U.S. patent application number 16/471032 was filed with the patent office on 2019-12-26 for chemical wall-treatment method that reduces the formation of coke.
The applicant listed for this patent is CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS), INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON (INSA LYON), TOTAL RAFFINAGE CHIMIE, UNIVERSITE CLAUDE BERNARD LYON 1. Invention is credited to Christel Augustin, Sophie Cazottes, Francois Dupoiron, Claude Duret Thual, Philippe Steyer, Nicolas Vache.
Application Number | 20190390362 16/471032 |
Document ID | / |
Family ID | 58401766 |
Filed Date | 2019-12-26 |
![](/patent/app/20190390362/US20190390362A1-20191226-D00000.png)
![](/patent/app/20190390362/US20190390362A1-20191226-D00001.png)
![](/patent/app/20190390362/US20190390362A1-20191226-D00002.png)
![](/patent/app/20190390362/US20190390362A1-20191226-D00003.png)
United States Patent
Application |
20190390362 |
Kind Code |
A1 |
Augustin; Christel ; et
al. |
December 26, 2019 |
Chemical Wall-Treatment Method That Reduces the Formation of
Coke
Abstract
The invention relates to a process for the treatment of a wall
made of Fe--Ni--Cr metal alloy of an industrial reactor which
reduces the formation of coke on the said wall when it is subjected
to operational conditions favourable to coking, the metal alloy
comprising, within its structure, carbides, some of which show on
the surface. The process comprises a chemical surface treatment
stage, during which at least a part of the carbides initially
present in the alloy is dissolved by electrolytic decomposition,
under conditions suitable for dissolving at least a part of these
carbides, in particular chromium carbides.
Inventors: |
Augustin; Christel; (Le
Havre, FR) ; Cazottes; Sophie; (Bourgoin Jallieu,
FR) ; Vache; Nicolas; (Lyon, FR) ; Steyer;
Philippe; (Four, FR) ; Duret Thual; Claude;
(Genilac, FR) ; Dupoiron; Francois; (Francheville,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOTAL RAFFINAGE CHIMIE
INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON (INSA LYON)
UNIVERSITE CLAUDE BERNARD LYON 1
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) |
Courbevoie
Villeurbanne
Villeurbanne
Paris |
|
FR
FR
FR
FR |
|
|
Family ID: |
58401766 |
Appl. No.: |
16/471032 |
Filed: |
December 19, 2017 |
PCT Filed: |
December 19, 2017 |
PCT NO: |
PCT/EP2017/083566 |
371 Date: |
June 19, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25F 3/06 20130101; C23C
8/02 20130101; B24C 1/04 20130101 |
International
Class: |
C25F 3/06 20060101
C25F003/06; C23C 8/02 20060101 C23C008/02; B24C 1/04 20060101
B24C001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2016 |
FR |
1662911 |
Claims
1.-15. (canceled)
16. A process for the treatment of a wall made of Fe--Ni--Cr metal
alloy of an industrial reactor which reduces the formation of coke
on the said wall when it is subjected to operational conditions
favourable to coking, the metal alloy comprising, within its
structure, carbides, some of which can show on the surface, the
process comprising: a chemical surface treatment stage, during
which at least a part of the carbides initially present in the
alloy, in particular at the surface, is removed by electrolytic
dissolution.
17. The process according to claim 16, in which the chemical
treatment stage is carried out under conditions suitable for
dissolving at least a part of the carbides over a depth of at least
10 .mu.m.
18. The process according to claim 16, in which the chemical
treatment stage is carried out under conditions suitable for
dissolving at least a part of the carbides chosen from chromium
carbides, niobium carbide, when the alloy contains niobium, and
carbonitrides, when the alloy contains nitrogen.
19. The process according to claim 16, in which the chemical
treatment stage is carried out in an electrolysis cell comprising
an aqueous solution chosen from a solution of an alkali metal
hydroxide and a sulfuric acid solution.
20. The process according to claim 16, comprising at least one
other chemical treatment stage, during which at least a part of the
carbides initially present in the alloy, in particular at the
surface, and not dissolved during a preceding chemical treatment
stage is removed by electrolytic dissolution.
21. The process according to claim 16, in which: one chemical
treatment stage is a stage of electrolytic dissolution of chromium
carbides, another chemical treatment stage is a stage of
electrolytic dissolution of niobium carbides, the metal alloy
containing niobium.
22. The process according to claim 21, in which the electrochemical
dissolution of the chromium carbides is carried out and then the
electrochemical dissolution of the niobium carbides is carried
out.
23. The process according to claim 16, characterized in that it
additionally comprises, after the chemical surface treatment stage:
a mechanical stage of impact surface treatment, during which the
surface of the wall which has been subjected to the chemical
treatment is hammered by projection of particles under conditions
suitable for obtaining a permanent plastic deformation of the
surface.
24. The process according to claim 23, in which the particles used
during the mechanical treatment stage are chosen from aluminium
oxide particles, metal particles, beads made of material which is
inert under the said operational conditions, or nesosilicate
particles.
25. The process according to claim 23, in which the particles used
during the mechanical treatment stage have a mean diameter of 100
to 500 .mu.m.
26. The process according to claim 23, in which, during the
mechanical treatment stage, the particles are projected by a
gaseous fluid under a pressure of 200 to 400 kPa.
27. The process according to claim 16, in which the metal alloy
contains at least 5% by weight of iron, at least 18% by weight of
chromium, at least 25% by weight of nickel and at least 0.05% by
weight of carbon.
28. The process according to claim 16, characterized in that it
comprises, after the chemical treatment stage and optionally after
the mechanical surface treatment stage, when it is carried out: an
oxidation stage carried out under conditions suitable for forming a
layer of oxide(s) on the surface which has been subjected to the
chemical treatment and optionally the mechanical treatment, in
particular a layer containing one or more chromium oxides.
29. A process for the treatment of hydrocarbons under conditions
capable of bringing about the formation of coke, characterized in
that the hydrocarbons are brought into contact with a surface of a
wall made of Fe--Ni--Cr metal alloy, the said surface of the metal
wall being pretreated by a treatment process according to claim 15
so as to reduce the formation of a coke deposit.
30. A process for the treatment of hydrocarbons according to claim
29, in which the hydrocarbons are brought into contact with the
surface of the metal wall at a temperature of 800 to 900.degree. C.
Description
TECHNICAL FIELD
[0001] The invention relates to a process for the surface treatment
of a metal wall which has the effect of reducing the formation of
coke at the surface of this wall. More specifically, the invention
relates to a process for the surface removal of carbides from a
wall made of metal alloy, in particular by chemical treatment. The
invention also relates to the use of a metal wall treated by the
treatment process in a process for the treatment of
hydrocarbons.
STATE OF THE ART
[0002] The walls of the reactors of some units of the petrochemical
or chemical industry are sometimes subjected to very severe
operating conditions which can result in phenomena of coking. For
example, the manufacture of alkenes, monomers valued in the polymer
industry, is obtained by cracking oil-derived hydrocarbons at
temperatures of the order of 800 to 900.degree. C. In this type of
process, a mixture of hydrocarbons and steam is circulated at high
speed in reactors, generally formed of metal tubes, often made of
alloys rich in nickel and in chromium. The reactors are thus
subjected to high temperatures and to complex aggressive
atmospheres and the formation of carbon (coke) is observed at the
surface of the walls of the tubes, this formation being catalysed
by the iron and the nickel present in the metal alloy forming the
walls. This deposition of coke can result in a fouling of the
tubes, causing a head loss, a deterioration in the conductivity of
the walls and a decrease in the yields. It is then necessary to
shut down the unit in order to remove the coke formed, an operation
which is harmful to the productivity of the unit.
[0003] This is the reason why numerous research studies have been
carried out by manufacturers in order to limit the formation of
coke.
[0004] These research studies have made possible the implementation
of solutions comprising in particular: [0005] the formation of a
protective layer at the surface, in particular a layer of oxides,
[0006] the use of coatings which limit the formation of coking,
indeed which even increase the yields, [0007] the addition of
sulfur-comprising entities to the feedstock, which results in the
formation of metal sulfides having a protective role at the
surface, [0008] the creation of a specific design of the
reactors.
[0009] The formation of a protective oxide layer can in particular
be obtained by the use of appropriate alloys, for example rich in
chromium or in aluminium, or by oxidation pretreatments.
[0010] Despite the existence of these solutions, there still exists
a need for a simple and inexpensive treatment which makes it
possible to reduce the formation of coke.
SUMMARY OF THE INVENTION
[0011] To this end, there is provided a process for the treatment
of a wall made of Fe--Ni--Cr metal alloy of an industrial reactor
which reduces the formation of coke on the said surface when it is
subjected to operational conditions favourable to coking, the metal
alloy comprising in particular, within its structure, carbides,
some of which can show on the surface. For example, the metal alloy
contains at least 5% by weight of iron, at least 18% by weight of
chromium, at least 25% by weight of nickel and at least 0.05% by
weight of carbon.
[0012] The term "operational conditions favourable to coking" is
understood to mean conditions liable to bring about the formation
of coke on the surface. The parameters influencing the coking
comprise, for example, the temperature, the nature of the liquid or
gaseous fluids circulating inside the reactor and in contact with
the surface, the flow conditions of the fluids (turbulences).
[0013] According to the invention, the process comprises a chemical
surface treatment stage, during which at least a part of the
carbides initially present in the alloy, in particular at the
surface, is removed by electrolytic dissolution.
[0014] Surprisingly, while the formation of coke is essentially
catalysed by the presence of iron and of nickel, an improvement in
the resistance to coking of a surface made of metal alloy is
observed on using the treatment according to the invention. In
other words, the formation of coke is reduced in comparison with an
untreated surface.
[0015] Without wishing to be committed to a theory, the removal
from the surface of at least a part of the carbides by electrolytic
dissolution makes it possible to reduce the formation of coke.
[0016] Such a surface treatment stage exhibits the advantage of
being easy to carry out and relatively inexpensive.
[0017] Advantageously, several chemical stages of treatment by
electrolytic dissolution can be provided.
[0018] The process according to the invention can advantageously be
carried out in order to treat a wall of a reactor after the
manufacture of the reactor and before bringing it into service.
[0019] Advantageously, after the chemical treatment stage, a stage
of oxidation of the wall can be envisaged, which makes it possible
to further reduce the formation of coke. Without wishing to be
committed to a theory, the surface treatment appears to promote the
formation of a homogeneous oxidized layer and to thus reduce the
formation of coke.
[0020] The invention is more particularly suited to treating a wall
of a steam cracking reactor (furnace) or of any other plant in
which there is observed the formation of coke catalysed by iron,
nickel and optionally by other catalysing metal elements present in
the metal alloy of which the reactor is composed.
[0021] The invention thus also relates to a process for the
treatment of hydrocarbons under conditions capable of bringing
about the formation of coke, characterized in that the hydrocarbons
are brought into contact with a surface of a wall made of
Fe--Ni--Cr metal alloy, the said surface of the metal wall being
pretreated by a treatment process according to the invention so as
to reduce the formation of a coke deposit. In particular, the metal
alloy is preferably a metal alloy containing at least 5% by weight
of iron, at least 18% by weight of chromium, at least 25% by weight
of nickel and at least 0.05% by weight of carbon.
[0022] The process for the treatment of the hydrocarbons can be a
cracking process, in which the hydrocarbons are brought into
contact with the wall as a mixture with steam. Such a treatment is,
for example, carried out in a steam cracking reactor.
[0023] Advantageously, the hydrocarbons can be brought into contact
with the surface of the metal wall at a temperature of 800 to
900.degree. C., in particular as a mixture with steam.
DETAILED DESCRIPTION OF THE INVENTION
Metal Alloys
[0024] The treatment process according to the invention is intended
for the treatments of Fe--Ni--Cr metal alloys containing in
particular carbides within their structure. These carbides can show
on the surface, in other words be in contact with the gaseous
medium surrounding the alloy, and/or can be located in the
immediate proximity of the surface, for example from a depth of 1
.mu.m or more.
[0025] The presence of these carbides can be observed by
observations with a scanning electron microscope of the surface of
the wall and/or of sections of the wall.
[0026] Such carbides are formed by precipitation during the
manufacture of the wall. They can also appear in part in operation.
In a known way, carbides, which are particularly stable chemically,
are formed from the carbon present in the metal alloy. These
carbides can in particular be observed for a carbon content of the
metal alloy of at least 0.05% by weight.
[0027] This type of metal alloy is suitable in particular for use
at high temperature (heat resistant alloys).
[0028] Advantageously, the alloys treated are alloys exhibiting an
Fe--Ni--Cr matrix, optionally an austenitic matrix, within which
precipitate chromium carbides (Cr.sub.xC.sub.y), indeed even
niobium carbides (NbC), when niobium is present, and/or
carbonitrides, when the alloy contains nitrogen, and/or other
carbides optionally.
[0029] Such alloys thus comprise: [0030] at least 5% by weight of
iron, preferably from 10% to 50%, in a preferred way from 12% to
48%, by weight, at least 18% by weight of chromium, preferably from
19% to 42% by weight, [0031] at least 25% by weight of nickel,
preferably from 31% to 46% by weight.
[0032] In addition, these alloys comprise carbon, in particular
from 0.05% to 1% by weight of carbon, preferably from 0.08% to 0.6%
by weight.
[0033] Advantageously, in these alloys, the nickel or the iron can
be the predominant element. In general, the iron content is the
remainder to 100% of the contents of the other elements present in
the alloy.
[0034] The treated metal alloys can comprise other elements. They
can in particular comprise one or more of the following elements:
[0035] niobium, in particular in a content of 0.3% to 2.5% by
weight, preferably of 0.5% to 2% by weight, [0036] manganese, in
particular in a content of 0.01% to 2% by weight, preferably of
0.5% to 1.7% by weight, [0037] silicon, in particular in a content
of 0.5% to 3% by weight, preferably of 1% to 2.5% by weight, [0038]
nitrogen, in particular in a content of at most 1% by weight, in
particular of 0.01% to 0.5% by weight.
[0039] The metal alloy used can preferably be suitable for
centrifugal casting. It can in particular observe Standard EN 10295
relating to heat-resistant steel castings. This technique consists
in pouring the liquid metal into a mould driven with a rotational
movement around its main axis. Generally, the mould rotates at a
speed such that it creates a mean acceleration of the order of
several hundred and up to 1000 m/s.sup.2 or more, in some cases.
The moulds can be made of sand or of metal die, fitted to machines
having a horizontal, vertical or oblique axis. The parts obtained
by centrifugation have very good physical and mechanical
characteristics.
[0040] The treated wall can thus advantageously be produced by
centrifugal casting.
Chemical Surface Treatment Stage
[0041] More specifically, this stage is an electrochemical stage,
in particular a selective dissolution electrochemical stage.
[0042] During this stage, at least a part of the carbides initially
present in the alloy, in particular at the surface, namely the
carbides showing on the surface and/or located in the immediate
proximity of the surface, is removed by electrolytic
dissolution.
[0043] In particular, this stage is advantageously carried out
under conditions suitable for dissolving at least a part of these
carbides over a depth of at least 10 .mu.m (from the treated
surface), preferably of at least 20 .mu.m, more preferably of at
least 30 .mu.m, indeed even of at least 40 .mu.m.
[0044] In particular, the electrolytic dissolution conditions can
advantageously be suitable for dissolving one or more carbides
chosen from chromium carbides, niobium carbides, when the alloy
contains niobium, carbonitrides, when the alloy contains nitrogen,
indeed even other carbides, preferably chromium carbides.
[0045] In an alternative form or in combination, several chemical
surface treatment stages can be provided. Thus, the process
according to the invention can comprise at least one other chemical
treatment stage, during which at least a part of the carbides
initially present in the alloy, in particular at the surface, and
not dissolved during a preceding chemical treatment stage is
removed by electrolytic dissolution.
[0046] Advantageously, it is possible to carry out a chemical
treatment stage which is a stage of electrolytic dissolution of
chromium carbides and another chemical treatment stage which is a
stage of electrolytic dissolution of niobium carbides, when the
alloy contains niobium, for example in the amounts mentioned
above.
[0047] A washing stage can be provided between two successive
chemical treatment stages under conditions capable of removing the
traces of electrolytes from the treated surface. It can be one or
more stages of rinsing the wall with water, preferably distilled
water, optionally followed by one or more stages of rinsing with an
alcohol, for example ethanol. This washing can be followed by a
drying under conditions which make it possible to remove the
rinsing fluid or fluids from the wall to be treated.
[0048] In a specific embodiment, the electrochemical dissolution of
the chromium carbides is carried out and then the electrochemical
dissolution of the niobium carbides is carried out.
[0049] The chemical stage is carried out by placing the wall to be
treated at the anode of an electrolysis cell, the cathode being
formed of a conductive part (for example made of metal or
graphite), and by applying an electric potential through the
electrolysis cell.
[0050] The chemical treatment stage can be carried out in an
electrolysis cell comprising an aqueous solution of an alkali metal
hydroxide or an aqueous sulfuric acid solution.
[0051] A procedure for the dissolution of chromium carbides is, for
example, described in U.S. Pat. No. 4,851,093 A.
[0052] The electrolytic solution can thus comprise an aqueous
solution of a soluble metal hydroxide. This metal can be an alkali
metal, such as Na, K or Li, for example Na.
[0053] The electrolytic solution can comprise from 100 to 200 g/l
of alkali metal hydroxide, preferably from 120 to 150 g/l.
[0054] Preferably, the chloride content of the solution is less
than 10 ppm by weight.
[0055] A procedure for the dissolution of niobium carbides is, for
example, described in "Anode dissolution characteristics of
titanium, niobium and chromium carbides", 1971, V. Cihal, A.
Desestret, M. Froment and G. H. Wagner.
[0056] The electrolytic solution can thus be an aqueous sulfuric
acid solution, the sulfuric acid concentration of which can be from
1 to 10 moll.sup.-1, preferably from 2 to 9 moll.sup.-1. However,
the invention is not limited to these specific conditions: a person
skilled in the art is in a position to determine other suitable
concentrations of sulfuric acid, indeed even to use other suitable
electrolytic solutions.
[0057] The difference in electric potential applied to the
electrolysis cell can be from 4 to 8 volts or from 3 to volts,
indeed even from 3 to 5 volts. It may be preferable to avoid
greater differences in potentials in order not to generate too much
heat.
[0058] The current flow passing through the electrolysis cell can
vary according to the surface area to be treated. The current
density can typically be from 5 A/in.sup.2 (7750 A/m.sup.2) to 10
A/in.sup.2 (15 500 A/m.sup.2) of surface area of wall to be
treated.
[0059] The duration of the treatment can be variable, for example
from 4 to 50 hours or from 2 to 50 hours, for example from 2 to 30
hours, depending on the amount of carbides and/or on the depth of
wall which it is desired to treat.
[0060] The temperature of the electrolytic solution can vary from
ambient temperature up to approximately 85.degree. C. However, it
is preferable for the temperature of the solution to be kept below
60.degree. C.
Mechanical Stage of Impact Surface Treatment
[0061] When it is present, this stage is preferably carried out
after the chemical treatment stage described above. This impact
surface treatment is obtained by hammering the surface by
projection of particles under conditions suitable for obtaining a
permanent plastic deformation of the surface, in particular under
conditions suitable for obtaining covering of the carbides
initially present at the surface by permanent plastic deformation
of the surface.
[0062] The carbides initially present at the surface can show on
the surface and/or be located in the immediate proximity of the
surface, in particular located at a depth of 1 .mu.m and more from
the surface.
[0063] These projections of particles create, at the surface, a
small impression or crater. This surface is thus plastically
deformed in tension or in compression during the impact.
[0064] Usually, the objective of this type of impact surface
treatment is to compress the substance under the impacted surface:
this compressed substance tends to regain its initial volume,
resulting in high residual compressive stresses. This makes it
possible to significantly increase the lifetime of a part made of
alloy as virtually all of the fatigue and corrosion failures under
tension are initiated at the surface of such parts.
[0065] In the present invention, the impacts caused by the
projectiles will cover this surface with a uniform layer in
compression.
[0066] While the chemical treatment stage brings about the
formation of cavities, the permanent plastic deformation obtained
by the implementation of the mechanical treatment stage makes it
possible to fill these cavities in again, at least partially. In
addition, covering of the carbides initially present at the
surface--in other words covering of the carbides showing on the
surface and/or located in the immediate vicinity of the surface
before the mechanical treatment--which have not been dissolved by
the chemical treatment stage is also observed. Surprisingly, this
modification of the surface, which limits the access to the
carbides trapped inside the metal alloy, also makes it possible to
reduce the formation of coke.
[0067] Depending on the form of the particles and on the projection
power, such a surface treatment can be denoted by the terms "shot
peening" (use of beads), "sand blasting", "alumina blasting" (use
of corundum particles) or "shot blasting".
[0068] The particles can be diverse in nature (inorganic, metallic,
and the like) and of varied shapes (spherical or angular) and
dimensions. The particles can thus be chosen from aluminium oxide
(for example white or brown corundum) particles, metal particles,
beads made of material which is inert under the operational
conditions of use of the wall made of metal alloy, for example made
of glass or of aluminium oxide, or nesosilicate particles.
[0069] In particular, the nesosilicate (garnet) particles exhibit a
general formula A.sub.mB.sub.n(SiO.sub.4).sub.t, where A is a
transition metal or an alkaline earth metal and B is a transition
metal or a rare earth metal. In particular, A can be chosen from
Mg, Ca and Mn and B can be chosen from Y, Ce and La.
[0070] The particles can exhibit a mean diameter of 100 to 500
.mu.m.
[0071] Use may be made, by way of example, of glass beads with a
mean diameter of 100 to 200 .mu.m, of aluminium oxide particles
with a mean diameter of 250 to 500 .mu.m.
[0072] The particles can be projected by a gaseous fluid, for
example air, argon or other, under a pressure of 200 to 400 kPa (2
to 4 bars), preferably of 250 to 350 kPa (2.5 to 3.5 bars).
[0073] For glass beads, a pressure of 300 to 350 kPa can be used.
For aluminium oxide particles, pressures of 270 to 320 kPa can be
used.
[0074] The projection distance can be from 5 to 25 cm, for example
from 10 to 20 cm.
[0075] The projection time can be from 0.2 to 3 minutes, preferably
from 0.5 to 2 minutes (in particular for a surface area of a few
cm.sup.2).
[0076] Use may be made, by way of example, of a plant which can
contain approximately 40 litres of particles, projected using a
nozzle with a diameter of 7 to 8 mm, with compressed air under a
pressure of 2.5 to 3.5 bars.
[0077] Other projection conditions can nevertheless be envisaged
depending on the particles used in order to obtain the plastic
deformation of the surface as described in the subject-matter of
claim 5.
[0078] Advantageously, the impact surface treatment stage can be
carried out under conditions suitable for obtaining covering of the
carbides and/or closing of the cavities over a depth of at least 20
.mu.m, preferably over a depth of at least 30 .mu.m.
[0079] This mechanical stage is preferably carried out "under cold
conditions", in other words at ambient temperature, namely a
temperature ranging from 18 to 35.degree. C.
[0080] Oxidation Stage
[0081] This stage is carried out after the chemical treatment
stage, optionally after the mechanical surface treatment stage. It
is carried out under conditions which make it possible to form a
layer of oxide(s) on the treated surface of the wall, in particular
a layer containing one or more chromium oxides.
[0082] The oxidation conditions can be those generally used to form
a layer of oxide(s) on this type of alloy and known from the prior
art.
[0083] By way of example, the oxidation can be carried out at a
temperature of 800 to 1100.degree. C., under partial molecular
oxygen pressure of 10.sup.-6 atm to 0.2 atm, for a period of time
of 30 min to 5 h.
BRIEF DESCRIPTION OF THE FIGURES
[0084] The invention is now described by means of examples and with
reference to the appended non-limiting drawings, in which:
[0085] FIG. 1 diagrammatically represents an electrolysis cell
which can be used for the chemical surface treatment stage;
[0086] FIGS. 2 and 3 represent SEM photographs of sections of two
samples which have been subjected to a selective dissolution
electrochemical treatment;
[0087] FIGS. 4 to 6 are diagrammatic representations of the
observations in section of samples which have respectively been
subjected to: only a polishing (FIG. 4), an electrochemical
dissolution treatment (FIG. 5), an electrochemical dissolution
treatment followed by a mechanical surface treatment (FIG. 6);
[0088] FIGS. 7 to 9 are SEM photographs with a secondary electron
detector (applied acceleration voltage of 20 kV--FIGS. 7 and 9, or
25 kV--FIG. 8) of sections of samples, according to two
magnifications: [0089] a: magnification 35.times., scale of 500
.mu.m [0090] b: magnification 150.times., scale of 100 .mu.m.
[0091] FIGS. 7a, 7b show photographs of a reference sample, FIGS.
8a, 8b show photographs of a sample which has been subjected to an
electrochemical dissolution treatment, FIGS. 9a, 9b show
photographs of a sample which has been subjected to an
electrochemical dissolution treatment followed by a mechanical
alumina blasting treatment.
DETAILED DESCRIPTION OF THE FIGURES
[0092] FIG. 1 diagrammatically represents an electrolysis cell 1. A
difference in electric potential is applied between two electrodes
2, 3 immersed in an electrolytic solution 4. The positive terminal
is the anode 2, the site of an oxidation, and the negative terminal
is the cathode 3, the site of a reduction. A direct current
generator 5, connected to the anode 2 and to the cathode 3,
provides the current.
[0093] The substance to be dissolved has to be located on the anode
2 (+ terminal). The space between the two electrodes 2, 3 is, for
example, approximately 1 cm. For the cathode (the - terminal), a
simple metal plate can be used. The electrolyte 4 will, for
example, be a sodium hydroxide solution.
EXAMPLES
[0094] Samples made of metal alloy of HP modified 25-35 type and of
35-45 type were tested. These alloys are composed of an Fe--Ni--Cr
austenitic matrix within which niobium carbides (NbC) and chromium
carbides (Cr.sub.7C.sub.3) precipitate. The characteristics of the
metal alloys of the samples used are given in Table 1 below.
TABLE-US-00001 TABLE 1 Typical chemical composition (% by weight)
of the materials used Cr Ni Fe C Si Mn Nb HP 25-35 25 35 26 0.5 1.4
1.6 0.5 HP 35-45 35 45 15 0.5 2.5 1.6 0.4
[0095] The samples used are plaques with dimensions of 8.times.30
mm (samples C1 to C5) and 8.times.25 mm (samples C6 to C9) and with
a thickness of 2 mm obtained by electrical discharge machining to
the core of 5 cm portions of new steam cracking tubes, with an
initial thickness of 8 mm. The initial surface state is a crude
machining state.
[0096] The tubes from which the tested samples result were
manufactured by centrifugal casting.
[0097] Each tested sample was polished by means of SiC-based
abrasive papers in the following order of fineness: 600, 800, 1200
and 2400.
Characterization Techniques Used
[0098] Scanning electron microscopy (SEM) for observation of the
surfaces and of sections. The SEMs used are the Philips XL 20 SEM
and the Zeiss Supra 55 VP SEM. Ion beam cutting: cross sections are
produced by ion beam cutting with a beam of defocused ions. This
technique uses accelerated argon ions to tear off the material,
thus making possible a very fine and contamination-free surface
polishing. The samples are adhesively bonded to titanium masks
using a "silver lacquer" formed of fine silver platelets in
suspension in a solvent.
Example 1: Electrochemical Treatment of the Surface
[0099] In this example, the sample is subjected to an electrolytic
dissolution chemical treatment.
[0100] The sample to be tested is placed at the anode of an
electrolysis cell, such as described in FIG. 1, the cathode being a
metal plate made of stainless steel or of graphite, with dimensions
similar to or greater than those of the sample. The anode and
cathode are separated by a distance of approximately 1 cm, the
plates being substantially parallel inside the electrolysis
cell.
[0101] An electrolytic solution is prepared by dissolving, with
mechanical stirring, 135 g of NaOH (in the form of pellets) in 1 l
of distilled water and then the electrolysis cell is filled with
the solution obtained. The chloride content of the solution is less
than 10 ppm by weight.
[0102] A potential difference is applied between the anode (sample)
and the cathode.
[0103] Two series of five and four tests were carried out on HP
25-35 alloys, which were all polished before being placed in the
solution. The conditions used for each test are collated in Table
2.
TABLE-US-00002 TABLE 2 Parameters of the electrolytic
decompositions Current Sample Duration Voltage intensity name
(hours) (V) (A) Comments C1 20 6 15 Evaporation of the C2 2 8 16
solution before the end of 3 6 10 the duration C3 15 6.5 12 C4 15 6
10 C5 20 6 10 Depth of dissolution of the chromium carbides C6 2
3.72 5 approximately 30 .mu.m C7 5 3.3 4.14 approximately 50 .mu.m
5 4.75 8.34 approximately 50 .mu.m C8 15 4.4 8.3 approximately 72
.mu.m C9 24 3.45 5 approximately 100 .mu.m
[0104] Under the conditions tested, observation with an SEM of the
surface of the samples C1 to C4 shows a dissolution of the chromium
carbides Cr.sub.7C.sub.3 but no dissolution of the niobium carbides
NbC. The austenitic matrix remains intact.
[0105] It will be noted that the current intensity does not appear
to influence the depth of dissolution of the chromium carbides,
unlike the duration of the dissolution.
SEM Observation of the Samples in Cross Section
[0106] Sections of the different samples were observed with an
SEM.
[0107] FIGS. 2 and 3 are photographs of the sample C4 dissolved for
15 h (FIG. 2) and of the sample C5 dissolved for 20 h (FIG. 3). The
acceleration voltage applied for the measurement is 15 kV, the
magnification is 619.times. (FIG. 2) and 629.times. (FIG. 3) and
the scale is 10 .mu.m. In the sample C4, cavities are observed over
a depth of approximately 40 .mu.m, which appears to indicate the
existence of interconnected carbide networks. In the sample C5, the
cavities extend over a depth of 80 .mu.m. Chromium carbides still
exist between 50 and 80 .mu.m, which appears to indicate that the
network of carbides is not completely interconnected. Table 2
indicates, for the samples C6 to C9, the maximum depth down to
which dissolution of the chromium carbides was observed.
[0108] It is thus possible to influence the depth of the carbides
attacked by modifying the electrolysis conditions.
[0109] FIGS. 4 and 5 diagrammatically represent typical
observations of a section of an untreated sample (FIG. 4) and of a
sample which has been subjected to a chemical treatment (FIG. 5).
In these diagrams, the black parts correspond to the chromium
carbides and the grey parts to the niobium carbides.
[0110] Thus, the presence of niobium carbides (NbC), of chromium
carbides (Cr.sub.7C.sub.3) and of cavities is noted in FIG. 5.
Niobium carbides are observed in the cavities. Without wishing to
be committed to a theory, during the electrolytic dissolution, the
solution might spread by dissolving the chromium carbides resulting
from the interconnected networks but while retaining the niobium
carbides (NbC). In addition, it is observed that the cavities are
not completely empty. A chemical analysis by SEM/EDX (Energy
Dispersive X-ray Spectrometry) shows that the chromium carbides
have been partially dissolved. The presence of oxygen inside the
cavities is also observed, which leads it to be believed that there
is formation of oxide or of hydroxide, probably originating from
the electrolytic solution.
Example 2: Mechanical Surface Treatment/Shot Peening
[0111] A polished HP 25-35 alloy sample is subjected to shot
peening in a sleeve sandblasting chamber. The parameters used are
as follows: [0112] Particles: glass beads O 100-200 .mu.m [0113]
Projection distance: approximately 15 cm [0114] Duration of the
projection: 15 seconds for a sample of a few cm.sup.2 [0115]
Carrier gas: compressed air under a controlled pressure of 2.5 to
3.5 bars, nozzle diameter 6 to 8 mm, 40 litres of particles in a
closed circuit.
[0116] A sample M1 is obtained.
Example 3: Mechanical Surface Treatment/Sandblasting (Alumina
Blasting)
[0117] A polished HP 25-35 alloy sample is subjected to alumina
blasting in a sleeve sandblasting chamber. The parameters used are
as follows: [0118] Particles: brown corundum (Al.sub.2O.sub.3) O
250-400 .mu.m [0119] Projection distance: approximately 15 cm
[0120] Duration of the projection: 15 seconds for a sample of a few
cm.sup.2 [0121] Carrier gas: compressed air under a controlled
pressure of 2.5 to 3.5 bars, nozzle diameter 6 to 8 mm, 40 litres
of abrasive particles in a closed circuit.
[0122] A sample M3 is obtained.
Example 4: Chemical Treatment+Mechanical Treatment/Shot Peening
[0123] The sample C4 of Example 1 is subjected to the same shot
peening treatment as that described in Example 2. A sample CM4 is
obtained.
Example 5: Chemical Treatment+Mechanical Treatment/Alumina
Blasting
[0124] The sample C4 of Example 1 is subjected to the same alumina
blasting treatment as that described in Example 3. A sample CM5 is
obtained.
[0125] FIG. 6 diagrammatically represents the typical observation
of a section of an alloy sample which has been subjected to a
chemical and mechanical treatment. It is noted that the chromium
carbides are no longer in direct contact with the surface and that
the cavities formed by the electrochemical dissolution have been at
least partly closed for the majority of them.
Example 6: Coking
[0126] Coking tests were carried out on the samples C4, CM4 and CM5
prepared in Examples 1, 4 and 5, and also on a reference sample
simply polished. The samples were brought to high temperature in
the presence of a mixture of light hydrocarbons and of steam
(similar to industrial conditions). They were thus subjected to
conditions favouring the formation of coke.
[0127] Each sample was subjected to the following conditions:
[0128] 1. Rise in temperature under dry argon (O.sub.2 impurities
in the argon of approximately 3 ppm by volume) up to 900.degree. C.
(5.degree. C./min), [0129] 2. Pre-oxidation of the samples
900.degree. C., 1 h under synthetic air, [0130] 3. Flushing of the
furnace with dry argon 30 min, [0131] 4. Coking of the samples: 45
minutes 860.degree. C.: ethane+steam, [0132] 5. Flushing of the
furnace under dry argon 30 min, [0133] 6. Halting of the heating
system and slow cooling of the samples.
[0134] The samples were observed with an SEM. FIGS. 7a and 7b are
photographs (magnifications 35.times. and 150.times. respectively)
of the surface of the reference sample which has not been subjected
to any specific treatment besides the initial polishing. The
formation of coke at the surface is observed. FIGS. 8a and 8b are
photographs of the sample C4 which has been subjected to the
electrochemical treatment (magnifications 35.times. and 150.times.
respectively) and FIGS. 9a and 9b are photographs of the sample CM5
(magnifications 35.times. and 150.times. respectively).
[0135] The samples which have been subjected to a chemical
treatment exhibit overall less coke than the reference sample. Coke
is still observed over approximately 10% of the surface of the
sample.
[0136] A notable reduction in the amount of coke is also observed
for the samples which have been subjected to a mechanical treatment
after the chemical treatment (samples CM4 and CM5), as may be made
out in FIGS. 9a and 9b for the sample CM5.
Example 7: Electrochemical Treatment of the Surface
[0137] In this example, the sample is subjected to an electrolytic
dissolution chemical treatment in order to remove the niobium
carbides.
[0138] The sample to be tested is placed at the anode of an
electrolysis cell of the same type as that represented in FIG. 1
and described in Example 1.
[0139] A 7.2 moll.sup.-1 electrolytic solution of sulfuric acid
(H.sub.2SO.sub.4) is prepared, with which the electrolysis cell is
filled.
[0140] A first test was carried out on an HP 25-35 alloy with
dimensions of 8.times.25 mm and with a thickness of 2 mm, which was
polished before being placed in the sulfuric acid solution.
[0141] A potential difference of the order of 0.8 V is applied
between the anode (sample) and the cathode for 2 hours. The sample
is subsequently rinsed with distilled water and then with ethanol,
dried and stored in a case sheltered from scratches and from the
air in a desiccator.
[0142] A second test was carried out under the same electrolysis
conditions on a sample with the same dimensions and of the same
alloy subjected beforehand to an electrolytic dissolution of the
chromium carbides. The latter is carried out with a current density
of 5 Ain.sup.-2 (0.775 Acm.sup.-2) for 2 hours in a NaOH solution
(135 g in the form of pellets in 1 l of water). The sample obtained
is subsequently rinsed with distilled water and then with ethanol
and dried before being introduced into the sulfuric acid solution
for the dissolution of the niobium carbides.
[0143] An examination of the surface state by backscattered
electron scanning electron microscopy (mode of imaging sensitive to
chemical contrast) shows that there was no dissolution of the
niobium carbides in the case of the first test.
[0144] On the contrary, a dissolution of all the carbides (chromium
and niobium) is observed by examination of the surface state of the
sample subjected beforehand to a dissolution of the chromium
carbides. The absence of niobium carbides (NbC) at the surface was
confirmed with an SEM by an EDX (Energy Dispersive X-ray) analysis.
The chemical distribution of the niobium over the surface analysed
shows a few places locally rich in Nb but no niobium carbide
left.
[0145] Unlike a simple dissolution in sulfuric acid, the successive
electrolytic decomposition of the chromium carbides and of the
niobium carbides thus makes it possible to dissolve the NbCs at the
surface. Without wishing to be committed to a theory, the
electrolytic dissolution of the M.sub.23C.sub.6/M.sub.7C.sub.3
might come partially "to lay bare" the NbCs and to increase the
free surface area in contact with the electrolyte of the second
dissolution.
* * * * *