Titanium is an amazing metal. It's strong, springy, and corrosion resistant. This makes it a great support in electrochemical baths. It doesn't conduct electricity real great, but it has practically no chemistry, so it's used almost everywhere. Say you want to support something rather thin, like platinum leaf: you'll want it mechanically bonded, and plating is a pretty good way to accomplish this. But, er, if it's so corrosion resistant, how the hell is it going to get plated? That's the question...
Industry knows how to do this. Some vague starting points are: hydrogenation, acid fluoride bath, molten salt bath (usually cyanide!), etc. All lay somewhere between nasty and horribly corrosive, and all of them are proprietary trade secrets!
I seem to have developed a passable method of plating platinum onto titanium. It starts with a chunk of titanium metal...
This is the already-plated side of a 3 x 12" strip of 0.08" thick CP4 grade titanium. The edges are rough because this piece was plasma cut. I didn't bother sanding it at all.
This is the back side that I'm going to be plating. It has some smudges of platinum that stuck when the first side was plated; the rest is covered in oxide from annealing.
The first step is etching with NaOH. But not just any NaOH. An aqueous solution doesn't do jack squat to titanium. Think hotter! The white spots you see are solid NaOH pellets, melting onto the surface. The green seems to be some reaction with the crud on the surface, possibly sodium platinate. Wear goggles! The heat source here is my propane torch, which is just fine. Direct flame applied to NaOH does turn it to Na2CO3 quickly, but on this side it's just fine.
Once the lye is melted, roll it around to coat the surface. Then keep on heating. Around 400C or so, the oxide will dissolve (forming sodium titanate dissolved in NaOH) and the exposed metal will react, producing hydrogen bubbles. You need enough NaOH to cover the surface, dissolve everything and react with the metal some. Incidentially, the silvery glob seen on the top-right of the strip is an aluminum pellet, which comes with the "Rooto Number Two" drain cleaner I happen to be trying, since Red Devil Lye was removed from the shelves (the "war on drugs" wages on...those bastards!) This stuff claims "87% NaOH", but doesn't state what the "inert ingredients" are. It contains flakes, which are probably sodium nitrate, and the aluminum produces hydrogen for stirring action. Neither of which I need, of course.
When it gets really hot, the hydroxide starts moving around and foaming and may turn color a little. When it reaches this point at least once at all points on the surface, it's ready to wash off. This isn't quite ready: the center spot is etched, but not around it. See below. When done, pour off the excess molten NaOH onto a COMPLETELY DRY metal plate -- you don't want this shit splattering!
Here it is, washed off. The center is a dark gray color, while the rest is kind of not different at all. The scale has been dissolved revealing shiny metal, but it hasn't been etched. You can quench from full heat, but the spray contains horrible choking particles of alkali. Better to let it cool under 100°C and wash in hot water. There may be some white stuff that will need some rubbing to remove.
Remelting the excess and heating more, this is the completely etched surface. On to activation:
Before plating, the metal has to be cleaned and activated. This is done with a solution of about 10g ammonium bifuoride, 50ml of 31.45% HCl, and 100ml H2O. The solution has become green due to copper and iron contamination, which I don't care about.
Moderate heating starts the reaction. At about 40-50°C, the solution suddenly foams up, following the heat wave. Here, the entire surface has completely foamed over. I like to give one cleaning treatment like this, then a second activation step.
Here it is, washed off. The surface looks cleaner.
A little more solution and just a dab of heat (30-40°C), enough to get it just bubbling. At this point, the surface is practically free of titanium oxide and is ready to be placed directly into the plating bath. Stir it around strongly to dislodge any gunk from the etching process. Apply current immediately, do not rinse.
This is plating a different strip of titanium, but serves the same purpose. I've been using a solution of potassium hexachloroplatinate and hydrochloric acid, using a graphite bar for anode, which bubbles chlorine gas, since there's no metal to enter solution as usual when electroplating (not that a platinum anode would join the party anyway!). The waves coming from the cathode are hydrogen bubbles breaking the surface. This anode was plated at about 1A.
This is the same anode, after plating and annealing. This earlier attempt wasn't prepared properly, and the plating wore off after some time. Pictured is a perchlorate cell, which started at about 30A, 5V. It only lasted a week, the plating blistered off.
I've had mixed results. I seem to get a better deposit on the far side of the electrode (lower current density?), and a black fuzzy deposit on the near side, which usually rubs off easily. Some of the preparations I've done have in fact passivated the surface so thoroughly that platinum actually stuck -- quite nicely -- to the factory finish on the other side! I have so far discovered more ways to completely and utterly passivate titanium (and one way to destroy it) than I've found to activate it for plating! Some of my results stand up to a hearty wirebrushing, others flake off readily. Sometimes, the results which do stand up wear away, revealing a yellow oxide layer underneath, as if the platinum is porous (which... I suppose it could be). Annealing seems to be necessary to strengthen the Pt-Ti bond: sometimes I have a powdery deposit which rubs off black onto my fingers, which adheres perfectly after heating to 700°C! I don't know what effect there is from more or less heat. Other times, it seems like the layer is ruined by heating, possibly due to oxygen (or various oxygen radicals in the flame) diffusing through the thin Pt layer, oxidizing the Ti beneath. The best anode I've made yet still has a spotty deposit. I have yet to get more than 50% coverage, and I have no idea about life (the best I have so far has gone two weeks with no visible change in surface area).