Anodising of Aluminum Alloys

Pilot D-300 Concentrate is a 50% aqueous solution of 4-sulfophthalic acid. It is the primary component of the preferred electrolyte for the production of “hard” anodic oxide coatings on aluminum and aluminum base alloys. Such coatings, in addition to being highly abrasion resistant, are colored in metallic gray, brownish gray, black, bronzes, gold and dark brown colors, which are attractive for architectural and other uses of aluminum.

The colors obtained with the D-300 process depend on the base metal composition, current density, voltage, film thickness and electrolyte composition, particularly the sulfate and aluminum contents. Therefore, it is necessary to control all these factors carefully in order to obtain consistent color in the finished product.

The preferred electrolyte composition range is shown in TABLE 1, and TABLE 2 gives specific examples. From TABLE 1 it is shown that a small and controlled amount of sulfuric acid is necessary for the operation. The coatings produced are colored and have an extremely high abrasion resistance. Electrolytes A and C (TABLE 2) are preferred for producing colored coatings of high abrasion resistance at room temperature, but D and E, where the proportion of sulfuric acid is much higher, give light colored coatings unless the temperature is lowered to 10°C. Using example A at 24°C and 24 amp/sq. ft. (2.59A/dm²) a coating of 25μm is built up in 30 minutes during which time the voltage increases from 32-70 volts. The colors vary from dull gold on pure aluminum, through bronze on 6063 alloy to black on 7075 alloy. In TABLE 3 it is shown that many different colors can be obtained by varying the density of the coating on the alloys shown.

Decorative and protective oxide coatings can be efficiently produced by the D-300 process under a wide range of practical operating conditions, at moderate cost, with moderate voltage and cooling requirements.

 

TABLE 1 Operating Limits in the D-300 Process

Pilot D-300 Concentrate 70-130 g/l. Sulfuric Acid 3-10 g/l. Temperature 15.6-32.7ºC Current Density 12-36 amp/sq. ft. (1.3-3.9 A/dm²)

TABLE 2 Examples of the D-300 Process

Electrolytes A B C D E D-300 Concentrate g/l. 100 10 75 50 75 Sulfuric Acid g/l. 8 1 10 100 65 Water Balance

TABLE 3 Colors Obtained with the D-300 Process

Alloy Description Nominal Constituents (%) Color of Coating 1100 Normal impurities dull gold 5052-H34 2.6 Mg. 0.25 Cr pale yellow 6061-T6 0.6 Si, 0.3 Cu, 1 Mg, 0.25 Cr black 6063-T5 0.4 Si, 0.7 Mg bronze 7075-T6 1.6 Cu, 2.5 Mg, 0.3 Cr, 5.6 Zn black 8013 0.35 Cr gold bronze

 

The information and suggestions given are drawn from data we believe to be reliable, but in all cases the user should check and confirm the suggestions and results in their own use before proceeding further. None of the suggestions or recommendations constitute freedom from any patents that may be existent in the field or be issued.

© Pilot Chemical Company