Steel rusts as soon as it comes into contact with water and oxygen. However, if it is covered with a waterproof layer, it will not rust. Such layers can be, for example, an oil film, a lacquer layer or an electroplated layer made 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 out and have to be replaced regularly.
Lacquer coats also wear out when the pliers are used intensively, but such intensive use usually also prevents rusting at the places where the lacquer coats have been removed.
In general, you should store your pliers in a dry place. If condensation (e.g. in a service vehicle) cannot be prevented in your work area, it makes sense to use a chrome-plated version.
Almost all KNIPEX pliers are available chrome-plated.