Grain structure visible through anodising

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,

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

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!

Regards,

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.

A. In a side 5 gal bucket see what happens when you add 1 oz/gal sodium fluoride and a smidgen of hex chrome (from the chromate or the chrome plate). You should see a finer grain.

A. I see this exact problem with some customer parts (about 5%, couple thousand parts in total); my theory is that it has to do with extrusion process of the base material.

The parts I saw this on were milled on all sides made from 1" * 3" extrusions (6082), then this extrusion is cut into length, and machined, scotchbrite wheeled for cosmetic purposes on one side and perimeter; the defect shows up only on the scotchbrite wheeled surface. Remedy was to to lightly scotchbrite affected surfaces on parts after etching (straight NaOH 30 gr/l 60 °C 45 sec) and then anodize -- sometimes the "staining" still reappears after anodizing, but very slightly, almost not noticeable, mostly it isn't noticeable at all.

And the parts are hung on the fixture in multiples of 10 -- 9 might be fine, one might have this defect, the part may be like 6th counting from top (other times it will be near top of a rack, or bottom, it is totally random), so it isn't related to part spending more/less time in etch or rinses, etc.

So, as I said, my theory is that it is related to extrusion process, like parts cut from the beginning or end of the blank will be affected, and middle parts will have no problems whatsoever, and the effect seems to be more pronounced if the final machined surface was close to outside dimension of the blank, meaning very little amount of thickness was milled off, because the other side, where more material is being removed, shows no signs of this "staining"

Spending less time in etch helps, but sometimes etching is a prerequisite to get the desired texture.

A. In my opinion this is a coarse grain structure from the production of the semi-finished product, e.g., coarse grain during extrusion (often only partially, for example pressing end, only one portion is affected).
Sandblasting will only hide the problem. The surface is a bit rougher and so the coarse grain is not so good visible.
With a stronger etching or a thicker anodized layer, the coarse grain is also more visible.
It is important to improve the material quality. Improve extrusion process and quality control of the material.

A. Hi Othman. I'm having a lit bit of trouble understanding the question because you speak of problems in black anodizing, but at least 4 of the pics seem to be showing us clear anodizing.

In pic 1 the lower piece has what might be flow marks -- it's hard to see because of the reflection in the lighting; the upper piece seems to have some whiteness right at the end, but are you sure this is not mechanical damage from assembly?

In pic 2 I see a white spot, but also some fairly deep lines which look to me more lie scratches than flow defects.

In pic 3 (I guess this is "black" anodizing), I'm not sure what those 5 or 6 little pits are in the upper piece, and I find it rather strange that the white marks at the joint line up so well from the top and bottom piece. Are you sure these pieces aren't rammed together causing the damage?

In pic 4 there is so much reflection that I can't see much.

In pic 5, those large triangular marks on two components look to me like burning from excessive heat, poor positioning, or whatever.

I think we need a more deeply detailed description of what you feel the problems are and where they start. Further, you probably need to visit the anodizing subcontractor and 'walk the line' and learn exactly where the defects occur if you haven't. But hopefully another reader can be more insightful than I. Good luck.

Regards,

A. What Ted said PLUS, where two pieces are joined the gas evolution from the back side is diluting the solution at that joint, these are not "white spots" they are the anodic coating from continuous diluted anodizing solution. The fix is to completely join the two pieces with no gap and no edge sticking up to draw extra current.

A. Hmm. I assumed the tubes were anodized before joining. Maybe I was oblivious.

Regards,