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.

Claims

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 ?m 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 ?m 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 ?m, preferably 40-70 ?m, 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 ?m.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0032] The invention is illustrated by Examples 1-8 presented in Table 1 and FIG. 1 of the drawing which illustrates the structure of the obtained coating.

Example No. 1

[0033] The method of the invention of applying a thermodiffusion zinc-based coating was carried out by providing the coating to the steel pipes having the length of 8.5 meters and the diameter of 60 mm. The batch of pipes in an amount of 50 pieces was initially subjected to mechanical (abrasive) processing at the outer and inner surfaces thereof. Then, into the inner cavity of each pipe the first component of the saturating mixture is loaded in the form of a zinc powder with needle-shaped particles 3-8 ?m, mixed with a filler in the form of carbon black (carbon black) in an amount of 25 mass %. The pipes with the applied first saturating mixture component were assembled into tooling formed with the bearing surfaces. The pipes are fixed in a predetermined position to prevent direct contact with each other, as well as to prevent the movement of the pipes relative to each other when the container is moving. In the resulting assembly, the minimum distance between the tube surfaces to be treated was 3-5 mm. The pipes were loaded into the container together with the tooling.

[0034] A second saturating mixture component containing a zinc powder having spherical particles of 8-25 ?m in size mixed with a filler in the form of carbon black in an amount of 25% wt. is then loaded directly into the container. As a flux, urotropine was introduced into the saturating mixture in an amount of no more than 1% wt. of the composition of the saturating mixture. Urotropine was a tertiary amine. After the flux was introduced, the container was closed, the lid was sealed, and the vacuum was established in the cavity of the container. Non-oxidizing shielding gas was then injected into the cavity of the container at a pressure of 4 atm. The gas was inert relative to the components of the saturating mixture. Nitrogen was selected as the non-oxidizing gas. The container was then placed into the oven and heated to a temperature of 380? C. The heated container was held in the oven at the temperature range of 380-400? C. for 3 hours.

[0035] The container was then removed from the oven, cooled and opened. The industrial vacuum cleaner was used to remove zinc-saturating mixture residues from the container, after which step the tubes were removed. The quality of the protective coating obtained on the outer and inner surfaces of the pipes was controlled. The resulting zinc-based coating consisted of iron-zinc intermetallic compounds forming a thin layer of gamma phase (? phase) and a wider layer of dense delta phase (? phase) with the thickness approximately 60 ?m/microns and the microhardness of the coating surface of 3800 MPa, (HV400) with satisfactory continuity and density without any discontinuities or pores. The coating was formed having a uniform thickness along the entire length of the pipe at the outer and inner surfaces thereof. The resulting coating is shown in FIG. 1. The structure of the coating consists of an intermetallic compound based on the ?-phase containing 7-11.5% Fe with the remaining Zn, whereas an inner thin layer of ?-phase contains 28% Fe and the rest is Zn.

[0036] To further increase the operational resistance on the inner surface of the pipe cavity, the first two turns of the thread and the chamfer of the pipe have been coated with a polymer layer passivating the thermodiffusion zinc-based coating. The passivation layer of the polymer coating is produced by the hot curing of the epoxy-novolak phenolic two-component polymer composition.

[0037] To prepare the epoxy-novolak phenolic polymer composition, a paint material from a Majorpack series of paint in red glossy or white glossy paint was used as a base. As the second component of the two-component polymer composition was used a hardener for the paint material of the Majorpack series of paint: red glossy or white glossy is used, with a ratio of the base to the hardener in the range from 4:1 to 10:1.

[0038] The implementation mode of the method of the invention according to Example 1 is shown in Table 1. In addition, Table 1 also presents details of Examples 2-7 of the method, which include the same sequence of steps as in Example 1. The modes of embodiments of Examples 2-7 are characterized by different temperature and exposure duration of the container with the products being treated in the furnace.

[0039] From the data shown in Table 1, it can be seen that for a reduction in the exposure duration of the pipes in the temperature range of the thermodiffusion zinc-based coating of 300-425? C. in the production of coatings of the predetermined thickness of 60 ?m (with high quality properties), it was observed the effect of sharing a new flux-a tertiary amine and a filler selected from the group consisting of silica, wollastonite, carbon black, aluminum oxide, and copper alloys. Under these conditions, diffusion of the zinc vapor provided a uniform, tight coating of a predetermined thickness on both the inner and outer surfaces of the steel pipes, including the threaded portions. Note that in Example 2, the coating was applied to a batch of pipes of minimum diameter (the inner diameter of the pipes was 45 mm with a pipe length of up to 12 meters). Further, in Examples 3-7, additional tertiary amine flux was added urea, piperazine and others in an amount of 0.1-0.3% wt., which resulted in some slight increase in coating rate.

[0040] The intensification of the process of diffusion saturation of the surface of steel pipes in the present method of applying zinc-based coating by the gas thermodiffusion is achieved by replacing traditional activators with complexes of inorganic and organic substances, which at the operating temperatures decompose to activate zinc atoms and contribute to an increase in the saturation rate of the surface of the articles with a corrosion-resistant ?-phase.

[0041] Example 8 in Table 1 corresponds to the closes prior art known to the inventors, i.e. according to RU2738218. Comparison with this reference shows that the dwell time required to obtain a coating of a given thickness of 60 ?m was reduced from 3.5 hours in the reference to 3 hours in the present method. This means that the dwell when applying the thermodiffusion zinc-based coating was reduced by 14%, which corresponds to an increase in the productivity of the present method and a reduction in energy costs. This is because the duration of operation of the electric heaters of the furnace required for heating the container and holding at a selected temperature was reduced.

[0042] Furthermore, the advantage of the coating technology of applying the thermodiffusion zinc-based coating according to the present method with respect to RU2738218 and other known analogs is the possibility of forming a thermodiffusion coating at lower temperatures (below 425? C.). As shown in Table 1, a preferred temperature range is lowered to effect soaking while applying the thermodiffusion zinc-based coating to the pipes, which improves the processing quality of steel pipes. This is because the lower temperature of the application of the thermodiffusion coating does not guarantee the weakening of the high carbon steels when the tubes are directed for application of the coating after the heat treatment. As is known, the heating and holding temperature at the level above 427? C. for carbon steels is critical. This is because it corresponds to the transition of perlite to austenite, which entails a change in the microstructure of the steel and a decrease in strength. Thus, the use of the present method ensures that the strength group of the oilfield grade steel pipe is maintained after application of the thermodiffusion zinc-based coating thereto.

[0043] The corrosion resistance test of the pipes prepared according to Example 1 shows an increase in their corrosion resistance in a medium containing hydrogen sulfide and carbon dioxide at a pressure of up to 2 atmospheres and a temperature of 80? C. Exposure under these conditions shows that the resistance of the coated tube was 1500 days without the corrosive damage.

TABLE-US-00001 TABLE 1 Pipe Thread Additional Time coating coating Temp. Pressure, Tertiaryamine Flux Exposure/ thick- thick- Ex. ? C. Gas atm Filler Flux Component hours ness/?m ness/?m 1 380- nitrogen 4 soot Urotropine 3 60 20 400 2 340- Carbon 2 Aluminum Triethanolamine 3 60 25 360 dioxide oxide 3 360- argon 1.2 Carbon Triethanolamine Uria 3 62 20 380 charcoal (primary amine) 4 300- nitrogen 8 Aluminum Urotropine Piperazine 3 62 23 320 oxide (secondary amine) 5 340- Carbon 2 Copper Triethanolamine Tetrabutyl- 3 62 25 360 dioxide alloys ammonium stearate (ammonium salt of fatty acid) 6 400- Carbon 0.1 wollastonite Urotropine Lithium 2 40 20 425 dioxide chloride 7 320- nitrogen 1.2 Soot Triethanolamine Aluminum 6 125 20 340 chloride 8 400- argon 1.0 Silica Prototype: 3.5 60 25 420 zinc chloride