Note: Descriptions are shown in the official language in which they were submitted.
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A method of coating a sheet of steel
The present invention relates to a method of coating a
sheet of steel and to a coating for a sheet of steel and relates
particularly, but not exclusively, to a method of coating a
surface of a weight bearing sheet of steel for use on an aircraft
carrier.
Ships, such as aircraft carriers, are commonly made from
steel including the uppermost deck surface that acts as the
aircrafts' takeoff and landing runway. These deck surfaces are
commonly protected by applying a layer of organic paint.
However, such paint is unable to withstand the gas-wash produced
during the takeoff and landing of aircraft that use jet engines
to facilitate a vertical takeoff or landing or short takeoff and
vertical landing (STOVL). This gas-wash can result in a change
of surface temperature from ambient to 2000 C causing a patch
of the paintwork to be burnt away on each such manoeuvre. Such
damage then makes a deck surface vulnerable to corrosion.
A surface coating is required that is able to deal with the
rapid change in temperature including starting from ambient
temperatures in a range of -50 C to +50 C. The coating must
provide a nonslip or anti-skid surface allowing it to be safely
walked on, it must be sufficiently robust to withstand the force
applied to it during takeoff and landing and be easily repairable
in the event of damage.
Preferred embodiments of the present invention seek to
overcome the above described disadvantages of the prior art.
According to an aspect of the present invention there is
provided a method of coating a sheet of steel comprising the
steps:-
=
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p repar ing a surface of a sheet of steel that is to be coated;
applying a first metal deposition coating to said sheet of steel,
said first coating comprising aluminium;
applying a second metal deposition coating to said first
coating, said second coating comprising aluminium and at least
one first doping material; and
applying a barrier coating to said second metal deposition
coating.
By applying, to a sheet of steel, a first coating of
aluminium, a second coating of aluminium plus a doping material
and a barrier coating, a series of advantages are provided. The
sheet material is protected from corrosion and provides a
nonslip or anti-skid surface which is safe to walk on and provide
suitable friction resistant surface to operate an aircraft from.
The surface can be painted and yet is sufficiently resistant to
rapid changes in temperatures that the gas wash from a vertical
takeoff aircraft will not damage the surface or the steel. The
surface also provides sufficient frictional resistance to allow
the takeoff on landing of other aircraft.
In a preferred embodiment the surface of said sheet of
steel is prepared by grit blasting.
By grit blasting the steel surface a substantially oxide
free and suitably rough surface is created which ensures a secure
bond between the steel of the sheet material and the aluminium
of the first coating.
In another preferred embodiment grit blasting uses a grit
comprising brown fused alumina which is suitably angular and
robust to provide the required profile to optimise adhesion of
the aluminium of the first coating.
In a preferred embodiment the first coating further
comprises at least one second doping material which enhances the
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corrosion resistance of the first coating and/or facilitates the
ease of repair.
In another preferred embodiment second doping material
comprises at least one of zinc, molybdenum, gallium, tin and
indium. The addition of the dopants extends the life of the
sacrificial characteristics of the first coating layer in marine
environments and increases the compatibility of the first
coating with the repair regime.
In a further preferred embodiment first metal deposition
coating is applied to a thickness of between 75 and 200 pm.
The second metal deposition coating may be applied in a
plurality of stages including a first stage and a second stage
and wherein a particle accelerator used in the coating
applications operates so as to accelerate the particles more in
the first stage than the second stage.
The second metal deposition coating may also be applied in
a plurality of stages including a first stage and a second stage
and wherein a particle accelerator used in the coating
applications operates so that a surface produced in said first
stage has a surface roughness less than a surface roughness
produced in said second stage.
By applying the second metal deposition coating in a
plurality of stages with lower acceleration of the particles in
the second stage thereby leading to greater roughness of the
resulting surface, the advantage is provided that the bond
between the first metal deposition coating and the second metal
deposition coating are secure (due to the increased particle
velocity on impact in the first stage) but the uppermost surface
of the second metal deposition coating is sufficiently rough
(due to decreased particle velocity on impact in the second
stage) to assist in the provision of a nonslip or anti-skid
surface.
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In another preferred embodiment the second metal deposition
coating is applied to a thickness of between 150 and 300 um.
In a preferred embodiment the barrier coating comprises an
inorganic heat resistant coating material.
By using an inorganic heat resistant coating material for
the barrier coating, the advantage is provided that the barrier
coating assists in the protection of the first and second metal
deposition coatings and the high temperatures, resultant from
jet gas wash, do not damage the barrier coating. As a result,
the coating of the present invention can be regarded as
intumescent.
In another preferred embodiment the barrier coating
comprises a potassium silicate coating material.
A potassium silicate coating provides the above described
advantages and can be applied at a convenient thickness.
In a further preferred embodiment the barrier coating is
applied to a thickness of between 5 and 25 um.
By providing a barrier coating of the above thickness, the
advantage is provided that the barrier coating follows, and
substantially reproduces, the profile of second metal deposition
coating thereby assisting in the creation of the nonslip
surface.
In a preferred embodiment at least one of said first and
second metal deposition coatings are applied using at least one
of the metal deposition techniques of arc spray, wire spray,
twin wire arc spray, air plasma and cold spray.
According to another aspect of the present invention there
is provided a coating for a sheet of steel, the coating
comprising:-
a first metal deposition coating comprising aluminium and
applied to a prepared surface of a sheet of steel;
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a second metal deposition coating comprising aluminium and at
least one first doping material and applied to said first metal
deposition coating;
a barrier coating applied to said second metal deposition
coating.
The first coating may further comprise at least one second
doping material.
The second doping material may comprise at least one of
zinc, molybdenum, Gallium, tin and indium.
In a preferred embodiment the first metal deposition
coating has a thickness of between 75 and 200 pm.
The first doping material may comprise at least one of
titanium, chromium, manganese, boron, aluminium oxide, titanium
oxide, chromium oxide, manganese oxide, boron oxide, boron
nitride, boron carbide and tungsten carbide.
In a preferred embodiment the second metal deposition
coating is applied in a plurality of stages including a first
stage and a second stage and wherein a surface produced in said
first stage has a surface roughness less than a surface roughness
produced in said second stage.
In another preferred embodiment the second metal deposition
coating is applied to a thickness of between 150 and 300 pm.
In a preferred embodiment the barrier coating comprises an
inorganic heat resistant coating material.
In another preferred embodiment the barrier coating
comprises a potassium silicate coating material.
In a further preferred embodiment the barrier coating is
applied to a thickness of between 5 and 25 pm.
According to a further aspect of the present invention
there is provided a sheet material comprising:-
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a sheet of steel material; and
a coating as set out above applied to said sheet of steel
material.
In a preferred embodiment the steel comprises hardened steel.
In another preferred embodiment the steel comprises carbon
manganese steel.
According to another aspect of the present invention there
is provided an article comprising at least one sheet material
as set out above.
In a preferred embodiment the sheet material forms a weight
bearing surface.
In another preferred embodiment the article is any of a
helicopter landing platform, a submarine, an oil rig, a chemical
processing plant, an energy production plant, a shipping
container and a munitions box.
According to a further aspect of the present invention
there is provided an apparatus for applying a coating to a sheet
of steel, the apparatus comprising:-
a frame for supporting a plurality of metal deposition coating
applicators;
applicator moving means for moving said applicators relative to
said frame;
at least one applicator for applying at least one first metal
deposition coating and at least one second metal deposition
coating to a sheet of steel; and
at least one processor for controlling said applicators and said
applicator moving means.
By providing an apparatus described above, the advantage
is provided that large surfaces of sheet material, for example
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the deck of an aircraft carrier, can be coated accurately and
evenly producing a surface to a consistent standard.
In a preferred embodiment the applicator uses at least one
of the metal deposition techniques of arc spray, wire spray,
twin wire arc spray, air plasma and cold spray.
The apparatus may comprise a plurality of applicators for
applying at least one first metal deposition coating and at
least one second metal deposition coating to a sheet of steel.
= Preferred embodiments of the present invention will now be
described, by way of example only, and not and in any limitative
sense with reference to the accompanying drawings in which:-
Figure 1 is a flowchart showing a method of the present
invention;
Figure 2 is a schematic sectional representation of a sheet
material and coating of the present invention;
Figure 3 is a micrograph image of a prepared surface of
sheet material used in the present invention;
Figure 4 is a micrograph of a cross-section of a prepared
surface of sheet material used in the present invention;
Figure 5 is a micrograph of a cross-section of a prepared
surface of sheet material not suitable for use in the present
invention;
Figure 6 is a flowchart showing a method of the present
invention;
Figure 7 is a perspective view of an apparatus for applying
the coating of present invention; and
Figure 8 is another perspective view of the apparatus of
figure 7.
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Re ferr ing to figures 1 and 2, a method of applying a
coating 10 onto a sheet of steel 12 is undertaken in a series
of steps set out in figures 1 and 6, with figure 1 representing
a simplified version of the more complete process shown in
figure 6. At the
start of the process a sheet of steel
material 12 is prepared in order to receive the coating 10 that
will be applied to at least one surface thereof. The thickness
of the sheet of steel is dependent upon the application for
which it is to be used and is therefore not a constraint or
limitation of this invention.
Furthermore, the term sheet
refers to a single or multiple sheets of steel that are joined
together and it is not a limitation of this invention that the
sheets form a planar or substantially planar surface. The
purpose of the coating is to create a protective nonslip or
anti-skid surface to the steel. The steel is for use in any
suitable weight bearing situation carrying the weight of, for
example, a person, vehicle or aircraft.
The primary functions of the surface preparation (indicated
at 14 on figures 1 and 6) are to expose a substantially oxide
free surface of the steel and to create sufficient surface
roughness to ensure secure bonding between the steel 12 and
coating 10. An example of such a surface preparation technique
is grit blasting (indicated at 16) in which a grit, preferably
brown fused alumina, is accelerated in a stream of compressed
air at a pressure of greater than 80 psi and directed at the
sheet steel at an angle between 450 and 90 to the surface being
prepared.
An inspection takes place at step 18 and a pass or fail is
determined at step 20. The
step of inspection may include
examining a micrograph of the prepared surface and an acceptable
surface micrograph at 100 times magnification is shown in
figure 3 with the sharp edges shown therein providing an
indication of a surface that will create a good bond to the
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coating that will be applied thereto. Figures 4 and 5 are also
micrographs (at 100 times magnification) showing a cross-section
of the grit blasted surface. Figure
4 shows an example of an
acceptable grit blasted surface and figure 5 shows an example
of an unacceptable surface due to having insufficient roughness.
The table below set out the preferred critical roughness
parameters for the blasted surface, the roughness being measured
using roughness measuring techniques familiar to a person
skilled in the art.
Parameter Description Acceptable
Range (pm)
Ra The mean of the absolute values and 4 (normally
describes the average roughness 4-5.5)
R, The maximum peak height 15
Rku The average sharpness of the peaks 3
Rsm The mean width of the profile peak 500
elements
Rt The maximum peak to valley height in 30 - 60
the sampling length
Prior to the grit blasting at step 16, other surface
preparation techniques may be applied. These include degreasing
and paint removal and the latter may be undertaken. using grit
blasting but with another grit such as shot or aggregate. The
surface may also be tested for grease and for the presence of
chloride using a chloride indicator tape or similar techniques
familiar to a person skilled in the art.
Following the surface preparation in step 14, metal
deposition is undertaken at step 22. This metal deposition is
undertaken in two different coatings with two different
compositions of metal being deposited consecutively onto the
steel 12. A first metal deposition coating 24 is applied at
step 25 and comprises at least aluminium. The
first metal
deposition coating 24 may include a doping component, including,
but not limited to, metals such as zinc, molybdenum, gallium,
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tin and indium The first metal deposition coating 24 is applied
by arc spraying although other metal deposition/spraying
processes are also acceptable including, but not limited to,
wire spray, twin wire arc spray, air plasma and cold spray. The
first metal deposition coating typically has a thickness of
between 75 and 200 pm. The first metal deposition coating is
preferably applied as a series of layers bypassing the spray gun
over the same surface a number of times.
Once the first metal deposition coating 24 has been applied
a second metal deposition coating 26 is applied (at step 27) on
top of the first coating 24. The second metal deposition coating
comprises aluminium and at least one other doping component,
including, but not limited to, metals such as titanium,
chromium, manganese, boron, aluminium oxide, titanium oxide,
chromium oxide, manganese oxide, boron oxide, boron nitride,
boron carbide and tungsten carbide. The second metal deposition
coating 26 is applied using any of the same techniques used to
apply the first metal deposition coating 24. The second metal
deposition coating typically has a thickness of between 150
and 300 pm and is applied in two stages. The two
stages
preferably use the same metal combination but can be
distinguished by a difference in the acceleration of the metal
particles from the spray gun as the spray is applied. In the
first stage of the application, producing the first stage 28 of
second coating 26, the spray particles are accelerated from the
spray gun at a faster rate than those producing the second
stage 30 of second coating 26. In the first stage the increased
momentum of the fast-moving particles results in secure adhesion
between the second coating 26 and first coating 24 and high
density. However, the resultant temporary surface (indicated
at 32) at the end of the first stage of the second coating does
not have sufficient roughness to provide a good nonslip or anti-
skid surface. By slowing down and reducing the momentum of the
particles in the second stage (by reducing the pressure of the
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accelerant gas in the spray gun by roughly half) a rougher
surface (indicated at 34) is produced.
Once the first and second metal deposition coatings have
been applied and inspection takes place at step 29 and a pass
fail is determined at step 31. The inspection can include a
visual inspection, surface roughness measurements and a bend
test (where a sample of steel, not forming part of the main
surface being protected, is taken and bent using a predetermined
force around a predetermined diameter to ensure good adhesion
between the metal deposition coatings and the steel).
The coating 10 is completed by the application (at step 33)
of a barrier coating 36. The barrier coating is an inorganic
heat resistant material such as a potassium silicate sealant.
The sealant acts as a barrier preventing oxidisation of the
aluminium and is applied in a layer that is thinner than the
first and second metal deposition coatings at between 5 and 25 um
thick. This thickness is sufficiently small that the roughness
of the upper surface 34 of the second metal deposition coating 26
is retained thereby providing a good nonslip surface. The
barrier coating can be applied by brush, roller, spray gun (HVLP,
High velocity, low pressure) or dipping for smaller surfaces.
The barrier coating 36 can include a pigment in order to provide
a base coat colour as required.
Once the barrier coating has been applied a final
inspection step takes place at 35 and a pass fail is determined
at 37. If required, further barrier coatings containing other
pigments to produce other colours can be applied to the surface
of the barrier coating to give line marking or other demarcation
as required.
Spray guns of the type described above can be operated
manually to apply the metal deposition coatings. However, when
working over large surface areas it is preferable to use a
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robotic device of the type shown in figures 7 and 8. The
apparatus includes a frame SO which is placed onto sheet
material 12 and is manoeuvrable via wheels 52. An applicator,
in the form of spray gun 54, is provided to apply the first
metal deposition coating and the second metal deposition coating
in consecutive applications.
However, the applicator could
alternatively include two or more spray guns to apply the
separate metal deposition coatings and most preferably includes
three spray guns. The first spray gun applies the first metal
deposition coating, the second spray gun applies the first stage
of the second metal deposition coating and the third gun applies
the second stage of the second metal deposition coating. The
applicator is moved relative to frame 50 using an applicator
moving means in the form of robotic arm 56. It will be apparent
to person skilled in the art that other applicator moving means
could be used to control the movement of the applicator relative
to the frame, such as a flat-bed scanner. The movement of the
robotic arm 56 and use of the spray gun 54 is controlled by a
processor (not shown). The
processor enables the even
application of the coatings and can ensure that at the edges of
the patch of coating 10 (see figure 8) are feathered, by
controlling the speed of movement of the arm and/or rate of
deposition of the metal coating, so that patches of coating 10
can be separately formed and a consistent coating be applied to
the whole surface of the sheet steel material 12.
The sheet material is suitable for use in any of the
following applications, which are provided by way of example
only and not as a definitive list, a helicopter landing platform,
a submarine, an oil rig, a chemical processing plant, an energy
production plant, a shipping container and a munitions box. It
is generally the case that the application in which the coating
is being applied to the sheet of steel is already built when the
coating is applied. For example, if the coating is being applied
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to the deck of an aircraft carrier, the coating is applied once
the ship has been constructed.
It will be appreciated by persons skilled in the art that the
above embodiments have been described by way of example only and
not in any limitative sense, and that various alterations and
modifications are possible without departure from the scope of
the protection which is defined by the appended claims.
