Iron exposed to moist air or oxygenated water are corroded, leaving a red encrustation of iron oxide on the surface. Progressive rusting is a major source of failure of unprotected structural materials. It is aggravated by bad design which leaves moisture traps in the structure. Rust may pit small holes in a surface, or uniformly progress over its area. It may attack joints and crevices. Despite their universal vulnerability to penetration, iron and steel materials are insufficiently protected, both in manufacture and in maintenance

A corrosive environment of iron or steel only requires the presence of water together with either a dissolved acid gas or oxygen, or in some cases just dissolved salts (eg brine). The importance of rust is particularly evident in the automobile industry. In the UK it has been estimated that automotive rust costs some �260 million per year, decreasing the value of each automobile by �1 every week. In countries exposed to ice and snow on the roads, the use of salt to clear it accelerates rusting. In the UK, where the use of salt in this way is estimated to cause 50% of rusting, it therefore costs �130 million per year. In the USA, repairs and replacements due to corrosion and rust damage may be worth nearly 5% of the gross national product. High humidity countries or locations experience the worst rusting: Suriname, Abu Dhabi and Indonesia top one list of rust-prone climates. In 1990 it was reported that the 1,300 kilometre trans-Alaska pipeline (designed to be rustproof for 30 to 40 years) was seriously corroding because of failure to corrode it adequately. Repairs were expected to cost from $600 to $1,500 million