Composition for electroless platinum plating and process for platinum plating

Abstract

The subject-matter disclosed herein relates to a composition (PC) suitable for electroless platinum plating, a process for plating a coating of platinum onto a substrate based on the use of said composition (PC), an apparatus suitable for performing said process and a platinum plated article formed therefrom.

Description

TITLEComposition for electroless platinum plating and process for platinum platingDESCRIPTIONTECHNICAL FIELD

[0001] The subject-matter disclosed herein relates to a composition (PC) suitable for electroless platinum plating, a process for plating a coating of platinum onto a substrate based on the use of said composition (PC), an apparatus suitable for performing said process and a platinum plated article formed therefrom.BACKGROUND ART

[0002] Plating of metals is a process wherein metals are deposited on the substrates. Said process is typically used in several sectors to impart specific properties (for example to improve resistance to corrosion and / or abrasion) to the surface of a particular substrate. As known, various methods of plating have been developed, among which electroless plating is particularly appealing. Said process comprises the deposition of a coating from plating bath onto a substrate through a reduction reaction catalyzed by the metal being deposited. In particular, electroless plating is based on the autocatalytic oxidation of a reducing agent in a plating bath that provides the electrons for metallic ions reduction.

[0003] Generally, an electroless plating bath includes water, a compound containing the metal to be deposited onto a substrate (usually a platinum a salt), and a chemical reducing agent for the metal ions. The mixing of thereducing agent and the metal compound in the plating bath is usually performed by drop wise addition or by direct mixture.

[0004] One of the main advantages of electroless plating with reference to classic electroplating methods (which are based on the formation of an electrolytic cell wherein the plating metal represents the anode and the substrate represents the cathode) is represented by the fact that with this method (which does not involve the use of electrolytic cell) it is possible to plate a metallic coating onto a substrate which is uniform, even if said substrate has an irregular shape and / or a complex geometry. Indeed, as the distance between the various portions of the cathode and anode will be different in case of a substrate (i.e., the cathode) with an irregular or a particular geometry, classic electroplating methods often generate a coating characterized by a non- uniform thickness.

[0005] In addition, since no electrolytic cell is necessary, electroless plating allows to avoid the relevant costs and times associated with the necessity to optimize the anode design, since it is possible to avoid the complexity of shaped anodes which are necessary to ensure an acceptable coating thickness distribution in the electrolytic processes (indeed, the irregularity of the thickness of the coating obtained with classic electroplating methods can be reduced by optimizing the anode design). Furthermore, electroless plating allows to reduce the amount of platinum that must be used in the process, with advantages in terms of costs.

[0006] However, despite these advantages, electroless plating still presents some drawbacks. In particular, the plating bath formulations known in the art are characterized by low stability and plating rates, with the possibility to generate coatings with a thickness usually lower than 1-3 pm, a thickness which sometimes is not sufficient to impart the desired properties to the treated substrate. More specifically, said thickness values may not be sufficient inseveral applications, such as for example with reference to protective platinum coatings applied to gas turbine engine components. The blades, nozzles and airfoils which are commonly used in gas turbine equipment are typically made of nickel and cobalt based superalloys. Said materials are able to provide the necessary mechanical strength and / or resistance to degradation (such as for example oxidation and corrosion). Indeed, gas turbine components are exposed to high temperatures (>l,000°C) for extended periods of time. Platinum coatings are often used to increase the resistance to degradation of gas turbine components, thus allowing the latter to be exploited for long periods of time. In particular, gas turbine components are commonly coated with platinum aluminide coatings to inhibit oxidation and corrosion of the surface. However, due to the particular purposes, properties and shapes of gas turbine components, it is still felt in the sector the need to provide a protective platinum coating which is homogeneous and with a thick value higher than 3 pm that can be obtained in short time intervals (i.e., it is still felt the need to provide high deposition rates, with advantages in terms of costs). For example, patent publication US6391477B1 describes electroless acidic aqueous plating compositions comprising water soluble platinum salts and hydrazine hydrate. However, said compositions appears to be unstable and the deposition rates which are obtained are quite limited (indeed, as can be seen from Examples 1- 8, a platinum coating with a thickness less than 0.8 micron is usually obtained after 1 hour at 80°C), with disadvantages in terms of costs. The same observations apply for the compositions described in the scientific paper “Fabrication of platinum coating on continuous porous SiC-Si3N4 composites by the electroless deposition process" (Paul R.K Et Al., Journal Of Materials Processing Technology, Elsevier, NL) and in the patent publication US8354552B2.

[0007] Hence, there is the need to overcome the drawbacks which still characterize electroless plating methods.SUMMARY

[0008] According to an aspect, the subject-matter disclosed herein relates to an innovative composition (PC) comprising: a) A reducing agent; b) A water-soluble platinum salt; c) A pH regulator and d) Water; and having a pH of about 2.75 or less.In particular, said composition (PC) is suitable for electroless platinum plating.

[0009] According to another aspect, the subject-matter disclosed herein relates to the use of said composition (PC) in an electroless plating processes. In particular, said composition may be used in the plating bath. In other words, the plating bath may comprise or consist of said composition (PC).

[0010] According to still another aspect, the subject-matter disclosed herein relates to an innovative process for platinum plating comprising the following steps: a) contacting a substrate with the composition (PC) described above; b) performing electroless plating on said substrate in order to apply a coating on said substrate; c) Optionally, performing an aluminization step of the coating obtained in step b).

[0011] According to still another aspect, the subject-matter disclosed herein relates to an apparatus for carrying out said innovative process.

[0012] Finally, according to still another aspect, the subject-matter disclosed herein relates to an article comprising a plated substrate obtainable by the innovative process.DETAILED DESCRIPTION OF EMBODIMENTS

[0013] According to an aspect, the subject-matter disclosed herein relates to an innovative composition (PC) comprising: a) A reducing agent; b) A water-soluble platinum salt; c) A pH regulator and d) Water; and having a pH of about 2.75 or less. In a preferred embodiment, the composition (PC) comprises: a) A reducing agent; b) A water-soluble platinum salt; c) A pH regulator; d) Water; e) A complexing agent, wherein said composition (PC) has a pH between 0.7 and 0.95.

[0014] More specifically, it was surprisingly found that in the innovative composition (PC) according to the subject-matter disclosed herein the components (and in particular the water-soluble platinum salt b)) are sufficiently stable. Consequently, thanks to the innovative composition it is possible to generate coatings which are stable, uniform and with a thickness up to 7 pm, independently on the geometry of the substrate. Said properties can be advantageously exploited in several applications, such as for example in applying protective platinum coatings applied to gas turbine engine components. Indeed, thanks to the present invention:• the amount of platinum is optimized;• no anodes design and production are required;• the uniformity of platinum thickness is guaranteed on any geometry (hence complex geometries, such as for example multi-airfoil nozzles, can be easily coated).

[0015] Reference now will be made in detail to embodiments of the disclosure. The examples are provided by way of explanation of the disclosure and should not be construed as a limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure.

[0016] In a preferred embodiment, said composition (PC) has a pH of 1.5 or less. This because it was found that at this pH range it is possible to advantageously limit the reduction rate of Pt2+to Pt°.

[0017] More preferably, said composition (PC) has a pH not lower than 0.2. This because it was found that at pH values lower than 0.2 the coating applied to the substrate tends to peel out (even if it remains cohesive).

[0018] Even more preferably, said composition (PC) has a pH between 0.7 and 0.95. In the most preferred embodiment, said composition (PC) has a pH equal to 0.9.

[0019] Said pH values are referred to pH measurements performed on said composition (PC) at 25°C.

[0020] Indeed, it was found that working in said range of pH allows to decrease the reduction potential of the reducing agent a) in order to obtain a controlled deposition of platinum, with the best compromise in terms of coating tension (indeed, poor coating elasticity is associated to high fragility), bath stability and deposition rate.

[0021] In a preferred embodiment, said reducing agent a) is a hydrazine compound. More preferably, the hydrazine compound is hydrated, even more preferably the hydrazine compound is hydrazine monohydrate (N2H4 • H2O). Other suitable hydrazine compounds include hydrazine chloride and hydrazine sulfate. Moreover, further compounds already known in the art can be used as reducing agent, such as sodium borohydride (NaBIHLi) and formic acid (CH2O2).

[0022] The amount of reducing agent a) in the composition (PC) may range from about 0.1 to about 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 1 to about 5 g / L. Said amounts can be applied to all the compounds cited above. Indeed, high concentration values of reducing agent a) bring to stability problem of the aqueous solution bath, while low concentration values bring to lower deposition rates (since the start of the reduction reaction becomes more difficult). The ratio between the moles of reducing agent a) and the moles of platinum is comprised between 1 : 1 and 1 :0.01, more preferably comprised between 1 :0.5 and 1 :0.02, even more preferably equal to 1 :0.16.

[0023] The most preferable reducing agent a) is hydrazine monohydrate. In particular, it was found that for hydrazine hydrate the most preferable amount is 3 g / L. However, said amount can be applied also to the other compounds cited above. The ratio between the moles of hydrazine monohydrate and the moles of platinum is comprised between 1 : 1 and 1 :0.01, more preferably comprised between 1 :0.5 and 1 :0.02, even more preferably equal to 1 :0.16.

[0024] In a preferred embodiment, the water-soluble platinum salt b) is a compound which is able to release tetrachloroplatinate salts, such as for example potassium tetrachloroplatinate (K^PtCL) and sodium tetrachloroplatinate (JS^PtCL). Other suitable water-soluble platinum salts b) include Pt(II) and Pt(IV) salts which are soluble in water such as platinumsulphate (PtSCh), potassium hexachloropl atinate(IV) (K^PtCk); sodium hexachloroplatinate(IV) (Na2PtCle); tetraammineplatinum(II) nitrate (Pt(NH3)4(NO3)2(which is water soluble despite the precipitation of platinum oxide due to the agglomeration reaction of platinum complexes), diammineplatinum(II) nitrite (Pt(NH3)2(NO2)2, cisplatin (Pt(NH3)2Ch); ammonium tetrachloroplatinate(II) ((NH4)2[PtCh]). It is also possible to use compounds which are able to release ions Pt2+ions in the aqueous solution, such as platinum (II) chloride (PtCh) and hexachloroplatinic acid (H^PtCk).

[0025] The preferred amount of platinum salt composition (PC) may range from about 0.1 to about 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 1 to about 5 g / L. Said amounts can be applied to all the compounds cited above. Indeed, said values allows a good compromise between stability in the bath and the plating rate. In particular, high concentration values of water-soluble platinum salt b) bring to stability problem of the aqueous solution bath, while low concentration values bring to lower deposition rates.

[0026] The most preferable water-soluble platinum salt b) is tetrachloroplatinate, even more preferably potassium tetrachloroplatinate.

[0027] In particular, it was found that for potassium tetrachloroplatinate the most preferable amount is 3.9 g / L. However, said amount can be applied also to the other compounds cited above.

[0028] The pH regulator generally comprises any organic or inorganic acid such as hydrochloric acid, acetic acid nitric acid and combinations thereof. The amount of said pH regulator is any amount sufficient to maintain the pH of the plating bath within the range mentioned above. In a preferred embodiment, said organic or inorganic acid is chosen on the basis of the counter-ion of the platinum salt. For example, if platinum is present in the plating bath as platinum (II) sulfate, then sulfuric acid can be used as pH regulator.

[0029] The pH regulator may also comprise buffering agents in order to better keep the pH stable during the electroless plating process.

[0030] In a preferred embodiment, said composition (PC) further comprises a complexing agent. Said complexing agent allows to prevent the chemical reduction of the platinum ions which are present in the composition (PC) and, at the same time, it allows their selective chemical reduction on the substrate’s surface.

[0031] Said complexing agent may be for example an EDTA salt (and in particular its disodium EDTA) and / or EDTA in acid form. In particular, under the pH conditions described above, EDTA is present in the composition (PC) in pentaprotonated or hexaprotonated form, with a consequent increasing of its solubility in the composition. However, said complexing agent may be any already known complexing / chelating agent such as sodium phytate, organic acids (such as citric acid and glutamic acid), hydroxymethyl compounds, diethylenetriamine pentaacetic acid (DTPA), nitrilotriacetic acid, phosphonates, glycine, polysaccharides, polypeptides, histidine, polynucleic acids, macrolides, crown ethers, ionophores, and mixtures of the compounds cited above.

[0032] The amount of said complexing agent in the composition (PC) may range from about 0.1 to about 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 2 to about 8 g / L. Said amounts can be applied to all the compounds cited above. In a preferred embodiment, the amount of moles of said complexing agent in the composition (PC) may be based on the amount of platinum moles which are present in the composition (PC). In particular, the ratio between the moles of said complexing agent and the moles of platinum may be comprised between 4: 1 and 1 :4, more preferably between 2: 1 and 1 :2, even more preferably is equal to 1 : 1. Said amounts can be applied to all the compounds cited above. Indeed, high concentration valuesof complexing agent may bring to stability problems due to the precipitation of ethylenediaminetetraacetic acid.

[0033] The most preferable complexing agent is disodium EDTA. In particular, it was found that for disodium EDTA the most preferable amount is comprised between 3 and 5 g / L, more preferably equal to 3.3 g / L. However, said amount can be applied also to the other compounds cited above.

[0034] The composition (PC) may also comprise further additives on the basis of the particular application of the plating process, such as stabilizers (which may be useful to control the plating rate and prevent decomposition of the bath) and surfactants.

[0035] In a preferred embodiment, the composition (PC) comprises or consists of: a) A reducing agent; b) A water-soluble platinum salt; c) A pH regulator; d) Water and e) Optionally, a complexing agent;And said composition (PC) has a pH of about 2.75 or less (more preferably 1.5 or less, even more preferably between 0.7 and 0.95, still more preferably equal to 0.9). In a more preferred embodiment, the composition (PC) comprises or consists of: a) A reducing agent; b) A water-soluble platinum salt; c) A pH regulator; d) Water and e) A complexing agent;and said composition (PC) has a pH between 0.7 and 0.95, more preferably equal to 0.9. In a more preferred embodiment, the composition (PC) comprises or consists of: a) A reducing agent in an amount ranging from about 0.1 to about 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 1 to about 5 g / L; b) A water-soluble platinum salt in an amount ranging from about 0.1 to about50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 1 to about 5 g / L; c) A pH regulator; d) Water and e) A complexing agent in an amount ranging from about 0.1 to about 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 2 to about 8 g / L, and said composition (PC) has a pH between 0.7 and 0.95, more preferably equal to 0.9.

[0036] In a preferred embodiment, the composition (PC) comprises or consists of:• Hydrazine hydrate;• Potassium tetrachloroplatinate;• Hydrochloric acid;• Water and• Optionally, disodium EDTA;And presents a pH of about 2.75 or less (more preferably 1.5 or less, even more preferably between 0.7 and 0.95, still more preferably equal to 0.9). In a more preferred embodiment, the composition (PC) comprises or consists of:• Hydrazine hydrate;• Potassium tetrachloroplatinate;• Hydrochloric acid;• Water and• disodium EDTA salt and said composition (PC) has a pH between 0.7 and 0.95, more preferably equal to 0.9. In a more preferred embodiment, the composition (PC) comprises or consists of:• Hydrazine monohydrate;• Potassium tetrachloroplatinate;• Hydrochloric acid;• Water and• disodium EDTA salt and said composition (PC) has a pH between 0.7 and 0.95, more preferably equal to 0.9.

[0037] In a more preferred embodiment, the composition (PC) comprises or consists of:• Hydrazine hydrate in an amount ranging from about 1 to about 6 g / L, preferably from about 1 to about 4 g / L, more preferably equal to about 3 g / L;• Potassium tetrachloroplatinate in an amount ranging from about 1 to about 6 g / L, preferably from about 2 to about 4.5 g / L, more preferably equal to about 3.9 g / L;• Hydrochloric acid in an amount sufficient to reach a pH of the composition (PC) of about 2.75 or less (more preferably 1.5 or less, even more preferably between 0.7 and 0.95, still more preferably equal to 0.9);• Water and• Optionally, disodium EDTA in an amount ranging from about 0.1 toabout 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 2 to about 8 g / L, still more preferably equal to about 5.5 g / L. In a more preferred embodiment, the composition (PC) comprises or consists of:• Hydrazine hydrate in an amount ranging from about 1 to about 6 g / L, preferably from about 1 to about 4 g / L, more preferably equal to about 3 g / L;• Potassium tetrachloroplatinate in an amount ranging from about 1 to about 6 g / L, preferably from about 2 to about 4.5 g / L, more preferably equal to about 3.9 g / L;• Hydrochloric acid in an amount sufficient to reach a pH of the composition (PC) between 0.7 and 0.95 (still more preferably equal to 0.9);• Water and• disodium EDTA in an amount ranging from about 0.1 to about 50 g / L, more preferably from about 0.5 to about 10 g / L, even more preferably from about 2 to about 8 g / L, still more preferably equal to about 5.5 g / L.

[0038] In a more preferred embodiment, the composition (PC) comprises or consists of:• Hydrazine hydrate in an amount equal to 3 g / L;• Potassium tetrachloroplatinate in an amount equal to 3.9 g / L;• Hydrochloric acid in an amount sufficient to reach a pH of the composition (PC) equal to 0.9;• Water and• Optionally, disodium EDTA in an amount equal to 3.3 g / L.

[0039] The subject-matter disclosed herein also relates to a process for preparing said innovative composition (PC). In a preferred embodiment, said process comprises the following steps:a) add the complexing agent to water; b) add the water-soluble platinum salt to the solution obtained in step a); c) adjust the pH of the solution obtained in step b) with the pH regulator and d) add the reducing agent.

[0040] As written above, the subject-matter disclosed herein also relates to a process for plating various substrates with platinum through an electroless plating bath. Said process comprises or consists of the following steps: a) contacting a substrate with a plating bath comprising or consisting of a composition (PC) according to any one of the embodiments described above; b) performing electroless plating on said substrate in order to apply a coating on said substrate; c) Optionally, performing an aluminization step of the coating obtained in step b).

[0041] In particular, a substrate is put in contact (preferably immersed) with a plating bath comprising or consisting of a composition (PC) according to any one of the embodiments described above. When the water-soluble platinum salt dissolves, platinum ions are released into the plating bath. Said platinum ions are reduced by the reducing agent a) and are then deposited on the substrate surface (which catalyzes the oxidation reaction of the reducing agent), while the pH regulator allows to maintain the plating bath at the desired pH level. The adding order of the reagents in the plating bath represents an important aspect since it is directly related to the stability of the plating bath. In particular, it is important to add the reducing agent in the acid solution.For example, as also seen before, the plating bath may be prepared by adding to water the complexing agent, then the platinum salt and, once the pH hasbeen adjusted to the desired value with the pH regulator, the reducing agent.

[0042] The substrate is put in contact in the plating solution for a time sufficient to plate the coating of platinum with the desired thickness onto the substrate. Usually, the deposition rate (known also as plating rate) is about 0.5 to 1.5 pm / hour. The rate can be increased by modifying the plating conditions (such as by increasing the bath temperature and / or the concentration of water- soluble platinum salt b) and reducing agent a)).

[0043] Moreover, further water-soluble platinum salt b) may be added to the plating bath during the step b) of the plating process in order to maintain a certain concentration of platinum ions in the plating bath during said step b) (thus replacing the platinum ions which are deposited on the substrate surface) and, consequently, increase the plating rate of the process.

[0044] It is also possible to add the pH regulator during the step b) of the plating process in order to better keep the pH of the plating bath at the desired pH level during said step b).

[0045] Normally, the substrate remains in contact with the plating bath for a time interval comprised between 1 and 14 hours, more preferably between 2 and 6 (depending on the desired thickness that must be applied to the substrate surface). During this time interval, it is possible to perform metal turn over (MTO) in order to continuously supply platinum ions in the plating bath.

[0046] Once the desired coating thickness is reached, the substrate may be subjected to further steps. For example, the applied coating can be subjected to an aluminization step. For example, the platinum coating can be aluminized by pack cementation, vapor phase aluminization, chemical vapor deposition, slurry aluminization. As written before, platinum aluminide coatings can be advantageously used for gas turbine components. Indeed, with the composition (PC) according to the present invention it is possible to obtain an article withthe specifications suitable for the production of platinum-modified aluminides.

[0047] The substrate can be put in contact with the plating bath with several ways known in the art such as immersion, spraying, pouring, etc. More preferably, said substrate is put in contact with said plating bath by immersion.

[0048] The substrate may comprise or consist of any material which is compatible with the electroless plating methods such as metals, alloys and non- metals. In a preferred embodiment, said substrate comprises or consists of nickel and cobalt based superalloys. Indeed, as written above, gas turbine components are typically made of nickel and cobalt based superalloys. In a preferred embodiment, said substrate is a component of a turbine engine equipment such as blades, nozzles and airfoils.

[0049] In a preferred embodiment, the bath loading, expressed as the ratio between the volume of the plating bath and the total surface area of the substrate, is comprised between 2 and 30 mL / cm2, more preferably between 10 and 15 mL / cm2. In particular, it was observed that:• with a bath loading equal to 8 mL / cm2(without adding further platinum ions during the process) an average deposition rate of 0.71 pm / hour in the first 4 hours is obtained (with a deposition rate in the first hour equal to 1.26 pm / hour).• with a bath loading equal to 11 mL / cm2(without adding further platinum ions during the process) an average deposition rate of 0.9 pm / hour in the first 4 hours is obtained (with a deposition rate in the first hour equal to 1.55 pm / hour).• with a bath loading equal to 30 mL / cm2(without adding further platinum ions during the process) an average deposition rate of 1 pm / hour in the first 4 hours is obtained (with a deposition rate in the first hour equal to 1.33 pm / hour).

[0050] In a preferred embodiment, during the step b) (and optionally also during step a)) the plating bath is brought and / or maintained at a temperature comprised between 20°C and about 90°C, more preferably between 40°C and 75°C, even more preferably between 50°C and 60°C. Indeed, high temperature values bring to stability problem of the plating bath, while low concentration values bring to lower deposition rates (since the start of the reduction reaction becomes more difficult). The most preferred temperature value is 55°C.

[0051] As said before, the pH of the plating bath is 2.75 or less, more preferably 1.5 or less, even more preferably between 0.7 and 0.95, still more preferably between 0.8 and 0.9.

[0052] In a preferred embodiment, the substrate is immersed in the plating bath at a temperature comprised between 40 and 75°C, then electroless plating is performed for a time interval which is sufficient to reach the desired thickness of the coating applied to the surface of the substrate. Successively, the obtained plated substrate is removed from the plating bath (and may be subjected to further steps according to the specific applications).

[0053] As written above, the subject-matter disclosed herein relates also to an apparatus for carrying out the process described above.

[0054] In a preferred embodiment, said apparatus comprises a container wherein the plating bath comprising or consisting of a composition (PC) according to any one of the embodiments described above is formed or transferred. Said container may be realized in a non-metallic material in order to prevent the reduction of the metal ions comprised in the container.

[0055] In a preferred embodiment, said apparatus comprises elements performing various functions, such as for example controlling and / or modifying the pH and / or the temperature of the plating bath, removing the plated substrate once the desired coating thickness is reached, etc.

[0056] As written above, the subject-matter disclosed herein relates also to an article comprising a plated substrate obtainable by the process according to the process described above. In other words, the subject-matter disclosed herein relates also to an article comprising a substrate plated (at least in part) in accordance with the process described above. Said article may consists of a substrate plated (at least in part) in accordance with the process described above. As explained above, with the composition (PC) it is possible to obtain a platinum coating which has a uniform thickness (even if said substrate has an irregular shape and / or a complex geometry). Preferably, said platinum coating is higher than 3 micron.

[0057] In a preferred embodiment, said article is a gas turbine engine component, such as for example a blade, a nozzle and / or an airfoil. As written above, said components are typically made of nickel and cobalt based superalloys and are characterized by particular properties and shapes which are not often compatible with known standard electroless plating processes.

[0058] The following non-limiting examples serve to further describe the invention and its advantages.EXAMPLE 1

[0059] A GTD111 squared specimen was immersed in an electroless platinum plating bath according to this invention and with a bath loading of 10.8 mL / cm2. This bath contains 3.90 g / L of K^PtCL salt, 6.60 g / L of Na2EDTA (as dihydrate salt) and 3.00 g / L of N2H4 monohydrate and has a pH value equal to 0.85. In particular, the plating bath was prepared by adding to water the complexing agent, then the water-soluble platinum salt and, once the pH was adjusted to 0.85 with HC1 (pH regulator), the reducing agent (in order to guarantee the stability of the plating bath). Plating was done at 55.4° C for 4 hours (without metal turn over - MTO). A bright and uniform platinum coatingwith a thickness of 4 micron was obtained, with a resulting mean plating rate of 1 micron / h. C. The obtained platinum coating can be used as platinum layer for the manufacturing of Pt-modified aluminide coatings.EXAMPLE 2

[0060] An Inconel 625 squared specimen was immersed in an electroless platinum plating bath formulated according to this invention and with a bath loading of 17 mL / cm2. This bath contains 3.90 g / L of K^PtCh salt, 6.60 g / L Na2EDTA (as dihydrate salt) and 3.00 g / L of N2H4 monohydrate and has a pH value equal to 0.85. In particular, the plating bath was prepared by adding to water the complexing agent, then the water-soluble platinum salt and, once the pH was adjusted to 0.85 with HC1, the reducing agent (in order to guarantee the stability of the plating bath). Plating was done at 55.4° C for 4 hours without MTO. A bright and uniform platinum coating with a thickness of 3.2 micron was obtained with a resulting plating rate of 0.8 micron / hour. Coating can be further built-up at slower rate and a thickness of 6.2 micron can be obtained, without MTO, after 8 hours of deposition. The obtained platinum coating can be used as platinum layer for the manufacturing of Pt-modified aluminide coatings.EXAMPLE 3

[0061] To perform a deposition with MTO, a primary electroless Pt plating bath is formulated according to this invention and two additional solutions containing complexed metal ions and reducing agent (for the refilling of consumed reagents) are separately prepared and stored until use. The primary bath contains 3.90 g / L of K^PtCL salt, 6.60 g / L ofNa2EDTA (as dihydrate salt) and 3.00 g / 1 of N2H4 monohydrate and has a pH value equal to 0.85. In particular, the primary plating bath was prepared by adding to water the complexing agent, then the platinum salt and, once the pH was adjusted to 0.85with HC1, the reducing agent (in order to guarantee the stability of the plating bath). For performing the MTO, two further solutions were prepared: the first one contains 13.92 g / 1 ofNa2EDTA (as dihydrate salt) and 16,45 g / 1 ofK^PtCh salt in a volume of 22 mL of water, and pH was adjusted by adding 2.80 g of HC1 37 %v / v; the second one contains 9.01 g / 1 of N2H4 monohydrate in a volume of 15 mL of water. Deposition was performed by immersing a Rene N4 superalloy disc-shaped specimen in the primary electroless plating bath at temperature 55.4°C, with a bath loading of 10.80 ml / cm2. After two hours of deposition, both MTO solutions were added to the primary bath and plating was carried out for two additional hours. A bright and uniform Pt coating with a thickness of 5 micron was obtained with a mean plating rate of 1.25 micron / h. The obtained Pt coating can be used as Pt layer for the manufacturing of Pt- modified aluminide coatings for the protection of turbomachinery components.COMPARATIVE EXAMPLE 1

[0062] A plating bath comprising 2 g / L of K^PtCh salt and 3.00 g / L of N2H4 monohydrate with a pH of 3 was prepared. It was observed the following:• when N2H4 monohydrate is added in a composition comprising the platinum salt and the pH of the obtained solution is brought to pH 3 by adjusting the pH with acetic acid, a rapid reduction of Pt2+to Pt° (with the consequent precipitation of platinum particles) occurred;• when N2H4 monohydrate is added in a composition comprising the platinum salt and with a pH of 3 (a pH value which is obtained by adding acetic acid to the composition comprising the platinum salt), a rapid reduction of Pt2+to Pt° (with the consequent precipitation of platinum particles) occurred, with the pH value which changed from 3 to 4.9;• when N2H4 monohydrate is added in a composition comprising the platinum salt and with a pH of 2 (a pH value which is obtained byadding acetic acid to the composition comprising the platinum salt), a rapid reduction of Pt2+to Pt° (with the consequent precipitation of platinum particles) occurred, with the pH value which changed from 2 to 3.Hence, in all three cases the platinum reduction was observed at room temperature (25°C) and before the immersion of the substrate in the plating bath. Even immerging the substrate (a GTD111 squared specimen) in the plating bath, it was not possible to apply a coating on said substrate.COMPARATIVE EXAMPLE 2

[0063] A plating bath according to the prior art and comprising 1 g / L of H2PtCle pentahydrate, 11 g / 1 of CeHsChNas (as hydrate salt) and 3.7 g / L of N2H4 monohydrate with a pH of 2 was prepared using HC1 as pH regulator. It was observed the following:• When N2H4 is added to the solution comprising the platinum salt and the complexing agent, an immediate reduction of Pt2+to Pt° (with the consequent precipitation of platinum particles) occurred, giving no time to adjust the pH value;• When N2H4 is added to the solution comprising the platinum salt and the complexing agent with pH prior adjusted to 2, an immediate reduction of Pt2+to Pt° (with the consequent precipitation of platinum particles) occurred and pH value changed from 2 to 3.2.Hence, in both cases the platinum reduction was observed at room temperature (25°C) and before the immersion of the substrate in the plating bath. A substrate (a Rene N4 superalloy disc-shaped specimen) was also immerged in the plating bath and kept in the bath for 30 minutes: low-adherent Pt nanoparticles deposited on said substrate and it was not possible to apply a continuous and uniform coating.COMPARATIVE EXAMPLE 3

[0064] Abath comprising 3.9 g / 1 of K^PtCL, 3.3 g / 1 of Na2EDTA dihydrate and3 g / 1 of N2H4 monohydrate was prepared according to this invention, with a bath loading of 10.8 ml / cm2. The pH was adjusted to 1 using HC1 as regulator.The solution was heated to 55.4°C and exhibited a bright yellow color. A substrate (a Rene N4 superalloy disc-shaped specimen) was immersed in the plating solution. After 30 min of deposition, the solution started to turn grey and reduction of Pt2+to Pt° started to occur in the plating solution. After 1 hour of deposition, the solution appeared colorless and transparent (indicating that no more Pt2+ions were present in solution), and a black Pt deposit of particles was observed at the bottom of the beaker. A black, non-uniform deposition of mixed Pt layer and particles was observed on the substrate. It was not possible to evaluate thickness of the deposit due to its non-uniformity and extremely poor adherence, which precluded its handling.

Claims

CLAIMS1. Composition (PC) comprising or consisting of: a) A reducing agent; b) A water-soluble platinum salt; c) A pH regulator; d) Water; e) A complexing agent, wherein said composition (PC) has a pH between 0.7 and 0.95.

2. The composition (PC) of claim 1, wherein said composition (PC) has a pH between 0.8 and 0.9.

3. The composition (PC) according to at least one of the previous claims, wherein said reducing agent a) is a hydrazine compound, more preferably hydrazine hydrate.

4. The composition (PC) according to at least one of the previous claims, wherein said water-soluble platinum salt b) is tetrachloroplatinate, more preferably potassium tetrachloroplatinate.

5. The composition (PC) according to at least one of the previous claims, wherein said complexing agent is EDTA, more preferably EDTA in salt form.

6. The composition (PC) according to at least one of the previous claims, wherein the ratio between the moles of reducing agent a) and the moles of platinum salt is comprised between 1 : 1 and 1 :0.01, more preferably comprised between 1 :0.5 and 1 :0.02, even more preferably equal to 1 :0.16.

7. The composition (PC) according to at least one of the previous claims, wherein the ratio between the moles of said complexing agent and the moles of platinum may be comprised between 4: 1 and 1 :4, more preferably between 2: 1 and 1 :2, even more preferably is equal to 1 : 1.

8. The composition (PC) according to at least one of the previous claims,wherein:• said reducing agent is hydrazine monohydrate;• said water-soluble platinum salt is potassium tetrachloroplatinate;• said complexing agent is EDTA disodium salt dihydrate and• said pH regulator is hydrochloric acid.

9. A process for platinum plating comprising the following steps: a) contacting a substrate with a plating bath comprising or consisting of a composition (PC) according to any one of claims 1-8; b) performing electroless plating on said substrate in order to apply a coating on said substrate; c) Optionally, performing an aluminization step of the coating obtained in step b).

10. The process of claim 9, wherein the temperature of the plating bath during step b) is comprised between 40 and 75°C, preferably between 50 and 60°C, more preferably equal to about 55°C.

11. An article comprising a plated substrate obtainable by the process according to claims 9-10, wherein the coating of said plated substrate has a uniform thickness which is higher than 3 micron.

12. Process for preparing a composition (PC) according to any one of claims 1-8 which comprises the following steps: a) add the complexing agent to water; b) add the water-soluble platinum salt to the solution obtained in step a); c) adjust the pH of the solution obtained in step b) with the pH regulator and d) add the reducing agent.