Solder Analysis - What happens to Tin over time?

[BNE]

The company I work for has a 40 pound hot solder pot for solder dipping some of the assemblies we make. It is not used every day, but it is used regularly. The pot is left on 7 days a week, 24 hours a day. I know that this pot has been in operation having ONLY new solder added to it as needed for over a year. The temperature is held well above 750F.

The solder is 60% Tin, 40% Lead.

We were looking into the requirements for a new customer who was requesting a Military-Specification on the solder used and the solder dipping of the part. In order to confirm the contents of the pot Vs. the contents of our solder bars, I sent samples to the solder manufacturer to analyse. (I melted the new bar and poured a sample from it.) The samples were only labeled "A" and "B".

The results:

Sample "Pot"
Tin 60.40
Lead 38.55
Copper 0.107
Silver 0.810

Sample "Bar"
Tin 59.20
Lead 40.54
Coppe 0.024
Silver 0.0055

The rest of the elements reported were only at contaminate levels. The higher content of Copper and Silver are from the parts that we are solder dipping.

My conclusion from this test (for us bullet crafters) is that we are probably over stressing about losing Tin in our pots. It is highly unlikely that we are losing significant amounts of Tin during a casting session, if any at all.

BNE

[lightman]

You always come up with interesting and useful information. Thanks for posting this. I would have expected 750 to cook the solder out of the alloy.

[Yodogsandman]

How often is the pot fluxed?

[jsizemore]

A local electronics outfit has to use lead free solder for mil-spec manufacturing. They "tin" the leads from a common pot that never gets fluxed. They scrap any dross off the top and keep on going.

[mfraser264]

When the lead alloy is in a liquid state, it is prone to oxidation. The air around us has oxygen in it and with oxygen being a somewhat unstable element, it tends to attach itself easily with other elements, especially those that are not stable like lead and tin. The layer that is skimmed off the top of the pot after fluxing is the tin with extra oxygen and it is difficult to break the oxide off and return the tin. In the printing business they would add ingots that were higher than normal in tin to correct for the loss of tin over time. In the casting industry it is common to cover the pot and restrict the atmosphere from reaching the bath of molten metal or inject an inert gas such as CO2 to keep the atmosphere that contains oxygen from contacting the metal. A simple lid or cover placed on top of the pot when the pot is not is use will help reduce the tin oxidation. Another option is and I am not sure what to use for lead, is to place material that floats on the top of the molten metal and form a soft barrier to keep the oxygen from reaching the metal. This material did not combust and could be easily removed when ready for casting. We did this a lot in the steel casting industry. Hope this helps. In casting, oxygen is not a friend.

[alamogunr]

Thanks for posting those results. In my pre-retirement employment, we used a large solder pot that was left on 24/7 same as your sample. When they discontinued using the pot all the solder was removed and cast into various ingots. This particular solder was either 40/60 or 50/50(Tin/Lead). Rather than have them contract with a hazardous waste company, I convinced them to let me dispose of it. Since I didn't know the exact resulting composition, I assumed it was all 40/60. Based on the results in the OP, it is probably somewhat higher in tin content. I estimate that I ended up with approx 150-200 lbs of the stuff.

BTW I didn't charge anything for disposing of this "hazardous material".

[Nueces]

BTW I didn't charge anything for disposing of this "hazardous material".

Where do we get such men?

[BNE]

How often is the pot fluxed?

It is not fluxed. We will dip a part into a liquid flux prior to dipping the part, so some flux goes into the pot. Not a lot, because the pot is rarely used. Also, the operators take a spoon and scrape off the top of the melt and tap it into a scrap bucket. (Yes, I sporadically empty the scrap out of the barrel and save the company the cost of hazardous waste removal. It gets me about a pound a month.) I have taken this scrap, melted it down and used an XrF to analyze it. The XrF is not as accurate as the analysis shown above, but it is close. The scrap reads very close to the original 60/40 ratio. Very close.

My point in this post and some others I have posted is I am sure that tin oxidizes and I am fully aware of how nice and shiny the top of the melt gets when you put sawdust or wax on top and stir it in. Fluxing also helps with the "Oatmeal" stuff on top of the melt.
BUT, I think some people would have you believe that you are depleting your bullets of Tin if you don't stop and flux every 30 minutes. The data does not support this.

I'm a ceramic engineer, not a chemist or chemical engineer, so thenext statement is more conjecture than proven fact: I think the oxide layer on top of the melt is similar to the oxide layer on the surface of Aluminum. The oxide layer is protecting the rest of the melt from oxygen.

[RogerDat]

...........

I'm a ceramic engineer, not a chemist or chemical engineer, so thenext statement is more conjecture than proven fact: I think the oxide layer on top of the melt is similar to the oxide layer on the surface of Aluminum. The oxide layer is protecting the rest of the melt from oxygen.

I'm just an ex-factory rat, truck driver, computer geek and shade tree mechanic but that makes sense to my inner redneck and what I recall of chemistry. A layer of tin oxide that has all the available binding sites for oxygen in use seems like it would be a barrier to more oxygenation. I know that is how aluminum oxide layer works.

[alamogunr]

Originally Posted by alamogunr
BTW I didn't charge anything for disposing of this "hazardous material".

Where do we get such men?

Just doing my part to protect the environment. We shouldn't charge a fee for that.

[bangerjim]

Remember..........soldering and/or dipping for electrical connections it TOTALLY different than casting the stuff in a mold and getting PERFECT fills and exact sizes due to the Sn being present (lowering surface tension). They do not "flux or reduce" a wave solder machine or dipping pot as we do for casting. What we do is totally different, even though both processes are melting an alloy of Pb.

With the right flux (rosin/acid) one can easily get pure lead to stick to wire and metals. No Sn there at all. But I would not want to trust the hardness or longevity of a pure Pb solder connections at all.

Not really a comparison for our cavity-filling, precise size casting uses. But still interesting info.

thanks,

banger

[scottfire1957]

Good info, BNE.

[cajts]

yes very interesting. I may have to do some testing for myself