described as follows: Fe → Fe2+ + 2 The following redox reaction also occurs in the presence of water and is crucial to the formation of rust: 4 Fe2+ + O2 → 4 Fe3+ + 2 O2− In addition, the following multistep acid–base reactions affect the course of rust formation: Fe2+ + 2 H2O ⇌ Fe(OH)2 + 2 Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 as do the following dehydration equilibria: Fe(OH)2 ⇌ FeO + Fe(OH)3 ⇌ FeO(OH) + 2 FeO(OH) ⇌ Fe2O3 + From the above equations, it is also seen that the corrosion products are dictated by the availability of water and oxygen. With limited dissolved oxygen, iron(II)-containing materials are favoured, including FeO and black lodestone or magnetite (Fe3O4). High oxygen concentrations favour ferric materials with the nominal formulae Fe(OH)3−xO. The nature of rust changes with time, reflecting the slow rates of the reactions of solids. Furthermore, these complex processes are affected by the presence of other ions, such as Ca2+, which serve as electrolytes which accelerate rust formation, or combine with the hydroxides and oxides of iron to precipitate a variety of Ca, Fe, O, OH species. The onset of rusting can also be detected in the laboratory with the use of ferroxyl indicator solution. The solution detects both Fe2+ ions and hydroxyl ions. Formation of Fe2+ ions and hydroxyl ions are indicated by blue and pink patches respectively. Prevention Because of the widespread use and importance of iron and steel products, the prevention or slowing of rust is the basis of major economic activities in a number of specialized technologies. A brief overview of methods is presented here; for detailed coverage, see the cross-referenced articles. Rust is permeable to air and water, therefore the interior metallic iron beneath a rust layer continues to corrode. Rust prevention thus requires coatings that preclude rust formation. Rust-resistant alloys Stainless steel forms a passivation layer of chromium(III) oxide. Similar passivation behavior occurs with magnesium, titanium, zinc, zinc oxides, aluminium, polyaniline, and other electroactive conductive polymers. Special "weathering steel" alloys such as Cor-Ten rust at a much slower rate than normal, because the rust adheres to the surface of the metal in a protective layer. Designs using this material must include measures that avoid worst-case exposures since the material still continues to rust slowly even under near-ideal conditions. Galvanization Galvanization consists of an application on the object to be protected