White spots in black anodizing

A. Hi George. There are so many potential causes that it's hard to start. But if another shop is successfully processing them, my first question would be whether they are also using titanium racks.

Another thing to note is that while both illustrated parts exhibit little white dots, that 'morse code' pattern on the second part is certainly not some random chemical phenomena; it's obviously orderly and is certainly either damage to the part during processing or defective material.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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.

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.

A. This is the same thing that I am experiencing, the lab states it is Si segregation; is the material high in Si?

Maybe together we can help each other.

Regards

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 pre-dye 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.

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.

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.

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

A. David

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

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?

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.

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.

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.

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.

Regards,

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 fnishing.com. 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.

Regards,

  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 :-)

Regards,

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 sulphate out of the pores (which pops out at the hot seal), then rinse again.

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

A. Hello Kai.

Without actually seeing a picture of the condition that you are seeing, more than likely you are correct. The white spots do lend themselves to the issue of the welding. One thing that you may be able to try is after the black dye, return the parts back to the anodize tank for about 5 minutes at the same voltage as was used to process the parts. Many times the white spots will fill in after parts are dyed again. you might also try some black touch-up paint if that would be acceptable for your customer.

Other than that I would suggest that it is gong to be a condition that will have to be lived with.

A. Not wanting to start an argument, because the suggestion might be related to electrolytic coloring (which I have no experience with), but white spots on welded areas are usually a result of porosity, these pores may be very deep and trap sulfuric acid, which leaches out during dyeing (or even later during hot sealing) and prevents dye absorption or ruins the dye around the pore, and, if regular water soluble immersion dyes are involved, then I can't see how returning the part back to anodizing tank will improve anything at all...

Improve welding practices, longer rinsing times, perhaps the short 5% nitric rinse suggested by others may also help to get the sulfuric out, these would be my primary choices to try and solve the problem

This is if the defect is indeed caused by the trapped sulfuric acid and not some other contamination of the process, high magnification pictures with scale next to defect would help to understand better what might be the problem

A. Janis,

what you are proposing is 100% correct. The white spots are related to solution entrapment, but this is the same principle when it come to anodizing castings that have machined surface(s). The sulfuric becomes entrapped in the pores which become more visible after machining and leaches out at the most inopportune time. Returning the castings back to anodize for a few minutes generally improves the appearance minimizing the white spots and allowing for more dye absorption. we see this situation quite regularly and the results of returning to anodize tank for about 5 minutes and dyeing again improves the visuals dramatically and sometimes the white spots are dyed and completely disappear. there is no guarantee that the white spots will always react favorably to the additional treatment but it is a very real possibility that they will be minimized greatly.