Soldering to Stainless Steel

I figure this is more of a plumbing problem even though it is electrical. I need to attach some copper wire to some #10 SS bolt heads without disturbing the flatness of the bottom (thread side) of the head. The reason is to provide the best water tight seal I can of the head against the wall it penetrates.

I got some acid flux from a steel shop and used silver bearing solder (regular hardware store stuff) which I understood should work. No go. It never wet the metal.

If I can't get it soldered I am going to put a larger galvanized washer under the bolt and solder to that then a nut for pressure (and/or a toothed wire lug). I am not real happy with that sort of contact. Then put nuts and rubber washers on both sides of the wall penetration and use teflon tape of pipe dope on the threads to keep water from seeping in. This stuff will be under water and not handy to try to test or repair.

Any suggestions for the soldering or other approaches?

You cannot soft solder stainless. You may be able to silver BRAZE it , needing a much hotter flame from an acetylene torch. Propane or MAPP not hot enough.

What's the application? Why must you use stainless steel for carrying electricity? (if that's what the wires are for) - consider something like Monel, which can be soldered to, if need be.

You CAN soft solder stainless but you need to use some pretty harsh cleaning methods, like phosphoric or hydrochloric acid. Best bet is to use low temp silver brazing wire as Jimbo said. It's not the same as the silver bearing solder you bought and it's much more expensive. I've successfully silver soldered stainless with a Mapp torch and used ordinary brazing flux. I think the stuff has a melting point of 1100 degrees or so. Make sure you use a cadmium-free alloy for safety.

jimbo said:

You cannot soft solder stainless. You may be able to silver BRAZE it , needing a much hotter flame from an acetylene torch. Propane or MAPP not hot enough.

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You might get solder to stick somewhat, but for an important electrical connection jimbo is right.

I don't know your application, but most people try to use a double nut or washer set up and solder the wire or crimp it to a ring and put it under the head, a double nut and or washer should keep the head water tight. They ever do that on car engines when they put a bolt into a cooling passage.

If you put a copper flat-washer under the head, or a piece of annealed copper sheet like a gasket, then you can solder to the copper. The copper is soft enough to seal and you can solder to a tab that you leave on beyond the limits of the bolt head.

I've soldered holes in SS sinks, but I used a special SS solder for the purpose.

I did not phrase it properly in the initial post but the flux was for soldering SS. They guy I got it from agreed what solder I should use. I don't know what was wrong. Maybe wrong flux or as suggested, the solder.

Thanks for the ideas.

Do we get to learn what the application is?

If you wish.

I am building a set of sensor probes to control my 1500 gallon water storage pump. I have a slow well and this is buffer storage. I am using PVC pipe with Ts and elbows and flat caps. A hole through the cap holds a SS bolt that is the probe. When a bolt touches the water, it completes a circuit through a common point in the bottom of the tank. Electronics sense the current or lack of current and trigger relays. The relays in turn control low voltage for pumps.

There are two sensors to keep the tank full, one about half way down to tell me if it is getting low, two (one redundant) probes to protect the pressure pump in the house from low water (it really hates that), and one more completely inside the pipe near the bottom to tell me if it ever leaks enough to accumulate in the bottom of the pipe.

You may be making your probes more complex than necessary. If you use insulated wires (say #12 solid copper THWN, with a bit of copper sheet soldered to the ends if you need more area), with length to depth of water you want to sense, and a "ground" terminal at the bottom of the tank, then hook up to your electronics circuit to detect when each one is "grounded", you have all you need.

The primary reason for my approach is the corrosion resistance of ss.

There are also some other benefits. It is a stiff structure so turbulence won't move it around. It is an easy way to get wires out while maintaining isolation between my water and the surrounding dirt. It also simplifies eliminating the possibility of water running back into my garage where the controller is located.

And everything is rated for immersion in potable water.

Any pump supply or some plumbing supply houses have NSF Standard 61 rated for potable water use piggybacked normally open and/or closed float switches designed just for this type thing.

Maybe $30 each.

Us water treatment guys use them in tanks and aeration systems etc.. Some are used in sump pump installation and sewage grinder pump applications too. They are very reliable and heavy duty.

And you'd have no need for electronic controls and relays etc..

Although I'd use the float switches and wire up a 'monitor' with different colored lights 120vac panel if you want to 'see' the levels, you could also look into and use RV (recreational vehicle) tank sensors and their monitor.

Thank you Gary. I bought two of them. The variety that swing from their wire to provide hystresis. One was unreliable out of the box. Online dealer would not take either of them back.

The simple fixed float switches don't seem to do hystresis as I recollect from when I looked at them. This way keeps active devices out of the water tank. It was also harder to get wires in to a buried tank and keeping water both in and out with the float switches.

I have all the stuff to do this. Just working on getting the wires attached.