[time-nuts] Heated crystal? & Rb tube corrosion (FE-5680A)

[Magnus Danielson]

On 12/01/2011 11:18 AM, Attila Kinali wrote:
> On Thu, 1 Dec 2011 01:00:44 -0500
> "Steve ."<iteration69 at gmail.com>  wrote:
>
>> Note the corrosion around the cheaper metal parts (screws, spacers, shell).
>> I can't speak for the FE-5680A, but when i see something like this in the
>> instruments i maintain it's a tale-tale sign gas mitigation.
>
> I have to agree with Bill that there is no corrosion. At least i dont
> see any, but on the head of the Rb cell.
>
>  From the chemistry point,  Rb or the noble gas buffer in the cell will
> not corrode any metals if they would leak. Because noble gas are noble
> and nearly completely inert (at least at the temperatures we are talking
> about) and Rb is less noble than anything else you will find in there,
> hence will oxidize first.
>
> There is a very slimm chance that Rb salts (after Rb has been oxidized)
> could lead to an acidic reaction. But i can neither prove or disprove
> that with my limited chemistry knowledge.
>
> But in this case, it's not Rb or any of its salt that's the culprit.
> If you have a look at http://n1.taur.dk/fe5680a-2/IMG_1393.JPG
> you see that the corrosion around the Rb tube is at the spot where
> two transistors are soldered to the tube. The metal of the transistors
> is copper plated with tin and soldered with a tin based solder.
> What you'll get here is an sacrifacial anode effect, ie the copper
> does oxidize the less noble iron/steal.

You need to recall that the operational temperature of the rubidium lamp 
is about 130 degreed, so air and local metalurgy gets to have fun.

The rubidium needs to be in glass/quartz containers as the lamp needs to 
be transparent to the 100 MHz RF field used to lit the lamp and the 
rubidium reference needs to be transparent to the 6,8 GHz field. Also, 
rubidium being an alkali metal we would sure want it and it's buffer gas 
kept safe. As I recall it (but no guarantees for correctness) would 
rubidium and water form a fairly potent base rather than acid.

Cheers,
Magnus