Study On Electrochemical Properties Of Co-containing Martensitic Age-hardened Stainless Steels

As a new type of ultra-high strength stainless steel,martensitic age-hardened stainless steel containing cobalt has been applied in aerospace and other fields due to its excellent comprehensive properties.Cobalt?Co?alloying could improve the strength and hardness of stainless steel,but its effect on lath martensite matrix structure,reversed austenite structure and corrosion resistance of martensitic age-hardened stainless steel was seldom reported.The research on electrochemical corrosion behavior and corrosion mechanism of martensitic age-hardened stainless steel containing Co was not deep enough.In this paper,cobalt-containing and cobalt-free martensitic age-hardened stainless steel was studied.The effect of Co on the structure of martensite,reversed austenite and copper-rich precipitated phase was investigated by means of OM,SEM,TEM and XRD.The influence of Co on the corrosion resistance of martensitic age-hardened stainless steel was researched by potentiodynamic polarization curve,AC impedance spectroscopy and Schottky curve,and the electrochemical corrosion behavior of cobalt-containing steel at different temperature?30?,40?,50?,60??of 3.5%NaCl solution,different Cl^-concentrations?2%,3.5%,5%,7%?of NaCl and Na₂SO₄ mixed solution has been tested.The main results were obtained as follows:?1?After the addition of 6wt.%Co,the austenite grain size degraded from 50.02?m to 27.06?m after solution treatment,and the quenching lath martensite was refined.After aging treatment,thin film reversed austenite existed at the martensite lath boundary,and the precipitation of copper-rich phase was inhibited.?2?The electrochemical experiments show that the addition of Co element could improve the corrosion resistance of martensitic age-hardened stainless steel.With the addition of 6 wt.%cobalt,the self-etching current density decreased from 2.471?A to2.005?A,and the pitting potential elevated from 0.1190 V to 0.2685 V.The impedance spectra show that the impedance modulus of passive film was obviously higher than that of cobalt-free steel,and the pitting corrosion sensitivity was obviously reduced.?3?The passive film of martensitic age-hardened stainless steel was a double-layer structure.In the process of potential changing from negative to positive,the semiconductor characteristics of passive film transform from p-type semiconductor to n-type semiconductor.Compared with cobalt-free steel,passive film of cobalt-containing steel has lower flat-band potential,lower donor density N_D and acceptor density N_A,and more stable passive film,which was helpful to improve corrosion resistance.The main components of passive film were Cr₂O₃,Cr?OH?₃,Fe₂O₃ and FeOOH.The presence of cobalt-iron oxide improved the corrosion resistance of martensitic age-hardened stainless steel.?4?For 6 Co martensitic age-hardened stainless steel,the corrosion resistance decreased with the increase of temperature and Cl^-concentration.When the temperature was higher than 40?and Cl^-concentration was higher than 5%,the corrosion of 6 Co steel becomes more serious and the pitting sensitivity increased.?5?In the mixed solution of NaCl and Na₂SO₄,when the molar ratio of Cl^-and SO₄^2-was 1:3,the pitting corrosion resistance of the material was the strongest.When the molar ratio was 3:1,the pitting corrosion resistance of the material was the weakest.The corrosion resistance was the worst in 0.2 mol/L Na₂SO₄ solution.When the molar ratio of Cl^-and SO₄^2-was 1:3 and the corrosion potential was higher than the flat-band potential,the semiconductor properties of 6 Co steel passive film changed from n-type to p-type.As the film potential was 0.8 V,the N_D and N_A were the smallest,the oxygen vacancy and metal ion vacancy were the most in the passive film,and the corrosion resistance of the passive film was the best.