Corrosion in the context of cement and concrete typically refers to the degradation of metal reinforcements such as steel bars used within concrete structures. The process is electrochemical in nature, involving the transfer of electrons from high-energy areas of metal (anodes) to low-energy areas (cathodes) in the presence of an electrolyte, which in concrete is the pore water that contains dissolved ions. Corrosion is initiated when the protective passivation layer on steel, formed by the high alkalinity of concrete, is broken down, often by carbonation (reduction in pH) or by the presence of chloride ions, leading to rust and the eventual weakening of the steel and concrete.