Science and Research 33 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 Since nearly 50 % of rolling element bearing failures are caused by abrasive/ adhesive wear and moisture corrosion [5], lubricants often contain extreme pressure/ antiwear (EP/ AW) additives in combination with corrosion inhibitors (CI). However, the compatibility of CI with EP/ AW is assessed as detrimental [6-9]. First experiments show a negative influence of calcium sulfonate (CI) and a positive influence of zinc carboxylate (CI) on the wear protection of ZDDP (EP/ AW) in oil-lubricated rolling element bearings [10,11]. To further improve the wear protection performance of lubricants, a run-in process for lubricants containing ZDDP has proven to be 1 Introduction Rolling elements bearings are lubricated with oils or greases to reduce friction and wear. These lubricants are also intended for other task, such as preventing corrosion on the metal surfaces of the bearings. Since base oils alone cannot sufficiently meet these requirements, multiple additives are added to enhance specific lubricant properties. However, when various additives are used together, one additive might influence the performance of the other additive during the formation of the tribological boundary layer. These interactions can be either synergistic, improving the performance of an additive, or antagonistic, reducing the performance of an additive. For instance, zinc dialkyl dithiophosphate (ZDDP) shows improved performance when combined with some friction modifiers [1,2], whereas the presence of dispersants can slow down the formation of ZDDP’s protective tribological boundary layer [3,4]. Minimizing harmful interactions is essential to fully utilize the full potential of additives. Meanwhile, promoting beneficial interactions can enable a reduction in the overall concentration of additives required. Consequently, it is crucial to investigate the mechanisms of surface-active additives not only individually but also in combination with one another. Influence of corrosion inhibitors on the tribological boundary layer formation of ZDDP during run-in process and on wear protection Merle Reimers, Georg Jacobs, Silvia Richter, Marius Bürger, Florian König* submitted: 24.09.25 accepted: 25.6.26 (peer review) Presented at the GfT Conference 2025 Nearly 50 % of rolling element bearing failures are due to abrasive/ adhesive wear and moisture corrosion. Hence, rolling element bearings require lubricants that not only reduce friction and wear, but also protect against corrosion. To prevent these failures extreme pressure / anti wear (EP/ AW) additives are often combined with corrosion inhibitors (CI), although their compatibility is generally considered antagonistic. Dispersants are known to slow down the TBL formation of ZDDP. Hence, a previous study shows CI elements getting embedded into the TBL, a similarly reduced TBL formation rate is expected when CI is combined with EP/ AW. This possibly impair wear and corrosion protection when applying the same runin process as when using EP/ AW solely. This contribution aims to further elucidate the wear and corrosion protection of rolling element bearings lubricated with EP/ AW and CI containing oil using a run-in process. Therefore, the TBL formation under run-in conditions are studied. Keywords wear, additives, lubricant, additive interactions, extreme pressure / anti wear additive, corrosions inhibitor, run-in, tribological boundary layer formation Abstract * Merle Reimers 1 (corresponding author) Orcid-ID: https: / / orcid.org/ 0000-0002-8880-5561 Dr.-Ing. Georg Jacobs 1 Orcid-ID: https: / / orcid.org/ 0009-0009-7683-6350 Dr. rer. nat. Silvia Richter 2 Marius Bürger 1 Orcid-ID: https: / / orcid.org/ 0000-0003-0868-9140 Dr.-Ing. Florian König 1,3 Orcid-ID: https: / / orcid.org/ 0000-0002-4795-6451 1 Institute for Machine Elements and Systems Engineering RWTH Aachen University Schinkelstr. 10, 52062 Aachen, Germany 2 Central Facility for Electron Microscopy, RWTH Aachen University Ahornstr. 55, 52074 Aachen, Germany 3 Soete Laboratory, Technologiepark Zwijnaarde 46, 9052 Zwijnaarde, Gent, Belgium favorable [12]. Studies with dispersants have shown a reduced tribological boundary layer (TBL) formation rate of EP/ AW [3,4]. Since the additive elements of CI are embedded in the TBL of EP/ AW [10], a slow TBL formation rate is also expected when CI is added to EP/ AW. Consequently, this slower TBL formation rate might lead to an incomplete TBL formation during the run-in process and thus to ineffective wear as well as corrosion protection of the TBL when CI is used simultaneously with EP/ AW. This contribution aims to further elucidate the wear protection of rolling element bearings lubricated with EP/ AW and CI containing oil using a run-in process. In addition, the corresponding corrosion protection of the TBL will be analyzed to highlight the effect of a run-in process on the corrosion protection. 2 Materials and methods The oils including the base oil and additives used in this study are presented below. The test methods for wear and corrosion protection are further described as well. For a closer investigation of the TBL formed electron probe microanalysis (EPMA) are carried out. 2.1 Base oil and additives Three different oils are tested in this study. All oils consist of base oil and 1.0 wt.% EP/ AW. One oil contains no CI, while the two others contain different kinds of CI with a concentration of 0.5 wt.%. As base oil a mineral oil (API group I) of the viscosity class ISO VG 100 was used. The EP/ AW was a primary zinc dialkyl dithiophosphate (ZDDP) with a C8 chain containing of 8 wt.% phosphorus, 16 wt.% sulfur and 9.5 wt.% zinc. Either the first type of CI calcium sulfonate (dialkyl benzene sulfonate) containing of 2 wt.% calcium or the second type of CI zinc carboxylate (2-ethylhexanoic acid) consisting of 15 wt.% zinc was combined with the mineral oil and ZDDP. In this study, the abbreviation CaSulf for calcium sulfonate and ZnCarb for zinc carboxylate will be used. An overview of the oils investigated and shown in Table 1. 2.2 Wear protection test The wear protection of the different oils is evaluated with an axial thrust bearing test rig (FE8) according to DIN 51819-1 [13]. Based on the results in [11] the TBL formation is investigated in the run-in test (75 min -1 , 80 kN, 80 °C) by variating the test duration using 2 h, 10 h and 80 h. The effect of run-in will be analyzed by a run-in for 10 h at 75 min -1 , 80 kN, 80 °C followed by the standard wear critical test for 80 h at 7.5 min -1 , 80 kN, 80 °C. This procedure are based on the results of the study in [12]. The test conditions are summarized in Table 2. The wear masses of the rolling elements sets are analyzed for evaluating the wear protection. The test rig and test bearing 81212 with 15 rolling elements and PA66 cage are shown in Figure 1. 2.4 Corrosion protection test The corresponding corrosion protection will be analyzed with a novel corrosion protection test (CPT) which was introduced in [14] and bases on the corrosion test according to DIN ISO 7120 [15]. The CPT allows the inves- Science and Research 34 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 Base oil Mineral oil Table 1: Oils and Additives Test FE8 test condition run-in 75 min -1 , 80 kN, 80 °C 2 h 10 h 80 h run-in + wear test 75 min -1 , 80 kN, 80 °C, 10 h + 7.5 min -1 , 80 kN, 80 °C, 80 h Table 2: FE8 test conditions Figure 1: a) wear protection test setup and b) test bearing axial load heating and cooling drive 2 test bearings torque measurement oil supply +5 0 -5 0 +5 -5 0 -12.05 +13.70 length [mm] SRR [%] b) Test bearing (81212-PA15) a) Axial thrust bearing test rig (FE8 test rig) 1 wt.% EP/ AW 1.0 wt% ZDDP 0.5 wt.% CI - CaSulf ZnCarb FE8 test condition 75 min -1 , 80 kN, 80 °C 2 h 10 h 80 h 75 min -1 , 80 kN, 80 °C, 10 h + 7.5 min -1 , 80 kN, 80 °C, 80 h 75 min -1 , 80 kN, 80 °C Science and Research 35 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 tigation of the corrosion protection of TBL formed during the FE8 test. Instead of using unalloyed carbon steel as described in the standard, the rolling elements that were taken from FE8 after the run-in test serve as the test specimens for the CPT. During the CPT, these rolling elements are immersed in an oil-water emulsion with an oil-water ratio of 10: 1 at 60 °C for 24 hours. To maintain a uniform emulsion, a stirrer operates continuously at 1000 min -1 . Unlike the standard test, which employs an additive-containing oil, only base oil without additives was used in the CPT. This setup enables evaluation of the corrosion protection properties solely of the TBL formed in the FE8 in presence of EP/ AW and CI additives instead of the corrosion protection of the additives in the lubricants. The CPT test setup is illustrated in Figure 2a. The corrosion protection of the TBLs formed in the FE8 test are assessed visually using a corrosion level scale from 0 (no corrosion) to 4 (severe corrosion). For each FE8 test, two rolling elements are evaluated in the CPT and their individual ratings are averaged to determine the overall corrosion level of the test. The CPT evaluation scale with its five corrosion levels, including representative images, is presented in Figure 2b. 2.4 EPMA For further investigation, electron microscopic methods such as electron probe microanalysis (EPMA) are used by measuring the thickness and chemical composition of TBL on the rolling bearings formed in the FE8 tests. The methods are described in detail in [10]. For the convenience of the reader, we shortly summarize the relevant information. A line scan was performed across the whole rolling element width using a step size of 140 µm. The electron beam was defocused to a diameter of 20 µm. The calibrated and measured characteristic xray intensities were transferred to element-specific mass coverage by using a special thin-film algorithm based on the model of Pouchou and Pichoir [16]. The mass coverage is a qualitative dimension of the additive elements embedded in the TBL and a quantitative dimension of the thickness of the TBL. For easier comparison, the line scans are shown as averaged values. The complete line scan measurements are included in the appendix. 3 Results and Discussion First the results of the TBL formation will be shown and discussed. On the basis of these results the wear protection will be improved using a run-in process. 3.1 TBL formation The wear masses of the run-in test shown in Figure 3a differ not significantly as expected due to the formation of TBL. Only for the test with ZDDP and CaSulf the negative effect of CaSulf on the wear protection of ZDDP can be seen after 80 h test duration. However, in the case of TBL formation, the EPMA results over the different test durations are particularly interesting (Figure 3b). The TBL formed after 10 h differentiated barely from the TBL formed after 80 h in case of using the oil containing only ZDDP without any CI. When adding CI, significant differences could be observed of TBL formed over time. The full EPMA line scan results are added to the appendix (Figure 7). When adding calcium sulfonate the TBL formation rate slows down. This can be observed when comparing the total mass coverage at 10 h with 14.2 µg/ cm 2 comparted to at 80 h with 22.8 µg/ cm 2 . Proportionally more calcium is embedded at the start of TBL formation after 2 h and 10 h compared to after 80 h. The total calcium mass coverage does not increase after 10 h, but the ZDDP elements such as Zn, P and S increase further. This shows the first embedding of faster CI additive element and some ZDDP elements, followed by only the ZDDP components in the TBL. When adding zinc carboxylate, the TBL formation rate is slower compared to ZDDP solely. This can be observed by comparing the total mass coverage at 2 h with 3.2 µg/ cm 2 , at 10 h with 14.0 µg/ cm 2 and at 80 h with 21.9 µg/ cm 2 . After the first 2 hours the TBL formed consists mostly oxygen and some zinc. Most likely the zinc is provided by CI zinc carboxylate. After 10 h the first ZDDP elements sulfur and phosphorus and more zinc are embedded in the TBL. This shows that the CI additive elements are embedded first and then on top the ZDDP elements into the TBL. n = 1000 min -1 24 h T = 60 °C base oil + water (10: 1) Corrosion level 0 1 2 3 4 Corrosion no little medium strong extensive Examples FE8 test rolling element with TBL Figure 2: a) corrosion protection test (CPT) set up and b) evaluation of CPT a) CPT test set up b) CPT evaluation wards. Especially when adding zinc carboxylate, no ZDDP elements are embedded in the first 2 hours. Therefore, 10 hours are selected for the run-in process in the next step as good compromise with a TBL including both EP/ AW and CI. 3.2 Wear protection improvement with run-in As the results of the TBL formation show, a run-in process for 10 hours at 80 kN, 80 °C, 75 min -1 is used to form an initial TBL, which shall protect the rolling element bearing from wear in the harsh wear test. This was already successfully illustrated for ZDDP containing mineral oil in [17]. However, the TBL durability should decrease when adding CI due to the slower TBL formation as the previous results indicate. The wear masses obtained from wear tests with and without a run-in process are shown in Figure 5a. When using ZDDP and CI simultaneously, the wear masses are reduced using run-in. Accordingly, after the wear test with prior run-in, a homogeneous TBL can be observed, whereas without run-in Science and Research 36 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 That the CI additive elements are embedded first in the TBL can be also observed in regard of corrosion protection. The corresponding corrosion protection test after the run-in test for 2 h, 10 h and 80 h in FE8 are presented in Figure 4. The corrosion protection of the TBL formed in FE8 with ZDDP solely do not differ over time of TBL formation. When adding CI with both calcium sulfonate or zinc carboxylate, the corrosion protection at the beginning of TBL formation at 2 h and 10 h are improved compared to after 80 h. However, the corrosion protection of the TBL formed with ZDDP and CI decreases compared to ZDDP solely. Only the TBL formed in the first 10 hours with ZDDP and zinc carboxylate results in a good corrosion protection. This agrees with first embedding of the zinc carboxylate CI additive elements zinc and oxygen. To align, when adding CI, the TBL formation slows down compared to the TBL formation with ZDDP solely. Furthermore, the CI additive elements are embedded faster, while the ZDDP additive elements follow after- Figure 3: FE8 results of TBL formation with a) wear mass and b) EPMA results a) run-in test (n = 75 min -1 ) b) EPMA results Figure 4: Corrosion protection test (CPT) results after TBL formation for 2 h, 10 h, 80 h in FE8 with n = 75 min -1 showing a) corrosion level and b) rolling elements after CPT ZDDP ZDDP + CaSulf ZDDP + ZnCarb 2 h 10 h 80 h a) Corrosion level ZDDP ZDDP + CaSulf ZDDP + ZnCarb 2 h 10 h 80 h b) Rolling elements after CPT no TBL is present. The corresponding EPMA line scans are attached in the appendix (Figure 8). This shows a positive effect for the run-in on the wear protection for all oils investigated in this study. For a better understanding of the run-in effect when using EP/ AW and CI simultaneously, the TBL after run-in and after run-in followed by wear test are compared in Figure 5b. When using ZDDP together with calcium sulfonate, the TBL growth begins in the run-in and even further continues during the wear test. Especially noteworthy is the reduction of the total calcium mass coverage after the wear test, while ZDDP elements such as zinc, phosphorus and sulfur increase during the wear test. This shows the strong growth effect of ZDDP elements, when an initial TBL was formed as described in [17], but poor durability of the CI element calcium under the harsh conditions of wear test. When using ZDDP and zinc carboxylate simultaneously, the TBL growth begins slightly in the run-in and even further continues during the wear test. Mainly phosphorus is embedded further during the wear test. Due to the already good wear protection provided without run-in, only a slight increase of wear protection was achieved with run-in. However, the TBL after wear test without and with run-in differ significantly in their composition (appendix Figure 8). Without run-in a high oxygen containing TBL was formed, while with run-in the TBL consists more ZDDP elements but also more oxygen compared to ZDDP solely. The oxygen was embedded by zinc carboxylate in the run-in phase, which was shown in TBL formation results. Within the first two hours, oxygen and zinc are embedded in the TBL introduced be zinc carboxylate, followed by a topped layer of ZDDP elements zinc, phosphorus and sulfur. This top layer of ZDDP growth further after the run-in in the wear test as known for ZDDP [17]. The effect of run-in on the TBL formation and therefore on the wear protection reflects also on the corrosion protection. The results are shown in Figure 6. For all three additive combinations, the corrosion protection increases significantly when using run-in. However, for ZDDP Science and Research 37 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 Figure 5: FE8 results a) wear mass comparing with and w/ o run-in and b) EPMA results of TBL formed in 10 h run-in and after durability in wear test a) wear test (n = 7.5 min -1 ) b) EPMA results Figure 6: Corrosion protection test (CPT) results after FE8 wear-test with and without run-in showing a) corrosion level and b) rolling elements after CPT ZDDP ZDDP + CaSulf ZDDP + ZnCarb w/ o run-in with run-in a) Corrosion level ZDDP ZDDP + CaSulf ZDDP + ZnCarb w/ o run-in with run-in b) Rolling elements after CPT desministerium für Wirtschaft und Energie) on the basis of a resolution of the German Bundestag. References [1] Eickworth J, Aydin E, Dienwiebel M, Rühle T, Wilke P, Umbach TR. Synergistic effects of antiwear and friction modifier additives. ILT 2020; 72(8): 1019-25. https: / / doi.org/ 10.1108/ ILT-07-2019-0293. [2] Lu R, Shiode S, Tani H, Tagawa N, Koganezawa S. A Study on the Tribofilm Growth and Tribological Properties of Tribofilms Formed from Zinc Dialkyl Dithiophosphate (ZDDP) and Molybdenum Dialkyl Dithiocarbamate (MoDTC). Tribology Online 2018; 13(3): 157-65. https: / / doi.org/ 10.2474/ trol.13.157. [3] Papay AG. Antiwear and extreme-pressure additives in lubricants. Lubrication Science 1998; 10(3): 209-24. https: / / doi.org/ 10.1002/ ls.3010100304. [4] Barcroft FT, Park D. Interactions on heated metal surfaces between zinc dialkyldithiophosphates and other lubricating oil additives. Wear 1986; 108(3): 213-34. [5] SKF Evolution. Bearing damage analysis: ISO 15243 is here to help you; 2022 [cited 2025 August 28] Available from: URL: https: / / evolution.skf.com/ bearing-damageanalysis-iso-15243-is-here-to-help-you/ . [6] Mortier RM, Fox MF, Orszulik ST. Chemistry and Technology of Lubricants. Dordrecht: Springer Netherlands 2010. [7] Spikes HA. Additive-additive and additive-surface interactions in lubrication. Lubrication Science 1989; 2(1): 3- 23. https: / / doi.org/ 10.1002/ ls.3010020102. [8] Rounds FG. Additive Interactions and Their Effect on the Performance of a Zinc Dialkyl Dithiophosphate. A S L E Transactions 1978; 21(2): 91-101. https: / / doi.org/ 10.1080/ 05698197808982864. [9] Spikes H. Mechanisms of ZDDP—An Update. Tribol Lett 2025; 73(1). https: / / doi.org/ 10.1007/ s11249-025-01968-3. [10] Reimers M, Richter S, Bürger M, Jacobs G, König F. Wear protection of extreme pressure / anti wear additives influenced by corrosion inhibitors in oil-lubricated rolling bearings. Tribology International 2025; 210. https: / / doi.org/ 10.1016/ j.triboint.2025.110807. [11] Reimers M, Richter S, Jacobs G, Mayer J, König F. Influence of Corrosion Inhibitors on the Wear Protection of Extreme Pressure / Anti Wear Additives in Oil-lubricated Rolling Bearings. Tribologie + Schmierungstechnik 2025; 72 (3-4). https: / / doi.org/ 10.24053/ TuS-2025-0013 [12] Burghardt G, Wächter F, Jacobs G, Hentschke C. Influence of run-in procedures and thermal surface treatment on the anti-wear performance of additive-free lubricant oils in rolling bearings. Wear 2015; 328-329: 309-17. https: / / doi.org/ 10.1016/ j.wear.2015.02.008. [13] DIN 51819-1: 2016-12, Prüfung von Schmierstoffen_ - Mechanisch-dynamische Prüfung auf dem Wälzlagerschmierstoff-Prüfgerät FE8_ - Teil_1: Allgemeine Arbeitsgrundlagen. Berlin: Beuth Verlag GmbH. [14] Reimers M, Richter S, Jacobs G. Bürger M, König F. Wear and corrosion protection in roller bearings - Influ- Science and Research 38 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 and zinc carboxylate the increase is not as significant as for ZDDP solely and ZDDP with calcium sulfonate. This agrees with the relatively high wear mass of 30 mg. To align, the results of wear test with and without run-in show that when only a few ZDDP elements were embedded in TBL during run-in, the CI elements are displaced in the wear test and the ZDDP is mainly involved in the further formation of the TBL during wear test. This results in significantly better wear and corrosion protection when using run-in compared to no run-in. 4 Conclusions Lubricants often contain extreme pressure/ anti-wear (EP/ AW) additives in combination with corrosion inhibitors (CI) to protect rolling element bearings from wear and corrosion. First investigations show an insufficient TBL formation in standard FE8 wear test when using EP/ AW and CI simultaneously [10,11]. To further improve the wear and corrosion protection of rolling element bearings, a run-in process before the FE8 wear test was used. Therefore, first the TBL formation under runin conditions are studied. The following conclusion can be drawn on the basis of the results: - When adding calcium sulfonate, the CI additive element calcium is embedded faster while the ZDDP additive elements follow afterwards. - When adding zinc carboxylate, only CI additive element zinc and oxygen are embedded first and then on top the ZDDP elements are embedder into the TBL. - For all three oils the wear and corrosion protection were significantly increased when using run-in. - Mainly ZDDP is involved in the further formation of the TBL during wear test after run-in process. Therefore, only a few ZDDP elements need to be embedded in TBL during run-in process. Overall, the results of this study show a significant increase of wear and corrosion protection using run-in procedures at higher lambda ratios. Nevertheless, the best corrosion protection was achieved by ZDDP solely. Because 10 hours of run-in is not economical, the run-in duration has to be reduced to improve the economic efficiency. Therefore, further investigations need to be conducted to find the optimum run-in duration as well as operational conditions, which might differ between the additive combination. Acknowledgement The project (01IF22309N) is funded by the Federal Ministry of Economic Affairs and Energy BMWE (Bunence of ratio between extreme pressure / anti wear additive and corrosion inhibitor. Tribologiy International 2026; 220. https: / / doi.org/ 10.1016/ j.triboint.2026.111880 [15] DIN ISO 7120. Mineralöle und andere Flüssigkeiten: Bestimmung der Korrosionsschutzeigenschaften in Gegenwart von Wasser. Berlin: DIN Deutsches Institut für Normung e. V.; 2000 2000. [16] Pouchou JL, Pichoir F. Electron probe X-ray microanalysis applied to thin surface films and stratified specimens. Scanning Microscopy 1993; 1993(7): 167-89. [17] Burghardt G. Wirkung tribologischer Grenzschichten in Wälzlagern unter Mischreibung. Dissertation, Institut für Maschinenelemente und Systementwicklung, RWTH Aachen University 2017. Science and Research 39 Tribologie + Schmierungstechnik · volume 73 · issue 2/ 2026 DOI 10.24053/ TuS-73-0010 1 wt.% ZDDP 1 wt.% ZDDP + 0.5 wt.% CaSulf 1 wt.% ZDDP + 0.5 wt.% ZnCarb O P S Zn Ca Total wear test O P S Zn Ca Total run-in + wear test O P S Zn Ca Total run-in O P S Zn Ca Total wear test O P S Zn Ca Total run-in + wear test O P S Zn Ca Total run-in O P S Zn Ca Total wear test O P S Zn Ca Total run-in + wear test O P S Zn Ca Total run-in Figure 8: EPMA line scan results of TBL for run-in process 1 wt.% ZDDP 1 wt.% ZDDP + 0.5 wt.% CaSulf 1 wt.% ZDDP + 0.5 wt.% ZnCarb O P S Zn Ca Total 2 h O P S Zn Ca Total 10 h O P S Zn Ca Total 80 h O P S Zn Ca Total 2 h O P S Zn Ca Total 10 h O P S Zn Ca Total 80 h O P S Zn Ca Total 2 h O P S Zn Ca Total 10 h O P S Zn Ca Total 80 h Figure 7: EPMA line scan results of TBL formation Appendix