The Effect of Microalloying, principally with Vanadium, Processing Conditions and Microstructure on the Resistance to CO2 Corrosion

摘要

One driver for steel development over recent decades has been the engineering requirements of improved strength and toughness, combined with weldability, for the safe and cost effective recovery and transmission of oil and gas. This has been achieved through refinement of grain size by microalloy precipitation and thermomechanical processing. However, little attention has been paid to these significant changes in steel chemistry and processing on corrosion resistance, despite the dominance of economic carbon steel for construction in the oil and gas industries.The more common forms of corrosion are associated with the presence of H2S or CO2.CO2 in aqueous solution forms a weak acid sufficient to promote significant localized corrosion in transmission gas/oil pipelines and in well-head applications (‘down-hole’). Systematic study has identified the influence of a wide range of alloying elements and different processing conditions on the resistance of low-carbon steels to CO2 corrosion; strong carbide-forming microalloying elements such as Ti, Nb and V, along with Cr additions, and different levels of Mn,Si,Cu,Mo and Ni,have been explored, along with treatments simulating different processing conditions. The present study also emphasizes the role of V and Ti microalloying in improving the resistance of Cr-containing carbon steel to corrosion in carbonic acid and how this is influenced by microstructure and the metallurgical condition of the microalloying addition, in particular,the extent of precipitation. It is noted that some commercially available corrosion inhibitors contain V as a vanadate compound to interfere with the corrosion process and so it is suggested that V microalloying may also be beneficial if present in an appropriate form in the steel. That Ti also seems to play a role in corrosion in the steels studied is judged to be compatible with the thermodynamics of transition metal anions in the Ti-V-Cr group.