Very simply flux reacts with surface corrosion and washes it away, making the metal surface being soldered easier for the solder to wet and producing a more solid and reliable joint, both mechanically and electrically.
Acid flux is more aggressive and acts faster than rosin flux, but must be washed off completely with water as soon as the joint cools because the acid will slowly attack the metal of the joint. As electronic circuits usually should not be washed with water, only rosin flux should be used on electronics because it can be left on the completed joint without damaging it.
There are two types of soldering flux. Acid core and rosin core. The core type fluxes are inside the actual roll of solder. Both types of flux can also be bought in small cans and are brushed on to the area to be soldered. Acid flux is used on non electrical components that need to be soldered together. The rosin core flux is used on electrical and electronic components.
In both cases the flux is used to clean the oxide from the components that need to be soldered together. When the heat from the soldering iron is applied to the soldering site the flux starts to flow just before the temperature of the solder starts to flow.
A good solder joint is bright and shines whereas a bad solder joint is dull and rough. It is known as a cold joint and could have a high resistive value to it. To repair a cold joint all that has to be done is to apply a little flux and re-heat the joint until it becomes bright with a high shine.
In some solder it is. But even if it is in the solder you should still use flux.
flux acts as a wetting agent in the soldering process, reducing the surface tension of the molten solder and causing it to better wet out the parts to be joined.
Soldering iron, solder, flux
Yes: flux core solder is. No: acid core solder is not.
When circuit boards are assembled, the parts are soldered on. In order for the solder to make a good connection to the parts and to the board, a substance called flux is mixed into the solder. It removes oxidation and cleans up the surfaces of the different parts so that the solder can stick to them. After the board is assembled, there is still flux on the board. Flux remover dissolves the flux and washes it away.
Flux cleans the metal and helps the solder stick.
Flux is used to keep metals from oxidizing. Solder will not bond well with copper-oxides which are formed when copper is exposed to high temperatures (ie a soldering iron) the flux prevents the oxidation. If you were to solder without flux the solder joint would be weak and possibly completely nonconductive.
The majority of boat props are aluminum. You can't solder it, it has to be welded or heli arced.
You need flux to solder pipes properly. However many modern plumbers never solder, they use plastic or composite pipe like PEX.
Solder comes in 2 basic varieties, solid and rosin-core. Simple rosin-core solder is a tube of low-melting-point metal with flux filling the single core down the center. Multicore solder has multiple hollows in it, each filled with flux.
To facilitate the soldering process, a chemical material called flux can be used to remove oxidation from the surface, keep the air from oxidizing the surfaces during soldering, and to aid in the flow of solder through the joint. Fluxes can be found within the solder (cored solder) or separately in paste form.
Solder has an expiration date because of the flux content. It does not go bad-per say, but the flux will dissipate out of the solder after a while. It's still useable, but the flux content may be slightly diminished. Also, over time the solder oxidizes. For home or hobby use, it's fine. But in a manufacturing atmosphere, you need to be prepared to throw some away every year.