51316-7657-Corrosion Behavior of Ultra-high Strength Drilling Steel in Alkaline Brines Containing Hydrogen Sulfide at High Temperature

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Product Number: 51316-7657-SG

ISBN: 7657 2016 CP

Author: Serguei Lvov

Publication Date: 2016

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In-situ electrochemical measurements were used to study the onset of sour corrosion in ultra-high strength low alloy carbon steel (UHSLA) in alkaline brine solutions at 200 °C. The solutions were buffered by NaHCO3 / Na2CO3 / NaOH to three pH values which were calculated to be 8.1 9.8 and 10.8 at 200 °C using commercial modeling software. The partial pressure of H2S was equivalent to 10 psia (69 kPa) at 85 °C. Electrochemical methods included linear polarization resistance electrochemical frequency modulation electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry. After each corrosion test the corroded surface was analyzed by scanning electron microscopy (SEM) energy-dispersive X-ray spectroscopy and X-ray diffraction (XRD). The polarization resistances (Rpol) after 60 hours were much smaller at 200 °C than those at 85 °C and therefore the corrosion rates (CR) were one to two orders of magnitude higher at elevated temperature. At 200 °C Rpol first decreased and then increased with pH increase. Inversely CR increased from 0.85 mm/y to 2.84 mm/y when pH increased from 8.1 to 9.8 and then decreased to 1.82 mm/y at pH equal 10.8. CRs were also calculated using modeling software for comparison with experimental results. The CR from modeling was in a good agreement with experimental CR at pH 10.8 but one order of magnitude off at pH 8.1 and pH 9.8. EIS plots at pH l 9.8 did not change significantly at different stir rates. This indicated that mass transport in the solution did not influence the rate-determining step. The cathodic Tafel slope indicated that Tafel-Heyrovsky mechanism was a possible mechanism for the cathodic reactions. The SEM images showed that the corrosion product layers covered partially the surfaces at pH 8.1 and pH 9.8. The XRD detected the presence of the siderite (FeCO3)-like phase and pyrrhotite (FeS)-like phase in the corrosion products formed at pH 8.1 and maghemite (?-Fe2O3) at pH l 9.8. This showed a trend of surface products transitioning from iron sulfide to iron oxide which was consistent with the predictions from the Pourbaix diagram for Fe-H2O-H2S-CO2 at 200 °C.