Method of applying thermodiffusion zinc coating to steel pipes

Abstract

The invention relates to thermochemical treatment of metal products, in particular relates to the technology of applying protective anti-corrosion coatings, which may be used for applying zinc-based thermodiffusion coatings (TDC coatings) on parts of different shapes, preferably on steel pipes of oilfield assortment, couplings, as well as fasteners and other products. A method of the invention relates to application of a thermodiffusion zinc-based coating to steel pipes, and comprises the steps of loading the pipes, a saturating mixture containing two-component zinc powder, activator and flux into the container; hermetically closing the container; vacuuming; filling the container cavity with non-oxidizing gas; heating and maintaining the container at a predetermined temperature; subsequent cooling the container and removing the pipes, characterized in that the first component of the two-component zinc powder having needle-shaped particles of 3-8 microns in size is loaded into the internal cavity of the pipes, and a second component of two-component zinc powder having spherical particles of 8-25 microns in size is loaded directly into container, and exposure being carried out at a temperature of 300-425° C., while a flux of one or more tertiary amines is introduced into the saturating mixture in an amount of 0.1-1.0 mas %; a filler containing one or more components selected from the group consisting of silica, wollastonite, carbon black, aluminum oxide and copper alloys, in the following ratio components in mass %: flux 0.1-1.0; filler 25-45; two-component powder zinc-the rest. Also claimed is a steel pipe having a hollow body with thermodiffusion zinc-based coating, characterized in that the coating is obtained using the method described above. The technical result of the invention is achieved by reducing the duration of exposure of the pipes in the temperature range of the thermodiffusion process when obtaining the coatings of a given thickness with improved corrosion resistance durability, uniformity and density of the coating over the entire surface of the pipe, as well as reducing the energy costs and increasing productivity, while ensuring high strength of pipes processed by the present method.

1 . A method of applying a thermodiffusion zinc-based coating to steel pipes having inner cavities, comprising the following steps: loading the pipes and a saturating mixture into a container having a cavity, said saturating mixture comprising a two-component zinc powder, an activating agent and a flux;
 hermetically closing the container; vacuumizing; filling the container cavity with a non-oxidizing gas; heating and maintaining the container cavity at a predetermined temperature; subsequently cooling the container cavity and extracting the pipes; said method further comprising:   
       a first component of a two-component zinc powder having needle shaped particles with a size of 3-8 μm being loaded into the inner cavities of the pipes; and 
       a second component of the two-component zinc powder having spherical particles with a size of 8-25 μm is loaded directly into the cavity of the container, and exposure being carried out at a temperature of 300-425° C.; 
       wherein one or more tertiary amines in an amount of 0.1-1.0 mass??% are introduced into the saturating mixture as a flux; as an activating agent as filler is used comprising one or more components selected from the group consisting of silica, wollastonite, carbon black, aluminum oxide, and copper alloys, with the following ratio of the components by mass %: 
       0.1-1.0 flux; 
       25-45 filler and 
       two-component zinc powder-the rest. 
     
     
         2 . The method according to  claim 1 , wherein prior to loading into the container the pipes are machined at outer and inner surfaces thereof. 
     
     
         3 . The method according to  claim 1 , wherein prior to loading into the container the pipes are assembled into tooling, and the pipes together with the tooling are placed in the container. 
     
     
         4 . The method according to  claim 1 , wherein the flux composition further comprises one or more components selected from the group consisting of urea or derivatives thereof, piperazine or derivatives thereof, ammonium salts of fatty acids, chlorides, fluorides, bromides, iodides, sulfates and sulfates of fatty acids, as well as aluminum and lithium chlorides. 
     
     
         5 . The method according to  claim 1 , wherein the non-oxidizing gas is a gas selected from the group consisting of argon, nitrogen or carbon dioxide, which fills the container at a pressure of 0.1 to 8 atm. 
     
     
         6 . The method according to  claim 1 , wherein after the pipes are removed from the container, passivation of the thermo-diffusion zinc-based coating is performed by applying a polymer layer. 
     
     
         7 . The method according to  claim 6 , wherein the passivation of the thermodiffusion zinc-based coating on the inner surface of the pipes is performed by applying a polymer layer resulted from hot curing of epoxy or epoxy-novolak phenolic compounds, including two-component compounds. 
     
     
         8 . The method according to  claim 6 , wherein passivation of the thermo-diffusion zinc coating on the outer and inner surfaces of the pipes is performed by applying a polymer layer, the resulting hot curing of epoxy or epoxy-novolak phenolic compounds, including two-component compounds. 
     
     
         9 . A steel pipe comprising a hollow body with a thermodiffusion zinc-based coating, wherein the coating is obtained by the method of  claim 1 . 
     
     
         10 . The pipe according to  claim 9 , wherein said pipe is made in the form of a pump compressor pipe, with a body having a length of 8-12 m, an inner diameter of at least 45 mm and comprises the thermodiffusion zinc-based coating on the outer and inner surfaces having thickness of 20-140 μm, preferably 40-70 μm, with a microhardness of 2500-3800 MPa, said coating comprises iron and zinc intermetallic compounds. 
     
     
         11 . The pipe according to  claim 10 , further comprising a passivation layer of a polymer coating resulted from the hot curing of epoxy or epoxy-novolak phenolic two-component polymer compositions, the passivation layer located on the thermodiffusion zinc-based coating, preferably on the inner surface of the pipe. 
     
     
         12 . The pipe according to  claim 10 , wherein the body of the pipe is provided with threaded portions located at ends thereof, and wherein the thickness of the thermodiffusion zinc-based coating on the threaded surfaces of the threaded portions of the pipe body is preferably within a range of 20-25 μm.