Clear Anodizing Problems & Solutions

? Are the parts champagne color coming out of the anodize tank or after sealing? Some mid-temp seals can impart a slight color.
Q. If the parts look good coming out of the anodize tank you could try a hot DI water seal or high temp seal that advertises no color.

A. Two things to look into, first is that your dissolved aluminum content is pretty high. Most literature sites about 0.5-1.5 oz/gal (4-12 g/L) on dissolved aluminum content. May be worth your time to make up a fresh bath, just make sure to add back enough of the old bath to prevent burning. An option to test this theory would be to make up test cell in a 5 gallon bucket and coat a test panel. You can use frozen water bottles floating on the surface in a pinch to help control the temp as long as you had adequate solution circulation (I've used aquarium pumps in a pinch before).

Second thing, I would consider sending out a sample of your bath and make-up acid to a lab for metals analysis. Your make-up acid could have a metallic contaminant that is contributing to the color of the oxide layer (I'm thinking possibly zinc).

A. 202 Anodize refers to an AA Aluminum Association Spec

• 204 = 30 Min or equiv anodize
• 215 = 60 Min or equiv anodize
• 202 = Thickness to be specified

A. Alumilite 202 is an Alcoa designation for a 0.3 mil thick, clear anodic coating with a boiling water seal.

A. Back in the 50's & 60's ALCOA promoted a licensing system called Alumilite, and they originated the 2xx numbering system. The 204 and 215 which Mr. Kraft cites from the Aluminum Association pretty much follow the original Alumilite 204 and 215. According to an ALCOA reprint I have from 1965, the Alumilite 202 called for 0.3 mils of anodize with a boiling hot water seal.

Phil Johnson
- Madison Heights, Michigan

A. It is an industry wide problem that tables produced which state that certain alloys anodise OK do not distinguish or define what constitutes aesthetic acceptability, what is acceptable on an industrial machine tool may not be acceptable on a piece of jewelry.

The 'problem' you refer to is not at all unusual, indeed almost predictable. The yellowish hue you are seeing is almost certainly associated with the copper content in the alloy and not with anything that the anodiser is doing incorrectly. Aluminium alloys with copper in them are usually used because the copper makes them easier to machine, however, when anodised the copper atoms become incorporated in the anodic film and give a yellow hue to the finished article since they are not translucent in the way the aluminium oxide is. Generally the thicker the film the more noticeable the colour change as more copper atoms are incorporated. From a technical view-point the product is probably fine, (corrosion resistance, abrasion resistance etc) however with a product such as a watch I assume that there are aesthetic requirements as well and this is actually were your problem lies.

Possible ideas to help, ask your aluminium supplier to supply alloy at the low end of the copper content range, (allowable range for Cu is 0.15 to 0.4) and if the product allows anodise with a thinner film, but note that with a thinner film you will probably sacrifice technical performance for improved aesthetics!

Good luck!

A. The most frequent tech service advice to a telephone or finishing.com question is "GET THE CURRENT ON THE PART". Rack the part so that it gets its current from an aluminum bar contact and then use titanium merely to spring hold the part against the aluminum -- get the current on the part. Your two alloys should come out with the same clear shade if properly deoxidized properly contacted, 72 °F, 15%/wt Sulfuric Acid, under 12 gm/L dissolved aluminum, and for the same anodizing time.

A. Teri,

I believe you are battling the amount of Si thats in 6061, but not present in 6063. My only suggestions would be to either lower the current density when anodizing the 6061 (leads to lighter coatings), or, perhaps lower the temp when anodizing the 6063 (leads to darker coatings).

A. I should have done a little more research before I replied to this letter. While I was partially correct about the Si content in 6061 compared to 6063 (.4-.8 Si for 6061, and .2-.6 for 6063) the range for other alloying constituents ( most notably Cu, Cr, and Zinc) is also much higher for the 6061. All, of which, can lead to the color differences that you are seeing. Me thinks, that even if one tightly bolted into both alloys and put them through identical processing conditions for a type 2 class 1 process, that the color differences would still be evident (if I have some 6063 around the shop, I will run this test, photograph, and post the pictures). I used to use racks that had 6063 splines, and 6061 fingers, and after processing, the color difference was striking (although, I'm sure some current robbing was taking place).

Opinion! I feel the need to get on my soap box for a minute, so please indulge me. I wish that the aluminum manufactures would find a way to tighten up their processes, and give us a consistent, quality product, with each mill run they produce. In this day, where quality, and consistency, is of the utmost importance...this is an issue I wish would be addressed. Letter 14352 (by the way, Katrina, if you are reading this, I would suggest skipping or reducing the NaOH etch..this may reduce/eliminate the mottling you are seeing), along with many others, and my own personal experience, clearly indicates this need. I've come across differing tempers (hardnesses), differing colors, differing growth rates, mottling, grain structures -- you name it -- all out of the same alloy. And we, as anodizers, must deal with the end result, and try to explain it to our customers that its the aluminum that is perhaps causing the problems/inconsistencies, as opposed to our process. Which, a lot of times is very difficult -- after all, aluminum is just aluminum, right? WRONG! My company has dealt with the big manufacturers on this issue, and all that we get told, is that the Al is "in spec". It can be, at times, very frustrating. I suppose, if my specifications on a 2 mil hardcoat was anywhere from .0005" to .00035", my parts would always be "in spec" too ... but alas, I am held to a higher standard. I, for one, get a little tired of having to tweak my process to compensate for inconsistencies of different lots of aluminum for the parts that we manufacture and coat. The differences clearly show up with EIS, hardness, SEM/x-ray, and dielectric strength testing that I/we have performed...but yet the material is all "in spec".

Ok, I feel better now. Thanks for indulging me ... ya'll have a great week.

A. 1. Is heat involved ? 2. On some incoming parts, perform an ASTM seal test, if they pass, then leave in the plant the normal time for the fading to happen, if they fail the seal test then you know why. Most job shops do not control their seal and do not perform seal test, so YOU do the seal test and determine whether sealed, then come back to us.

A. Good day Terry.

See letter #18655 from David Hendrick regarding blasting and uniformity, or the lack of it.
There is very good info here at Finishing.com.
You just need to look for it.

Regards,

A. First be sure the sand blasting is UNIFORM because anodizing magnifies ALL defects. Be sure the sand blast media is designated for ALUMINUM only and not loaded with some night shift man's iron gun parts. The racking on all parts must deliver the current evenly to all parts on the rack. The agitation must be uniform, air does not hit parts displacing solution, and the solution moves as much behind the part as in front of then part. Back up stream the time in the etch tank is always the same.

A. Clearer anodizing: Higher temp, higher concentration and lower current density. You did not say 15% what: weight or volume? Try 75 °F, 195 g/L acid, 10 ASF, and experiment plus/minus those levels.

Thank you for your advice,
our materials which were racked in the same bar for anodizing are from the same heat number of billets used.
Supposed to be of same chemical compositions.

What I propose to do is taking 2 bars of different tone material and rework, start from degreasing until sealing.
To see the color tone will be same or different.

BTW, our sealing here is cold sealing at room temperature 25 to 29 degree.

A. Hi Gertjan. The discoloration is under the anodizing and is not removable. Although some people consider 6082 to be "the European equivalent" of 6061 (or maybe 'European substitute' would be a better term), others believe the small additional amounts of alloying ingredients which offer its higher strength make it less suitable for aesthetic anodizing (you can search the site for 'anodize 6082 aluminum' to see the 2nd and 3rd opinions on this subject).

Regardless, it is obvious that the composition or temper of the 'yellow/gold/brown' pieces is different than the clearer pieces and it has reacted differently to some step in the process. I do not think it is possible to resolve this problem without assuring that all the pieces are the same composition and temper. I would start by requesting certifications for the materials which turned brown; they might not even be a 6xxx aluminum :-(

Luck & Regards,

? Ted, correct me if my understanding is incorrect.
The word "discoloration" is something that of colour changing, not that parts coming out with different colour tone.

Hello John. Lim and Gertjan provided pics of their problems, so I don't think that semantics is critical in their cases. Lim called his problem 'different tone', and Gertjan called his problem 'discoloration', and I didn't/don't take issue with either person's use of their wording ... but I'm not the arbiter, and there perhaps could be cases where the precise meaning is critical :-)

Luck & Regards,

A. From the picture you attached, the brown colour area is the recessed area? how do you load the part? I mean you may take measures to avoid air pocket in the anodizing bath, but probably neglect air pocket problem in other tanks, especially the sealing tanks.
tell me whether another side of brown colour is in different colour tone?

A. I respectfully disagree with John that an air pocket could have caused this problem. The welding on the brown pieces is not brown, but even the edge of the brown pieces are brown. I'm confident that there is something different or wrong about the composition or temper of the plates that came out brown.

Luck & Regards,

A. Lim, Gertjan

I am going to second Marc Green's 2002 post.

The tolerances for a material (aluminum) to be in specification are very wide. In comparison, the tolerances that most anodizers hold is very tight in comparison.

Intel drove down their "Copy Exact" program in response to variations in the performance of their 'tools', much of which was due to variation in the materials used. Suppliers of key components (used in the 'tools' Intel uses to manufacture semiconductors) were required to purchase a mill-run (~80,000 pounds) of aluminum to ensure that variations in these key components was not material related.

Prior to implementation of the program, we could identify the different materials used in a single lot or run of product prior to anodizing, even though the manufacturer / machinist claimed they were the same material. After anodize, there was a pronounced difference in appearance and a bi-modal histogram of the thickness.

As the anodize is formed of the aluminum, different heat lots of the same alloy can produce different appearances. Heat lots will have different chemical composition and temper which can affect the appearance of the anodize. Lim's and Gertjan's photos are characteristic of variations in the material.

Lim; consider taking a sample length from each of the two different 'colored' extrusions and immersing them for an extended time, both in NaOH and HNO3; the HNO3 immersion might be hours not minutes. If the materials are the same, they both will appear the same after exposure. If not, the difference in the material may be cause for the difference in the appearance of the anodize. The results won't fix the problem but it may tell you where it is.

Also consider measuring the thickness to see if there is any correlation. If you are anodizing to 5-10 µm, the difference will be very subtle if you do not sample enough.

A. Hi again Gertjan. In the USA we usually call that step "desmut" or "de-oxidize" rather than "pickle".

But in brief, as you surely know but we'll say for the benefit of readers, aluminum alloys are not pure aluminum; they are alloys of aluminum and other materials like silicon, copper, magnesium, and zinc. In the etch step the aluminum dissolves but some of those other materials are left behind and become concentrated on the surface. One of the purposes of the desmut is to try to dissolve away those contaminating materials; so you must choose a desmut solution and operating conditions that work best. The discoloration caused by those alloying materials is also certainly proportional to the anodizing thickness, which is why hard anodizing is usually gray to charcoal in color rather than clear.

So the answer is yes, you can hopefully minimize the tonal differences by minimal etching, optimal desmutting, and minimal anodizing thickness. But when even the welding comes out the right color, the process is probably pretty good, and the material that came out brown pretty bad :-)

Luck & Regards,

Ted, we have every reason to suspect the material composition although Gertjan confirmed that there is no problem with the material.
I know that sealing process will shift the anodized colour tone to the lighter side, I don't know whether Gertjan has ever monitored the colour tone of parts just taken out of the anodizing bath.
Actually, if there is the material issue, in this case during pre-treatment (after etching) experienced worker should know already.

Hi Both,

I think we have lot to sort out and test with the material and process.
For now your info helped us thinking in the right way I guess. Thanks, and if there is anything new or solved we let you know here!

Greetings Gertjan

A. Gertjan

With respect to color versus thickness, for any given alloy, a thicker coating will have more 'color' for any given set of parameters. Parameters can be altered to lighten or darken the anodize for any given thickness. For any given thickness and associated parameters, alloy color can vary. For some (7xxx), the difference is pronounced. The purer the alloy, the less change per unit of thickness.

A. Hi Gertjan,
Let me know what is your desmutting bath's (neutralization) composition?

Is it only nitric acid based or sulfuric acid + oxidizing agent based?

According to your answer, I would advise how you can make a test.

Hi Willie,
thanks for the overview of thickness/material influence on different types of alloys.

@allaattin,
Don't know the details yet. The next production run (not sure when) will be tested more specifically. Then I will have more info.

A. Hi Janis,

Have a look through your records and see if this is unique to one particular heat lot of material, or a recurring issue across multiple lots; also if material from one mill, regardless of heat lot, is more prone to this defect.

Though I've never seen the 'sweating' effect you describe, I've certainly battled enough horribly striped or swirly patterned components- made of everything from plates, bars, and rods, to custom extruded shapes- with obvious material defects that can be lined back up like a puzzle after anodizing.

Hopefully someone with a good explanation of WHY the dark streaks appear; whether it's a region of a higher concentration of one of the alloying constituents, or a problem with non-uniform cooling of the initial ingot, will chime in and explain it! I'm no metallurgist, but you should talk to your material supplier and inform them of the issue. This is a material problem straight from the mill.

If finding a new material supplier, or even rejecting any more shipments of that particular heat lot, is practical; I'd do that.

A. Hi Janis. Is there any chance (considering the different thickness/shape) that the two components were machined on different equipment and that one of the machines used different machining fluid, or overheated it, such that it does not get fully removed in your caustic cleaner? Could you pre-scrub a few parts with a pumice solution to learn anything?

Luck & Regards,

A. Hi Janis,
I suspect your anodizing process parameters. Firstly, why don't you increase caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] amount until 65-70 gr/lt? Up to me, 40 gr/lt is low amount for keeping balance of dissolved aluminium/caustic soda. Also you can increase current density of anodizing bath from 1 amp/dm2 to 1,5-2 amp/dm2. If I were you I would test both, changing and surveying the result. Your problems which you mentioned "zebra effect" and "yellowish-browning" might be solved by this process changing. Also, as last advice, you would skip etching process as test and you can survey non-etched anodized parts for both defect aspect. At least you would understand where is the source of problem depending on results.

A. Janis

I agree with Rachel that you might want to look into the aluminum components coming from 2 different heat lots.

The "B" photo suggests (strongly) that they are different heat lots. The runs/zebra stripes mentioned is the grain structure. Most pronounced at the inside radius on the right where more of a single grain structure is exposed / elongated.

Both etching and anodizing will expose / highlight the grain. Reducing the etch time will help, but not eliminate it.

You might be able to utilize an eddy current thickness tester to identify the different lots prior to anodize. That would go a long ways towards convincing the manufacturer that root cause is the material used to make the parts.

The procedure will work best if you have a sampling of the same part number. Take one part and standardize / normalize the thickness tester on a specific surface for ZERO / base and for a 2 or 3-mil shim ( 5-mil maximum). All subsequent testing should be done on the same designated surface; part / grain orientation will affect the results.

Once standardized, measure ZERO and the shim value (used for standardizing) on each of the parts. The parts made from a heat lot other than that used for the standardization part may exhibit a shift in the mean values obtained (either for ZERO or the shim value or both). You might need to run the experiment using a different surface with respect to the grain orientation to get the best results.

I've suspected the quench rate and / or temperature as the variable that is out of control causing this issue.

Good luck

A. There is no question to me that the discoloration is from the Alloy used. It would not be the first time a metal supplier sent something without making sure that the alloy is the correct one.Different alloys of aluminum anodize with a different color Hue. AAC has a chart that defines the hue of the anodizing after it has been done. Some are not pronounced but can still be seen when next to another alloy.If you email me I can send the info to you or you can visit "the aluminum anodizing council"

A. Hi

Color differences can often be observed in alloy 6082. Partly on the composition, but also often on the structure.
The micro structure consists of primary and secondary phases. The secondary phases are mainly formed during the heat treatment. Different cooling conditions or different heat treatment processes can result in micro structural differences. Many secondary phases can cause the anodized layer to turn gray or brown. Differences can sometimes be noticed after etching (different coverings).

Structural differences can be determined by examining the microstructure on the metallographic section on the light microscope (special etching required).

Simple method: measurement of the electrical conductance, many secondary precipitations can cause a higher conductance. Differences in the conductance indicate structural differences.

Was the electrical conductance measured (MS / m,% IACS)?

A. Aluminum Finishing
All alloys of aluminum have different constituents. Some are high in copper others high in Zinc. It really depends on the alloy and what lot or pour it comes from. Now since the oxide is formed from the base aluminum the metals in the alloy will have some affect on the tone of the oxide created. This creates a problem when it comes to anodizing parts and trying to match color. The fact is that even if the correct alloy is used it is sometimes impossible to tell if the pour is the same. Aluminum is poured in large batches and therefore it becomes impossible to be sure that all the parts you intend to anodize are from the same pour. Distributors of aluminum have no need or requirement to insure all the parts are being made are from the same lot/pour, they just supply the aluminum based of size and maybe alloy.
There remains some ability to compensate based of pretreatment /post treatment and dyeing if needed. But it is important to remember that the constituents of the oxide will have a bearing on the final finish when anodizing. Compensating for differences in the lot usually is difficult because it shows up after the parts are done and they do not match what you expected. Here are some ways to help correct this problem they may or may not work depending on the problem:
1- Limit or eliminate the etch prior to de-ox.
2- Increase the De-ox time
3- Add a fluoride in the de-ox.
4- Test random samples of the batch before the final run and adjust time/process.
5- Drink heavily (it will not solve the problem but you will not care)

A. Dave

Perhaps the 'magic' batch has a temper of T6511. Maybe ask whoever manufactured the components for the material certs, or if the raw material / bar stock is still labeled from the mill.