Enhanced Corrosion Resistance and Surface ... - Springer Link

ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2017, Vol. 53, No. 4, pp. 733–742. © Pleiades Publishing, Ltd., 2017.

PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION

Enhanced Corrosion Resistance and Surface Characterization of Anodized 7075 Al1 Rasiha Nefise Mutlu*, Başak Doğru Mert, Sevgi Ateş, Seyran Gündüz, and Birgül Yazıcı Çukurova University, Science and Letters Faculty, Chemistry Department, 01330 Balcalı, Adana/Turkey *e-mail: [email protected] Received June 8, 2016

Abstract⎯In this study, we aim to enhance physical and chemical properties of 7075 aluminum via anodizing. For this purpose, convenient potential, current density, time, electrolyte type and its concentration were determined. The corrosion performances of non-anodized and anodized samples have been investigated in 3.5% NaCl by using EIS, polarization techniques. The surface morphology was determined with SEM. As a result, additive (0.5 M C2H6O2 + 0.08 M H3BO4 + 10–3 Al+3) used in 0.4 M H2SO4 was improved the quality of aluminium oxide by blocking pores, increasing the thickness and decreasing the oxide formation potential. It was achieved corrosion resistance. Keywords: aluminum, SEM, anodic films, corrosion, colloidal film, oxide pores DOI: 10.1134/S2070205117040165

1. INTRODUCTION Anodizing is an important electrochemical surface modification method of aluminium and other metals. Anodizing is regarded as a technique for improving corrosion resistance and mechanical properties of aluminium. Aluminum anodizing process is also used in hydrophobic surfaces [1, 2], electro-catalysis [3], energy storage [4, 5], composite [6], semi-conductor [7] works etc. Aluminum surface contacts with air, therefore oxide film that’s thickness 2–9 nm is formed and aluminium is covered with this film [8]. However, this oxide film is weak; do not provide adequate corrosion resistance [9]. Aluminum corrodes easily in an aggressive environment [10]. Therefore, anodizing is performed to form the thick adherent and mechanically strong oxide coating of aluminium and aluminium alloys. Different acid bath [11–17] is used on aluminium surface to form the anodic oxide coating. Some bath commonly used sulphuric acid [11], sulphamic acid [12] phosphoric acid [13], citric acid [14], boric acid [14], phosphoric acid [15], tartaric acid and [16] various combinations of these acids [17]. Oxide film properties, depends on the anodization process parameters (current, potential, electrolyte type, etc.) and the electrodes can be counted in the elemental composition of these parameters. Anodic oxide that is including two types layer barrier-type oxides and porous oxides can occur. 1 The article is published in the original.

Anodizing of aluminum in acidic electrolytes, like sulphuric is well-established to result in a two-layer structure: initial layer is a barrier-type aluminium oxide, followed by the spontaneous formation is a porous oxide layer, depend on the anodizing conditions [18, 19]. Besides current density, the voltage and sample surface condition, the electrolyte composition (concentration) is also an important factor that deeply influences the film morphology. For example, two or more different acids are often mixed to serve as a renewed electrolyte, by which the morphology and property of the produced film can be improved. The addition of other substances into the electrolyte, such as some organic compounds, is suggested an effective method to regulate the anodization process sand film morphology [20]. The aim of this study forms compact oxide film on aluminium by anodizing process. Anodization of 7075 aluminum was performed on aluminium anode, by electrolysis in sulphuric acid electrolyte that is nonhazardous solution sand relatively cheap. The most appropriate conditions (potential, time, thickness, the electrolyte concentration and electrolyte additive) were determined. The corrosion performance of nonanodized aluminum and anodized aluminium samples (AAO) was investigated in 3.5% NaCI solution by using electrochemical impedance spectroscopy, polarization techniques. The surface morphology was investigated with SEM.

733

734

MUTLU et al.

14 I, mA cm–2

12

b

10 a

8 6 4 2 0

5

10

15 20 E, V

25

30

35

Fig. 1. Thecurrent-potential curves of Al (without abraded (a), abraded (b)) in 0.4 M H2SO4 .

2. EXPERIMENTAL 2.1. Anodizing of Aluminum The cylindrical 7075 Al has 0.785 cm2 surface area and following composition (wt %); 0.50% Fe, 0.40% Si, 1.60% Cu, 0.30% Mn, 2.5% Mg, 5.60% Zn, 0.20% Ti, 0.30% Cr). Electrode was coated with polyester and electrical conductivity was provided by a copper wire. The surface was mechanically abraded with series of emery papers (100–1200 grit). Then, it was cleaned with distilled water and dried with filter paper. This pre-treatment method compared with without pre-treatment. Aluminum oxide were prepared by anodizing method with two electrode technique help with Matrix power supply (MPS-3003L-3) instrument. The mild steel sheet (with 2 cm2 surface area) was used as the counter electrode. Anodized was accomplished in 0.4 M H2SO4 with different concentrations of several additives (boric acid, ethylene glycol, AlCl3). For this purpose 0.0–1.5 M additive concentration range was used. The most effective electrolytes concentrations were determined using the current-potential curves were symbolized as Electrolyte type 0.4 M H2SO4

symbol S

0.4 M H2SO4 + 0.08 MH3BO3 + 0.5 M Ethylene glycol (C2H6O2)

SBG

0.4 M H2SO4 + 0.08 M H3BO3 + 0.5 M

SBGA

C2H6O2 + 10–3 M Al+3 (AlCl3)

Different potential (0–30 V) was applied for determine convenient anodizing potential from the current-potential curves and the current was recorded with the aid of multi meter. Then optimal anodizing current density values were detected and it applied during the 1800 s time. The produced aluminum oxide thickness was measured by Zeiss (Supra 55) SEM instrument at high vacuum and 10.00 kV EHT every process.

2.2. Corrosion Behaviour of Electrodes The corrosion behaviour of non-anodized (Al) and anodized aluminum (AAO) in different electrolytes (S, SBG and SBGA) were studied in 3.5% NaCl. The test solution was opened to the atmosphere, and the temperature was 298 K controlled with thermostat (Nuve BS 302 serial number 03-0033). The electrochemical measurements were carried out using CHI (604 D) electrochemical analyzer under computer control. The three-electrode configuration was used. A platinum sheet (with 2 cm2 surface area) and Ag/AgCl (3 M KCl) electrodes were used respectively as the auxiliary and the reference electrodes. Electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit potential. The frequency range was between 10–2–105 Hz and amplitude was 5 mV. The EIS parameters were calculated by fitting the experimental results to an equivalent circuit using ZView software. The polarization curves were recorded after 168 h of immersion time in corrosive test solution (3.5% NaCl). The scan rate was 1 mV/s. The surface morphology (before and after corrosion) of non-anodized and anodized electrodes was investigated by scanning electron microscope (SEM). The SEM images were taken using a Zeiss (Supra 55) SEM instrument at high vacuum and 10.00 kV EHT. 3. RESULTS AND DISCUSSION 3.1. Anodizing of Aluminum In Fig. 1, the current-potential curves of aluminum electrodes were obtained by two electrode system in 0.4 M H2SO4. In this study firstly, the surface was washed with distilled water and dried (aluminum without abraded, Fig. 1a) and secondly, the surface was abraded mechanically (Fig. 1b). When the current-potential curves obtained for both electrodes (without abraded-abraded) compared, the maximum current values were determined for the abraded electrode. Figure 1 showed that, natural oxide layer was resolved by the mechanic process. The trend of curves indicates that the growth behaviour is similar at the initial state of the anodizing. High current at low potential means energy savings for anodizing process. The current-potential curves were obtained with different solutions and various concentrations of H3BO3 and C2H6O2 and presented in Fig. 2 (in 0.4 M H2SO4 + 0.04 M H3BO4 + 0.25 M Glycol (a), 0.4 M H2SO4 + 0.08 M H3BO4 + 0.5 M glycol (b) and 0.4 M H2SO4 + 0.16 M H3BO4 + 1 M Glycol (c)). Figure 2 showed that the initial oxide formation current density values were almost 6 mA cm-2. The higher current values were determined for 0.4 M H2SO4 + 0.08 M H3BO3 + 0.5 M C2H6O2 solution (Fig. 2b) which prepares the ground for corrosion-resistant controlled oxide layer which applied by the potential of the corre-

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES

Vol. 53

No. 4

2017

735

16 14 12 10 8 6 4 2 0

b c a

5

10

15 20 E, V

25

30

35

Fig. 2. The current-potential curves of the aluminum electrodes in 0.4 M H2SO4 + 0.04 M H3BO4 + 0.25 M Glycol (a), (0.4 M H2SO4 + 0.08 M H3BO4 + 0.5 M glycol (b), 0.4 M H2SO4 + 0.16 M H3BO4 + 1 M Glycol (c)).

18 16 14 12 10 8 6 4 2

b(SBGA) c

I, mA cm–2

0

a

5

10

15

20 E, V

25

30

35

Fig. 3. Thecurrent-potential curves of the aluminum electrodes in SBG + 10–2 M AlCl3 (a), SBG + 10–3 M AlCl3 (b), SBG + 10–4 M AlCl3 (c).

I, mA cm–2

sponding current response causing the closure of the surface current in the solution to permit the passage spontaneous natural oxide layer and smoother by breaking. In Fig. 3, different Al3+ concentrations were added in the SBG electrolyte. Al3+ has the common ion effect on solubility of working electrode. It is known that some of the salt will be precipitated until the ionic product is equal to the solubility of the product. In this study, (0.4 M H2SO4 + 0.08 M H3BO3 + 0.5 M C2H6O2, SBG) and different concentrations of the (10–4–10–2 M) Al3+ (AlCl3) were added to the electrolyte in order to realize anodizing process on the surface of formed oxide layer more quickly. According to the findings; 10–2 M Al3+ additive which excessive increase in the current value for the oxide layer is reduced resistance to deterioration of the surface quality. 10–4 M Al3+ is the condition in which it will provide the desired properties of the coated surface conditions cannot be created. 10–3 M Al3+ coadded with the state of the surface of the anodizing process, as shown in Fig. 3 is formed at lower potentials. Thus, 10–3 M Al3+ is added (SBGA) anodizing solution has been identified as the most suitable (0.4 M H2SO4 + 0.08 M H3BO3 + 0.5 M C2H6O2 + 10–3 M Al3+). After optimum concentration conditions in anodizing treatment and optimal solution are determined, by obtaining current-potential curves in each solution (Fig. 4) are provided to occur in a more effective way of data. If we keep in constant current 6 mAcm–2, it is seen that to begin oxidation potential different for each solution. In Fig. 4a, the anodizing process initial potential in S electrolyte is about 20 V determined from current-potential curve. The high-potential increases production costs. The current-potential curve of SBG solution (Fig. 4b) is analyzed; beginning of anodic oxidation is approximately 12 V (SBG). It is seen that oxidation initial potential is reduced. When Al3+ (Fig. 4c) added into SBG solution, anodizing creation potential obtained from the current-potential curve is observed about 10 V. In order to increase the thickness of oxide formation, it is necessary to capture the low initial potential at constant current. The most suitable electrolyte is 0.4 M H2SO4 + 0.08M H3BO3 + 0.5 M C2H6O2 + 10–3 M Al 3+ (SBGA). Oxide formation is slow at 6 mAcm–2 current. Therefore, we expectthe fastest possible oxide formation and suitable oxide thickness without deformation at 15 mA cm–2 constant current. Thickness of anodic oxide of each aluminium electrodes anodized in the S, SBG and SBGA solutions was investigated by cross section end of the 1800 s anodizing time at 15 mA cm–2 current density. The findings are shown in Fig. 5.

I, mA cm–2

ENHANCED CORROSION RESISTANCE

18 16 14 12 10 8 6 4 2 0

c b a Oxide initiation potentials

5

10

15 20 E, V

25

30

35

Fig. 4. The current-potential curves during anodizing of aluminum in different electrolytes, S (a), SBG (b) and SBGA (c).

Figure 5a shows the typical cross-sectional view of the aluminum metal. Only the natural oxide layer on the section is seen undetectably. The Fig. 5b shows the cross section of electrode anodized in S electrolyte. It is observed that consists of approximate 2 μm thick oxide layer but the oxide layer does not cover the entire


Vol. 53

No. 4

2017

736

MUTLU et al.

= 1.991 μm

1 μm (b)

(a)

1 μm

= 3.187 μm

(a)

100 nm (b)

100 nm

(c)

100 nm (d)

100 nm

= 1.113 μm = 1.743 μm = 1.819 μm

(c)

1 μm (d)

1 μm

Fig. 5. SEM images of bare Al (a) and the cross sections of the aluminum oxide layer formed in the S (b), SBG (c), and SBGA (d) solution.

aluminium surface. The Fig. 5c shows the cross section of electrode anodized in SBG. The oxide layer does cover the entire aluminium surface. The thickness of the oxide layer is clearly seen that increase up to 3.18 μm. A large part of this thickness constitutes the porous layer. It is seen that the pores are closed by colloidal particles. This situation is given as represented in Fig. 7. This structure is significant that more corrosion resistant. The Fig. 5d shows the cross section of electrode anodized in SBGA. The entire surface is covered with barrier oxide layer and the porous oxide layer formed on its surface. These two layers are observed clearly. The total thickness is 4.68 μm. This structure is more stringent and protective. In addition, colloidal film on the surface of the nano-porous layer constitutes the third layer. Pores are clogged. This formationis given as represented in Fig. 7. Figure 6a shows the typical view of the aluminum metal surface. Only cracks on the section are seen. The Fig. 6b shows the surface appearance of electrode anodized in S electrolyte. It is observed that the formation of porous oxide layer but the oxide layer does not cover the entire aluminium surface. It is full of cracks the Fig. 6c shows the surface appearance of electrode anodized in SBG. The oxide layer covers the entire aluminium surface. It is homogeneous and smaller pores. It is seen that the pores are closed by colloidal particles but some parts are still open. This situation is given as represented in Fig. 7. This structure is significant that more corrosion resistant. The Fig. 6d shows the surface appearance of electrode anodized in SBGA. The entire surface is covered with perfectly homogeneous and uniform oxide layer. Pores are clogged with colloidal particles and covered with Al-alcohol colloidal film. This structure is more strin-

Fig. 6. SEM images of aluminum oxide porebare Al (a) and anodized Al in S (b), SBG (c), and SBGA (d) samples.

gent and protective. This formation is given as represented in Fig. 7. When considered all the anodizing process; Al 3+ (aq) is formed spontaneously manner in two electrode system in 0.4 M H2SO4 electrolyte. Aluminum is dissolved more actively at high potential but it is converted to AlOH2+, Al(OH) 2+ , Al(OH)3, Al2O3,

AlHSO 24 + , Al2(SO4)3 component ions in the solution with H2O (O2(g)). The current values chance slightly by these ions. Then, anodic oxidation process starts at surface according to following equations: 3+

Al → Al (aq) + 3e

−

(1)

E = 1.663 + 0.0197 log(Al 3+ ) + 2 Al 3+ (aq) + 3H2O ↔ Al2O3(s) + 6H (aq) ,

(2)

2+ + Al 3+ (aq) +H2SO4 → AlHSO 4(aq) + H (aq) ,

(3)

+ 2 Al 3+ (aq) + 3H2SO4 → Al2 (SO4)3(s) + 6H (aq) .

(4)

The cathodic reaction is usually oxygen and/or hydrogen reduction: +

−

2H (aq) + 2e → H 2(g) (PH2 = 1 atm) E H + H = − 0.059 pH

(5)

2

−

−

1/2O 2 + H 2O + 2e → 2OH (aq) (PO2 = 1 atm) (6) E O OH − = 1.23 − 0.059 pH 2

+ O 2 + 4H (aq) + 4e − . → 2H 2O (in acidic medium)


Vol. 53

No. 4

(7) 2017


737

Al-alcohol colloidal ﬁlms (Al (OHCH2CH2O)3)

Pore cells Anodizing solution anions

Barrier layer

–3 SO–2 4 , BO 3

Pores layer

Al+3

B+3S+4Cl–1H+1O–2

Fig. 7. The representative figure, anodizing solution anions in the formation of the barrier film to the participation and block the pores of the Al-alcohol colloidal films.

As seen, O2 reduction occurs at more positive potential (1.23 V) then hydrogen reduction. Because of the low solubility of oxygen in the solution, current is limited. After that, the hydrogen ions begin to reduction. For the SBG electrolytes in presence of H3BO3 + C2H6O2 additive in anodizing bath may gains the ionic conductivity to oxide incorporate into the oxide layer and oxide could act as electronic trapping canters (Fig. 2). The related reactions: + − Al 3(aq) + BO33(aq)

→ AlBO 3( s ) (AlHBO 3+, AlH 2BO 32 + ).

(8)

Furthermore transported by diffusion toward, the anode surface is reduced on the cathode surface in ethylene glycol, and performs colloidal Al-alcohol film on the electrode surface. In this event, the reaction is as follows [21]: + − Al 3(aq) + 3OHCH 2CH 2O (aq) → Al(OHCH 2CH 2O)3( s ) (Al-alcohol colloidal film).

(9)

Glycol reaction at cathode: − + + H (aq) .(10) CH2CH2(OH)2 + e– → OHCH 2CH 2O (aq)

During anodizing process in convenient electrolyte, occurred events were described in Fig. 7. The positively charged surface of electrode (Al) is coated

with the ions in the electrolyte; HSO 4− , SO 24 − , H 2BO13− , HBO 32 −, BO 33− and OHCH2CH2O–. Depending on the applied potential, overvoltage is defeated and on the surface Al (H2O)6 ads, 2+ [Al(HSO 4 )(H 2O)5]ads ,

+ , [Al(SO 4 )(H 2O)5]ads 2+ [Al(H 2BO 3 )(H 2O)5]ads ,

+ , [Al(H 3BO 3 )(H 2O)5]3ads + [Al(BO3)(H2O)5]ads, [Al(HBO 3 )(H 2O)5]ads , [Al(OHCH2CH2O)3(H2O)5] instead of products, form passes into solution (Eqs. (1)–(10)). The surface is uniform, smooth and it has same thickness. O2– and OH– anions coating/metal interface is carried the coating formation react with metal ions occurrence continuously. Coating formation continues as being 12–14 Å/V. Also located in the anodizing solution of anions can participate in the formation of the barrier film, the barrier layer, protector and also affect the electronic properties [22, 23]. Figure 7 is shown as a representative. Reaction (2) occurred after the formation of the current electrolysis process and is reduced to a fixed value (0.0061 A cm–2) reach. In the cathode H2 (g), the rate of formation is decreased from the Al2O3 (s) [24]. According to literature, Cl– (AlCl3) ions increased the hydrogen reduction rate in which aluminum anode becomes [25]. In SBGA solution, Cl– (AlCl3) active ions accelerate encroaching through internal Helmholtz layer to pass the aluminum dissolving into solution because of


Vol. 53

No. 4

2017

738

MUTLU et al.

EIS parameters of non-anodized (Al) and anodized (S, SBG and SBGA) samples after 168 h immersion in 3.5% NaCl Rs, Ω Bare Al Al Anodized in S Al anodized in SBG Al anodized in SBGA

7.8 17.2

R1, Ω cm2 842 1000

R2, Ω cm2 953.3

R3, Ω cm2 C dl1 , μF cm–2 C dl 2 , μF cm–2 C dl 3 , μFcm–2 L, H cm2 – –

9.6 1.07

– 10

– –

1714 –

–

0.81

3.5

–

–

283970

9.6 × 10–4

0.06

0.53

–

30.000 19.2

2000 155.000

5.1

536 159880

smaller diameter, Al+3 (AlCl3) addition environment increase the electrolytic conductivity and support to increase of thickness of anodized formation without Al+3 ion diffusion from metal/oxide interface to the oxide/solution. Figure 7 shows a representative. Thus constitutes the lower potential in the thick anodized layer. The current density increases with increasing potential and reaches a maximum, the steady pores and a barrier layer with constant wall thickness are formed [26]. According to Fig. 7, SBGA electrolyte selected for this purpose. 3.2. The Corrosion Behaviour Electrochemical impedance spectroscopy (EIS) is a method commonly used in the elucidation of electrochemical processes that occur at the metal/solution interface [27–31]. Applied to the metal surface of the electrochemical impedance method of altering the surface structure of the small-amplitude alternating current, it is likely to be knowledgeable about the resistance and structure formed in the metal surface. Resistance at the point where the curve starts is solution resistance (Rs). The resistance value obtained by extrapolating the horizontal axis of the curve is polarization resistance (Rp) which contains charge transfer resistance (Rct), diffuse layer resistance (Rdl), sealed anodic layers resistance (Rf) and all other accumulated kinds (Ra), (Rp = Rct + Rdl + Rf + Ra) [32, 33]. The first loop appeared at the high frequency region is attributed to the pore resistance (R1) against the corrosion process occurring within pores of coating, the second one appeared at low frequencies related to the oxide coating resistance (R2 and R3). The related capacitance is symbolized as CPE and given as fitted, then it exchanged to Cdl for calculations;

Z CPE =

1 , Y 0(J ω) n

(11)

where Y0 and n are frequency independent parameters and –1 ≤ n ≤ 1, ω is angular frequency for which imaginary part of the impedance –Z'' reaches it maximum; 1/ n

⎛ ⎞ (12) ω=⎜ 1 ⎟ . ⎝ RctY 0 ⎠ Also double layer capacitance (Cdl) should be given as; C dl =

Y 0ω n−1 , sin(nπ 2)

(13)

calculated Cdl values are shown in table. When polarization resistance for 2 hours was examined; the lowest polarization resistance (3300 Ω cm2) was determined in the uncoated electrode. The highest polarization resistance for 2 hours (130000 Ω cm2) was found to be of the anodizing electrolyte SBGA. After 24 hours the data obtained polarization resistance respectively uncoated electrode 2360 Ω cm2, coated electrode in S electrolyte 9400 Ω cm2, in SBG 49900 Ω cm2, while in SBGA solution determined to be 190000 Ω cm2. It is seen that highest Rp belong to the anodized aluminum in SBGA solution. After 24 hours, the polarization resistance of the anodized aluminum in SBG solution to be increasing according to 2 hours standby time can show to be closing the pores of the coating after the corrosion products formed on the aluminium surface. After 48 hours immersion time, natural oxide layer formed on the uncoated aluminum electrode surface and seems to have increased the value of Rp, when compared to 24 hours immersions time Rp. At the end of 48 hours, the highest increase in value of Rp belong to anodized aluminum in SBGA (from 190000 Ω cm2 up to 341000 Ω cm2) is clearly seen. Aluminum anodized in SBGA environment said to constitute a more stringent and protective structures compared to other media. The Rp values of coated electrodes in S, SBG and SBGA electrolyte are stable. It is seen that the highest resistance and the lowest capacitance values of oxide coatings obtained with the SBGA electrolyte. According to these data, it is seen


Vol. 53

No. 4

2017


739

I –400 000

Z '', ohm cm2

a

–1500

105

–1000 –500

–50

0

10

4

500 0 500 1000 2000 0.1 Hz 1500 2500 2 Z, ohm cm

–200 000

d –200 000

0.1 Hz b

0 0

100 000

Z '', ohm cm2

Z '', ohm cm2

–300 000

θ° –75

II 108

–2000

103

–25

102 0 101

c 200 000 300 000 Z, ohm cm2

100 400 000 10–2 10–1 100 101 102 103 Frequency, Hz

104

25 105

Fig. 8. Nyquist (I) and Bode(II) curves of EIS measurements of non-anodized (a) and anodized in S (b), SBG (c) and SBGA (d) Al samples after 168 h immersion in 3.5% NaCl (fitted curves were presented as solid line).

that the most effective anodizing electrolyte is SBGA after all immersion time. The EIS measurements of the non-anodized and anodized electrodes were obtained in 3.5% NaCl solution after 168 h immersion time was presented in Fig. 8. The non-anodized Al exhibits one single semicircle and inductive loop at low frequency domain [34]. The diameter of curve is almost 1795.3 Ω cm2. In the convenient equivalent circuit model, Rs is solution resistance, Rp is polarization resistance which contains charge transfer resistance, diffuse layer resistance, accumulation resistance, Cdl ( double layer capacitance) and the equivalent circuit consists of the inductive elements, RL and L (Fig. 9a). The inductive region is illustrated with pitting corrosion and ionic relation phenomenon within oxide layer. It is seen that the highest resistance and the lowest capacitance values of oxide coatings obtained with the SBGA electrolyte. The specific adsorption of Cl– and diffusion underlying natural oxide coating may cause continuation of dissolution. The removal of corrosion products via diffusion may keep surface open for further corrosion. The open pores may be plugged with corrosion products or ionic species, but in corrosive medium new pores may reform on the metal surface. The higher resistance values were detected for Al samples anodized in SBGA solution. The convenient equivalent circuits were given in Fig. 9 and calculated data were presented in table. The Nyquist-Bode diagrams of anodized Al samples in S and SBG electrolyte present two loops in Fig. 8. The first loop appeared at the high frequency region is attributed to the pore resistance (R1) against the corrosion process occurring within pores of coating, the second one appeared at

low frequencies related to the oxide coating resistance (R2). The anodized Al in SBGA electrolyte had better corrosion resistance than non-anodized and other anodized samples. The counter anions of H3BO3 and H2SO4 include oxygen atoms with unshared electron pairs, so they may plug aluminum oxide’s pores and these species could provide some kind of inhibitory effect along the oxide pores. It is also known that these anions could yield fairly insoluble compounds with aluminum. Furthermore the third loop (R3) was seen in low frequency region, which is attributed clogging Rs

(a)

Cdl1 L1

R1

R2 Rs

(b)

Cdl1 Cdl2 R1

(c)

Rs

R2

Cdl1

Cdl2

R1

R1

Cdl3 R3

Fig. 9. The equivalent diagram (168h) for non-anodized (a), anodized in S, SBG (b) and SBGA (c).


Vol. 53

No. 4

2017

740

MUTLU et al. 0 –1 a

log I, A cm–2

–2 –3

b

c d

–4 –5 –6 –7 –8 –0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1.0 1.2 E, V Ag/AgCl

Fig. 10. The anodic polarization curves of non-anodized (a) and anodized in S (b), SBG (c) and SBGA (d) Al samples after 168 h immersion in 3.5% NaCl.

of the pores with ions transition in oxide pores. The most probably during long immersion time oxide pores were developed and corrosion products were filled pores the case in point current reaches limit value because reactions participate under diffusion control. All impedance measurements obtained when evaluating the best in the short and long immersion time, stable, protective, homogenous and stable oxide layer is formed on SBGA electrolyte is observed (Fig. 8, table). As known; electrochemical impedance spectroscopy measurements provide information about the resistance change when the system is carried out in the non-polarized conditions for open circuit potential. For the same system polarized conditions also were examined with the current-potential curves and the obtained findings are described below. The polarization curves (Fig. 10) of non-anodized (Al) and anodized aluminum (S, SBG and SBGA) electrode were recorded after 168 h of immersion time in corrosive test solution (3.5% NaCl). The scan rate was 1 mV/sec. The anodic dissolution of non-anodized sample, it started lower potential values than anodized sample. The Ecorr values were –0.645; –0.566; –0.206 and 0.903 V (vs. Ag/AgCl) respectively. The anodizing treatment shifted the Ecorr value of Al to nobler potential values. In Fig. 9 during anodic polarization of non-anodized Al sample, current significantly increased from –0.645 to –0.385 V potential range (vs. Ag/AgCI) which is related with dissolution then from –0.385 to 1.2 V the slight changing was detected. Throughout the pore growth by dissolution of natural oxide layer, electrolyte reached the metal surface at the base of the pores. Then, pores were filled by corrosion products the case in point current reaches limit value

because reactions participate under diffusion control. Other side for anodized samples, it is clear that the oxide layers with relatively higher thickness have relatively higher resistance against electrolyte penetration. Especially anodized sample in SBGA decreased the current density almost 103 times lower values. The electrochemical measurements showed that the convenient anodizing electrolyte was SBGA. The SEM of all surfaces was presented in Fig. 11. Result of maximum amplification performed (50000) all traces which on the surface significantly were observed (Fig. 11a–11d). In Fig. 11a', with the 5000 enlargement the bare aluminum surfaces withstand corrosive environments and clearly shows the pitting corrosion [35]. Pitting corrosion is one of the most dangerous types of corrosion. When it starts once as an auto catalytic, it moves toward the surface as metal perforation depth and causes. Surface is not a completely smooth work as a result of abraded and lines that resulting from grinding were seen, (b) within S subjected to anodizing aluminum electrode processing aluminum oxide’s increasing proportion shows that small pores formed on the surface which is also seen as the coo regions with potholes. The obtained image on the surface of aluminum oxide (Al2O3) seems not to be a homogeneous layer of the sheet. Figure 11b' showed that S aluminum electrode of the tapering surface of the partially based on the oxide layer against corrosive environment (3.5% NaCl) has become rough aluminum surface pits and a heterogeneous layer is formed. SEM micrograph of electrode proposed in SBG Fig. 11c seems to be more uniform and rough then SEM micrograph electrode prepared in S Fig. 11b. Oxide layer (Al2O3) which formed on the surface indicates in Fig. 11c the presence of a porous structure with small pore size compared to H2SO4 electrolytes more homogeneous although it is not covered all surface. These areas are suitable to create pitting corrosion. Figure 11c', subjected to anodic oxidation process is observed that in SBG aluminum electrode surface is more durable than coating only in the medium containing H2SO4. Small pores closed once formed large pores, but the place where it is seen as vulnerable in terms of corrosion and cracks. Figure 11d it is observed that in SBGA in the aluminum electrode of the amplification process performed with other electrolytes compared to become more homogeneous appearance. Pores are quite frequent and small. The corrosion resistance of the barrier layer and porous layer, which will give us 3.5% NaCl solution in highly corrosive environments 168h being waited for the impedance and current-potential curves. Figure 11d' is subjected to anodic oxidation process in SBGA is more homogeneous according to other media (11a'–11c') and than a corrosive environment is affected.


Vol. 53

No. 4

2017


(a)

741

(a')

1 mm

200 nm

200 nm

1 μm

(b)

(b')

1 mm

100 nm

1 μm

200 nm (c)

(c')

1 mm

200 nm

200 nm

1 μm

(d)

(d')

1 mm

200 nm

200 nm

1 μm

Fig. 11. The SEM micrographs (50.000 KX-100 nm) of non-anodized (a) and anodized S (b), SBG (c) and SBGA (d) and (5.000 KX-1 μm) of non-anodized (a') and anodized S (b'), SBG (c') and SBGA (d') Al samples after 168 h immersion in 3.5%NaCl solution.

4. CONCLUSION Physical and electrochemical properties of 7075 aluminum alloy were enhanced via anodizing method. For this purpose convenient potential (About 20 V in the electrolyte S, About 12 V at SBG electrolyte, in the

electrolyte SBGA is about 10 V) and for each solution, anodizing time (1800 s) and current density (15 mA), electrolyte and its concentration were determined. Pre-treatment method was preferred as mechanical abraded resolved better the natural oxide. Anodized


Vol. 53

No. 4

2017

742

MUTLU et al.

sample in SBGA decreased the current density almost 103 times lower values and Rp value higher than the other and surfaces compared to other homogeneous and smooth. The electrochemical measurements showed that the convenient anodizing electrolyte was SBGA. Porous anodic alumina (Al2O3) on the aluminium surface with small pore and corrosion resistant barrier layer and blocking pores with used additive in 0.4 M H2SO4 anodizing solution has been provided. ACKNOWLEDGMENTS The Scientific and Technical Research Council of Turkey (TUBİTAK) (Project no. 214Z169) have financially supported this study. REFERENCES 1. Ochs, M., Ćosović, B., and Stumm, W., Geochim. Cosmochim. Acta, 1994, vol. 58, p. 639. 2. Song, M., Liu, Y., Cui, S., et al., Appl. Surf. Sci., 2013, vol. 283, p. 19. 3. Habazaki, H., Konno, H., Shimizu, K., et al., Corros. Sci., 2004, vol. 46, p. 2041. 4. Waller, G., Brooke, P., Rainwater, B., et al., J. Power Sources, 2016, vol. 306, p. 162. 5. Choi, M., Ham, G., Jin, B.-S., et al., J. Alloys Compd., 2014, vol. 608, p. 110. 6. Lu, Z., Ouyang, Z., Grant, N., et al., Appl. Surf. Sci., 2016, vol. 363, p. 296. 7. Zhu, T. and Chong, M.N., Nano Energy, 2015, vol. 12, p. 347. 8. Hunter, M. and Fowle, P., J. Electrochem. Soc., 1956, vol. 103, p. 482. 9. Leifer, J. and Mickalonis, J., Corrosion, 2000, vol. 56, p. 563. 10. Branzoi, V., Golgovici, F., and Branzoi, F., Mater. Chem. Phys., 2003, vol. 78, p. 122. 11. Bouchama, L., Azzouz, N., Boukmouche, N., et al., Surf. Coat. Technol., 2013, vol. 235, p. 676. 12. Knörnschild, G., Poznyak, A., Karoza, A., and Mozalev, A., Surf. Coat. Technol., 2015, vol. 275, p. 17. 13. Zhang, R., Jiang, K., Zhu, Y., et al., Appl. Surf. Sci., 2011, vol. 258, p. 586.

14. Stojadinovic, S., Vasilic, R., Nedic, Z., et al., Thin Solid Films, 2011, vol. 519, p. 3516. 15. Surawathanawises, K. and Cheng, X., Electrochim. Acta, 2014, vol. 117, p. 498. 16. Vrublevsky, I., Chernyakova, K., Ispas, A., et al., Thin Solid Films, 2014, vol. 556, p. 230. 17. Kao, T.-T. and Chang, Y.-C., Appl. Surf. Sci., 2014, vol. 288, p. 654. 18. Despić, A. and Parkhutik, V.P., in Modern Aspects of Electrochemistry No. 20, Springer, 1989, p. 401. 19. O'Sullivan, J. and Wood, G., Proc. R. Soc. London, Ser. A, 1970, vol. 317, p. 511. 20. Giovanardi, R., Fontanesi, C., and Dallabarba, W., Electrochim. Acta, 2011, vol. 56, p. 3128. 21. Zhang, G.A., Xu, L.Y., and Cheng, Y.F., Electrochim. Acta, 2008, vol. 53, p. 8245. 22. Ren, J. and Zuo, Y., Surf. Coat. Technol., 2004, vol. 182, p. 237. 23. Kelly, K.L., Coronado, E., Zhao, L.L., and Schatz, G.C., J. Phys. Chem. B, 2003, vol. 107, p. 668. 24. Mert, B.D., Yazici, B., Tüken, T., et al., J. Electroanal. Chem., 1993, vol. 357, p. 105. 25. Drazic D., Popic J., Hydrogen evolution on aluminium in chloride solutions, J. Electroanal. Chem., 1993, vol. 357, pp. 105–116. 26. Soekrisno, S. and Anggoro, B., Adv. Mater. Res., 2014, vol. 896, p. 253. 27. Suay, J., Gimenez, E., Rodrıguez, T., et al., Corros. Sci., 2003, vol. 45, p. 611. 28. Hakimizad, A., Raeissi, K., and Ashrafizadeh, F., Surf. Coat. Technol., 2012, vol. 206, p. 4628. 29. Huang, Y., Shih, H., Huang, H., et al., Corros. Sci., 2008, vol. 50, p. 3569. 30. Huang, Y., Shih, H., Daugherty, J., and Mansfeld, F., Corros. Sci., 2009, vol. 51, p. 2493. 31. Mohedano, M., Matykina, E., Arrabal, R., et al., Appl. Surf. Sci., 2015, vol. 346, p. 57. 32. Tuken, T., Yazıcı, B., and Erbil, M., Turk. J. Chem., 2002, vol. 26, p. 735. 33. Solmaz, R. Kardaş, G., Yazıcı, B., and Erbil, M., Colloids Surf., A, 2008, vol. 312, p. 7. 34. Deyab, M.A., J. Power Sources, 2014, vol. 268, p. 50. 35. Shih, T.-S. and Chiu, Y.-W., Appl. Surf. Sci., 2015, vol. 351, p. 997.


Vol. 53

No. 4

2017