ABSTRACT

In this study, a high-phosphorus Ni-P coating was prepared by electroless deposition method. The characteristics and corrosion property of electroless Ni-P coating were investigated by surface analysis techniques and electrochemical measurements, and compared with L80 steel, corrosion resistant alloys such as 13Cr steel, 316L stainless steel, Inconel and 28Cr steel. The results showed that the high- phosphorus Ni-P coating with amorphous structure contained 88.3 wt.% Ni and 11.7 wt.% P, and improved the corrosion resistance of L80 steel substrate by more than 90 % in Cl-containing medium. The corrosion resistance of Ni-P coating was close to 13Cr steel but lower than Inconel, 316L stainless steel and 28Crsteel. Nevertheless Ni-P coating, similar to Inconel and 28Crsteel, had better resistance to pitting corrosion than 13Cr steel and 316L stainless steel in Cl-containing environment.

INTRODUCTION

Major corrosion forms of SAGD system

Carbon steel is the most common material used as slotted liner in steam-assisted gravity drainage (SAGD) system due to its low cost. However, carbon steel slotted liner often suffers from severe corrosion in complex corrosive environments containing CO2, H2S, chlorine compounds and solid particles.1,2 In this regard, the corrosion forms of the slotted liner in SAGD system mainly include sweet corrosion (CO2 corrosion), sour corrosion (H2S corrosion), CO2/H2S synergistic corrosion, sulfide stress cracking (SSC) and erosion corrosion. Therefore, the reasonable selection of corrosion protection measure is particularly important for well balancing the corrosion resistance and cost.

Selection of suitable mitigation methods

Among various corrosion protection measures, corrosion resistant alloys (CRAs) such as stainless steels and high-nickel alloys have been developed to mitigate the CO2/H2S corrosion.3 However, for the environment that contains H2S, the resistance to sulfide stress corrosion cracking (SSCC) should also be taken into consideration in addition to the corrosion weight loss. According to the practice recommended by ISO 15156 standard, the highest partial pressure of H2S applicable to super 13Cr stainless steel is 0.01 MPa, and that to duplex stainless steel is 0.1 MPa.4 Therefore, Ni-based alloy is the better option for the slotted liner used in SAGD system with relatively higher partial pressure of H2S, but this will inevitably bring about higher material costs,5 which limits their application in SAGD system containing CO2 and H2S gases.

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