I am presuming you don't have a voltage regulator and you just plug and play. Regular socket wall produce 60w and above. Your cheap solder is running about 35 - 40w range. W/O a regulator it's just going to keep heating until it melts.

Oxidization happens much more often with cheap solder .... sooner or later the shape gonna bend. for better quality ones they have separate rod that act as a cooler between the inner and outer rod, the cheap one don't have.

You can still use it, just that its kinda leceh ...... what you need to do is only turn on when you are ready to use and wait a bit. Make sure you have a wet sponge holder and a wet sponge use it to cool and clear the tip, like rub against it after a few solder points. Turn it off after finish use and don't leave it unattended until its safe to do so.

Its like using a manual car, just make sure when its red hot the thing touch nothing .....that is when you have the solder holder to keep it in place.

Idk if there is any master sifu solder expert here, for me I already have regulator that I can adjust and also a safety volt moderator for my pc after thunder fry my com last time ....... I can adjust the voltage so I can just turn it on and use whenever I want. Originally I bought one also from mr.diy and plug directly to socket and notice the heat kept getting higher. Which is why I suggest a regulator.

But of course this are my personal experience so I am by no means an expert in all this. Its best if someone who has been doing this for a while share their experience here so everyone can learn.