Process and apparatus for the formation of a deposit by projection of a coating material on a substrate

A projection device for depositing by projection a fused coating material onto a substrate. The projection device includes a projection nozzle having a central part defining a central passage for the coating material to be fused and projected. The central passage opening at a distal end thereof into a central orifice. The central part being fitted in a peripheral part. Ejection openings of ducts of oxycombustible mixture being distributed around the central orifice according to at least two series which are alternatingly offset from one another with respect to a main axis of the nozzle. The ducts include channels which are milled in the periphery of the central part. The carrier gas contains between 1 and 10% oxygen with the remaining portion consisting of nitrogen. The carrier gas being supplied by a unit for the separation of inert gas from air by adsorption or permeation. The projection device is particularly suited for the formation of the anti-corrosive deposits based on zinc.

BACKGROUND OF INVENTION 
a) Field of the Invention 
The present invention concerns a process for the formation of a deposit by 
projection of a coating material on a substrate, comprising the steps of 
melting, by combustion of an oxycombustible mixture, the solid coating 
material and pulverizing and projecting the molten coating material by 
means of a flow of a carrier gas containing at least 90% of at least one 
inert gas. 
b) Description of Prior Art 
The thermal projection with a flame, combines a whole group of processes in 
order to modify the surface properties of a substrate by providing, on 
this surface, a deposit of a coating material, which is generally 
metallic. Through the combustion of the oxycombustible mixture, the 
coating material is progressively brought to its melting temperature and 
the carrier gas pulverizes the molten material into fine particles which 
are provided with a strong kinetic energy. The particles in liquid or 
pasty state hit the substrate which is initially prepared for this 
operation. The carrier gas actually used consists of compressed air and 
the yields (ratio between the weight of the coating material truly 
deposited on the substrate and the weight of the coating material 
effectively used) typically obtained, are of the order of 55 to 57% for 
the projection of zinc which is the metal most currently used for the 
production of anti-corrosive deposits, such as on metallic tubes. 
The Applicant has noted that the thermodynamic properties of the carrier 
gas play an important role on the value of the yield. Thus, the 
vaporization temperature of the coating material may be rapidly reached 
for particles of small diameter if the carrier gas has a high thermic 
conductivity. On the other hand, the formation of oxides on the particles 
during their travel between the melting zone and the substrate to be 
coated is exothermic and may thus lead to an excessive evaporation of the 
material to be projected. 
SUMMARY OF INVENTION 
It is a first object of the present invention to propose a process of the 
type mentioned above, which is easy to carry out and is flexible, enabling 
a notable improvement of the projection yield and which may be carried out 
with reduced operation cost. 
For this purpose, according to a more specific characteristic of the 
invention, the carrier gas comprises between 1 and 10% oxygen, typically 
between 2 and 8%, the remainder being nitrogen, this carrier gas being 
typically supplied by a unit for the separation of air by adsorption or 
permeation. 
According to this aspect of the invention, the carrier gas may be produced 
at low costs and, although a small quantity of oxygen remains present, the 
increase of the yield may reach 13%, for the projection of zinc. Such a 
carrier gas with a high proportion of inert gas enables indeed to decrease 
the reactivity of the medium along the path followed by the particles 
because of the reduction of the oxidation zone and therefore enables a 
reduction of the volume of coating material which is in combustion and a 
decrease of the quantity of oxidized particles which are unsuitable for 
good linking on the substrate. Moreover, the reduction of the volume of 
heat following a decrease of the volume of oxidized particles reduces the 
distance between the projection nozzle and the substrate without modifying 
the quality of the deposit, and therefore enhances concentration of the 
projection. 
In the known processes, the combustible gas consists essentially of propane 
and sometimes acetylene. In the case of propane, the oxycombustible 
mixture has a low specific power and combustion speed the flame obtained 
forming long tips and being overall too powerful. The increase of the 
volume of oxygen, to raise the specific power, or the increase of the 
overall flow of oxycombustible mixture only reduces the projection yield. 
On the other hand, acetylene has a high specific power and combustion 
speed resulting in short tips and a flame which is locally too powerful. 
The reduction of the amount of oxygen or of the overall flow 
oxycombustible mixture produces a substantial decrease of the rate of 
deposit. 
It is another object of the present invention to propose a process which is 
still improved by the use of performing gases which are better adapted and 
by an optimization of the distribution of the heating of the coating 
material to be melted. 
According to an aspect of the invention, the oxycombustible mixture is 
prepared by adding oxygen and a compound of propylene and methylacetylene, 
or a compound of ethylene and acetylene. 
These compounds, which are commercially available, have a specific power, a 
combustion speed and a length of tip which are intermediate between those 
of propane and acetylene. The flame obtained results in a better 
distribution of the calorific power around the wire of the coating 
material. 
According to another characteristic of the invention, the oxycombustible 
mixture is ejected towards the coating material along at least two series 
of ejection ducts which are radially offset with respect to the latter. 
It is still another object of the present invention to propose an improved 
projection device at low manufacturing costs which is particularly 
suitable for carrying out the processes defined above, comprising a 
projection nozzle having a main axis and including a central duct for 
supplying coating material, which opens through an orifice at one end of 
the nozzle, a plurality of ducts of oxycombustible material opening at the 
end of the nozzle by means of openings which are angularly distributed 
around the central orifice, and an annular duct for carrier gas 
surrounding the end of the nozzle, characterized in that the openings of 
the ducts of oxycombustible mixture are distributed according to at least 
two series which are offset from one another relative to the main axis. 
The known flame projection nozzles are monobloc and the mixture ducts 
consist of tubular passages bored in the nozzle and terminating into 
calibrated orifices of the same diameter which are distributed along a 
circle around the central orifice, according to an arrangement which is 
hard to produce and permits only a reduced number of adaptations. 
According to an aspect of the invention, the nozzle comprises a central 
part defining the central ducts and fitted in a tubular peripheral part, 
the mixing ducts being formed at the interface between the central and 
peripheral parts and emerging through openings which are distributed in at 
least a first and a second series, the distance between the main axis and 
the openings of the first series being larger than the distance between 
the openings of the second series and the main axis. The mixture ducts are 
advantageously formed by means of longitudinal channels which are milled 
in the periphery of the central part, which easily modulates the depth, 
the shape and the number of these mixing ducts, and to reduce the 
manufacturing costs. Such a projecting device has also been found to be 
more efficient and flexible in use than the known devices which use known 
gases, namely air as carrier gas and acetylene or propane as combustible 
gas.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows a projection nozzle consisting of a coaxial assembly of a 
central tubular part 1 mounted in a tubular peripheral part 2, this 
assembly being mounted coaxially in an end of a cylindrical support 3 
which itself is mounted in a body 4 of a projection gun. In the embodiment 
illustrated in FIG. 1, the peripheral part 2 includes a central truncated 
central bore passage 5 connecting, at the rear, by means of a radial 
shoulder 6, to an annular chamber 7 of widened diameter. The central part 
1 includes a central passage duct 8 and includes, at its periphery, two 
series of longitudinal channels 9a 9b of different depths, which alternate 
and are angularly distributed. The outside profile of the ribbed portion 
of the central part 1 substantially corresponds to the inner profile 5 of 
the peripheral part 2. In particular, the ribs between the channels 9a, 9b 
include a rear part of enlarged diameter 10 which is received in annular 
chamber 7, abutting the radial shoulder 6 but not extending on the entire 
axial extension of the annular chamber 7. The central part 1 includes a 
rear end 11 of reduced diameter while the peripheral part 2 includes a 
rear end 12 of widened diameter, these rear ends being received in a 
stepped front housing 13 of support 3 in which there is a central passage 
14 through which the coating material, in the form of homogeneous or 
compacted wire 15, associated with pulling means (not illustrated), moves 
in central duct 8 of the central member 1 to exit, at the front end of the 
nozzle, through a central opening 16, around which emerge the ducts 9A, 9B 
(FIG. 2). The duct 8 advantageously includes, in the vicinity of central 
orifice 16, a tubular lining 40 made of a material which is more resistant 
to wear, for example stainless steel. The tubular support 3 includes a 
plurality of longitudinally stepped ducts 18 emerging, at the downstream 
end, in the downstream end of enlarged diameter of the stepped housing 13 
and, at the upstream end, in an annular chamber 19 which communicates, via 
ducts provided in body 4, with a source of combustible gas 20, typically a 
compound of propylene and methylacetylene which is sold under the 
designation "TETRENE" or a compound of ethylene and acetylene which is 
sold under the designation "CRYLENE". The median portion of intermediate 
diameter in each duct 18 communicates, by means of a radial duct 21, with 
an annular chamber 22 which itself communicates, via internal ducts 
provided in body 4, with a source of oxygen 23. The oxycombustible mixture 
is formed in ducts 18 and is homogeneously distributed in the annular 
chambers 13 and 7 to feed, also in homogenous manner, the ducts 9a, 9b. 
The combination of the central nozzle part 1 and peripheral nozzle part 2 
is mounted and held against an internal shoulder of the housing 13 by 
means of a bolt 24 screwed on the front end of the support 3. In the front 
part of the bolt 24 there is provided an end sleeve 25 which surrounds the 
peripheral part 2 and defines an internal housing which ends, at the 
front, into a converging conical part 26 which surrounds the front end of 
the peripheral part 2 by providing, around the latter, an annular duct 27. 
Sleeve 25 is held and blocked into position in bolt 24 by means of a 
peripheral hood 28 screwed on the front end of body 4 by thus forming an 
annular chamber 29 around bolt 24 and the rear part of the sleeve 25. The 
front end of body 4 includes an annular chamber 30 which communicates, 
through an interior passage 31, with a source of carrier gas 32. Sleeve 24 
includes radial ducts 33 establishing communication between chamber 29 
provided in hood 28 and the annular space between the sleeve 25 and the 
peripheral nozzle part 2. The carrier gas from source 32 is uniformly 
distributed in annular chamber 30 and passes into the annular chamber 29 
while cooling bolt 24 and the rear part of sleeve 25, and from there, 
through ducts 33, into the annular chamber between the sleeve 25 and the 
peripheral part 2 towards the exit passage 26, while cooling the 
peripheral nozzle part 2. 
As better illustrated in FIG. 3, the design of the nozzle according to the 
invention obtains a stepped heating, the alternation of the mixing ducts 
9a, 9b to differently distribute the combustion tips 34a, 34b around the 
material to be molten 15. Ducts 9b which are closer to the axis of the 
nozzle, provide a strong heating 34b of the material to be molten at a 
short distance from the front face of the nozzle and produce a rapid 
temperature rise of the material 15. The other ducts 9a ensures a heating 
34a which is more remote from the end of the nozzle and contribute to a 
progressive temperature rise of the material to be molten 15. 
The design of the nozzle in two parts produces ducts of very different 
shapes, for example, as illustrated from left to right in FIG. 2, channels 
of rectangular, triangular or trapezoidal cross-sections. The ease of 
machining also increases the number of ducts and their angular 
distribution, and to thus improve the distribution of the heating on the 
material to be molten. It it thus also possible, as illustrated at the 
right of FIG. 2, to produce ducts by means of a combination of bored holes 
9b and channels of various depths 9a, 9a'. 
As mentioned above, the source of carrier gas 32 may consist of a storage 
of nitrogen or argon or a mixture of both. Advantageously, according to 
the invention, this source of carrier gas 32 consists of a unit for the 
separation of the gases from air by adsorption or permeation which is 
supplied with atmospheric air by means of a compressor 35, the permeate, 
which is made of oxygen enriched air, being evacuated at 36. 
By way of example, for the formation of an anti-corrosive deposit by 
projection of zinc, with a carrier gas consisting of 97% nitrogen and 3% 
oxygen and a projection of metal with a mass flow of about 20 kg/hour, the 
parameters are the following: 
carrier gas: 
pressure: 4.5-5.times.10.sup.5 Pa 
projection flow: 20-30 m.sup.3 /hour 
oxygen: pressure: 2-3.times.10.sup.5 Pa 
combustible gas: 
flow: 1000-1200 liters/hour 
pressure: 
compound "CRYLENE": 2.times.10.sup.5 Pa 
compound "TETRENE": 2.5.times.10.sup.5 Pa. 
Under these conditions, the projection yield, for zinc, is improved by 
about 9% as compared to the utilization of propane and compressed air. 
Although the present invention has been described with respect to specific 
embodiments, it is not limited thereto, but, on the contrary, is capable 
of modifications and variants which will appear to one skilled in the art.