Unraveling silica auto-condensation and dissolution chemistry in engine coolant fluids: Applications in automotive coolant organic additive technology (OAT)

Engine coolants are mixtures of water and glycols (most commonly monoethylene glycol, MEG), formulated with corrosion inhibitors to protect the system materials. Nowadays, organic corrosion inhibitors are generally used in combination with inorganic corrosion inhibitors, such as silicates. To effectively formulate such engine coolant, it is of crucial importance to understand the behavior of silicate in a “semi-aqueous” environment. Silicate polycondensation has been thoroughly studied in aqueous solutions and the key factors influencing its aqueous chemistry are well defined (i.e. pH, temperature, concentration, etc.). In this paper the polycondensation chemistry of silicate in a wide range of MEG/water mixtures is thoroughly studied and the influence of several experimental parameters, such as working pH, condensation time and the MEG:water ratio, is investigated. The presence of MEG in MEG/water mixtures enhances silicate auto-condensation to form amorphous silica, at all pH values studied. Silicate condensation starts to occur within 10 minutes of pH adjustment and is faster with increasing MEG. The lowest levels of active silicate were observed in the pH region 8.0 to 8.5 (most common pH for engine coolants). The behavior of silicate was also studied in a generic coolant (which contains organic corrosion inhibitors in addition to MEG). In both cases condensation is severely enhanced. Temperature-driven amorphous silica particle dissolution was studied in selected MEG/water mixtures and it was found that it is negligible at ambient temperature regardless of the MEG:water ratio, whereas it is substantial at 90°C in pure water. Delineating silicate chemistry in coolant matrices will be valuable information for designing betterperforming formulations for engine coolant applications.

1. M. Wysokowski, T. Jesionowski and H. Ehrlich, Biosilica as a source for inspiration in biological materials science, Am. Mineral., 2018, 103(5), 665–691. doi: 10.2138/am2018-6429

2. O.W. Flörke, H.A. Graetsch, F. Brunk, L. Benda, S. Paschen, H.E. Bergna, W.O. Roberts, W.A. Welsh, C. Libanati, M. Ettlinger, D. Kerner, M. Maier, W. Meon, R. Schmoll, H. Gies and D. Schiffmann, Silica, Ullmann’s Encycl. Ind. Chem., 2008, 32, 421–507. doi: 10.1002/14356007.a23_583.pub3

3. D.J. Belton, O. Deschaume and C.C. Perry, An overview of the fundamentals of the chemistry of silica with relevance to biosilicification and technological advances, FEBS J., 2012, 279(10), 1710–1720. doi:10.1111/j.1742-4658.2012.08531.x

4. Ν.C. Brady and R.R. Weil, The Nature and Properties of Soils, 14th Edition, Prentice Hall, Hoboken NJ, 2007.

5. P.V.D. Ven and J.P. Maes, The effect of silicate content in engine coolants on the corrosion protection of aluminum heat-rejecting surfaces, SAE Trans., 1994, 103, 270–278. doi: 10.4271/940498

6. L.B. Coelho, M. Lukaczynska-Anderson, S. Clerick, G. Buytaert, S. Lievens and H.A. Terryn, Corrosion inhibition of AA6060 by silicate and phosphate in automotive organic additive technology coolants, Corros. Sci., 2022, 199, 110188. doi: 10.1016/j.corsci.2022.110188

7. D.A. Washington, D.L. Miller, P.B. Valkovich, R.A. Armstrong, F.A. McMullen, M.J. Quinn, F.A. Kelly and E.J. McWilliams, Performance of organic acid based coolants in heavy duty applications, SAE Tech. Pap. Ser., 1996, 960644. doi: 10.4271/960644

8. D. Scott, J.V. Orth and D. Miller, Coolant Pump Failure Rates as a Function of Coolant Type and Formulation. SAE Tech. Pap. Ser., 1994, 940768. doi: 10.4271/940768

9. F. Verpoort, T. Haemers, P. Roose and J.P. Maes, Characterization of a surface coating formed from carboxylic acid-based coolants, Appl. Spectrosc., 1999, 53(12), 1528–1534.

10. P.V.D. Ven and J.P. Maes, The effect of silicate content in engine coolants on the corrosion protection of aluminum heat-rejecting surfaces, SAE Trans., 1994, 103, 270–278. doi: 10.4271/940498

11. B. Yang, P. Woyciesjes and A. Gershun, Comparison of extended life coolant corrosion protection performance, 2017, Conference: WCX™ 17: SAE World Congress Experience. doi: 10.4271/2017-01-0627

12. M. Jayalakshmi and V.S. Muralidharan, Inhibitors for aluminium corrosion in aqueous medium, Corros. Rev., 1997, 15(3–4), 315–340. doi: 10.1515/CORRREV.1997.15.3-4.315

13. W.T. Tsai, Y.H. Hon and J.T. Lee, Corrosion inhibition of aluminum alloys in heat exchanger systems, Surf. Coat. Technol., 1987, 31, 365–380.

14. O. Lopez-Garrity and G.S. Frankel, Corrosion inhibition of AA2024-T3 by sodium silicate, Electrochim. Acta, 2014, 130, 9–21. doi: 10.1016/j.electacta.2014.02.117

15. S.T. Amaral and I.L. Muller, Effect of silicate on passive films anodically formed on iron in alkaline solution as studied by electrochemical impedance spectroscopy, Corrosion, 1999, 55, 17–23.

16. D.B.V.D. Heuvel, E. Gunnlaugsson and L.G. Benning, Passivation of Metal Surfaces Against Corrosion by Silica Scaling, Proceedings, 41st Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, 2016, paper SGP-TR209.

17. J.C. Rushing, L.S. McNeill and M. Edwards, Some effects of aqueous silica on the corrosion of iron, Water Res., 2003, 37(5), 1080–1090. doi: 10.1016/S0043-1354(02)00136-7