Analyzing Wear Metals in Oil Under Ambient Air

A preventive measure to identify potential problems associated with an engine is to monitor the presence of wear metals in oils, lubricants and other machine fluids. Early detection of wear metals in greases and lubricants at an early stage helps avoid costly damage and downtime to high value engines, turbines, generators, gears and other equipment.

Instrument

The ARL QUANT’X EDXRF spectrometer employed for analyzing wear metals is provided with a latest-generation silicon drift detector (SDD) and a 50 kV, 50 W silver target X-ray tube. The ARL QUANT’X uses primary filtered radiation in order to excite the sample. Specifically designed to improve the peak-to-background for elements from F to Am, a set of nine filters ensures that the ARL QUANT’X is easily adaptable per element or application range.

Sample Preparation

The oil sample is measured alone by transferring 3 g of product into a sample cup of 32 mm outer diameter, covered with a 4 µm polypropylene film.

Excitation Conditions

The excitation condition used to carry out the analysis is shown in Table 1. In order to cover 14 elements, three different filter conditions have been used. A live time of 100 s is used for each condition. A typical spectrum achieved using condition Mid Zc of a sample containing a number of elements of interest at 50 ppm. Condition Mid Zc is optimized to excite lead, zinc and copper, and it can also excite several other wear metals typically monitored.

Table 1. Excitation condition used for wear metals in oil

Figure 1. Typical spectrum for wear elements at 50 ppm in oil using condition Mid Zc (30 kV, thick Ag filter) in an air atmosphere.

Calibration

Linear calibration curves that relate net intensities to concentrations are arranged using standards that are prepared using a Conostan S21 50 ppm standard and diluted using 75 cSt blank oil (also from Conostan). Three concentration levels: 10 ppm, 20 ppm and 50 ppm, and a blank were prepared to set up the curves and to determine the detection limits. The calibration curves achieved for nickel (Ni) and titanium (Ti) are shown Figures 2a and 2b. Root mean square errors (RMSE) of 0.2 ppm (Ni) and 0.6 ppm (Ti) are obtained. The RMSE values obtained for other elements of interest are shown in Table 2.

Figure 2a. Calculated versus given concentrations in the case of titanium (Ti).

Figure 2b. Calculated versus given concentrations in the case of nickel (Ni).

Limit of Detection

In order to determine the limit of detection (LoD), 10 XRF cups were filled with 3 g of the blank oil. The LoD is determined to be three times the standard deviation of the concentrations obtained for each element. The results are shown in Table 2.

Table 2. Concentration range, RMSE and LoD values for wear metals in oil.

Conclusion

The ARL QUANT’X EDXRF spectrometer’s unique capabilities provide accurate, fast and repeatable monitoring of wear metals in oils, lubricants and machine fluids. Analysis of these samples can be performed under ambient air which is convenient, reduces analysis time and offers savings on costly helium gas consumption.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.