SV: [sdiy] Solder won't stick to old PCB

Mon Sep 28 14:04:23 CEST 2009

One reason for my post was to make you post :) since you're the
resident crazy chemist. Thanks for the clarification. Most of my
knowledge of chemistry comes from hands-on experience. The residues
that I was talking about are mostly stuff that would be hard to
*mechanically* remove (by washing with water or cleaning off with a
sponge) because of the porous nature of oxide layers. That residue
could react badly with future *good* attempts at removing the oxides,
using correct compounds. As I understand it, if you then used an
acid-based derusting agent (and those I find in my limited knowledge
are the best), you'd get salts, correct me if I'm wrong. This fact
makes the reaction not only slower but would make it yield a lot of
'crap' which then needs further cleaning up.

I have had very good experience when de-rusting my first synth that I
got - a DX11 that had rusty dirt on the keyboard assembly's steel
frame - using a compound based on, if I remember correctly, phosphoric
acid. It doesn't attack metal, and dissolves rust like it's soap and
water. I don't remember if it generated any fume, but if so, they were
light enough for a teenager to sleep in the same room (maybe I kept
the window open?..)
As a bonus, it did not attack the rubber parts on the keyboard
assembly either (ones that were molded onto the frame)

D.

On Mon, Sep 28, 2009 at 12:51 AM, David G. Dixon
<dixon at interchange.ubc.ca> wrote:
> --- Warning! --- Pedantry alert! --- Warning! ---
>
>> toothpaste is a very strong base. It is alkaline and, if it reacts
>> with the oxides on metal, could leave salt (?) residues on the
>> surface.
>
> Alkaline things don't really react with oxides.  Oxides are, after all, just
> dehydrated hydroxides, which are alkaline.  To wit: consider the dehydration
> of lime, conducted in a calcining furnace (rotary kiln, fluidized bed, etc):
>
> Ca(OH)2 --> CaO + H2O
>
> Slaked lime (on the left) is a hydroxide, lime (on the right) is an oxide.
> These two wouldn't react with each other, because they are essentially the
> same thing.
>
> To make a salt requires adding an acid and base together.  For example,
> hydrochloric acid and caustic soda made table salt:
>
> HCl + NaOH --> NaCl + H2O
>
>> to remove oxides you need a reducing agent, such as an acid.
>
> While there are some reducing acids (and some oxidizing acids), acids are
> not, generally speaking, reducing.  Sometimes, people talk about the
> reducing power of acids in corrosion of reactive metals such as titanium,
> but in this case, the reduction is indirect.  The acid is actually eating
> away the protective ("passive") oxide layer of the metal, and the metal
> itself is then reducing water to form hydrogen.  One of my grad students
> almost blew up a titanium autoclave this way, by heating up a sulfuric acid
> solution to 220 deg C without having a suitable oxidant such as oxygen or
> cupric ions present to protect the titanium (by maintaining the TiO2 layer).
>
> A reducing agent is something which donates electrons.  An acid is something
> which donates protons.  (Conversely, an oxidizing agent accepts electrons,
> and a base accepts protons.)  Sodium borohydride (NaBH4) is a reducing
> agent, because it accepts electrons and is thereby converted to an oxide.
> In the presence of a suitable catalyst (or maybe not), NaBH4 will reduce
> water to hydrogen gas (a reaction exploited in fuel cells):
>
> NaBH4 + 2 H2O --> NaBO2 + 4 H2
>
> In this case, the boron is being oxidized from the -5 valence state to the
> +3 valence state, thereby donating 8 electrons to the proceedings (a very
> generous reducing agent, indeed!).  Those 8 electrons combine with 8 protons
> to form hydrogen gas, which can be burned.
>
> ---- End of pedantry alert --- It is not safe to go about your business ---
>
>
>> The eraser idea is very good imo.. haven't tried it myself, but I
>> heard it's good for reconditioning faders / pots
>>
>> D.
>>
>> On Sun, Sep 27, 2009 at 9:19 PM, John Alex Hvidlykke <john at hvidlykke.dk>
>> wrote:
>> > Well, one of the most effective cures for tarnished silver (but probably
>> not
>> > as good a The Dip) is regular toothpaste. Might as well try that on my
>> > oxidised component leads ;-)
>> >
>> > \John
>> >
>> >> -----Oprindelig meddelelse-----
>> >> Fra: synth-diy-bounces at dropmix.xs4all.nl
>> >> [mailto:synth-diy-bounces at dropmix.xs4all.nl]Pa vegne af David G. Dixon
>> >> Sendt: 27. september 2009 22:04
>> >> Til: 'Graham Atkins'; 'John Alex Hvidlykke'
>> >> Cc: 'Synth DIY'
>> >> Emne: RE: SV: [sdiy] Solder won't stick to old PCB
>> >>
>> >>
>> >> They used to sell this stuff for de-tarnishing silverware the easy way,
>> >> where you just dip the silver into this solution and it comes out
>> >> perfectly
>> >> shiny.  I think it might have been called "Dip-It".  I think that
>> >> infomercial guy who just died (Billy something-or-other) was hawking
>> >> something similar on TV recently.  I don't know what this stuff
>> >> is, but I'm
>> >> guessing it's some sort of reducing agent (sodium borohydride,
>> perhaps?).
>> >> When I was a kid, I used it on bronze coins, and it worked perfectly.
>>  I
>> >> think one could brush this stuff on oxidized copper traces/pads
>> >> to the same
>> >> effect.
>> >>
>> >> > Before now with badly corroded joints I've used a mildly abrasive
>> >> > paste like
>> >> > T Cut (For car paintwork) applied on a cotton bud, but clean well
>> >> > afterwards.
>> >>
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