Wear Metal Analysis in Used Oils- A potential application area of Atomic Absorption Spectroscopy

Collection of used automotive engine oil

Engines constitute an important power generation source in road transport vehicles, aviation and small scale power generation for industrial and agricultural applications. The efficiency of running of different types of engines depends on their regular maintenance and monitoring of condition of component parts. It is neither practical nor desirable to disassemble an engine every time and analyze components such as piston rings, pistons, bearings, engine casings, etc by conventional destructive or non-destructive means. Used oil analysis offers a viable solution to periodically monitor the working condition of engine components and help decide on corrective measures.

Lubricating oils serve as life-line of engines as they prevent excessive wear and tear of moving parts. However, over extended use wear and tear becomes unavoidable and wear metals find their way into the lubricating oils. Atomic Absorption spectroscopy, Optical Emission Spectroscopy and Inductively Coupled Plasma Spectroscopy are conventionally used in analysis of wear metals in used oils. Each technique has its own merits and demerits. Atomic Absorption Spectroscopy, however, provides a commercially viable option on account of its lower price and simplicity of operation.

Engine components are made from different metals and alloys depending on operational requirements. Over extended periods of use constant wear releases metals such as Cr, Cu, Fe, Al, Mg, Na, Ni, Mo, Pb, Si, Sn, Zn or Ti into the lubricating oil. A periodic monitoring of such metals can be helpful in concluding which components are suffering from more wear and undertaking preventive action before major engine breakdowns occur. The table below gives an indication of possible engine parts that have undergo damage due to excessive wear resulting from findings of analysis.

Element	Possibility of damage to component parts
Cr	Piston rings, Cylinder liners
Cu	Bearings, Valve guides
Fe	Ball and Roller bearings, Piston rings, Cylinder liners, Gears or due to rusting
Mo	Piston rings
Ti	Casings
Sb	Crankshaft and Camshaft bearings
Cd	Bearings

Key Analysis Considerations

Analysis of wear metals by Atomic Absorption Spectroscopy can provide reliable diagnosis of wear of engine parts provided following precautions are kept in mind during sampling and analysis.

• Sample withdrawn should be representative and homogeneous. It is advisable to take more than one sample- preferably 5 samples and homogenize before analysis.
• More frequent monitoring is needed depending on age and usage of an engine or if the content of any particular element is found to be unusually high.
• Sample should be withdrawn when engine is warm with dipstick end at same representative point
• Maintain log of oil changes or top-ups.
• Dissolution of oil samples directly in organic solvents such as xylene or methyl isobutyl ketone should be preferred over other methods involving filtration or ashing
• Standard and sample should be treated identically to maintain same characteristics , particularly viscosity and flow properties
• Adjustable nebulizer should be used to optimize conditions in the flame

It is essential to clean the nebulizer and spray chamber as per recommended procedures so that the Atomic absorption Spectrometer can be used for routine applications involving aqueous solutions.