- I use Burnley soldering paste. It states on the package containing zinc chloride
That's a very aggressive acid flux, and is only suitable for plumbing and sheet metalwork. It has corroded away the protective iron plating on the tip, and the solder has eroded the tip by dissolving copper from it (and probably zinc as well if its actually brass). It will also corrode fine copper wires so if you have used it to tin or solder any stranded wire, they will fail sooner rather than later, rotting away under the insulation next to the tinned end. It also rots PCB tracks. If you've used it on PCBs, you may be able to salvage them by scrubbing them with very hot (near boiling) water with a trace of detergent added, to remove the acid flux residue.
Throw that flux paste away (or put it with your plumbing tools) and get electronics grade RMA (Rosin) flux, free from zinc, mineral acids, chlorides or other halides.
You *may* be able to save the tip you are currently using, by taping over the vents to protect the catalyst, and washing the pointed section first with thinners to degrease it then in boiling water with a trace of detergent, scrubbing it with a toothbrush, taking care not to get any thinners or water into the vents and on the catalyst.
Eroded portasol tips can be rebuilt by filing the end flat, and drilling a hole to swage in a thick piece of solid copper wire, which should be very lightly tinned before you swage it in place, and must be a very tight fit in the hole before swaging the sides of the remaining conical part near the tip to retain it. The hole must not go through into the combustion chamber, so probe down the bore of the shank to get the depth to the solid part of the tip, then calculate the remaining length to the end of the tip and mark your drill bit accordingly. Tape over the catalyst while drilling and swaging to keep dirt and swarf out. Once the thick wire is in place, cut it off about 10mm from the rest of the tip, then run it at max heat for five minutes so the tinning on it alloys with the wire and the tip to fuse it in place, then turn it off, file the end to a hoof tip profile and tin it and its ready to use again. As it doesn't have any plating you'll have to keep it dressed to shape with a fine file as it erodes, and eventually, when it gets too short, drill it out again and repeat the rebuild. Erosion can be minimised by buying solder with an alloy composition containing 1% to 2% copper.