Abstract. Corrosion is a critical issue during the development of a gas field, especially wet gas or retrograde gas field. Corrosion affects the management system of a field and further impacts the amount of investment. Therefore, accurate prediction of corrosion rate is needed to plan an effective preventive action before going further to the development phase. One of the important parameters that should be noticed to create an accurate prediction is the formation of the passive layer. In CO 2 -H 2 S environment, there will be three possibilities of passive layer: FeS, FeCO 3 or no passive layer. In this study, we create mathematical models to determine the formed passive layer in each segment of the gas production tubing and pipeline. The model is built using Faraday's Law and Thermodynamic approach to account the passive layer formation at different temperature, pH, corrosion rate and partial pressure of CO 2 and H 2 S. From the simulation, it was found that there were three boundary conditions: no scale-FeS boundary, no scale-FeCO 3 boundary and FeS-FeCO 3 boundary. The first two boundaries evolved over a time as the concentration of Fe 2+ ions was increasing. However, FeS-FeCO 3 boundary remained steady as it was not affected by the addition of Fe 2+ ions. Using sample case study, few variations were noticed at production pipeline and tubing. It was caused by the gas composition, which contained high CO 2 and very low H 2 S. Boundary conditions only changed slightly over two days period. IntroductionCorrosion is defined as the destructive result of chemical reaction between a metal or metal alloy and its environment [18]. It occurs when a metal or metal alloy meets corrosive agents in aqueous solution at certain pressure and temperature. In gas field, corrosion is usually caused by CO 2 gas (sweet corrosion) and H 2 S gas (sour corrosion). The existence of corrosion in production system could affect the amount of production and lead to safety issues. Therefore, early prevention of corrosion is needed before bringing a gas field into production.In order to plan an effective corrosion management, one should first understand the fluid characteristic and environment condition (ambient temperature, location, etc) of a gas field. Moreover, an accurate prediction of future corrosion phenomenon is also critical. It can be assessed by creating a model that describes the corrosion phenomenon of a field accurately. and well-known companies to estimate the future corrosion rate in a production system. From all those models, a critical parameter that influences the assumptions inside the models and determines the success of the prediction is the type of passive layer.

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