Pickling a Copper Boiler

[Runner]

Hi all,

I am in the final stages of completing the boiler for my 3.5" gauge locomotive, the outstanding tasks are silver soldering the crown stays to the top of the boiler, the backhead and foundation ring. The boiler construction has been stalled for some months due to not having sufficient heat source to silver solder the remaining items, I have a 7kw propane torch which is struggling even with the lowest melting point silver solder (620 deg C). In anticipation of overcoming that heat source limitation, I have tried to clean the boiler by pickling it in sulphuric acid, it's 25% solution at the moment, I understand that 10% is the ideal. However, I have difficulty in particularly cleaning between the crown stays and the top of the boiler were manual rubbing is impossible. After a dip in the pickling solution and washing there is still a brown stain which appears after washing and areas of blackened copper inside the boiler. The outside is reasonably clean because I have been able to rub with a cloth when drying and or mild abrasive action. I am constantly admiring pictures of boiler construction posted on the Internet that show a healthy pink glow just by pickling. I am not leaving it in the pickling solution for too long an hour or so.

Please advise if there is some additional requirements to removing the stains both brown and black and if pickling is all that's required how long should I leave it in the pickling solution?

Thanks in anticipation

Regards,

Runner

 

[sunworksco]

I recommend using an ultrasonic cleaner vat with acid.
You may find a local shop that can ultrasonically clean your boiler.
This machine cleans airbrushes with mild solvent, too.
I highly recommend a propane/oxygen torch kit. It makes a much cleaner solder joint with less cleanup.
You will save a lot of money in the long of it.
Regards,
Giovanni

 

[lordedmond]

I recommend using an ultrasonic cleaner vat with acid.
You may find a local shop that can ultrasonically clean your boiler.
This machine cleans airbrushes with mild solvent, too.
I highly recommend a propane/oxygen torch kit. It makes a much cleaner solder joint with less cleanup.
You will save a lot of money in the long of it.
Regards,
Giovanni

If thats the torch your are recommending its not big enough for a 3 1/2 g boiler the op says he has a 7 kw burner , again its nowhere ner big enough look at a propane burner that is bigger than 45 mm in dia and run it at 4 bar

to use a ultrasonic cleaner with acid is foolish in the extreme ,

to the OP a 5 % solution of acid is more that strong enough do not leave it if any longer than 1 hour or you will leach the copper , citric acid is plenty good enough to do the job and safer

my final recommendation is to consult your local boiler inspector about thing relating to boilers , he/she should have inspected your work before now anyway to enable the issue of the insurance documents

Stuart

 

[Kmot]

Distilled white vinegar is what I use, recommended to me by a model engineer loco builder. Guess what, it works great and is cheap! ;D

 

[SignalFailure]

Try Citric Acid Monohydrate as well, cheap, does the job gently and AFAIK you need'nt worry about leaving your boiler in there for hours if necessary. About as hazardous as kmot's vinegar

(try using steel wool when washing off the pickle too)

Paul

 

[steamboatmodel]

I would go with the Citric Acid or the Vinegar, They would be safe in the cleaner, but I wouldn't use anything stronger.
Regards,
Gerald.

 

[GWRdriver]

For many years the traditional pickle preferred by many has been a dilution of sulphuric acid which is commonly sold as battery acid. I have also used diluted hydrochloric acid, known as Muriatic acid, mostly used these days for cleaning masonry and concrete. Both work well and the short-term pickling effects of sulphuric and hydrochloric are about the same but the working life and other aspects of the two are different. Sulphuric has a longer working life, hydrochloric loads up with copper sulphate quicker. If the pickle is strong enough and you give it enough time it will eventually go pink. Naturally you don't want to expose the copper to any more etching action than is necessary, but I've intentionally left stuff in the soup overnight in a properly diluted solution, and no damage was done.

It may be that what you need to do is administer a spot treatment with one of the stronger acids.

 

[Maryak]

I'm no chemist but I wonder is the staining, copper sulphate from a reaction with sulphuric acid. all the ingredients seem to be present ??? ??? ???

Maybe some chemical guy can set me straight please. :-\

Best Regards
Bob

 

[sunworksco]

I was refering to Runner's comment:
The boiler construction has been stalled for some months due to not having sufficient heat source to silver solder the remaining items, I have a 7kw propane torch which is struggling even with the lowest melting point silver solder (620 deg C).
...
The copper turns a pinkish color, when pickling, because of the alloys on the surface being removed.
Ultrasonic cleaning does clean very well and rapidly as long as the liquid is not too acidic.

I was recommending the torch setup for his soldering needs, not for the boiler burner.
Regards,
Giovanni

 

[steamer]

Hi Giovanni,

You'll need something with at least a 25mm tip, depending on the size of the part in question.

My Turbo Torch does very well up to about 3 or 4 " tube....and that's a big tip. But that's all it will do.

It hooks up to a 20 lb propane tank with it's own regulator also 2-4 bar.

Lordemond would be pretty close in the size range of 45mm tip if your going to work on items that size or bigger regularly.

Additionally, you can bury the remainder of the object in commercial kitty litter to insulate it so your not loosing so much heat to the surroundings. That helps a great deal.

Dave

 

[Runner]

Thanks very much for your replies, I didn't expect such a robust response. However, it appears not to have consensus as to the best solution (no pun intended) to use. I have had a dialogue with a lab technician that works for a manufacturer of batteries for conventional submarines and sent photographs of the staining, his recommendation was after washing to apply heat to speed up the drying process particularly inside the boiler. This didn't improve it much. The pickling solution is new so I don't think that copper sulphate is the problem, but I may be wrong. I initially used hydrochloric acid in the early stages of manufacture, since it was readily available from Bunnings (local hardware stores in OZ) but changed tack soon afterwards because it fumed a lot more than sulphuric acid particularly when putting hot objects in the bath and this fuming put a film of rust on all the exposed steel items in the shed. I tried bleach as a post pickling cleaner, but that didn't work.

Today I tried another pickling process using sulphuric acid, and it appeared that the side of the boiler that was nearest to the surface of the pickling solution went blacker and the underside which was remarkably pink, indicating that the sulphuric acid may have striated. This I wouldn't have expected since the depth of solution was less than 5 inches. So it agitation of some sort the way to go?

I have taken careful note of all your recommendations and will try them out. No doubt one will work, after all it's basic chemistry here at work.


Regards

Runner

 

[bearcar1]

Runner, it cold be that some of the sulphur ions present in the acid are being chemically induced onto the surface of the copper, or the acid is not pure and contains traces of some other metal. (much like plating without the voltage) My take is if you switch your technique to the more commonly used citric acid solution you will encounter better results. A good mechanical scrubbing of the surface and a thorough rinsing in distilled water before placing your pressure vessel into its new bath should also aid in getting the pink color you desire. Be certain that you wash/scrub and again thoroughly rinse whatever container you are using before hand in order to minimize any cross contamination from occurring when you change over to citric acid. And yes, a gentle agitation or aeration of the bath will speed things up a bit. A common aquarium pump is good for such things, simply place the air tube so that the bubbles are being directed 'around' the piece being pickled. Best of luck.

BC1
Jim

 

[Jasonb]

Runner you may find it easier to use two torches. One large one for general heating of the area, helps if someone else can hold this but it could be clamped in position. Then you use a second torch to work the immediate area.

Ideally use two bottles so you don't suffer from pressure drop as two large torches will soon chill a small/average bottle.

A smal brass wirebrush or scotchbrite pad can help the acid remove stubborn deposits. Also you say you got corrosion in teh shop when dipping hot items, you should really not be putting anything in the acid that is still hot enough to make it steam.

J

 

[Runner]

Jim thanks for the suggestion, Citric Acid what form does it come in and is it mixed in solution and if so what dilution?

Jason that was how I intended to overcome the problem by having two torches, the 7Kw propane torch to supply the background heat and use a MAPP gas torch and canister to focus on the area of concern. MAPP gas is supposed to produce a hotter flame than propane so I am hoping that it will do the trick. The propane gas bottle is large, but it will still only be providing gas for the 7kw torch. The MAPP gas bottle is a bigboy size large enough for at least 30 mins running. If I can't achieve sufficient heat in that time, it isn't going to work anyway. Does anybody have experience with MAPP gas?

Regards

Brian

 

[sunworksco]

I have used MappGas for over 30 years as well as oxygen/acetylene and oxygen/propane, have soldered and brazed over 10 miles of copper tubing.
I would consider finding a small oxygen bottle and propane bottle.
If you take your time and look around you can find them.
It is going to give the best value and fastest soldering.
I have an oxygen/acetylene for large pieces and oxygen/propane/butane setup for smalls.

 

[GWRdriver]

putting hot objects in the bath and this fuming put a film of rust on all the exposed steel

The conventional wisdom amongst boiler builders is that pickling is never done hot. It is a cold process, for the reasons you state and others having to do with personal safety. The very first step after brazing should be to hot quench the boiler in clean cold water. I do this quench as soon as I can get the finished boiler to a bucket of water and gently lower it in, preferably smokebox end first. If done firebox end first care must be taken NOT to stand in the line of sight down the flues as steam can propel a jet of scalding water out the flues. The other beneficial effect of hot quenching is the thermal shock and steam generated will blast off most of the copper oxide and flux left from brazing. The pickle should clean up the rest.