Electrochemical Corrosion Behaviors Of Cr₂AlC MAX Phase And Its Amorphous/Nanocrystalline Coatings In 3.5 Wt.%NaCl Solution

MAX phases(i.e.Mn+1AXn,where M is a transition metal,A is an A-group element,X is C or N,and n=1,2,3)are a kind of layered ternary compounds.Owing to their unique bonding characteristics and layered structure,MAX phases have a combination of metal-like and ceramic-like properties,such as high electrical and thermal conductivity,good machinability,high strength and elastic modulus,and good chemical stability.Therefore,MAX phases have great potential as structural/functional integrated materials.Among these MAX phases,Ti2AlC,Ti3AlC2 and Cr2AlC have aroused wide concern as protective coatings at high temperatures due to their excellent oxidation/hot corrosion resistance.Meanwhile,previous studies have shown that MAX phases have excellent corrosion resistance in aqueous solutions as well.However,to date,most of studies solely focused on the determination of their corrosion resistances,but the studies about the microscopic corrosion mechanism involving with the type and the formation of passive films is relatively few,let alone the corrosion resistance of MAX phase coatings in aqueous solution.On one hand,the bonding type in MAX phases includes ionic bond,covalent bond and metallic bond,and an element can exhibit-two different bond states due to different occupations in the crystal lattices of MAX phases.In addition,MAX phases also exhibit nanolayered structure.All of these characteristics make MAX phases differ from common ceramics and metals.On the other hand,amorphous M-A-X coatings with the same stoichiometric ratio as ideal MAX phases could be obtained readily in the preparation of MAX phase coatings.On such bases,Cr2AlC,as one of the most typical MAX phases,was selected in this study,and the corrosion behavior of bulk Cr2AlC and its amorphous/nanocrystalline coating in 3.5 wt.%NaCl solution was systematically studied.It is meaningful in theory to understand the relationship between the structure(such as nanolayered structure and amorphous strcture)and composition of materials with their corrosion resistance,further promoting the practical application of MAX phases as corrosion resistant coatings.The main conclusions are as follows:Al in bulk Cr2AlC was more prone to be attacked and more easily slipped out from the matrix into solution when immersed at OCP in 3.5 wt.%NaCl solution.However,in the case of polarization,more of the reaction products,riched in Al and O,between de-intercalated Al and electrolyte deposited on the sample surface.This is because that Al in Cr2AlC is weakly bonded,it was more prone to be attacked when Cr2AlC was immersed in the NaCl solution.On the other hand,the high activity and high oxygen affinity of Al in Cr2AlC promoted the reaction between Al and electrolyte.Meanwhile,the corrosion behaviors of Cr2AlC-xAl2O3(x=6.1 wt.%and 15.2 wt.%)composites in 3.5 wt.%NaCl solution were investigated for comparison.The results indicated that Al2O3 particles mainly distributed at the grain boundaries of Cr2AlC matrix,partly blocked the inward penetration of the electrolyte and inhibited the anodic dissolution process,thereby the addition of Al2O3 improved the corrosion resistance of Cr2AlC matrix.Amorphous Cr-Al-C coating with the atomic ratio of Cr:Al:C=2:1:1 was deposited on polycrystalline YSZ(yttria-stabilized zirconia)substrate by magnetron sputtering with a single Cr-Al-C compound target.During subsequent annealing in Ar atomosphere at 500℃-700℃,the microstructure of the as-deposited amorphous coating experienced from completely amorphous,to partially crystallized matter of(Cr,Al)2Cx solid solution,and finally to fully crystallized Cr2AlC.By comparing the corrosion performances of amorphous coating,partially crystallized coating and fully crystallized Cr2AlC coating in 3.5 wt.%NaCl solution,it was found that all of coatings exhibited passivation behavior,and all the passive films exhibited p-type semiconductor characteristics.Furthermore,the amorphous coating exhibited the best corrosion resistance with the lowest corrosion current density and passive current density,because its passive film possessed the highest content of Cr2O3.The corrosion resistance of the fully crystallized Cr2AlC coating was slightly inferior to amorphous coating.However,the partially crystallized coating exhibited the worst corrosion resistance due to the inhomogeneity of microstructure and the lowest content of Cr2O3 in its passive film.Various(Cr,Ti)-Al-C coatings with the atomic ratios of(Cr1-x+Tix):Al:C=2:1:1(x=0.10-1.0)were prepared by incorporating Ti element into amorphous Cr-Al-C coatings.It was found that when x≤0.35,the as-deposited coating remained amorphous structure;but when≥ 0.50,the nanocrystals appeared,and a(Ti,Cr,Al)C solid solution,i.e.Cr and Al were incorporated into Ti sites of TiC lattice,formed.Therefore,the incorporated Ti element promoted the crystallization of Cr-Al-C coating.The electrochemical testing results of different(Cr,Ti)-Al-C coatings in 3.5 wt.%NaCl solution indicated that the passive current density increased with increasing the content of Ti in the coating,this is because that Ti could promote crystallization and decreased the homogeneity of microstructure of the coating.Meanwhile,the content of Cr decreased with the increase of Ti content,which could also result in the decrease of passivation ability of the coating.In addition,the current density in the transpassive region was significantly reduced with the increase of Ti content.This may because that in this potential region,protective TiO2 passive film still kept stable,while stable Cr(Ⅲ)oxide could transform to soluble Cr(VI)oxide.The depth profiles in Ti3SiC2-Ti3AlC2 diffusion couple were determined by EPMA after heat treatment at 1100℃-1400℃.It was found that only Si and Al diffused mutually along opposite direction at the Ti3SiC2/Ti3AlC2 interface.Boltzmann-Matano method and Sauer-Freise method were applied to calculate the interdiffusion coefficients.The calculation results showed that the magnitude of interdiffusion coefficient of Si and Al was in the order of 10-13-10-11 m2/s in the whole temperature range,and the activation energy for the interdiffusion at Matano plane(≈Ti3Si0.5Al0.5C2)was 246 kJ/mol.Moreover,the results obtained by these two methods showed that with increasing the content of Si,the interdiffusion coefficient decreased monotonously at 1100℃-1300℃;but at 1400℃,this variation trend of the interdiffusion coefficient was no longer monotonous.This phenomenon possibly resulted from the formation of Ti5Si3 purity phase and numerous voids on Ti3AlC2 side.