Method for preventing corrosion and component obtained by means of such

Abstract

A method for preventing corrosion in a component of a turbo-machine having a metal substrate made of carbon steel, low alloy steel and stainless steel includes: a first deposition step of depositing a first metallic layer on the substrate by electroplating; a second deposition step of depositing at least a second layer of a nickel alloy on the first layer by electroless plating; at least one thermal treatment step after the deposition steps, said thermal treatment being applied at a temperature and for a time depending on the overall thickness of the layers, the value of said temperature being directly proportional to the thickness, the value of said time being inversely proportional to the temperature.

Claims

1. A motor-compressor casing, comprising: a metal substrate made of carbon steel, low alloy steel, or stainless steel; and a coating comprising nickel on the metal substrate, the coating comprising: at least a first metallic layer deposited by electroplating; and at least a second layer of a nickel alloy deposited by electroless plating, wherein the second layer of the nickel alloy comprises a first portion of the second layer and a second portion of the second layer, wherein the first portion of the second layer comprises a nickel alloy having substantially the same proportions of constituent metals as the second portion of the second layer, and wherein the thickness of the coating is between 150 m and 300 m.

2. A turbomachine comprising a motor-compressor casing according to claim 1.

3. The motor-compressor casing according to claim 1, wherein the first portion of the second layer has a thickness between 10 m and 25 m.

4. The motor-compressor casing according to claim 1, wherein the second portion of the second layer has a thickness between 20 m and 289 m.

5. The motor-compressor casing according to claim 1, further comprising: a third metallic layer on the second layer by electroplating; and a fourth layer of the nickel alloy on the third layer by electroless plating.

6. The motor-compressor casing according to claim 1, wherein the layers of the nickel alloy comprise 9% to 11% of phosphorus.

7. The motor-compressor casing according to claim 1, wherein the coating has a hardness value between 600 HV.sub.100 and 650 HV.sub.100 and a ductility value between 1.000% and 1.025%.

8. The motor-compressor casing according to claim 1, wherein the second portion of the second layer has a thickness equal to or greater than twice a thickness of the first portion of the second layer.

9. The motor-compressor casing according to claim 1, wherein the second portion of the second layer has a thickness between 20 m and 100 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other object feature and advantages of the present invention will become evident from the following description of the embodiments of the invention taken in conjunction with the following drawings, wherein:

(2) FIGS. 1A and 1B are two block diagrams schematically showing a first embodiment and a second embodiment, respectively, of a method for preventing corrosion according to the present invention;

(3) FIG. 2 is an assonometric view of a component of a subsea turbomachine according to the present invention;

(4) FIG. 3 is a section view of the component of FIG. 2;

(5) FIG. 4 is a section view of a component of a centrifugal turbo-compressor for onshore or offshore applications, according to the present invention;

(6) FIG. 5 is an enlarged view of the detail V in FIGS. 3 and 4;

(7) FIG. 6 is an enlarged view of the detail V in FIGS. 3 and 4, corresponding to a different embodiment of the present invention;

(8) FIG. 7A is a schematic view of a known-in-the-art plant for extracting gas from a reservoir;

(9) FIG. 7B is a schematic view of a plant for extracting gas from a reservoir, including a component of a turbomachine according to the present invention.

DETAILED DESCRIPTION

(10) With reference to the attached figures, a method for preventing corrosion in a component 1 of a turbo-machine 201 is overall indicated with 100. The component 1 has a metal substrate 5 made of carbon steel, low alloy steel or stainless steel.

(11) In the embodiment in FIGS. 2 and 3, the subsea component 1 is the casing of a subsea compressor.

(12) According to the embodiments in FIG. 4, the method of the present invention is applied to the casing of a motor-compressor operating onshore or offshore.

(13) Particularly, albeit not exclusively, the method of the present invention can be successfully applied to other components for subsea applications or operating in other type of humid environment, particularly when carbon dioxide (CO.sub.2) and/or hydrogen sulphide (H.sub.2S) and/or chlorides are present, provided that the method 100 comprises at least a first deposition step 110, a second deposition step 120 and a final thermal treatment step 140, as detailed in the following.

(14) The first deposition step 110 consists in depositing a first layer 2a of metallic nickel on the metal substrate 5 by electroplating.

(15) The first layer 2a is known in the art as nickel strike and has a thickness comprised between 1 to 10 m, providing activation for the following second step 120

(16) The second deposition step 120 consists in depositing a second layer 2b of a nickel alloy on the first layer 2a by electroless nickel plating (also known as ENP).

(17) According to an embodiment of the present invention, the nickel alloy used in the second deposition step 120 of the method 100 consists of a nickel-phosphorous alloy.

(18) According to a more specific embodiment of the present invention, the nickel-phosphorous alloy used in the second deposition step 120 includes 9 to 11% of phosphorous.

(19) According to other embodiments of the present invention, different nickel alloys are used, for example a nickel and boron alloy.

(20) According to an embodiment of the present invention (FIG. 1A and FIG. 5), the second deposition step 120 includes a first phase of depositing a first portion 20b of the second layer 2b and a second phase of depositing a second portion 21b of the second layer 2b. The thickness of the first portion 20b of the second layer 2b is comprised between 10 to 25 m.

(21) The thickness of the second portion 21b of the second layer 2b is equal or greater than the double of the second layer, i.e. equal or greater than 20 m.

(22) According to another embodiment of the present invention, the method 100 includes further steps of depositing further layers of the nickel alloy by electroless nickel plating, each layer having a thickness greater than the thickness of the previous one.

(23) According to another embodiment of the present invention (FIG. 1B and FIG. 6), the method 100, after the second deposition step 120 include a third deposition step 130 of depositing a third nickel layer 2c on the second layer 2b by electroplating and a fourth deposition step 135 of depositing a fourth layer 2d of nickel alloy on the third layer 2c by electroless plating. The third layer 2c is obtained by impulse electroplating and provides adhesion between the second and fourth ENP layers 2b, 2d. In addition, the third layer 2c avoids formation of pinholes porosity which often occurs in ENP layers having a thickness of more than 100 m.

(24) According to another embodiment of the present invention (whose results are not shown), the third and fourth deposition steps 130, 135 can be repeated more than one time in order to obtain a multilayer structure wherein each electroless-plating layer is deposited over a respective electroplating nickel layer.

(25) At the end of the electroless nickel plating, a nickel-based coating 2 on the metal substrate 5 is obtained.

(26) As described above, according to different embodiments of the present invention, the coating 2 may include one or more ENP layers.

(27) In the embodiment of FIG. 5, the coating 2 consists of the first and second layers 2a, 2b, the latter comprising a first and a second portion 20b, 21b, both obtained by electroless nickel plating.

(28) In the embodiment of FIG. 6, the coating 2 consists of the first, second, third and fourth layers 2a, 2b, 2c, 2d.

(29) In all cases the overall thickness of the coating 2 is between 70 m and 300 m.

(30) With reference to FIGS. 2 and 3, the coating 2 is applied to the inner side of the casing of a subsea motor-compressor. With reference to FIG. 4, the coating 2 is applied to the inner side of the casing of a motor-compressor for onshore or offshore applications.

(31) According to other embodiments of the present invention, the coating 2 is applied also on the outer side or on both the inner and the outer sides.

(32) After the deposition steps 110, 120, 130, 135 the method 100 includes a final thermal treatment step 140 applied by exposing the coating 2 to a heating environment, for example in heat treatment oven, at a temperature T and for a time t. The execution of the thermal treatment step 140 allows to get the desorption of the hydrogen incorporated in the coating during the electroplating process. Moreover, through the thermal treatment step 140 the layers of the coating 2, are made more resistant, adherent to each other and structurally homogeneous.

(33) The values of temperature and time data T,t are comprised between 100 C. and 300 C. and between 2 h and 6 h, respectively. The values of temperature and time depend on the overall thickness of the coating 2, the value of said temperature T being directly proportional to the thickness of the nickel coating 2, the value of said time t being inversely proportional to the thickness of the temperature.

(34) In one embodiment of the method 100 the values of temperature T and of time t are dependent on the value of the overall thickness of the nickel coating 2, according to the following table:

(35) TABLE-US-00001 thickness of time of heat temperature of coating 2 treatment heat treatment 150 m 2 hours 200 C. 120 m 3 hours 190 C. 100 m 4 hours 180 C.

(36) The above heat treatment allows to reach an hardness value between 600 HV.sub.100 and 650 HV.sub.100 and a ductility value between 1.000% and 1.025% in the nickel-based coating 2. The hardness of the coating 2 improves resistance to erosion or abrasion from solid particulate which may flow in the turbo-machine 201, in contact with the coating 2.

(37) The best hardness and ductility results are obtained when the thickness of the coating 2 is between 150 m and 300 m.

(38) According to other embodiments of the present invention, more than one final thermal treatment step are applied, provided that the above characteristics are reached in the coating 2.

(39) With reference to FIG. 7A a conventional plant 200a for extracting a liquid and/or gaseous hydrocarbon mixture from a natural reservoir 205 includes a wellhead 202, a dry or wet scrubber 207 downstream the wellhead 202, a filter 208 downstream the scrubber 207 and a traditional turbo-machine 201a, e.g. a traditional centrifugal compressor or a subsea motor-compressor. The scrubber 207 prevents pollutants and in particular corrosive substances, e.g. carbon dioxide (CO.sub.2) and/or hydrogen sulphide (H.sub.2S) and/or chlorides, to reach the turbo-machine 201a. The filter 208 prevents solid particulate to reach the turbo-machine 201a. With reference to FIG. 7B, a plant 200 according to the present invention for extracting the same hydrocarbon mixture from the natural reservoir 205 includes a pipeline 203 and the turbo-machine 201. The pipeline 203 directly connects the turbo-machine 201 of the present invention to the wellhead 202. This means that the anti-corrosive properties of the turbo-machine according to the present invention permit to avoid the use of scrubbers and filter systems upstream the turbo-machine.

(40) All the embodiments of the present invention allow to accomplish the object and advantages cited above.

(41) In addition the present invention allows to reach further advantages. In particular, the method above described allows to avoid the presence of through porosity in the coating.

(42) This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.