Reading wear metals and additive elements in used oil
Elemental analysis reports parts per million of each metal. Use the table to link an element to likely sources, and watch the trend and the size limits of the method.
How elemental analysis works
The oil sample is excited so each element gives off light of its own colours. The brightness of those lines is proportional to how much of the element is present, and the result is reported in parts per million (ppm). Wear metals show which parts are wearing, contaminants show what is getting in, and additive elements show what oil is in the machine. Two excitation methods are common in oil labs: a spark from a rotating disc electrode, and an inductively coupled plasma.
Which element points to which part
| Element | Wear source | Contamination or additive source |
|---|---|---|
| Iron (Fe) | Gears, shafts, bearings, cylinders, the most common wear metal | Rust, wear from many components |
| Copper (Cu) | Bronze and brass bushings, thrust plates, oil coolers | Oil cooler corrosion, some additives |
| Tin (Sn), lead (Pb) | Bearing alloys, bushings, solder | Lead from greases and seals |
| Aluminium (Al) | Pistons, pumps, thrust washers, housings | Dirt, some coolers |
| Chromium (Cr) | Shafts, rings, liners, stainless steels | Some coolants |
| Nickel (Ni), titanium (Ti) | Turbine blades, valve train alloys, bearing hubs | Not usual |
| Silver (Ag) | Silver-plated bearing cages and pins, silver solder | Cooler solder |
| Silicon (Si) | Seal materials | Airborne dust and sand, silicone anti-foam, some coolants |
| Sodium (Na), potassium (K) | Not wear | Coolant leak, salt water, grease |
| Calcium (Ca), magnesium (Mg), zinc (Zn), phosphorus (P) | Zinc from brass components | Detergent and anti-wear additives, hard water |
| Molybdenum (Mo), boron (B) | Piston rings | Friction modifiers, additives, coolant |
Reading the numbers
- Trend each element for the same machine. A rise or a change of slope matters more than the absolute value.
- Several elements rising together point to a component: for example iron with copper and tin in a bronze-bushed gearbox.
- A sudden step in additive elements, such as zinc, phosphorus or magnesium, means a different oil was added.
- Compare with the new oil. Additive elements in the fresh oil are the baseline.
- Top-ups and filter changes can lower the reading and hide a trend, so note them in the record.
The size limit you need to know
Elemental methods measure well only the fine and dissolved particles. The practical limit is about 5 micrometres for plasma methods and about 10 micrometres for the spark method. Large wear particles, which appear late in a failure, can be missed. A filter-based preparation that captures the particles on the electrode, a ferrous density test, or a particle count and shape analysis closes that gap. If iron is rising and large particles are suspected, add one of these tests.
A mix-up that showed up in the numbers
| Sample | Fe | Cu | Ag | Mg | P | Zn |
|---|---|---|---|---|---|---|
| First | 19 | 10 | 0 | 0 | 0 | 3 |
| Second | 21 | 10 | 0 | 0 | 9 | 3 |
| Third | 27 | 13 | 2 | 107 | 75 | 90 |
| Fourth | 25 | 30 | 10 | 220 | 110 | 123 |
These are four successive samples from a medium-speed diesel engine, in ppm. Magnesium, phosphorus and zinc appear from the third sample, so the oil formulation changed after a top-up with the wrong oil. Copper and silver then rise, because the zinc-containing oil attacked the silver-plated bearing parts. Vibration and temperature monitoring would not have seen this. The case is in the field cases.