Study On The Corrosion Resistance Of 34CrMo4 Steel By Surface Nanocrystallization Treatment

As one of the Cr-Mo steel,34CrMo4 steel is widely used in the oil and gas engineering field due to its good comprehensive performance.However,with the deterioration of oil and gas exploitation environment,and the increasing of sulfur oil and gas fields,the application of workpiece（part）has been restricted by many factors,such as corrosion and wear,and so forth.Therefore,improving and enhancing the comprehensive performance of 34CrMo4 steel is an important step to advance the application of one.Shot peening（SP）is a simple,inexpensive and effective way to improve the surface hardness,corrosion resistance and fatigue resistance of materials.This paper,which used the hardness tester,X-ray stress instrument,electrochemical workstation,optical microscope（OM）,field emission scanning electron microscope（FESEM）,transmission electron microscope（TEM）and other equipment,was systematically studied the effect and law of the hardness,stress,and corrosion resistance of 34CrMo4 steel by controlling shot peening time.At the same time,as the original material was elected to the specimen of SP 20min,and test samples with different stress effective depth were prepared by stripping.And then,the corrosion resistance property and corrosion behavior of samples were evaluated and discussed in the blank solution.At last,the corrosion property difference and change theorem of between the SP 50μm depth and untreated samples were investigated in the wet sulfide corrosion solution with different pH value.It can been seen that the surface grains of specimens after SP treatment are reached to nanocrystalline,which led to that hardness obviously increased and residual compressive stress（RCS）is introduced,and the effective depth is about to 400μm.The corrosion property of the surface of SP specimens are clearly inferior to the untreated specimen that the reason why the surface roughness of SP specimens increased,and the cracks are produced.The corrosion property results of the different stress depth samples in the blank solution indicate that corrosive current density of 50μm depth specimen is minimum,and polarization resistance is maximum,which value I_corrorr and Rp are 1.9μA·cm^-2,1999.3Ω·cm²,respectively.Nevertheless,I_corrorr and Rp of the untreated specimen are 5.37μA·cm^-2,1048Ω·cm²,respectively.It can be seen that the corrosion resistance of 50μm depth specimen by SP is better than the untreated specimen.According to the corrosion results of the 50μm depth specimens in the wet sulfide corrosion solution with different pH value indicate that I_corrorr of specimens gradually decreased,and Rp values increase with the increasing of pH value.Then,when pH value is equal to 6,I_corrorr and Rp are 5.37μA·cm^-22 and 3949Ω·cm²,respectively.The specimen surface by SP has a bit of corrosion produce when pH is 2,and the base metal could be observed in the local position.At the same time,it can be seen that the enrichment of Cl^-is an important factor for the corrosive pitting of 34CrMo4 steel.On the other hand,after the corrosion produce be washed,we can see that surface have a abundant cracks that the reason why the surface adsorption effect of SP specimen was strengthened,and the grain boundary number obviously increase that is easy to accelerating element diffusion,which caused hydrogen atom combine to molecule that trigger hydrogen induced cracking（HIC）.As pH value increases,the surface of sample is covered with the dense product film,which can be effectively prevented further internal corrosion,and the results are that corrosion resistance of the material is significantly improved.