edited by GEORGE L. GILBERT Denison University Granville. Ohio 43023
Dyeing of Anodized Aluminum SUBMITTEDBY
Leonard C. Grotz Universitv of Wisconsin Wa~reshaCowry Center 1500 Un rersw Drove W ~ J I R IW ~ 53186
George Wollaston Clarion State College Clarion. PA Preparation Three strips of Al sheet approximately 15 X 3 X 0.065 em, 750 ml of 0.5 M NasP01 in 800 ml beaker, 750 ml of 1 M H2S04in 800 ml beaker. dyeing solution consisting of 3.0 g Congo Red, 375 ml H20, and 375 ml ethyl alcohol in 800 ml beaker, distilled water in wash bottle. source of 15-20 volt DC electricity. Demonstration
Clean the three strips of A1 by placing them in the NasP04 solution for 5 min. Evolution of Hz gas will occur and some AI(OHL will nrecinitate. Rinse with a stream of distilled water fr& the wash hothe. Place two of the strips in the 1M HzSO4 and clamp them to the sides of the beaker with alligator clips connected to the DC source. Electrolyze a t 1 A for 30 min. Note the evolution of H?- eas - a t the cathode. Keeo the current a t or below I A to prevent too large a rise in temperature. Remove electrodes, rinse with water, and place them in the dyeing solution, preheated to 60°C, along with the nonelectrolyzed strip. After 5 miu, remove the strips and rinse with distilled water. The anodized strip acquires a deep orange color whereas the dye solution readily washes off of the cathode and the nondectrolyzed strip. Another common dye that may be used if desired is ammonium aurintricarboxylate
(aluminon reagent) which will dye the Al red. Heating the dyed Al in boiling water for 10 min improves the adhesion of the dye to the Al. Remarks The interaction between dyes in solution and an ordinary Al metal surface is minimal. This situation can be changed, however, and Al metal dyed by the electrolysis of an Al anode in a system where the A1 metal is the easiest substance to be oxidized. In this process, called anodization, the Al metal is oxidized to A13+ ion which then undergoes hydrolysis to form a film of hydrated A1203 on the surface of the A1 metal. anode A1
-
A13+
-
+ 3e-
+ 3Hs0 AI(0H)a + 3H+ 2AI(OH)3+ (x - 3)HrO AI2O3.XHZO APt
cathode 2Ht
+ 2e-
-
Hz
The basic oxide film on the surface interacts strongly with acidic dyes such as Congo Red, causing adhesion of the dye to the metal surface. Since the thickness of the deposit of aluminum oxide increases with electrolysis time, the amount of dye deposited and the degree of color developed also increase with electrolysis time. These reactions also serve as the basis for the use of hydrated A1203 as a "mordant" in the dyeing of fabrics. Aluminum hydroxide, formed in the pores of the cloth, forms a "lake" or complex salt with the dye. Heating the dyed hydrated Alsos removes part of the water of hydration, thereby forming a more dense film which seals in the dye. Anodization of Al is widely practiced in industry to improve the corrosion resistance of articles made of aluminum.
Volume 60 Number 9
S e ~ t e m b e r1983
763
