| Corrosion is increasingly becoming a major obstacle to the sustainable development of oilfields. Oil production fluid containing CO2, H2S, corrosive anions and scale cations as well as others cause steel corrosion under temperature and pressure during oil production. Serious corrosion of 13Cr steel having good corrosion resistance would be caused by the rising concentrations of H2S and CO2 with the development of oilfields. However, currently a few papers about corrosion mechanism and protection methods of 13Cr steel have been published. Therefore, it is imperative to comprehensively study the corrosion behavior of 13Cr steel used in oilfield and to put forward the relevant protection technology.In this work, the mechanism and main influence factors on 13Cr steel in CO2-saturated solution were systematically studied using weight loss method, electrochemical measurement techniques, SSRT and surface analytical technology. The inhibition effect and mechanism of the surfactants to control corrosion of 13Cr steel in CO2-saturated solution were discussed. The inhibition technique was expected to get on the basis of the discussion to provide theoretical method for industrial production. The main conclusions are as follows:1. Uniform corrosion of 13Cr steel and influencing factorsThe electrochemical control process of 13Cr steel corrosion in the CO2-saturated medium transformed from anodic passive to cathodic diffusion due to the presence of H2S. The free corrosion potential declined and the corrosion current density of 13Cr steel increased, the increase of the corrosion rate were also confirmed by weight loss method with the increase of H2S content.The electrochemical behavior of 13Cr steel in the CO2-saturated medium did not influenced by Cl-. With the increase of Cl" content, the corrosion rate of 13Cr steel changed in the shape of saddle and reached the maximum value at 20 g/L Cl-. The electrochemical mechanism of 13Cr steel in the H2S/CO2-saturated containing Cl-did not change but electrochemical reaction rate change apparently after adding Ca2+ and Mg2+The corrosion of 13Cr steel could be significantly promoted by temperature, in the range of 25℃-150℃, the corrosion rate of 13Cr steel in the H2S/CO2-saturated medium without Cl- changed in the shape of saddle and reached the peak value at 70℃. The corrosion rate of 13Cr steel in H2S/CO2-saturated medium containing 100 g/L Cl- increased linearly with the elevated temperature. And the composition of corrosion products on 13Cr steel was affected by temperature, mainly FeS at low temperature, while mainly Fe1-xS at high temperature.The effect of Cl" on corrosion rate of 13Cr steel depended primarily on environmental temperature. When temperature is not higher than 70℃, Cl- had little effect on the corrosion rate of 13Cr steel. However, when temperature is higher than 70℃, Cl-had significant effects on the corrosion rate of 13Cr steel, compared with Cl--free, the corrosion rate of 13Cr steel was promoted obviously, but the effect of temperature on the structure and composition of 13Cr steel corrosion film was not changed.2. Stress corrosion cracking of 13Cr steel and influence factorsThe susceptibility to SCC of 13Cr steel in CO2-saturated medium increased, the amount of plastic deformation and fracture time obviously shortened, but the mechanisms of ductile fracture did not change with the decrease of pH value.The impact of Cl" on SCC of 13Cr steel depended largely on pH value. There was little impact at higher pH value, but at lower pH value. The lower pH value shortened mainly the plastic deformation value and the fracture time of 13Cr steel SCC, but had little influence on fracture strength and fracture mechanism.The presence of H2S increased greatly the susceptibility to SCC of 13Cr steel and resulted in the change of fracture mode from ductile fracture in the range of plastic deformation into brittleness fracture in the range of elastic deformation. The fracture strength, strain value and fracture time of 13Cr steel declined sharply with the increase of H2S concentration.Although the mechanism of brittleness fracture in the range of elastic deformation could not be altered, the descent of susceptibility to SCC was obvious after adding 16-2-16 Gemini surfactant into the H2S-saturated medium. The fracture strength, fracture time and the amount of elastic deformation increased with the elevation of 16-2-16 Gemini surfactant dosage.The SCC mechanism of 13Cr steel did not change in the range of 10-200 g/L Cl", however, the mechanical performance parameters such as fracture strength, fracture time, elastic deformation and others had obviously been changed with the increase of Cl".Less than 150 g/L, Cl-played a inhibition role in SCC of 13Cr steel, the peak value of inhibition effect was at 50 g/L. When Cl- concentration was more than 150 g/L, SCC of 13Cr steel was accelerated.3. The inhibition technology of 13Cr steel in the H2S/CO2-saturated solution.The corrosion rate of 13Cr steel in the H2S/CO2-saturated medium exponentially reduced with the increase of surfactants dosage at 50℃and 150℃, after adding 12-s-12e and n-2-n Gemini surfactants, which indicated that the Gemini surfactants had good inhibition performance. The inhibition performance of Gemini surfactants was better at high temperature than that at low temperature.It showed the best corrosion inhibition performance that the n-2-n Gemini surfactants composed of the moderate lipophilic chain length (n) and the 12-s-12 Gemini surfactants composed of the appropriate number of carbon atoms in space group. The results showed that 12-6-12 and 16-2-16 Gemini surfactants had better inhibition performance in the studied medium and were superior to that of imidazoline surfactant.Both the Gemini surfactants and the imidazoline surfactant were cathodic corrosion inhibitors. They adsorbed on 13Cr steel surface according to Langmuir adsorption isotherm model. Their Gibbs free energy was less than zero, indicating that the adsorption process on the surface of 13Cr steel was spontaneous.
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