| In the process of oil and gas drilling,with the wide application of brine drilling fluid and the increase in the degree of salinity in the formation,the concentration of Cl-ions in the environment of oil and gas fields has continuously increased,and more and more chlorine ion corrosion problems have appeared in the downhole drilling materials.Therefore,it is necessary to study the corrosion behavior and protective measures of drilling tool materials in high temperature,high pressure and high concentration Cl-ions environment,and to lay a theoretical foundation for the study of corrosion rate prediction and protective coating for materials under similar working conditions.In this paper,the high temperature and high pressure reactor was used to simulate the high temperature and high pressure environment of oil and gas fields.The corrosion rate and electrochemical characteristics of 42CrMo in high temperature,high pressure and high concentration Cl-ions solution were studied by immersion test and electrochemical test technology,combined with surface analysis technique?SEM,XRD?on the morphology,composition and structural characteristics of the surface corrosion products of 42CrMo steel and its double chromium plating under different temperatures and different concentrations of Cl-ions,finally,the corrosion effects of high concentration of Cl-ions on 42CrMo steel and its double chrome plating under high temperature and high pressure were discussed,moreover,and the protective performance of double chromium plating under high temperature and high pressure is evaluated.Related research results are as follows:?1?The analysis of 42CrMo after immersion for 72h under different temperatures and different Cl-ions concentrations showed that when Cl-ions concentration is 100g/L and the temperature is 180?,the average corrosion rate of the sample is the highest;SEM and XRD studies showed that the surface of 42CrMo had different degrees of localized pitting corrosion and the surface layer deposits was CaCO3 corrosion scale and were mixed with a small amount of Fe3O4 corrosion products.?2?The electrochemical analysis of 42CrMo in different Cl-ions concentration of solutions at 120?showed that with increasing Cl-ions concentration,the polarization resistance obtained by the impedance spectrum equivalent circuit showed a trend of decreasing first and then increasing,and when the Cl-ions concentration was 100g/L,the value was the smallest,and the corrosion current density showed a parabolic trend of increasing first and then decreasing,and the maximum value was obtained when the Cl-ions concentration was 100g/L,which indicated that the aggressive Cl-ions has a dual role in solution.?3?The analysis of single hard chrome plating and double chrome plating for different immersion time with a temperature of 180°C and a Cl-ions concentration of 100g/L?the most severe corrosion condition of 42CrMo in this test?shows that when the immersion time increased to 168h,the single hard chrome plating's protective performance is greatly weakened,and its substrate has undergone severe localized corrosion,but the double chrome plating still has a good protective effect on the 42CrMo substrate.This is mainly due to the lower porosity and higher compactness of the bottom layer of milky white chromium layer in the double chrome plating,which further blocks the diffusion of the external corrosion solution through the micro-cracks in the surface hard chrome layer to the plating/substrate interface,so as to better protect the 42CrMo substrate from corrosion.?4?The electrochemical analysis of the double chrome plating at room temperature and atmospheric pressure with a Cl-ions concentration of 100g/L showed that the double chrome plating has positive open circuit potential,lower self-corrosion current density and higher polarization resistance relative to single hard chrome plating.It shows that the double chrome plating has a high corrosion resistance in a high concentration of Cl-solution. |
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