[xchcui]

Hi.

There are several types of fluxes which are used as a chemical cleaning agent in soldering and brazing tasks.The flux helps to improve the joint between the metals by removing oxidation from the metals to be joint,during the brazing activity.
My question is:
During the soldering/brazing activity,does the flux(one or more of its components)chemically bond to the metal-joint and become part of the joint(while,can not be removed by cleaning)?
or the flux forms only residues which can be removed(cleaned),entirely,from the joint(by water,alcohol depend on the type of the flux),while no chemicals of the flux is,actually,chemically bond to the joint?
I know that there are different types of fluxes:different activities,with metal halides,different PH,washed by water or cleaned by alcohol etc.
This question is general for all of them,while the main idea is if them(or parts of there components)chemically bonded to the joint?

Thanks.

[Enthalpy]

Hi xchcui, a soldered or brazed joint is by nature a bond between metals. The molten metal wouldn't even wet a non-metal like the flux. The flux' role is only to remove the layer of oxide and other non-metallic compounds from the surfaces.

That said, removing the residues of flux after soldering or brazing isn't always easy. While it's basically possible without losing any strength at the joint, it's a matter of accessibility, of chemical resistance of the other elements that must be kept, and of reactions of the burned flux with the new metal surfaces obtained by joining.

Residues of flux are often undesired even in tiny amount, as they promote corrosion (in music instruments for instance) or may contaminate nearby assemblies (especially optics in satellites) and then, perfect cleaning gets difficult. This has led satellite manufacturers to use flux-free solder for electronic boards, which in turn are a recipe for unreliable solder joints - to my opinion the wrong choice.

[xchcui]

So i understand that the flux is not chemically bonded to
the soldered metal and it is possible to clean,entirely,all the flux residues.
Refer to the accessibility aspect,Let say that i
have full accessibility.For example:if i solder piece of copper  sheet using flux and i have accessibility to clean the soldered joint from both side,you say(as much as i understand)that alcohol or water (depend of the flux type) will not always remove all the residues of the flux,since the burned flux might bonded strong to the surface?
But if i will sand the upper layer of the soldered joint with sand paper,will it remove ALL the residues?

[Arkcon]

You're asking for guarantees, and we can't give them to you.  The presence of excess flux can ruin a joint without chemically bonding.  A trace of flux molecules can form a bond on the surface of a joint, and still be harmless.  The best joint is well cleaned, but serviceable ones exist with sloppy work.

[xchcui]

What i am trying to figure out is that:
You said that a trace of flux molecules can form a bond on the surface.I understand that the bond is only on the surface and doesn't deep in the solder joint,does it?
So,if i sand the upper surface with a sand paper,it seems like i can remove all the residues,can't i?
Or,maybe,even if i will remove the upper layer of the joint solder by sanding,there might be more flux molecules deeper(the new surface after sanding) in the soldered joint.

[Enthalpy]

If soldering or welding succeeded, no flux is left in the depth, only metal.
- But if joining failed, sure, flux will be left in pits, crevasses...
- "No flux" is for mechanical engineers. If you analyse parts per billions, you can always find residues.

Sanding: consider instead a brush with metal hairs, used with a good solvent.

[xchcui]

My question was mainly about the corrosion that the flux residues cause.So,i understand that after cleaning with good solvent and metal's hair brush,the remain amount of residues will be negligible for causing corrosion.

I would like to take it further to another aspect in order to understand it:
If i will use a flux,which contains toxic compounds,while brazing a food utensil and if the brazing success,while i clean the residues well,as you suggested,will the remaining flux residues(parts per billions)on the food utensil constitute a problem,when it refer to food safe concern?

[xchcui]

Is there any problem with my question?
Did i exceed the forum rules?
or maybe there is a problem to answer that question ,since
it refer to food safe concern

[Enthalpy]

I was absent. I can't tell about other people's reasons.
Yes, food safety, and safety in general, makes everyone uneasy, sure.

You might try to estimate how much flux is used in the operation, how much is acceptable in the food, how much can remain after cleaning, measure how much passes in the food when using the utensil, and so on, and so forth. But then, errors (bad cleaning) do occur at the production, and health standards differ among the countries and over time.

Or you could choose a solder that accepts a non-toxic flux. Or even better, if the parts' shape enable it, braze without any flux. If you can clean the metal parts just before, for instance with a motor-rotated brush, brazing succeeds easily in most cases. That would clearly be my preferred solution.

Don't forget that many brazing alloys too contain elements unsuited to food contact, like cadmium or lead.

[xchcui]

Thank you very much for your help.
your explanation and the options that you advised,
help me to understand the issue.

Thanks.