24Cr2Ni4W Alloy Steel Black Phosphate Coating Preparation And Properties

In addition to the excellent performance of the general phosphatecoatings,black phosphate coating also could reduce the diffuse reflection of theinstrument surface, eliminate reflective to concealment and other optical properties,which in the instrument, military and other industries has been widely used.24Cr2Ni4W alloy steel whichcontains a variety of elements is a high performance material. The material afterphosphate processing is not enough black, poor adhesion, poor wear resistance, whichaffects the actual application. In this paper,24Cr2Ni4W alloy steel blackphosphating process and properties were studied.By comparing experiments and orthogonal experiments, developed manganesedihydrogen phosphate, Zn（H2PO₄）₂·2H2O, Zn（NO₃）₂·6H₂O, Mn（NO₃）₂as themain components of black phosphate solution,studied surface conditioning time,surface conditioning tempeture and concentration of surface conditioning liquidprocesses on the impact of phosphate film,used copper sulfate drip, brine, neutral saltspray test（NSS） and electrochemical test method to test the corrosion resistance ofphosphate coating and analyzed phosphate coating microstructure and composition byscanning electron microscopy （SEM）,X-ray diffraction （XRD）.Experimental results show that,on conditions of suface conditioning timeof70s,suface conditioning temperature of60℃,concentration of suface conditioning of1.6g/L, phosphate temperature of85℃,phosphate time of30min,weight of obtainedphosphate coating was about30g/m²,the average thickness about20μm, coppersulfate drip for about650s,no corrosion after brine for12h, no corrosion after neutralsalt spray test for6h; phosphate coatings crystallize uniform and fine;the majorcomponents of the phosphating film are Mn₂Zn（PO₄）₂·4H₂O、Zn2Fe（PO₄）₂·4H₂O andZn₃（PO₄）₂·4H₂O.In order to improve the corrosion resistance of black phosphating film, developed two kinds of phosphating sealed treatment agent: organic silane sealant and inorganicsodium silicate sealant,and researched the main component and process of sealant onthe impact of phosphate coatings corrosion resistance.Under the concentration of KH560silane sealant of3%,dipping time of15min,baking temperature of70℃,baking time20min,silicate and cerous nitratesynergy effectively sealed phosphate coatings,which made corrosion current ofphosphate film reduced,the value of ac impedance increased.Under the concentrationof sodium silicate of15g/L,the value of pH of7, sealing liquid temperature of80℃,sealing time of12min,baking time of20min, sealing liquid could form silicateprecipitation to seal phosphate coatings,which made corrosion current of phosphatefilm reduced.The corrosion resistance property of phosphate film after treated by silanesealant or sodium silicate sealant was better than it after treated by chromate,nocorrosion on neutral salt spray test for72h.