Colored anodized aluminum and electrolytic method for the manufacture of same

A method for electrocoloring an anodized aluminum product comprising the steps of:

• a) anodizing an aluminum-based substrate by immersing the same as an anode into an electrolytic bath containing sulfuric acid and passing a direct current through said bath between said anode and a metal counter-electrode in order to form a first porous aluminum oxide anodic layer having a non porous barrier layer between the bottom of the pores and the aluminum/aluminum oxide interface;
• b) cleaning the inner surfaces of the pores of said first porous aluminum oxide anodic layer and reducing the thickness of said barrier layer until reaching a desired thickness by treating the thus obtained anodized aluminum-based substrate in an electrocoloring bath containing sulfuric acid at a concentration lower than the concentration of the bath used in step (a) and at least one pigmentary metal salt, under direct current conditions, in order to dissolve the bottoms of the pores by current-assisted dissolution until the barrier layer is reduced to the desired thickness;
• c) electrolytically treating the anodized substrate within the same electrocoloring bath used in step (b) under alternating current conditions in order to form a second porous aluminum oxide anodic layer under the first anodic layer, said second anodic layer having finer pores than the first anodic layer;
• d) electrolytically treating the thus obtained product within the same electrocoloring bath used in step (b) under direct current conditions in order to form a third porous aluminum oxide anodic layer under the second anodic layer, said third anodic layer having fine pores with thicker walls than the pores of the second anodic layer;
• and
• e) electrolytically depositing pigmentary metals within the pores of said first, second and third porous aluminum oxide anodic layers and at least on portions of the interfaces between said layers within the same electrocoloring bath used in step (b) under alternating current conditions at higher voltages than in step (c) in order to prevent formation of additional anodic layers.

A method for electrocoloring an anodized aluminum product comprising the steps of:

• a) anodizing an aluminum-based substrate by immersing the same as an anode into an electrolytic bath containing sulfuric acid and passing a direct current through said bath between said anode and a metal counter-electrode in order to form a first porous aluminum oxide anodic layer having a non porous barrier layer between the bottom of the pores and the aluminum/aluminum oxide interface;
• b) cleaning the inner surfaces of the pores of said first porous aluminum oxide anodic layer and reducing the thickness of said barrier layer until reaching a desired thickness by treating the thus obtained anodized aluminum-based substrate in an electrolytic bath containing sulfuric acid at a concentration lower than the concentration of the bath used in step (a) and under direct current conditions, in order to dissolve the bottoms of the pores by current-assisted dissolution until the barrier layer is reduced to the desired thickness;
• c) electrolytically treating the anodized substrate within the same electrolytic bath used in step (b) under alternating current conditions in order to deposit a second porous aluminum oxide anodic layer under the first anodic layer, said second anodic layer having finer pores than the first anodic layer;
• d) electrolytically treating the thus obtained product within the same electrolytic bath used in step (b) under direct current conditions in order to deposit a third porous aluminum oxide anodic layer under the second anodic layer, said third anodic layer having fine pores with thicker walls than the pores of the second anodic layer;
• and
• e) electrolytically depositing pigmentary metals within the pores of said first, second and third porous aluminum oxide anodic layers and at least on portions of the interfaces between said layers from an electrocoloring bath containing sulfuric acid and at least a salt of one of said pigmentary metals under alternating current conditions at higher voltages than in step (c) in order to prevent formation of additional anodic layers.

A method for electrocoloring an anodized aluminum product comprising the steps of:

• a) anodizing an aluminum-based substrate by immersing the same as an anode into an electrolytic bath containing sulfuric acid and passing a direct current through said bath between said anode and a metal counter-electrode in order to form a first porous aluminum oxide anodic layer having a non porous barrier layer between the bottom of the pores and the aluminum/aluminum oxide interface;
• b) cleaning the inner surfaces of the pores of said first porous aluminum oxide anodic layer and reducing the thickness of said barrier layer until reaching a desired thickness by treating the thus obtained anodized aluminum-based substrate in an electrolytic bath containing sulfuric acid at a concentration lower than the concentration of the bath used in step (a) and under direct current conditions, in order to dissolve the bottoms of the pores by current-assisted dissolution until the barrier layer is reduced to the desired thickness;
• c) electrolytically treating the anodized substrate within the same electrolytic bath used in step (b) under alternating current conditions in order to deposit a second porous aluminum oxide anodic layer under the first anodic layer, said second anodic layer having finer pores than the first anodic layer;
• d) electrolytically treating the thus obtained product within the same electrolytic bath used in step (b) under direct current conditions for a short period of time in order to deposit a third porous aluminum oxide anodic layer under the second anodic layer, said third anodic layer having fine pores with thicker walls than the pores of the second anodic layer;
• e) repeating steps (c) and (d) as many times as desired to form additional pairs of superimposed anodic layers;
• and
• f) electrolytically depositing pigmentary metals within the pores of said first, second and third porous aluminum oxide anodic layers and at least on portions of the interfaces between said layers from an electrolytic bath containing sulfuric acid and at least a salt of one of said pigmentary metals under alternating current conditions at higher voltages than in step (c) in order to prevent formation of additional anodic layers.