Effect of Temperature, Bicarbonate, and MEG Concentrations on CO₂ Corrosion of Carbon Steels

The CO₂ corrosion of two mild steels has been studied as function of monoethylene glycol (MEG) and bicarbonate concentration at conditions where FeCO₃ precipitation was sparse. The steel coupons were anodically polarized to obtain a surface with protruding carbide typical for corroding carbon steel. The corrosion rate clearly decreased with increasing MEG concentration and somewhat with bicarbonate concentrations. The corrosion potential decreased with increasing alkalinity, but was little affected by the MEG concentration. The cathodic reaction was under mixed control when the alkalinity was less than 10 mmol/kg, but became more activation controlled at higher alkalinity. At low alkalinity and low pH, the anodic Tafel slope was determined to be 40 mV/decade to 50 mV/decade. In solutions with 50 wt% MEG at higher alkalinity, the Tafel slope was 60 mV/decade to 70 mV/decade.