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topic 14441

White spots in black anodizing

A discussion started in 2001 but continuing through 2018


Q. Can someone explain the complete process with respect to Black Anodizing alum. castings (Alloy 771.2)

The problem we are having is white spots (after plating) on the milled surfaces but not on the raw casting surfaces. Could this have something to do with inner cavities trapping the chemicals used during plating? I heard from my co-workers that if the part is not baked properly or long enough after the cleaning that chemicals would leach out and cause the white spots. (Looks like exposed alum. surfaces not plated.)

If this is inherent with alum. castings, can someone suggest a different alloy that's less prone to having porosity present.

Bill Velazquez
- Hauppauge, New York

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon


A. I have been anodizing for a little while and do not claim to know a lot but I have read and also experienced that cast alum. cannot be anodized, only forged alum. can be anodized.

Mike Lowery
- Huntsville, Alabama


A. Bill, yes, due to the porosity of castings, producing a uniformly colored anodize is difficult but usually the opposite of what you describe would hold true.That is the milled surfaces would look even and the natural cast surface would appear spotty.The spots on the milled surfaces may indicate contamination from chemistry of blind holes or poor rinsing before dye.

Most wrought materials will be significantly less porous than any casting.

Bill Grayson
- Santa Cruz, California, USA


A. This is the same thing that I am experiencing, the lab states it it is si/Segregation, is the material high in SI?

Maybe together we can help each other.


Wendell Goodson
- Garland, Texas


A. Good Afternoon Weldon,

I am glad I am not the only one experiencing this problem. The alloy currently being used is 771.2 and it has a concentration of .1 of Silicon.

By the way our foundry has recommended we try 356 T6. I personally do not believe it's the alloy but more the lack of internal controls at the foundry which leads to impurities and pin holes developing in the castings. But in any event we will give 356-T6 a try to see if we have positive results. The only other option is to machine from solid material. Come to think of it , 356 T6 has a much higher concentration of Silicon 6.5 to 7.5, so maybe high concentration of silicon may not be the cause. Where's all the platers out there, come on guys, you must have a solution to this problem or how about all you foundries, help us out here, please.

Bill Velazquez
- Hauppauge, New York


A. This is a common issue when dealing with porosity such as anodizing castings or jig plate, plating zinc die castings, etc. Certainly, castings can be anodized they just are not going to look as pretty as a nicely machined extrusion will. There are a number of techniques that can be used to mitigate the effects of porosity and how well they work depends upon the finisher's expertise and the nature of the porosity encountered.

One of the issues may not be the cleaning chemicals leaching out but sulfuric acid leaching out following the dying/sealing process as sulfuric is viscous, etc., and it will dissolve away the coating and remove the dye as it bleeds out. That is particularly common with what you have described.

One method is to rinse in a rinse containing ~5% nitric following the anodize bath prior to the dye bath. That replaces the sulfuric acid contained in the pores with nitric which is much easier to rinse out (less viscous) as well as giving one a better coating for dying with slightly softened, wider pores.

Another technique used is to "pump" the pores following each bath with alternating hot water rinses and cold water rinses. Sometimes a neutralization bath with 10% bicarb can be used following the anodize bath.

Another technique is to use a spray rinse above each rinse tank. The one that has worked best for me in the past is the nitric pre-dye rinse. The amount of material being etched during the cleaning process also impacts the porosity problem, it is best to only bubble etch if etching at all, etching off the "skin" usually increases the porosity problem. Sometimes it may be best to just use a brief triacid etch and avoid any alkaline cleaning and employ a nitric predye rinse.

There are finishing houses that have the expertise to deal with the problem you have encountered and rather than just trying wholesale what I have mentioned above I would suggest you try having your parts processed at a finish house that specializes in decorative color-match anodizing, or find someone who is used to black anodizing jig or tool plate. Some castings are not going to anodize as well as others not only due to the alloying constituents but because the casting process can lead to varying amounts of porosity within the same alloy being cast.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


Q. Hi,

I am having these tiny white spots on Black Anodized parts. They are not too many but many enough to get rejected in the quality check. Any creative thoughts as how to avoid this problem?


Hemant Kumar
Hemant Kumar
- Florida, USA

First of two simultaneous responses --  

A. Is your part a machined casting? Machining on castings invariably open up pores in the casting which entrap a small amount of sulfuric acid during anodizing and then bleach out the dye after dyeing and sealing. If this is the case, you might allow the parts to soak in a warm rinse for a while to help the acid leach out before dyeing. Don't make it too hot or you will seal the anodize. Even this might not work because you never know how much acid has soak into the pores. Sometimes the spots don't appear for a long time.

Good luck.

Guy Lester
- Ontario, California, USA

Second of two simultaneous responses --  

A. This is the most common tech service complaint. If you are using 7075 alloy, note that all salts of zinc are white.

Calcium buildup (evap., add, evap., add, etc.) in the dye or seal will leave behind white spots.

Chlorides anywhere will sometimes show a white spot.

Too high dissolved aluminum in the anodizing, like above 12 gm/L, will cause white spots.

Dirty rinse water can cause white spots.

Sulfate drag into the dye, especially with 7075, will cause white spots.

Plain nitric deoxidize on high alloy does not get rid of the non-aluminum alloying ingredients - use a proprietary iron based deoxidizer.

First thing to try: 5% Nitric ACid Dip between the anodizing and the dye.

Start looking for pits and white spots after the anodizing, then look at the dye, then look at the seal,

Take two aspirin and call me in the morning.

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


A. Another possibility would be that you are getting galvanic corrosion during the dyeing process.. make sure your work is completely insulated from the tank during dyeing.

Marc Green
Marc Green
anodizer - Idaho


A. You did not mention if you use DI water or tap water. If you use tap water for your operation. Chloride ions may be the reason to cause the pitting you have.

S. Y. Yuen
- Hong Kong, China


Q. We are having an ongoing issue with white spots on some pieces we black anodize. Parts are 6061 machined plates - not cast or jig plate. They come to us very clean. We have tried all different cleaning - soap only, no etch - heavy etch - etc.

The weird thing is that this is almost entirely confined to one account. We do black on 6061 all day long for others and do not see this issue. The customer tells me they use a water based machining oil and I spoke to the mfgr. of that oil and we see no problem.

We go through extensive rinsing & cleaning and still can't get rid of the issue. We have used both titanium & aluminum racks & it does not seem to make a difference - We have masked all holes, even when not needed & still no difference.

I have attached a pic - any input would be appreciated.


David A. Kraft
plating shop employee - Long Island City, NY USA


A. Probably:
1. dissolved aluminium content high;
2. current density high;
3. bath temperature low;
4. it's not 6061.

John Hu
- Singapore


A. David

It could be overaged aluminum, particularly if the spotting occurs only on non-machined surfaces. Try an acid (HF) etch before anodize.

Willie Alexander
- Colorado Springs Colorado


A. I'm sure you've considered this, but any chance that your customer's finishing operation could be embedding particles not compatible with anodizing in the part?

Marc Green
Marc Green
anodizer - Idaho

March 14, 2014

Q. We have a problem in black anodizing bath per attached pic.


As we have seen, there are lots of white spots over there, as if pores of surface not filled by black color. But interestingly, it occurred only on this profile. On another profile which is black anodized in same bath, we didn't see any [white spots]. All profiles are extruded 6063 series and electrocoloring bath's composition is 2 gr/lt Sn and 20 gr/lt sulphuric acid and 40-50 gr/lt stabilizer. But I don't suppose it's related to bath. So I want to ask generally;
1- is this problem related extrusion quality?
2- is this problem related our anodizing process?
3- our electrocoloring bath's duration (for black color) is about 20-25 minutes. Is that too long for black color? (we check electrical reasons).


alaattin tuna
- sakarya,turkey

November 18, 2014

Q. I am really hoping someone can help. We are having severe spotting on 356-T6 machined castings. This gets worse from time to time and the vendor always says it is porosity and there is nothing they can do, fix your casting.


From reading this thread does the test plan below seem logical?
1) run 10 parts using a 5% Nitric rinse and compare?
2) run 10 parts using pump hot cold alternating rinse and compare?
3) run 10 parts using 10% bicarb and compare
Is there anything else I should try other than a new vendor?

RFQ: I will take suggestions for the new vendor as well:-)

Paul Dykstra
- Shelbyville, Illinois, USA

November 19, 2014

A. The white spots are very probably caused by the pores in a casting. If you have visible pores, there is not much that the anodizer can do.
Vacuum impregnation would seal the pores, but they would not anodize, so you would have the same problem.
A new source of the castings might help.

James Watts
- Navarre, Florida

November 19, 2014

A. Hi Paul,

There are a couple of things you could do to try and improve the integrity of the castings. Firstly you can try Hot Isostatic Pressing (also known as HIPping). Secondly you could try vacuum impregnation (usually done with a sodium metasilicate type material).

Both of these techniques are designed to either reduce or mitigate the effects of porosity.

If you have a big problem with porosity then you need to look at the casting technique. I am not a casting expert so really can't advise other than to say try a consultant if this is really a big problem.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Ed. note: James' and Brian's postings preceded the addition of the photo to the thread. Their suggestions may or may not change after they see the graphic.

simultaneous November 19, 2014

Q. We use a lot of Aluminum castings. However, we only have one customer that requires Anodizing for corrosion resistance. In our opinion the porosity is not that severe. The strange thing is that porosity seems constant bu the spots come and go. I have not tried HIP'ing these parts but am familiar with the process. We have tried impreg and it did not help.

In the previous messages it seemed that improved rinse processes might help. Are you of the opinion that different rinsing processes will not help? I had high hopes that Nitric rinse, pumping or bicarb might be worth testing.

Paul Dykstra [returning]
- Shelbyville, Illinois, USA

November 20, 2014

A. Paul

I agree with James & Brian that the cause is the porosity of the casting.

Your proposed process tests seem reasonable. While not eliminating it totally, I think the processor could do a better job of minimizing the spotting. If you can't change the casting, maybe you can change to a processor who has more experience with castings.

Willie Alexander
- Colorado Springs, Colorado

November 20, 2014

A. Good day Paul.

I am by no means an expert on anodizing, especially on castings. I, too had problems with the porosity. I do not know your cleaning process, but this is what I focus on.
I try to deal with the 7% silicon and do ^NOTuse the etch for this purpose. The deox will vary, as you know, based on chemistry. I am thinking pores, I use 10-20 minutes hot D.I. prior to anodize to try and fill the pores and limit the time in anodize. Prior to dye, I use cold/WARM water sequence to clean/rinse the pores. Hope this helps.


Eric Bogner, Lab Tech.
- Whitby, Ontario, Canada

November 26, 2014

Good day Paul.

I had made an error in my last post. As I was reviewing;
"I try to deal with the 7% silicon and do (NOT) use the etch for this purpose."
Hope this clears things up.


Eric Bogner
- Whitby, Ontario, Canada

January 4, 2015

thumbs up signThanks to everyone for the feedback. I sort of lost track of this thread before the holidays. We will try some of the suggestions and I will report back the results!

Paul Dykstra [returning]
- Shelbyville, Illinois, USA

April 28, 2015

Q. Hi All,

My customer's part found having white patch on thread holes of the part after anodic coating as per MIL-A-87625F, CLASS 2 BLACK.

The material is Forged Aluminium 500.

I need some advice to tackle this matter.



Kabilan CHandra
- Singapore

April 2015

A. Hi Kabilan. As you see, yours is about the 25th posting on this one thread, and there have been photographs posted, numerous specific suggestions, recommendations of books, etc. It can be a complex matter which would require you to post a lot of data. Still ... threads are usually not anodized, and are therefore usually aluminum colored rather than black. Are you sure they are "white patches" as opposed to non-anodized aluminum patches? Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

Black spots on black dyed sulfuric anodizing with hot water seal

October 17, 2018

Q. Dear All
I am a technologist and Lab head Al finishing plant.

Black dyed sulfuric anodized with hot water seal produces white powder.
The sealing sol'n consists of RO water with dilute phosphoric acid and sodium hydroxide. Very light sol'n... pH adjusted to 5.5-6.4.

RO cascade rinse is performed before sealing.
The process initially runs fine, but the tanks lasts for a very short time until white powder appears on black parts.
I noticed that the pH rises with time.

I will be appreciative for any assistance.

Best regards

Shay Ben-Menachem
chemist - Israel

October 2018

probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert

A. Hello Shay. Are you sure you mean 'black spots'? From the context of your question it sounds like the problem is white spots, which is perhaps the single most common problem discussed on Robert Probert offers an article about it in our on-line library, and we have about 35 to 40 threads on the subject if you search the site with the term "white spots anodize".

But I am not familiar with your seal with H3PO4 and NaOH in it. You call it a hot water seal ... is it boiling? Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

October 18, 2018

Q. Hi Ted,

Thanks for the article, and prompt response.
I have indeed obtained white spots on Aluminum treated with Sulfuric Anodize and dyed in Black. Thus, the white powder is seen well on black background. I think the water seal is the problem.
I use a hot water seal (close to boiling) which contains very low concentrations of phosphoric acid and sodium hydroxide. The pH is adjusted to 5.5-6.4.
I have bought the Aluminum How-To book from your website; excellent book, and read it all. The website is the best in the field, and I highly appreciate the great work you personally do.

I will read the article.
By the way, how do I look for further articles at the website?

Many thanks Ted.

shay ben-menachem
Algat - Haifa, Israel

October 2018

thumbs up sign  Hi Shay. Thanks for the very kind assessment of our efforts. Although I've acquired some limited knowledge from books and by osmosis, my hands-on experience is in plating & anodizing equipment and shop design rather than troubleshooting appearance issues in anodizing. Still, as we both know, Robert Probert emphasizes how essential the purity of your seal tank is. Hopefully he'll come across this thread at some point and comment.

As for finding articles on this site, our search engine uses Google but restricts it to searching just this particular site ... I use it dozens of times a day myself :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

simultaneous October 19, 2018

Hi Shay,
Shay don't you use licensed chemical products in your anodizing line for hot sealing? This is the first time I heard of a hot sealing formula like can you use phosphoric acid and sodium hydroxide for sealing bath even if diluted? I hope I don't misunderstand...according to my knowledge hot sealing bath includes such as ISO amil alcohol, 2 butanol organic compounds. I suggest you choose a chemical supplier for anodizing line and also you can discuss some problems as this with them.

alaattin tuna
- sakarya,TURKEY

October 20, 2018

A. If I were to hazard a guess here, are you using phosphoric acid and sodium hydroxide as a method to control your seal pH? If so, I would stop that immediately and use acetic acid and ammonium hydroxide for your adjustments. I'm thinking your spots may be related to the use of NaOH.

Marc Green
Marc Green
anodizer - Idaho

October 20, 2018

A. Marc Green has your best answer. Phosphate is a terrible contaminant to any seal. Do what Marc says, but first go back to the anodizing tank, rinse, then 5% nitric acid to flush the viscous white aluminum sulfate out of the pores (which pops out at the hot seal), then rinse again.

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

November 25, 2018

thumbs up sign Many thanks to you all.
It has been very useful.
I will sure make the changes are mentioned.

Shay Ben-Menachem [returning]
Algat - Haifa Israel

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