Corrosion of High-Strength Carbon Steels in Siderite Supersaturated Water at Near Neutral pH

When carbon steel corrodes in anaerobic carbonated water, and the steel surface area to liquid volume is high, the concentration of ferrous and bicarbonate ions increases rapidly even though the corrosion rate is low. Such solutions with high bicarbonate concentration and a near neutral pH are believed to induce stress corrosion cracking of high-strength carbon steels. This work was conducted to investigate the solid precipitation in siderite supersaturated solutions. It was also an objective to measure the corrosion rate of high-strength carbon steel in solutions with high bicarbonate concentration at pH close to neutral. Preloading the solutions with ferrous ions and bicarbonate made it possible to measure desupersaturation and corrosion rate as function of time. The initial siderite supersaturation was more than 1,000 in the desupersaturation experiments. Despite this, the nucleation and growth of siderite was so slow that the solutions remained supersaturated for 100 h to 500 h at 10°C to 25°C. The ferrous ion concentration decreased from 1,200 mg/kg to 100 mg/kg in less than 24 h at 40°C, but did not reach equilibrium within 250 h. The precipitate was siderite at 5°C to 25°C. At 40°C, the solid was a mixture of siderite and chuckanovite in low salinity water and siderite with dissolved Ca2+ in artificial seawater. The corrosion rate of carbon steel at pH 6.7 to 7.2 at 25°C decreased to less than 0.01 mm/y in 5 h. Siderite precipitated and grew to a thin protective layer at the steel surface. Even though the solutions were highly supersaturated with respect to siderite throughout the experiments, the carbonate layer at the steel surface did not grow to more 5 μm thickness during 250 h to 400 h. The investigated steels were armor wires for flexible pipes. They have an oxide layer at the surface which is an inherent result of the manufacturing process. The presence of these oxides did not impede the formation of protective siderite layer.