The Hot Corrosion Behavior Of Ta In NaCl-KCl Melts And The Efffects Of Surface Treatments

Owing to its good molding and soldering properties,excellent high temperature mechanical properties and outstanding corrosion resistance compared to other metals,Ta is widely used in the nuclear industry and other pyrochemical industries.Although Ta has excellent corrosion resistance compared to other metals or alloys,in actual production practice,Ta material workpieces are still difficult to meet the requirements of the production of chloride corrosion resistance of materials.Therefore,it is urgent to study the corrosion behavior of Ta in molten salts and explore the strengthening methods of Ta to improve its hot corrosion resistance.In this paper,the hot corrosion behavior of Ta in an equimolar NaCl-KCl molten salt in air atmosphere was studied.Then,a series of surface modification was carried out on Ta,and the laser surface melting?LSM?were treated on Ta surface,diamond film were developed by hot filament chemical vapor deposition?HFCVD?,TaC film were prepared by surface carburizing method and Ta-Si coating and Ta-Si-Al coating were prepared by a halide activated pack cementation?HAPC?technique,further the effect of surface modification on corrosion behavior of Ta in the molten salt were investigated.The main results are as follows?The corrosion products of the pure Ta plate after corrosion in an equimola NaCl-KCl molten salt at 850°C were NaTaO₃,Na₂Ta₄O₁₁ and Ta₂O₅.Under the action of high temperature and corrosion stress,the surface grains of Ta tended to be deformed to form slip bands,and then there would be preferential corrosion along the slip steps.These preferential corrosions acted as pass tunnels for transporting corrosive media and cause the initiaton of intragranular cracks or even the intergranular cracks.The chemical reactions occurring during the corrosion process are summarized as follows.The reactions of the surface layer in the initial stage were mainly the reaction of chlorine molten salt,O₂and Ta to form NaTaO₃ or Na₂Ta₄O₁₁ and Cl₂.When the corrosion layer of Na-Ta-O grew thicker,the reactions in the subsurface layer were NaTaO₃ or Na₂Ta₄O₁₁ reacts with Ta and Cl₂ to form Ta₂O₅ and NaCl,and then Ta₂O₅ reacted with NaCl and O₂ to form NaTaO₃ or Na₂Ta₄O₁₁ and Cl₂.Owing to the corrosion layer barricading the melts,the surface layer Ta₂O₅ could not be completely and immediately converted into NaTaO₃ or Na₂Ta₄O₁₁.The consumptions of the matrix metal were mainly due to the formation of Ta₂O₅ or TaCl₅ by the reaction between Ta and diffusion O or Cl.Most of the Cl₂ formed during the reaction would escape into the molten salt and the air,and a small amount of residual involving in the corrosion reactions mainly play as a catalyst.Beside of high temperature chemical reactions,acid and basic dissolutions play important roles in the degradation of the oxide layer,which weaken the protection of the surface corrosion layer.In addition,electrochemical corrosion is also an important factor affecting corrosion.The failure of pure Ta sheet at 850?equimolar ratio NaCl-KCl molten salt were mainly the continuous formation and detachment process of surface corrosion layer and the accelerated failure process promoted by the preferential corrosion along the slip bands.The Ar protected LSM treatment formed a fine-grained layer on the surface,which on the one hand had a self-healing effect to protect the under-corroded metal,on the other hand,the fine-grained layer blocked the contact of the corrosion layer with the substrate,enhancing the bonds of the corrosion layer and the matrix,which was weakened by the substrate slip of untreated Ta,so the corrosion resistance in NaCl-KCl molten salt was greatly improved.Unprotected non Ar protected LSM treatment lead to a more serious oxidation on the surface comparing to Ar protected LSM,which might form a thick oxide layer covering the surface and tended to cause the initiation of grain boundary thermal cracks.In the subsequent corrosion process,intergranular corrosion would occur along the grain boundary crack to accelerate the failure.The chemical reaction of LSM treated Ta were similar to that of pure Ta,and the acid-base dissolution of the oxide layer was the main cause of the protective layer's failure.In addition,electrochemical corrosion was also an important factor in the consumption of metal.A 2?m diamond film has prepared on the surface of the Ta substrate by hot filament CVD.The film could slow down the corrosion rate by hindering the corrosive medium to contact with the metal in a short period.After the C film is consumed,the corrosion mechanism was consistent with that of the untreated Ta.Through the surface carburization treatment,a 3?m TaC was formed on Ta surface.On the one hand,TaC hindered the contact between the corrosive medium and the metal.On the other hand,the TaC was relatively stable,and the corrosion rate was slow,only about 0.7?m of TaC was consumed after the 8 h immersion test.Thus TaC film greatly improved the hot corrosion resistance of the Ta material.A Ta-Si coating and a Ta-Si-Al coating were separately prepared on the Ta surface by a halide activation embedding method?HAPC?.The surface compositions of the Ta-Si coating were mostly TaSi₂ and small amount of Ta₅Si₃,and the surface composition of the Ta-Si-Al coating is TaSi₂+Ta₅Si₃+Al₃Ta.The corrosion process of Ta-Si coating in NaCl-KCl molten salt is mainly promoted by oxidation reaction.Since Si has better affinity than Ta and O,corrosion process would lead to the formation of amorphous SiO2film on the corrosion surface.The SiO₂ amorphous film was well preserved,had good thermochemical stability and good bonding property with the uncorroded coating,thus it can effectively protect the underlying coating and the substrate.Similarly,the corrosion of Ta-Si-Al coating in NaCl-KCl molten salt produced an amorphous SiO₂+Al₂O₃ film on the surface of the coating,which could significantly improve the corrosion resistance of Ta to NaCl-KCl molten salt.