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10x25mm
With the abundance of oxygen on earth, iron wants to be an oxide, not elemental iron. All chemical systems seek their lowest Gibb's Free Energy state. For iron, that is iron (III) oxide (red rust or iron ore). Elemental (metallic) iron has the highest Gibb's Free Energy state. Iron (II,III) oxide (gun bluing) is in the middle.
The chemical kinetics of these transformations at room temperatures are sluggish – almost geological – unless an intermediate reaction which is thermodynamically favorable occurs. Iron reacts very quickly with Group VIIA halogens and Group VIA chalcogens. So if an elemental iron or iron (II,III) oxide (gun bluing) surface is exposed to strong ionic salts or acids in these families, the intermediate reaction is quick. The iron sulphide or chloride formed by these intermediate reactions have Gibb's Free Energy states between iron (II,III) oxide and iron (III) oxide. It is not thermodynamically stable in an oxygen containing environment. So a further ionic exchange occurs, with the sulphides and halides oxidizing down to iron (III) oxide.
All non geological oxidation of iron and steel surfaces is either electrolytic or pyrochemical. Combustion is not relevant to firearms corrosion, because it usually occurs above the melting point of iron.
In rainwater corrosion, some ionic molecule dissolved in the water produces an electrically conductive liquid surface film. Salt molecules containing halogens (most commonly NaCl & KCl) are the most common ionic molecules responsible for this kind of rust. The most common sources on firearms are sweat, salt, and corrosive priming residues. Water and other liquids composing the films come from humidity, sweat, rain, and immersion.
The corrosion mechanism is initial chlorine reaction with iron atoms to create iron (II or III) halide, or less commonly iron (III) halate. Chloride (most common halide) or chlorate is then almost immediately converted to iron (II or III) hydroxide drawing on free oxygen ions in the water. Finally the water departs and you are left with iron (II,III) oxide [bluing, uncommon without inhibition] or iron (III) oxide [rust or iron ore, common].
These reactions would not occur without electrolysis. The electrical currents are created by concentration cells (areas of different chemical concentration in the water solution) and topographical cells (microscopic peaks and valleys on the metal surface have different electrical potentials). Mirror polished metal surfaces corrode more slowly than rough surfaces. Prevent the transmission of electrical currents altogether and iron will not corrode, even in the presence of strong – otherwise highly reactive – ionic solutions. This is inhibition. Barium sulphonate is a strong inhibitor.
Most firearms are not deliberately exposed to strong reactive ionic solutions, so how do they alight on firearms surfaces? Ionic salts from perspiration are only a minor contributor to corrosion. Most of the really damaging ionic solutions are the excreta of bacteria, which are widely present in the environment and fingerprint residues. Add oils, fats, and water and the little buggers have a feast. They excrete strong, ionic sulphur compounds which trigger corrosion when wet. Almost all corrosion pitting on firearms surfaces occurs under erstwhile bacteria colonies. Barium sulphonate is a strong antimicrobial.
Summary:
Eliminate water and/or chlorine (and all the other less common halogen & chalcogen atoms), and electrolytic rust does not occur on steel surfaces. The other major corrosion mechanism on steel firearms surfaces is bacterial. Get a bacteria food (sugars, organic oils, fats, soaps, etc) on the surface of steel or iron and bacteria will consume it, then excrete various sulphurous acids which launch you back into electrolytic corrosion.
Or you can apply inhibitors and antimicrobials to the iron surface to prevent corrosion, but there must be a binder to adhere them to the iron surface. Barium sulphonate is the best inhibitor and antimicrobial. Wax is the best binder which can be readily removed.