Study on Effect of Buffer Capacity on Corrosion Performance of CRAs in Simulated Well Condition Available to Purchase

The methodology to simulate or realize an actual oil and gas field condition in laboratory tests is important to evaluate the actual corrosion performance of materials. The environmental factors such as partial pressure of H₂S and CO₂, temperature, concentration of chloride ion and pH of the solution are essential to define the severity of corrosion condition. To consider the initiation of localized corrosion such as pitting or cracking in Corrosion Resistant Alloys (CRAs), it is important to research the stability of the passive film such as depassivation pH of the materials. One of the most important environmental factors which may have an impact on the stability of the passive film is pH of the solution. The local pH in the vicinity of surface of metals might fluctuate due to the result of the hydrolysis reaction of H₂O combined with metallic cation which are released from the metal surface. It could be difficult to maintain a constant bulk pH value in a small volume vessel during the entire laboratory test duration (e.g. 720 hours), therefore, the application of buffer systems is widely adopted. However, the excessively stabilized buffer solution might give a different pH fluctuation behavior in the vicinity of the metal surface from the actual corrosion environments. In this research, an estimation of buffer capacity is conducted with some variations of buffering systems. At first, an SSC test is carried out on 13% Cr stainless steel with various buffer solutions at a specific pH level. Then, pH behavior was estimated by comparing a high pressure CO₂ condition with a normal pressure CO₂ condition with and without the buffer solution system. Finally, a suitable composition of the solution from the view point of buffer capacity is proposed by using thermodynamic calculations.