Passivity And Pitting Behavior Of Ultra High Strength Martensitic Stainless Steel

Ultra high strength martensitic stainless steels with excellent mechanical properties and moderate corrosion resistance are suitable for aircraft devices and ocean equipment applications such as steam generators,pressure vessels,cutting tools,offshore plate forms for oil extraction,etc.The corrosion behavior of ultra high strength martensitic stainless steel is becoming a hot topic.This Ph.D work has focused on the electrochemical behavior of the ultra high strength martensitic stainless steel with different microstructures,and the effects of pH,chloride ions and mold on the corrosion behavior,as well as the passive film stability and the pitting development.The microstructure of ultra high strength martensitic stainless steel consists of retained austenite and lath/plant martensite with M₂₃C₆ carbides distributed within the matrix and at the boundaries.The carbide grain size and the amount of retained austenite are affected by the tempering temperature.The carbides can induce pitting corrosion because larger carbide grains result in a lower pitting potential,thereby diminishing the steel's corrosion resistance.By contrast,retained austenite improves the corrosion resistance of ultra high strength martensitic stainless steel.The alloying elements Fe and Co are quickly dissolved and diffused out of the pits,while Cr and Mo are dissolved and then deposit in the pits.The ultra high strength martensitic stainless steel exhibits excellent passivation properties in 0.1 M Na₂S_O4 solution?pH 3?by passive films on the surface.The thickness of the passive film shows a linear relationship with the formation potential.The passive films formed at different potentials perform n-type and p-type semiconductor behavior.The donor density decreases quickly and then become stable with increasing formation potential.The passive films formed at higher potentials are more stable than those formed at lower potentials.The passive films are composed mainly of FeO,Fe₃O₄,Fe₂O₃,Cr₂O₃,Cr?OH?₃,NiO,and MoO₃.The passive films formed at higher potentials primarily comprise Fe oxides,whereas those formed at lower potentials contain primarily oxides and hydroxides of Cr.The ultra high strength martensitic stainless steel occurs pitting in the chloride solution,and the critical chloride ion concentration is about 1%.The pitting potential becomes lower while the chloride ion concentration increases.The Cl^-changes the passive film from the oxide and hydroxide of Fe,Cr,Ni,etc to the chloride of Fe²⁺,Cr³⁺,Ni²⁺.The matrix of ultra high strength martensitic stainless steel occurs pitting when the passive film is dissolved,and the pitting corrosion is aggravated as the increasing of the concentration of Cl^-.The effects of the concentration of Cl^-on the corrosion potential and passivation current density are slight.Some corrosion occurs on the surface of 300M steel,Aermet 100 steel and ultra high strength stainless steel after mold test,including Aspergillus flavus,Aspergillus niger,Chaetomium globosum,Penicillium funiculosum and Aspergillus versicolor.300M steel presents the severest corrosion with wide and shallow pits,Aermet100 steel is the next with narrow and deep pits,and the resistance to corrosion for ultra high strength stainless steel is the best.The mold presents individual and cumulate growth and the amount increases gradually on the surface of Aermet100 steel specimens,individual and dispersive on 300M steel,while it shows radial and netty growth and the amount of the mold increases greatly,and a microbial film is formed on the surface of ultra high strength stainless steel specimen.Mold could facilitate the corrosion process for 300M and Aermet100 steels and inhibit the corrosion process for ultra high strength stainless steel in some degree.When applied the active potential on the ultra high strength martensitic stainless steel,the oxidation reaction occurs on the surface and some oxide particles accumulation results that the sample surface roughness increases.However,when applied the passive potential,the passive film formed on the surface,and the surface roughness unchanges.Nano scale scratches on the surface of the ultra high strength martensitic stainless steel have no significant effect on the pitting corrosion.However,defect pits on the surface become larger with the extension of the time of constant potential polarization,and close pits expand,and eventually even are together to form a larger defect pits,then the samples are more likely to occur pitting.In the initial stage,the pits develop to the depth direction,and then to the breadth direction.