L51793 The Effects of Water Chemistry on Internal Corrosion of Steel Pipelines

Abstract

The ability to safely transport wet, untreated natural gases through pipelines offshore or at other inaccessible locations is an important factor in the development of new gas fields. The internal corrosion rate of steel pipelines varies in a complex way with the gas composition, specifically CO2 , O2 , and H2 S, and condensed water chemistry. Estimating the corrosion rate of steel at inaccessible locations from the analysis of the gas and water composition from an accessible location will enable a better determination of the need for corrosion inhibitors. Quantitative understanding of the corrosion rate of steel under these conditions will be key to an accurate risk assessment of pipelines from internal corrosion. Previous work examined the effects of gas composition and slow liquid flow conditions on corrosion of steel. A parametric equation was developed that essentially reflected the deleterious effects of CO2 and O2 , and the beneficial effect of H2S on corrosion (Lyle, 1997). While this study provided important information regarding the deleterious effect of oxygen in the gas phase, the effects of condensed water composition, especially scale-forming species such as calcium and magnesium, were not examined. Furthermore, a better thermodynamic and kinetic understanding of the effects of the gas and liquid phase composition was needed. The present project, while an extension of the previous project, breaks important new ground: (i) the water chemistry typically found in pipelines is included in the tests; (ii) the scale and corrosion product formation is examined using a thermodynamic speciation software; (iii) surface analysis of the corroded samples is performed using laser Raman spectroscopy in order to confirm the thermodynamic model prediction; and (iv) electrochemical tests are conducted to understand the corrosion kinetics. A discussion of the general literature on internal corrosion is presented in the next section, followed by a summary of results from previous SwRI projects in this area. The thermodynamic approach and results are presented in Chapter 2. The experimental results are presented in Chapters 3 and 4, respectively, along with a discussion of the results in terms of thermodynamic and kinetic framework. Chapter 5 summarizes the results of this project and provides recommendations for further investigations. Details of the thermodynamic calculations, electrochemical experimental results, and analyses of the corrosion products are presented in the appendices.