Experimental Research On Mechanism Of Aluminizing And Corrosion Resistance Of Aluminized Carbon Steel By Powder Pack Cementation

The iron-aluminium intermetallic compounds have good resistance to high temperature oxidation,high temperature sulfurization and good wear resistance.Compared with stainless steel,the iron-aluminium intermetallic compounds have the advantages of low cost,low density,high strength to weight ratio,have been applied in petroleum,chemical industry,electric power,aviation and other fields.There are a variety of processes currently used to prepare iron-aluminum intermetallic coatings on steel surfaces to protect the substrate from corrosion,solid powder embedded aluminizing has been widely adopted due to its low requirement for equipment,low cost and convenient operation.This paper adopts solid powder pack cementation method and chooses Q235,Q345,20#steels as the matrices.Firstly,the effects of the composition of the powder,aluminizing temperature and holding time on the quality of aluminized layer are studied.The experimental results show that adding graphite in the powder can improve the surface quality of the aluminized layer and increase the thickness of the layer,while the thickness of the aluminized layer increases first and then decreases with the increase of the aluminizing temperature,and has a parabolic relationship with the holding time.The three kinds of matrices basically follow the diffusion law.Considering the surface quality and thickness of the aluminized layer,the optimum aluminizing process parameters are determined to be under 900?holding for 4h.The composition of the powder is 15%aluminum powder,5%ammonium chloride,75%alumina,and 5%graphite.Secondly,the structure,element composition,phase composition and microhardness of aluminized layer are analyzed.It is found that the thickness of the aluminized layer is about 370?m,and it is even and dense,and there are no leakage areas and cracks.From the outside to the inside,it is aluminide layer,transition layer and matrix,respectively.The aluminide layer mainly contains two elements,Al and Fe,the atomic ratio remains at 7:3,the main phase is Fe₂Al₅,and the hardness of aluminide layer reaches 896Hv_0.1 which is much higher than that of the substrate.Finally,the aluminized Q235 steel is selected,and the corrosion resistance of aluminized steel in high temperature oxidation environment and different solution environment is studied through comparative tests.The results show that in high temperature oxidation environment,the weight gain of aluminized steel after 100h oxidation is less than 2 mg/cm²,which is about 1/158 of the original sample.In sodium chloride and ammonium chloride solution,the weight loss of aluminized steel after 96 h immersion is 1/7 and 1/6 of the original sample,respectively.The results of electron microscopy and elemental analysis show that in the above corrosive environments,a dense oxide film is formed on the surface of the aluminized sample,which effectively protects the substrate and improves the corrosion resistance of the material.In the aqueous sodium sulfide solution,the dissolution of aluminum in the aluminized layer causes the corrosion resistance of the aluminized steel to deteriorate.The above immersed experimental results are consistent with the results of electrochemical experiments.