| Recently,abnormal weather caused by global warming has frequently occurred,and gas discharged from iron and steel smelting industry is one of the pollution sources.Therefore,it is important to prolong the service life and reduce the use of steels for alleviating global warming.The corrosion of high-strength stainless steel is one of the factors that shorten its service life.In this study,AM355 high strength stainless steel is taken as the object.To provide the theoretical basis and data support for prolonging the service life of high-strength stainless steel,the passivation,pitting and intergranular corrosion mechanism of martensitic stainless steel are investigted.The main conclusions are as follows:In the study of the influence of environmental pH values on the electrochemical behavior of AM355 high strength stainless steel in different,the following conclusions are obtained.Pitting corrosion takes places on AM355 in a solution with pH = 1.5-11.A secondary passivation zone could be detected in a solution with pH = 13,and AM355 has a tendency to intergranular corrosion in this solution.Both of the passivation film and the secondary passivation film formed on the high-strength stainless steel AM355 exhibit the characteristics of an n-type semiconductor,and the secondary passivation film has a larger concentration of point defects than the passivation film.The liquid-phase in-situ atomic force microscopy is used to study the pitting corrosion mechanism of AM355 stainless steel.It is found that the MnS-oxide inclusions have the highest pitting sensitivity than other two inclusions(MnS inclusions and oxides)in AM355.The high pitting sensitivity of MnS-oxide inclusions is due to the synergy effect between MnS and oxide,that is MnS preferentially dissolves as immersed in corrosive conditions indicating the occurrence of metastable pitting,and then the presence of Cr-depleted regions adjacent to the oxide inclusions makes transition from metastable pitting to stable pitting corrosion easier.Electrochemical atomic force microscopy is utilized to study the nanoscale structure and growth process of the passive film formed on AM355.The passive film formed on AM355 stainless steel is composed of nanoscale oxide particles,and the diameter of the particles generated though polarizing at 0.1 VSCE in 0.1 M NaOH solution for 60 min is approximately 13.14 ± 3.52 nm.The growth process of passivation film can be summaried as nucleation of oxide particles,three-dimensional growth,and mutual extrusion at the boundary.This formation process of passive film is corresponding to variation of surface roughness.By comparing the chemical composition and nanostructure of passive film and secondary passive film of AM355,the following conclusions are obtained.Both of passive film and secondary passive film are consisted of iron and chromium oxide and hydroxide.The passive film is a Cr-rich oxide film which has a Fe/Cr ratio much larger than that in the metal matrix,while the secondary passive film is a Cr-depletion film.The passive film and the secondary passive are composed of nanoscale particles,and only in the secondary passive film can the nanoscale holes be observed.Electrochemical impedance analysis reveal that the boundary region between the particles in the passive film and the nanopores in the secondary passive film can be used as the diffusion path of ions to the metal matrix.The intergranular corrosion susceptibility of AM355 stainless steels with different ageing temperatures is obtained through DL-EPR testing and immersion experiment.When the aging temperature is in the range of 450 ? to 550 ?,the intergranular corrosion susceptibility of AM355 stainless steel increase with the aging temperature.And then the susceptibility decreases steeply as the aging temperature reaching to 600 ?.In the next temperature range(from 600 ? to 700 ?),the suscepitibility decrease with aging temperature continuously.After analyzing the varation of the microstructure of AM355 treated at different aging temperature,it could be induced that the turning point of the intergranular corrosion susceptibility at 550 to 600 ? is due to the precipitation of the reversed austenite.And the effect of the reversed austenite on the intergranular corrosion is summaried as:on one hand the reverse transformation of austenite can reduce the content of the actual Cr-depleted region at the grain boundary;on the other hand,the reversed austenite formed at the grain boundaries could prevent the expansion of intergranular corrosion along the grain boundaries due to the natural stability of revered austenite. |
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