Effect Of Pre-Chroming On The Cyclic Oxidation Behavior Of Hot-dip Aluminized Q235 Steel

Aluminizing of steel is a kind of surface protection technology,by using the method of thermal diffusion to get Al-Fe alloy layer on the steel surface.Aluminum-iron alloy layer has good corrosion resistance,its oxidation resistance and corrosion resistance is more outstanding especially under the condition of high temperature.Although aluminized Q235 steel has good high temperature oxidation resistance,the aluminized layer will fail because of the mutual diffusion of Al and Fe element under high temperature.It has strong impact on the application of the carbon steel.Pointing to this problem,this article chooses chrome pre-treatment on Q23 5 substrate surface,to delay the mutual diffusion of Al and Fe element,and prolong the service life of aluminized steel.Because Q235 steel is low carbon steel,the first thing,I use salt bath nitriding pre-treatment,and then chromed using salt bath to prepare the diffusion barrier layer(CrN)at low temperature.After the preparation of the diffusion barrier layer,I use one bath process of hot-dip aluminum at 750℃ by 10 min and diffusion at 900℃ by 3h.The organization of the aluminum layer obviously changes after the diffusion.Then conducting the high temperature oxidation experiment at 800℃ by 36d,to verify the impact of the salt bath chrome pre-treatment.The high temperature oxidation experiment shows:The organization of the aluminum layer obviously changes without chrome in the process of cyclic oxidation.Small holes in the layer before oxidation,which grew up empty gather and forming holes or cracks in the process of circulation oxidation.Wave linear cavity produces in the outer layer and transition layer interface,with the extension of time,it gradually turned into a flat shape defect.Before the cyclic oxidation the outer layer is mainly composed of Fe2Als phase and a small amount of FeAl2,Fe-Al phase;the inner layer is mainly composed of Fe-Al and A1 in Fe alpha solid solution phase and small amount of Fe3Al phase.As the cyclic oxidation,Fe2Al5,FeAl2 and Fe Al phase of outer layer are gradually transformed into Fe3Al phase with mutual diffusion of Al and Fe atoms.The inner layer is divided into two layers;a layer of Fe3Al phase,and another layer is composed of Al in Fe alpha solid solution phase.Before and after the condition of high temperature cycle oxidation,the organizational change of the layer is not obvious with chrome.The layer also appear a few empty,as the cyclic oxidation,which grew up,but no holes with chrome.Wave linear cavity did not appear between the outer and inner layer.Along with the cyclic oxidation,the layer obviously doesn’t change.This suggests that the chrome pre-treatment has effect on the mutual diffusion of A1 and Fe atoms.Whether chrome or not,the oxidation kinetics curve of the layer are roughly the same change trend.In the early days of the cyclic oxidation weight increase is larger,but follow the rules of oxidation weight gain,by the end of cycle oxidation oxidation kinetics curve smoothly with chrome.With chrome pre-treatment organizational stability of the layer is better than without chrome pre-treatment.In the absence of chrome pre-treatment,the hot-dip aluminized Q235 steel oxidizing failure in 30 days,but with chrome pre-treatment,hot-dip aluminized Q235 steel in cyclic oxidation 36 days still has good oxidation resistance.It suggests that the chrome pre-treatment slows down the mutual diffusion of Al and Fe atoms,increases the service life of the aluminum layer.