KEY CONCEPTS
• Goals for new electric vehicles (EVs) will drive the need for lubricant improvements. 
• Current copper corrosion inhibitors are very technical to select and use, expensive and toxic, all of which complicate enabling EV copper corrosion solutions. 
• While current copper corrosion testing is mainly qualitative and generally not applicable to the actual application environment, new and better test methods are being developed.

Just like internal combustion engines (ICEs), electric vehicles (EVs) still need lubricants; however, the key characteristics of those EV lubricants are different from the lubricants used in ICEs. Additionally, EV lubricant chemistries are evolving to enable more advanced EVs. 

When the lubricant comes into contact with the EVs’ electrical components (e.g., motor, electrical connections, circuits), it needs to provide corrosion protection for copper and other metals, good heat transfer and electrical insulating properties, resistance to oxidation as well as the usual characteristics needed by any lubricant (e.g., wide temperature range, antiwear protection). Equipment specific tests may need to be developed, as well as industry- standard tests.¹ 

Copper corrosion is a concern for a number of industries, including seawater desalination, power stations, heat exchangers, sheets and pipelines, shipbuilding and electronics.² However, copper corrosion is especially problematic for EVs as a short within the electronics or electric motor could disable the engine. 

While not a necessary part of antioxidant systems, copper corrosion inhibitors can aid in oxidative stability by pacifying copper that would otherwise catalyze oxidation. Despite benefits, copper corrosion inhibitor treat rates are limited due to toxicity. 

STLE member Greg Miiller is the vice president of engineering and new business development with the Savant Group in Midland, Mich. He notes that the corrosion and proximity of copper will be a multi-faceted challenge for next generation EV lubricants. Miiller says, “If coatings and the application process were all perfectly reliable, we would not be having this conversation.” However, knowing “that coatings can fail both in the initial application and also over time (potentially also related to the fluid), it becomes a true test of the fluid.” He says, “Assumptions were originally made as to the overall cause of the failures; however, it appears as though the varying chemistries impact both the coating and copper alike.” He notes that as EVs and lubricating fluids continue to evolve, improvements will continue to be developed. 

EV test requirements 
ASTM D130 (Standard Test Method for Corrosiveness to Copper from Petroleum Products by Copper Strip Test) is the standard method for testing copper corrosion; however, the method relies on operator judgment, which adds uncertainty, and there are known variations between labs as different testers may interpret the same strip as different colors. In addition, ASTM D130 has limitations for determining copper corrosion as the temperature and exposure times of the test do not necessarily match EV application parameters.¹ 



Juan J. Ayala, managing director at Ayalytical Instruments, Inc. in Chicago, Ill., says that the ASTM D130 copper strip corrosion test is “performed manually and requires a visual determination of not only colors but combinations of various colors to determine a rating.” This process makes the test results subjective and possibly subject to bias. The test requires that a copper specimen be prepared, tempered in a fuel or lubricant and then rated using the ASTM color guide. Ayala notes that 7%-8% of the population has some level of color vision deficiency, making relying on manual interpretations of the D130 results subjective and lacking in reproducibility between different experimenters. 

Both better lubricants and better test methods are being developed to meet anticipated new EV test requirements. 

Copper inhibitors 
STLE member Daniel Vargo is a senior research chemist with Functional Products Inc. in Macedonia, Ohio. He observes: “Copper corrosion occurs when a copper surface reacts with chemically reactive components such as acids or sulfur compounds.” These reactions can result in the formation of “ionic copper compounds, which being no longer part of the metal surface, can be removed.” This translates into corrosion of the copper surface, which can lead to equipment failure. EV lubricant chemistries are evolving to enable more advanced EVs. 

Vargo says, “These copper parts can be eroded by the action of water, oxygen and active sulfur compounds, among other components that can be present in a lubricant.” Another factor that can promote corrosion is the temperature of the metal part when the engine is running. Vargo says, “Copper passivators are film-forming agents in that they react chemically with the surface.” The resulting films are “unreactive to the effects of corrosive oxygen, nitrogen, active sulfur or acids in the lubricants, thus protecting the copper from corrosion.” 

When copper is used in the fabrication of metal parts (e.g., gears, bearings), then the lubricant used in that engine needs a copper corrosion inhibitor. Vargo notes, “A very low treat level of <0.1% is usually sufficient to inhibit copper corrosion” when there are low levels of exposed copper. 

Vargo notes that other components in a formulation can affect the performance of copper corrosion inhibitors. He says, “All performance additives have a certain degree of affinity for the metal surface.” These different performance additives (e.g., antiwear, extreme pressure, friction modifiers) can compete with the copper passivator for the metal surface. The formulator must judiciously select the optimum treat level for each additive. 

STLE member Brian Casey, R&D senior manager at Vanderbilt Chemicals, LLC in Norwalk, Conn., notes that identifying the molecules that are the most effective copper corrosion inhibitors depends on the application. He agrees with Vargo that “nitrogen-containing heterocycles such as triazoles can be very effective.” He also has found that “despite having sulfur, dimercaptothiadiazole (DMTD) copper corrosion inhibitors can be potent as well by actually forming a protective film or varnish.” 

Casey notes that non-aromatic copper corrosion inhibitors are most common in aqueous systems. He observes that a judicious pairing of copper corrosion inhibitor additives “in specific formulations can provide benefits by filling gaps in the performance profile, including wear, friction, corrosion, etc.” 

Some chemistries like tolytriazole carry aquatic toxicity hazards. Casey notes, “Flat, aromatic small molecule rings present a toxicity concern because they are able to intercalate between the base pairs of DNA.” One way to mitigate the risks of small molecules like tolytriazole is to react them with other components. He finds: “These reaction products can be engineered to provide the same performance but benefit from improved oil solubility, lower water solubility/bioavailability and aqueous stability.” 

EU Ecolabel program 
The EU Ecolabel system is a voluntary scheme for goods and services that demonstrate environmental excellence based on standardized processes and scientific evidence that tackles the main environmental impacts of products through their entire lifecycle. The EU Ecolabel program is managed by the European Commission and Member States following the Strategic Working Plan for the EU Ecolabel.³ 

Vargo notes, “There are few copper passivators on the market for Ecolabel applications.” He says, “Typically, copper passivators are triazole derivatives, benzo and tolytriazoles and thiadiazoles.” An example of a well-known thiazole is DMTD. For Ecolabel, Vargo says that the published EU Lubricant Substance Classification list (LuSC-list) shows the maximum treat level that can be used when formulating environmentally acceptable lubricant (EAL) products using approved copper passivators. These maximum treat levels are based on triazoles or their derivatives. 

Vargo says, “The performance of a corrosion inhibitor can change with different base fluids as they are a major component in a formulation.” The base fluid is especially important for EAL or Ecolabel applications. He says, “The common base fluids are triglyceride oils, synthetic esters, polyalkylene glycols (PAGs) or, to a lesser extent, certain low viscosity polyalpholefins (PAOs).” He notes that each of these fluids have their own advantages and drawbacks (e.g., oxidative stability, hydrolytic stability, polarity, additive solubility) depending on the application. It is important to test a particular corrosion inhibitor “in a suitable matrix with the other additives present to determine the optimum level of each component.” 

Current copper corrosion test methods 
The SAE J3200 is an SAE International information report, titled “Fluid for Automotive Electrified Drivetrains,” that describes new performance properties and possible test methods for lubricants that are intended for use in EV drivetrain components.⁴ The relevant lubricants are appropriate for use in electrified drivetrains (e.g., e-transmissions and e-axles). The report covers only geared systems where an electric motor is either immersed in powertrain lubricant or comes in contact with the lubricant. 

The SAE J3200 report is continuing to be refined for EV drivetrains, and Miiller says it “is a guide to those testing the capabilities of the EV fluids and lubricants.” He notes the hope that the report will be modified to keep up with the continuing developments in the EV field. He says the document can be improved by “adding new tests, modifying some existing tests and potentially evaluating values reported.” Two tests that can be used to assess copper corrosion are the wire corrosion test (WCT) and the conductive deposit test (CDT). The CDT and WCT have been launched and are currently in the process of completing an ASTM test method. 

Wire corrosion test 
The WCT involves placing thin copper wires of known length and hence resistance in the liquid and vapor phases of a lubricant and monitoring the change in resistance while maintaining a specific temperature for 72 hours. The test is very sensitive and can investigate the corrosion rate for lubricants under the actual EV temperature range. As would be expected, most often the corrosion rate increases with temperature, but surprisingly this is not always the case.⁵ This measures real-time change in resistance/voltage of wire rather than relying on subjective evaluations. 

Miiller notes that the WCT and the ASTM D130 are similar tests, but they can provide “distinctly different results.” He says, “The WCT provides excellent real- time information on the corrosion event as it takes place so the rate of the corrosion can be monitored at different temperatures.” In contrast, “The ASTM D130 copper corrosion test does not apply a voltage and only looks at a rating of the copper in the end, which is very subjective.” Miiller has found “that the WCT detects corrosion and failures when the D130 test does not.” In addition, “even adding an elemental test as an extended D130 only values soluble copper and does not address the corrosion remaining on the wire.” 

Conductive deposit test 
The CDT is actively used to assess risk to electronics and motors in contact with the EV lubricants.⁵ The purpose of the CDT is to identify if conductive deposits will form when copper is exposed to lubricant or vapor. Copper that is corroded from the surface can deposit as copper sulfide onto the exposed copper when exposed to components of the liquid or vapor phase. These deposits can potentially bridge the gap between conductors at different electrical potentials, creating a catastrophic short in the circuits or electric motors.¹ 

The CDT involves placing a test printed circuit board in a sample of the test fluid, which is then heated for a period of time ranging from hundreds of hours to thousands of hours while the circuit is monitored in real time.¹ If the resistance drops below a specific value, the fluid has failed by showing conductive-deposit-forming tendencies in either the vapor phase or the liquid phase or both.¹ Photographs of the end-of-test board are evaluated for deposit accumulation and dendrite formation. 

The extended D130 and vapor phase copper test extends the time the copper strip is exposed to the heated fluid. A visual rating is performed at the end of the test, where the amount of soluble copper seen in the solution is an indication of the copper that has leached out of the copper strip.¹ 

Miiller also has found: “The CDT will capture conductive deposit failures that neither of the other two tests (i.e., D130 or WCT) are able to directly assess, and this is important to avoid real field failures.” He finds that the CDT “is looking at corrosion, but with a focus on conductive deposits which can damage a system with an arcing event.” The CDT is a test where voltage is applied to the copper traces but also goes beyond just corrosion. 

Miiller says that both the CDT and WCT provide quantitative results, with both tests providing “a rate of corrosion or a measure of the propensity to form conductive deposits over time.” He observes: “The CDT offers an index value over time that can be utilized by the OEMs or specification setting organizations to determine critical limits.” 

Miiller notes: “Because of the history of the ASTM D130 test, it may be useful for comparative results.” In contrast, he finds: “The CDT and WCT offer superior testing applications and information that show correlation to the field, which will prove to be critical tests for the EV industry for many years to come.” 

New methods 
Ayala notes, “While copper strip testing continues to be a critical indicator of sulfur compounds present after the refining process, measuring copper corrosivity continues to be a challenge.” The reliance of current test methods on manual and visual assessments introduces human bias and error into the testing process, leading to the potential for inaccurate or poorly reproducible results. 

Ayala describes copper digital detection imaging (CuDDI), a method that is in development, that “uses a highly stable and electronically controlled LED light source, which is stabilized at approximately 4,500 K and diffused at a 45-degree angle to simulate the ‘daylight’ effect referenced in current methods.” Using this light source method creates a more consistent environment via standardized simulation. 



Ayala says the optical corrosion measurement employed in the method uses “the proposed charge-coupled device (CCD) camera-based solution that is trained using the ASTM color standards and logic defined in Table 1 of ASTM D130.” This means “the algorithm is fixed and not subject to variations in color perception or color blindness.” The measurements also are performed under tightly controlled lighting conditions, meaning under reproducible ambient lighting conditions. Currently, the D130 manual method lacks control parameters for ambient conditions, which matters when looking at reflective color. 

Ayala identifies CuDDI’s potential improvements over current test procedures and their end results as including: 
• The elimination of inherent bias with the manual rating 
• A voltage and current-controlled light box to create a consistent ambient light environment 
• The automatic detection of the copper strip size 
• A long-lasting LED light source 
• Autorotation of the copper strip for recording results for both sides. 

The draft optical CuDDI method has not yet been established as an ASTM test method. 

Conclusions 
Vargo notes, “The details of electric motor design will have a large impact on the lubricant system that will be needed.” In addition, “with the mandated development and increased interest in EVs, new copper corrosion inhibitors may need to be developed for the lubricant system.” Copper, with its high electrical conductivity, will be used extensively in EV motors. The needed lubricants must provide protection from copper corrosion, effective cooling, the necessary dielectric properties and must not degrade or need to be changed over the EV’s lifespan. 

With the growing EV fluid market, Casey notes, “The additive manufacturing business is eager to get feedback on how existing additives can help solve the needs of emerging markets like EV and to develop new offerings to address the specific demands for EVs going forward.” 

Along with refining and developing effective lubricants and additives, the testing process of predicting copper corrosion will continue to be a necessary field for research. 

REFERENCES
1. Click here.
2. Click here.
3. Click here.
4. Click here.
5. Hunt, G. (2017), “New perspectives on the temperature dependence of lubricant additives on copper corrosion,” SAE Int. J. Fuels Lubr., 10 (2), pp. 521-527, https://doi.org/10.4271/2017-01-0891.

Andrea R. Aikin is a freelance science writer and editor based in the Denver area. You can contact her at pivoaiki@sprynet.com.