Steel rusts as soon as it comes into contact with water and oxygen. However, if it is coated with a waterproof layer, it will not rust. Such layers can be, for example, an oil film, a layer of paint or a galvanically applied layer of a non-rusting material such as nickel or chrome.
An oil film is the simplest method of rust protection. This is particularly effective on smooth surfaces, which is why pliers surfaces are finely polished by grinding, sometimes even mirror-polished.
Oil films wear off and must be renewed regularly.
Paint layers also wear off when the pliers are used intensively, but such intensive use usually also prevents rusting in the areas where the paint layer has been removed.
In general, you should store your pliers in a dry place. If condensation cannot be prevented in your work area (e.g. in a service vehicle), it makes sense to use a chrome-plated version.
Almost all KNIPEX pliers are available in chrome-plated.