EXTERNALLY ELECTROLESS METHOD FOR PRODUCING A NICKEL ALLOY AND CORRESPONDING ELECTROLYTE
The invention relates to a method for producing a nickel-phosphorus alloy in which a metallic substrate is dipped into an aqueous electrolyte containing at least nickel cations, hypophosphite ions, stabilisers and complexing agents, characterised in that the electrolyte additionally contains pyrones of formula I or the derivatives or salts thereof, wherein R1 represents a hydrogen atom or a hydroxyl group, R2 a methyl group, an ethyl group or a hydroxymethyl group, R3 a hydrogen atom or a hydroxyl group, and R4 a hydrogen atom, a hydroxyl group or a methyl ketone group, and to a corresponding electrolyte.
This application claims priority to German Patent Application 10 2017 125954.6, filed Nov. 7, 2017, the entirety of which is hereby incorporated by reference.
FIELD OF THE INVENTIONThe present invention relates to a chemical, i.e. externally electroless method for producing a nickel alloy by electroless metal deposition in an aqueous electrolyte and to a corresponding electrolyte.
BACKGROUNDNickel phosphorus is generally deposited chemically as corrosion and wear protection on metallic materials in the form of a nickel coating. In contrast to nickel electrodeposition, no electricity flow is used for deposition.
These nickel alloys comprise a nickel phosphorus alloy which is used in particular in functional areas of use. The phosphorus deposited in the layer may be used to control the layer properties. In this case, a distinction is drawn between a high (10-14 wt. %), medium (9-12 wt. %) and low (3-7 wt. %) phosphorus content.
A method is known from EP 1 330 558 B1, for example, for producing a lead-free nickel alloy in which a metallic substrate is dipped into an aqueous electrolyte containing nickel cations, hypophosphite ions and bismuth ions in a concentration of bismuth of at most 0.3 ppm and antimony ions in a concentration of antimony of at least 10 ppm.
An electrolyte for producing lead-free, externally electrolessly produced nickel layers is known from U.S. Pat. No. 2,884,344. According to the method described therein, at least two cations are added to the electrolyte, these being selected from the group comprising antimony, arsenic and bismuth.
This autocatalytic deposition of nickel and co-deposition of phosphorus in the presence of sodium hypophosphite to produce nickel-phosphorus alloys has developed in the last few decades from a niche product to a major method used in the electroplating industry.
Due to the co-deposited phosphorus, the resultant layers exhibit considerably improved corrosion resistance and hardness.
Owing to their excellent corrosion resistance (requirements), nickel alloys which have a particularly high phosphorus content are desired.
SUMMARY OF THE INVENTIONProvided herein is a method for producing a nickel-phosphorus alloy wherein a metallic substrate is dipped into an aqueous electrolyte comprising nickel cations, hypophosphite ions, at least one stabiliser, and at least one complexing agent, and wherein the electrolyte further comprises a pyrone of formula I
or a derivative or salt thereof, wherein R1 is selected from the group consisting of a hydrogen atom and a hydroxyl group, R2 is selected from the group consisting of a methyl group, an ethyl group, and a hydroxymethyl group, R3 is selected from the group consisting of a hydrogen atom and a hydroxyl group, and R4 is selected from the group consisting of a hydrogen atom, a hydroxyl group, and a methyl ketone group.
Also provided herein is an aqueous electrolyte for producing nickel-phosphorus alloys comprising nickel cations, hypophosphite ions, at least one stabiliser, and at least one complexing agent, and further comprising a pyrone of formula I
or a derivative or salt thereof, wherein R1 is selected from the group consisting of a hydrogen atom and a hydroxyl group, R2 is selected from the group consisting of a methyl group, an ethyl group, and a hydroxymethyl group, R3 is selected from the group consisting of a hydrogen atom and a hydroxyl group, and R4 is selected from the group consisting of a hydrogen atom, a hydroxyl group, and a methyl ketone group.
DETAILED DESCRIPTION OF THE INVENTIONThe object of the invention is therefore that of providing a method for producing an externally electrolessly produced nickel alloy which brings about a particularly high rate of incorporation or co-deposition of phosphorus. A further object consists in providing an electrolyte suitable therefor.
It has been recognised according to the invention that by using an electrolyte additionally containing pyrones of formula I
or the derivatives or salts thereof (which liberate the corresponding compound in the aqueous medium), wherein R1 represents a hydrogen atom or a hydroxyl group, R2 a methyl group, an ethyl group or a hydroxymethyl group, R3 a hydrogen atom or a hydroxyl group, and R4 a hydrogen atom, a hydroxyl group or a methyl ketone group, the methods leads to high levels of phosphorus deposition and thus to nickel-phosphorus alloys with a high phosphorus content.
In other words, nickel-phosphorus alloys which have a phosphorus content of greater than 10 wt. % based on all the components of the alloy may be produced according to the invention. Such a high deposition rate is preferably achieved in that a phosphorus content of greater than 11 wt. % based on all the components of the alloy is achieved.
The pyrones of formula I may thus comprise inter alia ethyl maltol, kojic acid or dehydroacetic acid (DHA).
In a particularly preferred embodiment, R1 represents a hydroxyl group, R2 a ethyl group and R3 and R4 a hydrogen atom in a pyrone of formula I. In other words, the use of ethyl maltol as an addition is particularly preferred and particularly good results in terms of phosphorus contents are achieved with this pyrone. Phosphorus contents in the alloy of over 11 wt. % may thereby be achieved.
It has furthermore been found that if the electrolyte additionally contains ascorbic acid, iso-ascorbic acid or the salts thereof, in addition to a high phosphorus incorporation rate it is also possible to accelerate deposition of the phosphorus, and this without the assistance of sulfur-containing compounds or further semimetals, such as selenium and tellurium. In particular, a deposition rate of generally more than 12 μm, specifically more than 16 μm and ideally more than 20 μm per hour may be achieved.
The addition of ascorbic acid or iso-ascorbic acid or the salts thereof does not in any way impair stability. On the contrary, ascorbic acid stabilises the system, as may be expected of a bidentate ligand. On the other hand, ascorbic acid accelerates deposition in a similar way as sulfur compounds are known to do.
Alternatively, the known sulfur compounds may be used.
The two types of acceleration are not mutually exclusive and may be combined, i.e. sulfur-containing compounds of known type may also be used in addition to ascorbic acid. It is therefore possible to accelerate medium phosphorus methods to up to 40 μm per hour and high phosphorus methods to over 20 μm per hour.
It has likewise been found that if the electrolyte additionally contains a mono-hydroxycarboxylic acid or the salts thereof, a further improvement in phosphorus content may be achieved while maintaining the deposition rate.
It is preferable for the pyrones of formula Ito be present in an amount of between 5 ppm and the solubility limit, preferably between 0.1 g/l and 25 g/L, particularly preferably between 0.5 g/L and 20 g/L.
If the method is performed at an electrolyte temperature of at least 85° C., preferably at least 92° C., a marked increase in deposition rate may be achieved. This increase does not proceed continuously, but rather in stages. That is to say, three discrete temperature ranges form: (i) below 85° C. virtually no deposition takes place, as with the known methods, (ii) between 86 and 91° C. 50 to 70% faster deposition takes place compared with traditional methods and (iii) from 92° C. the deposition rate is at least doubled.
Moreover, representatives (for example ethyl maltol, kojic acid, DHA) of the stated substance class have pK values of between 5 and 10, which makes it possible to increase the pH of the coating solution to over 5 without increasing the concentration of free nickel ions in aqueous solution. This increase in pH promotes the reducing properties of the hydrophosphite, whereby the deposition rate together with the ascorbic acid is more than doubled overall without impairing phosphorus incorporation.
A particularly high deposition rate is achieved when the pH is increased to over 5 with simultaneous use of ascorbic acid.
The present invention accordingly also provides an aqueous electrolyte for producing nickel-phosphorus alloys containing at least nickel cations, hypophosphite ions, stabilisers, complexing agents and reducing agents, characterised in that the electrolyte additionally contains pyrones of formula I
or the derivatives or salts thereof (which liberate the corresponding compound in the aqueous medium), wherein R1 represents a hydrogen atom or a hydroxyl group, R2 a methyl group, an ethyl group or a hydroxymethyl group, R3 a hydrogen atom or a hydroxyl group, and R4 a hydrogen atom, a hydroxyl group or a methyl ketone group.
EXAMPLESThe following Examples illustrate the invention.
One litre of an aqueous solution consisting of 5 g/L Ni2+ cations, introduced as nickel sulfate, 40 g/L sodium hypophosphite monohydrate, 35 g/L iso-ascorbic acid, 45 g/L lactic acid, 35 g/L ethyl maltol, 2 g/L sodium saccharin and 1 ppm Pb2+ cations, introduced as lead acetate, were heated to 93° C. at a pH of 4.5 (adjusted with sodium carbonate).
A small (special) steel plate with a total surface area of 1 dm2, previously cleaned for 15 minutes by conventional commercial heat degreasing with ultrasound and then activated for 4 minutes in a 10 per cent hydrochloric acid solution, was coated therein for one hour. The small plate was weighed, the layer with semi-concentrated nitric acid was etched away, the weight was determined again and the nickel and phosphorus content in the etching solution was determined using ICP-OES (inductively coupled plasma atomic emission spectroscopy; instrument manufactured by Agilent Technologies (720 Series)) to DIN ISO 4527.
At a deposition rate of 17 μm per hour, the layer exhibited a P content in the alloy of 11.2 wt. % (based on all the components of the alloy).
A comparative example without hydroxy-gamma-pyrone compound with a solution consisting of 5 g/L Ni2+ cations, introduced as nickel sulfate, 40 g/L sodium hypophosphite solution, 22 g/L succinic acid, 45 g/L lactic acid, 5 g/L sodium saccharin and 1 ppm Pb2+ cations, introduced as lead acetate, resulted, at a pH of 4.5 and 93° C., in a deposition rate of 12 μm per hour and a P content in the alloy of 8.4 wt. %.
As in the above example, in further exemplary embodiments 2 to 9 one litre of an aqueous solution was heated which contained the contents of additives shown in Table 1 below.
A small (special) steel plate was coated therein in accordance with the above details and then examined.
The pH and temperature values established in these tests are listed in Table 2, together with the results obtained, namely the phosphorus content (P content in the alloy) and deposition rate.
Claims
1. A method for producing a nickel-phosphorus alloy wherein a metallic substrate is dipped into an aqueous electrolyte comprising nickel cations, hypophosphite ions, at least one stabiliser, and at least one complexing agent, or a derivative or salt thereof, wherein R1 is selected from the group consisting of a hydrogen atom and a hydroxyl group, R2 is selected from the group consisting of a methyl group, an ethyl group, and a hydroxymethyl group, R3 is selected from the group consisting of a hydrogen atom and a hydroxyl group, and R4 is selected from the group consisting of a hydrogen atom, a hydroxyl group, and a methyl ketone group.
- and wherein the electrolyte further comprises a pyrone of formula I
2. The method of claim 1 wherein R1 is a hydroxyl group, R2 is an ethyl group, R3 is a hydrogen atom, and R4 is a hydrogen atom.
3. The method of claim 1 wherein the electrolyte further comprises at least one ascorbic acid compound selected from the group consisting of L-ascorbic acid and salts thereof, iso-ascorbic acid and salts thereof, and substances that produce L-ascorbic acid or iso-ascorbic acid in the electrolyte.
4. The method of claim 1 wherein the electrolyte comprises the at least one ascorbic acid compound in an amount between 0.5 g/L and 100 g/L.
5. The method of claim 1 wherein the electrolyte further comprises a mono-hydroxycarboxylic acid or a salt thereof.
6. The method of claim 1 wherein the compound of formula I is present in a concentration of at least 5 ppm.
7. The method of claim 1 wherein the compound of formula I is present in a concentration of between 0.1 g/l and 25 g/L.
8. The method of claim 1 wherein the method is carried out at an electrolyte temperature of at least 85° C.
9. The method of claim 7 wherein the method is carried out at an electrolyte temperature of at least 92° C.
10. An aqueous electrolyte for producing nickel-phosphorus alloys comprising nickel cations, hypophosphite ions, at least one stabiliser, and at least one complexing agent, or a derivative or salt thereof, wherein R1 is selected from the group consisting of a hydrogen atom and a hydroxyl group, R2 is selected from the group consisting of a methyl group, an ethyl group, and a hydroxymethyl group, R3 is selected from the group consisting of a hydrogen atom and a hydroxyl group, and R4 is selected from the group consisting of a hydrogen atom, a hydroxyl group, and a methyl ketone group.
- and further comprising a pyrone of formula I
11. The aqueous electrolyte of claim 10 wherein the electrolyte further comprises at least one ascorbic acid compound selected from the group consisting of L-ascorbic acid and salts thereof, iso-ascorbic acid and salts thereof, and substances that produce L-ascorbic acid or iso-ascorbic acid in the electrolyte.
12. The aqueous electrolyte of claim 11 wherein the electrolyte comprises the at least one ascorbic acid compound in an amount between 0.5 g/L and 100 g/L.
13. The aqueous electrolyte of claim 10 wherein the electrolyte further comprises a mono-hydroxycarboxylic acid or a salt thereof.
14. The aqueous electrolyte of claim 10 wherein the compound of formula I is present in a concentration of at least 5 ppm.
15. The aqueous electrolyte of claim 10 wherein the compound of formula I is present in a concentration of between 0.1 g/l and 25 g/L.
16. The aqueous electrolyte of claim 10 comprising a compound of formula I wherein R1 is a hydroxyl group, R2 is an ethyl group, R3 is a hydrogen atom, and R4 is a hydrogen atom.
Type: Application
Filed: Nov 6, 2018
Publication Date: May 9, 2019
Inventors: André Egli (Luzern), Mathias Schnippering (Winterthur)
Application Number: 16/181,433
