Aluminum anodizing: solutions
anodizing process usually takes place in solution – electrolyte. As the electrolyte for aluminum anodizing apply a very large amount of chemical solutions. Most electrolytes are acidic, but known and alkaline electrolytes. The highest tonnage produced anodizing in sulfuric acid solutions, however, other acids is also used on an industrial scale for anodic coatings with special properties.
Chromic acid was used in the first commercial anodizing process in the twenties of the last century. The solution contains 30 to 50 g of chromic acid anhydride CrO3 per liter. Tension increases to 40-50 In, current density - in the range of 0,1 to 0,5 A / EP2. Temperature - about 40 ° C. Anodic coating has a small thickness, from 3 to 4 m, and usually matte gray or brownish color. Chromic anodizing coating is widely used, for example, for processing aircraft parts for the following reasons:
- A good basis for further painting.
- The oxide becomes a minimum quantity of aluminum, so the thickness of the product is almost unchanged - it is possible to process thin sheet metal parts.
- Loss of fatigue resistance less, than in sulfuric acid anodizing.
- After contact of chromic acid, for example, a riveted connection or overlap compound, much less threat of corrosion, than in sulfuric acid anodizing.
Anodizing aluminum sulphate
Safety sulfate anodizing It is used for almost all aluminum alloys, decorative - only part of the alloys. So, GOST 9.031-74 produce requires only decorative anodic coating on articles of aluminum marks AD0 and AD1, alloys AMts, AMg0,5, AMg2, AMg4, AD31, AD35, 1915 and 1935 GOST 4784-97. Anodising in sulfuric acid gives a translucent colorless coating thickness of up to 35 micron. Exterior coatings is strongly dependent on the initial quality of the aluminum surface.
QUALANOD prescribes the following technological parameters for sulfuric acid anodizing: sulfuric acid content - up to 200 g / l (150-200 g / l), current density for thickness 20 and 25 m – from 1,5 to 2,0 A / EP2, for the thickness 15 um - from 1,4 to 2,0 A / EP2, and thicknesses 5 and 10 um - from 1,2 to 2,0 A / EP2. Anodizing bath temperature should be no higher 21 ° C for thicknesses 5 and 10 m, and not more than 20 ° C – thicknesses 15, 20 and 25 μm .
A variety of sulfate anodization is a so-called "hard anodizing". It is conducted at a temperature ranging from minus 5 ° C to plus 5 ° C, and indeed very hard coating is obtained.
Anodizing of aluminum in the oxalic acid solution
When anodizing in oxalic acid solution obtained transparent solid, somewhat yellowish coating, that, especially in Japan, used in the construction of buildings. oxalic acid concentration in the solution is from 3 to 5 % (by mass), the current density - from 1 to 2 A / EP2, power - from 40 to 60 B and temperature - 18-20 ° C. anodic coating, obtained in the oxalic acid solution, has high resistance to wear - is twice as high, than a conventional sulfuric acid. This process was first integral of coating color, the acclaimed.
QUALANOD specifications include, along with pure sulfuric acid anodizing, anodizing in sulfuric acid with addition of oxalic acid: sulfuric acid content – to 200 g / l oxalic - about 7 g / l. This gives, in particular, the ability to keep the temperature of the anodizing bath at a temperature of up to 24 ° C, instead of the more stringent 20 ° C for the thickness of coatings 20 and 25 m for pure sulfuric acid anodizing.
Anodizing of aluminum in phosphoric acid solution
anodic coating, which was prepared in the phosphoric acid solution, have larger pores, than a conventional sulfuric acid anodizing. Therefore, these coatings are used as the basis for obtaining coatings by electrical deposition of metals on aluminum, and in the preparation of aluminum surfaces for adhesive bonding of parts in the construction of aircraft. This process is also known as a process Boeing. Its main parameters: concentration of phosphoric acid – 10-12 % (by mass), voltage – 10-15 In, temperature 21-24 ° C.
Electrolytes for the integral color anodizing
These electrolytes include a wide variety of solutions of organic acids, usually with small additions of sulfuric acid. The resulting oxide coatings are iznosstoykost doubled compared to conventional coatings Sulfuric. range of colors – from pale golden to bronze and then to black. These processes are used extensively in the manufacture of building products, such as windows, entrances to shops and building facades. Integral anodic coating is very resistant to discoloration, which distinguishes them from the adsorption of colored anodic coatings.
1. TALAT 5203
2. QUALANOD Specifications, edition 01.01.2015