. 
In comparison with the bare 
6061 Al alloy, all the PEO coated 
samples show a decreasing in 
corrosion current density and an 
increasing in corrosion resistance 
by at least two orders of magnitude, 
with a large positive shift of the 
corrosion potential. Corrosion 
current density of the PEO sample 
with CD of 5.0 A/dm
2
 is 7.93 10 7 
A/cm
2
 which decreases to 1.28 
10
 7
 A/cm
2
 and reaches the smallest 
value of 7.45 10 8 A/cm2 when 
the applied CD is 7.5 A/dm
2
 and 
10.0 A/dm
2
, respectively. The 
higher applied CD (12.5 A/cm
2
) 
leads to an increase of corrosion 
current density (2.22 10 7 A/cm2). 
It can also be seen the polarization 
resistance is increased with the 
increase of the applied CD and reaches the highest value of 8.80 106 cm2 as the applied CD 
is 10.0 A/dm
2
. So, the coating with the best corrosion resistance is under the conditions of 
applied CD 10.0 A/dm
2
, in which the corrosion current density Icorr is 434 times smaller and the 
polarization resistance is 418 times higher than those of the bare 6061 Al. The corrosion 
resistance is affected by key factors such as thickness, porosity, coherence [32]. The reduction of 
pores and increase of coating thickness with increase the applied CD 5.0 ~ 10.0 A/dm
2
 are the 
reasons of improvement in corrosion resistance. While the appearance of large pores and cracks 
Figure 6. Polarization curves of PEO coatings on 6061 Al-
alloy processed with the different current densities. 
Quang-Phu Tran, Van-Da Dao, Van-Hoi Pham 
706 
at the applied CD 12.5 A/dm
2
 deteriorate corrosion resistance despite higher coating thickness. 
Table 2. Electrochemical parameters extracted from potentio-dynamic polarization curves of bare 
6061 Al-alloy and PEO samples with the different applied current densities. 
PEO Current Density (A/dm
2
) blank 5.0 7.5 10.0 12.5 
Ecorr (V) 1.433 1.207 1.122 0.909 1.082 
Icorr (A/cm
2
) 3.24 10 5 7.93 10 7 1.28 10 7 7.45 10 8 2.22 10 7 
c (V) 15.105 7.736 7.946 5.488 8.178 
a (V) 1.752 3.695 2.861 2.084 3.735 
Rp (cm
2
) 2.11 104 1.37 106 7.14 106 8.80 106 5.01 106 
4. CONCLUSIONS 
In this study, the influence of applied current density was investigated on the layer 
formation and performance of PEO coating on 6061 Al alloy using a DC mode. The results 
indicate that the applied CD much affects to the characteristics of PEO coatings. The coating 
thickness increased from 12.7 m to 44.8 m as the applied CD increased from 5 A/dm2 to 12.5 
A/dm
2
 which showed the corresponding growth rate from 0.423 m/min to 1.493 m/min, 
respectively. Surface morphology is characteristic of reducing the number of bulges and 
increasing flattened regions on the coating surface with the increasing of the applied CD, 
however, cracks and large pores appeared on coating surface at the applied CD of 12.5 A/dm
2
. 
XRD analysis results indicate the main phases are Al, -Al2O3, W, and α-Al2O3. The peak 
intensity of W and α-Al2O3 phases are slightly increased with increasing applied CD. Hardness 
and corrosion resistance of PEO coatings are increased with the increasing of applied CD from 5 
A/dm
2
 to 10 A/dm
2
 which shows the highest value of hardness and polarization resistance of 
1290 HV and 8.80 × 10
6
 cm2, respectively. The higher applied CD causes a decrease in 
hardness and corrosion resistance due to the appearance of large pores and cracks. The 
integration results indicate that, no matter what the coating thickness is, formation rate, the size 
of surface pores, the occurrence of cracks all impact both mechanical and electrochemical 
properties of the PEO coatings. 
Acknowledgments: This research is funded by Vietnam National Foundation for Science and Technology 
Development (NAFOSTED) under grant number 103.99-2017.69, and by Hung Yen University of 
Technology and Education under grand number UTEHY.L.2020.33. 
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