Study On Waterjet Cavitation Erosion Of 304 Stainless Steel After Corrosion In NaCl Solution

Cavitation erosion has been frequently encountered in fluid machinery.Such a common phenomenon leads to a decrease in the operational efficiency of fluid machinery,and in severe cases,it results in the damage of fluid-wetted components.For the fluid machinery applied in marine and ship engineering,the combined effect of corrosion and cavitation erosion gives rise to complex damage patterns over the hydraulic components.Heretofore,the knowledge of both cavitation and cavitation erosion is far from sufficient.The studies on the combined effect of corrosion and cavitation erosion have rarely been reported.Plumbing the mechanism of the effect of corrosion on the cavitation erosion of metals and analyzing the influence of operating conditions on cavitation erosion enable a supplement of the theory of cavitation erosion,and will lend support to the design of the fluid machinery with high resistance to cavitation erosion.In this dissertation,experimental methods were used to study the cavitation erosion of 304 stainless steel specimens after they were corroded.The NaCl solution served as the corrosion solution.Cavitation erosion was produced through submerged waterjet.The effect of corrosion on cavitation erosion was systematically analyzed through the mass loss,microstructure,surface morphology,cross-sectional metallographic structure and microhardness of the specimens after cavitation erosion.Main works and conclusions of this dissertation are as follows:(1)The 304 stainless steel specimens were separately corroded in NaCl solutions of different concentrations.The corroded specimens were tested in a waterjet cavitation erosion test bench conforming to the ASTM G134 standard.Pure water was used as the liquid medium for nurturing cavitation.The surface of the cavitation-eroded specimens was observed and analyzed,and the effect of the concentration of the NaCl solution on cavitation erosion was explained.The results show that after the electrochemical testing,the corrosion tendency of 304 stainless steel attenuates firstly and is then strengthened with increasing concentration of the NaCl solution.During the waterjet cavitation erosion experiment,it is demonstrated that a short period of corrosion tends to delay the cavitation erosion,but the delay effect is first weakened and then reinforced as the concentration of the NaCl solution increases.As the concentration of the NaCl solution increases,the cumulative mass loss,the extent of plastic deformation and surface roughness of 304 stainless steel specimens decrease first and then increase at the same corrosion and cavitation time.Compared to the eroded specimen without being corroded,the corroded specimen exhibits less damage and small surface roughness after being eroded by waterjet cavitation.Compared with the original specimen,the eroded specimen is featured by preferred orientation of grains,small average grain size and grain refinement.Slipping is made more difficult by the continuous action of jet stress,and the orientation of grains is changed to an increasingly difficult direction to deform,and the crystal grains are rotated towards an increasingly stable state.(2)The 304 stainless steel was corroded in 3.5% NaCl solution for 24 h,and then the corroded specimen was tested using the waterjet cavitation erosion test bench.The waterjet cavitation erosion time was varied and the specimen surface was observed and analyzed.The influence of waterjet cavitation erosion time on the development of cavitation erosion was described.The results show that pits are formed near the black inclusions on the surface of 304 stainless steel specimen after it has been corroded in 3.5% NaCl solution for 24 hours.As the waterjet cavitation erosion is prolonged,the cumulative mass loss and cumulative mass loss rate of the specimen increase constantly,the area of the annular cavitation erosion area gradually increases,and the area of the central zone gradually reduces.At a waterjet cavitation erosion time of 60 min,apparent peeling-off of material is evidenced on the specimen surface and the non-uniform distribution of surface height is intensified,and the surface roughness is rather high.During this process,deformation twins gradually appear on the cross-sectional metallographic structure,and the deformation twin layer is gradually thickened,the upper edge of the cross section becomes rough,and eventually a sawtooth shape is engendered.Meanwhile,both the microhardness and the thickness of the hardened layer increase gradually with the extension of waterjet cavitation erosion.The cavitation erosion time of 45 min is a characteristic instant,which serves as a demarcation between cavitation strengthening and cavitation damage.(3)The 304 stainless steel specimens were separately corroded in 3.5% NaCl solution for different time and then tested using the waterjet cavitation erosion test bench.The surface observation and analysis of the obtained results were performed to explain the effect of corrosion time on the cavitation behavior.The results show that corrosion has a delaying effect on cavitation erosion,but the delaying effect gradually weakens with the increases of the corrosion time.After cavitation erosion for 45 min,the cumulative mass loss,the extent of cavitation damage and surface roughness value increase.Meanwhile,the cross-sectional cracks are gradually deepened but the thickness of the twin layer gradually decreases with the increase of corrosion time.Compared to the eroded specimen without being corroded,the corroded specimen exhibits slighter cavitation damage and low surface roughness after being eroded by the waterjet cavitation.