Oxidation is a natural process in which the oxygen in the air combines with iron or other metals to form an oxide. In some cases, this is beneficial: That dull gray patina that forms on your knives over time actually serves as a protective coating on the blade. Oxidation also appears as rust, however, and can leave unsightly orange stains on even stainless steel. When unwanted rust or oxidation appears on your knife blades, you have a few different options for removing it.
How Oxidation Forms
Oxidation forms on your knife blades when water (either liquid water or the water in the air) reacts with the iron in the blade. The amount of water and the iron content of the blade both play a big part in this. A knife left out in the rain is more likely to form ugly rust spots, and blades with a high iron content oxidize more easily than those with less iron. Limited water contact (such as contact with air humidity) is more likely to create a patina, while large amounts of water are more likely to create rust. Controlling moisture and water contact goes a long way toward preventing unwanted oxidation.
White vinegar contains acetic acid, a chemical compound that reacts with iron oxide (rust) and separates it from the metal that isn't rusted. Place the rusted knife in a bath of white vinegar for up to five minutes, then wipe the blade down with a clean cloth to remove any remaining vinegar. Soaking a cloth or paper towel in vinegar and applying it to the rusted portion of the blade is also an option if you can't easily soak the entire blade. Do not leave the blade in vinegar for longer than five minutes. Although acetic acid is most reactive with iron oxide, it will begin breaking down the iron in the blade is left for too long.
Baking Soda Removal
Sodium bicarbonate, better known as baking soda, is another chemical that reacts with iron oxide for easy rust removal. Make a paste from baking soda and water to break down the rust (though you can use lemon juice instead to add additional acid to your cleaning paste in cases of heavy rust). Clean the blade to remove any dirt or loose rust before applying the paste. Use a toothbrush or piece of steel wool to scrub the rust with the baking soda paste. You should be careful if you do use steel wool or other abrasive materials as they can scratch the metal surface. Once the rust is scrubbed free, wipe it clean with a dry towel.
Penetrating oils such as WD-40 or Liquid Wrench performs a number of functions around the house, including breaking down rust to loosen bolts and eliminate creaks in hinges. Apply the penetrating oil to the rusted portion of your knife blade and wipe it off, using a very fine grit sandpaper (such as 400 grit) to scrub the rusted area before wiping up the oil if the rust stain is significant. Reapply the oil and let it soak into the rust for a few minutes if rust remains after the first cleaning. Keep in mind that penetrating oils are hazardous if inhaled, ingested or brought in contact with the eyes. Always use these oils in well-ventilated areas and wear breathing and eye protection. As oil makes things slick, you should also use caution when handling sharp knives that you've sprayed with penetrating oil.
Once you've removed the unwanted oxidation from your blade, take steps to prevent future oxidation. Clean knives after use and dry them quickly to prevent rust from forming. Apply metal polish or mineral oil to protect the blade if your knife isn't used for food preparation. Inspect your knives regularly for signs of rust or unwanted oxidation and clean them occasionally to remove dust or other dirt that can trap moisture and lead to rusting.