Microstructure Evolution And Properties Analysis Of Ni-based Amorphous Deposits

Corrosion is one of the main reasons in material failure.Amorphous materials have good corrosion resistance due to the uniform compositions,less grain boundary defects,and so on.Electroless amorphous binary and ternary Ni-based deposits have been widely used as the anticorrosive coatings.Until now,many researches have been focused on the Ni-P series deposits.However,the influence of microstructure and contents in amorphous deposits on corrosion resistance still lacks of systematic researches.In this study,three types of electroless amorphous deposits were prepared by adjusting the plating bath compositions and plating process parameters.Three amorphous deposits were binary Ni-P?S1?,ternary Ni-W-P?S2?and Ni-Mo-P?S5?,respectively.Because the content of P element directly determines the type of crystalline structure,and its specific content will also affect the amorphous degree of amorphous coating,three amorphous deposits were prepared with almost the same P content in order to improve the comparability of the results.In this study,the other Ni-W-P?S3?and Ni-Mo-P?S4?deposits were also prepared to study the influence of alloying element amount on the properties of deposits.All the five deposits were listed as following:S1-Ni-12.35 wt.%P,S2-Ni-3.81 wt.%W-12.4 wt.%P,S3-Ni-3.43 wt.%W-4.23 wt.%P,S4-Ni-2.79 wt.%Mo-12.89 wt.%P.and S5-Ni-4.31 wt.%Mo-12.82 wt.%P,respectively.SEM observation results showed that the surface of these deposits held typical nodular morphology,and these deposits were dense and no obvious defects existed.X-ray analysis results of these electroless deposits showed that the matrix of these deposits had amorphous disorder structure and there were still certain amounts of micro-order clusters in amorphous matrix.Compared with binary Ni-P deposits,the addition of element W in ternary Ni-W-P deposits could promote the formation and growth of micro ordered clusters,while element Mo in ternary Ni-Mo-P deposits had little effect on micro ordered clusters formation and growth.DSC results showed that amorphous Ni-W-P and Ni-Mo-P deposits had higher exothermic peak temperature than Ni-P binary amorphous deposit.This meant element W and Mo could improve thermal stability of deposits than bianary Ni-P deposit with approximate P content.Binary Ni-P amorphous deposit had only one crystallization process corresponding to the amorphous crystallization transformation.Ternary Ni-W-P deposit showed two crystallization processes.The first exothermic peak corresponded to amorphous structure crystallization transformation.The second exothermic peak corresponded to the transition of the metastable NixPy mesophase to the stabilized crystal phase Ni₃P in the coating.And the second exothermic peak temperature was correlation with the type and amount of NixPy.Ni-Mo-P ternary deposit showed one crystallization processes corresponding to the amorphous crystallization transformation.Amorphous Ni-P,Ni-W-P,and Ni-Mo-P deposits were annealed at 200 ? and 250 ? for different time.The matrix of all deposits still held amorphous structure.However,the micro order clusters in amorphous matrix had been changed.The further analysis through calculating XRD curve showed that the size and atomic number of ordered clusters were increasing with the annealing time when the annealing temperature was kept,but there was some fluctuation in the initial stage of heating.The changing morphology of annealed deposits could be observed by TEM observations.Types of the nanocrystallines could be confirmed by diffraction rings.The changing trend of micro-particles amount could be analyzed by calculating exothermic peak areas in DSC curves.The microstructure evolution in the amorphous deposits could be effectively evaluated by combining these three analytical methods.Three types of amorphous coatings had undergone significant crystallization transition at 400 ? heat treatment.Under the condition of heat treatment at 300 ?.the binary amorphous Ni-P deposits began to transform from amorphous structure to crystal structure,but there was still a certain amount of amorphous structure in the matrix.The ternary amorphous Ni-W-P deposits gradually changed from amorphous structure to nano-structure.In the ternary amorphous Ni-Mo-P deposits,the S4 coating with less Mo content changed from amorphous structure to nano-structure.The S5 coating with more Mo content still retained the amorphous structure when the heating time was 1h.The deposits began to precipitate a small amount of nanocrystals and crystal phases on the base of amorphous matrix with the increasing of annealed time.The reason for the significant difference in the structural transition of the three types of amorphous depositis at 300 ? was that the addition of elements affects the stability of the amorphous structure in the deposits.Corrosion resistance of the three types of amorphous deposits in 5 wt.%sulfuric acid solution were studied..Binary amorphous Ni-P deposit had better corrosion resistance than Ni-W-P and Ni-Mo-P ternary amorphous deposits.Binary Ni-P amorphous deposit could form stable passive film in sulfuric acid solution.And the passive film was consisted of Ni₃?PO₄?2 and Ni?OH?2 through XPS results.This passive film hold n-type semi-conductor characteristic.The protective efficiency of this passive film was affected by passive potentials and passive time.The passive film had better density,smaller roughness and higher protection when the potential was 0.1 V and the passive time was 30 min.Ternary Ni-W-P and Ni-Mo-P amorphous deposits could not form stable passive films in sulfuric acid solution.The main reason was that the series of W and Mo oxides formed on the surface of the coatings hindered the continuous formation of Ni₃?PO₄?2 passivation film which has good corrosion resistance,and affected the thickness and composition uniformity of the passivation film.This instability was shown on the polarization curve without the presence of a stable passivation interval.Corrosion resistance of the three types of amorphous deposits in 3,5 wt.%sodium chloride solution were also tested.Ternary Ni-W-P and Ni-Mo-P amorphous deposits showed better corrosion resistance than binary amorphous Ni-P deposit.The polarization curves showed that the three amorphous deposits could not form stable passivation intervals in the NaCl solution,which was related to the strong corrosion resistance of the chloride ion.The reason why the ternary amorphous Ni-W-P and Ni-Mo-P deposits had good corrosion resistance in the NaCI solution than binary Ni-P amorphous deposits was that the fonned oxides due to the elements W and Mo in the corrosive medium had a certain hindrance effect on the penetration of chloride ions,which was conducive to improve the overall corrosion resistance of the deposits.Ternary Ni-Mo-P deposits could form stable passive film in sodium chloride solution after annealed at 200 ? and 250 ?.On the one hand,this was related to the heat treatment to promote the relaxation of the structure to make the distribution of Mo elements more uniform,so that the formation of Mo oxides in the corrosive medium was more uniform,which could more effectively prevent the penetration of chloride ions corrosion.On the other hand,the surface of the deposits could form a certain amount of molybdate ions,these molybdate ions easily adsorbed on the surface defects in the deposit,which helped to prevent the accumulation and intrusion of chloride ions.Electrochemical results of amorphous Ni-Mo-P deposit annealed at 250 ?for 1 h showed that the passive film hold n-type semi-conductor characteristic.The passive film had better corrosion resistance under-0.1 V and 30 min.And the passive film was consisted of Ni₃?PO₄?2,NiO,Ni?OH?2,MoO,MoO₂ and MoO₃ through XPS anaylysis.