Study On Ni-Mo Alloy Coating Prepared By HVOF And Its Corrosion Behavior In Hydrochloric Acid Environment

With the development of petroleum,chemical,medicine,medicine,clean energy and other industries,the media environment of related equipment become more complex.Therefore,the corrosion resistance of engineering materials has been put forward with higher requirements.In industrial production,hydrochloric acid is usually used as a reaction medium.Hydrochloric acid has strong reducing,with temperature,hydrochloric acid concentration,and oxidative ion concentration,such as(Fe³⁺,Cu²⁺),the corrosive increases.However,it is difficult to achieve excellent corrosion resistance using ordinary metals and alloys.The screw rotor is a key component in a dry screw vacuum pump,which is easily etched or rusted under harsh conditions such as hydrochloric acid,so that a suitable coating is an effective method for preventing the failure of the screw rotor.There are many types of nickel-based alloy materials,with excellent in performance,which have been widely used in the engineering field in the reducing medium environment.Thus,they are the primary choices for screw surface coating materials.（1）The new Ni-Mo alloy coating is prepared by supersonic flame spraying process,and the coating surface roughness and the XRD method,the coating phase can be analyzed and determined by the mechanical performance tester,Vickers microhardness instrument and Princeton electrochemical workstation.Finally,the experiment shows that the kerosene quantity,spraying distance and powder delivery rate will affect the coating properties and cause the change of powder heat and kinetic energy.When the spraying distance is relatively close,the powder particles are not completely melt,resulting in irregular small gaps in the coating.With the increasing distance of the spraying flame,the powder particles can be fully heated and melted in the process of flame spraying,and the binding strength and microhardness of the coating are improved.When the amount of kerosene is 28 L/h,the coating obtains the best tissue and mechanical properties,but when the spraying distance is 350mm,the coating powder obtains the maximum heat and kinetic energy.When the powder delivery rate is 80g/min,the binding performance of the coating reaches the optimum,but the porosity is not optimal,but consider 80g/min.（2）The corrosion rate of Ni-Mo coating increases gradually with the increase of hydrochloric acid concentration,temperature,and Fe³⁺concentration.Under the action of the gradually increasing concentration of hydrochloric acid solution,due to the low salt concentration of H⁺and Cl^- the coating is easy to form an oxide film,and the rate of point corrosion reaction on the coating surface is relatively low,but as the hydrochloric acid concentration continues to increase,the rate of coating point corrosion begins to decline.However,after the addition of trace excess Fe³⁺solution,the corrosion reaction rate of Ni-Mo coating can increase significantly and rapidly,accompanied by serious point corrosion,and the corrosion reaction rate remains basically fixed with the concentration of free Fe³⁺ions in the solution.（3）By annealing for continuous high temperature for many times,After the comparative analysis of the study and work results showed that,Annealing thermal insulation heat treatment can also significantly improve the coating binding performance and binding strength of coating materials,When the heating and cooling temperature reaches and exceeds about 500℃,The maximum coating binding strength of the coating material is about 70Mpa,And can make the porosity of the coating material can be greatly reduced,However,along with the continuous increase of the working temperature of the heat treatment,The coating is prone to simultaneous segregation between elements and the diffusion of a large number of free Fe element ions from the matrix to the coating surface,however,since this is thus so that the surface of the metal coating in high medium temperature zone（600～800℃）will also be in the inner surface of the grain boundary layer and the internal surface of the metal grain layer at the same time as the internal temperature.A large amount of Mo element ionic metal ions and a metal organometallic compound such asβphase Ni₃Mo,in addition,the elevated temperature,and also precipitates a large amount of a metal carbide containing a large amount of Mo element（Mo₂C）.In addition to greatly reducing the thermoplasticity and toughness of the coating itself,it can also indirectly have some adverse effects on its high temperature and corrosion resistance.