51316-7659-Effects of CO2 and H2S on Corrosion of Martensitic Steels in NaCl(aq) at Low Temperature

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Product Number: 51316-7659-SG

ISBN: 7659 2016 CP

Author: Serguei Lvov

Publication Date: 2016

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Corrosion studies were conducted for martensitic carbon steels in brine solutions at 4 °C and 10 MPa (1450 psi) which simulated the subsurface environments encountered in the Arctic. Three environments with a 5 wt% NaCl solution were used: (1) 0.33 mol of CO2 per mol of H2O in brine (2) 7.25 10-5 mol of H2S per mol of H2O in brine and (3) a mixture of 7.25 10-5 mol H2S and 0.33 mol CO2 per mol of H2O in brine. Two martensitic carbon steels were selected for the investigations: a high strength low alloy (HSLA) carbon steel commonly used for drill pipe (G41000) and a newly designed ultra-high strength low alloy (UHSLA) steel (G41300). In situ electrochemical measurements were performed using linear polarization resistance electrochemical impedance spectroscopy linear sweep voltammetry and cyclic voltammetry as well as redox potential (Eredox)-time measurements of the brine-based solutions. In addition mass loss coupons were exposed to the brines to serve as a comparison to the electrochemical results. Surface and cross-section analyses of the corroded samples were performed using scanning electron microscopy energy-dispersive X-ray spectroscopy and X-ray diffraction to identify corrosion products and investigate for signs of localized corrosion. Solution speciation corrosion rates and potential/pH dependences (Pourbaix diagrams) were calculated using commercial modeling software. Results of the in situ and ex situ measurements were compared against the solution speciation and corrosion rates from modeling for each environment as well as the predicted stable corrosion products in the Pourbaix diagrams. The corrosion mechanisms for HSLA and UHSLA steels in NaCl(aq)/H2S/CO2 at low temperature are proposed.