Effect of medium pH on oil well pipes

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At very low pH values and dissolved oxygen levels, water corrosion is primarily controlled by hydrogen depolarization. At low pH levels in the presence of oxygen, hydrogen depolarization and oxygen depolarization reactions occur simultaneously on the surface of carbon steel; in this case, what takes place on the surface is essentially an acidic reaction process, and the corrosion pattern is characterized as uniform corrosion. Studies have confirmed that an increase in pH value leads to a reduction in the corrosion rate. It is believed that when there is an abundance of HCO3 ions in the solution and very little Fe2+, at a temperature of 70°C and with a low concentration of iron ions (1–2 mg/L), the addition of NaHCO3 solution results in the corrosion rate decreasing to almost half of its original value as the pH value rises from 4.1 to 6.2. Due to the continuous increase in pH, carbon steel undergoes passivation, and a FeCO3 protective film may form; however, the presence of this film can lead to underscale corrosion and localized corrosion, making the effect of pH even more complex at this point.