Study On Corrosion Mechanism Of Brine And Corrosion Inhibitors

Oilfield waste-water reinjection cause severe corrosion to metal equipment and pipeline, which bring tremendous losses to the oil field. As human awareness of environmental protection and sustainable development thinking in depth, add a low toxicity, efficient, environmentally friendly corrosion inhibitor effective measures to control metal corrosion.Serious situation for Baolang oilfield injection and water system metal corrosion that based on the analysis of The water quality, examines the influence of various corrosive media Ca2+、Mg2+、Cl-、SO42-、HC03-、SRB、pH、O2to carbon steel A3, and analysis the corrosion mechanism of the single factor. The fatty acid alkanolamide was synthesized by the tetradecanoic acid, palmitic acid and stearic acid as raw materials and diethanolamine mass. There are three kinds of fatty acid alkanol amide phosphate salt surfactant were preapared by the phosphorylation reaction and alcohol amines reaction. Examining the corrosion resistance of the product in the mineralized water and compared with other corrosion inhibitors to select optimal corrosion inhibitor (Phosphate sodium salt of palmitic acid alkanolamide). The corrosion inhibitor is added100mg/L to reach the oilfield anti-corrosion requirements and discuss the inhibition mechanism of the type of surfactant formulations inhibitor.1. The influence of inorganic ions in mineralized water on steel A3was studied.(1) When chloride ions’concentration was low in the water (below6100mg/L), corrosion rate of steel A3increased with the increase of chloride ions’concentration. When Cl-reached6100mg/L at40℃, the average corrosion rate increased from0.17mm/a to0.38mm/a, which increased122.6%. When the high concentration of Cl-is (6500mg/L-30500mg/L), corrosion rate increase with the increase of chloride ions’ concentration, the corresponding corrosion rate increased15.6%and4.9%at different40℃and70℃.(2) When the concentration of Ca2+and Mg2+is lower than1000mg/L, the average corrosion rate increased with the increase of the concentration, but the variation is small. But when the concentration of Ca2+and Mg2+is above1500mg/L, corrosion rate increase slowly.(3) When the concentration of HCO3-is730mg/L at40℃, corrosion rate reach the highest speed0.23mm/a, but decreased later. Similarly, When the concentration of HCO3-is100mg/L at70℃, corrosion rate reach the highest speed0.28mm/a, but corrosion rate of the steel A3decreased later with the increase of the concentration.(4) The corrosion rate increased slightly with the increase of the concentration of SO42-. When its concentration rising above500mg/L, corrosion rate is essentially the same.(5) At25～35℃,the corrosion rate with SRB was higher than without SRB. the corrosion rate with SRB increased rapidly with increasing the content of Fe2+. The accelerating role of SRB to the corrosion of carbon steel was not significantly affected at60℃.(6) Other factors, pH value in the range of6to13, the corrosion rate is relatively small changes, corrosive minimum; when pH value was less than7, the corrosion rate is larger with the decrease of the pH value; when pH value was3, the corrosion rate of reached0.6044mm/a. Dissolved oxygen corrosion of steel A3had a greater impact. As the dissolved oxygen concentration increases, the limit of oxygen diffusion current density will increase, the reaction rate of oxygen ions will accelerate, and thus the oxygen depolarization rate of corrosion increases.2. the fatty acid carbon number C14, C16, C18Alkanolamide phosphates (C14N, C16N, and C18N) critical micelle concentration (cmc) were50to30mg/L, corresponding to the surface tension of cmc (ycmc) were between27.31,24.57and27.65mN/m. C16N has a better corrosion performance, followed by the C14N, C18N worst. Adding100mg/L, the product of C16N to the simulation of salt water, the polarization inhibition rate reached71.7%.Under the temperature of50for4days,the average weight loss method of corrosion rate was0.0716mm/a,satisified the Water Injection corrosion rate requirements.3. Analyzing chemical corrosion behavior of the fatty acid alkanol amide phosphates. The alkanolamide phosphate end of the electronegativity of O, N and P atoms of the three polar groups-amide, alcohol hydroxyl and phosphate groups-composition), Chelate is composed by N atoms in the polar groups act as a ligand and Fe that adsorbed on the metal surface to form a monolayer of chelating membrane, thus changing the electrical double layer structure of the metal, improve the metal ionization activation energy of the process; The other side end of the non-polar groups by C, H atoms, nonpolar groups to form a layer of hydrophobic membrane on the metal surface which play a roal in Corrosion Inhibition). The C16N Contain the low surface tension,good water solubility and corrosion inhibition efficiency;C18N with the long number of carbon chain and the adsorption capacity is poor, namely, Increasing the concentration of the aqueous solution. Due to the bad Water-soluble, a large number of molecules into the micelle of the surfactant thus the Inhibition effect is not obvious; On the contrary, the short number of carbon chain(C14N) which have a better Water-soluble, short alkyl chain and the surface arrangement is loose lead to the low corrosion Inhibition.4. Electrochemical polarization curves show that the corrosion current density decreased and the corrosion rate increased when the fatty acid carbon number of phosphate salt of fatty acid alkanolamide increased from14to16corresponding to blank solution. However, when the fatty acid carbon numbers continue to increase to18.The corrosion current density of corrosion current density is smaller compared with the blank, nevertheless, the degree descent is little and the corrosion rate almost unchanged.When inhibitor concentration increased from50mg/L to100mg/L the polarization resistance largen(C18unchanged virtually) implying the reason for increase of corrosion rate is that the alkanolamide phosphate surfactant adsorbed increase on the metal surface leading to the adsorption film more thicker and reduced the interface between corrosive medium and A3metal significantly; For the hydrophobicity of C18is more stronger, the monolayers is thin, the Adsorption capacity is limited respectively result to the polarization resistance no further increase and the corrosion inhibition rate inhibition efficiency keeping the low when adsorbed to a certain extent. When added the alkanolamide phosphates which content different concentrations but the same carbon number to produced water, the amplitude of corrosion potential of the A3steel is unchanged virtually. Although the maximum value is just about60mV, the Tafel slope of the anodic polarization curves increased significantly.To sum the surfactant is the mixed type inhibitor which suppress the anodic process.