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Anodizing different alum. types-causes changes in finish

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I have been anodizing for a few yrs in a small in house shop. we do 99% aluminum 2024 & 5052. here recently one of our jobs has been plating an extruded angle from 6061-t6. we are plating them using the same processes as before, but our results are totally different. the parts do not have any streaking as read in some of the other postings, but are a dull dark grey vs. the usual light alum color. what are we doing wrong? or is this a normal problem? thanks....

Martin Baker
small in house anodizer - Sweetwater, Texas

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What you are seeing is entirely normal. The anodised colour is different because of the different alloying elements used. I think the main thing causing a difference in your situation is the amount of Silicon in the alloy. 2024 contains 0.15%, 5052 0.25% and 6061 0.4-0.8% Si. This difference is enough to give your 6061 anodising a darker grey colour than you would get with the other two alloys.

John Reid
- Brisbane, Qld, Australia

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Let us know the acid purity, acid strength in grams per liter, anodizing temperature, anodizing current density, starting surface finish, cathode material, surface preparation - then we will make some suggestions.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

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Angle is 6061; round parts are 2021

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Acid purity » industrial
Acid strength » 15% by weight
Anodizing temp » room temp. non-heated (80-110F)
Anodizing current density- unknown - fixed battery charger [linked by editor to product info at Amazon] 12v
Steating surface finish- unknown (sanded finish)
Cathode material- lead
Surface preparation- sanded then washed with lacquer thinner [linked by editor to product info at Amazon].

Martin Baker
- Sweetwater, Texas

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The 2000 material is less conductive and requires more voltage to get the same amperage. Obviously your current density is lower on the 2024 so the color is different. You will have to control the voltage and current accurately to get the same color. YOu can raise the acid strength up to 16 or 17%/weight and partially get the colors closer together, but the best way is to go get a rectifier with meters and control.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como