Corrosion protection effect of rust and scales: A “metal protects metal” perception by considering the examples of hematite and magnetite

Abstract The present report proposes a contradictory remark on the idiom “Iron cuts Iron” that could be synonymized as “Metal cuts Metal” using the example of hematite and magnetite. Most metals and alloys are highly reactive, forming rusts and scales primarily consisting of metal oxides. The formation of metal oxide layers is known as passivation. Generally, passivated metallic surfaces provide anticorrosive protection by avoiding the attacks of corrosive species on the fresh metal surface; therefore, “Corrosion is a Necessary Evil (CNE).” Most passive films are protective, but not all, and their actual nature can be determined using the Pilling–Bedworth ratio (PBR). In dry conditions, passivation occurs spontaneously by the oxidation of metal atoms, but in aqueous media, some external additives, called passivators, are added to build the passive films. Herein, by considering examples of hematite and magnetite, the anticorrosive properties of rusts and scales are described in dry and wet (solution and coating) environments.