The Fast-Growth Mechanism of Magnetite on Carbon Steel in Oxidizing High-Temperature Aqueous Solutions

The corrosion of carbon steel in brines containing reducible cations (Fe³⁺, Ni²⁺, Cu²⁺) and/or oxygen and HCl at temperatures ranging from 200 to 270°C and for exposure times up to 400 hours is reported. The reducible cations and oxygen are found to cause autocatalytic corrosion, in which the corrosion rate increases with time before the rate eventually becomes constant. Specific cation effects are observed in accelerating the corrosion rate, which cannot be attributed to hydrolysis and the formation of acid. It is also found that the corrosion rate, which is not strongly dependent on oxygen concentration, increases by nearly an order of magnitude with each decrease in one unit of pH, particularly in acid solution, and that a critical concentration of reducible cation exists for the onset of autocatalytic attack. The extremely high corrosion rates observed during autocatalysis and in the linear regime (up to 0.5 m/year) are attributed to the gradual formation of an aggressive concentrated chloride solution within the porous magnetite film on the metal surface. The high corrosion rates also reflect the reduction of Fe³⁺, Ni²⁺, and Cu²⁺, which act as cathodic depolarizers, and possibly from the deposition of metallic Cu and Ni at the magnetite/solution interface to yield catalytic surfaces for hydrogen evolution.