flyerstevie - 11-12-2015 at 12:03
I am not well versed on many things, chemistry being one of them and I have run into a problem cleaning some stainless steel I am going to bond with a
thermoset structural adhesive. I have found a specification for doing the cleaning, ASTM D2651 which says "immerse for 15 minutes at 63 +/- C in the
following solution by volume: 100 parts 95% sulfuric acid to 30 parts saturated sodium dichromate solution". I've made up a couple of test baths but
the results of the bath make-up process are not quite what I anticipated. I've found everything from 49 g to 236 g as the solubility numbers for the
sodium dichromate in water from different sources. Can anyone help me and tell me what the amount of sodium dichromate is I need to add to make 1L of
the saturated sodium dichromate solution for this purpose?
fdsailor - 11-12-2015 at 12:19
Oh God - I'm not experienced enough to comment in detail (others much more learned than I), but.. for safety sake wanted to get a quick response it:
1) Solubility of sodium dichromate is a function of temperature, hence the ambiguity.
2) MOST important - it is HARD to safely add that little water to that much concentrated acid. Even if you do the recommended "add acid to water" if
you don't do something to cool it, the result is likely to boil and make a dangerous, splattering mess. I think the normal solution starts with
freezing the water part first, then adding the acid (probably cooled itself, but obviously not frozen, not that you would succeed at that anyways)
SLOWLY. With full face shield, etc.
3) I think you want to be careful in disposal of any of the chromium compounds. Look up hexavalent chromium for a scare. Also an allergen.
In general, look up "Chromic acid" for guidance.
diddi - 11-12-2015 at 13:08
can I suggest you try about 50% phosphoric acid?
j_sum1 - 11-12-2015 at 17:40
Yeah, flyerstevie. That sounds like procedure to avoid if at all possible. is there a problem with sandpaper? Clean and a nice locking interface.
jsc - 6-1-2016 at 13:06
Holy cow, what a pain.
First of all, that is not "cleaning". It is etching the surface to rough it so that the adhesive can grip on to it.
To create a saturated solution first bring the vessel and water to the stipulated temperature (63C), then mix in the solute (sodium dichromate). Keep
adding solute slowly until no more dissolves. Then filter into a clean container that is warmed to the right temperature.
careysub - 6-1-2016 at 13:50
Etching perhaps, but also (and probably mainly) "de-passivating" the surface.
aga - 6-1-2016 at 14:48
<strike>Sand</strike> health-conscious-media blast both surfaces.
Clean, quick, simple and roughens the surface.
If you're into metal work, you must somebody with a 'peening' machine which is also perfect for this.
If this level of cleaning isn't enough, the adhesive isn't appropriate.
Edit:
If you have a link to ASTM D2651 then we could take a look.
Likely that the temperature is 'standard' which means either 20 or 25 C depending who's 'standard'.
[Edited on 6-1-2016 by aga]