Localized CO₂ Corrosion in Horizontal Wet Gas Flow

This study investigates localized CO₂ corrosion on carbon steels in wet gas services both experimentally and theoretically. A 100 mm I.D., 40 meter long flow loop is employed to perform the corrosion studies along the top and the bottom of the pipe under stratified and annular flow conditions. Various corrosion monitoring techniques, including ER, LPR, and WL, and surface analysis techniques, including SEM/EDS, MM, XRD, and XPS are used during the experiments and for post-test analysis.

The parametric study involves the systematic investigation for the effect of temperature, CO₂ partial pressure, flow (including gas and liquid phase flow rates and flow regimes), pH, Cl^-, and oil on localized corrosion and formation of corrosion product films. When there is a very protective film or the surface is film free, localized corrosion does not occur; it only occurs when a partially protective film is formed.

In the present study, localized corrosion is found only at high temperature (90°C). It occurs in both Cl^- containing and Cl^-; free solutions (with different pitting density). It also occurs at lower pH (4.5∼6.0) while at pH 6.2 very protective films form and no localized corrosion is identified. CO₂ partial pressure affects film formation and thus the localized corrosion when a partially protective film is formed. Higher pressure, higher liquid and gas flow rate cause higher corrosion rate under film free conditions and also lead to annular flow, where the corrosion behavior on the top of the pipe is similar to the bottom. The presence of a hydrocarbon phase exhibits a favorable effect on preventing localized corrosion allowing acceptably high gas flow rates even in non-inhibited environments.

The theoretical study presents here includes the development of a solution super saturation model and a scaling tendency model, which are good tools for predicting localized corrosion. It is found that localized corrosion occurs when the solution is only slightly above the saturation point and when the scaling tendency is between 0.3 and 3.0.