Study On Surface Treatment And Corrosion Resistance Of 45 Steel

With the aggravation of energy and environmental issues,lightweight of automobiles has become a trend in the development of automotive industry.Carbon fiber reinforced polymer composites?CFRP?are expected to be widely used in lightweight vehicles due to their high specific strength.However,galvanic corrosion is likely to occur when CFRP is in contact with metals due to the high corrosion potential of CFRP.In terms of this issue,45 steel and 30%C_f/PA6 composite were taken as research object,an Al-Al₂O₃-silane composite coating with certain thickness was prepared on the surface of 45 steel via a composite surface treatment technology of hot dip aluminizing,anodizing and silane treatment to reduce the potential difference,increase the contact resistance and isolate the corrosive medium,which consequently improved the corrosion resistance of 45 steel.The effects of surface treatment parameters on the microstructure,morphology and phase composition of the coating were analyzed by optical microscopy?OM?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?;The corrosion resistance of the composite coating in 3.5wt%NaCl solution was evaluated by dynamic resistance polarization test,electrochemical impedance test and salt spray test;Based on these results,the correlation between coating structure,morphology and corrosion resistance was established.The main conclusions are as follows:A coating with Fe-Al alloy layer and aluminized layer was formed from internal to external of 45 steel surface after hot-dip aluminizing,.The alloy layer was irregularly jagged which was mainly composed of Fe₂Al₅ phase;the aluminized layer was mainly composed of Al and a bit amount of needle-like FeAl₃ phase.With an increase in hot dipping temperature and time,the diffusion ability of Al and Fe was enhanced,resulting in a significant increase in thickness of alloy layer.At the same time,the hot dipping temperature and time have a certain degree of influence on the surface quality and uniformity of the coating.The optimum parameters was a dipping temperature of750?and a dipping time of 1 minutes.After anodizing of hot-dip aluminized 45 steel,an aluminum oxide layer with certain thickness was formed on the surface of the aluminum layer only,while the microstructure and structure of the alloy layer was unchanged.The anodization parameter significantly affected the thickness and morphology of the alumina film which influenced the corrosion resistance of the oxidation film.The thickness of oxidation film increased with an increase in current density and oxidation time.Meanwhile,the size and area of the pores on surface of the alumina film also increased with increasing current density and oxidation time,therefore,the corrosion resistance of the material firstly increased and then decreased.After anodic anodized with a current density of 2A/dm² for 30 minutes,the self-corrosion current density of the hot-dip aluminized 45 steel in 3.5wt%NaCl solution decreased by 2 orders of magnitude compared with that of the material before anodizing while the charge transfer resistance was increased by 20 times,indicated that the corrosion resistance was significantly increased.The oxide film prepared under the optimal anodization parameter was sealed with KH560 silane for various treatment time,the results suggested that a silane film with certain thickness was formed on the surface of the alumina.When the sealing time was prolonged from 1 minutes to 5 minutes,the pores on the surface of Al₂O₃ gradually reduced,and the thickness of the silane film gradually increased.The silane film effectively hindered the penetration of the corrosive medium into the material which improved the corrosion resistance of the material.However,as the silane treatment time increased to 7 minutes,cracks were formed on the surface of silane film which degraded the corrosion resistance of the samples.Under the optimal silane treatment time?i.e.,5 minutes?,the self-corrosion current density and electrochemical impedance value of the samples decreased by one order of magnitude compared with the hot-dip aluminized-anodized sample,and the corrosion resistance was further improved.