how to tell a galvanised spoke from stainless

[alexnharvey]

I have a basic multimeter and will give the readings between various spokes at the weekend.

[cromo]

I for one would be interested to see the results!

[Brucey]

the voltage will indeed vary, but the current output by a bimetallic junction will vary with the contact area etc and in any case will be very low indeed. I have not tried this recently but when I've done such measurements in the past, I had to use an instrument designed specifically for the purpose, i.e. with a much higher input impedance than normal; a normal multimeter won't give accurate readings. However it might be good enough to tell the difference which is the main thing.

cheers

[alexnharvey]

DT stainless (1) to filed spoke (now quite rusty) (3), .51V
(1) to lightly sanded spoke (heavily darkened coating) (2), .57V
(1) to untouched plated spoke, .73V

Filed to sanded 79mV
Filed to untouched, .27mV
Sanded to untouched, 190mV.

All in the same salt water solution.

[cromo]

Assuming that stainless with stainless would give a very small reading of a few millivolts or zero, then the result of 0.73V of stainless with plated would seem to indicate that the method has some merit.
The comparison of plated to abraded is possibly less conclusive. I imagine when abrading a plated object it is difficult to remove all the coating. Small traces of zinc could affect the voltage measured.
This is after all a simple cell producing a voltage when two dissimilar metals are placed in an electrolyte. Maybe stainless with stainless and plated with plated could be measured to act as controls and that would confirm or refute the validity of the experiment.

[alexnharvey]

Plated (4) to plated (a new entrant, 5). Both lightly cleaned with washing up liquid to remove any oil. Spoke 4 has some darkening to the coating in spots. 1mV.

Sorry, that has got flipped and so is running from 4 at the top to 1, the stainless spoke at the bottom.

[cromo]

Thanks to Alexnharvey for taking those measurements!
It appears that it is possible to quickly tell a galvanised spoke from a stainless steel one if you have a meter that measures millivolts.

[alexnharvey]

Here's another photo update, possibly the final one, for my 1000th post here

The filed spoke (3) has continued to rust. As well as the immersed section, rust has now broken through the plating on the upper area which is just above the saltwater. Similarly rust has occurred on the sanded spoke (2) both under water and just above. We can see lots of rust on the bottom of the glass where spoke 3 rests and a little where spoke 2 rests. The untouched spoke (4) has dark spots all over the coating. Based on the others, I expect it would take some months for rust to break through on spoke 4.

To summarise, some of us can have a fairly good guess as to whether something is galvanised or stainless by eye. This is probably due to the difference in colour caused by the alloying elements in the stainless, and the colour of zinc. Explained by Brucey back on page 2 viewtopic.php?f=1&t=136305&start=15#p1459755

All stainless steel bicycle spokes discovered so far are polished.

The magnetic properties of spokes is also helpful. Stainless spokes are variable in their magnetism, although still less magnetic than a non-stainless spoke.

We can probably further confirm our by measuring the small voltage between a known stainless or galvanised spoke and our unknown spoke.

We can definitely confirm that by immersing the spoke in salt water for months, and abrading the zinc coating or plating will, as you might expect, reduce this timescale considerably.

One last thing. There is a very slight difference in taste, but I would not be very confident that I could accurately distinguish them by taste if I were blind.