I am tinkering with a prototype for an invention, and I need to attach
some pipes to a water tank. Ideally, I would want to weld it together,
but I want a cheap/easy/quick-and-dirty solution.

I'm not concerned about it being particularly strong or durable(it's
just a prototype), but it does have to be watertight. I think the tank
will be calvanized steel and the pipes are both copper and galvanized
steel.  

The reason I ask is because I've never heard of soldering steel
together (I know copper tubing can be soldered).

Hellow vonwao~

Yes, it is possible to solder steel.

Make sure your surfaces are clean by scraping them down with steel
wool and using H3PO4 acid (Phosphoric Acid) to clean them. (For places
to buy this online, check out the following Froogle Search:
http://froogle.google.com/froogle?hl=en&lr=&ie=ISO-8859-1&q=Phosphoric%20Acid&btnG=Search&sa=N&tab=wf
).

Then, use plumber's acid flux (available at hardware stores, or :
http://froogle.google.com/froogle?q=acid+flux&hl=en&btnG=Search ) for
the soldering. Ordinary tin-lead solder will work great.

For websites that describe this process (in brief) see:

* "It's Such a Stud," Land Rover:
http://www.fourfold.org/LR_FAQ/Series/FAQ.S.repairs.StudPlateFabrication.html

* "Soldering Stainless Steel," Candle Power Forum:
http://www.candlepowerforums.com/vb/showthread.php?t=45331

* "Radio Misc." Antique Radio Classified:
http://www.antiqueradio.com/radiomisc12-96.html


Kind regards,
Kriswrite
    
RESEARCH STRATEGY:
"solder steel"
"how to" "solder steel"
"soldering steel"

thanks, good info

There may be pipe fittings to attach steel or copper pipe to a tank.

Soldering seem a bit fragile to me.  Welding would be stronger, and
probably easier if you have any experience and access (rental) to a
electric welder.

"SIKA" brand silicone sealant. Knew a bloke who used the high
temperature type [ used in the mineral sands industry to seal
elevators that transport hot processed sand] to act as a head gasket
for his car, and without any actuak gasket, not just as a gasket
sealer. It was good for up to 1500 degrees and the seal was so good I
doubt if he would ever be able to get the [cylinder] head off again.
It also has very good flexing qualities for 90 degree joins.

Try Liquid Steel.

J-B Weld is what I meant although Liquid Steel may work as well.

Actually, these are great comments, thanks for all this info.  Perhaps
I should try welding, I did not think of renting a welder.

I will also look into the J-B Weld / liquid steel.  My only concern is
that it needs to withstand water pressure (connected to municipal
water), and I'm afraid it will leak.

Thanks for these comments.

DO NOT ATTEMPT TO WELD GALVANIZED STEEL! The fumes are instantly
FATAL! You must really know what you are doing to weld it. Soldering
works fine on steel as decribed above. Better would be to solder a
threaded fitting on and then screw the pipes on.