What is acid used in soldering?

[ee851]

In this technical how-to-solder video

the professor uses a plastic brush dipped in acid
to clean a tinned joint.  He said the purpose of this acid is to chemically remove the flux.

I never heard of this acid before.    I must have missed something somewhere along the way, eh?
What exactly is this acid ?   What is it called in the trade?
Should I use it ?

[mariush]

In this video, he says "acid brush" but what he means is that it probably has isopropyl alcohol on it, not really an acid. He just wipes it to remove grease from handling the wires with the fingers and other impurities.

Flux is somewhat of an acid, it helps remove the oxide layer that forms on the surface of copper and solder, (the oxide) making it harder to solder things properly.

The solder wire has some flux in a solid form in the center of the wire, which becomes liquid when the iron tip melts the solder, but a bit of extra liquid flux prevents the wire from oxidizing before it's soldered.

The wipes he uses are impregnated with isopropyl alcohol of high purity.

[ftransform]

Acid brush is a term you would typically hear when soldering copper pipe. They used aggressive acid based fluxes which are completely inappropriate for electronics use.

[Skimask]

If he would've done that in the 2M/CCR class I took, he would've failed with that wire 'cause the solder wicked up under the insulation.
Just sayin...

[JSnyder]

^ agreed on both counts, Acid brush is a general term for cheap, disposable brushes used for any number of jobs where you will most likely ruin, and throw away the brush after use. E.g. applying aggressive acid based fluxes to copper pipe for the purpose of soldering (plumbing) as ftransform mentioned above. The solvent in the video used to remove flux is isopropyl alcohol as per mariush.

[KD0CAC John]

Lets see if I can remember correctly .
Using acid is & was not used in any soldering of plumbing , its mostly semantics - as our knowledge base grows , we need / use more terms to get more specific .
There use to be , mostly in the automotive industry for radiators " acid-core-flux " .
Not use in plumbing , for some of the same reasons for not using lead solder .
Again as I remember part of the purpose of using acid-core-flux was soldering of older , used copper parts radiators , or pipe for other than potable water .
The acid-core-flux was to shorten labor times , as to not have to clean oxidized copper , the acid removed the oxidation .
?
I have not seem it around for a few decades .   

[G7PSK]

At one time the only flux was zinc hydro-chloride, this was made by dissolving zinc in hydrochloric acid, it was used for all soldering including electrical and electronic work right up and into the 1930's, past flux was made by mixing the liquid with flour of some type.

I have some printed instructions on how to make a good electrical solder joint from the 1920's I will scan them some time and post them, you don't know how good you have it until you read about heating your iron on the open fire and then removing the soot before any further procedures.

[IanB]

In general, the "acid" used in soldering fluxes is hydrochloric acid. This acid is not used directly, but is generated in situ from components in the flux that decompose when heated to produce the acid. Zinc chloride is the flux component generally used for this in mechanical soldering, but is considered too aggressive for electronic soldering. In electronic soldering, the activating component in RA and RMA fluxes also produces hydrochloric acid, but in much smaller amounts. Hydrochloric acid vapour is quite irritating to the lungs and mucous membranes which is why you should avoid breathing in flux fumes.

However, as others have said above an acid is not usually used for cleaning the finished solder joint. Usually some kind of aqueous or solvent based wash is used to clean away the flux residues.

[Gall]

At one time the only flux was zinc hydro-chloride, this was made by dissolving zinc in hydrochloric acid, it was used for all soldering including electrical and electronic work right up and into the 1930's, past flux was made by mixing the liquid with flour of some type.

When I was taught to solder there have been two kinds of fluxes listed in the textbook. One was zinc hydro-chloride (for soldering saucepans and samovars) and another one was pure (solid) rosin. I soldered almost everything with pure rosin until I started to work with smaller parts.