Research On The Technique Of Paste-method Sherardizing With Protection Layer

A technique of Paste-method sherardizing with protection layer was developed in this thesis. Based on this technique, effects of NH4Cl dosage, heating temperature and heating preservation time on the corrosion resistance, thickness, structure and composition of sheradizing layer were studied. Anti-corrosion performances were valuated through SST-090 salt spray test chamber, HDV-7C potentiostat and CHI660 electrochemical workstation, the organization and phase composition of sheradizing layers were observated using S-4800 scannig electron microscope and X-ray diffractometer,thickness was tested using metallurgical microscope and HCC-24 thickness detector.Experimental results show that the three processing conditions studied in this thesis have little influence on the initial corrosion current of the sherardizing layer. The thickness of sheradizing layer from magnetic method was about 50-60μm thinner than the thickness from micrograph method,thus the thickness of solid solution can be got from the D-value of the two measured thickness. It was found in experiments that the sherardizing layer mainly contains alloy phaseδandÏ„₂ and no pure zinc --ηphase. The sheradizing layer is too thick to detectÏ„1 phase andα-Fe phase.The thickness of the sherardizing layer can be detected without the sheradizing-aid agent NH4Cl. When the mass ratio of NH4Cl to zinc powder is 0.01, both the alloy layer and the solid solution layer got thicker and the corrosion resistance was improved; when the mass ratio is further increasing to 0.02, 0.03, and 0.04, the alloy layer got thinner gradually, while the solid solution layer got thicker gradually, at the same time the junction zone of the alloy layer and the solid solution layer got thicker, thus the corrosion resistance of coating deteriorated gradually. With the mass ratio of NH4Cl to zinc powder changed from 0 to 0.04, there is no obvious change of the composition of the alloy layer. It can be concluded from the above that the main function of NH4Cl is to promote the diffusion of zinc into iron.Temperature increased and holding time prolonged can thicken alloy layer and strengthen corrosion resistance. When temperature is from 400℃to 440℃, the phase composition does not change, while the holding time is prolonged from 3 h to 6 h,ζphase appears, which indicates that higher temperature is more conductive to the metal diffusion than the longer time.