Copper corrosion testing method

Abstract

The present invention provides a method for assessing copper corrosion potential of a lubricant fluid, comprising the steps of: a) placing a sample of said lubricant fluid in a vial wherein said sample partially fills said vial such that there is a space in the vial above the sample, the space is suitable for sampling a gas phase material in the space; b) sealing the vial; c) heating the sample in the sealed vial at a temperature of at least 80 DEG C for more than 12 hours; and d) analyzing the gas phase fraction using headspace GC-MS with full scanning and selected ion monitoring.

Description

Technical Field

[0001] The present invention is directed to a test method for easily evaluating the vapor phase copper corrosion potential of a lubricant fluid. Background Art

[0002] As energy conversion progresses, the demand for transportation that uses (at least in part) electric motors, or "electric motors," increases. Copper is an important component in electric motors because of its electrical conductivity, heat transfer capabilities, ductility, and cost. Within electric motors, copper windings are usually protected by a coating. However, at connection points and due to coating degradation, the copper may be exposed.

[0003] Typically, in current electric motors, there are two common configurations: dry motors, where the lubricant fluid used in the transmission remains separate from the motor, and wet motors, where the lubricant fluid is used to lubricate and cool the motor in addition to lubricating the gearbox. In wet motors, there is an increased risk that the fluid, in liquid or gas phase or both, comes into contact with the copper windings of the motor.

[0004] The development of new lubricant fluids suitable for electric motors has been the focus of much research. It is important that these fluids exhibit low levels of copper corrosion. Copper corrosion is believed to be caused by many factors. The presence of sulfur and nitrogen compounds in the base oil or from certain additives used in the lubricant fluid is known to contribute to corrosion because they degrade at higher temperatures. The formulation of new lubricant fluids for electric motors requires extensive, time-consuming testing of potential candidates to determine their copper corrosion potential.

[0005] Testing for copper corrosion in hydrocarbon products has historically been based on ASTM D130, in which a copper strip is immersed in a fluid (e.g., oil) and heated for several hours. The strip is then given a visual grade on a scale of 1a to 4c based on the level of discoloration. This method provides a clear distinction between high and low levels of copper corrosion, but does not provide the precision required to make comparisons between low levels of corrosivity.

[0006] ASTM D130 is a relatively old test that was not developed for testing lubricants used in electric motors. Many attempts have been made to improve the test for more modern requirements. For example, to increase the sensitivity of the test, the test can be extended by analyzing the copper concentration of the oil sample into which the copper strip has been immersed at the end of the test with inductively coupled plasma (ICP) to quantify copper loss. The test can also be modified to evaluate the vapor phase copper corrosion potential of the fluid by suspending the copper strip above the fluid sample or by only partially immersing the copper strip.

[0007] The so-called "energized corrosion and conductive deposit" test has been developed to test the compatibility of fluids with electronic components under energized conditions. The conductive deposit test (described in G. Müller; J. Bucci; G. Mueller; R. Pelz; T. Newcomb; A. Gangopadhyay, "Conductive Layer Deposits and the Development of a Effective Bench Test for Electric Vehicle Drivetrain", Society of Automotive Engineers (SAE International); mid-2021) uses a circuit board, half of which is immersed in the fluid and the other half is exposed to the vapor. The fluid is typically heated at 150°C and 5V DC is applied to the board. The resistance is monitored over time in the liquid and vapor phases. The pass / fail threshold is still being evaluated, but it is currently considered that a failure can be determined when the resistance decreases due to short-circuit deposits within 1000 hours.

[0008] Another copper corrosion test is described in G. Hunt, M. Gahagan and M. Peplow, Lubrication Science, Vol. 29, No. 4, pp. 279-290, 2017. In this test, two bare wires are placed in a test container. One bare wire is immersed in the test fluid, while the other bare wire is suspended above the fluid. A direct current (DC) of 1 mA is applied. The test fluid is maintained at the desired temperature, and the suspended wire is exposed to the vapor of the test fluid. The wire resistance is monitored over time. As each wire corrodes, some of the conductive copper metal is lost, so the resistance will increase. Corrosion can be detected by the change in resistance measured in the circuit due to the reduction in the conductive cross-sectional area. However, this method is difficult to set up and requires the installation of new wires for each sample.

[0009] There remains a desire to simply and quickly test the vapor phase copper corrosion potential of formulated lubricant fluids. In addition, it would be ideal if this test method could provide insight into the mechanism of copper corrosion caused by any tested fluid to aid in the further development of improved fluids. Summary of the invention

[0010] The present invention provides a method for evaluating the copper corrosion potential of a lubricant fluid, the method comprising the following steps:

[0011] a) placing a sample of the lubricant fluid in a vial, wherein the sample partially fills the vial such that a space exists in the vial above the sample, the space being suitable for sampling gas phase material in the space;

[0012] b) sealing the vial;

[0013] c) heating the sample in the sealed vial at a temperature of at least 80° C. for more than 12 hours; and

[0014] d) The gas phase fraction was analyzed using headspace GC-MS with full scan and selected ion monitoring. DETAILED DESCRIPTION

[0015] The present inventors have discovered that an accurate assessment of the amount of copper corrosion potential of a lubricant fluid can be readily assessed using gas chromatography-mass spectrometry (GC-MS) analysis of the gas phase above the sample of the lubricant fluid after the sample has been subjected to a pretreatment including heating the sealed sample for more than 12 hours. In particular, GC-MS can be used to identify the presence of one or more of hydrogen sulfide (H2S), ammonia (NH3), carbonyl disulfide (CS2) and carbonyl sulfide (COS). However, other corrosive species can be identified in a full scan and by selecting the correct ions for monitoring.

[0016] The method of the present invention provides a simple and rapid method for evaluating the vapor phase copper corrosion potential in lubricant fluids. The method has been shown to produce results that correlate well with methods known in the industry, which are rather complex and time consuming. The method of the present invention can be used, for example, to provide a rapid evaluation of a large number of candidate lubricant fluid samples.

[0017] In the present invention, a sample of lubricant fluid is placed in a vial so that the sample only partially fills the vial and there is space above the liquid sample. Typically, the sample will fill 5% to 20% of the vial volume, preferably less than 20%. Larger sample volumes increase the likelihood of leaks and sample loss during testing due to the creation of higher vapor pressures.

[0018] Sealing of the vial is suitably performed by use of a cap, such as an aluminium cap, incorporating a septum of a pierceable material, such as one or more of silicone, rubber and PTFE.

[0019] Headspace GC-MS is a known method. The term "headspace" refers to the fact that a sample is taken from the space above a liquid sample in a vial. In a typical headspace GC-MS method, the sample is sealed in a vial and then heated for a short time, such as 1 hour, to allow the gas phase and liquid sample to equilibrate. An aliquot of the gas phase is then taken out via a needle inserted into the vial and transferred to a gas chromatograph (GC) through a heated transfer line, where the substances are separated and then identified in a mass spectrometer (MS).

[0020] In the present invention, the lubricating fluid sample is pretreated, wherein the sealed sample is heated at a temperature of at least 80°C for at least 12 hours. Preferably, the temperature is at least 140°C, more preferably at least 170°C. Suitably, the pretreatment step is carried out at a temperature not exceeding 180°C. The pretreatment step is preferably carried out for at least 24 hours, more preferably at least 48 hours. It will be readily appreciated that the pretreatment step may be carried out at a higher temperature for a shorter time or at a lower temperature for a longer time. For example, a suitable pretreatment step may be carried out at 120°C for 72 hours or at 170°C for 24 hours.

[0021] After the pretreatment step, the gas phase above the liquid lubricant fluid sample is taken from the vial and injected into the GC-MS. The GC-MS separates the substances in the injected gas phase and analyzes them.

[0022] Suitable GC-MS conditions will be readily determined by the skilled artisan.

[0023] The use of selected ion monitoring in the present invention allows for the evaluation of target molecules known to contribute to copper corrosion. Selected ion monitoring (SIM) is a scan mode in mass spectrometry in which a single mass-to-charge ratio is selected rather than the full range for detection. This scan mode can increase sensitivity and remove interference from co-eluting materials, thereby enabling accurate evaluation of substances of interest. Of particular interest in the present invention are ammonia (mass-to-charge ratio m / z=17), H2S (mass-to-charge ratio m / z=34), carbonyl disulfide (mass-to-charge ratio m / z=76), and COS (mass-to-charge ratio m / z=60). However, the present invention can be readily applied to any selected ions associated with actual or suspected corrosive substances.

[0024] The area under the peak on the chromatogram is proportional to the concentration of the analyte of interest. The detector can be calibrated using standard compounds to establish a response factor between peak area and analyte concentration. Alternatively, the peak area of ​​the corrosive species in the lubricant fluid can be compared to the same measurement made on a standard lubricant fluid. Typically, the standard lubricant fluid is a known lubricant fluid with a known copper corrosion potential.

[0025] The invention will now be further illustrated by reference to the following non-limiting examples.

[0026] Example

[0027] Examples 1 to 12

[0028] To demonstrate the effectiveness of the method of the present invention, a variety of fluids suitable for use as lubricant fluids for wet electric motors were prepared and tested using the method. Each fluid contained a GTL base oil and at least one additive package.

[0029] In each example, the fluid samples were placed in headspace vials and preheated at 140°C for 48 hours or at 170°C for 24 hours.

[0030] Headspace samples were taken and analyzed in GC-MS. Conditions: injection duration 1 min; oven 45 °C for 8 min, ramping to 120 °C at 20 °C / min for 20 min; PoraBOND Q column (50 m × 0.53 mm × 10 um); inlet 180 °C; split 5:1; helium as carrier gas.

[0031] Use selected ion monitoring to evaluate specific vapor phase corrosives. Four potentially corrosive species were selected for evaluation using selected ion monitoring. These are ammonia, carbonyl sulfide, carbon disulfide, and hydrogen sulfide.

[0032] The formulation shown in Example 1, which is a known fluid with a known copper corrosion potential, was taken as a standard and the area under the GC trace of each potential corrosive substance was measured. For each corrosive substance, the area was set to 100. All other measurements were quantitatively compared to this standard. The results of these examples are shown in Table 1.

[0033] Table 1

[0034]

[0035] The effects of changes in formulation, including addition of vapor phase corrosion inhibitors (Examples 2 and 3) and changes in the additive package (Examples 4 to 9), on the generation of vapor phase corrosive species are clearly shown to be readily identifiable using the method of the present invention. This allows for a quick and simple assessment of the potential vapor phase copper corrosion of a formulated lubricant fluid.

[0036] The results of the method of the present invention were then compared to the results of some of the same samples using a known and more complex copper corrosion test method.

[0037] The samples in Table 2 were each evaluated using a conductive deposit test according to G. Müller; J. Bucci; G. Mueller; R. Pelz; T. Newcomb; A. Gangopadhyay, "Conductive Layer Deposits and the Development of a Effective Bench Test for Electric Vehicle Drivetrain", Society of Automotive Engineers (SAE International); 2021. The oil samples used in the conductive deposit test were then evaluated using inductively coupled plasma (ICP) to assess the amount of copper dissolved in the used oil and the sulfur loss from the fresh oil at the end of the CDT. The results of these tests are shown in Table 2 below.

[0038] Table 2

[0039]

Claims

1. A method for evaluating the copper corrosion potential of a lubricant fluid, the method comprising the steps of: a) placing a sample of the lubricant fluid in a vial, wherein the sample partially fills the vial such that there is a space in the vial above the sample, the space being suitable for sampling gas phase material in the space; b) sealing the vial; c) heating the sample in the sealed vial at a temperature of at least 80° C. for more than 12 hours; as well as d) The gas phase fraction was analyzed using headspace GC-MS with full scan and selected ion monitoring.

2. The method of claim 1, wherein selected ion monitoring is used to identify the presence of species comprising one or more of NH3, COS, CS2, and H2S.

3. A method according to claim 1 or claim 2, further comprising the step e) of comparing the results obtained in step d) with the results obtained by applying steps a) to d) to another lubricant fluid having known copper corrosion properties.

4. The method according to any one of claims 1 to 3, wherein in step c), the sample is heated in the sealed vial at a temperature of at least 100°C. 5 . The method according to claim 4 , wherein in step c), the sample is heated in the sealed vial at a temperature of 140° C. for 48 hours or at a temperature of 170° C. for 24 hours.

6. A method according to any one of claims 1 to 5, wherein selected ion monitoring is used to assess the presence of one or more of NH3, COS, CS2 and H2S.

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