Copper Corrosion Test Method

Abstract

The present invention provides a method for evaluating the copper corrosion potential of a lubricating fluid, the method comprising the steps of: a) placing a sample of the lubricating fluid in a vial, the sample partially filling the vial, whereby a space exists above the sample in the vial suitable for sampling gas phase materials 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; and d) analyzing the gas phase portion using headspace GC-MS with full scan and selected ion monitoring.

Description

[Technical Field]

[0001] The present invention relates to a test method for easily evaluating the vapor phase copper corrosion potential of lubricating fluids. [Background technology]

[0002] As the energy transition progresses, there is an increasing demand for transportation that uses, at least in part, electric motors or "e-motors." Copper is a key component in e-motors due to its electrical conductivity, heat transfer capabilities, ductility, and cost. Within e-motors, copper windings are typically protected by a coating. However, at connection points, the copper can become exposed due to coating degradation.

[0003] Typically, current e-motors come in two common configurations: dry e-motors, in which the lubricating fluid used in the transmission is kept away from the e-motor, and wet e-motors, in which lubricating fluid is used to lubricate and cool the e-motor in addition to lubricating the gearbox. Wet e-motors increase the risk of fluid, either in liquid or vapor phase, or both, coming into contact with the e-motor's copper windings.

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

[0005] Testing for copper corrosion in hydrocarbon products is traditionally based on ASTM D130, in which copper strips are immersed in a fluid, such as oil, and heated for hours. The strips are then visually rated on a scale of 1a to 4c depending on the level of discoloration. While this method provides a clear distinction between high and low levels of copper corrosion, it lacks the precision required for comparisons between low-level corrosivity.

[0006] ASTM D130 is a relatively old test and was not developed to test lubricants for use in e-motors. Many attempts have been made to improve the test for more modern requirements. For example, to improve the sensitivity of the test, it can be extended by analyzing oil samples immersed in copper strips for copper concentration at the end of the test with inductively coupled plasma (ICP) to quantify copper loss. The test can also be modified by suspending a copper strip above the fluid sample or by only partially immersing the copper strip to evaluate the fluid's gas-phase copper corrosion potential.

[0007] To test the compatibility of fluids with electronic components under energized conditions, a so-called "Electrical Corrosion and Conductive Deposit" test has been developed. This conductive deposit test, described in G. Muller; J. Bucci; G. Mueller; R. Pelz; T. Newcomb; and A. Gangopadhyay, "Conductive Layer Deposits and the Development of an Effective Bench Test for Electric Vehicle Drivetrains," SAE International, 2021, uses a circuit board with one half immersed in the fluid and the other half exposed to vapor. The fluid is typically heated to 150°C, and 5V DC power is applied to the board. Resistance is monitored over time in both the liquid and vapor phases. While the pass / fail threshold is still being evaluated, it is currently believed that a failure can be determined when resistance decreases due to short-circuit deposits within 1,000 hours.

[0008] A further 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 vessel. One is immersed in the test fluid, and the other 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 wires are exposed to the test fluid vapor. The resistance of the wires is monitored over time. As each wire corrodes, some conductive copper metal is lost, thus increasing electrical resistance. Corrosion can be detected as a change in the measured resistance in the circuit due to a decrease in the conductive cross-sectional area. However, this method is difficult to set up and requires installing a new wire for each sample.

[0009] There remains a need for a simple and rapid test for the vapor phase copper corrosion potential of formulated lubricating fluids. Furthermore, it would be desirable if the test method could provide insight into the mechanism of copper corrosion being 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 assessing the copper corrosion potential of a lubricating fluid, comprising: a) placing a sample of the lubricating fluid into a vial, the sample partially filling the vial, whereby a space exists above the sample in the vial suitable for sampling gas phase materials 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; d) analyzing the gas phase portion using headspace GC-MS with full scan and selected ion monitoring. DETAILED DESCRIPTION OF THE INVENTION

[0011] The inventors have found that accurate assessment of the copper corrosion potential of a lubricating fluid can be readily assessed using gas chromatography coupled with mass spectrometry (GC-MS) analysis of the vapor phase above a lubricating fluid sample after the sample has been subjected to a pretreatment that includes heating the sealed sample for more than 12 hours. In particular, GC-MS can be used to identify the presence of species including one or more of hydrogen sulfide (HS), ammonia (NH), carbonyl disulfide (CS), and carbonyl sulfide (COS). However, other corrosion species can also be identified in a full scan by selecting the correct ions for monitoring.

[0012] The method of the present invention provides a simple and rapid method for evaluating the vapor phase copper corrosion potential of lubricating fluids. It has been demonstrated to produce results that correlate well with known industrial methods that are significantly more complex and time-consuming. The method of the present invention can be used, for example, to provide rapid evaluation of a large number of candidate lubricating fluid samples.

[0013] In the present invention, a sample of lubricating fluid is placed in a vial so that the sample only partially fills the vial, leaving a space above the liquid sample. Typically, the sample will fill between 5 and 20% of the vial's volume, preferably less than 20%. A larger sample volume increases the likelihood of leakage and sample loss during testing due to the resulting higher vapor pressure.

[0014] Sealing of the vial is suitably achieved by using a cap, for example an aluminium cap, incorporating a septum of a pierceable material such as one or more of silicone, rubber and PTFE.

[0015] Headspace GC-MS is a known method. The term "headspace" refers to the fact that the sample is collected from the space above the 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 period of time, such as 1 hour, to allow the vapor phase and the liquid sample to equilibrate. An aliquot of the vapor phase is then collected through a needle inserted into the vial and passed through a heated transfer line to a gas chromatograph (GC), where the species are separated and then identified in a mass spectrometer (MS).

[0016] In the present invention, a sample of lubricating fluid is subjected to a pre-treatment in which the sealed sample is heated to 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 pre-treatment step is carried out at a temperature not exceeding 180°C. The pre-treatment step is preferably for at least 24 hours, more preferably at least 48 hours. It will be readily understood that the pre-treatment step can be carried out at a higher temperature for a shorter time, or at a lower temperature for a longer time. For example, suitable pre-treatment steps can be carried out at 120°C for 72 hours or at 170°C for 24 hours.

[0017] After the pretreatment step, a portion of the gas phase above the liquid lubricant fluid sample is removed from the vial and injected into a GC-MS, which separates and analyzes the species in the injected gas phase portion.

[0018] Suitable GC-MS conditions may be readily determined by one skilled in the art. The use of selected ion monitoring in the present invention allows for the evaluation of target molecules known for their contribution to copper corrosion. Selected ion monitoring (SIM) is a scan mode in mass spectrometry in which a single mass-to-charge ratio is selected to be detected instead of the entire range. This scan mode allows for improved sensitivity, removes interference from coeluting substances, and allows for accurate evaluation of the species of interest. Of particular interest in the present invention are ammonia (mass-to-charge ratio m / z = 17), HS (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 is readily applicable to any selected ion associated with an actual or suspected corrosion species.

[0019] The area under the peak on the chromatogram is directly 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 ​​a corrosion species in a lubricating fluid can be compared to the same measurement obtained for a standard lubricating fluid. Typically, the standard lubricating fluid is a known lubricating fluid with a known copper corrosion potential.

[0020] The invention will now be further described by reference to the following non-limiting examples. [Example]

[0021] Examples 1 to 12 To demonstrate the effectiveness of the method of the present invention, several fluids suitable as lubricating fluids for wet e-motors were prepared and tested using the method. Each fluid contained a GTL base oil and at least one additive package.

[0022] In each example, a sample of the fluid was placed in a headspace vial and preheated to either 140°C for 48 hours or 170°C for 24 hours. Headspace samples were collected and analyzed by GC-MS under the following conditions: injection time 1 min, oven temperature 45 °C for 8 min, gradient at 20 °C / min to 120 °C for 20 min, PoraBOND Q column (50 m × 0.53 mm × 10 um), inlet temperature 180 °C, split 5:1, helium as carrier gas.

[0023] Selected ion monitoring was used to evaluate specific gas-phase corrosives. Four potential corrosive species were selected to be evaluated using selected ion monitoring. These were ammonia, carbonyl sulfide, carbon disulfide, and hydrogen sulfide.

[0024] The formulation shown as Example 1, which was a known fluid with a known copper corrosion potential, was used as a standard, and the area under the GC trace was measured for each potential corrosive species. This area was set to 100 for each corrosive species. All other measurements were quantified relative to this standard. The results for these examples are shown in Table 1.

[0025] [Table 1]

[0026] It is clearly shown that the effect of changes in formulation, including the addition of vapor phase corrosion inhibitors (Examples 2 and 3) and changes in additive packages (Examples 4-9), on the generation of vapor phase corrosive species can be readily identified using the method of the present invention, allowing for a quick and simple assessment of the potential vapor phase copper corrosion of formulated lubricating fluids.

[0027] The results from the method of the present invention were then compared to results for some of the same samples using known, more complex copper corrosion test methods. Each sample in Table 2 was evaluated using the Conductive Deposit Test according to G. Muller; J. Bucci; G. Mueller; R. Pelz; T. Newcomb; A. Gangopadhyay, "Conductive Layer Deposits and the Development of an Effective Bench Test for Electric Vehicle Drivetrains," in SAE International; 2021. The oil samples used in the Conductive Deposit Test were then evaluated using Inductively Coupled Plasma (ICP) to assess the level of copper dissolved in the used oil and sulfur loss from the fresh oil at the end of the CDT. The results from these tests are shown in Table 2.

[0028] [Table 2]

Claims

1. 1. A method for assessing the copper corrosion potential of a lubricating fluid, comprising: a) placing a sample of the lubricating fluid into a vial, the sample partially filling the vial, whereby a space exists above the sample in the vial suitable for sampling gas phase materials 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; d) analyzing the gas phase portion using headspace GC-MS with full scan and selected ion monitoring.

2. Selected ion monitoring was performed using NH ３ , COS, CS ２ , and H ２ 10. The method of claim 1, wherein the method is used to identify the presence of a species comprising one or more of S.

3. 3. The method of claim 1 or 2, also comprising a step e) of comparing the results obtained in step d) with results obtained by applying steps a) to d) to further lubricating fluids with 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. 5. The method of 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. Selected ion monitoring was performed using NH ３ , COS, CS ２ , and H ２ The method of any one of claims 1 to 5, used to assess the presence of one or more of S.

Images