One of the biggest excavations on earth is in the former Mesabi iron district in northern Minnesota. We did it to feed our iron and steel industries to whom we owe our title as the world's greatest industrial nation. The ore mined was hematite—iron oxide, ferric iron, Fe2O3—the highest oxidized state of iron in equilibrium with our oxygen-enriched atmosphere. The iron-making process drives off oxygen (O2) and leaves pure iron (Fe)— 2(Fe2O3)+heat? 4Fe+3O2?—that combined with carbon (C) makes the steel used in our industry—mesh, structural reinforcing steel, prestressing strand, load transfer dowels, chairs, bolsters, forms, and connections of all sorts. But the iron in steel has a natural desire to return to its oxidized state, hematite. With a little help from water and oxygen its desire is fulfilled, and is accelerated when chloride—the greatest corrosion promulgator in the world—is there to assist.
The high pH of uncarbonated concrete (minimum 12.5) keeps steel from corroding when chloride contents are less than about 0.20% by portland cement mass. Even if it is, coatings such as epoxy can protect steel by directly isolating it from the concrete and chlorides, or indirectly by using corrosion inhibitors. Carbonated concrete provides no protection because of a pH drop to around 8 to 9.
In a lean oxygen environment, the corrosion product is magnetite (Fe3O4, also Fe2O3·FeO). With sufficient oxygen the corrosion product is hematite (Fe2O3). Whichever the situation, the corrosion products have greater volumes than iron, and when confined, create enough bursting pressures to rupture concrete. We recall a corrosion investigation made at the bottom of a deep water-cooling tank inset in the ground. The diver came up with a handful of black rust and it immediately began to release heat. The black stuff was magnetite, which was combining with oxygen to form hematite, an exothermic reaction so powerful the liberated heat was overbearing. The oxidizing magnetite was dumped to return to the depths of its existence in a lean oxygen environment. There are hand- and foot-warming products that contain iron shavings and chloride, activated when agitated with a little water to produce heat.
The association of concretes with corrosion goes beyond the electro-chemical oxidation of steel. For example, embedded aluminum (Al) conduit usually is tied to reinforcing steel and thus in direct electrical contact to the steel. Two dissimilar metals electrically connected—act like a battery—a galvanic cell activated in the presence of electrolytes, such as chloride from an accelerating concrete admixture. The aluminum is the sacrificial metal and corrodes to form hydrous aluminum oxides whose mineral names include gibbsite, bayerite, boehmite, all associated with the aluminum mineral ore bauxite and having the general formula Al2O3·XH2O.
We mentioned chlorides as the greatest promulgator of steel corrosion in the world, and that is whether or not the steel is embedded in concrete. Chlorine belongs to the halide elemental group, which also includes fluoride, bromide, iodide, and astatine. We have encountered steel corrosion in bromide-contaminated concrete, however that happened. Fluoride readily reacts with any loose calcium and immediately gets combined as calcium fluoride (mineral name fluorite), the stuff that protects teeth, and also is used in some concrete surface hardening treatments. Fluorite has a Moh's hardness of 4 by comparison to the much lower hardness of portland cement paste. Because it readily gets chemically incapacitated, it is not available to promote metal corrosion. So perhaps promulgators can be broadened to include the halide family, except fluoride.
We have taken you to the hills of the Mesabi, the furnaces of iron and steel making, to hand warmers, aluminum batteries, and the halide family of elements, all of which are part of the return of iron (and aluminum) to its oxidized state in our oxygen-rich environment. But remember, from whence they came they shall return.