Colouring Anodized Aluminium
Today there are various applications for aluminum anodizing., including:
- decorative surfaces, showing pleasing aesthetics
- corrosion resistant surfaces
- high hardness surfaces for increased wear resistance
- improved adhesion of top layers of organic paint
- functional surfaces with specified dielectric and optical properties
Decorative surfaces, obtained by anodizing aluminum, have a very high aesthetic value and can be used for various purposes, for example, decorative anodized surfaces of aluminum cases, used in consumer electronic devices, such as shown in pic. 1.
Fig. 1 – Some high-end design products produced by Bang and Olufsen A/S, Denmark:
(a) BeoLab 4000™ speakers and (b) BeoLab 8000™ speakers
that employ decorative anodising of Al for generating visually appealing surfaces [1 ]
In figures 2-5 shown basic data about anodizing, which is obtained by the method of sulfate anodizing. For other anodizing methods and anodic electrolytes see here.
Fig. 2 – the sequence of porous film formation is shown from an already formed anodic barrier film over the surface of Al.
1) The oxide growth (barrier film) in its initial stages is assumed to grow uniformly.
2) As time progresses, the migrating Al3+ from the metal surface to the top of barrier film get concentrated along certain regions and this results in localisation of the electric field. 3) 4) Consequently, lowering of the electric field in the surrounding regions takes place and a scalloped structure at the top surface of barrier film evolves due to the lower field assisted dissolution
[1] Nagayama & Tamura, Thompson, Jensen)]
Fig. 3 – Schematic representation of a porous anodic film showing the principal morphological features [2]
Fig. 4 – Typical Dimensions of the Anodic Cell (Sulphuric Acid Anodizing) [3]
Fig. 5 – Ideal structure of a porous anodic layer (size of the pore is shown as an example, 1 mkm = 1000 nm) [1]
Methods of colouring anodized aluminium
Aluminum and aluminum alloys can be painted in different colors and color, both in the course of the process anodizing, and after it. normal, the most popular methods for painting anodized aluminum profiles, which are used in sulfate anodizing, include (Figure 6):
- adsorption staining;
- electrolytic staining;
- integral staining;
- interference staining.
Fig. 6 – Four methods for coloring anodized aluminum [3]
Adsorptive coloring aluminum
This method is used for hundreds of different dyes. Aluminum product with a colorless anodized, has not filled, immersed in an aqueous (rarely alcohol) solution, usually, organic dye. The color intensity depends on the amount of the dye, absorbed anodized. absorption of the dye is only in the 3-4 micron in pore depth anodizing. The coating is then compacted (Fig.. 7).
For good coloring, as well as high corrosion resistance is required thickness of the anode layer is not less than 15 m. The concentration of the dye solutions may be from 0,2-0,4 g / L for light colors, to 10 g / l for saturated colors. Usually apply hot dye solutions - from 55 to 75 oC, and dyeing duration - from 5 to 15 minutes, saturated colors and may require 30 minutes. An important parameter for dye adsorption is pH, optimum range is generally from 5 to 6.
Fig. 7 -Anodizing pores, hydrate sealing film, and dye interaction during anodizing process [4]
Electrolytic coloring anodised aluminium
Electrolytic staining or "two-step anodization". The process involves immersing the aluminum articles with a colorless anodized sulfate, has not filled, in acidic solution with one or more metal salts, for example, tin sulfate.
The product is connected in electrical circuit with AC or DC. Under these conditions, at the bottom then anodizing the deposition of these same metals. The color depends on the composition of the electrolyte. Most of the metals used (tin, nickel, cobalt, etc.) give colors from light "bronze" to black, and copper – Red color. Color is virtually independent of coating thickness of the anode coating and depends essentially only on the amount of metal, deposited in the pores (figure 8).
Fig. 8 – The process of tin deposition in the pores of the anode coating
The electrolyte is based on stannous sulfate
Lead in 0,2 g / m2 gives bright "bronze", in a 2 g / m2 - saturated black. Properties similar to the properties of electrolytic coating a conventional anodic coating sulfate. Typical based electrolyte contains stannous sulfate 14-18 g / l stannous sulfate, 15-20 g / l sulfuric acid, and organic and inorganic additives. For colors of light "bronze" to black is required from time to 0,5 to 15 minutes. The main application of electrolytic coatings - aluminum extrusions and panels for building facades. Sometimes to get new shades combine adsorption and electrolytic coloring.
The interference aluminum coloring
Interference coloring is a kind of electrolytic coloring. This method allows to obtain a wide range of colors due to the effect of optical interference. Usually, between anodizing and electrolytic painting, an additional operation (bath) is required to treat the anodic coating to expand the bottom of the pores to increase the color intensity.
The amount of metal, deposited in the conventional electrolytic coloring, more, than in the standard interference coatings. However, in the latter case, the metal compactly "packed" at the bottom far. An interference effect occurs between two light-scattering layers: an electro-chemically deposited metal layer at the bottom of the pores and an interface between the oxide layer and aluminum, located directly behind him.
Of all the colors, obtained by this method, It is considered the most attractive gray-blue coating. This method of color staining does not yet have a wide demand for more sophisticated technology and a limited set of colors.
Integral aluminum coloring
When coloring the integral anodic oxide coating is colored itself during the anodizing process. Staining occurs or by conventional anodizing of aluminum alloys in special solutions of organic acids or with conventional anodizing in sulfuric acid of special aluminum alloy.
The oxide layer may be colored in a color of light "bronze" to black depending on its thickness. Since this method requires complicated and acids or exotic alloys such as, it almost completely supplanted by electrolytic coloring, at least, products, which is used in construction.
Sources:
- Optically Designed Anodised Aluminium Surfaces: Microstructural and Electrochemical Aspects / V.Ch. Go ahead – PhD Tesis, Technical Unuversity of Denmark – 2015
- TALAT Lecture 5203 Anodizing of Aluminium – 1994
- https://trimmers-pro.com/how-anodic-coating-on-aluminum-protect-against-corrosion/
