Repeat Anodizing

[mtarrant]

I have had a couple of instances where dye did not take evenly. I stripped the old dye using caustic soda (20%) and re-anodized. I use the same current and time the second time.
Problem is that when dying a second time the colour is much lighter. Anyone have a solution?

[sswee]

What grade of material?

What finish does the part have and how long are you stripping the part?

[mtarrant]

The material is 6063. Original finish was as bright as I could get with mechanical polishing. I did not etch prior to anodizing. I stripped only until all color disapeared. If not sealed, this is only about two minutes. If sealed, about 6 min. I then put in desmut for 15 min. Then reanodised for 95 min at 1 amp (area 30 sq inch). Then 15 min in backing soda solution. Finally distilled water wash and dye at 140 deg for 20 min. Color second time is about 1/2 the density as the first.

[mtarrant]

The material is 6063. Original finish was as bright as I could get with mechanical polishing. I did not etch prior to anodizing. I stripped only until all color disappeared. If not sealed, this is only about two minutes. If sealed, about 6 min. I then put in desmut for 15 min. Then reanodised for 95 min at 1 amp (area 30 sq inch). Then 15 min in backing soda solution. Finally distilled water wash and dye at 140 deg for 20 min. Color second time is about 1/2 the density as the first.

[M_D]

Chances are good that you didn't completely get all of the original anodizing removed, and the subsequent anodizing layer was inhibited from the remaing oxide layer. The same thing happens when parts aren't de-oxidized completely to begin with.

Obviously there is some little glitch in your process or chemicals, but the color might even out if you increased the anodizing time 15-20 minutes or increased the amps per square foot. Make sure your connections are good also.