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topic 45421

Black spots in anodized aluminum profile

A discussion started in 2007 but continuing through 2018


Q. Gents,

I'm working in an anodizing plant and we have one defect that I hope you can help me to solve it.

Sometimes we notice some black spots in the aluminum profiles. Those spots will not be shown before anodizing; you can see it only after anodizing process. The hardness is very low in these spots.

When we analyze the sample we got the same chemical composition in the good and defected area. We are producing 6063 with below analysis. Can you please clarify this defect to me and how can we avoid it?

Thanking you for your best cooperation.

Sultan Mohammed
Production Engineer - Dammam, Saudi Arabia


A. Hi Sultan,
It's an extruder's problem - this is known as HOT SPOTS defect, mostly in elliptical shape and in various sizes. You cannot do anything to it in your surface treatment facility. Extruders have to improve the cooling of hot extrusions during extrusion process. You can send an official complaint to the extruder for the replacement of those profiles.

Good luck

Srimay Basu
- Dubai, U.A.E.

May 26, 2008

Q. Does anyone know if these spots can also occur in aluminum forgings (in particular 7049) as they do in extrusions?

Rocco Varano
- Longueuil, Quebec, Canada

February 20, 2009

Q. I have the same problem with 7050-T6 sheet, do you think the reason is also these "hot spots"

Thank you

rene pantoja
- Seattle, Washington

April 10, 2009

Q. I am experiencing black spots on parts machined out of AL-7075 after the parts are clear coat anodized at 20 micron. What is causing the black spots and how can they be eliminated and overcome this problem?

Arthur Koo
employee - Hong Kong, China

July 14, 2010

A. Yes. I have encountered the same Black spot after anodizing.
There were no black spots before anodizing.

This may be due to heat accumulation during machining. I feel we need to use good quality cutting Fluid during Machining to avoid the same.

Shivakumar Yalasangi
- Bangalore, Karnataka, India

January 23, 2013

Q. Hello,

I am having issues with an aluminum profile extruded out of 6005 that we use on semi trailers. When we get the product it is perfectly fine, then we apply a swirl pattern to it with our brush machine. Next we seal the piece with an Alumaclear coating then clean the trailer. Once the trailers leave we have an issue with black/lines appearing at the seam lines. They occur anywhere from 12 hours to 1 year after the trailer leaves. We have tried to reproduce the effect at the factory and it never occurs. We were getting the phenomena before we started sealing hoping that Alumaclear would be our answer. Can anyone tell me what would cause this to happen after a period of time?

black lines on aluminum

Thank you for your time.

Kyle Kruzick
Purchasing - Knox, Indiana, USA

May 7, 2013

Q. Hello respected Anodizers,
We are doing 10% Sulfuric Acid Anodizing with Oxalic acid using titanium racks. Item is made of Aluminium rods, Type 2017.
Process sequence:
Degreasing (Caustic + Soda Ash at 40 degrees Cent.)
1 x rinsing in tap water
Chromic acid pickling
1 x rinsing in tap water
1 x rinsing tap water
Sodium dichromate passivation at 85 °C. (in Iron tank)
Sealing in Sealant at 85 ° (In iron tank)
1 x DI Water rinse.

We are frequently getting black spots on the surface of the items anodized which are in the form of small black pits when observed under a microscope.
As per our experience these are due to the oil stains or dirt left over during previous machining operations and left on the items due to ineffective cleaning.
Can some experienced anodizer please help us as to what are these black spots and how to get rid of them?

- Karachi, Pakistan

August 28, 2013

Q. Do you think if the same "hot spot" problem happened to this part, too? We outsourced these parts and hundreds of black spots appeared on the surface.


We use Al 6061 alloy to machine parts, but these parts were outsourced to China. They were supposed to use the same alloy. However, I realized that the parts before anodizing were more bright than our parts.

I was wrong in my previous message.
The parts were etched (NaOH) for 3 minutes, and then de-smutted (HNO3) for about an hour (I don't know why so long) when they developed these spots, which are actually small "craters". So they haven't even reached the electrolyte bath for anodizing. I'm attaching here a better picture.


I've just tried with another batch of parts (also Chinese, brighter parts) and there was no problem."

Alfred Maleczki
- Salatiga, Indonesia

September 5, 2013

A. Has enough nitric acid dragged over from the deox into the anodizing tank, or do you have high chloride contamination in the anodizing tank? You can make holes like this in 6061 if the anodizing solution is contaminated with nitric or chloride.

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
and co-author of The Sulfamate Nickel How-To Guide

April 17, 2014

Q. We are experiencing black spots on polished sheet metal (Aluminum 2219, T62) formed parts after anodize, which is outsourced. Before anodize parts are visually inspected for pits, black spots.

45421-4   45421-5

Can anyone tell us what the cause is and how to prevent it? Thanks in advance.

S Gunnam
Aerospace - Los Angeles, California

April 19, 2014

A. Parts look DA sanded for your finish. It's hard to say for sure, but the first place I'd start would be your sandpaper. It's possible you are embedding media from the sandpaper into your aluminum. Make sure you use sandpaper that has AL2O3 as the media, do not use Silicon Carbide sandpaper for anything that gets anodized.

Marc Green
Marc Green
anodizer - Idaho

April 23, 2014

thumbs up signThanks Marc. We use only aluminum oxide media.

S Gunnam [returning]
Aerospace - Los Angeles, California

April 24, 2014

A. S. Gunnam,

I'm not clear as to whether you're the end user of the product, the anodizer, or whether you're the manufacturer and you just outsource the anodizing. It would help to know what parts of the total process on this part that you have direct control over, so we'd know what your limitations are in solving the issue.

It sounds like we've ruled out the obvious, and easiest to fix potential problem with the sandpaper. I downloaded the pictures and blew them up a bit to try and get a close up. Of course, I lost a bit of resolution. It really appears to be some type of contamination, now, whether it's coming from the base metal, something in the machining process, or yes, perhaps something still could be in the finishing process, and finally, something going on in your anodizing process.

Are those black marks raised, or are they pitted? Are you getting continuity on the black marks? When I enlarged the pictures, they look like pits, but hard to say for sure.

Does the issue still appear if the parts are reworked?

It really looks like you're seeing a base material issue. Now the question would be, is the contamination in the material itself, or is it being embedded during subsequent machining or surface finishing operations.

I'm a bit spoiled as an anodizer, as my shop has SEM/EDX capabilities to analyze stuff like this. And that's how I'd start, would be to sacrifice a part, and run an EDX analysis on the black area, to identify what I suspect is some type of metallic contamination. Once it's identified, look at the process from start to finish to narrow down the source of the contamination, beginning with the material itself.

Solving issues like this by looking at pictures with extremely limited process information on the internet is practically impossible. I could list at least dozen or two potential causes. So all I can do is tell you where I'd start, and that would be with the EDX analysis.

Identify the contamination, and then find the source.

Marc Green
Marc Green
anodizer - Idaho

April 22, 2014

Q. Hi Guys,

We experiencing the similar black spot after anodizing. In our case the Aluminium parts 6061 been tumbled then sand blast at our customer side.
Then we'll do Chemical Polishing then anodizing. The black is more visible only after anodizing. Need some inputs very urgently.


Muraly Nehru
- Penang, Malysia

April 3, 2015

Q. How to avoid black spots on 2XXX & 7XXX series aluminium alloys after anodising? We are getting repeated issue.

We controlled the machining speed (reduced the speed, feed & depth of cut) and reduced anodising voltage ramp-up rate and protect them with rust preventive oil. But even after. we are not able to arrest the aluminium getting black spots after anodising.

SK Samy
Metallurgist - Bangalore, Karnataka, India

June 24, 2015

A. Typically, with 7000 and 2000 series you'll notice an extremely quick dye time. I've found that most of what I run of that alloy dyes in five to 7 minutes. I too have experienced the pits and it's fairly simple to avoid. Basically your parts are getting attacked from to much time in dye. I suggest when dying the parts monitor them (stand by the tank and look at them); when they are black, remove from dye, rinse well and seal also. If you leave for long duration in seal with this alloy you may also see white spots; all can be avoided with better attention to parts and tank chemistry. Good luck.

jake zees
metal finishing - marysville Washington

January 9, 2017

A. Gentlemen, this black spotting in this thread is traditionally due to the oxidizing of the aluminum which can occur through acid contamination in the pretreatment process. Several have noted that they have seen this prior to anodizing or prior to coating, and that would support creation of the defect in the pretreatment process.


Joe Morgan(25+ years in OEM Paint Engineering)
- Morton, Illinois USA

March 8, 2018

Q. Hello,
We are an aluminum door handle producer.
We use ALMG3 alloy with 3% Magnesium content.
We casting the door handles by gravity die casting with a small ladle.

We are facing a problem of black dots that are showing only after the anodizing process.

Please help us to find a solution.
Thank you,

Abdulmunem Daaboul
STE ALUCAST SARL - Fouchana, Tunisia

April 10, 2018

A. Hello Mr. Daaboul,
I am assuming that you have made it through all the great information in this thread and eliminated things like chlorides in your ano tanks as suspects...
Although I cannot answer your question, one simple thing you can do to give the community more clues and maybe help you, is to take a couple of your finished castings and cut them down the middle and then anodize the halves and see if the spotting appears in the centers of the castings as well. What this will at least tell you is whether your problem is arising only at the interface between the die and the part, or whether it is throughout the material. Tell us what material your die is as well, and also if any other materials might be introduced into the mold, intentionally or as contamination, and how you are cooling the casting, over how long a period of time, and how/if you are cleaning it off after it is unmolded.
Another piece of information would be whether you are then polishing down the castings, and with what type of abrasive, and how thick a layer are you taking off.
I must declare that I have ZERO real-world experience with the casting process; these questions are only from a basic theoretical understanding from an anodizer's perspective of where one might begin troubleshooting- I have certainly seen parts milled from castings behave badly when plated (poorly-done extrusions, too!), someone with more specific experience relating to your issue will no doubt see your question eventually :)
The more information you give, the more you will get in return! Good luck!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

Black spots in anodized aluminium EN AW 5754

October 17, 2018

Q. Hi all,
we are a middle-sized plant focusing on CNC machining. Part of our production are aluminium parts. Some of these parts are anodized (outsourced). For quite a long time we've been having a problem with small black spots on the surface. It happens to occur on EN AW 5754 material. Spots are mostly longer, in different layers of the material. We know it's not caused by the supplier of the anodizing, since we tried more suppliers with same result.


We also weren't able to find any correlation between content of any element in the material and the spots.
Sometimes re-anodising helps, sometimes not, and the spots appear on different areas.
Does anyone know what is the cause of these spots/how to get rid of them?
Thanks for any advice.

Libor Böhm
Quality manager - Nosislav, Czech Republic

October 17, 2018

A. Hi Libor, you say you've tried a different anodizing supplier ... but have you tried a different supplier for your aluminum stock?
Based on the information you gave, that the defect is in different layers, can sometimes be stripped and reworked but might pop up in another spot, and you are running a high Magnesium wrought alloy (Mg goes black like that) ... If those defect lines are always oriented the same, corresponding to the direction of extrusion, I would wager my paycheck on a material issue!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

October 18, 2018

Q. Hi Rachel, Changing of the supplier of the material was one of the first things we've tried. And not only the supplier, but more importantly, the manufacturer of the material. And it's still the same problem with all of them.

However, I definitely agree it's a material issue. Question is, what is wrong? What particular quality of the material is causing these black spots? I've been looking into composition of each batch and trying to find some feature which would be common for the batches where spots appeared. So far without success. You mentioned Mg. This particular material can contain 2,6 to 3,6% of Mg according to standard. But there is no significant difference in Mg between OK and NOK material batches.

Thanks for engaging!

Libor Böhm [returning]
Quality manager - Nosislav, Czech Republic

simultaneous October 19, 2018

A. Hello Libor,

These fine lines (we say gray lines) are usually present in the material. Etching and anodizing partially dislodge the fine parts. These are coating defects that are darkly visible. They are often oxide lines with deposits of alloying constituents (Mg) and grain finer (AlTiB2). The problem is from the casting process.

Few lines can always be present on sheets of the type AlMg. If there are many such lines, it can be a material defect (casting process). Important is the specification and the quality of the material. Regarding gray lines, there is no clear indication of the composition. More important are otherwise the accompanying elements and important is a good casting process.

Joerg Beerli
- Hallau, SH, Switzerland

October 19, 2018

A. Hi Libor,
Can you share a couple of clearer pics and result of alloy? Also is this profile etched before anodizing? I suspect etching bath's behaviour against the Mg alloys. Can you share the sequence of baths in anodizing line? Based on your answer we can try something.


alaattin tuna
- sakarya,TURKEY

October 24, 2018

Q. Hello,
thank you Joerg and Alaattin for your posts. Basically, we all agree that it is a material issue. But it's not sufficient reason for raising an official complaint towards material producer. As you say, Joerg, specification is very important. I'm trying to find some specific material quality to look for (max. % of specific element for instance).
Here are some other pics of the defect.



Simplified sequence of the anodizing process we are using is here:
1. Alkaline degreasing
2. Pickling E0
3. Pickling E6
4. Alkaline rinse
5. Clarification (HNO3/H2SO4)
6. Alkaline rinse
7. Anodizing (H2SO4)
8. Acid rinse
9. Coloring
10. Rinse
11. Sealing
12. Rinse
13. Drying

Thanks for your opinions!

Libor Böhm [returning]
Quality manager - Nosislav, Czech Republic

October 25, 2018

A. Hi Libor,
I think the source of the problem is the alloy or casting process as per previous contributor's opinion. If you suspect it comes from the process then you can try 2 options:
1- you can skip only clarification bath's rinse (alkaline rinse as you describe)
(we call it "neutalization" bath instead of "clarification" bath). Also you had said "alkaline rinse" but I think it should be "acid rinse"
2- you can skip only E6 etching bath and its rinse

So, if you see again the black spots, then focus on material problem ... don't think anything else.

alaattin tuna
- sakarya,TURKEY

October 25, 2018

A. Regarding the 5754,
if you're using your own forming process on the product, look for sources of debris, like steel shavings, etc., that might get embedded into the soft surface. I've anodized quite a bit of 5745, have seen other issues, but nothing like this -- this reminded me of some 6xxx series parts that sat in RO water (high purity) overnight, they were cleaned, etched, deoxed and ready to be anodized, but it was very late in the day and had to be rescheduled for next morning, and those parts developed defects similar to these, very few spots, not regular at all, but deep craters (visible on high magnification) that were very very noticeable after anodizing.

Janis Ziemelis
- Riga, Latvia

Black streaking in anodized machined aluminum part

October 18, 2018

Q. Hi all,

I've got this part machined from 7050-T7451 aluminum, and after anodizing per Mil-A-8625 [link is to free spec at Defense Logistics Agency,] (Type II) its coming back with black streaks.


Now these parts are not aesthetic, so I'm not concerned by the appearance of the black streaks, but I am concerned about whether or not they pose a concern for corrosion resistance and material properties. So my question is, do imperfections in anodizing have a significant effect on corrosion resistance or material properties?


Bradley Roan
- Vacaville, California

October 20, 2018

A This is what happens when the material has been COLD DRAWN

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
and co-author of The Sulfamate Nickel How-To Guide

October 25, 2018

A. Regarding 7xxx,
there are quite a few posts about this very problem on here, and nothing very definitive. From my own experience, I see those gray lines when the part has been either reworked, the stock material has been sitting for a long time before a part was machined from it (years), or misalignment of Mars and Venus during anodizing. It should affect corrosion resistance -- how much, no one will give a definitive answer; put it in a salt spray cabinet to see how it behaves. But then again -- if corrosion resistance is so important, why 7xxx series? Is weight/size that much of an issue where you can't use a purer alloy that will anodize better and provide better corrosion resistance?

Here is a photo of a part made from 7075 I anodized:


There is a large bore in the part from left-right (during machining it may have been warmed up, though flood coolant was used), and the pattern seems to be somewhat oriented around it, suggesting a relation to internal stresses in the part, which supports what Mr. Probert said about cold drawing.

Janis Ziemelis
- Riga, Latvia

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