Research On Removing MCrAlY Coatings By Electrochemical Method And Oxidation Properties Of Recoated Coatings

MCrAlY coatings are widely used in aerospace,aviation,ships and other fields to protect the components against oxidation and corrosion during service at high temperature.For example,it is often desirable to prepare the underlying MCrAlY and ceramic layer on the surface of the aviation engine blades and vanes to enhance the binding force of the ceramic layer,and to improve their oxidation resistance.However,the coating process in the preparation and service quality often appears coating failure,thus affecting the performance of the blade,causing leaf scrapped.Due to the complexity of the blade manufacturing process and the high cost of the superalloy,the manufacturing cost of the blade substrate is much higher than that of the coating.If the unqualified blade can not be used again,it will cause great waste of resources.Therefore,under the premise of ensuring the original performance safety of the blade substrate,it has great practical significance and economic value for the rework the unqualified coating.In this work,NiCoCrAlY coating was prepared on K38G nickel-base superalloy by arc ion plating.The feasibility of chemical removal or electrochemical removal was discussed.The effects of temperature,concentration and other factors on the removal rate and result of the coating were discussed,and the removal process was finally determined.The XRD,SEM,EPMA and other analytical methods were used to investigate the microstructure of the superalloy substrates after the coating removed and the recoated coatings.The difference in oxidation behavior between the recoated coatings and the as-prepared coatings was compared.The interdiffusion behavior and the oxidation performance of recoating coatings were investigated.The main research results in this paper are as follows:(1)The chemical and electrochemical methods were used to remove the coatings separately.The removal rate and dissolution behavior of NiCoCrAlY coatings in different chemical solutions were investigated.It was difficult to quickly and effectively remove all coatings by chemical removal.By analyzing the potentiodynamic polarization curves and cross-sectional morphology of the coating and the substrate,the parameters of the electrochemical stripping coating process in the nitric acid system were determined.During the coating removal process,the aluminum-rich phase was preferentially dissolved.Each element was dissolved in the electrolyte in sequence to be present in the electrolyte in a stable cationic state.(2)The surface of the exposed substrate after coating removing was the interdiffusion zone formed during oxidation simulation.Therefore,the interdiffusion behavior of the elements in the oxidation simulation led to the change of surface composition and phase structure on the new substrate which was made different from the original K38G alloy.After recoating and annealing on the new substrate,the recoated simple was compared with the as-prepared coating.The result shows that the recoated coating was different from the as-prepared coating after annealing even Al element had an uphill diffusion during vacuum annealing,which led to the composition and the phase transformation of the recoated coating.(3)The isothermal oxidation and cyclic oxidation kinetics of the as-prepared coating and the recoated coating at 1000℃ were investigated.The weight gain of the recoated coating was less than that of the as-prepared coating.A dense and continuousα-Al2O3 scale,possessing wonderful oxidating resistance and spallation resistance,was formed on the surface during high temperature oxidation.Due to the existence of the interdiffusion zone before recoating,the newly formed interdiffusion zone of the recoated coating was deeper,and the TCP phase in the secondary reaction zone reprecipitation after apperearance.