I've never anodized titanium, but I do know a little about electricity and how it behaves in an electrolyte, I can probably be of assistance with your electrical issues.
I did a quick search on anodizing titanium, I couldn't find much of any technical value. What little I did find prompts these questions:
1. "The color is determined by the voltage"; assuming that is what's actually going on, I didn't see grey as a listed color. Grey is the natural color of the native oxide that will form on titanium in time.
2. "80V" colors the titanium purple, and the color range seems to stop there, how did you arrive at 100V?
3. Assuming 1. is true, there are going to be some serious electrical difficulties anodizing large surface areas of titanium. The electrical power required will be huge. All of the titanium anodizing stuff I found involved small pieces; jewelry, guitar picks, knife handles, things of that size. The power required isn't large.
4. How do you know that you actually anodized the titanium? An anodized film on titanium will be much thinner than an anodized film on aluminum. If you put an ohm meter across a DRY anodized surface; on aluminum the film will be an insulator, on titanium the measured resistance will be much lower, but still significantly higher than bare titanium. If you can't see a change in resistance before and after anodizing (be sure its dry first) you may have only etched the surface of the titanium and there is no film at all.
Anyway, you see how little I know about anodizing titanium. Let's get to the electrical issues.
What you describe with your larger pieces are the classic symptoms of butting heads with Ohm's Law, V=IxR.
The resistance in your case (R) is the resistance of your electrolyte (TSP in water) the resistance of the cathode, and the resistance of the titanium work (the anode). The resistance of the cathode and the work gets lower as these get larger; there is more in contact with the electrolyte as they get larger, its inversely proportional to surface area. As an anodic film forms on the work, the resistance of the work increases, this is why the current is very high at first and drops with time. Assuming you aren't overloading your power supply first.
V=IxR can be re-arranged to I=V/R. As you can see, the current goes up as the voltage goes up if the resistance is the same. When you started at a lower voltage, and then increased it after some resistance formed (the anodize) you were able to keep the current down to a level that your power supply could handle. This is called "ramping" in aluminum anodizing.
Back to 1. again. It's more likely that not the voltage but the current density and anodizing time control the color in titanium anodizing. In all things electrical, current does the work. Voltage only serves to overcome the resistance so that the desired current can flow. An anodic layer is formed by current, not voltage. If you were anodizing titanium wire, which will have small surface areas, and if they're all about the same size, its easy to see how one could think that its voltage that controls the color.
I need to find the current density range for anodizing titanium (not the voltage). If we knew this, you could get better results by controlling the current, and anodizing for a much longer time at much lower power levels.
Your specific electrical questions:
Cathode distance is a factor, but not the major one. Increasing it improves the uniformity of the anodic film.
The cathode surface area should be the equal or larger than the surface area of the work, larger doesn't hurt. It doesn't need to be on all tank surfaces.
You need better voltage and current control, not less of it. A Variac will only reduce the voltage and current, it has no capability to regulate it. Your going to need a real power supply to get any consistency in your results.
Your process time is very short because your current density is very high, this is why I need the anodization curves for titanium. If you anodize for longer times at lower current (and voltage) this will solve your power supply problems, and eliminate the lethal voltage hazard you face with 100V in a laundry tub, at 100 times the current required to kill a man. BTW, its current that kills, not voltage. You need enough voltage to make the current flow through a vital organ and the game is over.
From what I can gather, stainless steel is what you should be using as a cathode material.
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