Firescale?

I had to silver-solder two small components together today. To get a proper solder flow I had to apply heat directly to the part of one component that will later be visible. It ended up covered with black gunk. Is this what they call firescale?

I tried removing it with steel wool with only partial success. In the end a judicial application of 400 grit sandpaper did the trick without obliterating the pattern underneath.

If this is firescale I understand Sparex (sodium bisulphate) is the thing to clean it. Am I correct?

Hey Michael,

Short answer is yes. But it depends a lot on exactly what it is you are doing, but the following is a guide line and used when building model locomotive boilers of copper, after all the joints are done.

"To help clean the piece after soldering, it is quenched in "pickle", a moderately strong sulfuric acid bath, or a pretty concentrated solution of sodium bisulfate. Hydrochloric (muriatic) acid is better for copper and brass. (Platter's call it "bright dip".) Use two containers, one for the pickle, the other for rinse water. Pickle will eat holes in cotton and other vegetable fibers, and it can splash when a hot piece is dropped into it. Wear an apron. Don't pick up a piece in the pickle with iron tongs. Some dissolved copper will plate onto the silver because of the galvanic action. Copper tongs are standard; stainless steel might work. A hook of copper wire might suffice, and don't hesitate to use your bare hands/ fingers if you have no broken skin. "

I don't advise the latter, just in case!! Acid burns can easily go right to the bone, as with lye also.

Take care. Brian Lawson, Bothwell, Ontario.

ps...whatcha makin ??

This one is a miniature sundial

Next time cover everything with white (borax) flux to keep the air off, but now try soaking the piece overnight in water and seeing if any of the crud loosens.

Question: Why do they recommend sulfuric or sodium bisulfate first, if muriatic is the preferred pickle? I'm not trying to be difficult, mostly just curious as to what other materials model loco builders might use for boilers.

Also, does the pickle work fine at room temperature?

Joe

Hey Joe,

I'm no authority, but the suggestion was that the muriatic is "best" for copper and brass, rather than just best all-round. Besides, around here the sulphuric acid is a little less common at the local stores than the muriatic, and to me slightly less corrosive generally.

Temperature only affects pickle time as far as I know, but heating an acid causes undesirable vapours.

XXXXXXXXXXXXXXXXX

FWIW,

Now days many of us who are building copper boilers for live steam use a phosphor-copper rod. It melts at a slightly lower temp than traditional "silver brase" has a wider range of temp between "solidus" and "liquidus", runs like water when you want it to and most important requires no flux, the phosphor makes it self fluxing so no "pickle" is required between re-works. But this is only good for copper-copper work and one thing to beware of though it not to get it too hot, you burn the phosphor out and yer screwed.

It goes by many names, at my welding shop it's a UniWeld product called UNI-1000

--.- Dave (who's staring at the 4 foot long 8 inch diam copper piece for the 2-10-2's boiler.)

It would sound that the sulfur acids are used for organic fluxes and oils.

HCl is useful to 'clean' copper metals since it strips a layer off in a chloride and floats off leaving a bare and clean surface. e.g. clean a penny. (and old one).

The chemical reaction is hot by itself. It also creates 'movement' in the reaction and thus heats locally.

DON'T DO THIS UNLESS YOU WANT TO HAVE AN OVERFLOW... A HCl or FeCl bath when nearly depleted - is depleted and heated by adding aluminum. Fire and sparks and gas release. Hot reaction.

Mart> >

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You may want to check with the manufacturers of your silver brazing alloy. Here in the UK, model engineers (and professionals AFAIK) have mostly abandoned sulphuric and hydrochloric pickling as unnecessarily dangerous.

The standard EasyFlo 2 flux used here can be removed with hot water and that is what the suppliers (Johnson Matthey) recommend.

Many modellers here use citric acid solution. Resonably readily available and harmless - leave the work in it for a few hours and it will do as good a job as sulphric or hydrochloric without the risks.

I asked my chemistry teacher about this reaction after sticking a pencil with an aluminium sleeve retaining the eraser into FeCl etching solution and noting the violent reaction. She said it was the same type of reaction as thermite but with chlorine instead of oxygen swapping from iron to aluminium.