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Odd Preciptate and Scarring during Titanium Anodization

Lotaxi

Aluminum
Joined
Jun 9, 2020
No clue if this is the right forum for this question, but since I'm a machinist by profession this is where I know to go.
I've recently been playing around with making some small things for myself in my off hours, and since I have a fair bit of titanium laying around I figured I'd try to learn how to do some type 3 anodization. Since I went through the trouble of pushing my boss to help me develop a small electropolishing cell for the shop, I figured I could use the power supply to anodize as well.

I've been playing with the electrical anodization of grade 5 titanium (alloy 6Al4V), and the colors I'm getting are pretty great. I've successfully developed the entire voltage spectrum of colors, and quite vibrantly at that. I've noticed an issue with my process, however. My workpieces have some odd pitting and scarring on their surfaces that develops during the growth of the oxide layer. I have no idea what the cause might be. In my testing, I've been trying to build a general color vs voltage chart so that I can know what to expect from a given voltage within my setup. I typically set my power supply to a specific voltage, contact it with the anode to color my workpiece, record the final color that the voltage gives, then increase the voltage and repeat. After a while, the workpiece will begin to develop these dull gray scars that will also sprout an odd, white, fluffy stuff. If disturbed, the fluff will slough off and float around the electrolyte solution.

The uploader in the post form is saying my pictures are too big, so here's a link to some pictures of what I'm talking about.

I'd rather not use a hydrofluoric acid based etchant, so I'm using a heated solution of Sodium Fluoride and Ammonium Persulfate to etch my workpieces. It seems seems to work quite well, and I know it's stripping the surface of the titanium because if I toss an anodized piece into the etchant I can watch the color sequence reverse itself over time. I'm rinsing in distilled water, then anodizing in an electrolyte of Trisodium Phosphate (TSP) in distilled water using a titanium mesh cathode with a niobium anode contactor. With the power supplies we have, voltage and amperage capacities are not an issue. TSP seems to be a very common electrolyte for this purpose, so I'm very confused about what might be my issue.

Does anyone have any thoughts on what might cause these issues?
 
So first of all, your fluorides, while not as dangerous as HF, are still dangerous and you should always be wearing gloves, and then disposing of it accordingly.

Are you using only distilled water in making up your electrolytes? It sounds like localized depassivation of the titanium.
 
So first of all, your fluorides, while not as dangerous as HF, are still dangerous and you should always be wearing gloves, and then disposing of it accordingly.
I will not drink it, no matter how strong my teeth will become.

Essentially I've just figured out my own version of Multi-Etch, which is rather safe. Works quite well.

That said, it's still chemistry and I follow basic lab safety practice.

Are you using only distilled water in making up your electrolytes? It sounds like localized depassivation of the titanium.
Distilled only, yes. Another interesting point is that when the corrosion starts the oxide layer stops forming. That's why you can see the piece with both gold and purple despite the voltage being all the way up around 90V. If there's localized depassivation, then there would be preferential current flow and that would stop the oxide formation. That makes sense. What do?

Have you tried electropolishing the Ti before the rest of the process? If there's contamination that the etchant didn't remove, perhaps the EP first step will help.
Contamination is definitely an issue I might need to solve, but that's one of the purposes for the etch. We run a lot of different materials in our machines, so if there's anything that stays on the surface after my ultrasonic desmut/degrease bath and a bath in the etchant, then I need to figure out how to kill it. Only thing is that I don't have a polishing bath set up for Ti. The chemistry we run is Sulfuric/Phosphoric. Like I mentioned, I'd rather not play with HF if I can help it, so what other electrolytes might work for small-scale EP on Ti?
 
Only thing is that I don't have a polishing bath set up for Ti. The chemistry we run is Sulfuric/Phosphoric. Like I mentioned, I'd rather not play with HF if I can help it, so what other electrolytes might work for small-scale EP on Ti?
DamnifIknow. Any chance you're dealing with shoddy Ti that's processed poorly? Do you have samples from known alternate batches you can play with?
 
an area of interest

was wondering if your'e familiar with or run ( or if they are even applicable to anodizing vs plating) of Hull cell tests.
 
Is your TSP really Tri-Sodium-Phosphate? Many of the off the shelf TSP products are no longer truly TSP.
 
This is fascinating.
I found this and it might help you.

They show a citric acid schedule if you want to stay away from nasty stuff.


Also, its possible that distilled water is not adequate. DI water might get you there. Small quantity DI units can be bought or made. Pour your distilled through to get mo pure water.
 
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+1 on DI water. Your current density may be too high. The shape of the cathode needs to match the shape of the part to get even current density and film growth. There can be situations where rate of loss equals or exceeds rate of growth due to the insulating effect of the film. I seem to remember you need to monitor voltage and current to detect when this is occurring.
 
Is your TSP really Tri-Sodium-Phosphate? Many of the off the shelf TSP products are no longer truly TSP.
Bought it from a chemical supplier when I bought the APS and NaF, so in theory it is. Only burned like 10$ on it, so I think my next step is to go for a sulfuric acid electrolyte. We have a bunch because we use it in our electropolishing setup, so I may as well make myself a 1M solution and try that.
Your current density may be too high.
I'm shoving current with the supply we use for electropolishing, so that's a pretty good possibility. I've been looking into getting my hands on a higher wattage rheostat to put into the loop so I can control it better. The supply only does constant amperage by way of dropping voltage, so that's not helpful.
The shape of the cathode needs to match the shape of the part to get even current density and film growth.
It's purely cosmetic, so I'm ok with uneven coloration. Adds a bit of character to what I'm playing with. If I can figure out how to consistently and intentionally get that rainbow gradient radiating from the center out to the ends that you can see from one of the workpieces, I'll be quite happy.
There can be situations where rate of loss equals or exceeds rate of growth due to the insulating effect of the film.
The moment the attack happens, it freezes the oxide growth entirely to preferentially attack the corrupted area, so that makes a fair bit of sense.
I seem to remember you need to monitor voltage and current to detect when this is occurring.
I'm using a constant voltage process, and the ammeter on the power supply I'm using won't read out anything under an amp. Will need a separate ammeter.
 
It's purely cosmetic, so I'm ok with uneven coloration. Adds a bit of character to what I'm playing with. If I can figure out how to consistently and intentionally get that rainbow gradient radiating from the center out to the ends that you can see from one of the workpieces, I'll be quite happy.
The issue with electrode shape being not a constant distance, the current density probably won't be even. The areas closest to the electrode may be burning or having other problems and the rest anodising ok.
A good diagnostic is to use a flat piece with a flat electrode to work out your current densities and times to get the colour you want.
 
Is your power supply straight DC? Sometimes fancy shit switches the polarity back and forth which is used in refining to speed dissolution of the anode.
 
Sometimes fancy shit switches the polarity back and forth which is used in refining to speed dissolution of the anode.
Far as I can tell in the documentation it's straight DC, but I guess it's worth a call to the company to make sure.
The issue with electrode shape being not a constant distance, the current density probably won't be even.
It most certainly won't. For my decorative purposes, I don't need process controls that tight so I'm ok with it.
The areas closest to the electrode may be burning or having other problems and the rest anodising ok
Unfortunately it appears to be the places furthest away that are having the issues.
A good diagnostic is to use a flat piece with a flat electrode to work out your current densities and times to get the colour you want.
This is probably a good idea. I think I'll stick with the mesh electrode I'm currently using for the surface area, but the flat workpiece isn't too bad an idea. I'm also testing a different degreasing method and a different anodizing compound.

Another data point I've noticed: the moment the corruption process starts, I'm also seeing a complete halt of oxide growth. If it begins at 50V, for example, I can increase to 100V and not see any appreciable color change. The corruption speeds up, though, leading me to think that it's a chemical incompatibility. The lack of an oxide layer after the dunk in the etchant may mean that the aluminum in the alloy is reacting with the trisodium phosphate to produce aluminum phosphate. With the current electrolyte, I'm using an electrochemical process that will split phosphate ions off the TSP. It makes sense that if the aluminum starts to take a liking to the phosphate ion that the oxygen is no longer being dumped onto the titanium.

I see a lot of people doing art with this process using commercially pure alloys that conform to grades 2 and 4, so it might just be that the electrolyte people use is best suited for those alloys. Next step is to try a sulfuric acid electrolyte.
 
the 100v is probably breaking down the oxide layer. With a CC supply, the voltage gets pushed up as the insulating oxide layer builds to maintain the current.
 
are you etching the parts before ano? to me it sounds like you are not due to missing the higher volt colors. it also sounds like maybe you are driving too many amps. my ano. box only uses 2 amps max. works just fine on balisong handles and folding knife parts.
shocking or not eagle one etching wheel cleaner works great for the etch and bigger Co them me reached out about how i was getting clean colors. they changed from the multi etch they were using. for my ano fluid i have an old box of cascade dish washer powder that in use (before they stopped putting phosphates in the mix). works great 3rd pic shows a test piece that i etched then sanded half of so you can see the fresh surface only runs into the 40-50 volt light blue range.

Screenshot 2023-01-24 at 22-18-00 Lloyd Harner (@lloydharner) • Instagram photos and videos.pngScreenshot 2023-01-24 at 22-12-47 Lloyd Harner (@lloydharner) • Instagram photos and videos.pngScreenshot 2023-01-24 at 22-19-57 Lloyd Harner (@lloydharner) • Instagram photos and videos.png
 
are you etching the parts before ano? to me it sounds like you are not due to missing the higher volt colors. it also sounds like maybe you are driving too many amps. my ano. box only uses 2 amps max. works just fine on balisong handles and folding knife parts.
shocking or not eagle one etching wheel cleaner works great for the etch and bigger Co them me reached out about how i was getting clean colors. they changed from the multi etch they were using. for my ano fluid i have an old box of cascade dish washer powder that in use (before they stopped putting phosphates in the mix). works great 3rd pic shows a test piece that i etched then sanded half of so you can see the fresh surface only runs into the 40-50 volt light blue range.

View attachment 385012View attachment 385013View attachment 385014

I’d love to hear more about your process. Got any pictures you can share?
 
are you etching the parts before ano? to me it sounds like you are not due to missing the higher volt colors.
I'm using a heated mixture of sodium fluoride and ammonium persulfate. It strips stuff pretty good :)

The higher colors weren't getting reached unless I started out with the high voltage. If I ramped at all, the corruption would eventually start. The moment it started, the coating would freeze in place and no longer progress.
it also sounds like maybe you are driving too many amps.
Absolutely I am. The cell is pulling less than an amp, but when I get to the high voltage colors I'm seeing sparks and scarring. I've been trying to find a solution for this, but with the chemistry and size limitations of my cell I'm not seeing many options past hard fixturing. Because my pieces are small, I'd like to just use the touch probe I've been working with, but I just bought some niobium grippers that I can dunk so we'll see what happens there.
for my ano fluid i have an old box of cascade dish washer powder that in use (before they stopped putting phosphates in the mix).
Funny enough I think it was the phosphates in the TSP that were causing my problem. I'm using 6Al4V titanium, and I think the electrochemical process was pulling phosphate ions off the TSP molecule and letting it attack the aluminum in the alloy to form aluminum phosphate. I'm pretty sure that was the fluffy particulate that was appearing.

I’d love to hear more about your process. Got any pictures you can share?
Not the guy you replied to, but I did have better success with an electrolyte change. I'll make a better post once I've got my process down a little better, but here's the most recent results I've gotten.
 
DamnifIknow.
I know it's been almost a year, but I figured out a safe, small scale electropolishing chemistry. Ethylene glycol and potassium chloride gives me a nice, bright polish in about 5 minutes at about 20V. Takes some vigorous stirring, but other than that it's a pretty simple method.
 








 
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