Electrolyte components play essential role in the PEO treatment and many studies have
been focused on optimisation of the electrolyte composition to obtain required coating characteristics [72-74]. A suitable electrolyte not only acts as a medium to conduct current and transmit the energy required to initiate and sustain the PEO process as well as provide oxygen required for the oxide formation process, but also promotes metal passivation and produces an insulating layer, which is essential to trigger the dielectric breakdown responsible for the onset of plasma discharge events. Components of the electrolyte solution influence coating characteristics, including surface morphology, thickness and phase composition and causing to different structure and performance coatings. Yerokhin et al [8] classified the electrolyte components which can be used to produce oxide coatings in aluminium alloy in the PEO process as follows:
1. Electrolytes that provide fast dissolution of aluminium such as NaCl, NaClO3, NaOH,
and NaNO3.
2. Electrolytes that provide slow metal dissolution as H2SO4, Na2SO4.
3. Electrolytes that promoting metal passivation in close range of voltage such as H3PO4.
4. Electrolytes characterised by complex behaviour such as potassium fluoride and sodium fluoride.
5. Electrolytes promoting weak passivation of the metal.
6. Electrolytes promoting strong metal passivation, e.g. H3BO3, H2CO3, H3PO4
Fig. 2.11 shows typical polarisation curves of Al between the current and voltage in the six groups of electrolytes considered for the PEO treatment [8].
The electrolyte solutions from groups 4-6 make sparking voltage easy to obtain, which is highly helpful for production of ceramic coatings using PEO process.
Fig. 2.11 Different electrolytes solutions tested for PEO treatment of aluminium alloy [7].
The last groups of electrolytes were categorised into four sub-groups regarding to their contribution to the coating composition as follows;
i. Electrolytes that include only oxygen into the coatings;
ii. Electrolytes including anionic and cationic components that incorporate other elements into the coating;
iii. Suspensions that provide transportation of particles which contribute to the coating composition
Electrolytes from sub-group (ii) provide possibilities to change the coating properties and enable coating formation by substrate oxidation and electrolyte substances depending on the substrate surface. So, these electrolytes are considered to be the most promising in PEO processing for different applications. Therefore, to assist the conditions required for dielectric breakdown, electrolyte additives such as silicates and phosphates which promote metal passivation are widely used as the basic ingredients of the PEO electrolytes. The solutions may
22
studies [8, 74-78] have analysed the effects of electrolyte composition, concentration and pH on the PEO treatment of aluminium. Kai Wang et al. [74] produced PEO coatings on Al alloy in silicate and aluminate solutions with and without sodium fluorosilicate, and found that main phases are γ-Al2O3 and α-Al2O3; furthermore, mullite particles were generated around the plasma discharge channels, especially in silicate electrolytes. Also the surface morphologies of PEO layers produced in aluminate electrolytes resulted pancake structures while the volcano structures formed in the layer prepared in silicate electrolytes. Alhosseini et al [77] studied PEO treatments of 6061 Al alloy in electrolytes with two concentrations of KOH, 2 and 4 g/l and similarly for Na2SiO3. It was observed that the increase in concentration
of KOH causes augmentation of the electrolyte electrical conductivity and consequently reduces the sparking voltage, which promotes formation of PEO coatings with finer features. Higher concentrations of Na2SiO3 in the PEO electrolyte result in increased Si content in the
coating, which causes to increase in the coating thickness and corrosion potential. Guohua et al. [79] produced ceramic coatings on aluminium substrate by PEO in two kinds of electrolytes (silicate and phosphate solutions). Alpha, gamma and mullite phases were found in the coatings. Silicon located mainly in the outer region of the coating, while phosphorus distributed homogeneously across the coating.
Investigations into the influences of different processing parameters such as treatment time, electrolyte concentration and current mode on the PEO coating characteristics provide significant amount of detailed information. However, there are specific requirements in the direction with the required applications that give a range to control the process parameters. For instance, many studies reported that the coating porosity and average pore diameter can be increased by either increasing the supplying energy density (high voltage/current density, longer pulse time) or by increasing the electrolyte conductivity or by increasing PEO treatment time.
Appropriate electrolytes not only enhance metal passivation which is necessary to induce discharge events via dielectric breakdown but also affect, as a conductive medium, the current distribution during the PEO process and provide essential oxidising agents to form the coating [47]. Alkaline electrolytes containing phosphate and silicate anions in presence of potassium hydroxide are commonly used in PEO process to produce oxide layers on
aluminium [80-83]. Table 2.4 shows some of electrolyte compositions which used in PEO process.
Table. 2.4 Electrolyte compositions used to create PEO coatings on aluminium alloys and coating phases formed.
The electrolyte components and their concentrations change during the treatments, strongly influencing the properties of resulting coatings [89]. For instance, Wang et al. [74], in discussion of electrolyte effects on the characteristics of PEO coatings stated that the addition of Na2SiF6 may accelerate the growth of the oxide films and substantially increase
the hardness by enhancing the density of the coating. Lee et al. also reported that the dielectric breakdown voltage of the oxide layer decreases with increasing of sodium silicate and the time to reach dielectric breakdown be shortened on the time-voltage curve. On the other hand, it has been reported that the breakdown voltage was found to be higher in electrolyte containing phosphate than in silicate electrolyte [47]. Also Polat et al. in [76] reported that both dense and porous layer of coatings thickness increase with increasing of sodium silicate concentration in the electrolyte, However the increase in the thickness of the dense layer is less than that in the outer layer of the coatings. Due to the incorporation of silicon into the oxide, it has been found that increasing concentration of sodium silicate leads to an extension of the inner dense layer, which is composed mostly of γ and α-Al2O3 phases,
with some complex Al-Si-O phases [8]. These results also consistent with Jong Kim et al. [78] who studied PEO coatings formed on Al alloy in solutions of KOH with different concentrations of Na2SiO3, and found that the thickness and roughness of the ceramic oxide was markedly
increased by silicate additions. Extensive research of these effects has been performed for to
Substrate Electrolyte composition Phase composition of coating
Al [84] 30 g/l Na2SiO3; 10-40 g/l NaOH α-Al2O3, γ- Al2O3, mullite, Al2SiO5 Al 2024 [85] 2-5 g/l Na2SiO3; 3-5 g/l NaOH;
1 g/l organic agent α-Al2O3, γ- Al2O3
Al 2017A [86] 0-8 g/l Na2SiO3; 2 g/l KOH α-Al2O3, γ- Al2O3, mullite
Al 2024 [87] 20 g/ l Na2SiO3 γ- Al2O3 dominant, α-Al2O3, mullite, δ- Al2O3
24
Alumina exists in a number of crystalline phases, with α, γ, and θ polymorphs being most abundant. The α phase of alumina is a thermodynamically stable phase and possesses good mechanical properties. Combined with chemical inertness, this makes it one of the most technologically important ceramic materials [92, 93]. However alumina is normally considered brittle below ∼ 1000°C [94, 95].
Earlier literature onPEO of aluminium alloy is summarised in Table 2.5. The scientific studies include characterisation of growth and corrosion behaviour of PEO coatings on aluminium produced in different electrolytes under different current modes. Therefore, from the above it’s concluded that the composition of the electrolyte plays a very important role in the PEO process. It can effect a wide range of coating properties such as the morphology and microstructure, growth rate and composition, strength of adhesion to the substrate, micro- hardness, and tribological properties. Although published studies on PEO electrolytes has been on the development of composition and concentration to achieve desirable coating properties, still extensive researches on the effects of the electrolytes compositions nowadays. As treatment time parameter the electrolyte composition effects on the PEO process of AL foil will be discussed in details next chapters.
Table 2.5. Conditions of PEO treatment of aluminium reported in literature.
Ref. Author Substrate Electrolyte Parameters Study aim and results of electrolytic characteristics influences
[46] Snizhko et al 6082 Al alloy 0.5–2.0 g/l solutions of KOH
Current density applied is 46.7-140.7 mA/cm2 Overall current efficiency of the oxide film growth
ranges within the 10–30% and reduces significantly with increasing KOH concentration. Achievement of high voltage. Lower electrolyte concentration promotes oxide film formation and hinders Al dissolution.
[70]
Dong –Gun et al 7075 Al alloy 8 g/l Na2SiO3
2 g/l Na2SiF6
2 g/l NaOH
AC voltage at 50–Hz frequency (200 V) with constant 260 V DC.
Four different treatment time; 5, 30,45 and 60 min.
Increase the pore size with the increase of time. Ceramic coatings mainly consist of mullite, α-Al2O3
and γ-Al2O3 phase.
The thickness of the coating increases with the increase in time. However, the ceramic
coatings formed in 45 min 60 min have more porous coatings than coatings formed in 30 min.
[74]
K. Wang et al 6061 Al alloy 0-8 g/l Na2SiO3
0-8 g/l Na2AlO2
0-0.5 g/l Na2SiF6
2 g/l NaOH
Applied voltage; 200 V AC; (60 Hz), 260V DC- treatment time; 5 min-
γ-Al2O3, α-Al2O3 and mullite the main phase.
Addition of Na2SiF6 can accelerate the growth of the oxide layers and increase the microhardness by enhancing the layer density.
[86]
Aytekin Polat et al 2017A Al alloy - 2 g/l KOH, dis. water
- 4 g/l ·Na2SiO3·5H2O, 2 g/l KOH, dis. water - 8 g/l Na2SiO3·5H2O, 2 g/l KOH, dis. Water
Current density 0.150 A/cm2
Treatment time; 150 min.
Conductivity (mS/cm); 7.74, 11.2, 18.1
Increasing the sodium silicate concentration in the electrolyte leads to increase both the thickness of the dense and porous outer layer of the coatings. The coating produced in the electrolyte with low sodium silicate concentration has higher microhardness values and better wear resistance than the one formed in the electrolyte with high sodium silicate concentration and in the electrolyte without sodium silicate.
26 [96]
R O Hussein et al 1100 Al alloy 8 g/l Na2SiO3, with KOH Constant DC and unipolar pulsed DC regimes
Frequencies varied from 0.2 to 20 kHz Current density 0.15 A/cm2
Treatment time; 0.5-65 min. Conductivity (mS/cm); 5.2 and PH ;12
The plasma was characterized by OES, and the aluminium oxide coating morphology for different current modes was determined using SEM.
[97]
Wei-Chao Gu et al Al foil 0.033M Na2SiO3
0.008M Na6P6O18
0.025-0.5M NaOH
DC power source (600 V, 20 A) Current density 50 A/dm2
Treatment time; 30-60 min. Temperature ; 303 K
Analyse the influence of electrolyte concentration and composition on PEO coating properties. The high discharge voltage also results in higher content of α- Al2O3 phase in the coating prepared in the phosphate
electrolyte than that in the coating prepared in the silicate electrolyte.
[98]
Shifeng liu et al 5005 Al alloy An aqueous alkaline
electrolyte, i.e 4 g/l KOH. Red mud as additive.
-150 to 500V, (200Hz) Current density 50 A/dm2
Treatment time; 60 min.
Possible to produce complex ceramic coating on 5005 Al alloy using PEO with red mud as an electrolyte additive. The coating surface was the colour of red mud.Thickness 80 µm was formed.
[99]
J.A. Curran et al 6082 Al alloy 3-5 Na2SiO3
3-5 g/l Na4P2O7
1-2 KOH
AC power was applied with a 50 Hz
The voltage was in the 400–600 V in the anodic half-cycle. 150–250 V in the cathodic half-cycle.
Current density; 1 KA/m2
Coatings were produced on Al substrate and characterised using profilometry, scanning electron microscopy, X-ray diffraction and nanoindentation.
[100]
X Nie et al 6082 Al ally 2-10 g/l Na2SiO3 Voltage;400-600 V, current density; 100 mA/cm2,
treatment time; 90-150 min, Temp.; 343-353 K
α-Al2O3 and γ-Al2O3 were formed; α-Al2O3 is the
dominant phase in these coatings. PEO coatings appear excellent resistance to abrasive wear and corrosion.
[101]
C.B. Wei et al 2024 Al alloy Na2SiO3 in distilled water
with other additives.
Pulsed DC, node voltage was sustained at 260 V during the measurement. Treatment time 25 min.
The current flowing through the anode plate was monitored to investigate the distribution of the anode current and the effects of distance between the cathode and specimen.
[102]
YongJun Guan et al 2024 Al alloy 20 g/L Na2SiO3 The ratio of positive current to negative current is
set to one with a current density of 0.3 mA/mm2.
Transient signal gathering system is used to study the current, voltage, and the transient wave during the PEO process. SEM, OM, XRD and EDS are used to study the coatings evolution of morphologies, composition and structure.
[103] C.S. Dunleavy et al 6082 Al ally 3-5 Na2SiO3
3-5 g/l Na4P2O7
1-2 KOH
Current density; 1500 A/m2 ; treatment time 20 min. The composition, temperature and electron density
of the plasma formed during PEO processing are inferred from characteristics of the emission spectra.
[104]
Hongping et al 6061 Al alloy 10-20 Na2SiO3
1-5 KOH
The ratio of positive current to negative current is
set to one with a current density of 8 A/dm2. Reveal the growth process of PEO coatings on
aluminium alloy by analysing conducting behavior and V-I characteristics of an aluminium electrode.