Determination of Trace Metals in Lubricating Oils

น้ำมันหล่อที่ไม่ดี (มีสารปลอมปน) สามารถนำสู่การสึกหรอของชิ้นส่วนได้ โดยเฉพาะส่วนประกอบของเครื่องยนต์และกังหัน เป็นต้น ดังนั้นเราจะตรวจสอบหาโลหะแปลกปลอมในน้ำมันหล่อลื่นได้อย่างไร

เขียนโดย Prof Ricardo Aucelio (Pontificia Universidade Catolica do Rio de Janeiro)
อ่านเพิ่มเติมได้ที่ http://www.scitopics.com/Determination_of_Trace_Metals_in_Lubricating_Oils.html

Engine and turbine components undergo continual wear under normal operating conditions; this is minimized by the use of lubricating oils. Lubricating oils from petroleum are mainly composed of paraffinic, naphthenic and, to a lesser extent, aromatic hydrocarbons. Several additives, including metallo-organic ones, are also part of the final composition of commercial lubricating oil. Wear has both physical (friction between metallic parts, high temperature and pressure) and chemical (corrosion) sources. Chemical wear may produce not only metallic particles but also soluble metallo-organic species, whereas physical wear generates metallic particles of varying sizes (up to a few micrometers). Part of the metallic debris is not retained by filters and collectors, remaining in the lubricating oil and being transported through the whole system. Increasing amounts of some key elements in the lubricating oil may indicate the extent of the wear of wetted components. For instance, an abrupt increase of Ni, Sn or Cr indicates corrosion in bearings, valves and pistons, Fe indicates corrosion in various parts, Na indicates oil contamination with anti-freeze fluids and so on. The diagnosis based on elemental analysis of used lubricating oils may indicate the need for preventive maintenance of engines and turbines before irreversible damage occurs. Besides bringing economic benefits, such diagnoses save lives. The Spectroscopy Oil Analysis Program (SOAP) of the United States Air Force was the pioneer program based on this type of monitoring.Elements such as Ag, B, Ba, Bi, Ca, Cd, Co, Cr, Fe, Hg, Mg, Mo, Ni, P, Sb, Se, Sn, Ti and Zn, are also deliberately introduced in small portions to lubricating oils to address requisites for special applications. In these cases, metallo-organic compounds containing these elements act as additives, improving lubricating capability and properties such as antioxidant, anticorrosive, dispersing, antiwear, and others. The improvement of performance of the oil is dependent upon the amount of the additive introduced; therefore, these amounts must be strictly controlled. Sensitive techniques are required for analysis of used lubricating oils since the capability of monitoring small concentration changes in the key elements is needed. The complexity of the oil matrix, its viscosity and the high organic load impose serious difficulties for elemental analysis. Such determinations are also an analytical challenge because all wear metals (solid particles of different sizes and metallo-organic species) must be accurately determined for information purposes. Apart from the existence of a few electroanalytical and XRF methods for the determination of Zn, Cu, Pb, Fe, Cr, Ni, As and Cd in lubricating oils, the majority of analytical methods reported in the literature are based on atomic spectrometric techniques such as FAAS, ET AAS, DC or ICP OES, ICP MS and AFS (From Aucélio R.Q. et al. (2007, The Determination of trace metals in lubricating oils by atomic spectrometry. Spectrochim. Acta ., 62: 952-961).

Engine and turbine components undergo continual wear under normal operating conditions; this is minimized by the use of lubricating oils. Lubricating oils from petroleum are mainly composed of paraffinic, naphthenic and, to a lesser extent, aromatic hydrocarbons. Several additives, including metallo-organic ones, are also part of the final composition of commercial lubricating oil. Wear has both physical (friction between metallic parts, high temperature and pressure) and chemical (corrosion) sources. Chemical wear may produce not only metallic particles but also soluble metallo-organic species, whereas physical wear generates metallic particles of varying sizes (up to a few micrometers). Part of the metallic debris is not retained by filters and collectors, remaining in the lubricating oil and being transported through the whole system. Increasing amounts of some key elements in the lubricating oil may indicate the extent of the wear of wetted components. For instance, an abrupt increase of Ni, Sn or Cr indicates corrosion in bearings, valves and pistons, Fe indicates corrosion in various parts, Na indicates oil contamination with anti-freeze fluids and so on. The diagnosis based on elemental analysis of used lubricating oils may indicate the need for preventive maintenance of engines and turbines before irreversible damage occurs. Besides bringing economic benefits, such diagnoses save lives. The Spectroscopy Oil Analysis Program (SOAP) of the United States Air Force was the pioneer program based on this type of monitoring.Elements such as Ag, B, Ba, Bi, Ca, Cd, Co, Cr, Fe, Hg, Mg, Mo, Ni, P, Sb, Se, Sn, Ti and Zn, are also deliberately introduced in small portions to lubricating oils to address requisites for special applications. In these cases, metallo-organic compounds containing these elements act as additives, improving lubricating capability and properties such as antioxidant, anticorrosive, dispersing, antiwear, and others. The improvement of performance of the oil is dependent upon the amount of the additive introduced; therefore, these amounts must be strictly controlled. Sensitive techniques are required for analysis of used lubricating oils since the capability of monitoring small concentration changes in the key elements is needed. The complexity of the oil matrix, its viscosity and the high organic load impose serious difficulties for elemental analysis. Such determinations are also an analytical challenge because all wear metals (solid particles of different sizes and metallo-organic species) must be accurately determined for information purposes. Apart from the existence of a few electroanalytical and XRF methods for the determination of Zn, Cu, Pb, Fe, Cr, Ni, As and Cd in lubricating oils, the majority of analytical methods reported in the literature are based on atomic spectrometric techniques such as FAAS, ET AAS, DC or ICP OES, ICP MS and AFS (From Aucélio R.Q. et al. (2007, The Determination of trace metals in lubricating oils by atomic spectrometry. Spectrochim. Acta ., 62: 952-961).

Further reading

Aucélio R.Q. et al. (2007). The Determination of trace metals in lubricating oils by atomic spectrometry. Spectrochim. Acta ., 62: 952-961.

Varnes A.W. (1985). Analytical methods for the analysis of petroleum products, Spectrosc. 1: 28-33.

Sychra V. et a.. (1981) Analysis of petroleum and petroleum products by atomic absorption

K.J. Eisentraut (1984) Spectrometric oil analysis. Detecting engine failures before they occur, Anal. Chem. 56: 1086A-1091A.

Vahaoja P. et al. (2005) Determination of Wear Metals in Lubrication Oils: A Comarison Study of ICP-OES and FAAS, Anal. Sci. 21: 1365-1369.