Iron rust is caused by electrolytic action between the various constituents of iron or steel in the presence of moisture and impurities. It is a continuous process; a coating of rust does not protect the metal underneath.
The principal requirements of a rust-prevention process as applied to automobiles, aircraft and other machined and hardened parts are that it
Prevent rusting under normal use
Prevent the spreading of rust
Make no change in dimensions or fits
Make no alterations in physical properties
Be permanent for the life of the part
Be easy and quick of application
Be commercially practicable as to cost
Of the most familiar rust-proofing processes, the cold, the hot and the high-temperature, the last is eliminated by requirements (3) and (4), while the cold processes and also japanning are eliminated by (2), (3) and (5). There remain three hot processes, the Parker, the Coslett and the Guerini. These are discussed and compared in detail for a great variety of applications. It is not to be expected that any one process will meet all requirements. The engineer must select the one best adapted to the requirements of each machine or part.