Colouring And Sealing of Anodic Oxide Films

Colouring And Sealing of Anodic Oxide Films

Aluminium and its alloys after anodic oxidation treatment, generated a layer of porous oxide film on its surface, after colouring and sealing treatment, you can get a variety of different colours, and can improve the corrosion resistance of the film, wear resistance .

Schematic diagram of electrolytic colouring process:

 Oxide film colouring

(1) Inorganic pigment colouring

mainly physical adsorption, i.e. inorganic pigment molecules are adsorbed on the surface of microporous of the film layer to be filled. The method of colouring hue is not bright, and the combination with the substrate is poor, but the sun resistance is better. The dyes used for inorganic pigment colouring are divided into two kinds, and the metal after anodic oxidation should be impregnated in two kinds of solutions alternately until the number of reactive products (pigments) of two kinds of salts in the oxidized film meets the required hue.

(2) Organic dye colouring

 the mechanism is more complex, generally believed to have physical adsorption and chemical reaction. Organic dye molecules and alumina chemical combination of the following ways alumina and dye molecules on the phenolic group to form covalent bonds; alumina and dye molecules on the phenolic group to form hydrogen bonds; alumina and dye molecules to form complexes. Organic dyestuffs have a wide range of bright colours, but have poor sunlight resistance. It is better to use distilled water or deionised water to prepare the dyeing solution than tap water, because the calcium and magnesium ions in tap water will be coordinated with the dye molecules to form coordination compounds, which will make the dyeing solution obsolete.

(3) Electrolytic colouring

is the anodic oxidation of aluminium and its alloys into the electrolytic solution containing metal salts for electrolysis, through the electrochemical reaction, so that the heavy metal ions into the pores of the oxide film to metal atoms, deposited in the bottom of the pore on the non-porous layer and colouring (Figure 5.10). The coloured oxide film obtained by the electrolytic colouring process has the advantages of good abrasion resistance, sunlight resistance, heat resistance, corrosion resistance and stable and long-lasting colour, and is now widely used in aluminium profiles for architectural decoration. The higher voltage and longer time used for electrolytic colouring, the darker the colour will be.

Sealing treatment of oxide film

After anodic oxidation of aluminium and its alloys, no matter whether it is coloured or not, it is necessary to carry out the sealing treatment in time, the purpose of which is to fix the dyes in the micropores, preventing the exudation, and at the same time, improve the film's abrasion, sunlight, corrosion and insulating properties. Sealing methods include hot water sealing method, water vapour sealing method, dichromate sealing method, hydrolysis sealing method and filling sealing method.

(1) The principle of hot water sealing method

is to use the hydration of amorphous AL2O3: AL2O3+nH2O=AL2O3-nH2O

Where  is 1 or 3. When AL2O3 hydration for a hydrated alumina AL2O3-H2O, its volume can increase by about 33%; generated trihydrate alumina AL2O3-3H2O, its volume increased by about 100%. As a result of the hydration of AL2O3 on the surface of the oxide film and the pore wall, the volume increases and closes the film pores.

Hot water closed process for the hot water temperature of 90 ~ 100 ° C, pH 6 ~ 7.5, time 15 ~ 30 min. closed water must be distilled water or deionised water, and can not use tap water, otherwise it will reduce the transparency of the oxide film and colour.

(2) The principle of water vapour closure method

Is the same as the hot water closure method, but the effect is much better, but the cost is higher.

(3) Dichromate closure method

Is carried out in potassium dichromate solution with strong oxidising property and at higher temperature. When the anodized aluminium pieces are fed into the solution, the oxide film and the aluminium oxide of the pore walls have the following chemical reaction with potassium dichromate in the aqueous solution:

2AL2O3+3K2Cr2O7+5H20=2ALOHCrO4+2ALOHCr2O7+6KOH

The alkali aluminium chromate and alkali aluminium dichromate precipitates generated and the alumina monohydrate and alumina trihydrate generated by the hot water molecules together with alumina seal the micropores of the oxide film. The formula and process conditions of the sealing solution are as follows: potassium dichromate 50~70g/L; temperature 90~95 °C; time 15~25 min; pH value 6~7.

The oxide film treated by this method is yellow in colour and has good corrosion resistance. It is suitable for the closure after anodic oxidation of aluminium alloy for the purpose of protection, and is not suitable for the closure of colouring oxide film for the purpose of decoration.

(4) Hydrolysis closure method

Refers to the following hydrolysis reaction after the extremely dilute solution of nickel salt and cobalt salt is adsorbed by the oxide film:

NI2+ + 2H2O=NI(OH)2+2H+

Co2+ + 2H2O = Co(OH)2+2H+.

The generated nickel hydroxide or cobalt hydroxide is deposited in the micropores of the oxide film, thus closing the pores. Because a small amount of nickel hydroxide and cobalt hydroxide is almost colourless, so this method is particularly suitable for the closure of the colouring oxide film.

(5) Filling closed method

In addition to the above described closed method, anodic oxide film can also be used organic substances, such as transparent varnish, molten paraffin, various resins and dry oils, etc. closed.