That was the short explanation of a complex case. There are, however, more sides to this story, and many more nuances to be made. We also have tips to prevent rust, even on stainless knives. There are types of steel that are completely stainless, and still stay sharp. Ready for the long explanation? Here we go!
To understand why different types of steel have different features it is important to know how a type of steel is constructed. Steel is namely not only made from iron. For the most part, yes, but only iron will not last as a knife. That is why different components are added per type of steel. Carbon, chromium, nickel, molybdenum, silicon, tungsten, cobalt, manganese and many other options. By playing with the percentages of these components steel manufacturers produce their different alloys. Some alloys are very tough, others very hard. Some alloys already start to rust when you look at them, other can be put on the bottom of the ocean without anything happening.
Knife steel contains carbon. It is the component that ensures that you can sharpen your knife razor-sharp, and it also ensures that the knife retains this sharpness. That is why many knife brands have been using terms such as 'high carbon steel' for years. For many a quality feature. And in some way this is true. Many lower alloys namely contain less carbon which means that the type of steel does not retain its sharpness as well.
As such the more carbon in a type of steel the better. Unfortunately carbon also has another feature: it makes the steel rust. So a lot of carbon is great to retain sharpness, but at the same time it also increases the risk of rust or discolouration.
In types of knife steel you often find carbon contents of 0.5% to 1.5%. There are also peaks to 3%, or the other way to 0.15%, but those are really special types of steel