Patent Publication Number: US-2018045215-A1

Title: Impeller for rotary machine, compressor, supercharger, and method for producing impeller for rotary machine

Description:
TECHNICAL FIELD 
     The present disclosure relates to an impeller for a rotary machine, a compressor provided with the impeller, a supercharger, and a method for producing the impeller. 
     BACKGROUND ART 
     An internal combustion engine for an automobile, a diesel engine in particular, is often provided with an exhaust gas recirculation (EGR) system. A part of exhaust gas is introduced into a compressor for a supercharger mounted to an internal combustion engine provided with an EGR system, and thus erosion is likely to occur in the compressor impeller due to droplets contained in the exhaust gas. Thus, as a countermeasure against erosion, Ni—P based plating is applied to a compressor impeller made of an Al alloy or the like. 
     Further, a stress due to a centrifugal force generated from high-speed rotation and a stress due to a thermal expansion difference between a Ni—P based plating layer and an Al alloy are generated in a compressor impeller of a supercharger. Thus, a plating layer is required to have not only an anti-erosion property but also an anti-crack property (fatigue strength) and an anti-separation property (interface strength). 
     Once a crack develops on a plating layer, the crack advances to a base material and may break the base material. 
     Patent Document 1 discloses applying Ni—P based alloy plating to a compressor impeller of a supercharger mounted to a ship diesel engine equipped with an EGR system, to improve an anti-erosion property and an anti-corrosion property. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2014-163345A 
     SUMMARY 
     Problems to be Solved 
     While the thickness of a plating layer could be increased to improve the anti-erosion property of the plating layer, a plating layer with an excessively-increased thickness is more likely to separate from the interface of a base material and has a greater risk of generation of fatigue cracks on the surface of the plating layer. On the other hand, a coating layer with a reduced thickness is less likely to develop fatigue cracks, but the anti-erosion property may decrease. 
     As described above, the anti-erosion property and the anti-crack property are incompatible, and it is difficult to balance these two properties. 
     In view of the above problem of typical art, at least one embodiment of the present invention is to enable forming a plating layer that has an anti-erosion property and an anti-crack property (fatigue strength) in a good balance, for an impeller for a rotary machine. 
     Solution to the Problems 
     (1) An impeller for a rotary machine according to at least one embodiment of the present invention comprises: a base material of the impeller comprising Al or an Al alloy; and an electroless plating layer disposed so as to cover the base material, the electroless plating layer forming a surface layer of the impeller. The electroless plating layer comprises a Ni—P based alloy having an amorphous structure, the Ni—P based alloy having a P content rate of not less than 5 wt % and not more than 11 wt % in the electroless plating layer. 
     With the above configuration (1), the electroless plating layer has an amorphous structure and thus has a high strength and an improved anti-erosion property. Furthermore, the electroless plating layer contains P of not less than 5 wt % and not more than 11 wt %, thus having a high Vickers hardness and an excellent anti-crack property (fatigue strength), which makes it possible to suppress generation of cracks on the impeller. 
     Moreover, the electroless plating layer can be formed uniformly, for instance, in terms of the layer thickness, and thus it is possible to exert the above properties uniformly over a broad range. 
     (2) In some embodiments, in the above configuration (1), the electroless plating layer has a layer thickness of not less than 15 μm and not more than 60 μm. 
     If the layer thickness of the electroless plating layer is less than 15 μm, it may be difficult to exert the anti-erosion property and the anti-crack property sufficiently. On the other hand, even if the layer thickness is increased to exceed 60 μm, the effect to improve the anti-erosion property and the anti-crack property is limited, which increases the plating time and costs. 
     With the above configuration (9), it is possible to achieve the anti-erosion property and the anti-crack property with the electroless plating layer having a layer thickness of not less than 15 μm, and it is possible to reduce the plating costs with the electroless plating layer having a layer thickness of not more than 60 μm. 
     (3) In some embodiments, in the above configuration (1) or (2), the electroless plating layer has a Vickers hardness of not less than 500 HV and not more than 700 HV. 
     With the above configuration (3), the electroless plating layer has a Vickers hardness of not less than 500HV and thus can exert an anti-erosion property, while having a Vickers hardness of not more than 700HV and thus being able to exert a high anti-crack property. 
     (4) In some embodiments, in any one of the above configurations (1) to (3), a fracture ductility strain of the electroless plating layer is not less than 0.5% (not repeated but once). 
     With the above configuration (4), if the fracture property strain is not less than 0.5%, it is possible to form a plating layer having a high anti-fatigue fracture property, and thus it is possible to satisfy the allowable repetitive number in a low-cycle fatigue test. Accordingly, it is possible to suppress generation of cracks of an impeller and improve the lifetime of an impeller. 
     (5) In some embodiments, in any one of the above configurations (1) to (4), the impeller is a compressor impeller of a supercharger. 
     With the above configuration (5), the compressor impeller having the above configuration is used as the compressor impeller of the supercharger that rotates at a high speed, and thereby it is possible to improve the anti-erosion property and the anti-crack property (fatigue strength) of the compressor impeller. Accordingly, it is possible to achieve a long-life compressor impeller. 
     (6) A compressor according to at least one embodiment of the present invention comprises a compressor impeller which comprises the impeller according to any one of the above (1) to (5). 
     With the above configuration (6), providing a compressor impeller with a high anti-erosion property and anti-crack property (fatigue strength) makes it possible to extend the lifetime of the compressor. 
     (7) A supercharger according to at least one embodiment of the present invention comprises: the compressor according to the above (6); and a turbine for driving the compressor. 
     With the above configuration (7), providing a compressor including a compressor impeller with a high anti-erosion property and anti-crack property (fatigue strength) makes it possible to achieve a long-life supercharger that can bear high-speed rotation for a long period of time. 
     (8) In some embodiments, in the above configuration (7), the compressor is disposed in an intake passage of an internal combustion engine. The turbine is configured to be driven by exhaust gas from the internal combustion engine. A part of the exhaust gas is circulated to the intake passage at an upstream side of the compressor. 
     As in the above configuration (8), in a supercharger provided for an internal combustion engine including an EGR system, intake air containing exhaust air that contains droplets and has a high erosion property is introduced into a compressor of the supercharger. 
     With the above configuration (8), the supercharger having the above configuration (7) has the above configuration (6) and is provided with a compressor having a high anti-erosion property and anti-crack property (fatigue strength), and thereby it possible to achieve a long-life supercharger that can bear high-speed rotation for a long period of time. 
     (9) A method of producing an impeller for a rotary machine according to at least one embodiment of the present invention comprises: a step of forming an electroless plating layer as a surface layer of the impeller comprising Al or an Al alloy, so as to cover a base material of the impeller. The electroless plating layer comprises a Ni—P based alloy having an amorphous structure, the Ni—P based alloy having a P content rate of not less than 5 wt % and not more than 11 wt % in the electroless plating layer. 
     A compressor impeller produced by the above method (9) has the electroless plating layer formed on the surface. The electroless plating layer has an amorphous structure and thus has a high strength and an excellent anti-erosion property. Furthermore, the electroless plating layer contains P of not less than 5 wt % and not more than 11 wt %, thus having a high Vickers hardness and an excellent anti-crack property (fatigue strength). 
     Moreover, the electroless plating layer can be formed uniformly, for instance, in terms of the layer thickness, and thus it is possible to exert the above properties uniformly over a broad range. 
     (10) In some embodiments, the above method (9) further comprises a step of cutting out a test piece from the impeller on which the electroless plating layer is formed, and using the test piece to evaluate a fracture ductility of the electroless plating layer. Hardness and ductility of a plating layer changes depending on plating treatment conditions such as the total area of an object to be plated by a plating solution during plating treatment, and the relative velocity between the flow of the plating solution and the object to be plated. 
     According to the above method (10), the fracture ductility is evaluated by using a test piece cutout from the compressor impeller on which the electroless plating layer is formed, and thus it is possible to accurately evaluate the fracture ductility of the electroless plating layer of the an actual impeller. 
     (11) In some embodiments, in the above method (10), the test piece is collected from a region on a back surface of the hub of the impeller, the region being a projection of a blade root portion of the hub on the back surface of the hub. 
     While a stress is generated in an impeller due to a centrifugal force caused by rotation, for instance, the greatest stress is generated at the blade root portion of the impeller, as shown in  FIG. 14 . 
     With the above configuration (11), the test piece is collected from a region of a projection of the blade root portion of the hub on the back surface of the hub, and thus it is possible to obtain the fracture ductility under the severest stress conditions. 
     (12) In some embodiments, the above method (10) or (11) further comprises a step of changing a plating condition of the electroless plating layer if the fracture ductility is smaller than a threshold. 
     According to the above method (12), the plating conditions of the electroless plating layer are changed on the basis of the result of the fracture ductility, and thus it is possible to set the fracture ductility of the electroless plating layer to be not less than a threshold. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, it is possible to improve both of an anti-erosion property and an anti-crack property (fatigue strength) of an impeller, and thereby extend the lifetime of the impeller and apparatuses including the impeller. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a system diagram of a diesel engine provided with a supercharger according to an embodiment. 
         FIG. 2  is a schematic cross-sectional view of a compressor impeller according to an embodiment. 
         FIG. 3  is a diagram showing a relationship between the P content rate and the anti-erosion property of an electroless plating layer. 
         FIG. 4  is a diagram showing a relationship between the P content rate and the LCF fracture lifetime of an electroless plating layer. 
         FIG. 5  is a diagram of an example of a cyclic load in an LCF test. 
         FIG. 6  is a diagram showing a relationship between the crystal structure and the anti-erosion property of an electroless plating layer. 
         FIG. 7  is a diagram showing a relationship between the crystal structure and the LCF fracture lifetime of an electroless plating layer. 
         FIG. 8  is a diagram showing a relationship between the layer thickness and the anti-erosion property of an electroless plating layer. 
         FIG. 9  is a diagram showing a result of a corrosion test on an electroless plating layer. 
         FIG. 10  is a diagram showing the fracture ductility of an electroless plating layer. 
         FIG. 11  is an explanatory diagram of a method of testing the fracture ductility with a test piece. 
         FIG. 12  is a flowchart of a method of producing a compressor impeller according to an embodiment. 
         FIGS. 13A and 13B  are diagrams showing a section where a test piece is cut out from a compressor impeller,  FIG. 13A  showing a side cross-sectional view of a compressor impeller and  FIG. 13B  showing a front view of the same. 
         FIG. 14  is a perspective view of a strain distribution which is generated in the compressor impeller. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components. 
       FIG. 14  is a diagram of a compressor impeller  100  of a supercharger provided for an automobile internal combustion engine, coated with a typical Ni—P based plating layer, shown with an analysis result of a distribution of strain generated in the compressor impeller projected on a back surface  102   a  of a hub  102 .  FIG. 14  shows that the greatest strain, that is, stress, is generated in a region  102   b  of the hub  102 , where the root portions of blades  104  are projected. This stress is mainly generated by a centrifugal force generated when the supercharger rotates at a high speed, and is further combined with a stress due to a thermal expansion difference between the Ni—P based plating layer and a base material comprising an Al alloy, for instance. 
     As depicted in  FIG. 1 , a supercharger  12  according to at least one embodiment of the present invention is provided for an in-vehicle internal combustion engine, for instance, a diesel engine  10  equipped with an EGR system. 
     The supercharger  12  includes an exhaust turbine  14  which is disposed in an exhaust passage  20  of the diesel engine  10  and which is rotated by exhaust gas “e”, and a compressor  16  which operates in conjunction with the exhaust turbine  14  via a rotational shaft  13 . The compressor  16  is disposed in an intake passage  22 , and supplies the diesel engine  10  with intake air “a”. A part of exhaust gas is circulated to the intake passage  22  at an upstream side of the compressor  16 . 
     As an exemplary embodiment, as depicted in  FIG. 1 , a high-pressure EGR system  24  has a high-pressure EGR passage  26  branched from the exhaust passage  20  at the upstream side of the exhaust turbine  14  and connected to the intake passage  22  at the upstream side of the compressor  16 . 
     In the high-pressure EGR system  24 , a part of the exhaust gas “e” discharged from the diesel engine  10  is returned to the intake passage  22  at the inlet side of the diesel engine  10  via the high-pressure EGR passage  26 . 
     In an exemplary configuration, an EGR cooler  28  and an EGR valve  30  are disposed in the high-pressure EGR passage  26 . 
     In an exemplary embodiment, a low-pressure EGR system  32  has a low-pressure EGR passage  34  branched from the exhaust passage  20  at the downstream side of the exhaust turbine  14  and connected to the intake passage  22  at the upstream side of the compressor  16 . 
     In the low-pressure EGR system  32 , a part of the exhaust gas “e” discharged from the diesel engine  10  is returned to the intake passage  22  at the inlet side of the compressor  16  via the low-pressure EGR passage  34 . 
     In an exemplary configuration, an EGR cooler  36  and an EGR valve  38  are disposed in the low-pressure EGR passage  34 . 
     In an exemplary embodiment, an air cleaner  40  is disposed in the intake passage  22  at the upstream side of the compressor  16 , and an inter cooler  42  is disposed in the intake passage  22  at the downstream side of the compressor  16 . 
     Further, an exhaust bypass passage  20   a  is connected to the exhaust passage  20  so as to bypass the exhaust turbine  14 . A waste valve  44  is disposed in the exhaust bypass passage  20   a,  and an actuator  44   a  for adjusting the opening degree of the waste valve  44  is provided. 
     Further, a DPF filter  48  for capturing particulate matter in the exhaust gas, and an oxidation catalyst  46  for oxidizing NOx in the exhaust gas to NO 2  and combusting the particulate matter captured by the DPF filter  48  by oxidation of NO 2  are disposed in the exhaust passage  20  at the downstream side of the exhaust turbine  14 . 
     A compressor according to at least one embodiment of the present invention is the compressor  16  provided for the supercharger  12  depicted in  FIG. 1 . The compressor  16  includes a compressor impeller  50  disposed on an end of the rotational shaft  13  inside a compressor housing (not depicted). The compressor impeller  50  has, for instance, a configuration as depicted in  FIG. 13 . 
     As schematically shown in  FIG. 2 , the compressor impeller  50  includes a base material  52  comprising Al or an Al alloy and an electroless plating layer  54  formed on the surface of the base material  52 . The electroless plating layer  54  comprises a Ni—P based alloy having an amorphous structure and containing P of not less than 5 wt % and not more than 11 wt % in the layer. 
     The electroless plating layer  54  has an amorphous structure and thus has a high strength, thus being able to exert a high anti-erosion property. Furthermore, the electroless plating layer  54  contains P of not less than 5 wt % and not more than 11 wt %, which makes it possible to achieve an excellent anti-crack property (fatigue strength) while having a high Vickers hardness. Accordingly, it is possible to achieve both of the anti-erosion property and the anti-crack property. 
     Moreover, the electroless plating layer  54  is an electroless plating layer and thus can be formed uniformly, for instance, in terms of the layer thickness, and thus it is possible to exert the above two properties uniformly over a broad range. 
     As depicted in  FIG. 2 , the intake air “a” may contain a foreign substance such as a droplet L. For instance, if the low-pressure EGR system  32  depicted in  FIG. 1  is employed, the exhaust gas “e” containing a water droplet L is circulated via the low-pressure EGR passage  34  and is supplied to the compressor with the intake gas a. As described above, even if the intake air “a” contains a foreign substance (e.g. droplet L), the electroless plating layer  54  has a good anti-erosion property and a good anti-crack property, thus being resistant to erosion by the exhaust gas “e” and being capable of suppressing generation of cracks. 
       FIG. 3  is a test result showing a relationship between the P content rate and the anti-erosion property of the electroless plating layer  54 .  FIG. 4  is a test result showing a relationship between the P content rate and the low-cycle fatigue (LCF) test fracture lifetime of the electroless plating layer  54 . The low-cycle fatigue (LCF) is a fatigue fracture that develops on a member when such a great cyclic load that causes plastic deformation is applied to the member. 
       FIG. 5  is a diagram of an example of a cyclic load applied to a compressor impeller in an LCF test, where x-axis is time and y-axis is rotation speed of a supercharger equipped with the compressor impeller. A change in the rotation speed of the supercharger changes the cyclic load applied to the electroless plating layer  54 . 
     As depicted in  FIGS. 3 and 4 , the anti-erosion property rapidly decreases when the P content rate exceeds 11 wt %, while the LCF fracture lifetime decreases when the P content rate is less than 5 wt % or more than 11 wt %. 
     From the above result, the electroless plating layer  54  contains P of not less than 5 wt % and not more than 11 wt % to balance the anti-erosion property and the LCF fracture lifetime. 
       FIG. 6  is a test result showing a relationship between different crystal structures and the anti-erosion property of the electroless plating layer  54 .  FIG. 7  is a test result showing a relationship between different crystal structures and the LCF fracture lifetime of the electroless plating layer  54 . The “crystallization” in the drawings means that the electroless plating layer  54  having an amorphous structure is crystallized by heat treatment or the like. 
     As depicted in  FIGS. 6 and 7 , when the electroless plating layer  54  is crystallized, the anti-erosion property and the LCF fracture lifetime deteriorate rapidly. 
     From the above result, the electroless plating layer  54  is formed so as to have an amorphous structure to improve the anti-erosion property and the LCF fracture lifetime. 
     In an illustrative embodiment, the electroless plating layer  54  has a layer thickness of not less than 15 μm and not more than 60 μm. If the layer thickness of the electroless plating layer  54  is less than 15 μm, it may be difficult to exert the anti-erosion property and the anti-crack property sufficiently. On the other hand, even if the layer thickness is increased to exceed 60 μm, the effect to improve the anti-erosion property and the anti-crack property is limited, which rather increases the plating time and costs. 
     Accordingly, it is possible to achieve both of the anti-erosion property and the anti-crack property when the electroless plating layer  54  has a layer thickness of not less than 15 μm, and it is possible to reduce the plating costs when the electroless plating layer  54  has a layer thickness of not more than 60 μm. 
       FIG. 8  is a test result showing a relationship between the layer thickness and the anti-erosion property of the electroless plating layer  54 .  FIG. 9  is a test result showing a relationship between the layer thickness and the anti-corrosion property of the electroless plating layer  54 . 
     As depicted in  FIG. 8 , the electroless plating layer  54  cannot exert the anti-erosion property when having a layer thickness of about 1 to 2 μm, but can exert a high anti-erosion property when the layer thickness is in the range of from 15 to 60 μm. The lines A, B, and C in  FIG. 9  show the progress of corrosion on the electroless plating layer  54  for different corrosion environments.  FIG. 9  shows that the requirement lifetime can be satisfied when the electroless plating layer  54  has a layer thickness of not less than 15 μm, even in the severest corrosion environment. 
     In an illustrative embodiment, the electroless plating layer  54  has a Vickers hardness of not less than 500 HV and not more than 700 HV. In this case, the electroless plating layer  54  has a Vickers hardness of not less than 500 HV and thus can exert an anti-erosion property, while having a Vickers hardness of not more than 700 HV and thus being able to exert a high anti-crack property. 
     In an illustrative embodiment, as depicted in  FIG. 10 , if the fracture ductility strain of the electroless plating layer  54  having the above configuration is not less than 0.5%, the fracture lifetime in a LCF fracture test clears an allowable repetition number and a crack does not occur. 
     Accordingly, the electroless plating layer  54  having the above configuration is a plating layer with a high anti-fatigue fracture property, thus being capable of suppressing generation of cracks of an impeller and of improving the lifetime of an impeller. 
     The fracture ductility is measured by a test as depicted in  FIG. 11 , for instance. In  FIG. 11 , both ends of a test piece T having a plate shape with a rectangular cross section are placed on support bases  60  so that a side on which the electroless plating layer  54  is formed faces down. Subsequently, a load F is applied downward by placing an indenter  62  on an upper surface of the test piece T at the center in the axial direction to generate a predetermined strain. The above operation is repeated while changing the load until the plating layer fractures. 
     The compressor impeller  50  having the above configuration is used as the compressor impeller of the supercharger  12  that rotates at a high speed, and thereby it is possible to improve the anti-erosion property of the compressor impeller  50  and to restrict development of cracks, thus improving the lifetime of the compressor  16  and the supercharger  12  provided with the compressor  16 . 
     Furthermore, even if the supercharger  12  is provided for the diesel engine  10  having the low-pressure EGR system  32  and the intake air “a” that contains exhaust gas containing droplets and having a high erosive property is introduced into the compressor  16 , the supercharger  12  can endure high-speed rotation for a long time and the lifetime can be improved. 
     A method of producing an impeller for a rotary machine according to at least one embodiment of the present invention comprises a step (S 14 ) of forming the electroless plating layer  54  on a surface of the compressor impeller  50  formed of Al or an Al alloy, so as to cover the compressor impeller  50 , as depicted in  FIG. 12 . 
     The electroless plating layer  54  comprises a Ni—P based alloy having an amorphous structure and containing P of not less than 5 wt % and not more than 11 wt % in the electroless plating layer  54 . 
     The compressor impeller  50  produced by the above method has the electroless plating layer  54  formed on the surface. The electroless plating layer  54  has an amorphous structure and thus has a high strength, thereby achieving an excellent anti-erosion property. Furthermore, the electroless plating layer contains P of not less than 5 wt % and not more than 11 wt %, thus having a high Vickers hardness and an excellent anti-crack property (fatigue strength). 
     Moreover, the electroless plating layer  54  can be formed uniformly, for instance, in terms of the layer thickness, and thus it is possible to exert a high anti-erosion property and a high anti-crack property (fatigue strength) uniformly over the entire range of the plating layer. 
     In an illustrative embodiment, as depicted in  FIG. 12 , prior to step S 14 , the method further comprises a step S 12  of cutting out a test piece from the compressor impeller  50  having the electroless plating layer  54  formed thereon, and using the test piece to evaluate the fracture ductility of the electroless plating layer  54 . 
     In other words, as depicted in  FIG. 13 , the test piece T is cut out from the compressor impeller  50  to be used to evaluate the fracture ductility. 
     Hardness and ductility of a plating layer changes depending on plating treatment conditions such as the total area of an object to be plated by a plating solution during plating treatment, and the relative velocity between the flow of the plating solution and the object to be plated. 
     Since the fracture ductility is evaluated by using the test piece T cutout from the compressor impeller  50  on which the electroless plating layer  54  is formed, it is possible to accurately obtain the fracture ductility of the electroless plating layer  54  of the actually-produced compressor impeller  50 . 
     In an illustrative embodiment, as depicted in  FIG. 13 , the test piece T is collected from a region  56   b  on a back surface  56   a  of a hub  56  of the compressor impeller  50 , the region  56   b  being projection of a blade root portion of the hub  56  on the back surface  56   a  of the hub  56 . 
     While a stress is generated in the compressor impeller  50  due to a centrifugal force caused by rotation, for instance, the greatest stress is generated at the blade root portion of the hub  56 , as shown in  FIG. 14 . 
     By collecting the test piece T from the region  56   b,  it is possible to obtain the fracture ductility under the severest stress condition. 
     In an illustrative embodiment, as depicted in  FIG. 12 , if the measured fracture ductility is less than a threshold (S 16 ), the method further comprises a step S 18  of changing plating conditions for forming the electroless plating layer  54  (e.g. relative velocity between the flow of the plating solution and the object to be plated, plating time, etc.) 
     Accordingly, the plating conditions of the electroless plating layer  54  are changed on the basis of the result of the fracture ductility, and thus it is possible to set the fracture ductility of the electroless plating layer  54  to be not less than a threshold. 
     In an illustrative embodiment, as depicted in  FIG. 12 , a pretreatment S 10  is performed on the test piece T that is cut out prior to step S 12 , as shown in  FIG. 12 . 
     The pretreatment S 10  includes: an alkali degreasing step S 10   a  of removing grease or the like adhering to the surface of the test piece T with an alkali solution or the like; an etching treatment step S 10   b  of removing a passive state layer (alumina layer) formed on the surface of the degreased test piece T by using an acid solution or an alkali solution; and a smut removing step S 10   c  of removing smut which is C and Si less soluble to acid or the like remaining in the form of black powder after the etching treatment. 
     In a plating layer forming step S 14 , as an illustrative embodiment, the surface of the test piece T is plated with Zn, and then Zn is replaced with a Ni—P based alloy, thereby forming the electroless plating layer  54 . 
     In an illustrative embodiment, after the plating layer forming step S 14 , performed are a step S 20  of finishing the surface of the test piece T and a check step S 22  of checking the finished test piece T. 
     INDUSTRIAL APPLICABILITY 
     According to at least one embodiment of the present invention, it is possible to form an electroless plating layer on an impeller for a rotary machine, whereby it is possible to achieve both of a good anti-erosion property and a good anti-crack property (fatigue strength), and thereby improve the lifetime of the impeller and apparatuses including the impeller. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10  Diesel engine 
           12  Supercharger 
           13  Rotational shaft 
           14  Exhaust turbine 
           16  Compressor 
           20  Exhaust passage 
           22  Intake passage 
           24  High-pressure EGR system 
           26  High-pressure EGR passage 
           28 ,  36  EGR cooler 
           30 ,  38  EGR valve 
           32  Low-pressure EGR system 
           34  Low-pressure EGR passage 
           40  Air cleaner 
           42  Inter cooler 
           44  Waste valve 
           44   a  Actuator 
           46  Oxidation catalyst 
           48  DPF filter 
           50 ,  100  Compressor impeller 
           52  Base material 
           54  Electroless plating layer 
           56 ,  102  Hub 
           56   a,    102   a  Back surface 
           58 ,  104  Blade 
           60  Support base 
           62  Indenter 
         C Crack 
         S Strain 
         a Intake air 
         e Exhaust gas