Passive layer (reinforcing steel)

Due to chemical reactions on the surface of metal with substances from its environment, so-called passive layers are formed. The typically brown oxide layer (rust) on the surface of untreated reinforcing steel, results from the reaction of the metal with the oxygen (oxidation) from the air and/or water. The increasing thickness and compactness of the oxide layer prevents the oxygen from penetrating deeper into the steel, so that the corrosion process stops itself. High alkalinity, value ≥ 10, of the concrete keeps the oxide layer stable. If alkalinity decreases to a pH value ≤ 10, the corrosion process is restarted.