Still i thought the point of rosin was to be mildly corrosive and remove rust to let the solder flow easily to the joint ?
The reason rosin is nature's flux is because these long rosin molecules are acidic when liquid. But you have to heat up the rosin to make it acidic. After the rosin cools down, it solidifies and becomes inert. It is also waterproof, so the metal salts that are left after the acid eats away the oxides are safely bound up in the rosin residue.
"Corrosive" doesn't necessarily equal acidic. An acid can't eat unoxidized metal. It selectively removes the metal oxide layer. So if the residue contains active acid and it also allows oxygen penetration to the metal, it is corrosive. If it contains halides (like chlorides/flourides), it is corrosive for an additional reason which is more specific to iron alloys. The halides penetrate and embed into the surface of iron and induces rusting on any exposure to oxygen and moisture. This provides oxidized metal for any active acid to eat away. And it also can cause connections to break, mechanically, because red iron oxide is bulky and spongiform and pushes itself apart.
So i bought this "TE410/500"
https://www.tme.eu/en/details/te410_500/fluxes/cynel/
The link says this is rosin based and halide free. And it can be cleaned with alcohol. It sounds perfectly fine to use on PCB's. I would imagine you can probably leave it on there with no ill effect in most applications. There's nothing there to suggest that this flux is (too) corrosive for electrical work. The datasheet and the "no clean" designation, and the placement of this product in this specific store ALL SUGGEST that this flux is designed and appropriate for what you're doing. If you don't trust that this is the case, then stick with rosin flux.
and this is the RF800/1 link:
https://www.tme.eu/nl/details/rf800_1/vloeimiddelen/ag-termopasty/
Is that German?
the difference in price is because the RF800/1 is non corrosive..
Don't put too much stock into this answer. People ask questions, and store owner must provide response. In another universe, you would get answers like "I don't know. These are just two different SKU's to me. We make a X percent margin on this one. We make Y percent on the other. People like variety and are pretty stupid. Some people like A because it's the first one they tried, and it worked fine and they will use it for 20 years until they die. Other people have tried B and they will swear by it until the day they die. We have to stock them all because having a variety of products is appealing to consumers. And there is a significant segment of the population which perceives higher cost as attractive. If you present them with 3 options, they automatically desire to have the most expensive one. Whereas if we only stocked that expensive one, it would be perceived as a ripoff."
In short, there may be some important and significant differences between the two fluxes. The guy that answered your question might even know what they are. And he might have told you, correctly. But if he is talking out of his ass, he is always going to say something that justifies the cost of the more profitable (or at least more expensive) product. If not to sell you, at least to satisfy that you got a prompt response and weren't left hanging. But this guy probably spends at least 2 hours a day answering questions of customers buying thousands of different products which he has never even used.
R, RMA, RA have a very specific definition. A manufacturer can make RMA flux and sell it as their own special "no clean." But unless it meets specs for RMA, they can't take any flux that works just like RMA and sell it as RMA, unless it is actually RMA. R, RA, RMA designators hold more "meaning" than "no clean," even if some no cleans are more or less the same as rosin fluxes. So if you are skeptical about what you have, you might want to buy a mil spec rosin flux for a baseline comparison.
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