International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
Effect of water absorption in bearing greases on wear and corrosion
125
2022
Ivan Delic
Michael Adler
Karl Adam
Franc Bardin
ict2310251
23rd International Colloquium Tribology - January 2022 251 Effect of water absorption in bearing greases on wear and corrosion Ivan Delic AC2T research GmbH, 2700 Wiener Neustadt, Austria Adler Michael AC2T research GmbH, 2700 Wiener Neustadt, Austria Karl Adam 2voestalpine Stahl GmbH, 4020 Linz, Austria Franc Bardin TotalEnergies SE, 69360 Solaize, France 1. Introduction Water as well as aqueous solutions and emulsions are commonly used as coolants in industrial machinery. Insufficient sealing of bearing components can result in a contamination of the lubricant with coolant. Therefore, the focus of this work is set on the effects of water on grease performance and bearing materials. To generate a better understanding of grease-water interaction, experiments with water-grease blends were performed. Properties including water absorption capacity and water retention in dependence of temperature and time and the effect on standardized grease properties such as cone penetration or oil separation were investigated. The impact on corrosion inhibition was evaluated using standardized tests as well as an in-house corrosion test developed to determine the effects on standstill corrosion. Friction and wear properties were elaborated using selected tribometrical test devices following standardized procedures for comparability of the results. As the gathered findings provide insight into the behaviour of water contaminated grease, they possibly allow a better assessment of grease lifetime and maintenance intervals of grease lubricated components. 2. Methods and material All investigations were carried out on two commercially available Calcium sulfonate complex greases (CaSX) with different base oil viscosity. The greases were chosen as CaSX greases are commonly used in water rich environment due to good performance with water ingress as reported by Bosman et al. [1]. For the evaluation of water absorption capacity, grease water mixtures with different water contents were prepared using a quarter scale grease worker. 60 double strokes were found to be sufficient for good mixing. Deionized water was used for the experiments. The maximum amount of water mixable into the grease was evaluated optically by formation of water droplets in the mixture. The optical result was confirmed using Karl Fischer titration. Subsequently, the mixtures were used for the characterization of water retention. Droplets of grease were put on glass plates and the water content was continuously evaluated using Karl Fischer titration. Investigations on the change in consistency due to water ingress were performed using similar grease water mixtures. Further, the change in oil bleed due to water ingress was assessed using a modified filter paper test where a defined volume of grease is put in a cylindrical form and put on filter paper. The corrosiveness of the grease-water mixtures was assessed using a modified copper corrosion test according to ASTM D4048. To evaluate the protective properties against standstill corrosion, cylindrical rollers made of bearing steel (DIN ISO 1.3505) were exposed to mixtures of synthetic sea water and grease. Wear protection and extreme pressure properties were investigated using a 4-ball test rig and a high frequency reciprocating rig (HFRR or SRV) according to corresponding ASTM standards (ASTM D2266 ; D2596; D5706; D5707). 252 23rd International Colloquium Tribology - January 2022 Effect of water absorption in bearing greases on wear and corrosion 3. Results and discussion Figure 1: Grease water mixtures immediately after mixing (top) and after 4 days at room temperature (bottom) After the described experiments, water absorption capacity was found to be about 55 % mass regardless of base oil viscosity. Regarding water separation, it was observed that the water content was mainly reducing in surface near areas while the water content in the centre of the droplets remained constant over the investigated time-period. As expected, the decrease was faster with increasing water content. Similar results were observed with increased temperature and during other experiments. The tests show that discoloration occurs with water ingress. The penetration depth of both greases decreased with water ingress (indicating the grease becoming more rigid) until water contents of about 40 % were reached. An increase of penetration depth and a rougher grease structure were observed at that point. For lower water concentrations a linear correlation regarding penetration depth is reported. Figure 2: Penetration depth in dependence of water content for selected calcium sulfonate complex grease Oil bleed was determined gravimetrically after the grease and paper were exposed to increased temperature. In both cases the oil bleed was decreased with water ingress. A higher decrease was observed for the grease with a higher base oil viscosity. The results were compared to standardized oil separation methods where similar trends were observed. Copper corrosion was found to be more severe for grease without water contamination. However, circular structures were observed on copper platelets for all mixtures. This indicates evaporation and contact loss between the grease and material surface. Regarding standstill corrosion, good protection was maintained for all inspected mixtures. Significant corrosive attack was only detected with oversaturated mixtures where a formation of water droplets was observed. Water ingress resulted in lower wear protection for all investigated greases. A decrease in weld load. Similar results were found for the weld load. 4. Conclusions and outlook High water absorption capacity makes CaSX greases suitable for the use in water rich environments. The capacity can mainly be ascribed to the thickener structure forming inverted micelles as described by Bosman et al. and others [1,2]. That behaviour was confirmed by comparison of different base oil viscosities. Thickening and reduction in oil bleed can also be attributed to the formation and interaction of micelles in the thickener structure. The described properties are believed to have a significant impact on tribological performance of the grease as described by Cyriac et al. [4]. Corrosive attack was mainly observed in the presence of free water as described in literature [1]. Evaporation of water during copper corrosion tests is linked to circular structures on the copper surface. Similar structures were observed in bearing investigation regarding corrosive damage. Another correlation with the corrosion behaviour described in literature is given in the behaviour of the standstill corrosion experiments. Formation of free water is also linked to worsened wear protection as described by Cyriac et al. [4] and Huddedagaddi et al. [5]. As most properties responsible for wear and corrosion protection are affected by water ingress, an implementation of a monitoring method for the water content of greases is of great importance. The optical changes of the greases could be of use in the assessment of the usability regarding water contamination. Further investigations on tribological performance are currently being carried out. The idea is to link degradation of grease performance to simple grease properties to allow for a quick check of grease during operation. Obtained limits to water contamination can be used in the implementation of an inline monitoring to prevent critical failure. 5. Acknowledgement This work was funded by the project COMET InTribology1, FFG-No. 872176 (project coordinator: AC2T research GmbH, Austria). References [1] Rob Bosman & Piet M. Lugt (2018) “The Microstructure of Calcium Sulfonate Complex Lubricat- 23rd International Colloquium Tribology - January 2022 253 Effect of water absorption in bearing greases on wear and corrosion ing Grease and Its Change in the Presence of Water”, Tribology Transactions, 61: 5, 842-849 [2] Pierre Belot, “Complex Calcium Sulfoante Grease - A unique approach to industrial grease lubrication”,7th Lubricating Grease Conference, Cochi, India, Feb. 2005 [3] Gurt, Alan & Khonsari, Michael. (2020). “An Overview of Grease Water Resistance”. Lubricants. 8. 86. 10.3390/ lubricants8090086 [4] Cyriac, F., Lugt, P.M., Bosman, R. et al. Impact of “Water on EHL Film Thickness of Lubricating Greases in Rolling Point Contacts”. Tribol Lett 61, 23 (2016) [5] Channabasappa B.Hudedagaddi, Anirudh G.Raghav, Angela M.Tortora, Deepak H.Veeregowda, “Water molecules influence the lubricity of greases and fuel”, Wear, Volumes 376-377, Part A, 15 April 2017
