The Effect of Salt Concentration on Aqueous Strong Acid, Carbon Dioxide, and Hydrogen Sulfide Corrosion of Carbon Steel PDF Download

Author: Fazlollah Madani Sani
Publisher:
ISBN:
Category : Acids
Languages : en
Pages : 670

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Book Description
Aqueous brines are often produced during hydrocarbon recovery from geological reservoirs as an unwanted by-product. Degree of salinity is always an issue in produced water. In the USA, salt concentration in waters produced from conventional oil and gas wells falls in the range of 1 g/l (~ 0.1 wt.%) to 400 g/l (~ 28 wt.%). Besides salts, CO2 and H2S are ubiquitous in the production stream. Dissolution of these gases in produced waters results in evolution of corrosive species, such as CO2(aq), H2S(aq), H2CO3(aq), H+(aq), HCO-3(aq), and HS-(aq) that cause severe corrosion problems for carbon steel; primary material used in the construction of oil and gas pipelines. Combination of aqueous salts with dissolved CO2(aq) and H2S(aq) and their related species, has always been a great concern for pipeline operators in terms of corrosion problems. A large body of research exists on CO2 and H2S corrosion of oil and gas facilities, mostly at low salt concentrations; up to 3 wt.%. However, only a limited number of studies has investigated CO2 corrosion at high salt concentrations and to the best of this author’s knowledge, this number is zero for H2S corrosion. In the present study, the effect of salt (NaCl) concentration on aqueous uniform strong acid, CO2, and H2S corrosion of carbon steel is investigated. The key parameters in the corrosion process that are influenced by salt concentration are identified: transport phenomena (solution density, solution viscosity and diffusion coefficients of dissolved species), solution chemistry, and electrochemistry of the underlying reactions. Models have been reproduced and developed to account for the effect of salinity (up to ~ 5 m NaCl) on transport phenomena and solution chemistry. The Smolyakov and the square root (Kohlrausch law) equations were chosen for correcting the diffusion coefficients for the effect of temperature, and salt concentration, respectively, using new coefficients obtained in this study. The mixed solvent electrolyte (MSE) model, which is the only comprehensive solution chemistry model available in the literature for the H2O-NaCl-CO2-H2S systems is reproduced. The MSE model is used to calculate the equilibrium activity and activity coefficients of dissolved species important in aqueous strong acids, CO2, and H2S corrosion of carbon steel. pH measurements at 1 bar total pressure and temperatures below 80oC showed that the autogenous pH of aqueous CO2- and H2S-saturated solutions decreased with increasing NaCl concentration. The reason for the decrease in the solution pH was attributed to the increase in the activity coefficient of the H+ ion. The effect of salt concentration on electrochemistry and rate of strong acid, CO2, and H2S corrosion was studied by performing eight sets of experiments in aqueous N2-, CO2-, and H2S-satuared solutions at temperatures below 80oC, 1 bar total pressure, solution pH values between 3 to 5, and different NaCl concentrations, ranging from 0.1 wt.% (0.017 m) to 20 wt.% (4.27 m). The weight loss and linear polarization resistance corrosion rate measurements showed that the corrosion rate for all three types of corrosion generally decreased with increasing NaCl concentration. The analysis of potentiodynamic polarization sweeps indicated that increasing NaCl concentration decreased the rate of the cathodic H+ ion reduction reaction continually, while the rate of the cathodic water reduction reaction was mostly increased. The rate of anodic dissolution of iron in the active region increased at lower NaCl concentrations and then switched trend and decreased at higher NaCl concentrations. In strong acid corrosion, salt concentration seemed to have no effect on the mechanism of active dissolution of iron.