Copper-based lubricant for reducing hydrogen wear and related methods

Abstract

The invention relates to a lubricant for protecting friction surfaces from hydrogen-mediated destruction, comprising an organic copper salt (e.g. copper oleate), a succinimide derivative, an aromatic

Description

Technical Field This invention relates to lubricant compositions for reducing hydrogen wear and use thereof. The invention relates to the field of mechanical engineering and primarily to the bearing industry. It addresses the problem of the destruction of friction surfaces by white structure flaking (WSF) and the formation of white etching cracks (WEC), which is catalysed by high-intensity electric fields, leading to the mass destruction of bearing assemblies in wind turbines, traction motors of locomotives, and other machines. The inventors propose that the aforementioned wear problems (WSF and WEC), as well as micropitting of friction surfaces, are caused by hydrogen-induced wear of steel friction surfaces, with electric fields acting as a catalyst for hydrogen generation in the friction zone and facilitating hydrogen penetration into the surface layers of the steel friction surface. Background Art There are four well-known wear mechanisms that exist in industrial applications: adhesive, abrasive, corrosive, and surface-fatigue. However, there is another wear mechanism that has gone largely unaddressed: hydrogen wear. It is important to distinguish between hydrogen embrittlement and hydrogen wear. The phenomenon of hydrogen embrittlement, which has been well known for over a century, is a reduction in the ductility of a metal due to absorbed hydrogen. It usually manifests itself in terms of singular sharp cracks. This process typically occurs due to contact in hydrogenous media and is promoted by high temperatures, atmospheric pressure, high loads and deformation, and cathodic processes. As such, hydrogen embrittlement is a common issue for steelmakers. In contrast, hydrogen wear only occurs during sliding and rolling contacts that occur in rotating machinery such as gears and hydraulics. The high contact pressures cause a higher rate of hydrogen absorption, and therefore, an abnormally high concentration of hydrogen. As hydrogen invades the surface it increases the internal pressure and causes cracks to form. Through repeated cyclic stresses these cracks grow and eventually produce the formation of wear particles. Unlike hydrogen embrittlement, where a different law of hydrogen distribution applies, the phenomenon of hydrogen wear creates an unusual pattern of failure. This pattern of failure can be described as the self-organised destruction in the material's top layer during friction; characterised by a considerable non-equilibrium of processes and by joint impact of co-factors that contribute to absorption of hydrogen in the top layer and destruction of this layer. In the hydrogen wear process, hydrogen can diffuse deep into the surface and concentrate there, making the top layers fragile and provoking more intense deterioration. Factors that determine the intensity of hydrogen absorption include the composition and state of the medium, the treatment of the metal surface, its chemical composition, its atomic structure, its stresses and deformations (in particular, deformations of the metal lattice which raises its energy level), the hydrogen saturation time, and the conditions of hydrogen desorption from metal. Further research has shown that hydrogen wear has increasingly become a significant factor in the wear of machine parts in a vast array of industries, from transportation, aviation, and farming to mining, chemical production, and microbiological production equipment. Another significant factor is that hydrogen imparts higher fragility to products of hardened steel, especially those used under alternating loads such as crankshafts, piston rings, cylinders of internal combustion engines, and bolted joints. In some cases, hydrogen impact during friction determines the service life of the friction part. This last point has significantly changed our understanding of the nature of friction and wear. The thermal, electrical, and magnetic effects of friction, all of which control the hydrogen concentration in metals, are now proven to be factors that determine deterioration. Additionally, during desorption and lubrication, hydrogen can occupy numerous adsorption centres, and enters the top layer in much larger quantities - its concentration in steel rises rapidly to a level above equilibrium. At the same time, defects become more numerous with deformation. This effect will go on until internal pressure finally causes the destruction of steel along all emerging and interconnected cracks. While hydrogen wear is not as well documented as hydrogen embrittlement, it is no less important in understanding how hydrogen invades the metal surface of machine parts, and how this invasion leads to premature failures. Research is ongoing to determine the causes of this high failure rate, but it is generally concentrated on the amount of energy required to remove hydrogen ions from either water or from the base oil. When one considers the sheer size of turbines, the amount of torque that goes through their gearboxes, and the power that flows through their blades, the energy is extreme. Consequently, cracks have appeared on the inner ring of high-speed shafts and high-speed intermediate shafts of bearings. If left ignored, these cracks will extend to the gearbox itself and eventually lead to premature failures. Three factors appear to be working together in hydrogen wear: non-metallic inclusions, sliding, and hydrogen. However, as the wear phenomenon occurs below the surface it is impossible to detect with the naked eye, and by the time it reaches the surface the damage is done. Some researchers have called this phenomenon white etching cracking. The formation of wear particles when the cracks grow is sometimes called white structure flaking. An example of hydrogen wear can be witnessed in the wind turbine industry. It has been found that 60% of the industry's gearbox failures are caused by the WEC phenomenon, in particular in large multi-megawatt (MW) wind turbines. Operators now consider it the single most expensive failure mode for all wind turbine components. Various technologies of electrical insulation are used in bearing assemblies, gearboxes, etc., to reduce the negative impact of electric fields on friction surfaces, but they do not solve the problems of WEC and WSF formation. Some researchers have directed their efforts to electrically insulating the friction surfaces, see for example RU 2 319 869 C2, which achieves this by applying a coating of alumina with titanium, yttrium and magnesium oxides to bearings. Rll 2 683 406 C1 protects bearings of electrical machines from damage from electric currents by gas-plasma spraying a powder material consisting of aluminium oxide and titanium dioxide onto the bearings. Further attempts at insulating films are disclosed in US 6,513,986 B2. JP2002048145A and CN100354542C teach the application of a ceramic coating. JP2003120688A discloses an electrical insulating polyphenylene sulfide resin containing glass fibers for application to bearings. While the activity of hydrogen in the subsurface zone is known to be the result of a complex interaction between several mechanical, electrical, operational, and chemical factors, the impact of tribochemical reactions - the chemical changes that occur to a lubricant and a lubricated surface when separated by a thin tribofilm, generated as a result of interactions between lubricant additives and lubricated surfaces is not well studied. In particular, there is a need for a lubricant composition which can ameliorate the above-mentioned hydrogen-mediated effects. Summary of Invention Provided is a lubricant for protecting friction surfaces from hydrogen-mediated destruction comprising an organic copper salt, a succinimide derivative, an aromatic amine and a base oil. The inventors have found that such a composition is surprisingly effecting in preventing hydrogen-mediated wear. Succinimide derivative The detergent component is a succinimide derivative. Succinimide is an organic compound with the formula (CH2)2(CO)2NH. The succinimide derivative acts as a dispersant in the lubricant composition. The succinimide derivative may be selected from any succinimide-based dispersant, optionally selected from polyisobutylene succinimides, often referred to as PIBSI, a polyalkenyl succinimide or an alkenyl succinimide. The succinimide derivative may optionally be a bisuccinimide. The succinimide derivative may optionally be boronated. A particularly preferred succinimide derivative is polyisobutylene bis-succinimide. The succinimide derivative is present in an amount of from 0.5 to 3 wt%, or 1 to 2 wt%, or about 1.5 wt%. Good results have been achieved with 1.5 wt% polyisobutylene bis-succinimide. Commercially available succinimides are often provided in an oil soluble solvent such as polyisobutylene or mineral oil. The solvent may be a mixture of oil-soluble solvents such as polyisobutylene and mineral oil. It has been found that the presence of these solvents does not hinder the efficacy of the lubricant. Aromatic amine The antiknock component is an aromatic amine. Any aromatic amine suitable for use as an antiknock additive in a lubricant composition, or in a fuel composition for a spark ignition internal combustion engine, may be used in this invention. Aromatic amine compounds suitable for use in this invention include aniline and alkyl-substituted aniline compounds, 1,2,3,4 tetrahydroquinoline, diphenylamine and alkyl-substituted diphenylamine compounds such as butyldiphenylamine, octyldiphenylamine and di-octyl-diphenylamine, 2 ethylhexyl-4-(dimethylamino)benzoate, indoline, N,N-dimethyl-1,4-phenylenediamine, o-toluidine, p-toluidine, p-anisidine, p-phenethidine, and the like, and mixtures thereof. Butylated octylated diphenylamine, also known as BODPA or butyl octyl diphenylamine, optionally sold under the name TL57 Butyl Octyl Diphenylamine is the preferred aromatic amine for this composition. The aromatic amine is present in an amount of 0.1 to 2 wt%, or 0.1 to 1 wt% or about 0.5 wt%. Good results have been achieved with 0.5 wt% butylated octylated diphenylamine. Optionally the aromatic amine is diphenylamine or a substituted diphenylamine. As used herein, the term "substituted" or “substituent” is intended to indicate that one or more (for example 1 -10 or 1,2, 3, 4, or 5; in some embodiments 1, 2, or 3; and in other embodiments 1 or 2) hydrogens on the group indicated in the expression using “substituted” (or “substituent”) is replaced with a selection from the indicated group(s), or with a suitable group known to those of skill in the art, provided that the indicated atom’s normal valency is not exceeded, and that the substitution results in a stable compound. Suitable indicated groups include, e.g., alkyl, alkenyl, alkynyl, alkoxy, halo, haloalkyl, hydroxy, hydroxyalkyl, aryl, heteroaryl, heterocycle, cycloalkyl, alkanoyl, alkoxycarbonyl, amino, alkylamino, dialkylamino, trifluoromethylthio, difluoromethyl, acylamino, nitro, trifluoromethyl, trifluoromethoxy, carboxy, carboxyalkyl, keto, thioxo, alkylthio, alkylsulfinyl, alkylsulfonyl, and cyano. Additionally, non-limiting examples of substituents that can be bonded to a substituted carbon (or other) atom include F, Cl, Br, I, OR', OC(O)N(R')2, CN, CF3, OCF3, R', O, S, C(O), S(O), methylenedioxy, ethylenedioxy, N(R')2, SR', SOR', SO2R', SO2N(R')2, SO3R', C(O)R', C(O)C(O)R', C(O)CH2C(O)R', C(S)R', C(O)OR', OC(O)R', C(O)N(R')2, OC(O)N(R')2, C(S)N(R')2, (CH2)o-2NHC(0)R', N(R')N(R')C(O)R', N(R')N(R')C(O)OR', N(R')N(R')CON(R')2, N(R')SO2R', N(R')SO2N(R')2, N(R')C(O)OR', N(R')C(O)R', N(R')C(S)R', N(R')C(O)N(R')2, N(R')C(S)N(R')2, N(COR')COR', N(OR')R', C(=NH)N(R')2, C(O)N(OR')R', or C(=NOR')R' wherein R’ can be hydrogen or a carbon-based moiety, and wherein the carbon-based moiety can itself be further substituted. The term "halo" or "halide" refers to fluoro, chloro, bromo, or iodo. Similarly, the term "halogen" refers to fluorine, chlorine, bromine, and iodine. The term "alkyl" refers to a branched or unbranched hydrocarbon having, for example, from 1-20 carbon atoms, and often 1-12, 1-10, 1-8, 1-6, or 1-4 carbon atoms; or for example, a range between 1-20 carbon atoms, such as 2-6, 3-6, 2-8, or 3-8 carbon atoms. As used herein, the term “alkyl” also encompasses a “cycloalkyl”, defined below. The term "cycloalkyl" refers to cyclic alkyl groups of, for example, from 3 to 10 carbon atoms having a single cyclic ring or multiple condensed rings. Cycloalkyl groups include, byway of example, single ring structures such as cyclopropyl, cyclobutyl, cyclopentyl, cyclooctyl, and the like, or multiple ring structures such as adamantyl, and the like. The cycloalkyl can be unsubstituted or substituted. The term "heterocycloalkyl" refers to a saturated or partially saturated monocyclic, bicyclic, or polycyclic ring containing at least one heteroatom selected from nitrogen, sulfur, oxygen, preferably from 1 to 3 heteroatoms in at least one ring. Each ring is preferably from 3 to 10 membered, more preferably 4 to 7 membered. The term "aryl" refers to an aromatic hydrocarbon group derived from the removal of at least one hydrogen atom from a single carbon atom of a parent aromatic ring system. The term "heteroaryl" refers to a monocyclic, bicyclic, or tricyclic ring system containing one, two, or three aromatic rings and containing at least one nitrogen, oxygen, or sulfur atom in an aromatic ring. The heteroaryl can be unsubstituted or substituted, for example, with one or more, and in particular one to three, substituents, as described in the definition of "substituted". Copper salt of an organic acid The composition comprises a copper salt of an organic acid. An organic acid is an acid which comprises carbon-hydrogen bonds and may be a carboxylic acid. A carboxylic acid is an oxoacid having the structure RC(=O)OH. Copper salts of organic acids with a carbon number of C15 to C18 may be used. As a copper salt of an organic acid (anti-wear agent), the lubricant may include a salt of copper and carboxylic acid (C15-C18), such as a copper salt of oleic acid. Copper salts allow the formation of protective copper films on the metal surfaces under pressure. The more reducing metals from the reactivity series such as silver, gold or platinum may react in the same way, although they are less abundant and more expensive than copper. A particularly preferred copper salt of an organic acid is copper oleate. The copper salt of an organic acid is present in an amount of 0.1 to 1 wt%, or 0.1 to 0.5 wt%, or about 0.3 wt%. Good results have been achieved with 0.3 wt% copper oleate. Optionally the lubricant is free from metal salts of inorganic acids. As used herein an “inorganic acid” is an acid derived from one or more inorganic compounds, as opposed to organic acids which are acidic, organic compounds. An inorganic compound is a chemical compound that lacks carbon-hydrogen bonds. Common inorganic acids are hydrofluoric acid HF, hydrochloric acid HCI, hydrobromic acid HBr, hydroiodic acid HI, nitric acid HNO3, phosphoric acid H3PO4, sulfuric acid H2SO4, boric acid H3BO3, perchloric acid HCIO4, hydrogen cyanide HCN. Optionally the lubricant is free from zinc dithiophosphate. Optionally the lubricant is free from alcohol. As used herein an “alcohol” is an organic compound that carries at least one hydroxyl (-OH) functional group bound to a saturated carbon atom. Optionally the lubricant is oil-soluble. Also provided is a method for protecting friction surfaces from hydrogen-mediated destruction, wherein the method comprises introducing the a lubricant outlined above to the friction surfaces. Generally, the hydrogen-mediated destruction comprises one or more of current-induced destruction, electric field catalysed destruction, micropitting, macropitting, white structure flaking (WSF), and the formation of white etching cracks (WEC). Generally, the friction surfaces are subject to sliding and rolling contact. Optionally the friction surfaces are bearing assemblies, gearbox components, wheelsets of railway rolling stock, and rail tracks. Base oil The base oil is selected from polyalphaolefins (PAO), Very High Viscosity Index oils (VHVI), and esters. VHVI have a viscosity index of over 110. Good results have been achieved with a mixture of polyalphaolefins. Other features The lubricant is free of inorganic acids. The lubricant is preferably free of copper, cobalt, lead, tin and nickel salts of inorganic acids. Many previous applications use these salts for the formation of a protective metal film under pressure. However, the present inventors have found that these salts of inorganic acids lead to the formation of damaging strong inorganic acids in situ over time. The lubricant is free of resins such as epoxy resin. This aids the formation of a protective copper film on friction surfaces under pressure. The lubricant is homogenous and all components of the lubricant are oil-soluble. The lubricant is free of particles including abrasive particles. This prevents the clogging of the oil channels of the lubrication system of internal combustion engines. The lubricant is free from metal salts of inorganic acids. The lubricant is free from sulfur and phosphorus and zinc dithiophosphate (ZDDP), which contains zinc and phosphorus. While ZDDP has been proven to extend machine longevity, its negative effects on the environment have become a critical issue. Zinc and phosphorus induced neurotoxicity has been shown to play a role in neuronal damage and death associated with traumatic brain injury, stroke, seizures, and neurodegenerative diseases. The lubricant is free from alcohols. The lubricant is oil-soluble. Optionally the lubricant comprises, consists essentially of or consists of: an organic copper salt 0.1 to 1 wt%, a succinimide derivative 0.5 to 3 wt% an aromatic amine 0.1 to 2 wt% and a base oil. Optionally the lubricant comprises, consists essentially of or consists of: an organic copper salt 0.1 to 0.5 wt% a succinimide derivative 1 to 2 wt% an aromatic amine 0.1 to 2 wt% and a base oil. Optionally the lubricant comprises, consists essentially of or consists of: an organic copper salt 0.3 wt% a succinimide derivative 1.5 wt% an aromatic amine 0.5 wt% and a base oil. Optionally the lubricant comprises, consists essentially of or consists of: copper oleate 0.1 to 1 wt%, a succinimide derivative 0.5 to 3 wt% butylated octylated diphenylamine 0.1 to 2 wt% and a base oil selected from polyalphaolefins (PAO), Very High Viscosity Index oils (VHVI), and esters. Optionally the lubricant comprises, consists essentially of or consists of: copper oleate 0.1 to 0.5 wt% polyisobutylene bis-succinimide 1 to 2 wt% butylated octylated diphenylamine 0.1 to 2 wt% and a base oil selected from polyalphaolefins (PAO), Very High Viscosity Index oils (VHVI), and esters. Optionally the lubricant comprises, consists essentially of or consists of: copper oleate 0.3 wt% polyisobutylene bis-succinimide 1.5 wt% butylated octylated diphenylamine 0.5 wt% and a base oil selected from polyalphaolefins (PAO), Very High Viscosity Index oils (VHVI), and esters. Optionally the lubricant comprises, consists essentially of or consists of: copper oleate 0.3 wt% polyisobutylene bis-succinimide 1.5 wt% butylated octylated diphenylamine 0.5 wt% and a mixture of polyalphaolefins. Brief Description of Drawings Fig. 1 shows the micropitting rig (MPR) instrument and method used to determine damage on a friction surface. Fig. 2A shows the readout of the MPR for the lubricant of the present invention Fig. 2B shows the subsurface analysis of the test roller after the MPR run of Fig. 2A Fig. 3A shows the readout of the MPR for a commercially available lubricant composition (comparative example) Fig. 3B shows the subsurface analysis of the test roller after the MPR run of Fig. 22 (comparative example) Fig. 4 shows the data from Fig. 1 and Fig. 2 superimposed Fig. 5 is the modified MPR used for the tests Description of Embodiments Fig. 1A shows the micropitting rig instrument 100 and the method used to determine damage on a friction surface in the Examples. A micropitting rig (MPR) 100 is used to simulate conditions that the parts and lubricant would experience in an accelerated manner. The MPR has three counterface rings of equal diameter surrounding and in contact with a smaller diameter test roller. There are three contacts per revolution, so results can be obtained in days rather than weeks or years. The data obtained, shown in the exemplary graph indicated by reference 103, shows time on the x axis and acceleration, a measurement of vibration, on the y axis. Vibration can be measured in CLA acceleration or P / P acceleration. For the Examples described herein, the rig stopped automatically when vibrations reached a threshold indicative of a pit, or at approximately 200 M cycles. The present inventors set the threshold at 1.6g CLA or 10g P / P. In Fig. 1 the start of pit formation can be seen where the acceleration starts to increase, and is indicated by reference 106. The test run is stopped when the vibration reaches the threshold of 1.6g CLA, indicated by reference 109. For the Examples disclosed herein, a current was applied to the apparatus. This is to mimic the effect of stray currents passing into the gear box. The current was passed from the central test roller to the bottom right ring. The current was a constant current of 250 mA. The test roller was AISI 52100 roller, AISI 52100 rings (0.1pm Ra). “M” indicates million. The test roller is then removed and images such as exemplary image 112 are made, noting the position of any pits. Imaging includes subsurface analysis. The method can be found in the publication Gould, B et al, The effect of electrical current on premature failures and microstructural degradation in bearing steel, International Journal of Fatigue, Volume 145, 2021, 106078, ISSN 0142-1123, https: / / doi.Org / 10.1016 / j. ijfatigue.2020.106078 Fig. 1B shows the subsurface analysis preparation method. The test roller 201 is mounted in a resin puck 203 and placed in a slicing apparatus 300. The top surface, deemed excess material, is removed by turning. The remaining surface is analysed to a depth of between 0.5 mm and 1 mm in slices of approximately 0.1 mm thickness. Analysis entails visual analysis with the aid of magnification. Examples Example 1 A test roller (AISI 52100 roller, AISI 52100 rings (0.1pm Ra)) was introduced into a micropitting rig along with a lubricant. AISI 52100 is a steel alloy. The lubricant had the following composition: copper oleate 0.3 wt% is polyisobutylene bis-succinimide 1.5 wt% butylated octylated diphenylamine 0.5 wt% the remainder is a base oil comprising a mixture of polyalphaolefins (PAO) The rig was started and an electric current of 250 mA was applied to the apparatus. The current varied slightly between 245 mA and 255 mA. The current was passed from the central test roller to the bottom right ring. The rig was stopped at 152 M cycles without reaching the threshold. As shown in Fig. 2A and Table 3, the acceleration remained below 2 g P / P for the whole run. Other relevant parameters of the run are: Roller speed: 1495 mm / s Ring speed: 1300 mm / s Load 500 + / - 2 N Following the MPR, the test roller was subjected to subsurface analysis. This was done to a depth of 0.74 mm over 7 slices, see Table 1. Table 1 Slice Height of puck (mm) Distance through roller wear track (mm) Slice thickness (mm) 1 15.86 0 (Set as datum) 2 15.72 0.14 0.14 3 15.61 0.25 0.11 4 15.48 0.37 0.12 5 15.41 0.44 0.07 6 15.30 0.55 0.11 7 15.11 0.74 0.19 Very little damage was observed on the test roller in Example 1, see Fig. 2B. Two very small surface cracks appeared in the roller of Example 1. Furthermore, subsurface analysis showed no pits and no WECs. Example 2 (Comparative Example) The procedure of Example 1 was carried out but an industry standard ISO 320 gear oil was used. The rig was stopped at 73.5 M cycles due to high acceleration of approximately 7.43 g P / P, as shown in Fig. 3A and Table 3. Significant damage was observed on the test roller in Example 2, see Fig. 3B. Surface cracks were readily observable. The details of the subsurface analysis of Example 2 are set out in Table 2. Table 2 Slice Height of puck (mm) Distance through roller wear track (mm) Slice thickness (mm) 1 10.41 0 (set as datum) 2 10.27 0.14 0.14 3 10.18 0.23 0.09 4 10.04 0.37 0.14 5 9.94 0.47 0.10 6 9.88 0.53 0.06 7 9.75 0.66 0.13 Subsurface analysis revealed bigger micropits, WECs in cracks and WEAs without cracks in the roller of Example 2. Specifically, WECs were observed in slices 2, 3 and 6. Table 3 sets out the data of MPR runs using the lubricant of Example 1 and Example 2. Table 3 Comparison of Example 1 and Example 2 Lubricant Number of contact cycles, million (M) Final acceleration P / P (g) Example 1 152.077 0.624275 Example 2 73.509 7.432988 Fig. 4 shows the data from Examples 1 and 2 overlaid, clearly demonstrating the significant increase in acceleration incurred for the commercial lubricant composition compared to the composition disclosed herein. The above data shows that the lubricant of the composition protected the steel alloy from the hydrogen mediated effects arising from the combination of contact and electric currents. Fig. 5 shows the modified MPR used for the tests. Parts of the door are electrically isolated to prevent additional stray currents interfering with the measurement. Reference Signs List micropitting rig (MPR) 100 exemplary graph 103 start of pit formation 106 point where vibration reaches the threshold 109 exemplary image 112 test roller 201 resin puck 203 slicing apparatus 300 The invention is defined by the appended claims.

Claims

1. A method for protecting friction surfaces from hydrogen-mediated destruction, wherein the method comprises introducing a lubricant to the friction surfaces, the lubricant comprising an organic copper salt, a succinimide derivative, an aromatic amine and a base oil.

2. The method of claim 1 wherein the succinimide derivative is polyisobutylene bis-succinimide.

3. The method of claim 1 or 2 wherein the aromatic amine is diphenylamine or a substituted diphenylamine.

4. The method of claim 1 to 3 wherein the aromatic amine is butylated octylated diphenylamine.

5. The method of any one of claims 1 to 4 wherein the organic copper salt is copper oleate.

6. The method of any one of claims 1 to 5 wherein the base oil is selected from selected from polyalphaolefins (PAO), Very High Viscosity Index oils (VHVI) and esters, a mixture or at least two base oils selected from polyalphaolefins (PAO), Very High Viscosity Index oils (VHVI), and esters, or optionally a mixture of polyalphaolefins.

7. The method of any one of claims 1 to 6 wherein the succinimide derivative is present in an amount of 0.5 to 3 wt%, or 1 to 2 wt%, or about 1.5 wt%.

8. The method of any one of claims 1 to 7 wherein the aromatic amine is present in an amount of 0.1 to 2 wt%, or 0.1 to 1 wt% or about 0.5 wt%.

9. The method of any one of claims 1 to 8 wherein the organic copper salt is present in an amount of 0.1 to 1 wt%, or 0.1 to 0.5 wt%, or about 0.3 wt%.

10. The method of any one of claims 1 metal salts of inorganic acids.to 9 wherein the lubricant is free from11. The method of any one of claims 1 zinc dithiophosphate.to 10 wherein the lubricant is free from12. The method of any one of claims 1 alcohol.to 11 wherein the lubricant is free from13. The method of any one of claims 1 to 12 wherein the lubricant is oil-soluble.

14. The method of any one of claims 1 to 13 wherein the lubricant comprises:an organic copper salt 0.3 wt%a succinimide derivative 1.5 wt%an aromatic amine 0.5 wt%and wherein the remainder is a base oil.

15. The method of any one of claims 1 to 14 wherein the hydrogen-mediated destruction comprises one or more of current-induced destruction, electric field catalysed destruction, micropitting, macropitting, white structure flaking (WSF), and the formation of white etching cracks (WEC).

16. The method of any one of claims 1 to 15 wherein the friction surfaces are subject to sliding and rolling contact.

17. The method of any one of claims 1 to 16 wherein the friction surfaces are bearing assemblies, gearbox components, wheelsets of railway rolling stock, or rail tracks.

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