Caustic Soda Etch

[mtarrant]

I want the final job to be as bright as possible. I polish to a high shine with Caswell buff compound and so far have just used a degrease , desmut and then into the acid tank.

It is very hard to get an even colour.

Should I use a short caustic etch after mechanical polish to even up the surface? Will this make much differenct to the bright finish?

[M_D]

There are a lot of factors that can enter into the equation, but the method you are trying can and will work when done within certain parameters.

The first question, is what kind of aluminum are you working with, and what is the alloy? Cast aluminum is tougher than wrought or extruded, and some alloys are harder to work with than others.

The part needs to be free of oxide, which a good buffing should do. The parts need to be completely clean, or you may see uneven color. You need adequate agitation in the anodizing tank, the part needs to stay relatively cool without hot spots. Certain shapes and sizes of parts might require some consideration of the electrical contact point to ensure even current flow.

Without knowing more details, only guess can be made about your problem.

[M_D]

As far as the caustic etch, it will dull the surface relative to solution strength, time in etch bath, and temperature. It may help the uneven color problem though, depending on what the problem really is caused by.

[mtarrant]

Thanks M_D, the object is machined from 2x1 inch 6063 extrusion, wet sanded and polished by hand on a buffing wheel with Caswell Cut and Color compound.
I will try a short custic dip, say 3 minutes, in warm 20% caustic and as see if this improves the result.

[M_D]

You should be able to get a deep glossy finish on 6063 when all is right. I still don't have enough knowledge about your process to make any good guesses as to what the problem is. A good photo may be of some help.

There are many different things that could be causing the uneven color, from not having the part clean enough, some problem in the anodizing process, or even contamination once anodized but before or during dying.

What are you cleaning the part with, and does water sheet afterward?

How are you anodizing the part? (process specifics)

Are you using good clean water for rinses, and rinsing thoroughly after each step?

Are you carefull to not touch the part once cleaned, and is there any chance any solution has been contaminated with oil or grease of any kind?

What color of dye, and what kind of dye?

Dye PH?

Can you see the uneven color before sealing?

Have you sucessfully anodized other parts with good results using the same process?

[mtarrant]

I'll try to detail the process I'm using.
1. Wet sand the part (basiclly it's a rectangular block with flat surfaces) down to 1200 grit.
2. Machine buff on cotton stiched wheel with Caswell cut and buff. This gives a prety good shine but is hard to get the 'shine' completely uniform
3. Degrese with turpentine based degreser to remove the polishing residue.
4. Then attach aluminium welding rod as a hanger and leave in simple green for 15 min.
5. Rinse with distilled water from a pump up garden type pressure prayer. At this point it will pass the water break test.
6. Let hang in desmut (sulphuric + nitric acid + distilled water) for 15 min.
7. Spray rinse with distilled water.
8. Hang in anodising tank and anodise at 1 amp from CC power supply for the required time as per 720 rule. I pump compressed air through the tank to form a constant bubbling acid bath.
9. Spray rinse over the acid tank and then soak 15 min in backing soda + distilled water solution
10. Spray rinse again and suspend in dye at 140 degrees for 15 - 20 min. Note I adjust the PH of the dye with white vinegar (acetic acid) to the Caswell recommend level when dye is hot.)
11. Spray rinse again and then suspend in 180 deg Caswell sealer.

Apart from the dye and sealer, both of which are in electric earthenware slow cookers, the other tanks are all at room temperature.

I have had the best result wil Caswell Deep Red. Not absolultly perfect but pretty good. The variable color is usually with the Caswell deep blue. Bronze was also quite good but again had some variable reflectvity.

I still suspect that the colour variation is more due to uneven brightness rather than contamination as the latter is really obvious. Hence the question over a light etch between the degrease and desmut stages. I also have a suspusion that the time in the anodizing tank might be too low due to a degree of complication in calculating the true surface area of the part. Is it critical where the hanging wire contacts the part and the orientation in the tank. I try to suspend the part in the middle about 4 inch from the lead cathode which runs along the bottom and up both sides of the tank. Cathode surface area is about 5 times the area being anodized.

[mtarrant]

I'll try to detail the process I'm using.
1. Wet sand the part (basically it's a rectangular block with flat surfaces) down to 1200 grit.
2. Machine buff on cotton stitched wheel with Caswell cut and buff. This gives a pretty good shine but is hard to get the 'shine' completely uniform
3. Degrease with turpentine based degreaser to remove the polishing residue.
4. Then attach aluminium welding rod as a hanger and leave in simple green for 15 min.
5. Rinse with distilled water from a pump up garden type pressure prayer. At this point it will pass the water break test.
6. Let hang in desmut (sulphuric + nitric acid + distilled water) for 15 min.
7. Spray rinse with distilled water.
8. Hang in anodizing tank and anodize at 1 amp from CC power supply for the required time as per 720 rule. I pump compressed air through the tank to form a constant bubbling acid bath.
9. Spray rinse over the acid tank and then soak 15 min in backing soda + distilled water solution
10. Spray rinse again and suspend in dye at 140 degrees for 15 - 20 min. Note I adjust the PH of the dye with white vinegar (acetic acid) to the Caswell recommend level when dye is hot.)
11. Spray rinse again and then suspend in 180 deg Caswell sealer.

Apart from the dye and sealer, both of which are in electric earthenware slow cookers, the other tanks are all at room temperature.

I have had the best result wil Caswell Deep Red. Not absolultly perfect but pretty good. The variable color is usually with the Caswell deep blue. Bronze was also quite good but again had some variable reflectvity.

I still suspect that the colour variation is more due to uneven brightness rather than contamination as the latter is really obvious. Hence the question over a light etch between the degrease and desmut stages. I also have a suspusion that the time in the anodizing tank might be too low due to a degree of complication in calculating the true surface area of the part. Is it critical where the hanging wire contacts the part and the orientation in the tank. I try to suspend the part in the middle about 4 inch from the lead cathode which runs along the bottom and up both sides of the tank. Cathode surface area is about 5 times the area being anodized.

[Sid03]

Only quickly read over your post.....

caution with the pump up garden sprayer.....oils from the pump can make there way into your chemicals. bad news.

you dont need to let the part in the baking soda solution for 15 minutes. It should nuetralize the acid very fast. Move it around in there for a couple seconds....when it doesnt bubble anymore you should be good to go. Some guys prefer not to even use that step.

after the baking soda, I would dunk AND spray rinse the parts again, not just spray rinse.

ive heard of the simple green effecting the shine as well. investing in some of caswells degreaser wouldnt be a bad idea....and heat it.

[M_D]

There is a good chance the baking soda rinse is causing your problem, I would just rinse with good quality water. We dip our racks in a RO water rinse tank for 10-30 seconds, then spray rinse. Baking soda caused us problems like you decribe, once we quit using it 90% of our problems went away. I certainly wouldn't leave it in a strong solution that long.

I almost asked if you were using a soda rinse before, but since there are several other things that could cause the same results I wanted to get more details.

A heated cleaner works faster and more effectively.

The sodium hydroxide etch probably will not help you, if you etch the parts long enough to remove enough material it will dull the part. The etch is good for de-oxidizing the aluminum (much better than the desmut/"de-oxidizer"), and helps to even out a matt finish. If the part is free of oxide and you don't want a flatter finish, it is of little use.

We don't desmut parts that don't have smut, so parts that are buffed or are being anodized with a bright fresh machined finish get pre-cleaned of excessive cutting oil, buffing compound, etc., then dipped in the heated cleaner only for the time needed to make water sheet, rinsed in water, and anodized.

[mtarrant]

Thanks M_D,
I have just finished a run as follows:
1. Polish to high shine
2. Short rinse in solvent based degreaser to remove polish residue. (I can't buy Caswell degreaser due to air freight)
3. Soak in warm Simple Green and agitatate until water sheets ok
4. Distilled water rinse
5. Anodize
6. Spray rinse
7. Suspend in distilled water tank for about 5 minutes.
8. Spay rinse
9. Dye at 140 deg
10 Seal at 180 degree

The result using violet dye is perfect. Perhaps the baking soda was the culprit.