PRODUCTION METHOD OF A COATING LAYER FOR A PIECE OF TURBOMACHINERY COMPONENT, THE COMPONENT ITSELF AND THE CORRESPONDING PIECE OF MACHINERY
DESCRIPTION
Technical field
This invention relates to a production method regarding a coating layer for a turbomachinery component. It also relates to the component itself and the piece of machinery where the component is installed.
Background information and prior art
Turbomachinery impellers are crucial components because they interact with the process fluids
and also because they endure continuous mechanical, chemical and thermal stress.
These components are traditionally produced with "heavy" alloys so that a high degree of
durability is ensured during operation.
By "heavy" alloy is usually meant a metal based alloy: the metal has a high atomic number, such
as iron, nickel, cobalt etc. Stainless steel and in general all superalloys (having a nickel, cobalt or
many other metals as a base), are all heavy alloys.
Usually the component, or the material it is made of, based on the specific use, undergoes
mechanical, thermal and/or chemical treatments in order to modify the internal or the superficial
structure, or in order to create a superficial coating which will enhance mechanical, chemical
and/or thermal resistance. '"
Usually a nickel-plating procedure follows to create a protective coating against "corrosion".
"Corrosion" can briefly be defined as a typical process during which a metal undergoes an initial
degradation which is followed by a recomposition with other elements. Metals are at a higher
energetic level than the corresponding minerals and, quite because of this, in specific conditions
of the environment, metals are prone to undergo a transformation or degradation called
"corrosion". The corrosion process can be classified based on the different chemical/physical
processes that characterise it: for example the chemical corrosion in a dry environment, called
"purely chemical corrosion", or the intercrystalline/intergranular corrosion, or the
galvanic/electrochemical corrosion in a wet environment or others thereof.
The nickel-plating is a specific superficial treatment which aims at modifying the superficial
characteristics of the material which is being processed (such as hardness, resistance to external
agents thereof) which allows the deposition of nickel atoms on the surface which needs to be
treated.
Nickel coating has a very low porosity and consequently the process described above firmly
seals the base material in order to preserve it from the aggression of external agents, avoiding
corrosion.
Therefore, the protective capacity against corrosion of the coating is good, even if it also depends
on the type of metal on which it is applied, depending on the specific porosity, roughness and
surface condition of the metal; a high concentration of phosphorus (chemical symbol "P"),
exceeding 10%, seems to enhance the resistance against corrosion.
It is also possible to perform a thermal treatment (annealing) on the coated part, to increase its
hardness and wear resistance, in this last case, though, the resistance against corrosion decreases.
A major drawback linked to the use of nickel-plating to coat the centrifugal impellers of pieces
of turbomachinery is that these impellers undergo radial expansions, due to the centrifugal force,
when in use. Therefore the nickel coating might dilate creating small cracks or fractures in which
the corrosion process might start.
While a new family of tridimensional centrifugal impellers in steel was being developed, the
necessity to use lighter alloys to build them arose, especially in some applications, in order to
reduce production costs and to enhance the performance and mechanics of the machinery on
which they can be fitted.
Another interesting improvement is the increase in rotational speed of the same impellers when
using materials having a higher specific resistance than steel: both titanium and aluminium as
well as magnesium based light alloys have this
characteristic due to their low density.
One of the main disadvantages in using these lighter alloys to create centrifugal impellers is that
they are subject to be eroded by the fluid which, flowing at high speeds, can cause the erosion,
especially if the fluid contains liquid or solid particles. The erosion, usually not significant in
case of impellers made from the traditional heavy alloys, becomes very significant and
potentially catastrophic for impellers in light alloy, due to the low hardness and resistance to
erosion which characterise these materials.
The damage is also worsened by the rotational speed of the impellers, the higher the speed, the
stronger the erosion: this problem limits the use of light alloys, such as aluminium, to build
impellers
having a high rotational speed.
"Erosion" can briefly be defined as a phenomenon which entails the gradual removal of material performed by gas, fluid or liquid external agents, which can act jointly or after an alteration generated by chemical or physical processes. "Abrasion" can also be defined as a specific
eroding phenomenon which entails the gradual removal of material performed by solid external
agents.
A further difficulty is that the coatings for centrifugal impellers must also, in general, be
"machineable" in the easiest and smoothest way to limit production costs. By "machineable" it is
meant their capability to be created through specific devices (electrochemical baths or others),
which will completely coat the surfaces of the complex geometrical shapes of the impellers; this
applying especially to tridimensional closed impellers. Furthermore, these coatings will have to
ensure a high superficial hardness to ensure the resistance and the preservation of the coating
itself
also for long operational periods and also ensure resistance against the eventual impact of foreign
bodies.
Another downside is that the deposition of the coating layers must be carefully controlled to
ensure project tolerances and at the same time to avoid unacceptable faults in the finished
product, such as stains, coating delamination and failures, to remain within the border values
typical for the specific coating.
Thus, at this moment, still withstanding the progresses made by technology, it is problematic and
necessary to create turbomachinery centrifugal impellers which will be lighter and more resistant
to adapt to specific applications but which, at the same time, need to ensure at least the same
resistance against solid particles and liquid drops erosion as the one ensured by "heavy" alloys.
Purpose and summary of the invention
The main purpose is the creation of a method aimed at producing a turbomachinery impeller in a simple and cost effective way, thus overcoming, at least partially, the above mentioned issues.
Another purpose is to create an impeller with better specifications and a piece of turbomachinery
where the impellers will be mounted.
A specific purpose is also to use a specific coating which will eliminate some of the drawbacks
mentioned above, creating, at the same time, a finished product having better specifications than
the ones currently used.
In practice these purposes can be achieved through the method indicated in Claim 1, with an
impeller and a piece of turbomachinery indicated in Claims 6 and 8 respectively and the use
described in Claim 9.
The technical advantages of this invention are listed in the Claims listed below.
A main aspect of this invention is to set a method to produce a turbomachinery impeller which
will at least include the following steps:
- create a "light" alloy impeller,
- coat the impeller with at least one layer of nickel-plated coating
All throughout the document and the Claims attached a "coating layer" will mean a coating layer
which will incorporate intermediate layers or to which more intermediate layers will be added;
thus, the coating will incorporate many layers one on top of the other which will at least partially
penetrate into one another.
A "light" alloy will mean an alloy having a metal base which has a low atomic number, such as
aluminium, titanium, magnesium etc.
A very convenient application of the invention is the one in which the light alloy is aluminium
based, depending on the specific use.
n the application which better fits this invention, the nickel-plating will be made through "chemical nickel plating".
A "chemical nickel plating" is, generically, a process which uses the direct action of a reducing agent in a process bath on nickel ions which will be deposited and which will activate a nickel chemical reduction autocatalytic process; such reduction is caused by sodium hypophosphite (N2H2PO2 × H2O). The mechanical component, immersed in the process bath, will serve as a catalyst. Such deposition can be achieved on any material (even if not an electrical conductor) being it metal, glass, ceramic or plastic.
In particular, and considering the main reagents in the process bath, the following chemical relation can be established:
(1) H2PO2- + H2O → H2PO3- + H2
(2) Ni2+ + H2PO2- + H2O → Ni + H2PO3- + 2H+
The hypophosphite ions in an aqueous solution are catalytically oxidised to become phosphite ions releasing gaseous hydrogen and at the same time the nickel cations are catalytically reduced to nickel metal by the hypophosphite ions in the presence of water, while the hypophosphite ions are oxidised and become phosphite ions releasing, at the same time, hydrogen ions. Being the nickel a catalyst both for the first and for the second chemical reaction, the process is "self-triggered".
The process bath might include more elements or substances depending on the specific application, such as, for example, organic chelants, buffer solutions, exciting agents, stabilising agents, pH regulators or wetting agents in order to achieve an acid or alkaline bath, or a fluoride based or ammonia based bath, or others thereof.
This procedure allows the production of a nickel coating with a constant thickness (which
eliminates the need of correction after deposition) regardless the geometrical shape of the part,
thus avoiding the typical drawbacks involved in electrolytic procedures.
In a very convenient application of this invention, this layer of coating protects the impeller in
light alloy, aluminium alloy and others, from erosion. In this case the nickel plating is applied on
impellers used in pieces of turbomachinery which include process fluids at a high risk for
causing erosive phenomena,, such as gasses with liquid or solid particles in suspension.
The invention might be used in industrial applications such as gas and oil extraction sites,
because the gasses which gush from the well might contain liquid or solid particles.
Further benefits of the chemical nickel plating performed on a light alloy impeller, especially if
aluminium based, but not exclusively, arise from the fact that the adherence of the coating on the
base material, the hardness and the wear resistance are outstanding; it is also possible to enhance
the hardness of the coated part performing further treatments (for example, a thermal annealing
or others thereof) which will increase the resistance of the component against erosion.
According to another aspect, the invention can be seen as the creation of a turbomachinery
impeller in light alloy coated with at least one layer of a protective nickel coat, preferably
chemical nickel plating.
A further aspect sees the invention as regarding a piece of turbomachinery where at least one
impeller of the same type as the ones described above is mounted.
An additional aspect sees the invention as regarding the use of a layer of coating as the ones
described above, to protect at least partially from the erosion an impeller in light alloy, especially
if aluminium based, but not necessarily, of a piece of turbomachinery.
An advantage of the method implemented in the invention is that it becomes possible to coat a
light alloy mechanical component using a protective coating in a simple and cost effective way,
so that it will be possible to effectively mount it on a piece of turbomachinery, especially if the
fluids involved in the process are highly erosive.
Another advantage entailed, is that it becomes possible to easily coat a component which has a
very complex surface to be treated, such as, for example, the one of a tridimensional impeller of
a centrifugal compressor or of an expander.
Another advantage is that producing the centrifugal impellers in a light alloy, significantly
reduces the mass of the component, decreasing the mechanical stress and the vibrations in the
rotor of the machine. Further advantages deriving from mass reduction are the increase of the
number of turbomachinery stages and/or the increase of rotational speed.
Another advantage is that costs and production times are exceptionally lower, thus enhancing
productivity.
Another advantage is that the high quality manufacture, due to the fact that nickel deposition is
easily manageable, is extremely even and delivers a constant thickness.
Another advantage arises from the fact that the method is very versatile, because it can be
implemented through automated processes in combination with possible partial manufacturing
work, such as painting or others thereof.
Another advantage is that it becomes easy to obtain a finished component having the theoretical
fluid dynamics studied in the project, eventually keeping into account specific superficial
increase coefficients.
Ultimately, the invention described above, allows the use of light alloys, especially if aluminium
based, to create impellers for centrifugal compressors or expanders enjoying the advantages
listed above. Further convenient specifications and ways to produce the invention are indicated in the attached Claims and will be described further below in a few examples indicating possible applications.
Brief description of the drawings
The numerous purposes and advantages of this invention will be more evident for the experts in
this field if they refer to the schematic drawings attached, which show practical non restrictive
examples.
In the drawing:
Figure 1 shows a schematic section, not drawn to scale, of a possible realisation of a protective
coating following the procedure described in the invention;
Figure 2 shows a section view of a mechanical component showing a protective coating, created
following the procedures described in the invention;
Figure 3 shows a schematic section of a piece of turbomachinery on which the mechanical
components described in the invention were mounted;
Figure 4 shows an explanatory graph of the results of some erosion tests performed on a set of
samples, some of them coated with the procedures described in the invention, others with
commercial alloys to confront them.
Detailed description of some applications of the invention
In the drawings, (to equal numbers correspond equal parts in all of them), a coating 1, as indicated in the invention, please see Fig. 1, is applied through chemical nickel plating on the surface to be treated 3S belonging to a centrifugal impeller 3 made out of light alloy.
The impeller can be of any kind, such as, for example centripetal, mixed flow or others. Fig. 2 shows a partial section, not drawn to scale, of a centrifugal impeller 3 for a centrifugal compressor, coated with the above mentioned coating 1 as indicated in the invention and mounted on a shaft 5: please note that the surface 3S of the impeller 3 is both external and internal (internal channels), exception made for the hole 3F in which the shaft 5 is mounted. In particular, the impeller 3 drawn in the picture is a three dimensional closed impeller; obviously the impeller could be of any other type, an open three dimensional impeller for example, or a closed two dimensional impeller or an open one or any other type. Please note that figures 1 and 2 and not drawn to scale and that the thickness of layer 1 was drawn only for explanatory reasons.
Fig. 3 shows schematically a generic centrifugal compressor 10 which includes a stator box 12 inside of which the shaft 5 is free to rotate; the shaft rests on a set of bearings which offer support 14 and on which a series of centrifugal impellers 3 were mounted. The impellers have been coated 1, and each one of them is mounted for each stage of the compressor 10. On the box stator channels were carved 16 which allow the process fluid to reach the exit of the first impeller towards the second one of the next stage and so on, until the fluid will exit the machinery from the compressor 10.
Please note that this compressor is just an example, and that the invention can be used in another type of centrifugal compressor or in another centrifugal piece of turbomachinery, such as a pump or an expander or other types of devices. To lay the protective coating 1 the procedure conveniently suggests immersing the impeller 3 in a process bath containing an aqueous solution of reagents.
"he chemical baths contain at least the following reagents: nickel salts, sodium hypophosphite reducers mixed with an aqueous solution. The reaction is triggered spontaneously as soon as the impeller is immersed in the. bath and slowly the impeller 3 will start being covered with the thin layer 1 in nickel.
It is possible to set the thickness of the coating, preferably from 50 to up to 100 microns or more, properly regulating the duration of the immersion of the impeller in the bath (once the deposition speed is known).
It is also possible to apply more layers on the nickel one, such as paints or resins or other similar products depending on the specific application.
It is also possible to use specific elements or substances, such as tungsten carbide, DLC, chrome carbides, lactic acid or others, dissolved in the chemical bath based on the specific application. It is possible to prepare the surface of the impeller 3 for the following treatments implementing a few preliminary treatmentsjsuch as shot peening to reduce the tension and enhance the strain resistance of the material; degreasing of the impeller with solvents or detergents or vapour or immersing the part to perform chemical degreasing treatments; masking of areas of the surface which will not be coated, for example the hole in which the shaft will be mounted, or other treatments thereof.
In the most convenient application of the invention, the light alloy which the mechanical component 3 is made of is an aluminium based alloy.
The following tables indicate, as an example, the composition of the aluminium alloys 7175-T74 and 7050-T7452 (following the definition of the international regulations ASTM B 247 M) which can be used to produce component 3; obviously this is just an example and the light alloy specifications can differ both in the percentages and in the components used.
(Table Removed )
Fig. 4 shows an explanatory graph of the results of some erosion tests performed following the standard indicated by the regulations ASTM D 968-93 on several samples, in which: the X-axis indicates the quantity of sand used in Jitters and the Y-axis indicates the thickness of the eroded sample, based on normalised values (where 100% indicates the maximum erosion value obtained in the test).
In particular, the line 4A shows the results of the test for a sample in an alloy a in steel without coating; line 4B shows a sample made of an aluminium based alloy coated with a layer as indicated in the invention; line 4C shows a sample in an aluminium based alloy coated with a layer of hard anodisation which is Typically used to coat aluminium and the fourth line 4D shows a sample of an aluminium based alloy without coating.
Please note that in this graph the sample made of aluminium based alloy without coating, shows resistance values against erosion caused by solid particles which is significantly lower than the one of steel; please also note how, after the application of the coating, as indicated by the invention, it is possible to give the aluminium a resistance to erosion which is similar to the one of steel and much higher than the hard anodisation coating applied on aluminium to enhance hardness.
It is agreed that the illustration is only an indication and that it does not, in any way, limit the possibilities of the invention, which can vary in form and ways always being pertinent to the foundation at the base of the invention itself. The possible presence of ref. numbers in the
attached Claims has the only aim to make reading easier both when related to the previous text and when referring to the attached drawings, and does not limit, in any way, the scope of protection.

CLAIMS
1. A method for creating a turbomachinery impeller characterised in that it comprises at least the following procedures:
- creating the impeller in a light alloy;
- coating the impeller with at least one layer of nickel-plated coating
2.The method as claimed on Claim 1, characterised in that the nickel plating is performed through chemical nickel-plating.
3. The method as claimed on Claims 1 or 2, characterised in that the nickel-plating at least entails the following phase; immerse the impeller in a process bath which at least contains the sodium hypophosphite ions in aqueous solution.
4. The method as claimed on Claim 3 characterised in that, based on the specific application in which the impeller needs to be used, more elements will be added such as, organic chelants, buffer solutions, exciting agents, stabilising agents, pH regulators or wetting agents.
4. The method according to all the Claims stated above, characterised in that the light alloy used is aluminium or titanium or magnesium, or any light metal.
6. An impeller for a piece of turbomachinery characterised in that it has been created with a light alloy and it has been coated by at least one layer of coating created following all the previous Claims.
7. The impeller in Claim 6, characterised in that it is associated to a piece of turbomachinery which works thanks to a process fluid which triggers erosive phenomena, such as, for example, a gas containing liquid or solid particles.
8. A piece of turbomachinery characterised in that it includes at least one centrifugal impeller according to Claims 6 or 7.
9. Using a layer of chemical nickel coating to protect at least partially a turbomachinery impeller; the impeller is made of light alloy, such as, for example, aluminium or titanium or magnesium or any other light metal based alloy.
10. A method for creating a turbomachinery impeller, substantially as herein described with reference to accompanying drawings and example.
11. Using a layer of chemical nickel coating to protect at least partially a turbomachinery impeller, substantially as herein described with reference to accompanying drawings and example.