Modeling The Pitting Corrosion Lifetime Of HP-13Cr Stainless Steel In Extremely Aggressive Environment

In recent year,supermartensitic HP-13Cr stainless steel(HP-13Cr SS)was taken into consideration owing to its excellent mechanical properties and outstanding CO₂ corrosion resistance in ultra-high depth,ultra-high temperature and ultra-high pressure.However,the pitting corrosion always occurred in service,due to extreme aggressive corrosion environment caused by temperature/CO₂ pressure-flow rates-stress.Hence,the pitting lifetime at different fators were constructed based on the study of the nature of corrosion scales for HP-13Cr SS in extremely aggressive environment.Meanwhile,the influence weight of single and the synergy factors were determined by the factorial design method in extremely oilfield environment.Hence,in this paper the multi-degree pitting corrosion lifetime prediction model for HP-13Cr SS in extremely aggressive environments was constructed,with a view to providing necessary technical reference and theoretical direction for the oil and gas production in northwestern China.The pitting corrosion of HP-13Cr SS HP-13Cr in the extremely aggressive environment was investigated from the corrosion scales formed perspective,by means of microstructure characterization and high-temperature-high-pressure electrochemical measurements.The results of these studies indicated that the precipitation of Cr(OH)₃ is the dominating factor affecting on the formation of corrosion scales,and its effect can be categorized based on two compromising aspects.On the one hand,Cr(OH)₃ precipitation contributed to the increase of scales thickness.On the other hand,it inhibited the precipitation of Fe CO₃ due to the hydrolysis of Cr³⁺.Because of these reasons,the corrosion scales undergo significant microstructural changes,i.e.,from monolayer(95 ^oC/2.8 MPa)to bilayer(120 ^oC/3.2 MPa and 150 ^oC/3.6MPa),then to single layer(180 ^oC/3.8 MPa).Therefore,the corrosion-resistance performance of corrosion scales decreased with increasing temperature and CO₂ pressure,wherein the decreasing pitting potential and repassivation potential accompanied with the increasing density and diffusivity of acceptor in the scales.Also,the pitting susceptibility of HP-13 Cr SS increasing with raising the tempweature and CO₂ pressure.A new prediction mechanism model for pitting corrosion lifetime considering both pit initiation and pit growth was presented.The pit initiation was modeled based on a combination of the Sridhar model and Macdonald model,and the repassivation potential(E_rp)was defined as the critical potential.The pit initiation time can be divided into the time in which the open circuit potential(OCP)exceeds the E_rp,and the pit induction time,t_ind,when OCP>E_rp.The pit growth was modeled using the Markov process and extreme value statistics were used to describe the maximum pit depth distribution.If the time–consuming pit initiation process is neglected,it results in unacceptable errors in the pit growth kinetics parameters obtained.Thus,an acceleration method,the pre-initiated pits method,was developed to eliminate this negative effect.The proposed model was validated using experimental data on the pitting corrosion of HP-13Cr SS in vatious temperature and CO₂ pressure and reproduces the experimental observations with high fidelity at 180 ^oC/3.8 MPa after immersion 30 days.Menwhile,the universality of this mechanism pitting lifetime prediction model was verified used 304SS,and the pitting lifetime prediction model in 3.5 wt.%Na Cl solution at 30 ^oC was constructed.The prediction results are in good agreement with the long-term immersion maximum pitting depth results,which further proved the reliability of this model.A multi-degree freedom prediction model for pitting corrosion lifetime of HP-13Cr stainless steel under the oilfield environment coupling with high temperature-high CO₂ pressure,various flow rates and complex stress distribution was presented in this work.The pitting lifetime prediction parameters were determined by means of combining pitting lifetime mechanism models of single factor and weight coefficients.Meanwhile,the pitting lifetime prediction models were modified under flow rate and stress with the idea of high thoughput,based on the constructed temperature and CO₂ prediction mechanism models.The results indicated that flow rates at the range of 0.3 m/s?1 m/s altered the flow state within the pits,resulted in inhabition of pitting growth.Meanwhile,the promoted effect of tensile stress on the pitting growth is attributed to the local deformation induced around a pit,but,the effect of compressive stress on the pitting growth was not significant.In this experiment,the self-designed stress corrosion rotating cage was used to realize the corrosion test under the coupling of temperature/pressure-flow rate-stress,and the weight coefficients of single factors and their interaction effects were determined by factorial factorial design method.The results showed that the influence weight of temperature/CO₂ pressure,stress and the couping between temperature/pressure and stress to the maximum pit depth were52.54%,28.20%,and 19.26%.However,the effect of flow rates in the range 0.3 m/s?0.7m/s on the maximum pit depth was not significant.According to the influence weight and the sub-model,the the pitting growth kinetic parameters were calculated in extremely aggressive environment.Then,the prediction results were compared with the six-year-served field data,which indicates that the mechanistic-chemometrics model has high validity and high accuracy.