Study On The Corrosion Behavior Of Austenitic Stainless Steel For Oil Drilling

The petroleum industry has been played an important role in promoting the upgrading of related industries and stimulating economic growth. Austenitic stainless steel material has been widely used in oil exploitation as a non-magnetic drill collar because of its good physical properties, mechanical properties and corrosion properties. It will make sense to study the corrosion properties for judging the corrosion sensitivity and optimize the material production process of austenitic stainless steel.Electrochemical Potentiokinetic Reactivation (EPR) is generally used for evaluation of intergranular corrosion due to its non-destructive, rapid and quantitative features. Electrochemical impedance spectroscopy (EIS) technique can help to obtain more information which can provide the theory basis detecting corrosion of austenitic stainless steel.In order to study the influences of solution treatment and sensitization treatment on the micro structure and corrosion behavior of austenitic stainless steel, the OM,SEM,EPR, EIS were used in the paper, and determine the appropriate solution treatment and the intergranular corrosion sensitive temperature of austenitic stainless steel. Besides,EIS technique and Oxalic acid etching method are applied to detect intergranular corrosion and equivalent circuit for intergranular corrosion is studied and corresponding parameters are decided accordingly.The relativity between equivalent circuit parameters and intergranular corrosion sensitivity is discussed.The results of the research showed that the microstructure of untreated specimens consist of a large amount of twin crystal and nitrides containing granular Cr and V in the structure. When the samples were treated at 950-1100?for 30min,the microstructures were austenite completely, and the grain size became more and more bigger and the grain became more and more evenly with the increase of the solution temperature. The reactivity R decreases and the radius of resistive arc in the etching solution increases with the increase of solution treatment temperature gradually, and the corrosion resistance will be the best when the solution temperature is 1100 ?.The oxalic acid etching method shows the corrosion degree increases with the increase of etching time in the 10% oxalic acid solution. The etched surface of the specimens showed severe grain boundary corrosion, corrosion channel is in the grain boundary and most of the grain is surrounded by corrosion ditch, which shows intergranular corrosion process have a relationship with etching time.The grain size of specimens are the smallest when the sensitization treatment is 800 ? for 6h by the same solution treatment, and the reactivation rate of EPR was the highest and its corrosion morphology was more serious than the other sensitization temperature, and the radius of capacitive arc is the smallest in the corrosion medium when the specimens were solution treated at 800 ? for 6h.The austenite grain size increases with the increase of sensitization time and the matrix is austenitic structure with twin crystal disappearing, then the precipitates began to increase at the grain boundary and the Cr2N of the pearlite tissue was precipitated when the specimens were solution treated at 800 ? for 12h, at the same time,the reactivity of the samples increased and intergranular corrosion susceptibility increased with the increase of the sensitization time.The research of the factors that influenced the pitting resistance of specimens shows that, the capacity of resist pitting increase with lower the immersion time or the concentration of NaCl, but decrease with the value of pH decreasing.Summarizing effects of solution treatment and sensitization parameters on the microstructure and corrosion properties of austenitic stainless steel, this paper suggests the suitable solution temperature was 1100?,and the corrosion sensitivity temperature is 800 ? .The corrosion resistance decreased with sensitization time increased, and the steel should not be used in the sensitized temperature range.