I bought a low end soldering iron of eBay - it was advertised as a "15 in 1 Soldering Iron Kit", and was advertised as temperature adjustable. It has a dial to adjust the temperature with numbers in degrees Celsius.
I put a new chisel tip from the kit in it, turned it up to 350 degrees Celsius, tinned the iron, and successfully soldered a couple of joints with lead-free electronics flux-core solder. Then I cleaned it on the damp sponge that came with the kit, and waited for it to heat to a temperature where it would melt the solder again. But by the time it reached that temperature, it had already gone black and the solder just balled and rolled off the tip. Wiping it on the sponge again cooled it, and the same problem was there when it heated back up.
I tried adding no-clean electronics flux from a flux pen to some balls of solder, and melted them into a blob, and it stuck to a tiny part of the tip. I tried letting the tip sit in the ball for a little while, and it still kept rolling off and not tinning. I wiped it on the damp sponge again, and repeated the flux treatment a couple of times, but the tip has developed some kind of yellow mineral-like crusty coating that isn't coming off with the sponge or the flux (and I'm wondering if it is possibly even from the sponge) - and now I can also see a reddish colour that I think might be the copper core.
This is a magnified view of what the tip looks like now (it is cool - red is not glowing metal):
I'm assuming that this one day old tip is completely wrecked now? It was a cheap iron / tip, but I'm getting a more expensive soldering station soon, and I want to make sure I don't ruin the tips the same way. I never used the soldering iron above 350 degrees Celsius, I had the tip on the iron while it was on for less than an hour total, I never scratched it on anything or used much pressure, and I don't think it was exposed to any contaminant (unless it was the flux), and I tried to keep it tinned as soon as it reached a temperature where that was possible - what else could I be doing wrong? Or is it just that the cheap iron is reaching temperatures that are higher than my selection (it does only have two wires from the PCB inside it to the element, despite being sold as temperature adjustable - so I assume it doesn't have a true thermostat and only adjusts the power / current / voltage based on the dial)? Or is it a poor quality tip that was always going to fail after a use or two?