What should be used first to clean a corroded soldering iron tip?

Using flux to clean a corroded soldering iron tip is beneficial because flux is a substance that helps to remove oxidation and prepares the surface for better conductivity. When a soldering iron tip becomes corroded, it often has oxidized materials that can impede effective heat transfer and soldering performance. Applying flux can aid in breaking down these oxides and enhancing the removal process.

Unlike the other options, which have different cleaning properties or purposes, flux specifically works to restore the effectiveness of the soldering iron. Alcohol can dissolve some residues but is not specifically designed for oxidation, sandpaper may damage the tip by abrading too harshly, and water is not effective for removing corrosion. Hence, using flux first helps in maintaining the tip's integrity while effectively reducing corrosion.