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topic 14352

Grain structure visible through anodising


Q. I am currently having a problem with black anodising grade 6061 aluminium. The parts are cylindrical with and OD varying between 1 and 3cm. When the parts are black anodised the coating replicates the grain structure underneath and appears mottled, this is not passing quality inspection as the coating does not appear uniform (even though it most probably is). Can anything be done while anodising to prevent this or is there something that can be done to treat the aluminium to reduce the large sized grains evident on the surface? Or, would a different aluminium grade be better?

The parts also require turning, so that an easily machinable aluminium is required prior to the anodising.

Is the "mottled" effect due to the extrusion of the aluminium and the severe grain growth experience on the exterior of the rod because of this?

Any suggestions would be most welcome.

Katrina Parlevliet
- Ringwood, Victoria, Australia

simultaneous (2002)

A. Is your alkaline etch loaded with zinc from previously running 7075? Is the alkaline etch too high in dissolved aluminum. What are you deoxidizing in, how long, concentration, contamination level. Is the anodizing solution over 12 gm/L of dissolved aluminum? Can you see the mottled look after anodizing and before dying, if so try dipping in 5% Nitric Acid before dying. Is there high calcium in the dye? Is the dye tank stainless steel, then reverse the galvanic cell any which way you can. Try dying a piece in a beaker in the lab in a new make-up to see if the dye tank is contaminated.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como


A. Your anodizer is over aggressive in his etching, probably using a hot caustic or strong nitric / hydrofluoric acid etch. Suggest using NO caustic etch and only chromic/sulfuric deox prior to anodize. If indeed the grain is from the extrusion process, might try one of many acid "matte" etch products which tend to be elevated temperature fluoride baths.

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser
Syracuse, New York

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A. Katrina,

I think that your evaluation is correct. I do not believe that it is anything to do with the anodising.

6061 is a 'hard' alloy and can be difficult to extrude AND achieve required mechanical properties.

I do not think that you can be confident to ALWAYS have a grain free appearance - the effect is merely cosmetic.

Try keeping the etch time to the absolute minimum.

Also, 6061 is one of the best free machining 6000 series alloys, especially in T6 condition.

If you do not need high mechanical properties, you are probably aware that you could use 6063 or 6060 - this will be far less likely to have the grain effect, BUT, machinability would be compromised.

Best regards,

Martin Webb
- New Zealand


A. Are you sure your material is 6061? 7075 has a strong tendency to show grain structure when etched and anodized.

Guy Lester
- Ontario, California, USA

Sandy feel and grainy look along edges on thin aluminium anodised plates

May 6, 2015

Q. We have just moved premises and have had a small natural anodising job. The sulphuric acid solution is 10% (analysed), solution temperature is 24 °C and the voltage is measured at 4.5 V.

we are getting a very sandy rough finish to the surface with extreme roughness along the thin edges. We are using titanium jigs.

We are using lead anodes and there is a large deposit of white fine crystals on the anodes and in the bottom of the the tank. The tank is polypropylene.

Part looks fine after cleaning and activation

Can you suggest what the problem might be?

Much obliged

Steve Horwood
plating shop owner - Cape Town, South Africa

May 2015

Hi Steve. When you say you "just moved premises", you seem to be implying that you were anodizing successfully until the move ... or am I misreading you? If that is what you are implying, were you able to retain the employees who were delivering good parts before the move?

Three more quick notes: first, it's usually hard to make much progress here until you tell us the alloy, since aluminum alloys behave so differently when anodizing. Second, you say the part looks fine "after cleaning and activation"; what do you mean by activation? Third, satisfactory anodizing at 4.5 V sounds very unlikely to me.

I think we may be able to help you work through this, but we'll have to clearly understand exactly what you are doing before we can tell you what it is that is perhaps being done wrong. Thanks!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

May 7, 2015

A. Couple of observations, but I agree with Ted, we should be able to figure this one out with more info.

1. 24 °C is a little high but not terrible. 21 °C would be preferable.

2. As Ted mentioned 4.5 volts is not enough. For Type 2 anodizing, you'll want a rectifier capable of 21 volts.

3. It almost sounds as if your tank is wired wrong, especially since you mention "lead anodes". You do realize that your lead (ideally you would use aluminum, not lead) acts as the cathode, and should be hooked up to the "-" (negative) output of your rectifier, right? It almost sounds like you wired backwards, and are plating onto your part, instead of anodizing.

Marc Green
Marc Green
anodizer - Idaho

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Grain Marks on Etched 6082 Aluminum Surface

November 2, 2015

Q. I have a problem with grain marks on the Aluminium.
We use Aluminium 6082T651 which is CNC Machined. No grain marks visible after machining. Thereafter we Etch and Natural Anodise. Well, just after the part gets anodised^etched we start to see the lines/grains of the material. This is not very pleasing and we require a more satin matte finish (the Apple aluminium finish)

non-etched aluminum -a etch aluminum -b

No brushing or polishing is done to the part.

Material is Alu 6082 T651 (Nedal Aluminium Germany)
Caustic soda Etch = 60 g/l
Alu = 100 g/l
Additive (Al 5000) = 30 g/l (Metachem Germany)
Temperature = 65 °C

Juan Swart
User - Centurion, Pretoria, South Africa

simultaneous November 3, 2015

A. Use a less aggressive etch. Addition of something like gluconic acid or sodium glutamate might help.

James Watts
- Navarre, Florida

November 4, 2015

A. Two comments:

The "Apple finish" is achieved by mechanical polishing prior to anodizing. In addition to the natural color, they are also sometimes dyed bright colors after anodizing. To duplicate, you will also have to polish prior to anodize.

The patterns shown on your parts are from the etching. To minimize, use lower temperature, lower concentration or shorter time.

jeffrey holmes
Jeffrey Holmes, CEF
South Carolina

November 5, 2015

A. Juan

You don't say how long you etch the part, but if you are using it just to clean, cut back to the minimum.

Willie Alexander
- Colorado Springs, Colorado

simultaneous November 6, 2015

Q. Thanks for the support guys.

Willie Alexander - The sample a posted was etched for 5 minutes. Al concentration was low so I added more aluminum to make the solution less aggressive. However, I also did a 3 and a 1 minute cycle where the 3 did look the same. The 1 min sample looked less etched, less grain marks but the shininess of the aluminum is visible, so in other words it doesn't have that desired matte finish to it.

Jeffrey Holmes - I did however played around with the solution. I added more aluminum to it so it is more or less 100g/l.
Temperature again was at 65"C and my etching time this time was 10min. The effect to the first 5 min sample looks exactly the same.

James Watts - I will try to get the other etching solutions and see what they do to the same Aluminum.

I'm also waiting on some glassware to test my solution properly.

I'm also setting up a wet shot blast using a GB60 (glass bead).
I will post the results once the test is complete to share the knowledge and experience.

My only concern still is the grain lines, will it always be visible? Is it the type of alu (6082) or just the polishing required prior to the etching and anodizing?
Polishing on my products is difficult due to it so small and complex shapes.


Juan Swart [returning]
- Centurion, Guateng, South Africa

November 6, 2015

A. Good day Juan.

Why are you etching?
The parts are machined.
I would think degrease/soak clean and deox only.
What is your deox?

Eric Bogner
Aerotek Mfg. Ltd. - Whitby, Ont., Canada

simultaneous November 8, 2015

A. Unless your etch concentration is very low, 5 minutes sounds extremely long to me. I'm surprised your machining tolerances aren't affected by such a long etch. Why are you going so long?

Marc Green Marc Green
anodizer - Idaho

November 9, 2015

A. Juan

The glass bead may prove to be more appropriate if the natural grain is objectionable. Reduce your etch time to the minimum if you go that route.

The glass bead however may introduce other cosmetic flaws if not performed uniformly.

Willie Alexander
- Colorado Springs, Colorado

November 9, 2015

Q. Eric - We require a smooth matte "Apple finish" to describe it best on our parts. The internal surfaces/faces of our parts are not that important but the outer faces and surfaces are what is important.
The color so far is good but the issue is the material grain lines that we experience after etching.
Please also see earlier posts, that will explain everything again.

Juan Swart [returning]
- Centurion, Guateng, South Africa

November 9, 2015

Hi Juan,
You are on the right track with the glass beading. We do that here regularly to hide the large grain structure that can pop up when you don't want it on extrusions or plate.
I am not familiar with the'wet' blasting (we just use compressed air), but you will have to do some experimenting with how long you etch the parts once they have been blasted. We are usually in the neighborhood of 3 min to 5 min in our caustic no-dump etch bath.
I have attached some pictures of parts cut off a 3" dia rod of 6061. The bad part with a 5 min etch and anodize and the glass beaded part with a 5 min etch and anodize.


Hope you have blast working on your parts.
Jeff Holly

Jeff Holly
- Toronto, Ontario, Canada

simultaneous November 10, 2015

Dear Marc Green,
I did tests with different concentrations. The first been the stronger and the last test the lower concentration.
I also did different times, however, the chemical supplier recommended a low concentration at a timing of 10-15 minutes. The final result (grain marks) can be seen of all of them.
The 1 min etch in a low concentration is not enough but you can also see the grain marks.

Willie Alexander,
Thanks for the heads up. I'm very close in testing the glass bead blasting. Will play around with it.

Jeff Holy,
Thanks for sharing your story. The wet shot bead blast consists of a mixture of water and glass beads, it gives you a finer and slower process, if that make sense.
I noted that you use a 6061 Alloy. I'm in contact with other companies and organisations and they recommended that I change my alloy grade from a 6082 to a 6063. The visible grain is because of the higher Magnesium content in the 6082.

So, that is my next test.

Thanks for all the information and advise.

Juan Swart [returning]
- Centurion, Guateng, South Africa

November 10, 2015

A. Good day Juan.

Letter #18655 from David Hendrick offers a unique approach regarding surface finish. What concerns me is the fact that your parts are machined, and obviously there are tolerances to be adhered to.

Best of luck.


Eric Bogner.
Lab Tech.

Eric Bogner
Aerotek Mfg. Ltd. - Whitby, Ont., Canada

December 8, 2017

Q. Hello, we have a trouble with grain structure visible on AL-7022 machined parts. From what I've read it is caused by too long etching. We need to find a way to cover the grain structure. In many cases rubbing it with liquid paraffin into the surface makes it a little bit darker and hides some defects. In this case, however, it is not entirely enough. Does anyone have and idea what might work? Re-anodising is sadly not possible due to tolerances that must be adhered to.

Here's what it looks like:

Petr Simek
- Prague, Czech Republic

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