Note: Descriptions are shown in the official language in which they were submitted.
METHOD OF APPLYING THERMODIFFUSION ZINC COATING TO
STEEL PIPES
FIELD OF THE TECHNOLOGY
The invention relates to the chemical and thermal treatment of metal
products, in particular it relates to the technology of applying protective
anti-
corrosion coatings and can be used to apply zinc-based thermodiffusion
coatings to parts of different shapes, e.g. oilfield grade steel pipes,
couplings,
fasteners, and other articles.
BACKGROUND OF THE INVENTION
Various embodiments of the technology for application of
thermodiffusion zinc-based coatings to steel products are known in the art,
with
the challenging problem being the ability to provide a uniform dense coating
on articles of complex shape, for example, on the inner surface of the
elongated
pipes or tubes.
Russian Federation Patent RU2500833, MIC: C23C 10/36, published
December 10, 2013, discloses a method for applying an anticorrosion coating
on metal products, including pipes, by their thermodiffusion zinc-plating. The
method involves loading articles/workpieces/parts into a sealed container
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arranged in a muffle furnace; loading a saturating mixture containing zinc
powder and an inert filler; mixing the mixture and articles; filling the
container
with an inert gas and heating to a temperature of 350-450 C for a time
sufficient
to diffuse the zinc vapor onto the surface of the workpieces to form a
protective
s layer of a predetermined amount. In this case, the workpieces are placed
in the
container in a regular manner using tooling with support surfaces, and the
powder saturating mixture contains zinc crystals with a purity of 0.97-0, 99%
needle-shaped with an effective surface area coefficient of 10. The saturating
mixture has a particle size distribution in the range of 3-7 ,m, and its mass
is
1-4% of the mass of the treated parts or 130-140% of the mass of the required
coating on the surface of the treated parts.
The drawback of this method is the complexity of obtaining a uniform
coating layer on the inner side of products such as long pipes.
From Russian Federation Patent No. RU2180018, IPC: C23C 10/28,
C23C 30/00, published February 27, 2002, it is known A Method of
Manufacturing a Powder Mixture For Applying Thermodiffusion Zinc Coating,
comprising activating the powder mixture with ammonium chloride while
allowing the use of micron-sized zinc powder with spherical particles, flake-
like shape or elongated oblong shape.
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The use of a powder mixture of this composition does not provide a
dense uniform coating of sufficient thickness for the anti-corrosion
protection
of oilfield steel pipes.
It is also known to provide a gas reaction medium in a sealed container
for diffusion galvanizing by introducing an activator, which decomposes when
heated to active gases, into the powder mixture. For example, from the
description of the Russian Federation Patent Application No. RU2539888, IPC:
C23C 10/36, published January 27, 2015, there is known a method for
Termodiffusion Galvanizing Of Steel Products, comprising providing a powder
mixture composition for thermal diffusion zinc plating comprising a zinc
powder, an inert filler and an activator, and processing the steel products in
said
composition by heating to a temperature of 420 C. The following is added to
the composition of the powder mixture for thermodiffusion zinc related
processing (mass%): 25-75% zinc powder and 75-25% inert filler, and 0.5-
0.8% carbon tetrachloride from the mass content of zinc powder is added as an
activator. In this method an increase in the saturation capacity of the powder
mixture is achieved by replacing the previously used ammonium chloride with
a more effective activator - tetramine methane. When heated, the activator
decomposes into carbon and chlorine. The carbon then reacts with atmospheric
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oxygen and restores the oxides on the surface of the steel parts. The free
chlorine atoms react with zinc, forming volatile zinc chlorides, which are
then
exchanged into exchange reactions, as a result of which zinc from the volatile
compounds passes into the coating composition on the surface of the parts.
This
s makes it possible to obtain coatings of a given thickness on hard-to-reach
surfaces of parts.
The drawback of this method is the high chemical aggressiveness of free
chlorine, which is released during thermal decomposition of
tetrachloromethane and causes rapid wear of the equipment.
From the description of the Russian Federation useful model RU 27664,
IPC Fl6L 15/08 published February 10,2003, it is known a tubing or drill
string
pipe, comprising a coupling and an adapter at the threaded ends, which is
characterized in that on the threaded surfaces of the sleeve and the adapter a
diffusion powder zinc coating with a thickness of 25+ 5-10 mu m is provided.
The disadvantage of the known technical solution is that the diffusion
powder zinc coating is applied only on the sleeve, and the long tubing pipe of
the protective coating does not have the coating. Given that the zinc-based
coating refers to the tread type, i.e. it protects the base metal from
corrosion by
its own dissolution, this means that when in contact with the steel surface,
zinc
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forms a galvanic couple, wherein the sacrificial anode is a sacrificial anode.
Therefore, the arrangement of the tubing string with alternation of pipes with
a
surface of different materials is undesirable. In the present case, there is
an
alternation of the steel and zinc-plated surface of the pipe, which inevitably
s leads to the fact that accelerated dissolution of zinc will start at the
interface of
dissimilar metals and a corrosion process will develop.
Russian Federation Patent RU2284368, IPC: C23C 10/52, Fl 6L 58/08,
discloses a method for creating a protective diffusion coating at the outer
and
inner surface of the pipe and its threaded sections, as well as a pump
compressor
pipe (tubing), produced by this method. The patent specification refers to oil
assortment pipes, namely, tubing with a diameter of 60-114 mm and casing
pipes with a diameter of 114-508 mm The method includes treating the
threaded portions and adjacent surfaces of the tube by isothermal holding in a
diffusion mixture comprising a metal powder and an inert filler powder, after
which cooling is carried out in the air. This method uses a diffusion mixture
containing a metal powder consisting of a mixture of zinc, copper and
aluminum powders with grain size of 0, 1 ¨ 0.5 mm, with the following content
of components in the diffusion mixture (mass %): Zinc 25-40, copper 0.045-
0.075, aluminum 0.175-0.225, inert filler-the rest. The isothermal holding is
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carried out for 1, 0-3.0 hours at a temperature of 440 10 c to obtain a
protective coating with a thickness of 30-80 m containing the following
components (mass %): iron 6-15, zinc 84.1-93.4, copper 0.4-0.6, aluminum 0,
2-0.3, wherein the coating has a microhardness defined by the reduced
footprint
s of the tetrahedral pyramid in the range of 4500-5250 MPa.
The disadvantage of this method is the inability to obtain a dense uniform
coating on the entire surface of the pipe. This is because the method includes
processing only threaded sections and adjacent surfaces of the pipe, and not
all
of the pipe as a whole. The use of relatively large metal powders with a grain
size of 0.1-0.5 mm can result in an uneven and porous coating, which also
reduces corrosion resistance. Furthermore, the coating technique of this
coating
comprises holding for 1-3 hours at a temperature of 430-450 C. However, this
temperature for carbon steels is critical in terms of austenite transition to
austenite, which occurs already when the temperature exceeds 427 C. Thus,
when applying the thermodiffusion coating in this way, it is possible to
change
the microstructure of the steel of the pipe being treated, which can lead to a
loss
of its strength, which increases the risk of accident when operating the
oilfield
grade pipes.
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The prior art known to the inventors includes Russian Federation Patent
No. RU2738218, IPC: C23C 26/00, published 09.12.2020, which discloses a
method for applying a zinc coating to metallic articles by thermodiffusion
zinc
plating, includes loading the workpieces into a sealed container. The
saturating
zinc-containing mixture is then loaded into the container, the container
cavity
is filled with an inert gas and heated. As a saturating zinc-containing
mixture,
a two-component zinc mixture is loaded, while the first component in the form
of an acicular zinc powder with a size of 3-5 microns is loaded directly into
the
container, and the second component in the form of a spherical zinc powder
with a size of 20-25 microns is loaded into a capsule with walls that are
destroyed at a temperature of 400 20 with walls, the capsule is placed in a
container simultaneously with the products being processed, after which the
zinc flux-zinc chloride is loaded into the container, an inert process gas and
an
activating agent for intensifying the adhesion process are supplied. The
galvanizing process is carried out in two steps, first by heating to a
temperature
of 350-380 C to form a zinc inner layer on the articles by adhesion of the
acicular zinc to the surface of the workpiece, and then after heating to a
temperature of 400 20 C and the destruction of the capsule material, said
ball-shaped zinc powder is released to form an outer coating layer.
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The drawback of this technical solution is the need for each
implementation of the method to produce a new special capsule with the walls
breaking under heating to a temperature of 400 20 C. Thus, in order to
reliably ensure the release of zinc powder from said capsule, it is necessary
to
s heat the container in the furnace with an exposure at a temperature above
420
C, which can affect the change in the microstructure of the steel of the pipes
being treated and reduce their strength.
SUMMARY OF THE INVENTION
The proposed technical solution is aimed at overcoming the drawbacks
of the known prior art, as well as solving the problem of expanding an arsenal
of technology, allowing the protective thermodiffusion zinc-based coatings to
be applied to the long-length steel pipes usable in the oilfield industry, to
completely cover their outer and inner surfaces as well as the threaded
sections
of the pipes.
The technical result achieved by the present invention is to reduce the
duration of pipe exposure in the temperature range of thermal diffusion
galvanizing when producing coatings of a given thickness with improved
corrosion resistance properties, to improve uniformity and density of the
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coating on the entire surface of the pipe, as well as to reduce energy
consumption and to increase productivity while ensuring the high strength of
pipes processed by the method of the invention.
In order to resolve the above-noted problems, the method of the
s invention proposes applying the thermodiffusion zinc-based coating on
steel
pipes including: loading pipes into a container; loading a saturating mixture
containing a two-component zinc powder, an activating agent and flux;
hermetic closure of the container, its vacuum evacuation; filling of the
container cavity with non-oxidizing gas; heating and holding at a
predetermined temperature; subsequent cooling of the container and extraction
of pipes. In the method, the first component of the two-component zinc powder
having needle-shaped particles with a size of 3-8 microns is loaded into an
internal cavity of the pipes, and the second component of the two-component
zinc powder is loaded directly into the container; the exposure is carried out
at
a temperature of 300-425 C, wherein one or more tertiary amines are
introduced as fluxes into the saturating mixture, and an activating agent is a
filler 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 components (mass %):
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Flux 0.1-1.0
25-45 filler
a two-component zinc powder¨the rest.
According to the present invention, the method of applying the
thermodiffusion zinc-based coating is carried out primarily for coating steel
pipes or tubing having the linear or drilling steel pipes up to 8 -12 m in
length,
wherein prior to loading into the container the outer and inner surfaces of
the
pipes are machined.
To obtain a uniform and quality coating layer, before loading into the
container the pipes are assembled into the tooling having support surfaces to
allow for regular placement of the pipes, wherein the pipes are placed into
the
container together with the tooling.
According to the present method, the flux composition may further
comprise one or more components selected from the group comprising 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.
CA 03236537 2024- 4- 26
In the method of the invention a non-oxidizing gas is preferably a gas
selected from the group consisting of argon, nitrogen or carbon dioxide,
which.
This gas, after the evacuation operation fills the container at a pressure
within
the range of 0,1-8 atm.
s After the pipes are removed from the container, the thermodiffusion
coating is passivated by applying a polymer layer. Passivation makes it
possible
to realize a synergistic effect of the protection in case of damage to the
polymer
layer. In the case of damage to the polymer layer, zinc from the iron-zinc
intermetallic compound forms sparingly soluble substances preventing the
development of under film corrosion at the iron-zinc intermetallide-polymer
layer interface.
Further, the problem of formation of asphalt-resin-paraffin deposits
(ARPD) on the inner surface is known to the oil pipes. Passivation of the
inner
surface of the pipes having a thermodiffusion zinc-based coating by applying a
smooth polymer layer reduces the mass of the ARPD by 30-40%.
The passivation of the zinc-based diffusion coating may be performed
on both the inner and outer surfaces of the zinc related processing of pipes
or
tubings. However, it is preferred to perform the passivation on the inner
surfaces, which are exposed to the greatest impact of the corrosive
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environment. The passivation procedure is performed by applying a layer of
the polymer composition upon the subsequent hot curing. Epoxy paints or
epoxy-novolak phenolic two-component polymer compositions are used to
apply the polymer layer.
As a result of implementing the present method, the pump-compressor
pipes are obtained as a finished product. The outer and inner surfaces of such
pipes are provided with a thermodiffusion zinc-based coating having a
thickness of 20-140 microns (preferably 40-70 microns) with a microhardness
in the range of 2500-3800 Mpa. The coating includes intermetallic compounds
of iron and zinc of variable composition from FenZn to Fe4Zn, forming layers
of gamma phase (y-phase) and delta phase (6-phase), providing corrosion
resistance of the coating.
As a result of implementing of the method of the invention, a
thermodiffusion zinc-based coating is produced having a predetermined
thickness with improved corrosion resistance properties, uniformity and
density of the coating Such coating can be obtained on the steel pump or
compressor pipes and on other steel pipes for the oil industry on the entire
outer
and inner surfaces of the pipes. The length of such pipes is typically within
the
range of 8-12 m, with the inner diameter being no less than 45 mm and no more
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than 1000 mm. These parameters are typically determined by the capabilities
of existing equipment.
To further improve corrosion resistance and durability in the severe
operating conditions, the pipes are further provided with a passivating layer
of
s the polymer coating, which is received as a result of hot curing of epoxy
or
epoxy-novolak phenolic two-component polymer compositions. The
passivation layer is placed on top of, or in addition to the thermodiffusion
zinc-
based coating, preferably on the inner surfaces of the pipes.
The pump-compressor pipes for connecting to the column can be
provided with threaded portions located at the ends of the pipe, wherein the
thickness of the thermodiffusion zinc-based coating on the threaded surfaces
of
the threaded portions of the pipe is preferably 20-25 Am. The latter is
determined by the requirements for the parts threaded connections.
DETAILED DESCRIPTION OF THE INVENTION
The invention is illustrated by Examples 1-8 presented in Table 1 and
Figure 1 of the drawing which illustrates the structure of the obtained
coating.
Example No. 1
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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
s 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 gm, 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.
A second saturating mixture component containing a zinc powder having
spherical particles of 8-25 gm 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
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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
s 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.
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 (y phase) and a wider layer
of dense delta phase (6 phase) with the thickness approximately 60 inn/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
CA 03236537 2024- 4- 26
FIG. 1. The structure of the coating consists of an intermetallic compound
based on the 6-phase containing 7-11.5% Fe with the remaining Zn, whereas
an inner thin layer of y-phase contains 28% Fe and the rest is Zn.
To further increase the operational resistance on the inner surface of the
s 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.
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.
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
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different temperature and exposure duration of the container with the products
being treated in the furnace.
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
s zinc-based coating of 300-425 c. in the production of coatings of the
predetermined thickness of 60 pm (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.
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
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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 6-phase.
Example 8 in Table 1 corresponds to the closes prior art known to the
s 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
iim
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.
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
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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
s
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.
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.
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Ex Temp. Gas Pres- Filler Tertiaryamine Additional Time
Pipe Thread
sure, Flux Flux Exposure
coating coating
= C atm
Component /hours thickness thickness
/gm
/gm
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)
340- Carbon 2 Copper Triethanolamine Tetrabutyla 3
62 25
360 dioxide alloys mmonium
stearate
(ammoniu
m salt of
fatty acid)
6 400- Carbon 0.1 wollastonite Urotropine Lithium 2 40
20
425 dioxide chloride
7 320- 1,2 Soot Triethanolamine Aluminum 6 125
20
340 nitrogen chloride
8 400- argon 1.0 Silica Prototype: - 3,5 60
25
420 zinc chloride
TABLE 1
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