Fabrication Of Corrosion Resistance Carbide Layer On Stainless Steel In Molten Salts And Its Formation Mechanism

Stainless steel(SS)is widely used as structural material,but its applications are largely restricted by the corrosion behavior in high-temperature and humid environment,which will decrease the mechanical properties and the service life.Numerous strategies were adopted to control the corrosion of SS to overcome such drawbacks.Among of which,coating a corrosion resistance layer on SS is a promising option.Polymer materials,inert metals,inorganic materials and ceramics are the common materials used for functional coating.Due to its properties of temperature resistance,anti-corrosion and attractive ensemble physico-chemical properties,such as abrasion resistance,conductivity and catalytic activity,inorganic materials and ceramics,especially carbon and carbides have gained extensive attention in the past decades.The awkward is that carbon and carbides coating are hardly prepared in the aqueous solution due to its chemical inertness.It is often obtained by physical or chemical vapor deposition methods.However,vapor deposition usually requires high-cost equipment,extreme conditions and it is difficult to form coating on complex structural matrix.Moreover,the low binding strength between layer and substrate is a common problem.Therefore,there is an urgent need to develop new methods that suitable for fabricating SS with carbon and carbide inorganic materials film.In this study,we fabricate a carbon and carbide layer on 316 L SS via electrochemical deposition method.During the process,it takes advantage of the electrochemical property of the molten salts and the feasible solubility of inorganic materials and metals in it.The in-situ reaction of carbon(C)and calcium metal(Ca)as well as titanium metal(Ti)and high-valent titanium in molten salts to prepare the electrolyte with stable physical and chemical properties for electroplating.A dense and anti-corrosion carbon layer is fabricated on SS by optimizing parameters of deposition.Subsequently,the transformation from C to titanium carbide(TiC)is achieved by electrochemical deposition of titanium on carbon layer in electrolyte.Theoretical calculation is used to stimulate the mechanism of the transformation process.The measurements of corrosion indicate that the anti-corrosion performance of SS coated with C or TiC layer has been significantly improved.Specifically,the main contents are as follows:(1)Fabricate C layer on SS by using carbon powder as raw material in FLiNaK(LiFNa F-KF,46.5-11.5-42mol%)molten salts.The results demonstrate that calcium carbide is synthesized by the dissolved calcium metal and carbon powder in molten salts.The synthesized C22-is used to supplement the consumed Ca C2 by deposition and maintain the concentration of C22-constant during the electroplating process.Thereby,the influence of physical and chemical properties fluctuation of electrolyte is significantly suppressed,and a dense carbon layer was obtained.The effects of deposition potential,temperature,time and composition of molten salts on formation were carefully investigated.By optimizing the conditions,a C film with compact morphology,controllable thickness and good combination to the matrix was obtained.The results of polarization tests indicate that C layer significantly improves the anti-corrosion performance of SS matrix.Compared to bare SS,the density of corrosion current and corrosive rate were dramatically suppressed by a factor of more than 7.9 and 16.8 respectively for C coated 316 L sample.(2)Fabricate TiC layer by “two-step deposition”.The C layer is transformed to TiC layer by depositing Ti on C layer prepared on SS in FLiNaK molten salts.Taking advantage of the reaction between Ti4+ and Ti in FLiNaK molten salts,a stable molten salts electrolyte containing low-valence Ti2+ ions is firstly formed.The reactions of Ti2+ and effects of experimental conditions on quality of the layer were investigated,such as the applied potential,the density of current,the temperature of the deposition and the time of deposition.The results indicate that the reduction of Ti2+ to Ti is a one-step reaction.The process of the transformation from C to TiC is containing two steps,namely,the deposition of Ti on C layer and the interdiffusion of C-Ti inter-phases,in which the diffusion reaction is the decision step.The deposition rate is determined by the applied potential and the density of current,while diffusion rate is influenced by the temperature.Balancing the rates of deposition and the deposition time is beneficial to the transformation process.High-quality TiC layer was fabricated by optimizing the deposition conditions.The results of polarization curve manifest that the anti-corrosion performance of SS matrix is improved by TiC layer.(3)First-principle calculation is used to investigate the adsorption and diffusion behaviors of Ti and C atoms,and simulate the formation process of TiC layer.The results of binding energy and electronic properties indicate that Ti atom is combined stably to H site on surface by forming a Ti-C bond based on the structure characteristics of C layer,and then grow into a Ti layer.Subsequently,the interdiffusion takes place motivated by repulsion of atoms,concentration gradient,Gibbs free energy,etc.During the initial diffusion process,C atom with smaller radius and lower barrier than those of Ti atom is the main reacted composition.The lattice deformation and defects caused by atom diffusion contribute to the transformation from C and Ti to TiC lattice,until the TiC phase is fully prepared.It is speculated from calculation that the formation of TiC during the electroplating is the result of lattice reconstruction reaction caused by atom diffusion after deposition.