Corrosion Study of Nitrided X-52 Carbon Steel in C02 Environment

Carbon Dioxide (C02) corrosion is one the most studied fonn of corrosion in oil and
gas industry. Carbon is preferred material of construction for offshore pipelines due to
economy, availability and strength but this material are not corrosion resistant. As
such the use of carbon steel is usually implemented with the corrosion control such as
corrosion allowance, corrosion inhibitor and coating. Since corrosion involves
interaction between metal surface and the environment, surface alteration by surface
hardening method such as nitriding could protect the metal from corrosion. The
objective of the project is to study the performance of different nitrided carbon steel
based on 450°C, 480°C and 520°C in C02 environment. The corrosion rates of the
nitrided samples were investigated at pH 3 and pH 4 by linear polarization resistance
and electrochemical impedance spectroscopy. At ph 4, corrosion rate of sample
nitrided at 450°C and 480°C decreases by 30%. However the corrosion rate increases
by 50% for sample nitrided at 520°C. This may be due to the poor surface
preparation of the sample and uneven nitride layer fails to protect the sample. At pH
3, the corrosion rates of nitrided samples at 480°C and 520°C were higher than
untreated sample by 3% for sample nitrided at 480°C and by 70% for sample nitrided
at 520°C. For the sample nitrided at 450°C, the corrosion rate was similar as the
untreated sample. The slight decrease and increase in the corrosion rate at pH 3
indicate that the presence of nitriding layer was detrimental. This negative effect can
be linked to porous nitriding layer that lead to galvanic coupling between the nitriding
layer and bare metal. Further studies need to be conducted to confirm this possibility.