Corrosion Behavior And Mechanism Of Ni-Mo Low Alloy Steels In Tropical Marine Atmosphere

The addition of alloying elements Ni and Mo to the low alloy steel can obviously enhance the compactness of the film,and thus improving its corrosion resistance in corrosive environment.In the tropical marine atmosphere,the penetration of chloride ion makes the corrosion product film loose and porous,promoting the contact between electrolyte and steel matrix,and further accelerating the corrosion of steel.Unfortunately,the influence mechanism of Ni and Mo on the corrosion resistance of low-alloy steel,the interaction between Ni,Mo and chloride ion,and the composition and distribution of corrosion products of Ni-Mo low-alloy steel in different chloride ion deposition rates are still unclear,which greatly limits the development and application of low-alloy steel in tropical marine atmospheric environment.In this paper,a series of new low alloy steels containing different content of Ni and Mo are designed.The corrosion behavior and mechanism of Ni-Mo low alloy steels in tropical marine environment are investigated by electrochemical measurement combined with microscopic morphology analysis,phase analysis and other analytical methods.The influence mechanism of alloy element composition and chloride ion deposition rate on the corrosion resistance of low alloy steel is proposed.It provides an important reference for the development of new corrosion resistant low alloy steel suitable for severe tropical marine atmospheric environment.The main contents are as follows:(1)In the simulated high-Cl-tropical marine atmosphere(the NaCl content was 1wt.%),the low alloy steels with Ni content of 1.72wt.%?2.87wt.%and Mo content of 0.34wt.%?1.23wt.%had good corrosion resistance.The NiFe2O4,Mo6+and ?-FeOOH in the inner corrosion product film had significant effects on the corrosion resistance.With the increase of Ni content and Mo content,the corrosion rates of low alloy steels decreased gradually.Ni and Mo gradually changed from uniform distribution in the inner and outer films to enrichment in the inner film.The dominant composition phase of the inner corrosion product film gradually changed from ?-FeOOH to ?-FeOOH.For the Ni-containing steels,the NiFe2O4 content increased gradually,and the corrosion product film changed from the double-layer structure with sparse outside and dense inside to the single-layer structure with dense inside.For Mo-containing steels,the content of Mo6+ increased gradually.The protective effect of Mo6+on the corrosion product film was closely related to corrosion potential Ecorr,charge transfer resistance Rct and protective ability ?/?.(2)The higher the content of Ni and Mo in the steel matrix,the more obvious the synergistic effect of Ni and Mo on the corrosion resistance of low alloy steel.When the content of Ni or Mo in the steel matrix increased,on the one hand,it could promote the enrichment of Mo and Ni in the corrosion product film.On the other hand,it could obviously promote the formation of corrosion products with strong protection,such as ?-FeOOH,Mo6+and NiFe2O4.The adsorption of Mo6+on the electronegative inner film of NiFe2O4 made the corrosion product film more compact and enhanced its electrochemical stability.The product film had the cation selective permeability,which can effectively prevent the penetration of Cl-into the film,so as to improve the corrosion resistance of Ni-Mo low alloy steels in tropical marine atmospheric environment.(3)The change of Cl-concentration in the corrosive environment had a significant effect on the protection and stability of corrosion product film of low alloy steel.With the increase of Cl-concentration,the inhibition effect of Cl-on NiFe2O4 in the product film of Ni-advanced steel and Mo6+in the product film of Mo-advanced steel increased gradually.The composition phase of the inner corrosion product film changed from ?-FeOOH to ?-FeOOH,that of the outer film changed from ?-FeOOH to ?-FeOOH.Ni and Mo gradually changed from uniform distribution in the inner and outer films to enrichment in the inner film.In low-Cl-concentration environment,the formation of NiFe2O4 and Mo6+in the inner film made the corrosion product film compact and stable,and the corrosion resistance of steel was higher.In high-Cl-concentration environment,the content of NiFe2O4 in the product film of Ni-advanced steel and Mo6+in the product film of Mo-advanced steel decreased rapidly.When the NaCl content in the corrosive environment was more than 2wt.%,the corrosion product films of the two low alloy steels lost stability and protection,and the corrosion resistance was lower.(4)During the on-site exposure in tropical marine atmosphere,the corrosion resistance of preferred economical 0.73wt.%Ni-0.34wt.%Mo low-alloy steel had good corrosion resistance.With the increase of exposure time,the corrosion rates of the carbon steel and low-alloy steel first increased and then decreased.The composition of carbon steel had no obvious change.The temperature,humidity and chloride deposition rate mainly affected the composition of carbon steel in the early stage of exposure,but not the composition of the later exposure.A large amount of NiFe2O4 and Mo6+ were formed in the corrosion product film of low alloy steel,which improved the compactness and protection of the product film.The reason for the change of corrosion rate was the formation of nano-sized ?-FeOOH in the corrosion product film.The composition phase of the inner corrosion product film changed from ?-FeOOH to ?-FeOOH.But there was no obvious change in the composition phase of the outer film,which was mainly ?-FeOOH.