Odd Preciptate and Scarring during Titanium Anodization

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.

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.

snowman said:

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.

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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.

snowman said:

Are you using only distilled water in making up your electrolytes? It sounds like localized depassivation of the titanium.

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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?

Milland said:

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.

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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?

Lotaxi said:

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?

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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?

Milland said:

Any chance you're dealing with shoddy Ti that's processed poorly?

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I mean it's certed, but technically that's always a possibility.

Milland said:

Do you have samples from known alternate batches you can play with?

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I've got stuff from different mill heats, yeah. I'm about to run a part from one of the others. I'll see what happens.

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.

+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.