Iron, in its assorted forms, is exposed to a wide variety of atmospheric environments. Iron tends to be highly reactive with most of them because of its natural tendency to form iron oxide. When it does resist corrosion, it is due to the formation of a thin film of protective iron oxide on its surface by reaction with oxygen in the air.
This film can prevent rusting in air at 99% relative humidity, but a contaminant such as acid rain may destroy the passivity of the film and permit continued corrosion. Thicker films of iron oxide may act as protective coatings and, after the first year or so, could reduce the corrosion rate.
Although the corrosion rate of bare steel tends to decrease with time in most cases, the difference in corrosivity of different atmospheres for a particular alloy is tremendous. In a few cases, the corrosion rates of ferrous metals have been reported to increase with time, and careful analysis of the exposure conditions generally reveals that an accumulation of contaminating corrosive agents has occurred, thus changing the severity of the exposure.
It is generally conceded that steels containing very low amounts of copper are particularly susceptible to severe atmospheric corrosion. In one test over a 3½-year period in both a marine and an industrial atmosphere, a steel containing 0.01% copper corroded at a rate of 80 µm/y (3.1 mpy), whereas increasing the copper content by a factor of five reduced the corrosion rate to only 35 µm/y (1.4 mpy). Further additions of small amounts of nickel and chromium reduced the corrosion rate to ~10 µm/y (0.39 mpy). Other tests comparing grey cast iron, malleable iron, and low-alloy steels indicated that their corrosion resistances were approximately the same.
Plain-cast iron appears to have a corrosion rate about one half that of 0.2% copper steel in a marine atmosphere. One must be careful in citing such comparisons to stipulate the precise composition of the carbon steel (CS) because the corrosion behavior of CS is influenced so markedly by small variations in copper and phosphorus content. After five years of exposure in an industrial atmosphere, a structural CS showed a penetration of ~20 µm (0.8 mil), a copper structural steel ~10 µm (0.4 mil), and a low-alloy steel ~4 µm (0.15 mil).
It is impossible to give a corrosion rate for steel in the atmosphere without identifying the composition, location, and specific environmental factors. If one can relate exposure conditions to those described in the literature, a fairly good estimate can be made of the probable corrosion behavior of a selected material. However, all aspects of the exposure of the metal surface must be considered. For example, a high-strength, low-alloy (HSLA) steel may show an advantage in corrosion resistance of 12:1 over CS when freely exposed in a mild environment. As the severity or the physical conditions of exposure change, the HSLA steel will show less superiority, until in crevices or the backside of structural forms in progressively more corrosive atmospheres, it will be no better than CS.
Very little needs to be said about the behavior of stainless steels (Types 200 and 300), which contain high percentages of nickel and chromium, except that they can keep their shiny aspect without tarnishing for many decades. The steels containing only chromium (Type 400) as the principal alloying constituent tend to rust superficially, but the others are relatively free from surface atmospheric corrosion. However, many of these alloys are susceptible to stress corrosion cracking in many common environments.
This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 111-114.