pstock

How can one identify titanium tubes on a frame, assuming there is no sticker identifying the materials?
I ask because a friend sent me a photo of a bike nearly identical (at least in terms of cosmetics) to my Titanium Eddy Merckx but which apparently is Scandium.
comparing them side by side however I see they are not really that similar. The Scandium frame seems to have larger tube diameters.
And where mine says "Titane" I think this other one says "Team SC".

But say you were just looking at a naked frame.
Titanium not steel? I presume that a magnet does not stick to a titanium frame?
Titanium and not Aluminum/Scandium? tube dimension? (would they weigh about about the same?)

ever curious, Peter

unterhausen

many people can tell from any place where bare metal is exposed. You probably could do it using ultrasonics, but I'd have to think about that.

tuz

Yes I think Al or Scandium frames have generally bigger diameters than Ti. Also, their weld beads are usually fatter.

bobbycorno

WAY fatter. As a rule, the thicker the tube wall, the larger the weld bead needs to be. Al tubes are almost without exception thicker that Ti or steel, hence the bigger weld beads (like 5mm vs 2-3mm).

Also, bare Ti has a "dirty gray" color to it, where Al is more neutral to gray-white.

SP
Bend, OR

NoReg

What about hardness, you can cut aluminum with a knife. Yeah, I know that could require at a minimum that someone turn their back.

pstock

right, but let's assume that whoever owns the bike is A) not going to want you scraping part of the frame bare and B) not want you hacking away at it with a knife (nor let you take it to the lab to put under the spectrometer).

here are photos of the two bikes that got me scratching my head.
in the end, I figure it was just the frame cosmetics that confused me (because the frame sticker on the Scandium model is pretty clear.)

https://picasaweb.google.com/lugwork/TwinMerckxes#

But I expect it is as easy to slap a "Titanium" sticker on some frame as it is to slap a "531" sticker on. So, if you don't want to trust the stickers, how do you know?

was there any rhyme or reason to their paint schemes? or did they probably do their coffee mugs in the same patterns that week?

Mark Kelly 

Measure the outside diameter of the bottom bracket shell where the BB threads are. A titanium one will be about 40mm, an aluminium one will be significantly larger, around 44mm.

Falanx

The easiest non-destructive way is to find yourself a short, *clean* aluminium tube, take out the forks and headset, rub a little bit of scourer or scotch pad down inside the head tube and then press the short bit of aluminium to the interior wall of the headtube, making a good contact.

Break out a digital multimeter and look for a voltage, of more than 0.3v. See one, it's Ti, or a stainless steel/steel. No real big voltage difference, aluminium alloy.

NoReg

Hey buddy, there is a whole place for threads on electric bikes.

unterhausen

I'm going to have to try Falanx's method. Although the method proposed by Mark Kelly is probably more reliable

Mark Kelly 

Terrific idea, needs one modification: add a salt bridge between the metals. With a salt bridge you'll be able to measure the potential difference reasonably easily.

The contact potential (with no salt bridge) is pretty well impossible to measure, for reasons related to the energy distribution which causes the contact potential in the first place.

A salt bridge, preferably in gel form, will solve this because you've then got a Galvanic potential which is much more robust. Getting a salt bridge in gel form is surprisingly easy: "Personal lubricant" works OK, but adding a bit of common salt to the lube works even better.

The cell can be made using simple alfoil as the test electrode: you must hold the foil so it contacts the lube but not the Ti. Doing a quick test using this method using the salted lube I measured a potential of about 180 mV with an ordinary Fluke multimeter. This is lower than the 290 mV difference between the standard electrode potentials but you'd expect some losses with this crude setup, plus I'm not sure what the "true" potential should be given that the Ti is an alloy (with Al and V).

Falanx

I can get a goodly readable contact potential between copper plate, and nickel particles in a rubber matrix, let alone between to clean pieces of metal, although, yes, a salt bridge would help even moreso :-)

I recommend tomato ketchup/catsup as a contact electrolyte. Or katsu or teriyaki sauces.


The presence of less than ten percent alloying ingredients that in no way alter the crystal structure of the alloy is negligible.


Shush :-p

Mark Kelly 

I'll take your word for that but I think the rubber matrix might actually be helping rather than hindering.

In practice, I've never been able to measure contact potential directly with a multimeter and as far as I know it's impossible in theory also.

Here's why: As you know contact potential is dependent on the difference in work functions between the metals. Let's say your multimeter test leads are pure copper. There are then three metal to metal junctions in the circuit: Cu / Ti, Ti / Al and Al / Cu so the net difference in work functions around the circuit is thus Cu - Ti + Ti - Al + Al - Cu which can be rearranged as Cu - Cu + Ti - Ti + Al - Al which is obviously zero.

In the 20s and 30s a lot of work went into measuring the work function of metals because it governs the emission from hot cathodes and I am quite sure no-one was able to do it directly. I have pretty much every important book on electronic theory and practice up until 1960 in my library and they are unanimous on the difficulty of accurately measuring work function.

As always if you can show me where I'm wrong I'm all ears.

Derailed

Um... you're talking about electronic effects, right? Very small changes in composition most certainly can dramatically affect electronic characteristics -- the crystal structure is not as important as electronic structure. (I know very little about metallurgy -- you may well be right in this specific case -- but I think it is silly to dismiss a concern about the effect of changing the composition because, allegedly, the crystal structure is the same. If that were a reasonable argument, wouldn't it negate much of what is known about semiconductors?)

Mark Kelly 

Hey Falanx

You are dead right - Kecap as salt bridge works really well (if you don't mind the mess).

I cleaned a chunk of 6Al4V I had sitting around and a piece of ordinary Al, each with the Norton Abrasive pad I use to give a brushed finish to Ti frames.

The PD measured with the Kecap salt bridge is 265 mV. This is over 90% of theoretical value, which is pretty good. On the run before I took the phot I got 280 mV which is even better. If you press too hard with the probes the metal surfaces contact and the PD drops to zero. It might be worth trying a piece of tissue paper soaked in Kecap, should eb more stable (and less messy).

I recleaned the surfaces and tried to directly measure contact potential. The PD with direct contact of two clean surfaces is less than the resolution of the instrument (0.1 mV).

Falanx

Compare and contrast the conductivity of metals with by definition, overlapping valence and conduction bands, and semiconductors, with their several orders of magnitude less charge carriers. In the case of solid solutions in metal, the minor alteration in lattice parameter with other metals with similar valence, in the case epecially of Ti alloys is the controlling factor. In the case of silicon, or gallium arsenide, you'd be doping a very poor conductor with electron sinks or providers, grossly atering electronic behaviour with just a few ppm impurity, but only because the material is instrinisically piss-awful at conducting.

Falanx

You've never worked with flurorsilicone rubbers, I'm guessing ;-) hellish material. Why we chose it for a conductive EMC gasket, I'll never know...

Falanx

I maintain, their is a culinary solution to every engineering problem :-D

NoReg

Probably more true than the idea that there is a culinary solution to most nutritional problems. Got put on a low salt diet, and I am now willing to believe that most everything on the supermarket shelves would work as a salt bridge.

Derailed

Thanks for the educational response -- I stand corrected. I should have known this, but I didn't, so I really appreciate you taking the time to explain it in enough detail that I can understand why you're right.

Falanx

I'm sorry to hear you just got your sense of taste robbed, man.

Are you allowed to sub in potassium alternatives to sodium?

Falanx

No worries man. As a friend of mine once said, while deftly and expertly robbing an audience member of a small, crisp, folding one in a magic trick;

"You guys, you come here for fun. But me, I do this for a living..." ;-)

pstock

Me again, the Ti Pest.
So here's one I am looking at long distance and cannot verify in person.
It has a Titanium sticker on it but it's not very convincing.
just the look could be aluminum.
any clues?

GrayJay

Surface finish of the metal does not look like alu to me. If it is not Ti, then my next guess would be that it is a steel bike with a nickel plate finish.

unterhausen

it's titanium. I really don't think it would be worth it to try to make aluminum look like that