Study On The Structure And Corrosion Resistance Of Cr₂N Deposited On The Surface Of Martensitic Stainless Steel

Cr Nx is a kind of metal coating material with high hardness,high wear resistance and high corrosion resistance,so it is expected to be applied to the surface modification of proton exchange membrane fuel cell?PEMFC?metal bipolar plate.The feasibility of depositing Cr₂N coatings on martensitic stainless steel substrates by pack cementation is studied in this paper.The pack powder mixtures used for the process consisted of Cr₂N as a source of both N and Cr,NH₄ Cl as activator and Al₂O₃ as inert filler.By controlling the content of Cr₂N,holding time and holding temperature in turn to study the influence of technological conditions on the structure of Cr-N coating,a group of optimum pack cementation process conditions were determined.The corrosion resistance of the coatings was evaluated by corrosion test.The coating structure of the outermost Cr₂N layer with rich Cr diffusion layer can be obtained on the 440 A martensitic stainless steel substrate by using the pack powder mixture 30Cr₂N-2NH₄Cl-68Al₂O₃?wt.%?at 1100 ?for 4 h.The pack cementation process has great influence on the coating structure:?1?When the ratio of Cr₂N powder in the powder mixtures was more than 30%,the powder sintered seriously after the reaction.When the proportion of Cr₂N powder was increased successively by 5 %,the thickness of Cr₂N coating on the surface increased,and when the ratio of Cr₂N powder was 30 %,the thickness of Cr₂N coating on the surface was the thickest.And there are long stripe precipitates rich in Cr in the secondary outer layer.It is determined that the best pack cementation powder is 30Cr₂N-2NH₄Cl-68Al₂O₃?wt.%?;?2?After determining the composition of the pack cementation powder,the surface Cr₂N coating formed at the holding time of 1-8 h is smooth and compact,except for the incomplete structure and many defects of the surface coating formed at the holding time of 0 h.After holding time is more than 2 h,there are long stripe precipitates rich in Cr in the secondary outer layer.The total thickness of the coating is proportional to the holding time t 1 / 2,which accords with the general law of the total thickness and time of the coating in the thermal diffusion reaction deposition;?3?When the optimum holding time is 4 h,the thickness of Cr₂N coating on the surface increases with the increase of holding temperature at 1000 ?,1050 ?,1100 ?.At 1100 ?,there are long stripe precipitates rich in Cr in the secondary outer layer.In this case,the thickness of the outermost Cr₂N layer is 17 ? m,and that of the Cr rich diffusion layer is about 19 ? m.Therefore,the best process conditions of the pack cementation are as follows: at 1100 ? for 4 h in a powder pack 30Cr₂N-2NH₄Cl-68Al₂O₃?wt.%?.Selection of coating samples under this condition for corrosion resistance test.The electrochemical results showed that in the simulated corrosion solution of PEMFC,the self-corrosion potentials of original stainless steel sample,coated stainless steel sample were-0.623 V and 3274 ?A·cm-2,-0.212 V and 0.0362 ?A·cm-2 respectively,and the corrosion current of the coating sample is lower than the cathodic potential at the anode potential,suggestion that the coating sample has stronger corrosion resistance;In the immersion test period,the coated stainless steel sample had no weight loss for 800 h at 60 ? in 0.5 mol/L H₂SO₄ + 2 mg/L F– solution,while the original stainless steel sample dissolved rapidly at weight loss of 0.252g/h.On the other hand,the coated stainless steel sample had no weight loss for 13500 h at room temperature in 0.5 mol/L H₂SO₄ + 2 mg/L F– solution,while the original stainless steel sample dissolved at weight loss of 0.007 g/h.It is fully indicated that the Cr₂N coating on the surface of the sample has a good protective effect on the substrate;The results of four-probe resistivity test show that the resistivity of the surface Cr₂N coating is equal to that of the martensitic stainless steel substrate and has good electrical conductivity.In conclusion,the electrical conductivity and corrosion resistance of Cr₂N coating on martensitic stainless steel have met the requirements of proton exchange membrane fuel cells for bipolar plates.