The life of offshore steel structure in the oil production units is decided by the huge corrosive degradation due to , S2？, and Cl？, which normally present in the oil field seawater. Variation in pH and temperature further adds to the rate of degradation on steel. Corrosion behavior of mild steel is investigated through polarization, EIS, XRD, and optical and SEM microscopy. The effect of all 3 species is huge material degradation with FeSx and FeCl3 and their complex as corrosion products. EIS data match the model of Randle circuit with Warburg resistance. Addition of more corrosion species decreases impedance and increases capacitance values of the Randle circuit at the interface. The attack is found to be at the grain boundary as well as grain body with very prominent sulphide corrosion crack. 1. Introduction The severe corrosion of the submersed structures in the oil field at the production site and crude oil transportation is unpredictable and is a major component of the total corrosion loss in oil and gas industries. The corrosion species in the aqueous oil field seawater are , S2？, Cl？, , and O (Table 1) [1] which are also influenced by the variation of pH and temperature. and S2？ are formed from CO2 and H2S of the oil in the aqueous environment. And Cl？, , and O are present in the seawater. Besides these parameters, there are fluid dynamics of sea water and suspended solids and sands, influencing the erosion corrosion of the marine structures. Crude oil and natural gas can carry various high-impurity products which are inherently corrosive. In the case of oil and gas wells and pipelines, such highly corrosive media are carbon dioxide (CO2), hydrogen sulfide (H2S), and free water [2]. Table 1: Ions present in typical oil field seawater. The effect of any individual parameter on corrosion rate has been studied extensively [3–6]. But the conjoint effect of the above mentioned parameters and interfering effects and interactions are complex and are not very well understood. The salts and sulfide compounds dissolved in crude oil can provoke the formation of a corrosive aqueous solution whose chemical composition involves the presence of both hydrochloric acid (HCI) and hydrogen sulfide (H2S) [3, 4]. Corrosion mitigation in the oil field industry has traditionally been performed by combining methods for measuring the corrosion rates such as corrosion coupons and regular pipeline inspections with prevention strategies [6]. But that required years to get empirical results and could not be applied to other geographical locations of different sea water
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