Effects Of Carbon Content In Carbon Steel On The Microstructures And Performance Of The Coatings Via Simultaneous Chromising And Nitriding By Pack Cementation Process

Chromium nitride coatings have been applied to surfaces of different grades of steels,which increase their surface hardness and hence tribological performance.Nowdays,they have also been investigated for use as corrosion resistant coatings on metal alloy bipolar plates in PEMFC where the coating must also have low interfacial electrical contact resistance.This study aims to prepare chromium nitrides coatings in different carbon content of carbon steels via simultaneous nitriding and chromiuming by pack cementation,to investigate the the effects of processing conditions and carbon content in steel substrates on the microstructure of the coatings produced.By calculated with the assistance of the Chem Sage computer program in combination with SPS database for thermochemical analysis,the pack powder mixtures used for the process consisted of Cr2 N as a source of both N and Cr,NH4 Cl as activator,Al2O3 as inert filler and the coating process was in the temperature range of 1000?1100 °C.By pack cementation in a pack 30Cr₂N-2NH4Cl-68Al2O3?wt.%?at 1100 °C for 4 h,three different structure of coatings were obtained to use pure iron respectively,steel Q235,steel 45#as substrate:?1?The coating structure which use pure iron as matrix was a top Cr2 N with a thickness of approximately 1.5 ?m and a large Cr-enriched diffusion layer underneath with a thickness of over 50 ?m.The Cr atomic concentration ratio is 40% in the Cr-enriched diffusion layer;?2?The coating structure which use steel Q235 as matrix was a top Cr2 N with a thickness of 10 ?m and a Cr-enriched diffusion layer underneath with a thickness of approximately 30 ?m.In the Cr-enriched diffusion layer,the Cr-carbide was precipitated and the Cr atomic concentration ratio is 20%;?3?The coating structure which use steel 45# as matrix was a top Cr2 N with a thickness of 16 ?m,an inner Cr-carbide layer with a thickness of 8 ?m and a Cr-enriched interdiffusion zone underneath with a thickness of 15 ?m.Accordingly,the carbon in the matrix promotes Cr deposition and Cr2 N deposition.The diffusion of C to the substrate surface reacts with Cr,which promotes the reactive Cr deposition on the substrate surface;Cr-carbide hinders the diffusion of surface Cr to the substrate and increases the Cr atom concentration on the substrate surface;Furthermore,if the content of C in the matrix is enough,Cr-carbide will form a continuous Cr-carbide layer on the surface of the substrate,the Cr-carbidelayer layer can promote the deposition of Cr2 N layer.The coating process has a great influence on the coating structure.The higher the ratio of Cr2 N powder,Cr2 N surface layer is thicker;the longer the holding time on steel 45#,the top Cr2 N layer and inner Cr-carbide layer thickness growth and the phase of inner Cr-carbide layer was?Cr,Fe?7C3 changed to?Cr,Fe?23C6.The microhardness test showed that the surface hardness of the top Cr2 N layer on steel 45# was as high as 1600 HV.Electrochemical test results show that the corrosion potentials and corrosion current of the matrix of steel 45# and Cr2 N coating were-0.521 V and 230.63 ?A/cm2?-0.448 V and 10.89 ?A/cm2.Coatings showed secondary passivation phenomenon when polarization voltage exceeded 0.3 V,The corrosion current was 0.5 ?A/cm2.The corrosion resistance of samples was greatly improved.Immersion test show that coating samples of steel 45# has produced pitting corrosion due to defects in the four corners.By pack chromising and nitriding,on the 420J2 stainless steel and 430 stainless steel substrat,the coating structure was a top Cr2 N with a thickness of approximately 2 ?m and a large Cr-enriched diffusion layer which the Cr atomic concentration ratio is high.The top Cr2 N layer was highly textured with a strong?111?XRD reflection.Electrochemical test in simulated PEMFC working environment solution show that the Cr2 N coating on 420J2 tainless steel has the best corrosion resistance,the corrosion potential increased by 0.4 V,the corrosion current density is reduced by 2 orders of magnitude reached to 0.16 ?A/cm2,the corrosion resistance has been greatly improved and it has met the requirements of bipolar plate on corrosion resistance.