Perhaps the greatest threat to the long-term durability of reinforced concrete is corrosion of the steel (primarily composed of iron, or Fe) which expands due to the greater volume of the corrosion byproducts and then cracks the concrete and debonds from the concrete matrix. Billions of dollars in damage occurs each year due to this simple reaction, and our apparent inability to stop it. So what causes this reaction?
1) All metals, except gold and platinum, are unstable and will corrode. Reinforcing steel is made primarily of iron, which is highly corrosive. There are several things that can lead to corrosion of steel in concrete, with the most common being chloride—such as the chloride in table salt (sodium chloride). These chlorides come from sea water, deicing salts, chlorides used in the mix as an accelerator, or salty aggregate or water. Another common culprit that can cause corrosion is carbonation caused by atmospheric CO2 penetrating the concrete.
2) Corrosion is an electrochemical reaction (a corrosion cell) with positive charges flowing through the moist concrete and negative charges (electrons) flowing through the reinforcing steel. The cathode is a spot where water and oxygen have gotten through the concrete to the steel—perhaps at a crack. The anode is where the steel corrodes and forms corrosion byproducts that can crack the concrete.
3) Good sound concrete has a pH in the pore solutions of 13.0 to 13.5, which is much more alkaline than something like Drano. In this environment, the steel forms a thin layer on its surface that we call a passivating layer. This layer protects the rebar and prevents the anode from forming so no corrosion can occur.
4) When chloride ions or carbonation penetrate the concrete and get down to the reinforcing steel, they lower the pH and attack the passivating layer. If there is also moisture and oxygen (at the cathode) then the steel begins to corrode. The corrosion byproducts have no tensile strength, reducing the strength of the reinforcing steel, and have greater volume than the original steel, causing cracks.
5) Chlorides and carbonation will penetrate the concrete—it’s only a question of time. But with good concrete cover (2 inches) and dense (low permeability) concrete, that could take centuries. And if the concrete is covered and dry, then it can’t corrode because the ionic migration of positive charges can’t take place.