Knights protected their armor through a variety of means, not all of which prevented it from rusting, but simply prevented it from rusting past a certain point. One of the more popular methods of protecting steel was known as "bluing," which is a technique that is still used today! The are two forms of bluing: hot and cold.
Hot bluing occurs when the surface of the steel is heated and then rapidly quenched instead of allowing it to cool slowly. ("Quenched" means to cool it by dipping it in water or oil.) The act of quenching also contributes to the ratio of hardness (brittleness) to flexibility the steel would have. There were a variety of methods depending on the quality of steel and how much temper you wanted it to have, and was something of an art form; you could ruin a perfectly good piece of steel if you weren't careful. But basically, the goal was to create a thin outer layer that was impregnated with oil that would wear away over time but would otherwise slow down the oxidation process that causes rust. The end process caused the steel to take on a bluish color, hence the name "bluing." However, bluing was not perfect; it would slow down rust, but not stop it. This meant you took very good care of your steel (or at least your servants, squire, page, etc. did) by cleaning, oiling, and polishing it with every use.
Cold bluing is a bit different. Instead of heat and oil, a chemical was used to cause the steel to rapidly rust. (For example, urine.) That's right, they intentionally caused the steel to rust, taking care to make sure that it rusted evenly. They would then rub oil into the rusty metal, let it rust some more, apply more oil, let it rust some more, and so forth. The end result was a very deep black or brown colored surface that basically helped slow down the rusting process. Like hot blued steel, you still had to take very obsessive care of your steel. Hope I helped u.. Plzz mark as the BRAINLIEST answer..