Question about soldering technique

Normal solders won't "stick" to aluminium. The reason is in the chemistry but (although we can't see it) aluminium and iron and carbon steel too are very reactive metals.

Before we dive into the fix, let's look at the problem:

If you've ever painted a steel car panel only to see rust develop in just weeks on a piece of steel you thought was totally clear, you've seen this. Or if you've seen "thermite" (which is powdered aluminium and iron) used you've seen it. Or watched that mesmerising slow motion of someone grinding steel on a well with those sparks flowing like a fountain. 

I suspect those flashes we see on movies these days where "bullets" are hitting stuff *even glass* - are using little "balls" of raw iron too.

So what's going on?

Get ready as this is going to shock you. Take a piece of iron or (non-stainless) steel and polish it with some abrasive. If you had a microscope capable of it you'd see that the surface of your polished metal is actually covered in a layer of iron oxide: rust. This happens so fast that we can't normally see it. Those sparks from the grinding wheel aren't caused by friction they're caused by the raw iron coming into contact with the atmospheric oxygen and almost instantly igniting into a tiny fireball. 

The reaction is exothermic and when it occurs at a small enough scale (on those tiny particles coming off the grinding wheel) it's self-sustaining.

For this reason, industries where explosive gasses may be present (petrochemical plants, oil refineries etc.) use a lot of stainless steel. The additives in the steel don't just stop it from rusting (which is the non-technical term for oxidation) but they also stop a strike from creating a spark and blowing everyone to kingdom come. Such accidents are a thing of the past and one of the largest in America happened at a flour mill... as crazy as it sounds, very fine powders of sugar, flour and such can create an explosive atmosphere.

Mill City Museum and site of "The Great Mill Disaster" – Minneapolis, Minnesota - Gastro Obscura (atlasobscura.com)

I've done this experiment with a number of products and it's quite shocking just how violent the explosion can be. 

Now that's what I call 'magic smoke' baby!

Needless to say that you should never, ever try this at home. I'm an idiot with no sense of personal safety and I've had stuff (literally) blow up in my face more than once. But that also gave me an idea to improve the safety systems that prevent these accidents - perhaps my greatest achievement except that the patent process quite literally put me financially underwater. Anyway...

Aluminium oxide forms on aluminium in the same way and at the same speed. We tend to think that aluminium doesn't corrode like steel but it does - and effectively instantaneously, just like steel does. The difference is that aluminium oxide is white and it's hard to see.

So why is your solder not "sticking"? 

It's because it's literally rolling over the oxide layer. And that's assuming you've got the metal hot enough. Even a fairly thin piece of Al. will rip the heat right out of your iron. Soldering Al. is usually done with a butane torch or a full-blown welding kit made for the purpose.

"But Marc, there can't be an oxide layer there because I can't put a meter on it and get a measurement. It conducts electricity"  

This is true but the reason we can take a resistance measurement on iron, steel, while the surface is shiny is because the meter probe can pierce the atoms thin layer of oxide that forms in the fraction of a second after the surface is polished. We can't see it, but it's there.

TL;DR

I'm not sure how Matt did this but generally the consensus is to not do it. There are some special fluxes that do work (but they are highly corrosive and very toxic so I'm not going to mention them here). One method that appears to work for some is to polish the area under the cover of a mineral oil (baby oil) or perhaps isopropyl alcohol.

Both of which are quite flammable in their own right!

The idea is to prevent the oxide layer from forming in the first place and relying on heat from the iron to evaporate the quenching material while allowing the solder and flux to to its job. Zinc-tin solder should be used. Old farts like me (and Big Clive) a fantastic YouTuber with an amazing beard use lead-tin stuff but unless you have a fume extractor or don't value your brain too much (or the health of any future children) go right ahead.

I'm kidding. Please stick to the safer option.

<RANT MODE>

Lead is nasty crud and it gets into our germ cells (sperm potentially and particularly eggs in women exposed to the crap) which means it ends up in our kids can since it causes DNA damage... This is a problem I've seen in Argentina where farmers routinely use lead based insecticides to protect their crops and there is even natural lead compound in the water. American smokers have been exposed to high levels of lead in cigarettes for many years as a result of importing tobacco from these regions too.

While this is an extreme example - consider this. When a woman ingests even a small about of lead, the lead gets into her eggs and if she becomes pregnant with a girl, the lead gets into the unborn baby's ovaries passing the DNA damage down another generation. That's how serious we have to be about this stuff and why I beg everyone to recycle their tech (particularly the older stuff which is brimming with lead) properly. We all have a duty to protect the environment for future generations.

</RANT MODE>

 

Posted by: @uberman

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I'm new to soldering metal and I've been having some trouble ... Even though all the signs point to it being the metal plate since it's literally a solid 36x21x4mm plate of aluminium...

OK - they use brass in the video, not Aluminum.  Normally you do not try to solder aluminum (for several reasons).  You would normally weld or braze it with a torch.  TO braze it, you need some brazing rod.

If you really want to solder it, you might try:  Sanding the spot you want to solder with 240 or 320 grit sand paper and them immediately cover with flux before trying to solder it.  Basically, Aluminum oxidizes extremely fast so you can't get solder to stick.  You sand it to get a fresh surface and use the flux to lock out the oxygen before you solder.  Then you solder, preferably with Zink/Tin solder and a very hot iron or torch.  Once it is tinned, it should solder much easier - but you could try to solder it direct.  They make something called Aluflux, but I have never used it.  It might have nitric in it and that would do the trick.  You can try it, but personally I would just switch to brass.

There are other ways to solder aluminum, but if you are just beginning, I might shy away from them for now.  Many of them are "lets see if this works" kind of things.

Hope that Helps.

Teh Manta