Formation and Surface Structural Features of Crystalline Ceramic Nanocomposite Coatings on Aluminium Using Lithium Sulphate with Silicate Additive

doi:10.1007/s40195-014-0034-6

Abstract

Abstract:

We have synthesized a series of the ceramic coatings by anodization of aluminium using lithium sulphate and sodium silicate additive. Our experiments show that the present coatings are nanocomposites in nature, consisting of a mixture of nanocrystalline alumina, silica, aluminium silicate and mullite; the formation of alumina was similar to conventional anodizing technology, while the formation of mullite was attributed to an addition of sodium silicate. The microhardness of the coatings progressively increased with the increasing current density up to 0.2 A/cm², which could mainly be attributed to the decrease of porosity in the interfacial region of the oxides up to the range. From the performance of the coatings against corrosion (Tafel/Nyquist plots), it was inferred that the coatings fabricated by lithium sulphate–sodium silicate bath have enhanced corrosion resistance (R_p = 3.12 kΩ), as well as better microhardness value than that of the lithium sulphate bath alone (R_p = 660.96 Ω) which confirm the perception that the silica particles included in the anodized alumina matrices randomly. Presence of Al, Si and O indicated that the electrolyte components had been intensively incorporated into the coatings.

Key words: Anodization, Nanocomposite, Ceramic, Lithium sulphate

M. Mubarak Ali, V. Raj. Formation and Surface Structural Features of Crystalline Ceramic Nanocomposite Coatings on Aluminium Using Lithium Sulphate with Silicate Additive[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(2): 185-197.

Figures/Tables 13

Fig. 1 Voltage–time response of the coatings obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

Fig. 2 Influence of current density on microhardness of the coatings obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

Fig. 3 SEM images of the coatings formed on aluminium from lithium sulphate bath by anodization at various current densities: a 0.1 A/cm2, b 0.15 A/cm2, c 0.2 A/cm2, d 0.25 A/cm2

Fig. 4 SEM images of the coatings formed on aluminium from lithium sulphate–sodium silicate bath by anodization at various current densities: a 0.1 A/cm2, b 0.15 A/cm2, c 0.2 A/cm2, d the focused view of the circle in c

Fig. 5 Influence of current density on microstructure (using atomic force microscope) of the coatings obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

Table 1 Influence of current density on surface statistics of the coatings obtained from lithium sulphate bath at (10 ± 1) °C for 30 min

Current density (A/cm²)	Mean distribution (nm)		Average roughness (nm)		Root means square roughness
Current density (A/cm²)	Lithium sulphate	Lithium sulphate–sodium silicate	Lithium sulphate	Lithium sulphate–sodium silicate	Lithium sulphate	Lithium sulphate–sodium silicate
0.10	309.26	117.32	46.18	29.05	58.42	36.15
0.15	417.27	93.10	136.58	18.78	168.92	24.56
0.20	103.57	80.25	8.89	12.88	12.56	16.78
0.25	857.43	–	289.76	–	329.02	–

Fig. 6 Influence of current density on pore parameters (number of pores, area, diameter and length of the pores) of the coatings obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

Table 2 Influence of current density on elemental composition (at.%) of the coatings obtained at (10 ± 1) °C for 30 min in lithium sulphate and lithium sulphate–sodium silicate baths

Current density (A/cm²)	Element	Lithium sulphate bath (at.%)	Lithium sulphate–sodium silicate bath (at.%)
0.10	O	72.57	72.40
	Al	23.58	22.73
	S	3.86	3.32
	Si	–	1.54
0.15	O	72.63	72.82
	Al	23.27	21.67
	S	4.10	3.50
	Si		2.00
0.20	O	73.34	74.46
	Al	22.41	21.00
	S	4.25	4.43
	Si		0.10
0.25	O	72.20	–
	Al	23.95	–
	S	3.85	–

Fig. 7 Influence of the current density on XRD pattern of the coatings obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

Fig. 8 Influence of current density on corrosion resistance of the coatings using Tafel polarization technique obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

Table 3 Calculated Tafel polarization parameters for the coatings formed from lithium sulphate bath at various current densities

Current density (A/cm²)	I_corr (mA)		E_corr (mV vs. SCE)		R_corr (mile/year)
Current density (A/cm²)	Lithium sulphate	Lithium sulphate–sodium silicate	Lithium sulphate	Lithium sulphate–sodium silicate	Lithium sulphate	Lithium sulphate–sodium silicate
0.10	1.08 × 10^-9	3.67 × 10^-10	-374	630	4.65 × 10^-1	1.57 × 10^-1
0.15	7.59 × 10^-10	8.15 × 10^-11	460	667	3.25 × 10^-1	3.49 × 10^-2
0.20	7.96 × 10^-11	7.79 × 10^-11	900	743	3.41 × 10^-2	3.34 × 10^-2
0.25	1.75 × 10^-9	–	-588	–	7.53 × 10^-1	–

Table 4 Influence of current density on impedance parameters of the coatings obtained from lithium sulphate bath at (10 ± 1) °C for 30 min

Current density (A/cm²)	R_s (Ω)		C_dl (μF)		R_p
Current density (A/cm²)	Lithium sulphate bath	Lithium sulphate–sodium silicate bath	Lithium sulphate bath	Lithium sulphate–sodium silicate bath	Lithium sulphate bath (Ω)	Lithium sulphate–sodium silicate bath (kΩ)
0.10	2.9 × 10^-3	9.73 × 10^-7	54.35	32.24	433.59	1,320
0.15	0.22	5.45 × 10^-6	21.31	23.66	454.50	1,770
0.20	0.57	0.05	8.98	11.73	660.96	3,120
0.25	1.24 × 10^-7	–	36.95	–	4.52	–

Fig. 9 Influence of current density on corrosion resistance of the coatings (Nyquist plots) obtained from lithium sulphate a, lithium sulphate–sodium silicate b baths

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