摘要
Abstract
Abradable seal coating(ASC)has a porous,multi-layer and multi-phase structure.When the ASC is in a corrosive environment,the galvanic corrosion of the substrate-bonding coating or the bonding coating-top coating becomes an important reason for accelerating the failure of the ASC system.At present,NiAl bonding coating is usually used as bonding layer in ASC system,but the research on galvanic corrosion between NiCrAl bonding coating and substrate is lacking.In this study,NiCrAl coatings were prepared on FeCoNi superalloy and(1Cr11Ni2W2MoV)stainless steel substrates by atmospheric plasma spraying(APS).The measurement methods,including scanning electron microscopy(SEM),3D confocal laser scanning microscope(CLSM),X-ray diffraction(XRD)and electrochemical tests(open circuit potential test(OCP),potentiodynamic polarization curve test and galvanic corrosion test),were used to characterize the corrosion behaviors of NiCrAl bonding coating,and the galvanic corrosion behaviors of NiCrAl bonding coating on FeCoNi superalloy and stainless steel substrates in 3.5wt.％NaCl solution.The results showed that partial phase transformation of NiCrAl powder occurred after spraying,and the formation of Al2O3 increased the corrosion resistance of the coating.The NiCrAl coatings for different immersion time were observed by CLSM,and the results of CLSM were processed by image method.The results showed that the porosity and surface roughness of NiCrAl coating decreased firstly and then increased.Combined with the micro-morphology and energy spectrum of NiCrAl coating surface for different immersion time,this was due to the accumulation of corrosion products on the surface of the coating in the pores at the early stage of immersion,and the cracking and shedding of corrosion products at the later stage of immersion.After 240 hours of immersion,clear corrosion channels were observed at the cross section of the coating,indicating that the substrate had been corroded.The experimental results showed that when the corrosive medium reached the interface between the coating and the substrate through the corrosion channel,local micro-galvanic corrosion became the main cause of corrosion.The results of the potentiodynamic polarization curve analysis indicated that,compared with NiCrAl-FeCoNi superalloy,NiCrAl-stainless steel system had lower corrosion potential.
The results of the galvanic corrosion testing indicated that the NiCrAl coating was preferentially corroded as the anode,and the stainless steel and FeCoNi superalloy were used as the cathode.When the NiCrAl coating formed galvanic couples with stainless steel and FeCoNi superalloy substrates,the average galvanic current density for the NiCrAl-FeCoNi superalloy couple was 7.08 μA/cm2,while the average galvanic current density for the NiCrAl-stainless steel couple was 13.56 μA/cm2.Galvanic current data of the NiCrAl-FeCoNi superalloy couple and the NiCrAl-stainless steel couple were decomposed into both a S8 crystal(smooth coefficient)and(d1 to d8)d series crystals(detail coefficients)by the orthogonal Daubechies wavelets of the fourth order(db4),and the electrochemical activity of the couples was analyzed through a comparison of the energy distribution plots(EDP)of the wavelet coefficients.After 240 hours of immersion,the corrosive medium penetrated the coating and caused corrosion to the substrate due to the continuous degradation by the corrosive medium as well as the galvanic corrosion of the NiCrAl coating.The results indicated that the galvanic current of the NiCrAl-FeCoNi superalloy couple exhibited lower energy in the d series crystals during discrete wavelet analysis.This result meant that the NiCrAl-FeCoNi superalloy galvanic couple had low electrochemical activity.关键词
大气等离子喷涂/NiCrAl涂层/电偶腐蚀/小波分析Key words
APS/NiCrAl coating/galvanic corrosion/wavelet analysis分类
金属材料
