A soldering iron oxidizes if it is left to burn continuously.

When the tip oxidizes, it is difficult to solder. Heat does not transfer to the soldering target.

If the surface oxidizes excessively, as in the image, tip refresher or tip recovery cannot cope.

To prevent oxidation of the tip, take the following two points. 

• When using a temperature-controlled soldering iron, keep the temperature below 350°C and as low as possible.  Especially above 400℃, the tip will oxidize faster.
• Coat the tip with solder when returning it to the soldering stand. Plating the tip with solder prevents oxidation and extends its life.

This is true of all soldering tools. Desoldering tools as well. 

It is important to prevent all tools from oxidizing. This increases life and efficiency.