| In this article, using the experiments of static high temperature and high pressure corrosion weightlessness experiments, combining with microscopic analysis methods such as SEM, EDS, XPS, XRD etc,research the corrosion behavior of35CrMo steel after quenched and tempered which is raw materials of downhole drilling tools and35CrMo steel of surface treatment by electroless plating Ni-Fe-P under the conditions of CO2partial pressure1MPa,2MPa,3MPa,4MPa,temperature of30℃,60℃,80℃,100℃, which analog Daqing Oilfield oil and gas wells environment of CO2-rich. The results are as follows:(1)35CrMo steel CO2corrosion is a very serious corrosion in the analog the Daqing Oilfield downhole medium, temperature and pressure environment. Minimum corrosion rate is1.7933mm/a, the maximum value reaching7.0098mm/a, which is far greater than the the NACE standard in predetermined very serious corrosion rate;(2) The impact of the temperature on the corrosion rates is fairly obvious. The corrosion rate increases monotonically with temperature increasing in the low temperature region of30℃to-60℃,and increases monotonically with temperature decreasing in the high temperature zone of80℃to-100℃.The minimum value of corrosion rates at30℃is1.7933mm/a., the maximum of corrosion rate at60℃is7.0098mm/a.C02partial pressure has a small effect on static corrosion rate of the steel35CrMo:Overall, CO2partial pressure greater, the higher the rate of corrosion, but changes slightly; To same temperature, the corrosion rate reaches the maximum under4MPa; dense corrosion product film is not formed on the substrate under80℃,1MPa.Corrosion rate of35CrMo steel decreases first and then increase with CO2partial pressure increasing.The effect of time on the corrosion rate is fairly obvious. In the process of etching time from12h to96h, the corrosion rate first increases and then decreases,and the maximum is7.0098mm/a at72hours.(3) The temperature can affect the CO2corrosion rate of the35CrMo steel by changing corrosion mechanism,corrosion product film morphology,structure and thickness.The main component of corrosion product film is FeCO3and Cr(OH)3,which is lamellar, thin and loose. The composition and structure of the corrosion product film did not change much in addition to the significant increase in thickness and a large area off. When the temperature reaches80℃, corrosion product film of IMPa have no substantive changes. The corrosion products film of Crystal-like is obvious under2MPa,3MPa,and4MPa.The corrosion products of the surface conversion completly to the crystal. And the mainly composition of particles is FeCO3,becoming smaller and denser. CO2partial pressure can not change the corrosion mechanism, its impact on corrosion is mainly manifested in:when the grain-like corrosion product film have formed, corrosion rate can be changed by changing the product film density, thickness and grain size due to the CO2partial pressure changing. The corrosion time can also change the the35CrMo steel corrosion mechanism and corrosion product film morphology. structure and thickness, thereby affecting the rate of corrosion reaction. In corrosion reactions12h,35CrMo stee1corrosion product film is thin, loose.and the main component is Cr(OH)3. When the the corrosion reaction time reach48h, the corrosion product film is divided into two levels,Upper crystal,mainly composed of FeCO3,lower for a sheet-like product, the composition of Cr(OH)3.72h, the upper particle disappears, and with reaction time prolonging, the upper granular corrosion product film appears again, when the time reaches96h, particulate corrosion products covering almost the entire lower corrosion products.(4) Using electrochemical impedance technology studied the TWIP steel corrosion products film under the condition of different temperature, CO2partial pressure and corrosion reaction time, found that:As the temperature rises, CO2partial pressure increase and extension of corrosion time, the impedance value of product film is increasing. At80℃,the Nyquist plot of the product film have a capacitive arc at high frequency and low frequency region, and there is a Warburg impedance at intermediate frequency area. That is, the product film is very dense expect small amount of holes at80℃. At100℃, a capacitive loop appears in the low frequency region, and the Warburg impedance disappeared, showing the product film is very dense, almost no holes. As the CO2partial pressure increases, the Warburg impedance from scratch, and then gradually disappear, and the capacitive arc radius is increasing.As the CO2partial pressure increases,the porosity of the corrosion product film decreased, corrosion product film thickness and density increase. As the the corrosion reaction time extended, low-frequency capacitive becomes large, impedance value is increasing, With the extension of the corrosion time, the compactness of the corrosion product film gradually. That is to say:With the extension of the corrosion time, the compactness of the corrosion product film gradually increase, porosity decrease, the product film thickness increase and the corrosion rate will begin to decrease when the corrosion scale can play a corrosion protective effect.(5)35CrMo steel of Surface treatment by electroless plating Ni-Fe-P was characterized by SEM, EDS, XRD and EIS test methods.Results show that:the surface of electroless plating Ni-Fe-P treatment35CrMo steel can significantly improve its CO2corrosion resistance. Electrochemical test results show that:the Ni-Fe-P coating has a strong tendency to pas. |
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