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Graining technology by electrolytic etching on the surface of aluminum alloys
Materials Chemistry and Physics 77 (2002) 170–178
Graining technology by electrolytic etching on the surface of aluminum alloys M.Z. An∗ , L.C. Zhao, Z.M. Tu Department of Applied Chemistry, Harbin Institute of Technology, P.O. Box 411, Harbin 150001, PR China Received 2 May 2001; received in revised form 29 September 2001; accepted 9 October 2001
Abstract The electrolytic etching solution of an aluminum alloy is comprised of the species forming a barrier layer on the surface of the aluminum alloy and the species etching the barrier layer. The experiments showed that in the sodium metaboricate—boric acid electrolysis system with sodium metaboricate as the main component, sodium carbonate as the etchant and sodium phosphate as the enhancing bond-agent— the electrolytically etched graining on the surface of the aluminum alloy are true to life, the depth of the mark is moderate. After the electrolytically etched graining is anodized and is then submitted to electrolytic coloring, the color comparison is obvious, and extremely similar to wood graining. The XRD diagrams show that the electrolytically etched surface of the aluminum alloy consists of aluminum and alumina, which mainly localized in the graining zone and exists in ␣-Al2 O3 and ␥-Al2 O3 . Having been anodized, SO4 2− is introduced into the oxidized film; the pore density and the pore diameter of the multi-porous layer of the oxidized film in the graining zone are bigger than those in the grainless zone. Thus the inner depositing amounts in the micro-pores of the oxidized film in the graining zone are big, which would show dark color. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum alloy; Electrolytic etching; Oxidation; Electrolytic coloring
1. Introduction Since the 1980s, some new treatment techniques of the surface of aluminum alloys have been developed on the basis of the traditional oxidation—coloring due to the need of building industry and the requirements for the protection and the decoration of aluminum alloy materials, such as interference electrolytic coloring [1], electrolytic multicoloring [2], electrolytic coloring for imitation stainless steel [3], electrolytic coloring of simulating wood graining [4], pattern electrolytic coloring [5] and microarc oxidation and coloring [6], etc. Electrolytic coloring of simulating wood graining is to make the surface of an aluminum alloy, show similarly natural wood graining with color and luster, and three-dimensional images by means of electrolytically etched graining; such aluminum materials treated by electrolytically etched graining method are anti-wear and anticorrosive and can be applied to the fields of building industry and furniture and electric equipment production. Having been treated by means of electrolytically etched graining, the aluminum alloy shows not only the color and ∗ Corresponding author. Tel.: +86-451-641-3721; fax: +86-451-641-5527. E-mail address: [email protected] (M.Z. An).
luster and the natural image of the wood materials but also the metallic characteristics such as anti-wear, anticorrosive, fire-resistant, sturdy and durable, scarcely deformed, damp-resistant and insect-resistant. The application of aluminum alloys relaxes the problems of lack of timber and the decrease of the cover percentage of forest. Traditional methods of the wood graining treatment of aluminum alloys contain mechanical method, printing method and squeeze film method, etc. Mechanical method is to extrude aluminum materials into strips and wires by means of wiredrawing and squeeze film, which are then submitted to anodization and coloring. The method shows the disadvantages of processing being extremely difficult, high cost, the formed wood graining being too regular and the images being not true to life. Printing method is to print patterns directly on the surface of aluminum alloys. It is obvious that the effect of the method is poor. Squeezing film method is to make the aluminum alloy oxidized and coated by electrophoresis pass through an roller with a binding agent on it, and the adhesive foil with patterns like wood graining is then pressed on the surface of the aluminum alloy treated as mentioned above by means of a squeezing film machine. The production speed of squeezing film method is very low, and it is difficult that the adhesive foil is strongly packed on the surface of the aluminum devices
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with complex shapes and the cost is very high. Electrolytic etching method has overcome all kinds of the disadvantages mentioned above and has opened up vast vistas for the application. The theoretical investigation of the application of electrolytic etching technique to the graining on the surface of an aluminum alloy has not been reported. However, the theoretical investigation of the conventional electrolytic oxidation and coloring of aluminum alloys have been reported [7–13]. The theoretical exploration of the electrolytically etched graining on the surface of an aluminum alloy can be done by drawing lessons from the experience about the conventional oxidation and electrolytic coloring of aluminum alloys. This paper deals with the mechanisms of the electrolytically etched graining on the surface of the aluminum alloy by means of the analysis of the microstructure and chemical states, and on the basis of the investigation of the electrolytically etched graining decoration technology.
2. Experimental techniques Electrolytically etched graining is mainly used to produce the materials of building industry; Al–Mg–Si alloys are extensively used in building industry and therefore the Al–Mg–Si alloy was selected as the researched electrode. The aluminum alloy was cut into small strips with a dimension of 100 × 50 × 2 mm3 , the stainless steel strip with the same dimension to that of the aluminum alloy strip was used as the counter-electrode. An a.c. powerstat was used as the power source for the a.c. electrolytically etched graining and electrolytic coloring, and a direct current stabilized power source was used as the power source for the anodization. The technological process of electrolytically etched graining is as follows: alkalinity chemical degreasing—hot water washing—water washing—impregnated corrosion— water washing—electrolytic etching graining—water washing—removing film—water washing—anodization—water washing—electrolytic coloring—water washing—drying— electrophoresis coating—water washing—drying. The alkalinity chemical degreasing and impregnated corrosion are conventional pretreatments. The alkalinity chemical degreasing solution and operating conditions are as follows: NaOH 8–10 g dm−3 , Na3 PO4 ·12H2 O 45–55 g dm−3 , Na2 SiO3 10–20 g dm−3 , temperature 60–70 ◦ C, time 3–5 min. The impregnated corrosion is operated in 40–50% HNO3 solution at room temperature for 1–3 min. The effect of the electrolytically etched graining on the surface of the aluminum alloy is determined by visual method, i.e., to observe carefully the depth, density, uniformity and lightness of it. The morphology of the electrolytically etched grained surface of the aluminum alloy was observed with a model HITACHI S-570 scanning electron microscope. Since the alumina film was not conductive, a gold layer was predeposited on the grained surface of the aluminum alloy before
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the analysis by evaporation. The structure of the grained surface film of the aluminum alloy was analyzed with a model X’Pert MRD Philips X-ray diffractometer, and a Cu target (λ = 0.1542 nm) was used, scanning range was 2θ = 15–90◦ with a set scanning step-length 0.02◦ , scanning rate was 8.00◦ min−1 . The composition and chemical state of the grained surface of the aluminum alloy by electrolytic etching were examined with a model SHIMADZU ESCA 750 X-ray photoelectron spectrometer. Since the transmitting depth of an X-ray photoelectron spectrometer is only several nanometers, the grained surface film of the aluminum alloy is thinned with metallographic abrasive paper in order to analyze the colored film.
3. Results and discussion 3.1. Selection of electrolytic etching solution According to the principle of electrolytically etched graining, the species to form the barrier layer and the etchant are optimized to prepare the electrolytic etching solution by means of current density vs. potential, and potential vs. time curves. It is known from literature [14] that organic acids and their corresponding salts, such as citric acid, tartaric acid, gluconic acid, malic acid, oxalic acid, EDTA, etc. carboxylic acids or amino-acids and their corresponding alkaline salts and soluble alkaline-earth metal salts, and mineral acids and their corresponding salts, such as boric acid, phosphoric acid, pyrophosphoric acid, polyphosphoric acid and their corresponding soluble salts can form a thin barrier layer on the surface of the aluminum alloy. The species showing etching effect include halides (fluorides, chlorides), CO3 2− and Cu2+ . Fig. 1 shows the potential vs. the current density of the aluminum alloy for its electrolysis in the solutions of the species forming the barrier layer (the set concentration being 0.1 mol dm−3 ). From Fig. 1, it can be seen that tartaric acid and boric acid can form a barrier layer on the surface of the aluminum alloy extremely easily and there is scarcely polarized current. The polarized currents of sodium citrate and sodium metaborate are relatively high, showing that they hardly form a thick barrier layer only when they have been mixed with other species, and they can form a barrier layer with a moderate thickness so as to meet the requirement of electrolytically etched graining. Fig. 2 shows the potential vs. time of the aluminum alloy for its electrolysis in the solutions of the species forming the barrier layer under the set current density being 2 A dm−2 . From Fig. 2, it can be seen that the shapes of the potential vs. time curves in different electrolytic etching solutions are the same although the end potentials are different. At the very beginning of the electrolysis, the potential sharply increases; 1 min later, it begins to decrease, and then increases slowly and reaches a constant value at last. The explanation of that
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Fig. 1. Current density vs. potential of the aluminum alloy for its electrolysis in the solutions of the species forming the barrier layer (concentration: 0.1 mol dm−3 ).
process is that at the very beginning of the electrolysis, the potential sharply increases because of the rapid formation of the compact barrier layer; the barrier layer is punctured at once, which makes the aluminum substrate uncovered, the potential decreased, and then a new barrier layer is formed; the potential increases again, after a period of time, the balance of the formation and the dissolution of the barrier layer is obtained and the potential reaches a stable value. Fig. 3 shows the current density vs. potential of the aluminum alloy for its electrolysis in solutions of the etchants. From Fig. 3, it can be seen that the etching effects of the four kinds of the etchants are very strong except sodium carbonate. Theoretically, sodium carbonate, sodium phosphate, copper sulfate, sodium chloride and sodium fluoride all can be used as the etchant; however, the etching effect of an etchant is related to the conditions of its application that which etchant is used should be determined by experimental results. Fig. 4 shows the potential vs. time of the aluminum alloy for its electrolysis in different etching solutions. The potential vs. time curves of the aluminum alloy
for its electrolysis in the etching solutions are different from those in the electrolyzing solutions of the species forming the barrier layer. In Fig. 4, the potential is lower at the beginning of the electrolysis, but rises slowly as time goes on, and reaches a constant value at last. The potential changes scarcely with the time, showing that the aluminum alloy is dissolved steadily. The optimum electrolytic etching solution can be prepared by the optimum combination of the species forming the barrier layer and the etchant. It was found that the electrolytic etching effect of the electrolytic etching solution made from sodium metaboricate as the species forming the barrier layer, sodium carbonate as the etchant, boric acid as the buffer agent and sodium phosphate as the enhancing bond-agent is excellent. It was found from the experiments that with increase of the content of sodium metaborate, the grains deepened gradually. When the content of sodium metaborate was 5 g dm−3 , the etched grains were the most true to life; when the content was further increased, the grains became blurred and
Fig. 2. Potential vs. time of the aluminum alloy for its electrolysis in different solutions of the species forming the barrier layer (concentration: 0.1 mol dm−3 ).
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Fig. 3. Current density vs. potential of the aluminum alloy for its electrolysis in etching solutions (concentration: 0.1 mol dm−3 ).
shallow. When the content of boric acid was lower, the grains became dense and shallow; with the increase of the content of boric acid, the grains deepened and the distance between the grains was increased. When the content of boric acid was 2 g dm−3 , the grains became deeper, the thickness of the grains and the distance between the grains were moderate; when the content of boric acid was further increased, the grains became shallow, the over-corrosion on the surface of the aluminum alloy took place. With the increase of the content of sodium carbonate, the grains deepened obviously. When the content of sodium carbonate was 3 g dm−3 , the etched grains were the most true to life, the depth of the grains were suitable, and the distances between the grains were moderate. When the content of sodium carbonate was too high, the surface of the aluminum alloy was completely eroded and no grains formed on the surface of the aluminum alloy. With the increase of the content of sodium phosphate, the grains deepened gradually. When the content of sodium phosphate was 2 g dm−3 , the grains were distinct and the
etching effect was excellent. When the content of sodium phosphate was too high, the grains became shallow inversely and blurred. With the increase of the current density, the distances between the grains became narrow and deepened; when the current density was too high, the complete dissolution of the surface of the aluminum alloy took place, the grains showed big distances and became shallow. In general, the current density was selected as 3 A dm−2 . With the rising of the temperature from the lower temperature, the distances between grains increased and the grains became thicker; at higher temperatures, the distance between the grains were still bigger and the grains became thinner. Generally speaking, at lower temperatures, the etching effect was better, the temperature was selected as 15 ◦ C. At such a temperature, it is unnecessary to heat the solution in the production process. In summer, the current density can be raised so as to guarantee the quality of the product without temperature drop. This is become the influence of the current density on the grain distance is opposite to that of the temperature on the
Fig. 4. Potential vs. time of the aluminum alloy for its electrolysis in etching solutions (concentration: 0.1 mol dm−3 ).
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Fig. 5. SEM micrographs of the electrolytically etched grains on the surface of the aluminum alloy: (a) graining zone; (b) grainless zone.
grain distance. The electrolysis time depends on the current density; if the current density is lower, the electrolysis time is relatively longer and vice versa. From what have been said above, we can sum up the compositions and operating conditions of the optimum electrolytically etched graining for the aluminum alloy: sodium metaborate (NaBO2 ·4H2 O) 5 g dm−3 ; boric acid (H3 BO3 ) 2 g dm−3 ; sodium carbonate (Na2 CO3 ) 3 g dm−3 ; sodium phosphate (Na3 PO4 ·12H2 O) 2 g dm−3 ; current density (J∼ ) 3 A dm−2 ; temperature (T) 15 ◦ C; time (t) 15 min. Grain depth of 0.01–0.03 mm, grain width of 0.5–1.0 mm and the distance between grains of 1–3 mm are obtained on the surface of aluminum alloy from the above technology. Having been electrolytically etched, the aluminum alloy has to be submitted to the removal of the film in the film-removing solution made from sodium carbonate and sodium dicarbonate, and the anodization in a solution of sulfuric acid; finally, the golden yellow coloring of it by means of silver salt and the reddish-brown coloring of it by means of copper salt are carried out.
3.2. Microstructure of the electrolytically etched grains The SEM micrographs of the electrolytically etched grains on the surface of the aluminum alloy (Fig. 5) show that the oxidized film in the graining zone is thick and more loose, may be porous layers exist; in the grainless zone, the oxidized film is very thin, only forming a poreless barrier layer. The SEM micrographs of the anodized surface on the aluminum alloy after having been electrolytically etched as shown in Fig. 6 show that a compact porous layer formed in the graining zone and the pore density was big; although there was also a porous layer in the grainless zone, the pore density was small and the pore diameter was small. This indicates that anodization makes the films in the graining zone and the grainless zone thick, but the microstructure in the grain zone is obviously different from that in the grainless zone, i.e., the pore density and the pore diameter of the oxidized film in the graining zone are all bigger.
Fig. 6. SEM micrographs of the anodized surface on the aluminum alloy after having been electrolytically etched: (a) graining zone; (b) grainless zone.
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Fig. 7. SEM micrographs of the electrolytic coloring surface of the aluminum alloy after having been electrolytically etched and anodized: (a) graining zone; (b) grainless zone.
SEM micrographs of the electrolytic coloring surface of the aluminum alloy after having been electrolytically etched and anodized as shown in Fig. 7 show that the cell-like species formed in the graining zone. This is the result of the cross-linking of the deposited metal colloidal particles and its compound in the adjacent micro-pores in the oxidized film; while in the grainless zone, the cross-linking hardly takes place due to the small pore density of the oxidized film, thus the black cell-like species is not observed. 3.3. Structure of the electrolytically etched graining surface of the aluminum alloy Fig. 8 shows the XRD diagrams of the grained surface of the aluminum alloy by electrolytic etching. From Fig. 8,
Fig. 8. XRD diagrams of the grained surface of the aluminum alloy: (a) electrolytically etched; (b) not electrolytically etched.
we can see that the electrolytically etched graining surface of the aluminum alloy consists of aluminum covered by the extremely thin film of Al2 O3 , which is comprised of ␣-Al2 O3 and ␥-Al2 O3 . This is because the exploring depth of the XRD is rather bigger than the thickness of the film of Al2 O3 , the diffraction peak of Al is very strong. The comparison of part a with part b of Fig. 8 indicates that Al atom preferentially orientates on the crystal plane (2 2 0). However, the electrolytic etching does not change the crystallization state of the aluminum substrate; it is possible to etch the surface of the aluminum alloy electrolytically and it is believed that this phenomenon is attributed to the orientations of the crystal planes of the substrate. The relative intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 are raised by 10%, meaning that the contents of ␣-Al2 O3 and ␥-Al2 O3 in the graining zone are much higher than those in the grainless zone, i.e., the oxidized film in the graining zone is mainly comprised of ␣-Al2 O3 and ␥-Al2 O3 ; of course, an extremely thin oxidized film (Al2 O3 ) exists in the grainless zone. Fig. 9 shows the XRD diagrams of the electrolytically etched and anodized surface of the aluminum alloy. It is known from Fig. 9 that the surface of the aluminum alloy is still comprised of Al, ␣-Al2 O3 and ␥-Al2 O3 , but the preferential orientation of Al on the crystal plane (2 2 0) is more obvious, it becomes the strongest peak. This is because the surfacial Al of the aluminum alloy is changed into Al2 O3 during the process of anodization according to the analysis mentioned above, which makes the preferential orientation of Al in the substrate more obvious. The comparison of Fig. 8b with Fig. 9b indicates that the anodization does not show big effect on the XRD diagrams in the grainless zone, only makes the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 stronger. This is the result of the increase of the content of Al2 O3 on the surface of the aluminum alloy due to the increase of the thickness of the oxidized film caused by anodization. The comparison of Fig. 8a with Fig. 9a indicates
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the content of Ag in the film is very low. The comparison of Fig. 9b with Fig. 10b shows that on the grainless surface, the preferential orientation of Al on the crystal plane (2 2 0) is more obvious due to the coloring and the intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 are increased compared with those of Al on the crystal plane (1 1 1). The comparison of Fig. 9a with Fig. 10a shows that the electrolytic coloring makes the relative intensity of the diffraction peak of Al2 O3 increased, which may be that the coloring metal deposits on the pore wall or the pore bottom in the porous oxidized film, which acts the sealing effect on the aluminum substrate and hence makes the intensity of the diffraction peak of Al decreased. Thus the intensity of Al2 O3 is relatively increased. Fig. 9. XRD diagrams of the electrolytically etched and anodized surface of the aluminum alloy: (a) electrolytically etched; (b) not electrolytically etched.
that preferential orientation of Al on the crystal plane (2 2 0) is more obvious in the graining zone due to anodization, while the intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 are not changed compared with those of Al on the crystal plane (1 1 1). The increase of the content of Al2 O3 due to anodization is not shown in the XRD diagrams of the aluminum alloy. Fig. 10 shows the XRD diagrams of the electrolytically colored sample. The comparison of part a with part b of Fig. 10 indicates that the XRD diagrams of the electrolytically etched grained sample is generally similar to that of grainless sample; the difference of which is that the intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 in the electrolytically etched sample are relatively stronger, indicating that the contents of those components are relatively higher in the graining zone. In the XRD diagrams there is no diffraction peak of coloring component Ag, this is because
Fig. 10. XRD diagrams of the electrolytically etched, anodized and colored surface of the aluminum alloy by means of Ag salt: (a) electrolytically etched; (b) not electrolytically etched.
3.4. Chemical composition of the grained surface of the aluminum alloy by electrolytic etching Fig. 11a shows the XPS spectrogram of the electrolytically etched surface of the aluminum alloy. The surfaces of the graining zone and grainless zone are all comprised of Al, O and C, where C is the pollutant. In order to elucidate the chemical states of the surfacial Al and O, the high-resolution XPS analyses should be carried out. Fig. 11b shows the high-resolution XPS spectra of Al 2p on the electrolytically etched surface of the aluminum alloy. By considering the XPS spectra of Al 2p and O 1s and reading the relative handbook, the analytical results can be obtained and are listed in Table 1. From Table 1, it can be
Fig. 11. Typical XPS spectrogram (a) and high-resolution XPS spectra of Al 2p (b) on the electrolytically etched surface of the aluminum alloy.
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Table 1 XPS quantitative analysis of the electrolytically etched graining surface of the aluminum alloy Sample
Binding energy (eV)
Composition
Al 2p (theory: 74.0, 71.3)
O 1s (theory: 531.0)
Not electrolytically etched surface Surface Sputtered for 5 min
73.726, 71.011 73.937, 71.060
531.03 530.83
Al2 O3 , Al Al2 O3 , Al
Electrolytically etched surface Surface Sputtered for 5 min
73.822 74.024, 70.793
531.22 531.85
Al2 O3 Al2 O3 , Al
seen that the grainless zone of the aluminum alloy consists of Al and Al2 O3 and the content of Al2 O3 is relatively high, indicating that aluminum is easily oxidized to form Al2 O3 in atmosphere. After being sputtered by Ar+ stream for 5 min, the content of Al was found to be obviously increased, indicating that the oxidized film is very thin. The electrolytically etched surface of the aluminum alloy is only comprised of Al2 O3 , after being sputtered by Ar+ stream. It was found that there was a few amounts of Al to exist, this indicates that the electrolytically etched surface of the aluminum alloy is mainly comprised of Al2 O3 , as it is sputtered by Ar+ stream, the oxidized film in the grainless zone is relatively thin and is easily etched away, thus the analysis shows the existence of Al. In the graining zone, the oxidized film is relatively thick. Therefore the surface is mainly comprised of Al2 O3 , and this is consistent with the XRD results. Table 2 lists XPS analytical results of the electrolytically etched grained and electrolytic colored surface of the aluminum alloy. From Table 2, it can be seen that the silver in the coloring film exists in colloidal Ag particles with a certain amount of Ag2 O and the coloring film contains a certain amount of SO4 2− . The above-mentioned analyses indicate that after the surface of the aluminum alloy was grained by electrolytically Table 2 XPS quantitative analysis of the electrolytic coloring film on the electrolytically etched surface of the aluminum alloy Sample
Not electrolytically etched surface (eV)
Electrolytically etched surface (eV)
Composition
Al 2p Theory Experiment
74.0 74.1
74.0 74.2
O 1s Theory Experiment
531.0 531.2
531.0 531.3
Ag 3d5/2 Theory Experiment
367.9, 368.2 368.0
367.9, 368.2 368.1, 368.5
Ag Ag2 O
S 2p3/2 Theory Experiment
169.9 170.1
169.9 170.2
SO4 2−
Al2 O3
etching, an Al2 O3 film formed and the oxidized film in the graining zone was thicker. Having been anodized and electrolytically colored by means of silver salt, the electrolytically etched surface of the aluminum alloy was covered by a thicker Al2 O3 film. The coloring metal silver exists in a mixture of colloidal silver and Ag2 O in the oxidized film, and at the same time it was found that SO4 2− existed in the Al2 O3 film, which provided the experimental evidence for exploring the mechanisms of the anodization and coloring of the aluminum alloy.
4. Conclusion The electrolytic etching solution of the aluminum alloy has been prepared from sodium carbonate as the etchant, sodium phosphate as the enhancing bond-agent, boric acid and sodium metaboricate by the experimental combination of the species forming a barrier layer on the surface of the aluminum alloy and the species etching and dissolving the former. The surfacial grains of the aluminum alloy obtained by being electrolytically etched in the above-mentioned electrolytic etching solution are distinct, natural and smooth; the electrolytic etching solution is reliable and steady, easily adjusted and the operation is simple and feasible. In the microstructure, anodization technology makes the difference between the electrolytically etched graining zone, and the grainless zone increased, and the pore density and the pore diameter of the porous layer of the oxidized film in the graining zone are bigger; the electrolytic coloring metal particles in the micro-pores of the oxidized film in the graining zone undergo cross-linking, forming bigger cell-like species and hence showing a dark color. The electrolytically etched surface of the aluminum alloy consists of Al and Al2 O3 , where Al2 O3 is mainly located in the graining zone and exists in ␣-Al2 O3 and ␥-Al2 O3 ; the Al2 O3 film in the grainless zone is very thin. The anodization does not show a significant effect on the structure and the composition of the surfacial oxidized film, but introduces SO4 2− into the oxidized film. This is the result of the electric migration of the anions in the anodizing solution. The electrolytic coloring metal silver deposits in the micro-pores of the porous layers of the oxidized film in a mixture of silver colloid and Ag2 O.
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Graining technology by electrolytic etching on the surface of aluminum alloys M.Z. An∗ , L.C. Zhao, Z.M. Tu Department of Applied Chemistry, Harbin Institute of Technology, P.O. Box 411, Harbin 150001, PR China Received 2 May 2001; received in revised form 29 September 2001; accepted 9 October 2001
Abstract The electrolytic etching solution of an aluminum alloy is comprised of the species forming a barrier layer on the surface of the aluminum alloy and the species etching the barrier layer. The experiments showed that in the sodium metaboricate—boric acid electrolysis system with sodium metaboricate as the main component, sodium carbonate as the etchant and sodium phosphate as the enhancing bond-agent— the electrolytically etched graining on the surface of the aluminum alloy are true to life, the depth of the mark is moderate. After the electrolytically etched graining is anodized and is then submitted to electrolytic coloring, the color comparison is obvious, and extremely similar to wood graining. The XRD diagrams show that the electrolytically etched surface of the aluminum alloy consists of aluminum and alumina, which mainly localized in the graining zone and exists in ␣-Al2 O3 and ␥-Al2 O3 . Having been anodized, SO4 2− is introduced into the oxidized film; the pore density and the pore diameter of the multi-porous layer of the oxidized film in the graining zone are bigger than those in the grainless zone. Thus the inner depositing amounts in the micro-pores of the oxidized film in the graining zone are big, which would show dark color. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum alloy; Electrolytic etching; Oxidation; Electrolytic coloring
1. Introduction Since the 1980s, some new treatment techniques of the surface of aluminum alloys have been developed on the basis of the traditional oxidation—coloring due to the need of building industry and the requirements for the protection and the decoration of aluminum alloy materials, such as interference electrolytic coloring [1], electrolytic multicoloring [2], electrolytic coloring for imitation stainless steel [3], electrolytic coloring of simulating wood graining [4], pattern electrolytic coloring [5] and microarc oxidation and coloring [6], etc. Electrolytic coloring of simulating wood graining is to make the surface of an aluminum alloy, show similarly natural wood graining with color and luster, and three-dimensional images by means of electrolytically etched graining; such aluminum materials treated by electrolytically etched graining method are anti-wear and anticorrosive and can be applied to the fields of building industry and furniture and electric equipment production. Having been treated by means of electrolytically etched graining, the aluminum alloy shows not only the color and ∗ Corresponding author. Tel.: +86-451-641-3721; fax: +86-451-641-5527. E-mail address: [email protected] (M.Z. An).
luster and the natural image of the wood materials but also the metallic characteristics such as anti-wear, anticorrosive, fire-resistant, sturdy and durable, scarcely deformed, damp-resistant and insect-resistant. The application of aluminum alloys relaxes the problems of lack of timber and the decrease of the cover percentage of forest. Traditional methods of the wood graining treatment of aluminum alloys contain mechanical method, printing method and squeeze film method, etc. Mechanical method is to extrude aluminum materials into strips and wires by means of wiredrawing and squeeze film, which are then submitted to anodization and coloring. The method shows the disadvantages of processing being extremely difficult, high cost, the formed wood graining being too regular and the images being not true to life. Printing method is to print patterns directly on the surface of aluminum alloys. It is obvious that the effect of the method is poor. Squeezing film method is to make the aluminum alloy oxidized and coated by electrophoresis pass through an roller with a binding agent on it, and the adhesive foil with patterns like wood graining is then pressed on the surface of the aluminum alloy treated as mentioned above by means of a squeezing film machine. The production speed of squeezing film method is very low, and it is difficult that the adhesive foil is strongly packed on the surface of the aluminum devices
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M.Z. An et al. / Materials Chemistry and Physics 77 (2002) 170–178
with complex shapes and the cost is very high. Electrolytic etching method has overcome all kinds of the disadvantages mentioned above and has opened up vast vistas for the application. The theoretical investigation of the application of electrolytic etching technique to the graining on the surface of an aluminum alloy has not been reported. However, the theoretical investigation of the conventional electrolytic oxidation and coloring of aluminum alloys have been reported [7–13]. The theoretical exploration of the electrolytically etched graining on the surface of an aluminum alloy can be done by drawing lessons from the experience about the conventional oxidation and electrolytic coloring of aluminum alloys. This paper deals with the mechanisms of the electrolytically etched graining on the surface of the aluminum alloy by means of the analysis of the microstructure and chemical states, and on the basis of the investigation of the electrolytically etched graining decoration technology.
2. Experimental techniques Electrolytically etched graining is mainly used to produce the materials of building industry; Al–Mg–Si alloys are extensively used in building industry and therefore the Al–Mg–Si alloy was selected as the researched electrode. The aluminum alloy was cut into small strips with a dimension of 100 × 50 × 2 mm3 , the stainless steel strip with the same dimension to that of the aluminum alloy strip was used as the counter-electrode. An a.c. powerstat was used as the power source for the a.c. electrolytically etched graining and electrolytic coloring, and a direct current stabilized power source was used as the power source for the anodization. The technological process of electrolytically etched graining is as follows: alkalinity chemical degreasing—hot water washing—water washing—impregnated corrosion— water washing—electrolytic etching graining—water washing—removing film—water washing—anodization—water washing—electrolytic coloring—water washing—drying— electrophoresis coating—water washing—drying. The alkalinity chemical degreasing and impregnated corrosion are conventional pretreatments. The alkalinity chemical degreasing solution and operating conditions are as follows: NaOH 8–10 g dm−3 , Na3 PO4 ·12H2 O 45–55 g dm−3 , Na2 SiO3 10–20 g dm−3 , temperature 60–70 ◦ C, time 3–5 min. The impregnated corrosion is operated in 40–50% HNO3 solution at room temperature for 1–3 min. The effect of the electrolytically etched graining on the surface of the aluminum alloy is determined by visual method, i.e., to observe carefully the depth, density, uniformity and lightness of it. The morphology of the electrolytically etched grained surface of the aluminum alloy was observed with a model HITACHI S-570 scanning electron microscope. Since the alumina film was not conductive, a gold layer was predeposited on the grained surface of the aluminum alloy before
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the analysis by evaporation. The structure of the grained surface film of the aluminum alloy was analyzed with a model X’Pert MRD Philips X-ray diffractometer, and a Cu target (λ = 0.1542 nm) was used, scanning range was 2θ = 15–90◦ with a set scanning step-length 0.02◦ , scanning rate was 8.00◦ min−1 . The composition and chemical state of the grained surface of the aluminum alloy by electrolytic etching were examined with a model SHIMADZU ESCA 750 X-ray photoelectron spectrometer. Since the transmitting depth of an X-ray photoelectron spectrometer is only several nanometers, the grained surface film of the aluminum alloy is thinned with metallographic abrasive paper in order to analyze the colored film.
3. Results and discussion 3.1. Selection of electrolytic etching solution According to the principle of electrolytically etched graining, the species to form the barrier layer and the etchant are optimized to prepare the electrolytic etching solution by means of current density vs. potential, and potential vs. time curves. It is known from literature [14] that organic acids and their corresponding salts, such as citric acid, tartaric acid, gluconic acid, malic acid, oxalic acid, EDTA, etc. carboxylic acids or amino-acids and their corresponding alkaline salts and soluble alkaline-earth metal salts, and mineral acids and their corresponding salts, such as boric acid, phosphoric acid, pyrophosphoric acid, polyphosphoric acid and their corresponding soluble salts can form a thin barrier layer on the surface of the aluminum alloy. The species showing etching effect include halides (fluorides, chlorides), CO3 2− and Cu2+ . Fig. 1 shows the potential vs. the current density of the aluminum alloy for its electrolysis in the solutions of the species forming the barrier layer (the set concentration being 0.1 mol dm−3 ). From Fig. 1, it can be seen that tartaric acid and boric acid can form a barrier layer on the surface of the aluminum alloy extremely easily and there is scarcely polarized current. The polarized currents of sodium citrate and sodium metaborate are relatively high, showing that they hardly form a thick barrier layer only when they have been mixed with other species, and they can form a barrier layer with a moderate thickness so as to meet the requirement of electrolytically etched graining. Fig. 2 shows the potential vs. time of the aluminum alloy for its electrolysis in the solutions of the species forming the barrier layer under the set current density being 2 A dm−2 . From Fig. 2, it can be seen that the shapes of the potential vs. time curves in different electrolytic etching solutions are the same although the end potentials are different. At the very beginning of the electrolysis, the potential sharply increases; 1 min later, it begins to decrease, and then increases slowly and reaches a constant value at last. The explanation of that
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Fig. 1. Current density vs. potential of the aluminum alloy for its electrolysis in the solutions of the species forming the barrier layer (concentration: 0.1 mol dm−3 ).
process is that at the very beginning of the electrolysis, the potential sharply increases because of the rapid formation of the compact barrier layer; the barrier layer is punctured at once, which makes the aluminum substrate uncovered, the potential decreased, and then a new barrier layer is formed; the potential increases again, after a period of time, the balance of the formation and the dissolution of the barrier layer is obtained and the potential reaches a stable value. Fig. 3 shows the current density vs. potential of the aluminum alloy for its electrolysis in solutions of the etchants. From Fig. 3, it can be seen that the etching effects of the four kinds of the etchants are very strong except sodium carbonate. Theoretically, sodium carbonate, sodium phosphate, copper sulfate, sodium chloride and sodium fluoride all can be used as the etchant; however, the etching effect of an etchant is related to the conditions of its application that which etchant is used should be determined by experimental results. Fig. 4 shows the potential vs. time of the aluminum alloy for its electrolysis in different etching solutions. The potential vs. time curves of the aluminum alloy
for its electrolysis in the etching solutions are different from those in the electrolyzing solutions of the species forming the barrier layer. In Fig. 4, the potential is lower at the beginning of the electrolysis, but rises slowly as time goes on, and reaches a constant value at last. The potential changes scarcely with the time, showing that the aluminum alloy is dissolved steadily. The optimum electrolytic etching solution can be prepared by the optimum combination of the species forming the barrier layer and the etchant. It was found that the electrolytic etching effect of the electrolytic etching solution made from sodium metaboricate as the species forming the barrier layer, sodium carbonate as the etchant, boric acid as the buffer agent and sodium phosphate as the enhancing bond-agent is excellent. It was found from the experiments that with increase of the content of sodium metaborate, the grains deepened gradually. When the content of sodium metaborate was 5 g dm−3 , the etched grains were the most true to life; when the content was further increased, the grains became blurred and
Fig. 2. Potential vs. time of the aluminum alloy for its electrolysis in different solutions of the species forming the barrier layer (concentration: 0.1 mol dm−3 ).
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Fig. 3. Current density vs. potential of the aluminum alloy for its electrolysis in etching solutions (concentration: 0.1 mol dm−3 ).
shallow. When the content of boric acid was lower, the grains became dense and shallow; with the increase of the content of boric acid, the grains deepened and the distance between the grains was increased. When the content of boric acid was 2 g dm−3 , the grains became deeper, the thickness of the grains and the distance between the grains were moderate; when the content of boric acid was further increased, the grains became shallow, the over-corrosion on the surface of the aluminum alloy took place. With the increase of the content of sodium carbonate, the grains deepened obviously. When the content of sodium carbonate was 3 g dm−3 , the etched grains were the most true to life, the depth of the grains were suitable, and the distances between the grains were moderate. When the content of sodium carbonate was too high, the surface of the aluminum alloy was completely eroded and no grains formed on the surface of the aluminum alloy. With the increase of the content of sodium phosphate, the grains deepened gradually. When the content of sodium phosphate was 2 g dm−3 , the grains were distinct and the
etching effect was excellent. When the content of sodium phosphate was too high, the grains became shallow inversely and blurred. With the increase of the current density, the distances between the grains became narrow and deepened; when the current density was too high, the complete dissolution of the surface of the aluminum alloy took place, the grains showed big distances and became shallow. In general, the current density was selected as 3 A dm−2 . With the rising of the temperature from the lower temperature, the distances between grains increased and the grains became thicker; at higher temperatures, the distance between the grains were still bigger and the grains became thinner. Generally speaking, at lower temperatures, the etching effect was better, the temperature was selected as 15 ◦ C. At such a temperature, it is unnecessary to heat the solution in the production process. In summer, the current density can be raised so as to guarantee the quality of the product without temperature drop. This is become the influence of the current density on the grain distance is opposite to that of the temperature on the
Fig. 4. Potential vs. time of the aluminum alloy for its electrolysis in etching solutions (concentration: 0.1 mol dm−3 ).
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Fig. 5. SEM micrographs of the electrolytically etched grains on the surface of the aluminum alloy: (a) graining zone; (b) grainless zone.
grain distance. The electrolysis time depends on the current density; if the current density is lower, the electrolysis time is relatively longer and vice versa. From what have been said above, we can sum up the compositions and operating conditions of the optimum electrolytically etched graining for the aluminum alloy: sodium metaborate (NaBO2 ·4H2 O) 5 g dm−3 ; boric acid (H3 BO3 ) 2 g dm−3 ; sodium carbonate (Na2 CO3 ) 3 g dm−3 ; sodium phosphate (Na3 PO4 ·12H2 O) 2 g dm−3 ; current density (J∼ ) 3 A dm−2 ; temperature (T) 15 ◦ C; time (t) 15 min. Grain depth of 0.01–0.03 mm, grain width of 0.5–1.0 mm and the distance between grains of 1–3 mm are obtained on the surface of aluminum alloy from the above technology. Having been electrolytically etched, the aluminum alloy has to be submitted to the removal of the film in the film-removing solution made from sodium carbonate and sodium dicarbonate, and the anodization in a solution of sulfuric acid; finally, the golden yellow coloring of it by means of silver salt and the reddish-brown coloring of it by means of copper salt are carried out.
3.2. Microstructure of the electrolytically etched grains The SEM micrographs of the electrolytically etched grains on the surface of the aluminum alloy (Fig. 5) show that the oxidized film in the graining zone is thick and more loose, may be porous layers exist; in the grainless zone, the oxidized film is very thin, only forming a poreless barrier layer. The SEM micrographs of the anodized surface on the aluminum alloy after having been electrolytically etched as shown in Fig. 6 show that a compact porous layer formed in the graining zone and the pore density was big; although there was also a porous layer in the grainless zone, the pore density was small and the pore diameter was small. This indicates that anodization makes the films in the graining zone and the grainless zone thick, but the microstructure in the grain zone is obviously different from that in the grainless zone, i.e., the pore density and the pore diameter of the oxidized film in the graining zone are all bigger.
Fig. 6. SEM micrographs of the anodized surface on the aluminum alloy after having been electrolytically etched: (a) graining zone; (b) grainless zone.
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Fig. 7. SEM micrographs of the electrolytic coloring surface of the aluminum alloy after having been electrolytically etched and anodized: (a) graining zone; (b) grainless zone.
SEM micrographs of the electrolytic coloring surface of the aluminum alloy after having been electrolytically etched and anodized as shown in Fig. 7 show that the cell-like species formed in the graining zone. This is the result of the cross-linking of the deposited metal colloidal particles and its compound in the adjacent micro-pores in the oxidized film; while in the grainless zone, the cross-linking hardly takes place due to the small pore density of the oxidized film, thus the black cell-like species is not observed. 3.3. Structure of the electrolytically etched graining surface of the aluminum alloy Fig. 8 shows the XRD diagrams of the grained surface of the aluminum alloy by electrolytic etching. From Fig. 8,
Fig. 8. XRD diagrams of the grained surface of the aluminum alloy: (a) electrolytically etched; (b) not electrolytically etched.
we can see that the electrolytically etched graining surface of the aluminum alloy consists of aluminum covered by the extremely thin film of Al2 O3 , which is comprised of ␣-Al2 O3 and ␥-Al2 O3 . This is because the exploring depth of the XRD is rather bigger than the thickness of the film of Al2 O3 , the diffraction peak of Al is very strong. The comparison of part a with part b of Fig. 8 indicates that Al atom preferentially orientates on the crystal plane (2 2 0). However, the electrolytic etching does not change the crystallization state of the aluminum substrate; it is possible to etch the surface of the aluminum alloy electrolytically and it is believed that this phenomenon is attributed to the orientations of the crystal planes of the substrate. The relative intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 are raised by 10%, meaning that the contents of ␣-Al2 O3 and ␥-Al2 O3 in the graining zone are much higher than those in the grainless zone, i.e., the oxidized film in the graining zone is mainly comprised of ␣-Al2 O3 and ␥-Al2 O3 ; of course, an extremely thin oxidized film (Al2 O3 ) exists in the grainless zone. Fig. 9 shows the XRD diagrams of the electrolytically etched and anodized surface of the aluminum alloy. It is known from Fig. 9 that the surface of the aluminum alloy is still comprised of Al, ␣-Al2 O3 and ␥-Al2 O3 , but the preferential orientation of Al on the crystal plane (2 2 0) is more obvious, it becomes the strongest peak. This is because the surfacial Al of the aluminum alloy is changed into Al2 O3 during the process of anodization according to the analysis mentioned above, which makes the preferential orientation of Al in the substrate more obvious. The comparison of Fig. 8b with Fig. 9b indicates that the anodization does not show big effect on the XRD diagrams in the grainless zone, only makes the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 stronger. This is the result of the increase of the content of Al2 O3 on the surface of the aluminum alloy due to the increase of the thickness of the oxidized film caused by anodization. The comparison of Fig. 8a with Fig. 9a indicates
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the content of Ag in the film is very low. The comparison of Fig. 9b with Fig. 10b shows that on the grainless surface, the preferential orientation of Al on the crystal plane (2 2 0) is more obvious due to the coloring and the intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 are increased compared with those of Al on the crystal plane (1 1 1). The comparison of Fig. 9a with Fig. 10a shows that the electrolytic coloring makes the relative intensity of the diffraction peak of Al2 O3 increased, which may be that the coloring metal deposits on the pore wall or the pore bottom in the porous oxidized film, which acts the sealing effect on the aluminum substrate and hence makes the intensity of the diffraction peak of Al decreased. Thus the intensity of Al2 O3 is relatively increased. Fig. 9. XRD diagrams of the electrolytically etched and anodized surface of the aluminum alloy: (a) electrolytically etched; (b) not electrolytically etched.
that preferential orientation of Al on the crystal plane (2 2 0) is more obvious in the graining zone due to anodization, while the intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 are not changed compared with those of Al on the crystal plane (1 1 1). The increase of the content of Al2 O3 due to anodization is not shown in the XRD diagrams of the aluminum alloy. Fig. 10 shows the XRD diagrams of the electrolytically colored sample. The comparison of part a with part b of Fig. 10 indicates that the XRD diagrams of the electrolytically etched grained sample is generally similar to that of grainless sample; the difference of which is that the intensities of the diffraction peaks of ␣-Al2 O3 and ␥-Al2 O3 in the electrolytically etched sample are relatively stronger, indicating that the contents of those components are relatively higher in the graining zone. In the XRD diagrams there is no diffraction peak of coloring component Ag, this is because
Fig. 10. XRD diagrams of the electrolytically etched, anodized and colored surface of the aluminum alloy by means of Ag salt: (a) electrolytically etched; (b) not electrolytically etched.
3.4. Chemical composition of the grained surface of the aluminum alloy by electrolytic etching Fig. 11a shows the XPS spectrogram of the electrolytically etched surface of the aluminum alloy. The surfaces of the graining zone and grainless zone are all comprised of Al, O and C, where C is the pollutant. In order to elucidate the chemical states of the surfacial Al and O, the high-resolution XPS analyses should be carried out. Fig. 11b shows the high-resolution XPS spectra of Al 2p on the electrolytically etched surface of the aluminum alloy. By considering the XPS spectra of Al 2p and O 1s and reading the relative handbook, the analytical results can be obtained and are listed in Table 1. From Table 1, it can be
Fig. 11. Typical XPS spectrogram (a) and high-resolution XPS spectra of Al 2p (b) on the electrolytically etched surface of the aluminum alloy.
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Table 1 XPS quantitative analysis of the electrolytically etched graining surface of the aluminum alloy Sample
Binding energy (eV)
Composition
Al 2p (theory: 74.0, 71.3)
O 1s (theory: 531.0)
Not electrolytically etched surface Surface Sputtered for 5 min
73.726, 71.011 73.937, 71.060
531.03 530.83
Al2 O3 , Al Al2 O3 , Al
Electrolytically etched surface Surface Sputtered for 5 min
73.822 74.024, 70.793
531.22 531.85
Al2 O3 Al2 O3 , Al
seen that the grainless zone of the aluminum alloy consists of Al and Al2 O3 and the content of Al2 O3 is relatively high, indicating that aluminum is easily oxidized to form Al2 O3 in atmosphere. After being sputtered by Ar+ stream for 5 min, the content of Al was found to be obviously increased, indicating that the oxidized film is very thin. The electrolytically etched surface of the aluminum alloy is only comprised of Al2 O3 , after being sputtered by Ar+ stream. It was found that there was a few amounts of Al to exist, this indicates that the electrolytically etched surface of the aluminum alloy is mainly comprised of Al2 O3 , as it is sputtered by Ar+ stream, the oxidized film in the grainless zone is relatively thin and is easily etched away, thus the analysis shows the existence of Al. In the graining zone, the oxidized film is relatively thick. Therefore the surface is mainly comprised of Al2 O3 , and this is consistent with the XRD results. Table 2 lists XPS analytical results of the electrolytically etched grained and electrolytic colored surface of the aluminum alloy. From Table 2, it can be seen that the silver in the coloring film exists in colloidal Ag particles with a certain amount of Ag2 O and the coloring film contains a certain amount of SO4 2− . The above-mentioned analyses indicate that after the surface of the aluminum alloy was grained by electrolytically Table 2 XPS quantitative analysis of the electrolytic coloring film on the electrolytically etched surface of the aluminum alloy Sample
Not electrolytically etched surface (eV)
Electrolytically etched surface (eV)
Composition
Al 2p Theory Experiment
74.0 74.1
74.0 74.2
O 1s Theory Experiment
531.0 531.2
531.0 531.3
Ag 3d5/2 Theory Experiment
367.9, 368.2 368.0
367.9, 368.2 368.1, 368.5
Ag Ag2 O
S 2p3/2 Theory Experiment
169.9 170.1
169.9 170.2
SO4 2−
Al2 O3
etching, an Al2 O3 film formed and the oxidized film in the graining zone was thicker. Having been anodized and electrolytically colored by means of silver salt, the electrolytically etched surface of the aluminum alloy was covered by a thicker Al2 O3 film. The coloring metal silver exists in a mixture of colloidal silver and Ag2 O in the oxidized film, and at the same time it was found that SO4 2− existed in the Al2 O3 film, which provided the experimental evidence for exploring the mechanisms of the anodization and coloring of the aluminum alloy.
4. Conclusion The electrolytic etching solution of the aluminum alloy has been prepared from sodium carbonate as the etchant, sodium phosphate as the enhancing bond-agent, boric acid and sodium metaboricate by the experimental combination of the species forming a barrier layer on the surface of the aluminum alloy and the species etching and dissolving the former. The surfacial grains of the aluminum alloy obtained by being electrolytically etched in the above-mentioned electrolytic etching solution are distinct, natural and smooth; the electrolytic etching solution is reliable and steady, easily adjusted and the operation is simple and feasible. In the microstructure, anodization technology makes the difference between the electrolytically etched graining zone, and the grainless zone increased, and the pore density and the pore diameter of the porous layer of the oxidized film in the graining zone are bigger; the electrolytic coloring metal particles in the micro-pores of the oxidized film in the graining zone undergo cross-linking, forming bigger cell-like species and hence showing a dark color. The electrolytically etched surface of the aluminum alloy consists of Al and Al2 O3 , where Al2 O3 is mainly located in the graining zone and exists in ␣-Al2 O3 and ␥-Al2 O3 ; the Al2 O3 film in the grainless zone is very thin. The anodization does not show a significant effect on the structure and the composition of the surfacial oxidized film, but introduces SO4 2− into the oxidized film. This is the result of the electric migration of the anions in the anodizing solution. The electrolytic coloring metal silver deposits in the micro-pores of the porous layers of the oxidized film in a mixture of silver colloid and Ag2 O.
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