In multiphase gas pipelines constructed from carbon steel, there is a threat to the system integrity due to top-of-line corrosion. In wet gas pipelines, produced water has a tendency to condense at the top-of-the pipeline as the temperature and pressure decreases, causing corrosion at the 12 o''clock position. This form of corrosion can be very aggressive because the pH of the water may be very low and the water might also not contain any corrosion inhibitor. Preventing top-of-line corrosion in multiphase systems with inhibitors is very challenging. Inhibitors must be transported with the gas phase, then partitioned into the condensed water when necessary and form a protective barrier on the metal surface. A further challenge is the evaluation of inhibitor efficacy in preventing top-of-line corrosion. Traditional laboratory tests (kettle tests, RCE, flow loop, etc.) are not suitable for this purpose as they are only used to determine the corrosion rate of the bulk water phase. Therefore, alternative top-of-line corrosion test methods are needed. This paper details the work performed to develop a top-of-line corrosion test method suitable for assessing inhibitor performance. Using the novel test method, several products have been developed that are effective in preventing top-of-line corrosion and their test results are provided. Furthermore, a hypothesis is proposed to explain the top-of-line corrosion inhibition mechanism.
In multiphase gas pipelines, corrosion inhibitors are widely used to mitigate top-of-line corrosion. The inhibitors can be applied by several techniques including batch treatment using a traditional or spray pig and continuous injection 5. In addition, neutralizers can be applied to the pipelines to raise the system pH between 6.5 and 7.5 6. Several authors have described laboratory test methods used for evaluating top-of-line corrosion inhibitors. These include corrosion flow loops, modified RCE and modified corrosion glass cells.